# Locating and Measuring Earthquakes

```Section 10.2
 Modified
Mercalli Scale: ranks earthquakes
from intensity: 1 (barely felt) to 12 (total
destruction. It measures the destructions
that the earthquake caused
 Richter Scale: Used to measure the
magnitude of an earthquake. Each time you
go up 1 number, the earthquake is 10x
stronger
 Magnitude is the measure of an earthquake
based on the strength of the seismic sound
waves emitted by the earthquake.
 Seismographs:
detects and records waves
produced by earthquakes
 Scientists use data from seismogrphs to
locate earthquakes’ epicenter and to
measure their magnitudes
 Seismogram: A record sheet that is placed on
a drum attached to the base.
 The attached pen does not shake because it
is attached to a weight
 Seismograph
 Check this bridge out!
P
waves produced by an earthquake always
arrive at a seismograph station before the S
waves
 The farther a seismograph station is from the
epicenter, the greater the difference in the
arrival times of the P and S waves
 Seismologists
use the difference in the
speeds of P- and S-waves to locate the
epicenters of earthquakes
1.
2.
Subtract the arrival time of the P-waves
from the arrival time of the S-waves (this
can be expressed in hours, minutes and
seconds, such as 08:06:40 or just hours and
minutes
On the clean edge of a sheet of paper,
make two marks to show the observed
interval along the travel-time scale on the
vertical axis of the “Earthquake P-wave and
S-wave Time Travel” graph
3. Keeping the edge of the paper vertical, slide
the marks on the edge of your paper along
the P- and S-curves of the travel-time scale
until the marks match up with both curves
4. Carefully follow the marked edge of your
paper straight down to the horizontal axis to
find the distance to the epicenter
5. If you draw a circle around the recording
station with a radius equal to that distance,
the epicenter should be located somewhere
within that circle.
6. To find the exact location, follow the
procedure above to find the distance from
the epicenter to at least three seismic
recording stations.
7. The epicenter is the point at which the
circles intersect or meet
8. If you find that there is a small triangle, the
epicenter is probably near the center of that
triangle
 To
find origin time, subtract the travel time
from the arrival time. To find origin time,
subtract the travel time from the arrival
time. It is like if a friend who lives a half
hour away, arriving at your house at 10am.
Based on that information, you know your
friend started out at 9:30 am. Example: if
the epicenter is 4000 km away, the travel
time must be 7 minutes. If an earthquake
was recorded at 2:25 pm, and the P-waves
took 7 minutes to get there, the earthquake
must have actually happened at 2:18 pm
 If
an epicenter is located 4000 km away, how
would the arrival times of S- and P- waves
compare on seismograms at your location?
 An earth quake occurred at 2:00:00. When
would the first P-waves arrive at a seismic
recording station 8000 km away?
```
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