Explore the relationship between the fracking industry and seismicity

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Locating an Earthquake Epicenter
During an earthquake, seismic waves are sent all over the globe. Though they
may weaken with distance, seismographs are sensitive enough to still detect these
waves. In order to determine the location of an earthquake epicenter, seismographs
Magnitude Typical Maximum Modified Mercalli Intensity
1.0 – 2.9 I
3.0 – 3.9 II – III
4.0 – 4.9 IV – V
5.0 – 5.9 VI – VII
6.0 – 6.9 VII – IX
7.0 and above VIII or above
Figure 13.8 | A comparison of magnitude versus intensity scales for
earthquakes.
Author: Randa Harris
Source: Original Work
License: CC BY-SA 3.0
Page | 320
Introductory Geology Earthquakes
from at least three different places are needed for a particular event. In Figure 13.9,
there is an example seismogram from a station that includes a minor earthquake.
Once three seismographs have
been located, find the time interval
between the arrival of the P-wave
and the arrival of the S-wave. First
determine the P-wave arrival, and
read down to the bottom of the seismogram to note at what time (usually
marked in seconds) that the P-wave
arrived. Then do the same for the
S-wave. The arrival of seismic waves
will be recognized by an increase in
amplitude – look for a pattern change
as lines get taller and more closely
spaced (ex. Figure 13.10).
By looking at the time between
the arrivals of the P- and S-waves,
one can determine the distance to the
earthquake from that station, with longer time intervals indicating longer distance.
These distances are determined using a travel-time curve, which is a graph of Pand S-wave arrival times (see Figure 13.11).
Though distance to the epicenter can be determined using a travel-time graph,
direction cannot be told. A circle with a radius of the distance to the quake can
be drawn. The earthquake occurred somewhere along that circle. Triangulation is
required to determine exactly where it happened. Three seismographs are needed.
A circle is drawn from each of the three different seismograph locations, where the
radius of each circle is equal to the distance from that station to the epicenter. The
spot where those three circles intersect is the epicenter (Figure 13.12).
Figure 13.9 | This seismogram is read from left to right and top to bottom. Note the small
earthquake that is marked, and the resulting change in wave amplitude at that point.
Author: USGS
Source: USGS
License: Public Domain
Figure 13.10 | An example seismogram with the
arrival of P and S waves included. Note how the
arrival of waves is marked by an increase in the
wave height (known as amplitude) and by more
tightly packed waves. This example does not
include time along the bottom, but those in the
lab exercise will.
Author: User “Pekachu”
Source: Wikimedia Commons
License: Public Domain
Page | 321
Introductory Geology Earthquakes
Figure 13.11 | A travel-time graph that includes the arrival of P-waves and S-waves. Note that these
curves plot distance versus time, and are calculated based on the fact that the Earth is a sphere. Curves
vary with the depth of earthquake because waves behave differently (i.e. their velocities change) with
depth and change in material. This particular curve is used for shallow earthquakes (<20 km deep) with stations within 800 km. The S-P curve refers to the difference in time between the arrival of the P-wave and S-wave. If you noted on your seismogram that the P-wave arrived at 10 seconds, and the S-wave arrived at 30 seconds, the difference between arrival times would be 20 seconds. You would read the 20 seconds off the y-axis above to the S-P line, then drop down to determine the distance to the epicenter. In this case, it would be approximately 200 kilometers

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