Earthquake Codes IBC

A review of seismic hazarddescription in US design codesand proceduresJames E Beavers

Mid-America Earthquake Center, Urbana, USA

SummaryA review of seismic hazard description in UnitedStates design codes and procedures is presentedin this paper. This review includes: history ofseismic hazard maps; development and use ofseismic hazard maps; and use and adoption ofseismic building codes in the US. The reviewincludes discussion of two paradigm shifts. The

first paradigm shift occurred in the 1970s whenseismic hazard was described as contour maps ofprobabilities of peak ground accelerations beingexceeded. The second paradigm shift occurred inthe 1990s and led to the development of a newconcept for describing the use of seismic hazardmaps in US seismic design codes.

Key words: seismic history; seismic codes; seismic hazard; seismic design; seismic maps

Prog. Struct. Engng Mater. 2002; 4:46–63 (DOI: 10.1002/pse.106)

Introduction

SEISMICITY

The contiguous US is considered as having twoseismicity regions, interplate and intraplate. In simpleterms, the interplate region is considered as west ofthe Rocky Mountains and the intraplate region east ofthe Rocky Mountains. The area east of the RockyMountains is often referred to as the central andeastern United States (CEUS). Technically, the USGeological Survey (USGS) defines the separationbetween the interplate and intraplate regions as anoverlapping area between 1008 and 1158 West[1].

A map of historical seismicity in the US is shown inFig. 1. The interplate region of the US is well knownfor its seismicity, including such earthquakes as the1906 San Francisco (M7.7), 1933 Long Beach (M6.4),1952 Kern County (M6.7), 1971 San Fernando (M6.7),1989 Loma Prieta (M7.2), and 1994 Northridge (M6.7).However, the largest earthquakes in the contiguousUS occurred in the intraplate region of the CEUS inthe New Madrid Seismic Zone (NMSZ). Theseearthquakes occurred in the 19th century, on 16December 1811, 23 January 1812 and 7 February 1812,having moment magnitudes of M8.1, M7.8, and M8.0,respectively[2,3]. Two other historically largeearthquakes have also occurred in the CEUS, 1755Cape Ann (M6.8), off the coast of Boston,Massachusetts, and 1886 Charleston, South Carolina(M7.3). Prehistoric large earthquakes have also been

identified in the intraplate region of the NMSZaround 1450, 900 and earlier[4,5] and of the Charleston,South Carolina area between 6000 BC and 500 AD[6].During the 20th century no earthquakes haveoccurred in the CEUS with a moment magnitudelarger than a mid-five.

RISK

As stated in the Earthquake Hazards Reduction Act[7]:‘All 50 States are vulnerable to the hazards ofearthquakes, and at least 39 of them are subject tomajor or moderate seismic risk.’

Like many parts of the world, as populationsincrease, risk increases. For example, when the NewMadrid earthquakes of 1811–1812 occurred, thecontiguous US population was 7 million withthe geographic centre of population 1600 km from theNMSZ, just northeast of Washington, DC. Today, thatpopulation has grown to 280 million, with 219 millionliving in the CEUS[8]. Ironically, the geographic centreof population is now a few km northwest of theNMSZ. To support the populated area of the CEUS,there are over 75 million housing units, 3.5 millioncommercial buildings and 350 thousandmanufacturing establishments[9,10]. Over 25% of thetotal US energy consumption is transported interstateby pipeline or river barge throughout the CEUS, andover 80% of the US electrical energy production isconsumed in the CEUS[11,12]. The largest populated

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Copyright & 2002 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2002; 4:46–63

area near the NMSZ is Memphis and Shelby County,Tennessee, with a population of 897 472[8]. In 1820,eight years after the New Madrid earthquakes,Memphis and Shelby County had a total populationof 354[13]. Based on the population growth from 1820to 1830 (354 to 5652), the population in 1811 and 1812would have been only a few, if any.

The 1989 Loma Prieta earthquake resulted in 63deaths and $10 billion in damage, and the 1994Northridge earthquake resulted in 57 deaths and inexcess of $20 billion in damage. While these losses aresignificant, losses in the CEUS from a repeat of an1811–1812 New Madrid earthquake have beenprojected as being several thousand deaths[14,15] and$200 billion in damage. The interplate region of the UScan be considered a seismicity region of frequentearthquakes with moderate-to-high consequences, whilethe intraplate region of the CEUS can be considered aseismicity region of infrequent earthquakes with highconsequences. Because future earthquakes can occuranywhere in the US the adoption of seismic buildingcodes containing well-defined descriptions of seismichazard is required to reduce seismic risk.

SEISMIC DESIGN CODES AND PROCEDURES

During the 20th century there were three prevalentregional building codes in the US. Developed in theearly to mid-1900s, these codes were the UniformBuilding Code (UBC) of the International Conferenceof Building Officials (ICBO), the National BuildingCode (known as the BOCA Code) of the BuildingOfficials and Code Administrators (BOCA), and theStandard Building Code (SBC) of the Southern

Building Code Congress International (SBCCI).Typically, the UBC was used west of the MississippiRiver, the BOCA Code was used in the uppermidwest and northeast, and the SBC was used in thesouth.

A fourth code existed during the 20th Century,known as The National Building Code, published bythe American Insurance Association[16]. This code wasfirst published in 1905 as the Building Coderecommended by the National Board of FireUnderwriters[17]. Seismic design provision did notenter this code until the 1976 edition recommended bythe successor to the National Board of FireUnderwriters, the American Insurance Association.The 1976 edition was the last publication of TheNational Building Code, as the American InsuranceAssociation ceased to promote it after 1976[18,19].

Other seismic design procedures in the form ofregulations, standards and guidelines were developedin the US in the mid to late 1900s. The Atomic EnergyCommission (AEC)[20], now split into US NuclearRegulatory Commission (NRC) and US Departmentof Energy (DOE), and the US EnvironmentalProtection Agency (EPA)[21,22] prepared regulationsfor seismic design of nuclear facilities and wastedisposal landfills. The AEC[23], National Bureau ofStandards (now National Institute of Standards andTechnology, NIST)[24], American National StandardsInstitute (ANSI)[25], American Society of CivilEngineers (ASCE)[26,27], Institute of Electrical andElectronics Engineers[28], US Army, Navy, and AirForce[29] and DOE[30,31] prepared various standardsand guidelines. Each regulation, standard orguideline specified some form of seismic hazard

Fig. 1 Historical seismicity of the contiguous USA

SEISMIC HAZARD DESCRIPTION IN US CODES 47


description; however, except for the AmericanNational Standard A58.1 developed by ANSI[25] andlater replaced by ASCE 7[27], all other regulations,standards and guidelines were generally developedfor special cases and have not been referenced by USbuilding codes.

