Eart 110N The Dynamic Earth Lab
Lab#4—Location of Earthquakes
Objectives: To find the location and origin time of earthquakes using seismograms.
Introduction: In 1892 a seismologist working in Japan made the 40 seismometers placed in
observatories around the world that would produce data that led to an explosion in our
understanding of the deep structure of the earth. Before we had the luxury of the internet,
seismologists operating a seismic station used to send postcards on a weekly or monthly basis to an
international data center in England with the time that they first observed an earthquake along with
the location of the station. The first thing they saw from the earthquake was the P wave and
sometimes they were able to pick the S wave (which is typically larger in amplitude and has a
lower frequency). In England they would collect these postcards and figure out where and when
these earthquakes happened. They would start out by placing the earthquake at the station with the
earliest arrival and use a travel-time table to see if arrivals at other stations matched the location.
They would then improve their estimate of location and start time step by step in an iterative
process (which turns out to converge very well in only 2 iterations). The way that the travel time
tables were created was by tracking the arrival of different phases at stations with different
distances from an earthquake with a known location.
Yes, this is kind of a chicken and egg problem, and that problem still exists today though we have
gotten much better at it. In today's lab you will make crude estimates of regional and teleseismic
earthquake locations and the phase velocity of Pn in the same way that early seismologists did.
First, some background information and vocabulary:
The radiation patterns of seismic waves are consistent with origin of earthquakes by slip on faults.
The epicenter is the point on the surface of the earth directly above the hypocenter.
The hypocenter or focus of an earthquake is the point within the earth where the first rupture
occurs. The first seismic waves travel outward from this point.
At least three seismograms (records of ground motion recorded by seismographs) are needed to
locate hypocenter and time of rupture.
Earthquakes are classified by focal depth:
Shallow
< 70 km
intermediate 70 < depth < 320 km
deep
320 km < depth < 700 km
In class: do everything except the abstract, working in groups of 5 on big globe to find epicenter in
part 2b.
At home: write up your lab neatly and show your work for interpolations and error calculation.
Regional earthquake location
1a. How is a travel time table made? Look at the seismograms in Figure 1a. These are recording
the P wave at stations 355 km and 506 km away from an earthquake. Pick the time when the P
wave first arrives and calculate the P wave velocity [(distance 2 - distance 1)/(time2 - time 1)].
This velocity is approximately the velocity of a moho refracted wave called Pn, which is a P wave
that hits the moho at the critical angle and propagates along it as a 'head wave' at the velocity of the
upper mantle. This is the first arrival for distances ~100-1000 km. Next use Figure 1b, a travel
time curve created by a seismologist (me), to measure the velocity of Pn. This curve uses more
data and is likely a better representation of actual velocities. Assume that this velocity is the true
velocity and compare it to your crudely calculated Pn velocity by calculating the relative error =
(actual - measured)/actual. Express this as a % error by multiplying by 100.
1b. Next you will locate a significant earthquake that happened in the western United States. First
you will measure the time it took for a P wave to travel from the earthquake (Figure 2) and then use
the travel-time table (Table 1) to see how far that station is from the hypocenter. The seismograms
have been plotted so that time = zero is when the earthquake rupture began. Once you have the
distances to each of the stations, use a compass and the map of the stations in California (Figure 3)
to draw circles around each station whose radii are equal to the distance calculated. The circles
should converge at one point, the epicenter. For your abstract, think about the geologic setting of
where the earthquake happened (e.g,. was it likely a subduction zone earthquake, or some other
kind ...).
Global earthquake location
2a. In this part of the lab you will locate the epicenter and depth of a teleseismic (= distant)
earthquake. First, using Figure 4, measure the S-P residuals (the difference in travel time of S and
P waves) for any three stations that you like. The advantage of this method using travel-time
residuals is that it does not require knowing when the earthquake rupture started; it only uses
differences in travel time of different phases recorded at any given station. The P wave is the first
large amplitude arrival. The S wave is in the grey box. To get the S-P time, pick similar features to
measure in the P and S waves. For example, if the P wave's first big amplitude is up, then pick the
first big upswing in the S wave and measure the difference in time between the two. Using the first
two columns in Table 1, find the distance of the epicenter to the seismograph, using interpolation to
get a more accurate estimate. Then, using the approximate locations of these seismographs from
Figure 5, take a piece of string and chalk to draw circles on the big globe around the seismograph
locations whose radii are the epicenter-seismograph distance in degrees. You can use more than 3
stations to improve your location. Look at Figure 2.2 in Fowler or your tectonic map from lab 1
and note tectonic environment where this earthquake occurred.
2b. Finally, you will calculate the depth of the earthquake by using pP-P residuals (the difference
in travel time of the P wave and its (short) reflection off of the surface in the broad region
surrounding the epicenter). Figure 6 shows the same seismograms as Figure 5, but zoomed in so
that you can make more accurate measurments of pP-P times. Make these measurements in the
same way you measured S-P times and for the same seismograms. In table 1 find the appropriate
epicenter-seismograph distance and, using interpolation, find the depth of the earthquake
hypocenter.
For example: if the S-P times is 423.4 seconds, then the epicenter-seismograph distance is 55.5
degrees. If your pP-P time is 70 seconds then you know that the depth of the earthquake is about
350 km. But assume we are able to measure these time differences very accurately using digital
seismograms on a computer, and find the depth using interpolation.
dist S-P time pP-P time (sec)
(deg) (sec) depth (km) 300 400
55.0 420.4
56.0 426.4
64.3 82.4
64.6 82.7
(here the TA can explain)