Ground Shaking
Ground shaking is a term used to describe the vibration of the ground during an earthquake. Ground shaking is caused by body and surface seismic waves. As a generalization, the severity of ground shaking increases as magnitude increases and decreases as distance from the causative fault increases. Although the physics of seismic waves is complex, ground shaking can be explained in terms of body waves, compressional, or P, and shear, or S, and surface waves, Rayleigh and Love. P waves propagate through the Earth with a speed of about 15,000 miles per hour and are the first waves to cause vibration of a building. S waves arrive next and cause a structure to vibrate from side to side. They are the most damaging waves, because buildings are more easily damaged from horizontal motion than from vertical motion. The P and S waves mainly cause high-frequency vibrations; whereas, Rayleigh and Love waves, which arrive last, mainly cause low-frequency vibrations. Body and surface waves cause the ground, and consequently a building, to vibrate in a complex manner. The objective of earthquake-resistant design is to construct a building so that it can withstand the ground shaking caused by body and surface waves.

In land-use zoning and earthquake-resistant design, knowledge of the amplitude, frequency composition, and the time duration of ground shaking is needed. These quantities can be determined from empirical data correlating them with the magnitude and the distribution of Modified Mercalli intensity of the earthquake, distance of the building from the causative fault, and the physical properties of the soil and rock underlying the building. The subjective numerical value of the Modified Mercalli Intensity Scale indicates the effects of ground shaking on man, buildings, and the surface of the Earth.

When a fault ruptures, seismic waves are propagated in all directions, causing the ground to vibrate at frequencies ranging from about 0.1 to 30 Hertz. Buildings vibrate as a consequence of the ground shaking; damage takes place if the building cannot withstand these vibrations. Compressional and shear waves mainly cause high-frequency (greater than 1 Hertz) vibrations which are more efficient than low-frequency waves in causing low buildings to vibrate. Rayleigh and Love waves mainly cause low-frequency vibrations which are more efficient than high-frequency waves in causing tall buildings to vibrate. Because amplitudes of low-frequency vibrations decay less rapidly than high-frequency vibrations as distance from the fault increases, tall buildings located at relatively great distances (60 miles) from a fault are sometimes damaged.

Taken from: Hays, W.W., ed., 1981, Facing Geologic and Hydrologic Hazards -- Earth Science Considerations: U.S. Geological Survey Professional Paper 1240B, 108 p.

Surface Faulting
Surface faulting -- the differential movement of the two sides of a fracture at the Earth's surface-- is of three general types: strike-slip, normal, and reverse. Combinations of the strike-slip type and the other two types of faulting can be found. Although displacements of these kinds can result from landslides and other shallow processes, surface faulting, as the term is used here, applies to differential movements caused by deep-seated forces in the Earth, the slow movement of sedimentary deposits toward the Gulf of Mexico, and faulting associated with salt domes.

Photograpch credit: unknown

Death and injuries from surface faulting are very unlikely, but casualties can occur indirectly through fault damage to structures. Surface faulting, in the case of a strike-slip fault, generally affects a long narrow zone whose total area is small compared with the total area affected by ground shaking. Nevertheless, the damage to structures located in the fault zone can be very high, especially where the land use is intensive. A variety of structures have been damaged by surface faulting, including houses, apartments, commercial buildings, nursing homes, railroads, highways, tunnels, bridges, canals, storm drains, water wells, and water, gas, and sewer lines. Damage to these types of structures has ranged from minor to very severe. An example of severe damage occurred in 1952 when three railroad tunnels were so badly damaged by faulting that traffic on a major rail linking northern and southern California was stopped for 25 days despite an around-the-clock repair schedule.

The displacements, lengths, and widths of surface fault ruptures show a wide range. Fault displacements in the United States have ranged from a fraction of an inch to more than 20 feet of differential movement. As expected, the severity of potential damage increases as the size of the displacement increases. The lengths of the surface fault ruptures on land have ranged from less than 1 mile to more than 200 miles. Most fault displacement is confined to a narrow zone ranging from 6 to 1,000 feet in width, but separate subsidiary fault ruptures may occur 2 to 3 miles from the main fault. The area subject to disruption by surface faulting varies with the length and width of the rupture zone.