Local city and state codes were also developedduring the 20th century. Cities such as New York, LosAngeles, Philadelphia, Charleston, South Carolina,and Chicago either adopted a version of regionalcodes or developed their own code. The firstseismic codes in the US were developed and adoptedat the local level in Santa Barbara and Palo Alto,California, following the 1925 Santa Barbaraearthquake[32].

On a regional basis, the first written seismic designprovisions occurred in the 1927 (first) edition of theUBC[33]; however, these provisions did not becomemandatory until 1961. Adoption of seismic provisionsinto the BOCA and SBC Codes did not occur until thelatter part of the 20th century. Seismic designprovisions were included in the SBC in 1976[34] byreferencing ANSI A58.1[35]; however, these provisionswere not mandatory unless local authorities requiredseismic design, which was rarely the case. In 1988, theSBCCI adopted the 1982 ANSI A58.1[36] seismicprovisions into the main body of the SBC making theprovisions mandatory.

The BOCA Code was first published in 1950[37] andincluded seismic design provisions. The BOCAprovisions were similar to the 1927 edition of the UBCrequiring a lateral load be considered ranging from 5to 20% of the weight of the building, depending onheight. In the next edition of BOCA[38] seismic designprovisions were added that were similar to the 1955edition of the UBC, including using seismic hazarddescription in the form of the 1949 US Coast andGeodetic Survey (USCGS) seismic hazard map;however, seismic design exemptions were placed inthe 1955 edition of BOCA that continued for over 20years. As late as the 1978 edition[39], one exemptionstated: ‘Earthquake loading shall not be required . . .when the building complies with . . . the following . . .

is located where local experience or the records . . . donot show loss of life or damage to property, regardlessof zone.’ Owing to a relative lack of seismic activity inthe CEUS during the 20th Century, this statementallowed exemption from seismic design in buildingswere this code had been adopted.

Early history of hazard descriptionin US building codes

SEISMIC DESIGN

In his chapter on Municipal Building Codes forProtection against Earthquake Damage, John R.Freeman[40] made the following statement: ‘The

preceding pages plainly show that the art ofconstructing earthquake-resisting buildings is still inthe formative stage, that there are differences ofopinions among experts and that there is muchdeficiency in important data.’ He goes on to say: ‘Themovement did not become active in the United Statesuntil after the Santa Barbara disaster in 1925, and hasnot yet gone far in new researches, but has promotedmuch activity toward better building codes inCalifornia.’

When the first seismic design provisions wereintroduced into the 1927 edition of the UBC followingthe 1925 Santa Barbara, California earthquake (M6.8)no seismic hazard maps of the US were in existence.The seismic load F was simply determined to be alateral load on a building equivalent to 7.5% (10% onpoor soils) of the building’s weight W as shown inEq. (1).

F ¼ 0:075W (1)

The seismic provisions in the 1927 UBC werespecified under Section 2311, Earthquake Regulations;however, this section refers the user to the Appendix.In the Appendix under the section Refer to Sec. 2311 itis stated: ‘The following provisions are suggested forinclusion in the Code by cities located within an areasubject to earthquake shocks.’ As a result, theprovisions were not mandatory. It was not until afterthe 1933 Long Beach, California earthquake, in whichmany schools collapsed, that the provisions becomewidely accepted. The Field Act of 1933 mandatedseismic design of schools in California and the RileyAct of 1933 mandated seismic design for buildings inCalifornia[41,42].

The simplified approach to seismic design asdeveloped for the 1927 edition of the UBC essentiallyremained in place until the 1961 edition of the UBCwas published; however, in 1928 Heck[32] developed aseismic map that resulted in seismic zonation ofeleven western US states into three zones havingequal probability. This zonation map was adoptedinto the 1935 edition of the UBC (Fig. 2), resulting inthe lateral force equation being changed to:

F ¼ CW (2)

where the value C became a minimum value forseismic Zone 1. The minimum value of C for buildingswas either 0.02 or 0.04, depending on soil capacity. ForZone 2, C was to be doubled and for Zone 3, C was tobe multiplied by four.

In 1952, ASCE published Lateral Forces of Earthquakeand Wind[43] that gave Eq. (3) for seismic design,representing the first dynamic approach to theproblem:

V ¼ CW (3)

where C was defined as a function of the inverse ofthe structure period; however, this concept was notadopted into the UBC until 1961 when the lateral force

EARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS48


equation became:

V ¼ ZKCW (4)

where C is a numerical coefficient based on theinverse of the cube root of the structure period, K anumerical coefficient based on building type and Z anumerical coefficient based on seismic zonedetermined from the accompanying seismic hazardmap.

In 1945, ANSI published ANSI A58.1[25] that, for thefirst time, considers earthquake loads in a standard.The lateral force equation was the same as Eq. (2)while C was taken as 0.1 for each story of a building.Values of C for parts of buildings were either 0.2 or1.0, depending on the part. A seismic hazarddescription was included in the Appendix in the formof a map showing earthquake locations of destructiveintensity; however, seismic hazard description playedno direct role in the standard. Seismic designrequirements in the next edition of ANSI A58.1[44]remained basically the same, including the seismichazard description, except the values of C weremodified.

INTRODUCTION OF SEISMIC HAZARD MAPS

Although Heck developed one of the first seismichazard maps, the first seismic zone map of thecontiguous US was developed in 1948 (Fig. 3) by the(USCGS)[45]. The hazard was defined into four zoneswith Zone 3 representing major damage and Zone 0representing no damage. The map identified many

Fig. 3 1948 seismic probability map of the US

Fig. 2 1933 seismic hazard map of the western USA. Portionsof this work are reproduced from the 1935 edition of the UniformBuilding Code (TM), copyright 1935 edition, with thepermission of the publisher, the International Conference ofBuilding Officials (ICBO). ICBO assumes no responsibility for theaccuracy or the completion of summaries provided.



areas in the US as having significant seismic hazard,including the CEUS. The NMSZ and Charleston,South Carolina areas were assigned as having thesame seismic hazard as the highest zone of California.According to Algermissen[46], Roberts & Ulrich[45]

developed the map on the premise that similarearthquakes will occur in the future where they haveoccurred in the past. Although the 1948 map was notwidely published until 1950, the ICBO adopted themap into the 1949 edition of the UBC[47] for use in thedesign of buildings.

The 1948 map was revised in 1949[48]. The revisionsresulted in a lowering of the seismic hazard inCharleston, South Carolina area from Zone 3 to Zone2, and raising the seismic hazard in Puget Soundregion of Washington from Zone 2 to Zone 3 (Fig. 4).Following the development of the revised 1949 map,the UBC adopted the 1949 map into the 1952 editionof the UBC[49]. Uncertainty about seismic hazarddescription continued to exist when the Charleston.South Carolina area was upgraded back to Zone 3 inthe 1958 edition of the UBC and downgraded back toZone 2 in the 1961 edition[32].

