Earthquakes

Figure 10.2: Elastic rebound.

Earthquakes occur when a critical amount of stress is applied to the crust. According to the elastic rebound theory, rocks can bend elastically up to a point, until they finally break. The rocks then snap apart, releasing energy in the form of seismic waves (Figure 10.2). The plane defined by the rupture is known as a fault, and the rock layers become offset along it. An excellent example of this kind of offset can be found along the San Andreas Fault at Point Reyes National Seashore. The Earthquake Trail, which begins at the Bear Valley Visitor Center, follows the trace of the 1906 San Francisco earthquake to a picket fence along the fault (Figure 10.3). The fence was once connected, but today it is separated by a 6-meter (20-foot) gap. Looking at the photo, one can visualize how the foreground moved to the left, while the background moved to the right. Although the San Francisco earthquake was quite large, most of the damage to the city was actually caused by a fire, which is a common secondary hazard of earthquakes.

There are two ways to measure earthquakes: magnitude and intensity. Magnitude (M) is the measure of the energy released by the earthquake, whereas the intensity is what people actually experience. The first scale used to measure magnitude was the Richter scale, which measures the amplitude of a seismic wave at a defined distance from the earthquake. The Richter scale is somewhat limited, however, because it cannot accurately measure or compare large earthquakes. In 1979 Thomas C. Hanks and Hiroo Kanamori developed the Moment Magnitude scale, abbreviated M_w. Both the Richter scale and the Moment Magnitude scale use the numbers 1 - 10 to measure the amount of energy released. Although it is possible to have a magnitude smaller than 1, and there is technically no upper limit, this is the range commonly used when reporting earthquakes to the public. The United States Geological Survey (USGS) describes earthquakes as minor (M3.0 - 3.9), light (M4.0 - 4.9), moderate (M5.0 - 5.9), strong (M6.0 - 6.9), major (M7.0 - 7.9) and great (M8.0 or higher). These scales are logarithmic, meaning that a M9.0 earthquake would have 10 times the amplitude, and release 32 times the energy of a M8.0 earthquake. Accordingly, an M9.0 quake would have 100 times the amplitude and 1024 times the energy of a M7.0 earthquake. The largest earthquake in US history was the 1964 Alaskan Earthquake, which had an M_w of 9.2.

Figure 10.3: The picket fence provides clear visual proof of the potential significance of this type of offset.

The magnitude of an earthquake does not tell us how much damage is done by the waves in a particular area. The amount of shaking and damage is known as the earthquake’s intensity, and it can be measured by the Modified Mercalli Intensity (MMI) scale. This scale uses the Roman numerals I - XII to describe the effects of the earthquake in a particular location. For example, near the epicenter of a small earthquake, or at a location far from a large earthquake, the intensity may be described with an MMI of II: “Felt only by a few persons at rest, especially on the upper floors of buildings. Delicately suspended objects may swing.” The MMI scale is a subjective gauge compared to the Moment Magnitude scale. The USGS has attempted to improve the accuracy of MMI shake maps by soliciting data from the public. Figure 10.4 shows the intensities felt by the 1964 Great Alaskan Earthquake (when data were collected through mail questionnaires). Today, after experiencing an earthquake, anyone may go to the USGS earthquake website and describe the effects at the “Did You Feel It?” page.

Figure 10.4: Intensity map of the 1964 Great Alaskan Earthquake.

Alaska has more earthquakes than all of the other states combined. California is second, with only about half as many as Alaska. The following table lists the number of earthquakes of M3.5 or greater from 1974 to 2003, according to the USGS Earthquake Hazards Program.

Cascadia

Although Washington and Oregon have not experienced many major earthquakes in the recent past, they are located above the Cascadia subduction zone (Figure 10.5). Since the Cascadia subduction zone is as large as the subduction zone that created the 2004 Sumatra Earthquake and subsequent tsunami, it could potentially generate an M8.0 or higher earthquake. It did just that in the year 1700, when the tip of the overriding plate (North America), which had been buckling upward under the stress of the ongoing collision, flattened out when the plates suddenly slipped past one another. The resulting earthquake caused the submergence of coastal land and the drowning of cedar forests along 965 kilometers (600 miles) of California, Oregon, Washington, and southern British Columbia. Places formerly above sea level dropped down, pushing enormous waves all the way across the ocean, and creating a tsunami that struck Japan. Written records of the damage to Japan’s coastal villages allow us to pinpoint the day and time this earthquake occurred—the evening of January 26, 1700.

Figure 10.5: The Cascadia subduction zone and its related seismic occurrences.

If the Cascadia subduction zone were to partially rupture today, it could create an M8.2 earthquake, and if it were to rupture along its entire length, it could generate an M9.0 earthquake. Geologists have recently forecast a 37% chance of an M8.2 or higher event within 50 years, and a 10 - 15% chance of an event of M9.0 or higher. Unlike Alaska, which had a population of only 763,000 in 2013, Washington and Oregon have a combined population of over 10 million people, most of whom live near the coast. Needless to say, Cascadia inhabitants would benefit greatly by preparing for a large earthquake and tsunami.

San Andreas

The San Andreas Fault, a 1304-kilometer (810-mile) long plate boundary (Figure 10.6), is not a single fault but a fault zone because it does not move all at once. Instead, great lengths of the fault rupture while other sections remain locked. Some sections do neither, but instead creep along without generating significant earthquakes.

Southern California is particularly susceptible to a devastating earthquake, in part because it has been so long since a major or great quake has occurred. Over 150 years ago, the M7.9 Fort Tejon earthquake caused 360 kilometers (225 miles) of the San Andreas Fault to rupture. The southernmost section of the San Andreas has not ruptured in the last 300 years. In response to this growing hazard, geologists have created a “Great ShakeOut” earthquake scenario. This scenario models the effects of a M7.8 earthquake on a 300-kilometer (180-mile) rupture from Bombay Beach at the Salton Sea to Lake Hughes north of Los Angeles. Both the Fort Tejon segment and the southernmost segment of the San Andreas fault zone are considered at risk for a major earthquake at any time, and either one could generate an earthquake that would wreak havoc on the lives of the 23 million inhabitants of Southern California. Earthquake preparedness drills based on this scenario have been taking place in southern California since 2008, and they attract millions of participants every year.