1

Name:_____________________ EPS 50 - Lab 7: Earthquakes and Seismic Risk Grotzinger and Jordan, Ch. 13 Introduction: Earthquakes are natural phenomena. The ground shaking is caused by a sudden release of strain energy stored in rocks. In an earthquake, the stored elastic strain energy is released through slip across a fault, and a portion is radiated as seismic waves. Seismic waves tell us about the nature of the earthquake process itself, but they also provide a direct sampling of the deep interior of the earth. Much of what we know about the structure of the earth is a result of studying seismic waves. Objective: In this lab, you will learn to pick P and S waves from seismograms, locate an earthquake, calculate the local (Richter) magnitude, and perform a basic seismological hazard analysis. Figure 1 demonstrates why this is particularly relevant to you. Lab is worth 100 pts.

Figure 1: Fault systems in the Bay Area Figure 2: The travel-time curve.

2

Part 1: Earthquake Location Once an earthquake occurs, the most important task of seismologists is to find the

earthquake location. The location of the earthquake is the foundation of earthquake studies, and any further analysis (e.g., magnitude estimation, focal mechanism, damage levels, etc.) relies on the precision of the earthquake location. Knowing the locations of earthquakes is also essential when probing the interior of Earth. Seismologists can interpret the Earth structure by using waveform and travel-time data when they know the earthquake sources (locations). Various methods are used to locate earthquakes, but the basic idea is simple triangulation. To apply this we need the arrival times of the direct P-wave and S-wave measured in absolute time scale. Historically seismometers equipped with clocks were used to read the time of each wave arrival. However, in different stations or countries the clocks might not be synchronized. Today seismologists use GPS that have an accuracy of 5 microseconds in tandem with Internet to collect waveforms and correlate them into absolute time scale from thousands of seismic stations around the world.

As was discussed earlier in lecture, P waves are the primary compressional waves and also the fastest seismic waves. S waves are secondary waves characterized by traveling shear disturbances (sideways motion). After accurate measurements of arrival time are made, they are compared to predicted arrival times from an accepted model of the earth (i.e., Figure 2, the travel time curve). The goal is to minimize the difference between the observed and predicted wave arrival times. The closer these are in value, the more accurately the earthquake location and occurrence time (origin time) have been determined. Figure 2 illustrates the different wave speeds by comparing travel-time vs. distance (travel time curve) for a representative model of the crust beneath the San Francisco Bay area. Notice that P waves always take less time than S waves to arrive at the same point, and the time separation between the P- and S-waves increases with increasing distance. This separation is a very important general property used to locate earthquakes. If a measurement of the S minus P time is made (Ts-p), the distance between the source and the particular recording station may be determined. Several estimates of source-station distance using this method along with triangulation will provide a good approximation of the earthquake location.

For the event we will look at in this lab, you are given two seismograms for each of the five stations shown in Figure 3. One seismogram recorded the ground motion in the east-west (EW) direction and the other, in the north-south (NS) direction. You may use both records to help identify the P-wave and S-wave arrival times, but you really only need one record to determine the P/S-arrivals since they should be the same for both the EW and the NS components. The P wave is the first to arrive and therefore the easiest to determine. The S wave arrival is more difficult to determine, because it may be perturbed by the preceding P wave. However, the S wave can be distinguished by an increase in amplitude, and wider duration (longer period) pulses. The x-axis in the records is the time in seconds starting from 05:00:0.0 UTC, and the y-axis is the amplitude in mm.

3

1) Locate and circle five stations on the map in Figure 3. Use colored pencil to trace over the fault systems. Use a different color for each fault system and label the main systems. (2 pts)

Figure 3. Location of the five seismic stations.

4

2) Based on what you have learned about wave propagation, which of the five stations is the closest to the event and why? Use Figure 4 on page 11 to answer this question. (2 pts) 3) On Figures 5 through 9 (pages 12 - 16) record the P-wave and S-wave arrival times for each station into the table below using only the NS OR EW records (note that BHE = east-west, BHN = north-south). Indicate which one you use on the table. Show your picks in the seismograms by drawing an arrow on the figure, then justify your pick with a sentence (also on figure). (10 pts) 4) Calculate the Ts-p times for the five stations and enter them into the table. (2.5 pts) 5) Use the travel time curve to estimate the distance of the earthquake source from each of the

stations, and enter the distances into the table. (5 pts) 6) Use the travel time curve to determine a P-wave travel time and use that time to mark the

origin time of the earthquake and record this into the table. (2.5 pts) TABLE 1 Station P-wave time

(hr:min:sec) S-wave time (hr:min:sec)

Ts-p (sec)

Distance (km)

Origin Time (hr:min:sec)

Component NS or EW?

