Page 438 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

402 part III The Earth–Atmosphere Interface
reached far under China, causing frequent earthquakes. As evidence of this ongoing strain, the January 2001 quake in Gujarat, India, occurred along a shallow thrust fault that gave way under the pressure of the northward-pushing Indo-Australian plate. More than 1 million buildings were destroyed or damaged during this event. In October 2005, a 40-km segment of the 2500-km fault zone that marks the plate boundary snapped, hitting the Kashmir region of Pakistan with an M 7.6 quake that killed more than 83 000 people.
Tectonic earthquakes are those quakes associated with faulting. Earthquakes can also occur in association with volcanic activity. Recent research suggests that the injection of wastewater from oil and gas drilling into subsurface areas is another cause, leading to episodes of induced seismicity (discussed with fault mechanics ahead).
Earthquake Anatomy
The subsurface area where the motion of seismic waves is initiated along a fault plane is the focus, or hypocenter, of an earthquake (see example in Figure 13.18). The area at the surface directly above the focus is the epicentre. Shock waves produced by an earthquake radiate outward through the crust from the focus and epicentre. As dis- cussed earlier, scientists use the seismic wave patterns and the nature of their transmission to learn about the deep layers of Earth’s interior.
A foreshock is a quake that precedes the main shock. The pattern of foreshocks is now regarded as an impor- tant consideration in earthquake forecasting. An af- tershock occurs after the main shock, sharing the same general area of the epicentre; some aftershocks rival the main tremor in magnitude. For instance, on the South Island of New Zealand an M 7.1 quake struck in Septem- ber 2010 with an epicentre 45 km from Christchurch, causing USD $2.7 billion in damage and no deaths. Seventeen days later, a 6.3 aftershock with an epicentre just 6 km from the city produced building collapses, 350 deaths, and more than USD $15 billion in damage. These events show that the distance from the epicentre is an important factor in determining overall earthquake effects on population centres.
In 1989, the Loma Prieta earthquake hit south of San Francisco, California, east of Santa Cruz. Damage totaled USD $8 billion, 14 000 people were displaced from their homes, 4000 were injured, and 63 were killed. Unlike previous earthquakes—such as the San Francisco quake of 1906, when the plates shifted a maximum of 6.4 m relative to each other—no fault plane or rifting was ev- ident at the surface in the Loma Prieta quake. Instead, the Pacific and North American plates moved horizon- tally approximately 2 m past each other deep below the surface, with the Pacific plate thrusting 1.3 m upward (Figure 13.18). This vertical motion is unusual for the San Andreas fault and indicates that this portion of the San Andreas system is more complex than previously thought.
Earthquake intensity and Magnitude
A seismometer (also called a seismograph) is an instru- ment used to detect and record the ground motion that occurs during an earthquake. This instrument records motion in only a single direction, so scientists use a combination of vertical-motion and horizontal-motion seismometers to determine the source and strength of seismic waves. The instrument detects body waves (trav- elling through Earth’s interior) first, followed by surface waves, recording both on a graphic plot called a seis- mogram. Scientists use a worldwide network of more than 4000 seismometers to sense earthquakes, and these quakes are then classified on the basis of either damage intensity or the magnitude of energy released.
Before the invention of modern earthquake instru- mentation, damage to terrain and structures and severity of shaking were used to assess the size of earthquakes. These surface effects are a measure of earthquake in- tensity. The Modified Mercalli Intensity (MMI) scale is a Roman-numeral scale from I to XII that ranges from earthquakes that are “barely felt” (lower numbers) to those that cause “catastrophic total destruction” (higher numbers; see Table 13.1). The scale was designed in 1902 and modified in 1931.
Earthquake magnitude is a measure of the energy re- leased and provides a way to compare earthquake size. In 1935, Charles Richter designed a system to estimate earth- quake magnitude based on measurement of maximum wave amplitude on a seismometer. Amplitude is the height of a seismic wave and is directly related to the amount of ground movement. The size and timing of maximum seis- mic wave height can be plotted on a chart called the Rich- ter scale, which then provides a number for earthquake magnitude in relation to a station located 100 km from the epicentre of the quake. (For more on the Richter scale, see earthquake.usgs.gov/learn/topics/measure.php.)
The Richter scale is logarithmic: Each whole number on it represents a 10-fold increase in the measured wave amplitude. Translated into energy, each whole number signifies a 31.5-fold increase in energy released. Thus, a magnitude of 3.0 on the Richter scale represents 31.5 times more energy than a 2.0 and 992 times more energy than a 1.0. Although useful for measuring shallow earth- quakes that are in close proximity to seismic stations, the Richter scale does not properly measure or differentiate between quakes of high intensity.
The moment magnitude (M) scale, in use since 1993, is more accurate for large earthquakes than is Richter’s amplitude magnitude scale. Moment magnitude con- siders the amount of fault slippage produced by the earthquake, the size of the surface (or subsurface) area that ruptured, and the nature of the materials that faulted, including how resistant they were to failure. Techni- cally, the M is equal to the rigidity of Earth multiplied by the average amount of slip on the fault and its area. This scale considers extreme ground acceleration (move- ment upward), which the Richter amplitude magnitude method underestimates.