Taken from: Hays, W.W., ed., 1981, Facing Geologic and Hydrologic Hazards -Earth Science Considerations: U.S. Geological Survey Professional Paper 1240B, 108 p.

Ground Failure

Liquefaction Induced
Liquefaction is not a type of ground failure; it is a physical process that takes place during some earthquakes that may lead to ground failure. As a consequence of liquefaction, clay-free soil deposits, primarily sands and silts, temporarily lose strength and behave as viscous fluids rather than as solids. Liquefaction takes place when seismic shear waves pass through a saturated granular soil layer, distort its granular structure, and cause some of the void spaces to collapse. Disruptions to the soil generated by these collapses cause transfer of the ground-shaking load from grain-to-grain contacts in the soil layer to the pore water. This transfer of load increases pressure in the pore water, either causing drainage to occur or, if drainage is restricted, a sudden buildup of pore-water pressure. When the pore-water pressure rises to about the pressure caused by the weight of the column of soil, the granular soil layer behaves like a fluid rather than like a solid for a short period. In this condition, deformations can occur easily.

Photograph credit: Loma Prieta Collection, Earthquake Engineering Research Center, University of California, Berkeley

Liquefaction is restricted to certain geologic and hydrologic environments, mainly areas where sands and silts were deposited in the last 10,000 years and where ground water is within 30 feet of the surface. Generally, the younger and looser the sediment and the higher the water table, the more susceptible a soil is to liquefaction.

Liquefaction causes three types of ground failure: lateral spreads, flow failures, and loss of bearing strength. In addition, liquefaction enhances ground settlement and sometimes generates sand boils (fountains of water and sediment emanating from the pressurized liquefied zone). Sand boils can cause local flooding and the deposition or accumulation of silt.

Lateral Spreads - Lateral spreads involve the lateral movement of large blocks of soil as a result of liquefaction in a subsurface layer. Movement takes place in response to the ground shaking generated by an earthquake. Lateral spreads generally develop on gentle slopes, most commonly on those between 0.3 and 3 degrees. Horizontal movements on lateral spreads commonly are as much as 10 to 15 feet, but, where slopes are particularly favorable and the duration of ground shaking is long, lateral movement may be as much as 100 to 150 feet. Lateral spreads usually break up internally, forming numerous fissures and scarps.

Damage caused by lateral spreads is seldom catastrophic, but it is usually disruptive. For example, during the 1964 Prince William Sound, Alaska, earthquake, more than 200 bridges were damaged or destroyed by lateral spreading of flood-plain deposits toward river channels. These spreading deposits compressed bridges over the channels, buckled decks, thrust sedimentary beds over abutments, and shifted and tilted abutments and piers.

Lateral spreads are destructive particularly to pipelines. In 1906, a number of major pipeline breaks occurred in the city of San Francisco during the earthquake because of lateral spreading. Breaks of water mains hampered efforts to fight the fire that ignited during the earthquake. Thus, rather inconspicuous ground-failure displacements of less than 7 feet were largely responsible for the devastation to San Francisco in 1906.

Flow Failures - Flow failures, consisting of liquefied soil or blocks of intact material riding on a layer of liquefied soil, are the most catastrophic type of ground failure caused by liquefaction. These failures commonly move several tens of feet and, if geometric conditions permit, several tens of miles. Flows travel at velocities as great as many tens of miles per hour. Flow failures usually form in loose saturated sands or silts on slopes greater than 3 degrees.

Flow failures can originate either underwater or on land. Many of the largest and most damaging flow failures have taken place underwater in coastal areas. For example, submarine flow failures carried away large sections of port facilities at Seward, Whittier, and Valdez, Alaska, during the 1964 Prince William Sound earthquake. These flow failures, in turn, generated large sea waves that overran parts of the coastal area, causing additional damage and casualties. Flow failures on land have been catastrophic, especially in other countries. For example, the 1920 Kansu, China, earthquake induced several flow failures as much as 1 mile in length and breadth, killing an estimated 200,000 people.

