Seismo Lab - Shoreline School District

Seismology Lab
SWHS Geology
Part I: Finding epicenters with seismic wave travel time curves
Refer to the following seismograms of an earthquake recorded at sites in Alaska, North
Carolina, and Hawaii. Estimate, to the nearest tenth of a minute, the times of first arrival
of the P-waves and S-waves at each station. Then subtract S from P to get S-minus-P.
1st P arrival
1st S arrival
Sitka, AK
Charlotte, NC
Honolulu, HI
Using the S-minus-P times and the travel time curves (on the next page), determine the
focus-to-seismograph distance for each station (in miles).
Focus-to-seismograph distance (mi)
Sitka, AK
Charlotte, NC
Honolulu, HI
Next, find the earthquake’s epicenter using the distances just obtained.
a. First use the geographic coordinates below to locate and mark the three seismic stations
on the map.
Sitka, AK
Charlotte, NC
Honolulu, HI
57N, 135W
35N, 81W
21N, 158W
b. Use a drafting compass to draw a circle around station whose radius is equal to the
focus-to-seismograph distance from #2 (use the scale to set the radius). The circles should
intersect at one point – the epicenter (if the do not quite intersect at a point, then find a
point that is equidistant from the three edges). Record the location of the epicenter below:
What is the origin time of the earthquake (at what time did it occur?)
At what time did the L-waves from this earthquake begin to arrive at the Sitka Station?
Part II: Finding epicenters without using travel time curves
For this part, you will determine the epicenter of the 2001 Nisqually Earthquake. Use the
three seismograms from seismic stations OSD, SEP and MBW to find the arrival time of the
P-wave for each station. Then use the P-wave arrival time and the Nisqually Earthquake
origin time to find the P-wave travel time for each station.
The Nisqually Earthquake occurred 2/28/01 at 18:54:32 GMT
P-wave arrival time1
Time (GMT) which P-waves arrive
(P-wave arrival time) – (Earthquake origin time)
P-wave travel time2
P=wave travel times can be converted into a distance if we know how fast P-waves travel.
For continental crust, the average P-wave velocity is 6.5 km/s. Using this information,
calculate the distance from each seismograph to the epicenter using the following equation:
Distance = P-wave travel time * P-wave velocity
Distance to Epicenter
On the map below, use a drafting compass to draw a circle around each seismographic
station, using the distances you calculated above as the radius for each circle.
How far from Seattle (in km) is the epicenter, and in what direction?
For many earthquakes, the arcs do not intersect at a point. Can you think of two possible
reasons why?
Part III: Earthquake Hazard and Rick Assessment
Figure 15.7 shows the kind of materials which buildings have been constructed on in San
Francisco. They include hard Fransican Sandstone, uncompacted beach and sand dunes,
and artificial fill. Imagine you have been hired to assess the risk of buying new
construction on points X, Y and Z.
a. What is the risk at location X? Why?
b. What is the risk at Y? Why?
c. What is the risk at Z? Why?
On October 17, 1989, just as game 3 of the World Series was about to start, a strong
earthquake occurred at Loma Prieta, CA and shook the entire San Francisco area.
Seismographs at locations X, Y and Z are shown in Figure 15.8.
Suggest a reason why more ground shaking occurred at sites Y and Z than at X.
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