CHAPTER 3 - EARTHQUAKES
Overview
The chapter begins with descriptions of the events, particularly destruction, associated with earthquakes recorded over the past 350 years in North
America. Seismic waves produce earthquakes from the sudden release of stored strain, usually along faults. The elastic rebound theory has been used to explain earthquakes by large amounts of strain building along faults, but small stress is a newly proposed model of fault behavior. Deep earthquakes are thought to be associated with subduction and mineral transformation.
Body waves, traveling through the earth's interior, originate at the earthquake focus, while surface waves originate at the epicenter. There are two types of body waves: P waves are compressional and vibrate parallel to wave propagation and S waves vibrate perpendicular to wave propagation. Surface waves cause most damage and have slowest travel time. Two types of surface waves are: Love waves that have no vertical displacement and vibrate perpendicular to wave propagation, and Rayleigh waves that behave like ocean waves describing elliptical paths. Both types of surface waves are destructive.
Seismometers record the motion of these seismic waves along x, y, and z axes of three dimensional space. Their record, a seismogram, is produced by a seismograph. Earthquake epicenters and foci are determined by analysis of seismographs that record the intensity of earthquake vibrations in three orthogonal directions or by triangulation of recording seismographs.
Earthquake strength is determined by reference to either the modified Mercalli scale (intensity) (Table 3.1), Richter scale (magnitude - Table 3.3) or moment magnitude (displacement along faults).
Most large earthquakes occur in western North America (Figure 3.13) along faults related to the converging North American and Pacific plates.
Earthquakes east of the Rockies are related to old diverging plate boundaries or failed rifts (aulacogens). Historic earthquakes east of the Rockies have had
Richter magnitudes of 5.0-6.0, and Mercalli intensities of VIII-XI (New York,
Massachusetts, South Carolina, and most significantly New Madrid, Missouri).
Effects of earthquake ground motion include: fire, landslides, liquefaction, permanent land surface displacement, aftershocks, and tsunamis (seismic sea waves).
The circum-Pacific belt contains 80% of the earth's shallow focus (0-70 km),
90% of intermediate focus (70-350 km), and 100% of deep focus (350-670 km) earthquakes. The Earthquakes occur along mid-oceanic ridges and in
Benioff zones associated with ocean trenches and the Mediterranean-
Himalayan belt.
First-motion studies of earthquakes provide information on the way rocks move along faults and are useful in understanding plate boundaries. Plate boundaries are defined in part by earthquake concentrations. Characteristic patterns of earthquake distribution and rock motion are associated with diverging, transform and converging boundaries, and provide information on subduction angles. Earthquakes that occur away from plate boundaries are termed intraplate earthquakes and usually occur along zones of weakened crust such as continental margins, aulacogens or ancient suture zones.
The chapter ends with a discussion of possible means for earthquake prediction, particularly seismic gaps and seismic history, and the potential for earthquake control using water as a lubricant.

Learning Objectives
1. Earthquakes are the sudden release of strain energy, usually along faults, but also associated with volcanism and mineral transformations. Elastic rebound theory accounts for this stored strain, but a weak-fault model, involving small stress has been suggested for some earthquakes, and not all earthquakes are associated with faults.
2. Earthquakes produce seismic waves. Body waves originate at the focus, the point of initial movement along a fault. Surface waves originate from the epicenter, point on earth's surface directly above the focus. P waves are compressional body waves vibrating parallel to wave propagation that arrive first at a recording station. S waves are transverse body waves that vibrate perpendicular to wave propagation and arrive after P waves. P waves pass through fluids, but S waves do not. Surface waves are slowest, but cause the most damage. They include Love waves (vibrate perpendicular to propagation) and Rayleigh waves (cause ground to move in elliptical path).
3. Seismometers detect seismic waves by measuring ground motion.
Seismograms are the records of earth motions produced by seismographs, which are recording seismometers.
4. Difference in arrival times of P and S waves at a seismograph tell its distance from the earthquake focus, using a travel-time curve. Location of the epicenter requires three component seismographs or three stations to triangulate distance from the focus. Depth of focus can be determined by analyses of seismograms: shallow< 70 km, intermediate 70-350 km, deep
350-670 km. Earthquakes do not occur below 670 km, and shallow focus account for 85% of the total earthquake energy released.
5. Earthquake strength is measured by either intensity (damage) or magnitude
(energy released). The modified Mercalli scale expresses intensities in Roman numerals (I-XII). The Richter scale determines magnitude by measuring the height of a particular wave on a seismogram. Recorded Richter magnitudes range from a little more than 0 to 8.6. It is logarithmic so that a difference of one on the scale is 10 times the ground vibration and 32 times the energy released. Moment magnitude, determined by rock strength, surface area of rupture, and amount of rock displacement along a fault in the field, is a new method of calculating magnitude, and is more accurate for magnitudes of 7 or greater. Moment magnitudes can exceed 9.0, the theoretical limit for the
Richter scale.
6. Most earthquakes in North America occur west of the Rocky Mountain region and are associated with movement between the North American and Pacific plates. Historic earthquakes east of the Rockies occur along old diverging plate boundaries and aulacogens. They have had Richter magnitudes of 5.0-6.0
(Adirondack Mountains, New York; Lawrenceville, Kansas, Quebec City,
Canada), and Mercalli intensities of VIII-XI (St. Lawrence River Valley,
Massachusetts, New York, South Carolina). The most widely felt earthquakes
(intensity XII) ever to strike North America occurred at New Madrid, Missouri,
1811-1812. A seismic hazard map is illustrated as Fig. 3.14.
7. Earthquake damage is caused by ground motion that topples buildings, produces fires (broken gas mains), landslides, liquefaction, permanent land displacement, and tsunamis (seismic sea waves). Foreshocks and aftershocks