Loss of Bearing Strength - When the soil supporting a building or some other structure liquefies and loses strength, large deformations can occur within the soil, allowing the structure to settle and tip. The most spectacular example of bearing-strength failures took place during the 1964 Niigata, Japan, earthquake. During that event, several four-story buildings of the Kwangishicho apartment complex tipped as much as 60 degrees. Most of the buildings were later jacked back into an upright position, underpinned with piles, and reused.

Soils that liquefied at Niigata typify the general subsurface geometry required for liquefaction-caused bearing failures: a layer of saturated, cohesionless soil (sand or silt) extending from near the ground surface to a depth of about the width of the building.

Taken from: Hays, W.W., ed., 1981, Facing Geologic and Hydrologic Hazards -- Earth Science Considerations: U.S. Geological Survey Professional Paper 1240B, 108 p.

Landslides
Past experience has shown that several types of landslides take place in conjunction with earthquakes. The most abundant types of earthquake induced landslides are rock falls and slides of rock fragments that form on steep slopes. Shallow debris slides forming on steep slopes and soil and rock slumps and block slides forming on moderate to steep slopes also take place, but they are less abundant. Reactivation of dormant slumps or block slides by earthquakes is rare.

Large earthquake-induced rock avalanches, soil avalanches, and underwater landslides can be very destructive. Rock avalanches originate on over-steepened slopes in weak rocks. One of the most spectacular examples occurred during the 1970 Peruvian earthquake when a single rock avalanche killed more than 18,000 people; a similar, but less spectacular, failure in the 1959 Hebgen Lake, Montana, earthquake resulted in 26 deaths. Soil avalanches occur in some weakly cemented fine-grained materials, such as loess, that form steep stable slopes under nonseismic conditions. Many loess slopes failed during the New Madrid, Missouri, earthquakes of 1811-12. Underwater landslides commonly involve the margins of deltas where many port facilities are located. The failures at Seward, Alaska, during the 1964 earthquake are an example.

The size of the area affected by earthquake-induced landslides depends on the magnitude of the earthquake, its focal depth, the topography and geologic conditions near the causative fault, and the amplitude, frequency composition, and duration of ground shaking. In past earthquakes, landslides have been abundant in some areas having intensities of ground shaking as low as VI on the Modified Mercalli Intensity Scale.

Taken from: Hays, W.W., ed., 1981, Facing Geologic and Hydrologic Hazards -- Earth Science Considerations: U.S. Geological Survey Professional Paper 1240B, 108 p.

Tsunamis

Photograph credit: NOAA/EDIS

Tsunamis are water waves that are caused by sudden vertical movement of a large area of the sea floor during an undersea earthquake. Tsunamis are often called tidal waves, but this term is a misnomer. Unlike regular ocean tides, tsunamis are not caused by the tidal action of the Moon and Sun. The height of a tsunami in the deep ocean is typically about 1 foot, but the distance between wave crests can be very long, more than 60 miles. The speed at which the tsunami travels decreases as water depth decreases. In the mid-Pacific, where the water depths reach 3 miles, tsunami speeds can be more than 430 miles per hour. As tsunamis reach shallow water around islands or on a continental shelf; the height of the waves increases many times, sometimes reaching as much as 80 feet. The great distance between wave crests prevents tsunamis from dissipating energy as a breaking surf; instead, tsunamis cause water levels to rise rapidly along coast lines.

Tsunamis and earthquake ground shaking differ in their destructive characteristics. Ground shaking causes destruction mainly in the vicinity of the causative fault, but tsunamis cause destruction both locally and at very distant locations from the area of tsunami generation.

Where Have Tsunamis Occurred Historically?

East Coast Historically, no tsunamis have been generated on the east coast, a consequence of the low level of seismic activity and the lack of vertical fault displacement. No tsunami occurred during the Charleston, South Carolina, earthquake of 1886, one of the largest earthquakes in the United States. In addition, none of the tsunamis occurring in the Atlantic Ocean region has significantly affected the east coast of the United States. The only tsunami known to have been recorded on the Atlantic Coast of the United States was generated by an earthquake off the Burin Peninsula of Newfoundland on November 18, 1929; it caused a wave height of 1 foot.