RESISTANCE TO SEISMIC DESIGN

Although the 1949 map was maintained in the UBCuntil the UBC was updated in 1970, in January 1952the USCGS made the following official statement[50]:‘The Seismic Probability Map of the United States,SMC-76, issued by the US Coast and Geodetic Surveyin 1951, has been withdrawn from circulation becauseit was found to be subject to misinterpretation and toogeneral to satisfy the requirements of many users.’

In place of the Seismic Probability Map, the USCGSoffered a map showing the distribution of important

earthquakes which was to be used as a guide inevaluating earthquake risk. Apparently this actionwas followed by protests of groups of businessmeninterested in getting lower risk ratings in theirlocalities[51]. It is not clear why the USCGS withdrewthe 1949 Seismic Probability Map[52]; however, in 1995the following statement was made[32]: ‘. . . the UnitedStates Geodetic Survey retracted a map in the 1950’sbecause business and scientific interests appliedpressure on the grounds that the map (like all science)was subject to misinterpretation.’ There also appearsto have been resistance by the public and other federalagencies to design buildings and facilities toearthquake loads where earthquakes were notgenerally known to occur.

In 1950, the AEC, now the US Department ofEnergy, was building uranium-enrichment plants tosupport the Cold War effort in Paducah, Kentucky,Portsmouth, Ohio, and Oak Ridge, Tennessee.Paducah, Kentucky, is 80 km northeast of theepicentre of the 7 February 1812 New Madridearthquake and located in Zone 3 of the 1949 SeismicProbability Map, while Oak Ridge, Tennessee, andPortsmouth, Ohio, were each located in Zone 1. ThePaducah plant process buildings were two storey inheight, each 13 ha in plan, and the associated processequipment was located on the second floor, 8.5mabove grade.

On 13 August 1951, a memorandum wasapparently sent to Mr Red Williams, Director ofProduction and Engineering titled: Design Criteria forthe Paducah Project. One month later, on 12 September1951, Mr Williams responded by issuing amemorandum titled: Design Criteria for the KentuckyArea, were it is stated[53]: ‘Zone 1 criteria were used inchecking for seismic shock on all buildings . . . On

Fig. 4 1949 revised seismic probability map of the US



21 December 1950, representatives of the AEC . . . USCorps of Engineers and participating architect-engineers met . . . The US Corps of Engineers, whohave done considerable construction in this area,stated that they did not consider seismic shock in theirdesigns. It was generally agreed by all concerned thatthe Zone 1 criteria would be adequate.’

The design to Zone 1 criteria was further justified amonth later in a memorandum on 12 October 1951, inwhich it was stated: ‘I suspect that low, sprawlingbuildings such as those under construction atPaducah, because of their great width, will have astrength somewhat in excess of that specified for Zone1.’ Eleven days later, 23 October 1951, a finalmemorandum was written on the subject: ‘I concur . . .this matter is now closed as far as Washington isconcerned and further action is not required by thisoffice.’

None of the memoranda mention the New Madridearthquakes of 1811 and 1812, and based on a study ofthe seismic hazard criteria of the AEC uranium-enrichment plants[53], it appears the process buildingswere not designed for Zone 1 seismic loads. In the late1990s the US Department of energy completed a $30million upgrade of the process buildings to bringthem into compliance with an earthquake peakground acceleration that has a 10% chance of beingexceeded in the next 25 years (250-year event).

The above actions may, or may not, have led to thewithdrawal of the USCGS Seismic Probability Map;however, they give the reader a sense of how thepopulace has considered and continues to considerthe earthquake threat in the US. In any event, on

21 July 1954, the AEC published seismic design criteriafor all construction in the US stating[54]: ‘Earthquakeloads should be in accordance with Section 2312 andAppendix, Section 2312 of the Pac. U.B.C. Inborderline cases, i.e., wherever a lower intensityseismic zone may be justified, a study should be madeof the earthquake history of the locality in question.’

Seismic hazard description and designstatus 1960–1985

MILESTONES

Three milestones occurred in the 1960s that influencedthe application of seismic design and thedevelopment of seismic hazard maps. The firstmilestone occurred in 1960 when the ICBO adoptedseismic design provision into the main body of the1961 edition of the UBC[55]. This action made seismicdesign provisions mandatory for those communitiesadopting the 1961 and later editions of the UBC.

The second milestone occurred in 1969 when theUSCGS presented a new seismic hazard map at theFourth World Conference on Earthquake Engineeringcalled the Seismic Risk Map of the United States[56]. Likethe 1949 Seismic Probability Map 20 years earlier, thenew map used the same four zones to describeseismic hazard (Fig. 5). This map had a significantimpact in the CEUS in that many Zone 0 areas becameZone 1 and many Zone 1 areas became Zone 2. As aresult, the description of seismic hazard throughoutthe CEUS had increased over what was previouslybelieved. For example, while the DOE facilities at

Fig. 5 1969 Algermissen seismic hazard map



Paducah, Kentucky, remained in Zone 3, the facilitiesat Oak Ridge, Tennessee, were now in Zone 2.

The third milestone also occurred in 1969 when theICBO adopted the 1969 USCGS seismic hazard mapinto the 1970 edition of the UBC[57]. On 21 May 1970,the Director of Construction of the AEC sent a letter toall field offices stating[58]: ‘This risk map has beenrecently modified . . . in the 1970 edition of theUBC . . . The ‘‘Southern Standard Building Code’’ isoptional for projects located east of the MississippiRiver . . . but where this code is used, the seismic riskprobability contained in the UBC should be applied tothe design of buildings.’At the time, the SBC, whichwas then called the Southern Standard Building Code,did not have provisions for seismic design[18].

SIGNIFICANT EARTHQUAKES

In addition to these milestones, two majorearthquakes occurred in the US between 1960 and1971 that had an impact on seismology andengineering in the US. The first was the 1964 Alaskaearthquake (M9.0) and second was the 1971 SanFernando earthquake (M6.6). Although the Alaskanearthquake stimulated the USCGS to revitalize itseismic hazard mapping programme that led to the1969 map[32], it did not capture the attention ofengineers and scientists as it should, perhaps becauseof the relatively remote location. Following the 1971San Fernando earthquake, a much greater responseoccurred among engineers and scientists. One reasonfor this response can be attributed to the fact that onedamaged hospital, the Olive View Hospital, had been

designed and constructed to the 1970 edition of theUBC. Because of the hospital’s extensive damage,caused by what is now referred to as soft storeyresponse, the building had to be torn down. Inaddition, many other buildings and structures,especially bridges and overpasses, performed poorly.As a result, a movement by federal agencies,engineers and scientists developed to better describethe seismic hazard, conduct new research on structureperformance and develop new seismic designprocedures.

