FAQ's about Earthquakes

No. The San Andreas Fault System, which crosses California from the Salton Sea in the south to Cape Mendocino in the north, is the boundary between the Pacific Plate and North American Plate. The Pacific Plate is moving in northwest with respect to the North American Plate at approximately 46 millimeters per year (the rate your fingernails grow). The strike-slip earthquakes on the San Andreas Fault are a result of this plate motion. The plates are moving horizontally past one another, so California is not going to fall into the ocean. However, Los Angeles and San Francisco will one day be adjacent to one another!

For further information, see: Earthquakes, Megaquakes, and the Movies.

Browse the following websites for more information:

• US Recent Earthquakes
• World-wide Recent Earthquakes
• World-wide Earthquakes in the Last 7 Days
• NEIC QED list
• Global Earthquake Information - IRIS Seismic Monitor
• United States - ANSS

Nowadays, thanks to the advent of science, it has been shown there is no connection between weather and earthquakes. Earthquakes are the result of geologic processes within the earth and can happen in any weather and at any time during the year. Earthquakes originate miles underground. Wind, precipitation and barometric pressure changes affect only the surface and shallow subsurface of the Earth. Earthquakes are focused at depths well out of the reach of weather, and the forces that cause earthquakes are much larger than the weather forces. Earthquakes occur in all types of weather, in all climate zones, in all seasons of the year, and at any time of day.

As with many natural phenomena, tsunamis can range in size from micro-tsunamis detectable only by sensitive instruments on the ocean floor to mega-tsunamis that can affect the coastlines of entire oceans, as with the Indian Ocean tsunami of 2004. If you hear a tsunami warning or if you feel strong shaking at the coast or very unusual wave activity (e.g., the sea withdrawing far from shore), it is important to move to high ground and stay away from the coast until wave activity has subsided (usually several hours to days). For more general information on tsunamis and what to do during a tsunami warning, please visit sites sponsored by FEMA, the National Weather Service, NOAA, and the USGS.

In 1931, there were about 350 stations operating in the world; today, there are more that 4,000 stations and the data now comes in rapidly from these stations by telex, computer and satellite. This increase in the number of stations and the more timely receipt of data has allowed us and other seismological centers to locate many small earthquakes which were undetected in earlier years, and we are able to locate earthquakes more rapidly.

The NEIC now locates about 12,000 to 14,000 earthquakes each year or approximately 50 per day. Also, because of the improvements in communications and the increased interest in natural disasters, the public now learns about more earthquakes. According to long-term records (since about 1900), we expect about 18 major earthquakes (7.0 - 7.9) and one great earthquake (8.0 or above) in any given year. However, let's take a look at what has happened in the past 32 years, from 1969 through 2001, so far. Our records show that 1992, and 1995-1997 were the only years that we have reached or exceeded the long-term average number of major earthquakes since 1971. In 1970 and in 1971 we had 20 and 19 major earthquakes, respectively, but in other years the total was in many cases well below the 18 per year which we may expect based on the long-term average.

A temporal increase in earthquake activity does not mean that a large earthquake is about to happen. Similarly, quiescence, or the lack of seismicity, does not mean a large earthquake is going to happen.

See NEIC's Earthquake Statistics webpage for the tables of earthquake counts by magnitude and year.

The earliest reference we have to unusual animal behavior prior to a significant earthquake is from Greece in 373 BC. Rats, weasels, snakes, and centipedes reportedly left their homes and headed for safety several days before a destructive earthquake. Anecdotal evidence abounds of animals, fish, birds, reptiles, and insects exhibiting strange behavior anywhere from weeks to seconds before an earthquake. However, consistent and reliable behavior prior to seismic events, and a mechanism explaining how it could work, still eludes us. Most, but not all, scientists pursuing this mystery are in China or Japan.

1975: For example, on November 29, 1975, a large magnitude-7.2 earthquake struck the Big Island of Hawaii at 4:48 a.m. It was centered about 28 kilometers southeast of Kilauea Volcano's summit caldera at a depth of 5 kilometers; the earthquake occurred within the volcano's south flank. The earthquake was preceded by numerous foreshocks, the largest of which was a 5.7 magnitude jolt at 3:36 a.m. the same morning, and was accompanied, or closely followed, by a tsunamis, massive ground movements, hundreds of aftershocks, and a short-lived eruption in Kilauea's summit caldera.

