Download Earthquake Lab The goal of this portion of the lab is to learn how reco

Name: ________________________
Block: _________________
Earthquake Lab
The goal of this portion of the lab is to learn how recording of earthquakes – seismograms – are
used to locate earthquakes, determine their magnitudes, and to understand the sense of fault
motion related to the earthquake.
Typical seismogram of a small earthquake recorded fairly near the source. The P-waves arrive
first and are later swamped by the arrival of the S-waves. The S-P interval (referred to as lag
time) is the time between the first arrivals of the P and S waves; it is used to determine the
distance from the epicenter. This interaction produces additional seismic waves (phrases) which
will be detected by seismographs. Refer to Chapter 10, pages 217-221 in your textbook for an
explanation on how seismographs work.
A. Locate the Epicenter
1. Measure the S-P interval from the 3
seismograms shown on the following page,
and enter the results in the table below. Then,
use the S-P travel time curve (follows the
seismogram) to determine the distance
between the station and the epicenter and enter
the date into the table.
2. Next, take a compass and set its radius to
the epicenter distances for each station and
draw circles or arcs around each station.
Ideally, the three circles/arcs should intersect
at a single point, which is the epicenter. If
they do not, you should go back and check
your work. Clearly mark the location of the
epicenter.
Station
Elko
Eureka
Las Vegas
S-P(sec)
Distance(km)
Las Vegas, Nev
Elko, Nev
Eureka, CA
P-waves travel between 6 and 13 km/s. Swaves are slower and travel between 3.5
and 7.5 km/s. In most regions, study of
numerous earthquakes with well-known
epicenter locations results in an empirical
S-P curve, such as the one shown to the
right.
B. Magnitude of the Earthquake
The magnitude of an earthquake provides a
convenient measure of its size scaled to a
small number usually less than 9. It is a
unitless number, derived from the
amplitude of ground motion associated with
S-waves, adjusted for the distance between
the epicenter and the seismograph station
(the amplitude generally drops off with increased distance from the epicenter). The table below
helps to put earthquake magnitudes into perspective.
1. Measure the maximum amplitude of
the S-waves from the 3 seismograms,
and enter the results in the table below:
add the distances from epicenter you
obtained in the previous problem.
2. Then, use the graphical device to the
right (called a nomogram) to determine
the Richter magnitude of the earthquake
as indicated by each of the three
seismograms. Ideally, they should all
converge on a single answer. If they do
not, you should go back and check your
work.
Station
Distance from
Epicenter (km)
Max S-wave
Amplitude (mm)
Magnitude
Elko
Eureka
Las Vegas
Analysis Questions: (Answer all questions in complete sentences)
1. Where is the epicenter for the earthquake? (Mention a direction and distance from the closest
major city). ____________________________________________________________________
______________________________________________________________________________
2. Is there a known fault in the vicinity of the epicenter that is likely to have slipped to create the
earthquake? If so, name the fault. _________________________________________________
3. Why did you need to draw three circles in order to locate the epicenter of this earthquake?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
4. Which type of seismic wave reaches the seismic station first? ____________________
Second? ___________________
5. How is the Lag Time between the arrival of the P and S waves used to calculate the location of
the earthquake? ________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
6. What is the relationship between the length of the Lag Time and the distance to the epicenter?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Use the diagram below to answer questions 7 and 8
7. Which station is the closest to the epicenter and why?
______________________________________________________________________________
______________________________________________________________________________
8. List the seismograph stations from closest to furthest from the epicenter.
______________________________________________________________________________
9. Determine the Richter magnitude of an earthquake that was located 500 kilometers from this
seismic station. ________________________________________________________________
Extension Activity
The graph below shows travel time in minutes and distance traveled for primary and secondary
waves. Primary and secondary waves start at the same time but do not travel at the same speed.
Study the graph. Use the graph to help answer the questions that follow.
1. How long does it take for a primary wave to travel 2000 kilometers? ____________________
2. How long does it take for a secondary wave to travel 2000 kilometers? __________________
3. How far does a secondary wave travel in 10 minutes? ________________________________
4. How far does a primary wave travel in 10 minutes? __________________________________
5. What happens to the time difference between primary and secondary waves as the distance
traveled gets longer? ____________________________________________________________
6. Suppose a primary and secondary wave both traveled a distance of 5000 kilometers before
they are picked up by a seismograph. Which wave will arrive first? _______________________
7. How much time lag at 5000 km will there be between these two waves? __________________
8. Suppose both a primary and secondary wave start together and travel for 5 minutes. Which
wave will travel farther? _________________________________________________________