precede and follow the main earthquake and they can cause destruction as well.
8. Vertical motion of the sea floor is most conducive to tsunami formation and most are associated with subduction zones. Tsunami wavelengths can reach
160 kilometers with speeds of 725 kilometers per hour. A breaking tsunami can reach 30 meters. Devastating tsunami have struck Hawaii, California,
Alaska, New Guinea, and Japan. An Early Warning System was developed after the 1946, Hilo, Hawaii, tsunami to minimize loss of life around the Pacific coast.
9. Most earthquakes are concentrated in the circum-Pacific belt, which produces 100% of deep focus, 90% of intermediate focus, and 80% of shallow focus earthquakes. The Mediterranean-Himalayan belt is the second major concentration of earthquakes. Benioff earthquake zones begin at ocean trenches and slope and deepen toward island arcs and continents. Benioff zones account for the world's deep and intermediate focus earthquakes.
10. First-motion studies determine whether the fault producing the earthquake was undergoing tension or compression.
11. Plate boundaries are identified and defined by earthquakes. Diverging plate boundaries produce rift valleys from normal faulting indicated by firstmotion studies. First motion studies indicate that transform boundaries experience shallow strike-slip motion. Converging boundaries are marked by either continental collision, or subduction. A subducting plate initially undergoes tension at a trench as it is bent, but may experience either tension or compression as it descends into mantle based on first-motion studies.
Subduction angles vary from gentle to steep, and plates may even break-up at depth. Deepest earthquakes may be caused by mineral transformations or dehydration. Intraplate earthquakes occur in areas away from plate boundaries underlain by weakened or thinned crust.
12. A variety of observations have been used to predict earthquakes: microseisms and changes in rock properties, such as magnetism, change in water levels in wells, radon emission, changes in geyser eruption intervals, surface tilting and elevation change, animal behavior and foreshocks. Most prediction is based currently on seismic history and identification of seismic gaps, areas quiet for long periods of time. Box 3.4 provides a detailed discussion of the analysis of seismic gaps associated with the San Andreas system.
13. Timed release of stored strain along faults could potentially allow the control of earthquakes. Lubricating the fault with water is one method to release small, timed earthquakes rather than a large unpredicted one.
Boxes
3.1 - IN GREATER DEPTH - EARTHQUAKE ENGINEERING
Buildings constructed of strong, flexible and light materials are the most resistant to seismic shaking, while those of unreinforced block or brick, particularly with heavy roofs and unconnected structural components, tend to fail The 1985 Mexico City earthquake killed 5000 people and caused $5 billion damage. Severe damage resulted when the two-second period of the seismic waves was amplified by the natural two-second period of the lakebeds underlying the city. As a result, the ground moved back and forth by 40 cm every two seconds, and did so 15-20 times. The amplified ground shaking was