ANSI A58.1

In 1972 ANSI updated the 1955 edition of ANSI 58.1.The seismic design lateral force equation specified inthe standard was the same as that in the 1961 editionof the UBC, i.e. Eq. (4); however, a slightly modifieddescription of seismic hazard was developed by theAmerican National Standards Committee. Thisdescription was similar to the original 1949 USCGSmap; however, seismic Zones 0 and 1 were combinedinto seismic Zone 1 and seismic Zone 1 west of 1108was upgraded to Zone 2 (Fig. 6). It is not known whythe 1969 USCGS map was not used for the update;however, recollection by Professor W.J. Hall (personalcommunication, 22 January 2002) points to concernsabout the USCGS description of seismic hazard incertain areas as being conservative. The 1972 editionwas then updated in 1982[36]. The 1982 edition seismichazard description was defined on the basis of theresults of a paradigm shift that was occurring inearthquake engineering, as discussed below.

Fig. 6 1972 ANSI seismic hazard map



PARADIGM SHIFT

In response to the poor performance of buildingsduring the San Fernando earthquake, it was evidentthat a paradigm shift needed to occur in USearthquake engineering. The USGS (which took overresponsibility for seismic hazard mapping in the early1970s), NBS and the National Science Foundation(NSF) became major players in promotion of thisparadigm shift. NBS and NSF funded the formation ofthe Applied Technology Council (ATC) in 1973 todevelop comprehensive national seismic designguidelines. At the same time, the USGS began areassessment of seismic hazard description, whichresulted in the 1976 seismic hazard map[59]. The mapitself was a paradigm shift because, for the first time,seismic hazard was now described in the form ofprobabilistic estimates of maximum accelerationcontours on rock (Fig. 7).

In 1978, ATC published national seismic designguidelines known as ATC 3-06[60]. Incorporated intothe guidelines were the first versions of engineeringdesign maps defining seismic hazard in the form ofeffective peak acceleration and effective peak velocitycontour maps. The maps represented modifications tothe Algermissen and Perkins (1976) map to betterrepresent earthquake engineering design parameters,and provided contours of estimated accelerationvalues in rock having a 10% probability of beingexceeded during a 50-year period (return period of500 years). The effective peak acceleration map is shownin Fig. 8.

One unique aspect of the effective peak map is that incertain areas of California the peak acceleration valueswere about half the Algermissen and Perkins mapvalues. As stated by Algermissen[46]: ‘The ATC

Effective Peak Acceleration map is very similar to theAlgermissen–Perkins acceleration map with theexception that the largest values of groundacceleration shown on the ATC map are 0.4 g inCalifornia, while the Algermissen–Perkins map hasaccelerations as high as 0.8 g in California. Thisimplies that the probability of exceedance of 0.4 g issomewhat underestimated within the 0.4 g contours ofthe ATC map. The ATC Effective Peak Velocity mapwas derived from the Algermissen–Perkinsacceleration map using principles and rules-of-thumboutlined in the report . . .. The ATC-3 report is anexcellent example of the use of recent research resultsin an interdisciplinary effort to produce new seismicdesign provisions.’

Algermissen was quite positive about theinterdisciplinary effort between engineers andseismologists to produce the new seismic designprovisions; however, as engineers and seismologistsbecame more knowledgeable about seismic hazard, itbecame apparent that judgemental truncation of peakacceleration values should not have been conducted.At the time, however, it was hoped that enhancedductility provisions would cover any shortfall createdby truncation[61]. In any event, the effective peakacceleration and velocity maps of 1978 became thebaseline for the NEHRP Provisions until the creationof Project 97 in 1994, as discussed below.

In the update of ANSI A58.1-82[36] the provisions onearthquake load drew considerably from the ATC3-06guidelines, but retained many features from the 1979edition of the UBC. For example, while ANSI A58.1-1982 included the effective peak acceleration map(Fig. 8), the effective peak acceleration contours weredefined as boundaries to five seismic zones as in the

Fig. 7 1976 Algermissen and Perkins seismic hazard map



UBC, Zones 0–5, for design purposes. Using the ANSIA58.1-1982, seismic design procedures becamemandatory in the 1987 edition of the BOCA Code andthe 1988 edition of SBC.

In addition to the inclusion of new and innovativeseismic hazard maps as a paradigm shift inearthquake engineering, the 1978 ATC 3-06 seismicdesign guidelines represented a truly nationalconsensus set of seismic design procedures. For thefirst time the US had a set of seismic design guidelinesthat provided seismic design and analysis proceduresthat went beyond the simplified static analysisapproach. The guidelines laid the foundation for theNEHRP Recommended Provisions[62] and led to theseismic provisions that are now included in the 2000edition of the International Building Code(IBC)[63].

LLNL AND EPRI

Beginning in the 1960s and continuing through 1980s,the US had an extensive programme of nuclear powerplant construction. A major safety issue was theseismic performance capability of a plant to withstandearthquakes. Some of the first landmark work onseismic hazard description was summarized by theAEC[23]. In the 1970s, the NRC began sponsoring theLawrence Livermore National Laboratory (LLNL) todevelop a probabilistic seismic hazard analysis(PSHA) methodology for the CEUS. In conjunctionwith sponsoring LLNL, NRC recommended that thenuclear power industry perform an independentstudy to provide a coordinated utility position onPSHA estimates and provide NRC with comparativeinformation. A consortium of nuclear power utilitiesfunded the Electric Power Research Institute (EPRI) toperform a seismic hazard study. In addition tofocusing on where earthquakes had occurred in the

historical past, LLNL and EPRI studies focused on thegeology and the likelihood an earthquake may occurin a specific tectonic province where no earthquakeswere known to have occurred.

By 1985 LLNL and EPRI had developed theirindependent methodologies[64,65]; however, there wasconsiderable disparity between results when bothmethodologies were used for the same site. For mostnuclear plant sites the LLNL methodology resulted ina significantly higher seismic hazard. Part of thisdisparity had to do with the way in which LLNLapplied the use of expert opinion. LLNL developed aseismic hazard model that used seismicityinformation from expert input (11 seismicity and 5ground motion experts) and a Monte Carlo simulationapproach, while EPRI seismicity information wasprovided by six teams of geoscience experts. In 1993,LLNL published the results of a study focused onimproving the elicitation of data and its associateduncertainty from the experts to better capture the trueuncertainty of the state of knowledge[66]. The resultsof this study showed a reduction in seismic hazardmore closely associated with EPRI results.

Issues of uncertainty and use of expert opinion inthe LLNL and EPRI methodologies continued toremain after publication of the 1993 LLNL document.Because comparative evaluations showed that thedifferences between PSHA studies were often nottechnical, but due to information gathering andassembly process, NRC, DOE and EPRI sponsored astudy to focus on integration and evaluation issuesthat should be considered in a PSHA[67,68]. In 1990,Reiter also discussed many issues of PSHA in hisbook Earthquake Hazard Analysis}Issues andInsights[69].

