What is a ground shaking hazard map & who uses it?

http://earthquake.usgs.gov/research/hazmaps/interactive/cmaps/custom2002_2006.php

• A ground‐shaking map shows the distribution of earthquake shaking levels that have a certain probability of occurring. p y g

• These maps assist engineers in designing buildings, bridges, highways, and utilities that will withstand shaking from earthquakesand utilities that will withstand shaking from earthquakes. 

• These maps are used to create and update the building codes that are now used by local governments to help establish construction requirements necessary to preserve public safety.

http://earthquake.usgs.gov/research/hazmaps/haz101/haz101.php#what

Who’s involved?

• United States geological Survey (USGS)

Who s  involved?

• United States geological Survey (USGS)

• National Earthquake Hazards Reduction Program (NEHRP)

• International Building Code (IBC)  

What is Peak Ground Acceleration (PGA)?

A ll i l h d h h d i h k illA small particle attached to the earth during an earthquake will “accelerate” back‐and‐forth during an earthquake.  

The maximum acceleration is the PGAThe maximum acceleration is the PGA.

Acceleration is chosen, because the building codes prescribe how much horizontal force a building should be able to withstand during an earthquake. This force is related to the ground acceleration. 

force

The magnitude of the “PGA” of the particle is compared to“the acceleration of gravity” or “g” which is a constant.

g = 9.81 m/s2

PGA = 35% g  or  0.35 g

The ground motions at a given location are those from all future possible th k it d t ll ibl di t f th t l tiearthquake magnitudes at all possible distances from that location.

The ground motion coming from a particular magnitude and distance is i d l b bilitassigned an annual probability.

An algorithm calculates the PGA at the site for all the earthquake locations and magnitudes believed possible in the vicinity of the site. 

Each of these magnitude‐location pairs is believed to happen at some average probability per year Small ground motions are relatively likelyaverage probability per year. Small ground motions are relatively likely, large ground motions are very unlikely.

PGA maps show, by contour values, the PGAs that have a common given probability of being exceeded in 50 years.p y g y

http://earthquake.usgs.gov/research/hazmaps/interactive/cmaps/custom2002_2006.php

Ground motions having 2 percent probability of being exceeded in 50 years

Bellingham

Ground motions having 10 percent probability of being exceeded in 50 years

BellinghamBellingham

20

PGA maps are constantly changing as newly discovered faults and seismic data are integrated into the PGA algorithms.

http://earthquake.usgs.gov/research/hazmaps/interactive/cmaps/custom2002_2006.php

CanadaUSA

Nooksack River

Boulder Creek Fault

Bellingham

Western Whatcom County

http://earthquake.usgs.gov/research/hazmaps/interactive/cmaps/custom2002_2006.php

PGA is what is experienced by a particle on the ground.

Spectral acceleration (SA) is approximately what is experienced by a building, as modeled by a particle on a mass-less vertical rod having the same natural period of vibration as the buildinghaving the same natural period of vibration as the building.

SA

(( ))

PGA

Response spectrumA response spectrum is simply a plot of the peak or steady-state response (displacement, velocity or acceleration) of a series of oscillators of varying natural frequency, that are forced into motion by the same base vibration or shock. From Wikipedia, the free encyclopedia

Sh t b ildi l th 7Short buildings, less than 7 or so stories, have short natural periods (0.2 to 0.6 sec)

Tall buildings have natural periods greater than 0.6 seconds

The decrease in oscillation due to the stiffness of the building is called damping

The building codes assume that 5 percent of critical damping is a reasonable value to approximate the damping of buildings for which

called damping.

earthquake-resistant design is intended.

SA

(( ))

PGA

Buildings having periods less than 0.6 seconds are less than 7 stories

The decrease in oscillation due to the stiffness of the building is called damping.

The building codes assume that 5 percent of critical damping is a reasonable value to approximate the damping of buildings for which earthquake-resistant design is intended.

SA

(( ))

PGA

0.2 sec Spectral Acceleration (%g) with 2% Probability of Exceedance in 50 Years

1.0 sec Spectral Acceleration (%g) with 2% Probability of Exceedance in 50 Years

3030

40

50

slope stability analysiss ope stab ty a a ys s

r

R

L = arc length

WFS = τLR

rW

τ

P d t ti th d

Earthquake effect on slope stabilityPseudo-static methodGround motion is horizontal and continuous such that a “static” horizontal force (Feq) is generated.

r

RFeq

Feq = aW

W

Feq aW

a = %g predicted by groundshaking

τ

Earthquake effect on slope stabilitya t qua e e ect o s ope stab ty

r

R L = arc length

F

r’

W

Feq

FS = LR

rW + r’Feq

τ

τ