particularly devastating to buildings with a natural two-second period as well.
These buildings were generally between five and 20 stories, and approximately
800 of them collapsed or were severely damaged by the earthquake and aftershock. Most of the city's 600,000 structures were not damaged demonstrating that proper design can lessen earthquake damage.
3.2 – IN GREATER DEPTH - WHAT TO DO BEFORE, DURING, AND AFTER AN
EARTHQUAKE
Earthquake preparedness can reduce damage and the chance of injury. Before an earthquake, anchor the foundation and chimneys and repair any cracks, check for hazards inside the home, learn how to turn-off the utilities, have disaster supplies on hand, and have an emergency communication plan.
During an earthquake, indoors - drop, cover and hold under a piece of heavy furniture, but leave an unsafe building, outdoors - move to an open area, in a vehicle - stop in a safe place and stay in car. After an earthquake, prepare for aftershocks, help injured people, check for utility damage, inspect chimneys, listen to radio for information, and don't travel.
3.3 – ENVIRONMENTAL GEOLOGY – WAITING FOR THE BIG ONE IN BRITISH COLUMBIA
The western coast of Canada is at considerable risk from earthquake activity associated with subduction of the Juan de Fuca plate below the North American plate along the Cascadia Subduction Zone. Three types of earthquakes could affect southwestern British Columbia including crustal earthquakes (originating within the
North American plate), intraplate earthquakes (originating within the Juan de Fuca plate) and subduction earthquakes (originating along the boundary between the plates). Geologic evidence indicates that great earthquakes (>M=8) have occurred on average once every 500-700 years. Rupture of the locked portion of the Juan de
Fuca-North American plate boundary could cause an earthquake in excess of M=9 but smaller intraplate earthquakes could also cause significant damage in the region depending on location of the epicenter. Vancouver is particularly prone to earthquake damage as large areas of the city are built on unconsolidated or waterlogged surficial sediments or lie close to steep slopes.
3.4 - ENVIRONMENTAL GEOLOGY - WAITING FOR THE BIG ONE IN CALIFORNIA
Although there has been considerable recent earthquake activity along the San
Andreas Fault system, concern about the "Big One," a magnitude 8, remains high for both the northern and southern portions of the system. The southern
San Andreas, including Los Angeles, seems the more likely candidate at the moment, since its last movement occurred in 1857 and its activity seems to reflect a 140-year cycle. The U.S. Geological Survey has assigned a 60% probability for a 7.5-8.3 magnitude earthquake striking the southern San
Andreas in the next 30 years. The northern portion of the fault is also overdue.
The U.S. Geological Survey gives the Hayward Fault, across the bay from San
Francisco, a 67% probability for a 7.0-7.5 magnitude earthquake in the next
30 years, although the San Andreas Fault in that area only rates a 10% probability for a magnitude 8 event during the same period.
Short Discussion/Essay

1. Why can't there be an earthquake with a magnitude greater than 9.0 on the
Richter scale?
2. Why are most earthquakes generated in the crust and not in the mantle?
3. Do all faults produce earthquakes, why or why not? Are all earthquakes associated with faults, why or why not?
4. Why do surface waves cause the most structural damage?
5. Why didn't the seismographs in California get much of a record of the Loma
Prieta earthquake?
Longer Discussion/Essay
1. Why were deep earthquake foci, now called the Benioff zone, so troubling to geologists before the Theory of Plate Tectonics?
2. Why is the New Madrid area in Missouri so active seismically?
3. Why are earthquakes so difficult to predict?
4. Explain the difference between earthquake intensity and earthquake magnitude and the bias inherent with the modified Mercalli scale.
5. Why are first motion studies important to understanding sea-floor spreading?
Selected Readings
Newsletters and free publications on earthquakes:
Earthquake Update. A monthly newsletter published by the Earthquake
Project, c/o NCPI, 10 Winthrop Square, Boston, MA 02110.
Earthquakes and Volcanoes. A bi-monthly publication of the U.S. Geological
Survey available as a yearly subscription from Superintendent of Documents,
U.S. Government Printing Office, Washington, D.C. 20402 (credit card subscriptions taken at 202-783-3238).
Probabilities of Large Earthquakes in the San Francisco Bay Region. 1990. U.S.
Geological Survey Circular 1053. Available free from Superintendent of
Documents, U.S. Government Printing Office, Washington, D.C. 20402.
Loma Prieta: A Selected Annotated Bibliography. 1990. Available free from
BAREPP, Metrocenter, 101 8th Street, Suite 152, Oakland, CA 94607.
The Next Big Earthquake. 1990. Available free from U.S. Geological Survey,
345 Middlefield Road, Menlo Park, CA 94025.
Other general articles:
Borcherdt, R.D. ed. 1994. "The Loma Prieta, California, Earthquake of October
17, 1989 - Strong Ground Motion," U.S. Geological Survey Professional Paper
1551.
Clague, J. and Turner, B., 2003. Vancouver, City on the Edge: Living with a dynamic geological landscape. Tricouni Press, Vancouver, 191pp.
Frohlich, C. 1989. "Deep earthquakes," Scientific America 260(1): 48-55.
Johnston, A.C. and Kanter, L.R. 1990. "Earthquakes in stable continental crust," Scientific American 262(3): 68-75.

Rial, J.A., Saltzman, N.G. and Ling, H. 1992. "Earthquake-induced resonance in sedimentary basins," American Scientist 80(6): 566-591.
Wallace, R.E. ed. 1990. "The San Andreas Fault System," U.S. Geological
Survey Professional Paper 1515.