Question

Provide brief explanations and discussion of ALL of the following:

1. Seismic P and S wave propagation and particle motion

2. The causes and effects of seismic wave site amplification

3. Isoseismal surveys (to estimate earthquake intensity)

4. How earthquakes are located from seismic recordings

Answer 1

A.

P-waves and S-waves are body waves that propagate through the planet. P-waves travel 60% faster than S-waves on average because the interior of the Earth does not react the same way to both of them.

P-waves are compression waves that apply a force in the direction of propagation. As the interior of the Earth is almost incompressible, P-waves transmit their energy quite easily through the medium and thus travel quickly.

On the other hand, S-waves are shear waves, which means that the motion of the medium is perpendicular to the direction of propagation of the wave. The energy is thus less easily transmitted through the medium, and S-waves are slower.

B.

Seismic waves are similarly modified by local geological conditions. These variations are known as site effects, and they can strongly influence the nature and severity of shaking at a given site. Site effects include the following factors.

Topography :

The features present at the Earth's surface have also been identified as having an influence on shaking intensity. Some studies of the distribution of the intensity of shaking experienced in an earthquake concluded that hilltop sites often shook at one intensity level higher than nearby sites with flatter topography.

Ground failure potential :

Ground failures can be as spectacular as a large landslide or much more subtle where sub-surface liquefied soils lead to differential settlement of a structure above, but the results of both types of failure can lead to large losses.

The total thickness of soil to bedrock.

Related to the direct amplification effects of soft Earth materials, the geometry of the soft deposits can further distort ground motion at soft rock sites. Seismic waves entering sediment-filled valleys, such as the Duwamish River Valley, can trap seismic energy such that it reverberates like sound in an echo chamber. This can lead to both higher amplitudes and longer durations of shaking. Because such effects are geometric in nature, they depend on the characteristics of the incoming wave, and it's direction of approach they can be very difficult to predict.

C.

In seismology, an isoseismal map is used to show lines of equal felt seismic intensity, generally measured on the Modified Mercalli scale. Such maps help to identify earthquakeepicenters, particularly where no instrumentalrecords exist, such as for historical earthquakes. They also contain important information on ground conditions at particular locations, the underlying geology, radiation pattern of the seismic waves and the response of different types of buildings. They form an important part of the macroseismic approach, i.e. that part of seismology dealing with non-instrumental data. The shape and size of the isoseismal regions can be used to help determine the magnitude, focal depth and focal mechanism of an earthquake.

D.

Unfortunately, Earth is not transparent and we can't just see or photograph the earthquake disturbance like meteorologists can photograph clouds. When an earthquake occurs, it generates an expanding wavefront from the earthquake hypocenter at a speed of several kilometers per second.

We observe earthquakes with a network of seismometers on the earth's surface. The ground motion at each seismometer is amplified and recorded electronically at a central recording site. As the wavefront expands from the earthquake, it reaches more distant seismic stations. When an earthquake occurs, we observe the times at which the wavefront passes each station. We must find the unknown earthquake source knowing these wave arrival times.We want to find the location, depth and origin time of an earthquake whose waves arrive at the times measured on each seismograms. We want a straightforward and general procedure that we can also program in a computer. The procedure is simple to state: guess a location, depth and origin time; compare the predicted arrival times of the wave from your guessed location with the observed times at each station; then move the location a little in the direction that reduces the difference between the observed and calculated times. Then repeat this procedure, each time getting closer to the actual earthquake location and fitting the observed times a little better. Quit when your adjustments have become small enough and when the fit to the observed wave arrival times is close enough.