The eruption began at 5:32 a.m. from a 500-meter long fissure on the caldera floor and ended by 10:00 p.m. According to scientists at the USGS Hawaiian Volcano Observatory, the eruptive activity "was apparently triggered by the 7.2 magnitude earthquake. The small volume and brief duration of the eruption suggest that the shallow magma might not have reached the surface under its own buoyant energy without a triggering mechanism apparently provided by the violent ground shaking."

1868: The largest historic earthquake (estimated between 7.5 and 8.1) on the Big Island occurred beneath the south flank of Mauna Loa Volcano on April 2, 1868. The earthquake was followed by a small eruption from Kilauea's southwest rift zone and from a fissure on the caldera wall that flooded the adjacent Kilauea Iki crater with lava. Also, within Kilauea's caldera, part of the floor subsided about 90 meters. This activity occurred nearly simultaneously with an eruption from the southwest rift zone of Mauna Loa volcano.

Source:

Macdonald, Gordon A., Abbott, Agatin T., and Peterson, Frank L., 1983 (2nd edition), Volcanoes in the Sea -- The geology of Hawaii: Honolulu, University of Hawaii Press, 517 p.

More Historic Examples

Mount Pinatubo, Philippines

Mount Pinatubo's huge explosive eruption on June 15, 1991, occurred within 11 months of a magnitude 7.8 earthquake that occurred about 100 kilometers northeast of the volcano. Many scientists have since asked, "Was the eruption triggered by, or otherwise related to the earthquake that had occurred on July 16, 1990?" A recent study by scientists of the Philippine Institute of Volcanology and Seismology and the U.S. Geological Survey Study suggests that there was indeed a relationship between the two events.

The study suggests that the "failure stress along faults of the Pinatubo area" after the big earthquake "were probably not a cause of Pinatubo's awakening. However, compressive stress on the magma reservoir and its roots was about 1 bar, possibly enough to squeeze a small volume of basalt into the overlying dacitic reservoir.

Alternately, strong ground shaking associated with the Luzon earthquake might have done the same or triggered movement along previously stressed faults that in turn allowed magma ascent."

Source:

Bautista, B.C., Bautista, L.P., Stein, R.S., Barcelona, E.S., Punongbayan, R.S., Laguerta, E.P., Rasdas, A.R., Ambubuyog, G., and Amin, E.Q., Relationship of Regional and Local Structures to Mount Pinatubo Activity in: Newhall, C.G., Punongbayan, R.S. (eds.) Fire and mud: Eruptions and lahars of Mt. Pinatubo, Philippines, Philippine Institute of Volcanology and Seismology, Quezon City and University of Washington Press, Seattle p. 351- 370.

Restless Calderas

A recent study of the historic activity at calderas from around the world showed that "caldera unrest occurred at least 79 times in close temporal association with regional earthquakes or, in a few instances, with swarms of regional earthquakes. By close temporal association we mean within a time span that is short in relation to the usual recurrence intervals of both the regional earthquakes and the unrest, usually within a few months or less."

"Fifty regional earthquakes (most M 6 and above) were followed within hours to months of unrest at nearby calderas... Twenty seven of these episodes culminated in eruptions, and three others are continuing without eruptions as yet (Rabaul, Wrangell, and Yellowstone)." Rabaul caldera in Papua New Guinea erupted in 1994.

The authors also found that "at least 27 regional earthquakes occurred within 100 kilometers of a restless caldera during or shortly after caldera unrest" and concluded "that magma bodies beneath young calderas often react to changes in regional tectonic strain, and that unrest at calderas is sometimes a general, long-range precursor to regional earthquakes."

Source:

Newhall, Christopher, G., and Dzurisin, Daniel, 1988, Historic Unrest at Large Calderas of the World: U.S. Geological Survey Bulletin 1855, vol 1, p. 19-20.

