FAQ's about Hazards

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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

A landslide is defined as, the movement of a mass of rock, debris, or earth down a slope. (Cruden, 1991). Landslides are a type of "mass wasting" which denotes any down slope movement of soil and rock under the direct influence of gravity. The term "landslide" encompasses events such as rock falls, topples, slides, spreads, and flows (Varnes, 1996). Landslides can be initiated by rainfall, earthquakes, volcanic activity, changes in groundwater, disturbance and change of a slope by man-made construction activities, or any combination of these factors. Landslides can also occur underwater, causing tidal waves and damage to coastal areas. These landslides are called submarine landslides.

Failure of a slope occurs when the force that is pulling the slope downward (gravity) exceeds the strength of the earth materials that compose the slope. They can move slowly, (millimeters per year) or can move quickly and disastrously, as is the case with debris-flows. Debris-flows can travel down a hillside of speeds up to 200 miles per hour (more commonly, 30 – 50 miles per hour), depending on the slope angle, water content, and type of earth and debris in the flow. These flows are initiated by heavy, usually sustained, periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions. More information can be found in USGS Fact Sheet numbers FS-071-00, Landslide Hazards (English Version), and FS-072-00, Peligros de Deslizamientos (Spanish Version).

Information on debris flows can be found in our Publications section.

Sources of Information for this FAQ:

(1). Cruden, D.M., 1991. A Simple Definition of a Landslide. Bulletin of the
International Association of Engineering Geology, No. 43, pp. 27-29.

(2). Varnes, D.J., 1996. Landslide Types and Processes, in Turner, A. K., and R.L. Schuster, Landslides: Investigation and Mitigation, Transportation Research Board Special Report 247, National Research Council, Wasington, D.C.: National Academy Press.

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.

There are about 1500 potentially active volcanoes worldwide, aside from the continuous belt of volcanoes on the ocean floor. About 500 of these have erupted in historical time. Many of these are located along the Pacific Rim in what is known as the "Ring of Fire." In the U.S., volcanoes in the Cascade Range and Alaska (Aleutian volcanic chain) are part of the Ring, while Hawaiian volcanoes form over a "hot spot" near the center of the Ring.

1. Niihau
2. Kauai
3. Oahu
4. Molokai
5. Lanai
6. Maui
7. Kahoolawe
8. Hawaii (Big Island)

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.

Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products -- lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A volcano is most commonly a conical hill or mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth. The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled. -- From: Tilling, 1985, Volcanoes: USGS General Interest Publication.

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".

The type of equipment and techniques we use to study volcanoes depends on the particular volcano topic we are investigating and on the experiment we are conducting. When specialized instruments are not available for a special study or for monitoring a specific type of activity, we design and build our own; for example the acoustic flow monitor (AFM) for detecting lahars and for studying flowing mixtures of water and rock debris under controlled conditions.

For studying and monitoring restless and erupting volcanoes, several onsite and remote methods are used to gather data that also help us answer four critical questions during a volcano emergency.

For reconstructing a volcano's eruptive history so that we can identify the type of activity most likely to occur in the future as well as the areas around a volcano that are likely to be effected by future eruptions, we use many geologic mapping and dating strategies. These include:

1. Identifying rock outcrops, formations, and features on the ground and identifying their exact location on detailed aerial photographs and topographic maps or in computerized geographic information systems (GIS).
2. Collecting dozens to hundreds of volcanic rock and ash samples from sites located on or near the volcano and also tens of kilometers downwind or downstream, and then using laboratory techniques for determining their chemistry and mineral compositions.
3. Determining the ages of as many rock deposits formed by past activity of the volcano by using several common methods:
    
   • Carbon-14 dating when a volcanic deposit either incorporated or came to rest on top of vegetation or organic-rich soil and sufficient carbon-bearing material can be found. It's based on the fact that living trees and other organic matter contain small amounts of carbon's radioactive isotope (atomic weight of 14). When a tree is killed by a volcanic deposit, its radioactive carbon begins to decrease by radioactive decay at a known rate. By measuring the 14C/12C ratio in the wood sample, its age can be calculated. This technique can adequately date deposits that are as old as about 50,000 years, and each date may have an error range of between a few tens to several hundred years. The most common technique for dating recent volcanic deposits, only a few scientific laboratories in the United States can perform the carbon analysis.
      
