Seismic Hazards

A fault is a fracture in the crust along which one side has moved relative to the other side. Faults can be very small or hundreds of miles long. The earth's crust is composed of huge plates that are in slow but nearly constant motion. Part of California is on the Pacific Plate, and part is on the North American Plate. The San Andreas Fault, which runs from the Salton Sea in Imperial County to Cape Mendocino in Humboldt County, is the boundary between these plates. Sometimes one block of the crust moves up while the other moves down, sometimes they move horizontally in opposite directions (that's what's happening with the San Andreas Fault; Los Angeles is creeping closer to San Francisco). Some faults are well known and easy to spot, such as the San Andreas. Others are underground, with nothing on the surface revealing their presence (a blind thrust fault). The 1994 Northridge earthquake was caused by a blind thrust fault. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

Earthquakes occur when the two sides of a fault slip suddenly against each other. The Pacific and North American plates move past each other about 1.5 inches a year. The friction between the plates causes stress, which is released when the blocks of crust slip suddenly along a fault plane. That releases waves of energy that travel through the ground, causing the shaking you feel. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

There are hundreds of identified faults in California; about 200 are considered potentially hazardous based on their slip rates in recent geological time (the last 10,000 years). More than 70 percent of the state's population resides within 30 miles of a fault where high ground shaking could occur in the next 50 years. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

Earthquake ruptures usually begin far under the surface of the Earth. The point of origin miles down is called the hypocenter. The epicenter is the point on the surface directly above the hypocenter. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture. (From http://earthquake.usgs.gov/learning/faq.php?categoryID=1)

The National Earthquake Information Center (U.S.) reports 12,000-14,000 earthquakes a year around the world, or 35 a day. Throughout the world, there are one "great" (magnitude 8.0 or more), 18 "major" (7.0-7.9), 120 "large" (6.0-6.9) and 1,000 "moderate" (5.0-5.9) earthquakes in an average year. Each year, California generally gets two or three earthquakes large enough to cause moderate damage to structures (magnitude 5.5 and higher). (From http://www.consrv.ca.gov/index/Earthquakes/Pages/qh_earthquakes.aspx

While there's no way to stop an earthquake, there are ways to build safer buildings and structures and otherwise be prepared for them. Building codes in California are updated often as new information comes in. While earthquakes are a deadly threat, there have been few earthquake-related deaths in California relative to places with less stringent codes or enforcement, such as Turkey and China. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

There are several ways to measure an earthquake, but the most common is magnitude. Scientists no longer use the original Richter scale, but an updated version. Earthquakes should be referred to as "magnitude X'' rather than "an X on the Richter scale." A magnitude 6.0 earthquake releases 32 times more energy than a magnitude 5.0 and nearly 1,000 times more energy than a 4.0. But that doesn't mean the ground shakes a thousand times harder in a 6.0 than a 4.0, because the energy is released over a much larger area. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

A magnitude 6.0 quake releases approximately as much energy as 6,270 tons of TNT, an M 7.0 199,000 tons, an M 8.0 6.27 million tons and a M 9.0 99 million tons. Of course, all that energy is not focused in one particular spot, but spreads out in waves. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

There are three major factors: magnitude, your distance from the fault, and local soil conditions. Magnitude is discussed above. As for distance, the seismic waves that cause the shaking become less intense farther from the fault. Certain soil conditions amplify the shaking; generally, the looser the soil, the greater the amplification. Although most of San Francisco escaped serious damage in Loma Prieta, those with unconsolidated landfill or soft soils (such as the Marina District) suffered serious damage. The ground motion in such areas was 10 times stronger than at neighboring sites on rock. (From http://www.consrv.ca.gov/index/Earthquakes/qh_earthquakes.htm)

A tsunami (pronounced soo-nah-mee) is a series of waves of extremely long wave length and long period generated in a body of water by an impulsive disturbance that vertically displaces the water.

The term tsunami was adopted for general use in 1963 by an international scientific conference. Tsunami is a Japanese word represented by two characters: "tsu" and "nami." The character "tsu" means harbor, and the character "nami" means wave. In the past, tsunamis were often referred to as "tidal waves." The term "tidal wave" is a misnomer. Tides are the result of gravitational influences of the moon, sun, and planets. Tsunamis are not caused by the tides and are unrelated to the tides; although a tsunami striking a coastal area is influenced by the tide level at the time of impact. (From http://nctr.pmel.noaa.gov/faq.php#db)

Tsunamis are most often caused by earthquakes and landslides. These push the water upward, sideways or downward to create the tsunami waves. Volcanic eruptions can also cause tsunamis. There is evidence than in the distant past, asteroids and comets striking the Earth have created enormous tsunamis. (From http://nctr.pmel.noaa.gov/faq.php#db)