BKS

CMB

FARB

MHC

PKD

7) Using your data from all five stations in Table 1, compute and report the average origin time

and the standard deviation. Show your work. (2 pts)

5

Part 2: Triangulation If we have only one seismic station, the earthquake source may be located anywhere

along the circle which has its center at the receiver station and a radius equal to the distance calculated using P and S wave arrival times. With two seismic stations, we are able to obtain two source-to-receiver-distances, i.e. two circles with different locations and radii. The location of the earthquake will be one of the two points where these two circles intersect. If we have three seismic stations and homogeneous topography (no lateral and vertical change in the Earth), the location of the earthquake will be the point where the three circles intersect. Unfortunately, regional geological structure is usually heterogeneous. As a result, three circles will not intersect at one point but over an overlapping area, and the earthquake may be at any point in this area. The size of the overlapping area represents the uncertainty of the earthquake location. 8) Using a compass and your estimated distances (from table) for each of the stations, sketch a circle (radius = source-station distance) on Figure 3. (5 pts)

9) Shade the region of intersection of the curves you have drawn and comment on the precision of the earthquake location. (5 pts) 10) Which fault did this earthquake probably occur on? Mark this fault strand with a distinct (heavy) line on Figure 3. (2 pts)

11) Using the map, estimate the distance from this earthquake to Berkeley. (2 pts)

6

Part 3: Earthquake Magnitude Charles Richter introduced the concept of “earthquake magnitude” in 1935 by creating the Richter magnitude scale. The number an earthquake receives from the Richter scale depends on the seismic energy it releases. It is the same number reported by the press and is therefore the earthquake magnitude number that is most familiar to the public. The Richter scale is an arbitrary scale that gives relative earthquake magnitudes rather than absolute magnitudes. Richter designed this scale such that a 1 mm ground motion amplitude recorded by a Wood-Anderson instrument located 100 km away from the earthquake source represents a magnitude 3 event. A 10 mm ground motion recorded by the same instrument in the same distance from the earthquake source represents a magnitude 4 event. In other words, a unit-increase in the Richter or Local magnitude means a 10-fold increase in amplitude recorded by Wood-Anderson instrument. The amplitude of ground motion for a local magnitude 5 event is 10 times larger than a local magnitude 4 event, but the local magnitude 5 event releases about 32 times more energy! The formula of the local magnitude scale is:

ML = log(A) - C(r), where ML is the Richter (local) magnitude, log(A) is the logarithm (base 10) of the observed Wood-Anderson amplitude in millimeters, and C(r) is a distance dependent correction term. Charles Richter compiled the following table of C(r) values, and used this information to develop the graph provided on the figure above. Figure 10: Richter magnitude

7

Use Figure 10 to help answer the following questions: 12) How many times greater is the amplitude of a magnitude 4.7 compared to a magnitude 4.2

earthquake recorded at a distance of 100 km? Show your work. (2 pts) 13) How many times more energy does a magnitude 8.0 earthquake release compared to a

magnitude 5.0 earthquake? Show your work. (2 pts) 14) What is the Richter magnitude of an earthquake that produces a 1 mm amplitude at a station

500 km away? Use the top portion of Figure 10 to make this estimate. (3 pts) 15) Using Figures 4 through 9 and 10, measure the peak amplitudes of each component for the

five stations and determine the individual ML using either the equation or the graph. Enter your measurements and calculations in Table 2. (30 pts)

TABLE 2 Station NS amplitude

(mm) EW amplitude

(mm) log(ANS) log(AEW) C(r) ML

(NS) ML

(EW)

BKS

CMB

FARB

MHC

PKD

16) Calculate the average magnitude of the earthquake and the standard deviation. (2 pts). Comment on the scatter of your estimated magnitudes. What are probable causes of the scatter? (3 pts)

8

Part 4: Beach ball diagrams

Seismologists study not only slip and slip direction of earthquakes but also the orientation and type of fault movement triggered by earthquakes. The term “focal mechanism” describes the behavior and deformation near the earthquake source. Seismologists estimate the focal mechanism based on the information from seismograms and represent it with a “beach ball” symbol on the map. A fault plane existing in the lower hemisphere (A in the left-hand-side figure) will intersect the hemisphere as a curve. The two lines on the beach ball are the stereographic projections of the fault planes onto the surface. The stress-field orientation at the time of rupture governs the direction of slip on the fault plane; the beach ball also depicts this stress orientation. In this scheme, the gray quadrants contain the tension axis (T), which is the minimum compressive stress direction, and the white quadrants contain the pressure axis

(P), which is the maximum compressive stress direction. The computed focal mechanisms show only the P and T axes and are not shaded.

The first three examples describe fault motion that is purely horizontal (strike slip) or vertical (normal or reverse). The oblique-reverse mechanism illustrates that slip may also have components of horizontal and vertical motion.

17) For the following faults, use arrows to draw the fault motion (show the motion of each block

of the fault). Name the type of fault that each block diagram represents. (6 pts) Fault type:____________ Fault type:____________ Fault type:____________

9

18) Draw the cross-section view (view from the side) and the map view (view from above) beach ball diagrams for faults illustrated below. Refer to the top figure on the previous page for cross-section view and map view beach ball diagram examples. (6 pts)

Strike-slip: Normal:

Reverse:

10

d) For the following beach ball diagrams indicate if they correspond to a strike-slip fault (SSF), normal fault (NF) or reverse fault (RF), or oblique fault (OF). (6 points)