   • Tree-ring dating when a volcanic deposit caused an unusual growth pattern of annual rings among trees growing at the time the deposits were emplaced. This technique can sometimes date deposits to an actual calendar year or to within a few years when used to on deposits of the past few hundred years.
      
   • Paleomagnetism in some volcanic areas where scientists have determined the yearly changes in the position of the Earth's magnetic pole over the past several hundreds or thousands of years and when the Earth's magnetic direction is preserved in volcanic rocks (usually lava flows and individual large rocks in pyroclastic flows); this technique usually yields ages with a range of between a few tens and several hundred years.
      
4. Representing the types and ages of volcanic rock deposits and/or identifying volcanic hazard areas around the volcano on a paper map or computerized geographic information system.

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.

The term "100-year flood," is used to describe the recurrence interval of floods. As the table below shows, the "100-year recurrence interval" means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year event will occur.

But, just because a 100-year flood happened last year doesn't mean that it won't happen this year, too. In other words, future rainfall and floods don't depend on the rainfall and floods that happened in the past. The past records are mainly used to show what kind of river flows can be expected. So, when you hear about a 100-year flood, at least you have a general idea that it does mean a BIG flood, and if you hear of a 200-year flood you know that it means one even BIGGER! As an example, in July of 1994, some places in south Georgia received more than 20 inches of rainfall in a few days -- the floods they produced were tremendous... way over the 100-year flood. At Senoia, Ga., the maximum amount of water flowing by the Line Creek gage was 2.4 times greater than the 100-year flood level.

There are many paths to becoming a volcanologist. Most share a college or graduate school education in a scientific or technical field, but the range of specialties is very large. Training in geology, geophysics, geochemistry, biology, biochemistry, mathematics, statistics, engineering, atmospheric science, remote sensing, and related fields can be applied to the study of volcanoes and the interactions between volcanoes and the environment. The key ingredients are a strong fascination and boundless curiosity about volcanoes and how they work. From there, the possibilities are almost endless. Learn more about volcano training and schools.

A magnetic storm is a period of rapid magnetic field variation. The causes of magnetic storms are explained, in general terms, in the Introduction to Geomagnetism page given on this website. Briefly, then, magnetic storms have two basic causes. First of all, let us be reminded that the Sun is always emitting a wind of charged particles that flows outward into space away from the Sun itself. Occasionally the Sun emits a strong surge of solar wind, something called a coronal mass ejection. When this gust of solar wind impacts upon the outer part of the Earth's magnetic field, the magnetosphere, the field is disturbed and it undergoes a complex oscillation. This causes the generation of associated electric currents in the near-Earth space environment, which, in turn, generate additional magnetic-field variations -- all of which constitute a 'magnetic storm'. The second cause of magnetic storms is the occasional direct linkage of the Sun's magnetic field with that of the Earth's. This direct magnetic connection is not the normal state of affairs, but when it occurs, charged particles, traveling along magnetic-field lines, can easily enter the magnetosphere, generate currents, and cause the magnetic field to undergo time-dependent variation. On occasion, the Sun emits a coronal mass ejection at a time when the magnetic-field lines of the Earth and Sun are directly connected. Then we can experience a truly large magnetic storm.

Go to the Natural Hazards Gateway, which includes:

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

Restless volcanoes can be very dangerous places, but it's possible to work safely around them if you're properly prepared. First and foremost, scientists protect themselves by working as a team to create a "safety net" in which all the important bases are covered. Like a professional driving team, a volcano-response team includes key staff who know the monitoring equipment extremely well, experts in several scientific disciplines who can interpret data coming back from the field, a spokesperson to communicate warnings and other information to public officials and the media, and a scientist-in-charge, or "driver," who assumes overall responsibility for team performance. As part of an experienced scientific team capable of quickly assessing the past behavior of a restless volcano, installing instruments to take its pulse, and analyzing all available information to understand what the volcano is doing, a modern volcanologist is prepared to work safely even in the hazardous environment of a restless volcano.

The USGS poster Geologic Hazards of Volcanoes depicts many of the hazards associated with a volcanic eruption