The use of LLNL and EPRI PSHA methodologiescontinue to be used for specific nuclear power plant

Fig. 8 1978 ATC effective peak acceleration map



sites or other high-hazard (nuclear or chemical)facility sites, e.g. the DOE Savannah River Site. Themethodologies were never considered as a tool to beused in US building codes; however, as discussedbelow, the results of the LLNL and EPRImethodologies have been used to calibrate and verifythe USGS Project 97 work.

Seismic hazard description anddesign status 1985–2001

NEHRP PROVISIONS

Following the 1971 San Fernando earthquake theUnited States Congress passed the EarthquakeHazards Reduction Act of 1977[7]. The Act becameknown as the National Earthquake HazardsReduction Program (NEHRP). The FederalEmergency Management Agency (FEMA), NIST, NSF,and USGS were assigned as partners in NEHRP.

In 1979 the Building Seismic Safety Council (BSSC)was formed within the National Institute of BuildingSciences. BSSC was funded by FEMA, under NEHRP,to take the ATC 3-06 document and transform it into aset of seismic building design provisions. Theseprovisions became known as the National EarthquakeHazards Reduction Program (NEHRP) RecommendedProvisions for Seismic Regulations for New Buildings andOther Structures. The first set of NEHRP RecommendedProvisions was published in 1985[62]. Since that time,the Provisions Update Committee (PUC) of the BSSChas been updating the NEHRP RecommendedProvisions every 3 years. Holmes[70] provides a briefdiscussion of changes in the Provisions from 1985through 1997.

When the 1985 NEHRP Recommended Provisionswere published, the seismic hazard maps contained inthe Provisions were the same maps as published in the1978 ATC 3-06 guideline document. AfterAlgermissen, Perkins and colleagues published the1976 seismic hazard map, the USGS continued towork on better descriptions of seismic hazard[71,72]. In1988 the PUC began the updating process of the 1991update of the Provisions. As part of the processdiscussion occurred about the need for updating thedescription of seismic hazard throughout the US,based on new knowledge gained in understandingseismic hazard during the past 10 years. New spectralmaps were introduced into the 1991 NEHRPProvisions appendix, including, for the first time, amap representing a 10% probability of spectral valuesbeing exceeded in 250 years (2500-year event). Thesemaps[73] were based on spectral accelerations for twoperiods, 0.3 and 1.0 s. Unfortunately, caveats wereplaced on the introduction of the maps, e.g. theyshould not be used for design. For design purposesthen, the 1991 NEHRP Recommended Provisions[74] were

published, using the same 1978 maps from the 1985NEHRP Provisions.

In preparation for the 1994 NEHRP RecommendedProvisions update, FEMA funded the BSSC to establisha Design Values Panel to update the maps. While thePanel made progress and resolved some issues, it wasnot able to reach consensus on a final product and the1978 maps were again used as the seismic hazarddescription of the US[75]. From the beginning of the1991 NEHRP Provisions update process in 1988 to thefailure of the Design Values Panel to reach consensusin 1994, Mittler et al. call this the ‘controversialPeriod’[32].

A SECOND PARADIGM SHIFT

As a result of the Design Values Panel’s failure toreach consensus, a second paradigm shift in USearthquake engineering occurred. The BSSC, FEMAand USGS began to take aggressive action in late 1993to resolve the problem. In December 1993, the author(at the time Chairman of BSSC) and Dr Walter Hays ofthe USGS developed a concept for resolving theDesign Values Panel issues called Project 97. TheFebruary 1994 BSSC Annual Meeting theme focusedon Project 97[76–79] and Project 97 was formallyaccepted by BSSC, USGS and FEMA[80–83]. Theprimary goals of Project 97 were to: (1) developnational seismic hazard maps that represent aconsensus baseline for seismic hazard descriptionthroughout the US, and (2) develop national seismichazard design values for use as consensus input forpublished provisions and guidelines for seismicdesign and evaluation using the seismic hazard mapsas the baseline. It was stated[81]: ‘With regard to the1997 update of the NEHRP Recommended Provisions,probably the most critical issue involves the seismichazard maps and an appropriate design procedurebased on those maps’.

While the two goals defined above were key toProject 97, the overall goal was to replace the morethan 20-year-old seismic hazard maps that everyoneacknowledged were out-of-date. To do so, it wasnecessary to improve both the maps and theprofession and public’s perception of them.

To initiate Project 97 the BSSC and USGS developeda partnership through the signing of a Memorandumof Understanding (MOU) in 1994. The USGS role inthe partnership was to develop a new probabilisticprocess for US seismic hazard description and wasrequired to address current ground motion issuesusing public forums and resolve these issues in amanner that would be considered fair andunbiased.

BSSC’s role in the partnership required thedevelopment of a seismic design procedure for use byengineers and architects based on, but separate from,the USGS seismic hazard maps. To do this, the BSSCformed a Seismic Design Procedures Group (SDPG).



BSSC was also charged with ensuring the SDPGprocess used to develop the procedures included amechanism that provided public input and the BSSCconsensus process.

To oversee Project 97, BSSC, with input from FEMAand USGS, appointed a Project ManagementCommittee (PMC) to guide the project. The PMCestablished a Resource Group consisting ofmembership from ATC, BOCA, Center for EarthquakeResearch and Information (CERI), FEMA, ICBO,National Center for Earthquake Engineering Research(NCEER), NIST, NSF, SBCCI, relevant StructuralEngineers Associations (i.e. California, Illinois,Washington), and USGS. The organizationalmanagement structure of Project 97 is shown in Fig. 9.For more details on many of the issues leading up tothe development of Project 97 see Mittler et al.[32].

USGS ACTIONS

By the time the BSSC formally presented the conceptof Project 97 to the earthquake engineeringcommunity, the USGS had already begun developinga new process for description of seismic hazard. Bythe summer of 1994, when the MOU was signed byUSGS and BSSC, USGS’s new seismic hazarddescription programme was well underway. A total ofseven USGS consensus building regional workshopswere held beginning in June of 1994 and ending inSeptember of 1996[1]. At these workshops USGSreceived input from seismologists, geophysicists,geologists, and structural and geotechnical engineers.

As part of its development process the USGS alsoheld a national workshop with ATC in September1995[84]. At the workshop four key seismic hazarddescriptive issues were addressed, all of which had

been discussed in one fashion or another, withoutclear resolution, at the 1994 and 1995 BSSC AnnualMeetings. The issues and correspondingrecommendations were:

* Issue 1: What ground motion parameters should bemapped?

