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, that 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 stations to triangulate their distances 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 the United State occur in the Rocky Mountain region because it is a converging plate boundary. 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 the United States occurred at New Madrid, Missouri, 1811-1812. A seismic risk map is illustrated as Fig. 16.13.

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 first-motion 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.

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 16.2 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.