Tsunamis can be generated when the sea floor abruptly shifts and vertically displaces the overlying water from its equilibrium position. Waves are formed as the displaced water mass attempts to regain its equilibrium. The main factor which determines the initial size of a tsunami is the amount of vertical sea floor deformation. Not all earthquakes generate tsunamis. To generate tsunamis, earthquakes must occur underneath or near the ocean, be large and create movements in the sea floor. All oceanic regions of the world can experience tsunamis, but in the Pacific Ocean there is a much more frequent occurrence of large, destructive tsunamis because of the many large earthquakes along the margins of the Pacific Ocean. (From http://nctr.pmel.noaa.gov/faq.php#db)

Any disturbance that displaces a large water mass from its equilibrium position can generate a tsunami. Generally tsunamis caused by landslides or volcanic eruptions dissipate more quickly than Pacific-wide tsunamis caused by some earthquakes and rarely affect coastlines distant from the source. (From http://nctr.pmel.noaa.gov/faq.php#db)

Tsunami waves are shallow-water waves with long periods and wave lengths. (A wave is classified a shallow-water wave when the ratio between the water depth and its wavelength gets very small. The speed of a shallow-water wave is equal to the square root of the product of the acceleration of gravity (32ft/sec/sec or 980cm/sec/sec) and the depth of the water.) Shallow water waves are different from wind-generated waves (the waves many of us have observed on the beach). Wind-generated waves usually have period (time between two succesional waves) of five to twenty seconds and a wavelength (distance between two successional waves) of about 50 to 600 feet (15 to 200 meters) A tsunami can have a period in the range of 10 minutes to 1 hour and a wavelength in excess of 700 km (430 miles). (From http://nctr.pmel.noaa.gov/faq.php#db)

The terms 'tsunami' and 'tidal wave' do not mean the same thing, though many use the words interchangeably. You will find the term 'tidal wave' (referring to tsunamis) is most often seen in reproductions of old news reports and older text books.

tidal wave

1. The wave motion of the tides.
2. In popular usage, any unusually high and therefore destructive water level along a shore. It usually refers to either a storm surge or tsunami.

tsunami

A series of traveling waves of extremely long length and period, usually generated by disturbances associated with earthquakes occurring below the ocean floor. (From http://nctr.pmel.noaa.gov/faq.php#db)

A tsunami is caused by a sudden displacement of water. The most frequent cause is an underwater earthquake but, less frequently, tsunamis can be generated by volcanic eruptions, landslides, or even oceanic meteor impact. The length of these waves, from one crest to the next, can be up to 200 km long, and they travel in the deep ocean at speeds around 700 km/hr. Their height in the open ocean is very small, a couple of meters at most, so they pass under ships and boats undetected.

So called 'rogue waves' are a bit more mysterious, and not very well understood. They are very high waves, tens of meters, perhaps. They are very short compared to tsunamis, less than a 2000 m, perhaps. They arise unexpectedly in the open ocean, and the generating mechanism is a source of controversy and active research. Some theories:

• Strong currents interact with existing swell to make them much higher.
• They are just a statistical aberration that occurs when a bunch of waves just happen to be in the right spot at the right time, so that they add together to make one big wave.
• If a storm 'prepares' the ocean, by making it very rough, and this is followed by a sudden intensification of the storm, then the wind can get a "better grip" on the ocean surface (i.e., wind energy is much more efficiently transferred to the water), and the monster waves can thus be created."

(From http://nctr.pmel.noaa.gov/faq.php#db)

Tsunamis are detected by open-ocean tsunami buoys and by coastal tide gages. These instruments report their information in real-time to tsunami warning centers (one center in Alaska, another in Hawaii, and a third to be installed soon in Puerto Rico).

Besides the direct observations, the amplitudes of tsunamis are also estimated from the size and type of earthquake that may have generated them. The warning centers receive earthquake (seismic) data from many sites as well as analyses of this information from earthquake centers. Since seismic waves travel much faster than tsunamis, the earthquake information is often available hours before the tsunamis are able to travel across the ocean. This is much help for people near the earthquake, however, since the local tsunami is there often within minutes. It is that easy to estimate the strength of a tsunami from the character of an earthquake. Hence, there are false alarms. It is hoped that the direct tsunami observations will reduce the number of these.

Local tsunamis are also generated by underwater landslides and volcanoes. There is some research being done to better understand these types of tsunamis and to develop early detection methods for them. (From http://nctr.pmel.noaa.gov/faq.php#db)