Recommendation: A 5%-damped elastic responsespectral value for seismic structural design. For soilfailure evaluations, peak ground motionparameters (peak acceleration, velocity, anddisplacement) should be mapped. It was alsorecommended that: magnitude and distancecontributions to the seismic hazard be determinedat selected locations; seismic source maps anddocumentation of seismic source parameters andground motion attenuation relationships bepresented; and results of the ground motionmapping be made available to users.

* Issue 2: What reference site conditions should beused as a basis for mapping?

Recommendation: Rock preferred over soil.* Issue 3: Should maps be based on a probabilistic

approach, a deterministic approach, or both?

Recommendation: Strong preference forprobabilistic-based, rather than deterministic-based, ground motion maps. The desiredprobability of exceedance levels for the maps were20, 10, 5, and 2% in 50 years (250-, 500-, 1000-, and2500-year return periods).

* Issue 4: How should uncertainty in both seismicsource characterization and ground motionattenuation be incorporated in the mappingprocess and in the interpretation of results?

Fig. 9 Project 97 organization chart



Recommendation: USGS results should becompared with existing site-specific analysisresults where detailed procedures for incorporatinguncertainty had been employed. Also detaileddocumentation of the hazard mapping andindependent formal peer review of the modellingand results must be done.

In December 1995 interim maps were placed on theUSGS website depicting probabilistic ground motionsand spectral response with 10, 5 and 2% probabilitiesof exceedance in 50 years, or return periods of 500,1000, and 2500 years. After considerable review andcomment, including comparing results with LLNLand EPRI PSHA methodologies, the final seismichazard maps[1,85] for Project 97 were placed on theUSGS website in June 1996 and the maps became partof the 1997 NEHRP Provisions. The acceleration andspectral values were based on a B–C boundarycondition, defined as firm rock where average shearwave velocities were defined as 760m/s in the top30m.

Today the USGS seismic hazard maps can be foundat http://geohazards.cr.usgs.gov/eq/. An end usercan insert a longitude and latitude and receive theseismic hazard in terms of probability of exceedance,peak acceleration and spectral accelerations. Inaddition, deaggeration of the seismic hazard can beconducted and six artificial time histories for a firmrock site can be obtained. The time histories envelopethe spectral shapes derived for the specified location,using the seismic design procedures developed by theSDPG. The next generation of USGS maps is in theprocess of being developed as further discussedbelow.

SDPG ACTIONS

To develop the seismic design procedures for usingthe USGS seismic hazard maps, the SDPG and PMCworked closely with the USGS to define the BSSCmapping needs and to understand how the USGSseismic hazard maps should be used to develop theBSSC seismic ground motion maps and designprocedures. The goals of the SDPG were as follows:

1. To replace the existing effective peak accelerationand velocity-related acceleration design maps withnew ground motion spectral response maps basedon new USGS seismic hazard maps.

2. To develop new ground motion spectral responsemaps within the existing framework of theProvisions with emphasis on uniform marginagainst the collapse of structures.

3. To develop design procedures for use with the newground motion spectral response maps.

The SDPG also made four policy decisions thatdeparted from past practice:

1. The USGS maps were to define maximumconsidered earthquake (MCE) ground motion foruse in design procedures.

2. The design maps were to provide an approximateuniform margin against collapse for groundmotions in excess of the design levels.

3. The design maps were to be based on bothprobabilistic and deterministic seismic hazarddescription.

4. The design maps were to be response spectraordinate maps and reflect the differences in theshort-period range of response spectra for areas ofthe US and its territories with different groundmotion attenuation and recurrence times.

A wealth of new data existed for the SDPG toevaluate when attempting to accomplish its goals andobjectives. One major technical advance made inPSHA was finding that the rate of change of groundmotion versus probability is not constant throughoutthe US (Fig. 10). For example, in Los Angeles,California, the rate of change of the 0.2-s spectralacceleration between the 2 and 10% probabilities ofexceedance in 50 years (2500-year earthquake versus a500-year earthquake) is about 1.7, while in Memphis,Tennessee it is 5.1. With current building code seismichazard design levels being defined as a 500-yearreturn period event, a building in Memphis wouldneed three times the conservatism in designcompared with a building in Los Angeles to preventcollapse in a rare, but real, event. As stated in the 1997NEHRP Provisions[86]: ‘The collective opinion of theSDPG was that the seismic margin contained in theProvisions provides, as a minimum, a margin of about1.5 times the design earthquake motions. In otherwords, if a structure experiences a level of groundmotion 1.5 times the design level, the structure shouldhave a low likelihood of collapse . . . The USGSseismic hazard maps indicate that in most locations inthe United States the 2 percent probability ofexceedance in 50 years ground motion values aremore than 1.5 times the 10 percent probability ofexceedance in 50 years ground motion values.’

Ground motion values for a 2500-year event beingmore that 1.5 times a 500-year event was notacceptable to the SDPG. In addition, the SDPG foundthe estimated maximum magnitude earthquakes forcoastal California had recurrence intervals less thanthe 500-year events on probabilistic maps. Given thislatter finding, the SDPG made the decision to developa procedure that would use the best estimate ofground motion from the maximum magnitudeearthquakes on seismic faults with high probabilitiesof occurrence (short return periods). The SDPGdefined these earthquakes as deterministic earthquakesand found the level of seismic safety using thedeterministic earthquake approach would beapproximately equivalent to that associated witha 2–5% probability of exceedance in 50 years for areasoutside coastal California. This led the SDPG to selectthe 2% probability of exceedance in 50 years as theMCE ground motion for use in design outside of



coastal California as acceptable to handle the disparityin seismic margin issues[86,87]. MCE ground motionsare uniformly defined by the SDPG as the maximumlevel of earthquake ground shaking considered asreasonable to design structures to resist.

To develop the MCE ground motions for use indesign the SDPG required two distinct steps:

1. The various USGS probabilistic seismic hazardmaps were combined with deterministic hazardmaps by a set of rules (logic) to create the MCEground motion maps that can be used to defineresponse spectra for use in design.

2. Design procedures were developed that transformthe response spectra into design values (e.g. designbase shear).

The response spectra represent actual groundmotion consistent with providing approximatelyuniform protection against the collapse of structures.Because of differences across the US, three regionswere defined, using a set of seismic design procedurerules:

1. Regions of negligible seismicity with very lowprobability of collapse of the structure.

2. Regions of low and moderate-to-high seismicity.3. Regions of high seismicity near known fault

sources with short return periods.

The CEUS contains regions identified in Regions 1and 2. For regions of negligible seismicity the conceptof the MCE earthquake ground motions is notrequired. However, the SDPG required someminimum protection against earthquakes in thenegligible seismicity region. For regions of low andmoderate-to-high seismicty the MCE ground motionsmust be utilized in the seismic design, and for regionsof high seismicity the deterministic earthquake

approach is used. The reader is referred to the NEHRPProvisions[86] for specific details of seismic design byregions.

