Earthquake location

1. Why is it important to locate earthquakes as accurately as possible?

Step 1: Single station approach.

Earthquakes generate seismic waves that propagate through the Earth and that are recorded by seismic instruments all over the world.
The relative amplitudes of the three components of ground displacement due to a P wave arrival at a single station can be used to get a rough idea of where the signal is coming from (and therefore, in which direction the earthquake occurred).
The difference in arrival time between the P and the S wave can be used to estimate the distance to the earthquake.

Use the recordings of vertical and horizontal ground displacements (in nanometers) of a large earthquake recorded by station KONO in southern Norway and combine the directional information with the distance information to get a rough estimate of this earthquake's location.

Figure 1. Three components of P wave displacement as recorded by station KONO in southern Norway.

2. Measure the amplitude of the P phase on the North-South component (North is positive) and on the East-West component (East is positive) from the records shown in Figure 1.
a. Plot these components in a coordinate system with the N-S direction as the y-axis and E-W the x-axis.
b. Determine from which direction the P wave arrived. Note that 'Up' is positive on the vertical component.

SeisGram2K mini-manual

Figure 2. To launch the Java program Seisgram2K.

• Expand the window for a larger image and use the top left triangles to change the amplitude scale.

• To zoom in, draw a box around the P arrival and/or use the arrows of the toolbar.

• To pick a seismic phase, click on 'Pick' and pick the onset with the cursor. The date and time are given as "yyyymmdd hhmm ss.sss" at the top of the figure. Click 'Close' to get out of picking mode. (WATCH OUT: You will lose the pick information at the top of the screen if you go to another window.)
  

•  In picking mode you can indicate uncertainties around your pick by clicking on the line of the pick you just made, holding the button and moving your mouse up. This will help you in a later exercise to get quantitative about your results.

3. For a teleseismic event (an earthquake that is very far away from the station where the seismogram is recorded), the P phase is visible most prominently on the vertical component, whereas the S phase is the first large arrival on the horizontal components.
a. Explain why this is the case and check that this also the case for the seismograms in figure 2.
b. Measure the arrival time difference between the P and the S. You may have to zoom in to pick times accurately.

Figure 3. Q = angular epicentral distance

4. Seismologists often talk about epicentral distances in degrees, instead of kilometers. Figure 3 shows a cross-section through the earth along the plane of a seismic ray. It shows how the epicentral distance can be defined by an angle. How many kilometers along the surface correspond to one degree in epicentral distance? (The radius of the earth is approximately 6371 km.)

5. Compare the value of the S-P time that you found to the graph in Figure 4 and determine the approximate distance to the earthquake in km.

Figure 4. Travel time difference between P and S as a function of epicentral distance. The values were calculated assuming an earthquake at the surface and 1-D reference Earth model ak135.

6. Use Figure 5 and the information from 2. and 5. to determine the approximate coordinates (latitude and longitude) of the earthquake.

Figure 5. Map centered on station KONO in southern Norway (red triangle). Blue dots show locations of large earthquakes that occurred during the period 1976 to 1993. Angles and distance can be highly distorted due to the map projection.

Check your answer here!

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