Earthquake Lab Handout

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University of Texas, San Antonio *

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1213

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Geology

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Dec 6, 2023

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pdf

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10

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Name: ________________________________________ Earthquake Lab Background Big Idea Earthquake waves occur naturally as a result of seismic activity such as movement along fault boundaries, volcanic eruptions, and movement of magma. The seismic waves radiate outward and are picked up and recorded at seismographs. These recordings can then be used to determine distance to the earthquake and ultimately with enough recordings exactly where on Earth the earthquake occurred. With enough earthquake data, scientists can then be able to better determine and advise the public about earthquake risks and prevention. Introduction When an earthquake occurs, the news often report on the location of the epicenter, which is the point on the Earth's surface directly above the focus, the point of actual movement and release of energy (Figure 16.1). The elastic rebound, or release of energy and snap back to a low energy condition, creates seismic waves that travel through the Earth and upon reaching the surface violet shaking and damage. These originate at the focus and move radially outwards in 3 types of waves; p-waves, s-waves, and surface waves (see below and Figure 16.4). The three waves create up and down movements on a seismograph as they pass through the recorders location. • P-waves: P stands for primary or pressure. They are the fastest and arrive first. They are a compression wave and move through the Earth in a push-pull fashion. • S-waves: S stands for secondary or shear. The travel slower than P-waves and therefore arrive second to a location. They are perpendicular wave and travel through the Earth in a side-to-side motion. • Surface waves/ L-waves: L stands for Love, named after A.E.H. Love who discovered them. L-waves travel along the surface in a complex motion. They have the largest amplitude of the waves and are the slowest. Due to the large amplitude, complex motion, and the location at the surface; these are the waves that cause the majority of damage and are therefore apply named Love.
Seismograms Seismograms as shown above (Figure 16.4) can be used to determine distance to an earthquake. This determination is done by first identifying the arrival times of the p and s waves and then using a travel time versus distance graph (Figure 16.5) developed for this purpose. Figure 16.4 is a seismogram recorded at a station in Australia. Seismic waves recorded there were from an earthquake whose epicenter was located 1800km away in New Guinea. The normal background recordings were a fairly steady horizontal line until the first pulse of seismic activity. The first pulse at 7:14.2 (14.2 minutes after 7:00) would be the arrival of the p-waves. The second pulse would be the arrival of the slower S-waves at 7:17.4. The third and largest pulse would be the arrival of the L-waves. Since these waves are the slowest and moving along the surface they did not arrive until 7:18.3. Although the sample graph (Figure 16.4) begins at 7:12, the earthquake actually occurred at 7:10 and 23 seconds, written as 7:10.4. Therefore, the travel times of the seismic waves were 3.8 min for the P-waves (7:14.2- 7:10.4), 7.0 min for the S-waves (7:17.4-7:10.4), and 7.9 min (7:18.3-7:10.4) as shown at the top of the table in Figure 16.5.
Earthquake Hazards and Risks An earthquakes impact to a specific building or other structure is highly dependent on the material upon which it is built. Seismologists and geologists spend large amounts of time investigating seismic wave travel through different materials. They then compare the surface damage to the underlying bedrock and rock strata. Several prediction models are developed and run in order to determine the best mitigation strategies, develop hazard maps, and revise building codes. Below in Figures 16.2 and 16.3 is an example for an area in San Francisco, CA. Shown are a simple geologic bedrock map and seismograms of an area affected by the Loma Prieta earthquake that occurred in 1989. It can be observed that although the seismograms came from locations that were very close in geographic distance, the underlying rock structure either dampened or amplified the surface affects and the damage. Relative Motion Relative motion experienced during an earthquake is most simply thought of is what kind of force was applied to the two sides of the fault, was it in compression or dilation (tension). These locations of relative motion can then be placed on a map around the fault in order to help determine exactly what happened during the earthquake. This is of importance in order to help determine what faults are there, how fast they are moving, and possible future earthquakes. This is a fairly simple procedure as consists of looking at the arrival of the very first P-wave. If the pen on the seismogram first moves upward then it indicates that the first motion was of compression or squeezing the rocks together. If on the other hand, first motion was down, then dilation, or rocks pulling apart, is indicated. Take a look up at Figure 16.3, what was the relative motion at all of the seismic stations? (First motion was up on all three so the relative motion in this area was in compression or squeezing the rocks together).
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