Following the work of the SDPG and the PUC, the1997 edition of the NEHRP Provisions was publishedin February 1998[86] containing state-of-the-art seismichazard description of the US. The 2000 edition of theNEHRP Provisions[88] was released without change tothe 1997 maps. The reader is referred to a theme issueof Earthquake Spectra[89] for more in-depth informationon the 1997 Provisions update.

ASCE-7

In 1988, ANSI combined with ASCE to update andredesignate ANSI A58.1-1982 to ASCE 7[27]. ASCE-7seismic hazard description remained as in ANSIA58.1-1982 with five seismic zones. ASCE 7 has sincebeen updated three times, in 1993, 1995 and 1998. Aspart of the 1995 update, the Earthquake Loads werebased on the 1994 NEHRP Provisions. The basis forseismic hazard description was the effective peakacceleration map (see Fig. 8) with some minormodifications to contour shapes in the 1994 NEHRPProvisions update. In 1998, ASCE 7 was updated basedon the 1997 NEHRP Provisions and the seismic hazarddescription represented the Project 97 design maps.The long-term goal of ASCE is for ASCE 7 to becomethe recognized standard by all earthquakeregulations, codes, standards, procedures andguidelines for basic seismic design[90], and ASCE 7-02is currently under development.

INTERNATIONAL BUILDING CODE

In 1994 ICBO, BOCA and SBCCI formed theInternational Code Council (ICC) to develop the IBCby the year 2000[63]. After much discussion among the

Fig. 10 Rate of change in seismic hazard



three code bodies and the BSSC, it was decided tobase the seismic provision of the IBC on the 1997NEHRP Provisions. To do this a Code ResourceDevelopment Committee (CRDC) was formed withinthe BSSC and funded by FEMA to convert theProvisions to code language. The seismic designprovisions are contained in Chapter 16, StructuralDesign, of the IBC, and the seismic hazard mapscontained therein are the same as those in the 1997NEHRP Provisions.

Issues: 2002 and beyond

SEISMIC HAZARD DESCRIPTION

As this review was being conducted, the USGSreleased drafts of updated versions of the 1996 seismichazard maps at http://geohazards.cr.usgs.gov/eq/.Again, the accelerations values represented bycontours on the maps are for a B–C boundarycondition having shear-wave velocities of 760m/s.The process of getting consensus throughout the USfor these updates was similar to that used in Project97. As stated by Frankel et al.[91]: ‘The changes in themaps have been fostered by feedback at severalregional workshops and a national workshopco-convened by the USGS and the AppliedTechnology Council. In general, the changes fromthe 1996 maps are incremental.’

Although the changes have been incremental,Frankel et al.[91,92] have included a significant amountof new knowledge to include, but not limited to: (1)adding four new attenuation equations for thewestern US; (2) incorporating new fault models forCalifornia; (3) including both aleatory and epistemicological uncertainty for characteristic faults; (4) addingthree new attenuation equations for the CEUS; (5)reducing recurrence intervals for the New Madridand Charleston, South Carolina earthquakes; and (6)lowering moment magnitudes of characteristicearthquakes in the CEUS (A. Frankel, personalcommunication, 31 January 2002). In addition, by thesummer of 2002 Frankel et al. plan to produce seismichazard maps for the CEUS based on hard rock, i.e.rock with shear wave velocities of the order of3000m/s.

Based on the above findings, from the author’sperspective, it is refreshing that input of newknowledge resulted in only incremental changes fromthe 1996 maps. This seems to demonstrate thatgeneral consensus is being achieved in the US forseismic hazard description at the B–C boundarycondition; however, there remain issues about howuncertainties have been addressed around theNMSZ[92]. Unfortunately, the B–C boundary conditionexists primarily in the western US, while hard rock tofirm rock conditions exists in the CEUS. In addition,most building foundations rest on a soil surface rather

than a rock surface. As a result, conversions of themaps values must occur to include site effects, i.e.building foundation characteristics.

SITE EFFECTS ON DESIGN GROUND MOTIONS

While consensus appears to have been achieved forthe B–C boundary condition, one of the largestuncertainties, or lack of consensus, exists in theconversion from the B–C boundary to otherfoundation conditions, especially for soil effects. Theuncertainty also increases depending on: (1) whetherthe site is a deep, or shallow, soil site; and (2) thenonlinear response of the soil as a function ofground motion levels. In the NEHRP Provisions thePUC, by consensus, account for this difference byclassifying foundations into a particular Site Class:A for hard rock; B for rock; C for dense soil and softrock; D for stiff soil, E for soil; and F for soft/unstablesoils.

Frankel et al. clearly define the B–C boundary asbeing a site on firm rock having shear-wave velocitiesof 760m/s. It is clear to the author that advances needto be made for conversions from the B–C boundary toother building foundation sites. As an example, thereis a site in the NMSZ at latitude 37.18 and longitude88.88 having top-of-rock shear-wave velocities of2770m/s[93], basically a hard-rock site. The site isoverburdened with about 100m of soil deposits withthe top 30m defined as a Site Class D soil[94]. Usingthe Site Class process of the NEHRP Provisions, hereincalled the general procedure, the MCE 0.2-s spectralvalue is 2.4 g, Ss, followed by an amplification factorFa of 1.0, resulting in a 0.2-s design earthquakespectral value, SDS, of 1.6 g. The MCE 1.0-s spectralvalue is 0.75 g, Sl, followed by an amplification factorFv of 1.5, resulting in a 1.0-s design earthquakespectral value of 0.75 g. Obtaining the top-of-soilzero-period acceleration (ZPA) design value Sa resultsin a value of 0.64 g.

A site-specific study[95] reveals a hard-rock ZPA of0.75 g (2% in 50 years). The Frankel et al. ZPA MCEmap for firm rock shows a value of 1.2 g at the samelocation. Converting the Frankel et al. 1.2 g to ahard-rock value results in a ZPA of 0.79 g (1.2 gdivided by 1.52)[1,6], a value close to 0.75 g (A. Frankel,personal communication, 20 February 2001). Attop-of-soil, the site-specific study resulted 0.2- and1.0-s period spectral values of 0.9 g and 1.0 g,respectively, and a ZPA of 0.5 g, deamplificationoccurring because of soil nonlinear response to thehigh rock motions; however, for small groundmotions at the site, less than 0.01 g, amplificationranging from 3.4 to 4.0 occurs[96].

When using the NEHRP Provisions process where asite-specific study has been conducted, it is stated:‘When site-specific procedures are utilized . . .themaximum considered earthquake spectral responseacceleration, SaM, at any period T, shall be taken fromthat spectrum.’