West Coast Tsunamis generated by earthquakes in South America and the Aleutian-Alaskan region have posed a greater hazard to the west coast of the United States than locally generated tsunamis. For example, the 1946 Aleutian tsunami produced waves heights of 12 to 16 feet at Half Moon Bay, Muir Beach, Arena Cove, and Santa Cruz, California. The 1960 Chilean tsunami produced wave heights of 12 feet at Crescent City, California. The 1964 Alaskan tsunami generated waves of more than 20 feet at Crescent City, California, where it caused $7.5 million in damage and 11 deaths. It also produced waves ranging from 10 to 16 feet along parts of the California, Oregon, and Washington coasts. In contrast, for example, the 1906 San Francisco, California, earthquake produced local tsunami waves of only about 2 inches. The largest known locally generated tsunami on the west coast was caused by the 1927 Point Arguello, California, earthquake that produced waves of about 7 feet in the nearby coastal area.

Alaska The combination of seismic activity in the Aleutian-Alaskan trench where the Pacific and North American tectonic plates collide and the vertical displacements of faults make this region of Alaska a source of tsunamis. The earliest recorded tsunami in this region was in 1788. Four major tsunamis were generated in 1946, 1957, 1964, and 1965; the 1964 Alaskan tsunami caused over $80 million in damage and killed 107 people.

Hawaii The Hawaiian Islands have experienced many destructive tsunamis because of their location in the Pacific Ocean where about 90 percent of all recorded tsunamis take place. Since 1819, more than 100 locally and distantly generated tsunamis have been recorded in the Hawaiian Islands with 16 of them causing significant damage. More than one-half of all tsunamis recorded in the Hawaiian Islands were generated in the Kuril-Kamchatka-Aleutian regions of the northern and northwestern Pacific. Tsunamis generated in that area produce the greatest waves on the northern side of the islands. About one-fourth of the historic tsunamis affecting Hawaii were generated along the western coast of South America. Tsunamis generated in the island areas of the Philippines, Indonesia, the New Hebrides, and Tonga-Kermadec have been recorded in the Hawaiian Islands, but they have not been damaging. The worst locally generated tsunamis were generated in 1869 and 1975 on the southeastern coast of the big island of Hawaii; they caused destructive waves of as much as 59 feet.

Taken from: Hays, W.W., ed., 1981, Facing Geologic and Hydrologic Hazards -- Earth Science Considerations: U.S. Geological Survey Professional Paper 1240B, 108 p.

(UC Berkeley)

For further information, see:
Univ. of Nevada article

A:

Natural Hazards Slides - NOAA National Geophysical Data Center
Steinbrugge Slide and Photograph Collection - UC Berkeley
UC Berkeley California Heritage Collection- UC Berkeley
The GeoImages Project - UC Berkeley
EQIIS Image Database - UC Berkeley
Metadata for Geologic Photographs - USGS Hazards collection
The Museum of the City of San Francisco - Bay area earthquake damage
Smithsonian Photographs Online
Library of Congress American Memory

(UC Berkeley)

A: The short answer is - a rolling motion means you are probably far away from the earthquake; a sharp jolting motion means you are probably close to the earthquake.

Three factors primarily determine what you feel in an earthquake:

1. Magnitude (you feel more intense shaking from a big earthquake than from a small one; big earthquakes also release their energy over a larger area and for a longer period of time. In most cases, only 10-15 seconds of shaking that originate from the part of the fault nearest you will be very strong
2. Distance from the fault (earthquake waves die off as they travel through the earth so the shaking becomes less intense farther from the fault)
3. Local soil conditions (certain soils greatly amplify the shaking in an earthquake. Seismic waves travel at different speeds in different types of rocks. Passing from rock to soil, the waves slow down but get bigger. A soft, loose soil will shake more intensely than hard rock at the same distance from the same earthquake. The looser and thicker the soil is, the greater the amplification will be, (e.g, Loma Prieta earthquake damage area of Oakland and Marina (SF) were 100 km (60 mi) and most of the Bay Area escaped serious damage).

The ground shaking produced by an earthquake is actually very complex--hard, gentle, long, short, jerky or rolling--and not describable with one number. Motions are described by the PEAK VELOCITY (how fast the ground is moving); PEAK ACCELERATION (how quickly the speed of the ground is changing); FREQUENCY (energy is released in waves and these waves vibrate at different frequencies just like sound waves); and DURATION (how long the strong shaking lasts).