Karymsky Volcano, Russia

For a recent example, see the May 1996 report on Karymsky Volcano on the Kamchatka Peninsula in Russia from the Smithsonian Institution's Bulletin of the Global Volcanism.

An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.

In California there are two plates - the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. The primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone. The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year.

Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great EQs when it finally releases.

For further information, see the USGS General Interest Publication "Earthquakes".

Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies. Most of these earthquakes were minor. The largest and most widely known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude 5.5 followed a series of smaller earthquakes. Injection had been discontinued at the site in the previous year once the link between the fluid injection and the earlier series of earthquakes was established. (Nicholson, Craig and Wesson, R.L., 1990, Earthquake Hazard Associated with Deep Well Injection--A Report to the U.S. Environmental Protection Agency: U.S. Geological Survey Bulletin 1951, 74 p.)

Other human activities, even nuclear detonations, have not been linked to earthquake activity. Energy from nuclear blasts dissipates quickly along the Earth's surface. Earthquakes are part of a global tectonic process that generally occurs well beyond the influence or control of humans. The focus (point of origin) of earthquakes is typically tens to hundreds of miles underground. The scale and force necessary to produce earthquakes are well beyond our daily lives. We cannot prevent earthquakes; however, we can significantly mitigate their effects by identifying hazards, building safer structures, and providing education on earthquake safety.

Go to the Natural Hazards Gateway, which includes:

• Earthquakes
• Floods
• Geomagnetism (solar flares, etc.)
• Hurricanes
• Landslides
• Seismic Zone Mapping
• Sinkholes
• Tsunamis
• Volcanoes
• Wildfires

A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.

Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slipslip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion. Faults which move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults.

The following definitions are adapted from The Earth by Press and Siever.

normal fault - a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.

thrust fault - a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being subducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.

strike-slip fault - a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.

A left-lateral strike-slip fault is one on which the displacement of the far block is to the left when viewed from either side.

A right-lateral strike-slip fault is one on which the displacement of the far block is to the right when viewed from either side.

For animations of each type of fault, see: IRIS.

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. 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
Soil Liquefaction Website - Univ. of Washington
California Geological Survey Seismic Hazards Maps

Five billion years ago the Earth was formed by a massive conglomeration of space materials. The heat energy released by this event melted the entire planet, and it is still cooling off today. Denser materials like iron (Fe) sank into the core of the Earth, while lighter silicates (Si), other oxygen (O) compounds, and water rose near the surface.

The earth is divided into four main layers: the inner core, outer core, mantle, and crust. The core is composed mostly of iron (Fe) and is so hot that the outer core is molten, with about 10% sulfur (S). The inner core is under such extreme pressure that it remains solid. Most of the Earth's mass is in the mantle, which is composed of iron (Fe), magnesium (Mg), aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over 1000 degrees C, the mantle is solid but can deform slowly in a plastic manner. The crust is much thinner than any of the other layers, and is composed of the least dense calcium (Ca) and sodium (Na) aluminum-silicate minerals. Being relatively cold, the crust is rocky and brittle, so it can fracture in earthquakes. (Univ. of Nevada)

This is a brief summary of our knowledge of the earth's interior. For further information, see: University of Nevada

A wide variety of maps are available from the USGS national mapping program in both paper and digital form. Check our list of earthquake and fault maps first. If you don't find what you are looking for there, the following links also have map information:

This Dynamic Planet: World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics
Dibblee Geological Foundation Maps - for some of California, with roads and towns
California Geological Survey
Other State Agencies

You can also contact your state geological survey and/or your county geologist. The Association of American State Geologists has compiled a directory of state geological organizations, which provides addresses, telephone numbers, and staff.

A temporary increase or decrease in the seismicity rate is usually just part of the natural variation in the seismicity. There is no way for us to know whether or not this time it will lead to a larger earthquake. Swarms of small events, especially in geothermal areas, are common, and moderate-large magnitude earthquakes will typically have an aftershock sequence that follows. All that is normal and expected earthquake activity.