According to Eq. (4.1.3.5) of the Provisions, thedesign spectral values are to be two-thirds SaM. Thus,top-of-soil ZPA Sa is 0.33 g (0.5 g� 0.67), while 0.2- and1.0-s spectral values become 0.60 g (0.9 g� 0.67) and0.67 g(1.0 g� 0.67), respectively; however, the NEHRPProvisions require site-specific study design spectralvalues to be greater than, or equal to, 80% of thespectral values determined in the general procedureshown above. This means the ZPA reduction by thissite-specific study is limited to 0.51 g (0.64 g� 0.8),rather than 0.33 g and the 0.2-s spectral accelerationreduction is limited to 1.28 g (1.6 g� 0.8), rather than0.60 g. The 1.0-s site-specific spectral value, 0.67 g, isgreater than 80% of the general procedure, i.e. 0.60 g(0.75g� 0.8).

The difference between 0.33 g and 0.51 g for ZPA,and 0.60 g and 1.28 g for 0.2-s spectral values, showssignificant conservatism in the NEHRP Provisions overthe site-specific study, while the difference between0.67 g and 0.60 g for 1-s spectral values appear to showsome non-conservatism. Progress has been made andnew advances in understanding site effects hasoccurred[97–99]; however, this example demonstratesdisparities that can occur for a particular site.

MAXIMUM CONSIDERED EARTHQUAKE

In the current IBC 2000 seismic design is based on theMCE defined as having a 2% chance in 50 years ofbeing exceeded (2500-year return period event). Manyengineers are questioning the use of, and logic for,such a long return period in design. The concept ofthe MCE began during the development of the ATC3-06 document. There was some concern that designingfor a 10% in 50-year exceedance value (500-year returnperiod event) did not capture the probable, but rare,earthquakes in the CEUS. In addition, there wasconcern about the lower attenuation rates in the CEUScompared with those in the western US. This issuewas debated and discussed at meetings throughoutthe 1980s and into the 1990s, and did not becomeknown as the MCE until the SDPG of Project 97coined the phase. The intent of the 2500-year eventwas briefly discussed above and is thoroughlydiscussed in the Commentary of the NEHRP Provisions;however, the difficulty in explaining this quantitativenumber is in its origin of being selected on ajudgemental basis. In both the 1997 and 2000 editionsof the NEHRP Commentary[86,88] it is stated: ‘Theapproach adopted in the Provisions is intended toprovide for a uniform margin against collapse at thedesign ground motion,’ however, this statement failsto express quantitatively what the performance of thebuilding will be if the 2500-year event occurs.

So what drove the SDPG to agree to a 2500-yearearthquake as the MCE? At the time there was greatuncertainty on the return periods of destructiveearthquakes in the CEUS; however, on the basis ofrecent paleoliquefaction research[4–6] it is now

believed that the return periods of such catastrophicevents as the 1811–1812 New Madrid and 1886Charleston, South Carolina earthquakes have returnperiods around 450–500 years. Does the logic theSDPG used in the mid 1990s still hold true? Will newseismic hazard curves in the eastern US becomesimilar to those in the western US coastal regions?Should the MCE be redefined as a more frequentevent earthquake than the 2500-year return periodearthquake?

ADOPTION OF SEISMIC CODES IN THE USEngineers and scientists have made great progress indeveloping a consensus approach to seismic hazarddescription, to include the development of the IBC;however, adoption of seismic codes in the US occursat state and local county and/or city levels. The factthat the earthquake engineering community hasagreed to the development and production of the IBCdoes not mean that buildings and structures are beingdesigned to seismic provisions.

Owing to the seismic frequency of the interplateregion of the US, most communities in the region haveadopted building codes containing seismicprovisions; however, because the seismicity of theCEUS was relatively quiet during the 20th century, thepopulace of the intraplate region appears to haveadopted a mindset that earthquakes do not occur inthe CEUS. As a result, building codes with seismicdesign provisions have not been demanded at thelocal level.

In 1993, Stevens et al.[100] conducted a survey onseismic code adoption and enforcement in the CEUSin seven states centred around the NMSZ: Arkansas,Indiana, Illinois, Kentucky, Mississippi, Missouri andTennessee. These seven states are members of theCentral United States Earthquake Consortium(CUSEC) headquartered in Memphis, Tennessee. Inall, 595 counties and 18 cities were surveyed.Response to the survey was a surprising 70%. Ofthose responding, 86% had no local building code and84% indicated no seismic code was being adopted. Ofthose communities having no seismic code in place,an overwhelming 85% had no plan to implement aseismic code.

In 1995, the Insurance Services Office (ISO),developed a Building Code Effectiveness GradingSchedule (BCEGS)[101]. BCEGS provides a rating ofcommunities across the US on how well they aredoing in building code adoption and enforcement.Building-code enforcement departments are classifiedon a scale of 1 to 10, with 1 being the highestclassification. In addition, there is a Class 99 rating forthose communities with building code enforcementdepartments that do not meet the minimum BCEGSrequirements. Communities with building codeenforcement departments getting ratings of 1 to 3receive maximum credit, 4 to 7 receive intermediate



credit, 8 and 9 receive minimum credit and 10 receiveno credit. Since the initial pilot programme in 1994,ISO has evaluated more than 3600 code enforcementdepartments in the US. The ISO evaluation resulted in25% of the code enforcement departments in westernstates achieving a classification of 1 to 3, while only1% of the code enforcement departments in theCUSEC states received the same classification. Inaddition, it was found that 70% of the CUSEC statescode enforcement departments received a Class 99rating, compared with only about 15% of those inwestern states.

Conclusions

Engineers and scientists made great achievements inseismic hazard description in the US during the 20thcentury. Excellent research has been conducted tobetter understand seismic hazard and how buildingsperform when earthquakes occur. As we move intothe 21st century we need to continue theseadvancements; however, the purpose of research is toreduce losses, including injury and loss of life, fromfuture earthquakes in the US and all parts of theworld. To do this, engineers and scientists must alsowork to have research implemented at the end userlevel. This requires engineers and scientists tobroaden their approach. We must develop techniquesthat facilitate understanding, we must be able tocommunicate to a larger audience and we must notforget that when the next disastrous earthquakeoccurs, we will again be asked again and again: Whydidn’t we know it was going to happen? Why did ourbuildings collapse?

Acknowledgements

The author wishes to thank S.T. Algermissen,Director, GeoRisk Associates, Inc. and W.J. Hall,Professor Emeritus, University of Illinois forreviewing this paper. Their constructive commentsand suggestions significantly improved the content ofthe paper. This research was supported by the Mid-America Earthquake Center under National ScienceFoundation Grant EEC-9701785.

This is the only known archival record of the AECmemos. It is also believed, although the type in thememo is too illegible to prove, that a third party to the21 December 1950 meeting consisted ofrepresentatives of the USCGS.

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James EBeaversMid-America Earthquake Center, 205 North Mathews,NCEL,Room1241,Urbana, IL 61801,USAE-mail: [email protected]



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