A:

Photograph credit: National Geophysical Data Center

Liquefaction takes place when loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking. Liquefaction occurring beneath buildings and other structures can cause major damage during earthquakes. For example, the 1964 Niigata earthquake caused widespread liquefaction in Niigata, Japan which destroyed many buildings (photo on left). Also, during the 1989 Loma Prieta, California earthquake, liquefaction of the soils and debris used to fill in a lagoon caused major subsidence, fracturing, and horizontal sliding of the ground surface in the Marina district in San Francisco.

For further information, see:
Earthquake Effects FAQ section (top)
Soil Liquefaction Website - Univ. of Washington
California Geological Survey Seismic Hazards Maps
EERI article

A:A: Observations of earthquake lights (EQL), mostly white to bluish flashes or glows lasting several seconds associated with moderate to large earthquakes, have been reported infrequently by observers since ancient times. It wasn't until the phenomenon was captured in photographs, taken during the Matsushiro earthquake swarm in Japan between 1965 and 1967, that the seismological community acknowledged their occurrence. A satisfactory theory to explain EQL, however, has been elusive and is still not agreed upon. Proposed mechanisms include piezoelectricity, frictional heating, exoelectron emissions, sonoluminescence, phosphine gas emissions, and fluid injection (electrokinetics), but the most recent theory suggests that EQL are caused by separation of positive hole charge carriers that turn rocks momentarily into p-type semiconductors (first and second references below).

The most extensive modern study of EQL observations comes from the Saguenay, Quebec, earthquakes of 1988-1989 (third reference below). At least 46 well-documented reports span the time from three weeks before the main shock to two months after. The general categories of observations include: (1) seismic lightning, (2) atmospheric luminous bands, (3) globular incandescent masses, (4) fire tongues, (5) seismic flames, and a newly-recognized category, (6) coronal or point discharges. The latter observations, resulting from one observer being in the right place at the time of the main shock, strongly support the positive hole theory.
Observations of earthquake lights during the 1995 M6.9 Kobe, Japan earthquake were documented in the fourth of the references below. There were 23 sightings within 50 km of the epicenter of a white, blue, or orange light all with an upper height of 200 meters and a linear dimension of 1 to 8 km. The types of phosphorescent phenomena were classified as: lightening with zig-zag lines, swelling shield-shaped sources, upward-extending fan-shaped sources, or a belt of lights (including arc-shaped sources).

While EQL sightings are often given more exotic labels, they are a recognized geophysical phenomenon that may one day contribute to the possibility of forecasting earthquakes in the few locations where they occur.

The following links and references provide additional information about earthquake lights:

• Freund, Friedemann T., Rocks that Crackle and Sparkle and Glow: Strange Pre-Earthquake Phenomena. Journal of Scientific Exploration, 17, no. 1, p. 37-71, 2003.
• Freund, Friedemann T. et al., Flow of Charges and Energy during Rock Deformation.
  Nature, 2003 (in press).
• St-Laurent, France, The Saguenay, Quebec, Earthquake Lights of November 1988 – January 1989. Seismological Research Letters, 71, no. 2, p. 160-174, 2000.
• Tsukuda, Tameshinge, Sizes and some features of luminous sources associated with the 1995 Hyogo-ken Nanbu earthquake, Journal of Physics of the Earth, 45, no.2, p. 73-82, 1997.
• Hough, Susan E., A volcano in North Carolina? A closer look at a tall tale,
  Seismological Research Letters, 71, no. 6, p. 704-708, 2000.
• Derr, John S., Earthquake lights: A review of observations and present theories.
  Bulletin of the Seismological Society of America, 63, no. 2, p. 2177-2187, 1973.
• Derr, John S., Luminous phenomena and their relationship to rock fracture.
  Nature 321, no. 6069, p. 470-471, 1986.
  Lockner, David A., Johnston, Malcolm J.S., and Byerlee, James D., A mechanism to explain the generation of earthquake lights, Nature 302, no. 5903, p. 28-33, 1983.
• Tilling, Robert I. et al., November 29, 1975 Kalapana Earthquake

contributed by John S. Derr, April 2003