United States earthquakes (by region or by state)
Largest earthquakes (for each state)
Earthquake Searchable Catalog
The ANSS (Advanced National Seismic System) Composite Catalog
Northern California
Southern California
Nevada
Pacific Northwest

Tsunamis are large, potentially destructive sea waves, most of which are formed as a result of submarine earthquakes, but which may also result from the eruption or collapse of island or coastal volcanoes and the formation of giant landslides on marine margins. These landslides, in turn, are often triggered by earthquakes. Environmental damage by these tsunamis include coral reef destruction, contamination of wells and other sources of fresh water by salt water, denudation of trees and other types of dry-land vegetation, accelerated beach erosion, and fish and other marine life fatalities due to abnormal wave action. The flooding and powerful wave action of the tsunami may potentially cause damage to man-made containment vessels of petroleum products, chemicals, and garbage landfills, resulting in toxic leakage, which in turn has the potential to pollute both coastal land and ocean environment.

Tsunami waves can be generated from displacements of water resulting from rock falls, icefalls and sudden submarine landslides or slumps. Major earthquakes are suspected to cause many underwater landslides, which may contribute significantly to tsunami generation. For example, many scientists believe that the 1998 tsunami, which killed thousands of people and destroyed coastal villages along the northern coast of Papua-New Guinea, was generated by a large underwater slump of sediments, triggered by an earthquake.

The 1964 Alaska earthquake caused 115 deaths in Alaska alone, with 106 of those due to tsunamis generated by tectonic uplift of the sea floor, and by localized subareal and submarine landslides. The earthquake shaking caused at least 5 local slide-generated tsunamis within minutes after the shaking began. For an eyewitness account of the tsunami caused by the movement and landslides of the 1964 Alaska earthquake, please see: http://www.npr.org/templates/story/story.php?storyId=5007860

Current research in the Canary Islands concludes that there have been at least five massive volcano landslides that occurred in the past, and that these same large events may occur in the future. These giant landslides have the potential of generating large tsunami waves, at close and also very great distances and would have the potential to devastate large areas of coastal land, as far away as the eastern seaboard of North America.

Rock falls and rock avalanches in coastal inlets, such as those that have occurred in the past at Tidal Inlet, Glacier Bay National Park, Alaska have the potential to cause regional tsunamis that pose a hazard to coastal ecosystems and human settlements. On July 9, 1958, a magnitude M 7.9 earthquake on the Fairweather Fault triggered a rock avalanche at the head of Lituya Bay, Alaska. The landslide generated a wave that ran up 524 m on the opposite shore and sent a 30-m high wave through Lituya Bay, sinking two of three fishing boats and killing two persons.

Source of Information:

Geist, E. L., 1998, Source characteristics of the July 17, 1998 Papua New Guinea tsunami: EOS, Transactions of the American Geophysical Union, v. 79 (supplement), p. 571.

Committee on the Alaska Earthquake of Division of Earth Sciences, National Research Council, Seismology and Geodesy, 1972,Plafker, George, USGS, U.S. Geological Survey Professional Paper. 543-I. Tectonics of the March 27, 1964, Alaska Earthquake.

Siebert, Lee, 2004, Landslides resulting from structural failure of volcanoes, Catastrophic Landslides:  Effects, Occurrence and mechanisms, Evans, Stephen G., and Jerome V. DeGraff, eds., Reviews in Engineering Geology, Volume XV, Geological Society of America, Boulder, Colorado USA.

Wieczorek, Gerald F., Jakob, Matthias, Motyka, Roman, Zirnheld, Sandra L., and Patricia Craw, 2003, Preliminary assessment of landslide-induced wave hazards:  Tidal Inlet, Glacier Bay National Park, Alaska, U.S. Geological Survey Open-file report 03-100.

The moon, sun, and other planets have an influence on the earth in the form of perturbations to the gravitational field. The relative amount of influence is proportional to the objects mass, and inversely proportional to the square of its distance from the earth. No significant correlations have been identified between the rate of earthquake occurrence and the semi-diurnal tides when using large earthquake catalogs. There have, however, been some small but significant correlations reported between the semi-diurnal tides and the rate of occurrence of aftershocks in some volcanic regions, such as Mammoth Lakes. (UC Berkeley)

For further information, see: University of California, Berkeley, Seismological Laboratory.

Please see Can It Happen Here?