Download Locating an Epicenter Model Answers for Analyze and Conclude

Locating an Epicenter
Model Answers for Analyze and Conclude Questions
Use these sample answers to correct your own answers. You do not have to have the exact answer, word for
word, but just like these answers, each question should be:
RESTATED (with correct grammar!)
ANSWERED both accurately and
THOROUGHLY
SUPPORTED with data and evidence
1. Of the three cities listed in the data table, the seismographs in Denver would have detected the earthquake
first. Because Denver is the closest of the three, the seismic waves would reach it first, then Seattle, then
Houston. You can see this on the map, but you can also tell by looking at the data. The difference in arrival time
is the shortest for Denver, 1 minute and 25 seconds. This corresponds to a distance of 800 km from the
epicenter. Houston, which is 2000 km away, would have detected the earthquake last of the three cities because
it is the farthest.
4. In order to locate an epicenter you need to have the data from three seismic stations. The seismograph at each
station is able to detect its distance from the epicenter by measuring the difference in arrival time between the P
and S waves. Since the speed of seismic waves is known, you can calculate the distance they must have
traveled. However, the seismic waves do not indicate which direction the earthquake occurred. You know that
the earthquake occurred at the distance, but it could be in any direction, so you can draw a circle around the
location of that receiving station with a radius of its distance from the epicenter. The epicenter is somewhere
along that circle. Data from only one station would give you infinite points upon this circle. Data from two
stations would give you two possible locations because the circles will intersect twice. When you have data
from three stations, the three circles indicating the distance from the three seismic stations will intersect at one
single point, which is the epicenter of the earthquake.
5. The epicenter is about 600 km southeast of San Francisco. According to the graph, this would result in a
difference in arrival time of 1 minute. [note: you may not have gotten this exact distance, but it should have
been close!]
6. As the distance from the epicenter of the earthquake increases, the difference in arrival time between P waves
and S waves gets longer. One can see this relationship in the graph. For example, when the earthquake’s
epicenter is 200 kilometers from the recording station, the S waves will arrive 20 second after the P waves. A
seismograph located 600 kilometers from the epicenter, will record a difference in arrival time of one a full
minute. This pattern continues as S waves arrive farther and farther behind P waves as they travel greater
distances.
7. If I were working as a seismologist and found the epicenters of many earthquakes in a region, I would infer
that this is an area of high seismic activity. I would expect to find faults there (breaks in the crust). (NOTE: We
hadn’t learned much about this yet, so I did not expect you to all get it. However, after Mr. Danko’s lesson
today, you should be able to answer this one much better!)
8. The epicenter of the earthquake was located in southeastern California. According to the Seismic Risk Map
on page 81 in the text (and posted on SchoolWires), this earthquake occurred in an area at major risk for
damage from earthquakes. Therefore, this earthquake was NOT a freak event, but a typical and expected
occurrence in this region. (A “freak” event means a rare or unusual one!)