Dia da Terra: #Richter "10": Where and When? #earthquake

sexta-feira, 5 de agosto de 2011

#Richter "10": Where and When? #earthquake

Dia da Terra

http://www.usgs.gov/

Q: What is the biggest earthquake ever?

A: Since 1900, the earthquake in Chile on May 22, 1960, is the biggest in the World with magnitude 9.5 Mw.

Probabilities.

Most earthquake probabilities are determined from the average rate of historical events. Assuming the annual rate is constant, one can make a probability statement about the likelihood of such an event in the next so-many years. These probabilities might range from one in thirty to one in three hundred.

For some faults, historical occurrences are not available, but rate of slip along the fault can be estimated. Assuming a particular magnitude, one can estimate the number of years it would take to accumulate the required amount of slip. This estimate can be used to give an annual rate and used in the same manner as historical rates. These probabilities might range from one in three hundred to one in three thousand.

Finally, given a large event, there is a declining rate of aftershocks. From this rate, the probability of an aftershock can be determined. These probabilities might be larger than one in thirty.

Predictions.

Predictions usually occur as a result of some event supposed to be indicative of an earthquake occurring in the near future. Such an event may be a swarm of small earthquakes, increasing amounts of radon in local water, unusual behavior of animals, increasing size of magnitudes in moderate size events, or a moderate-magnitude event rare enough to suggest that it may be a foreshock.

Unfortunately, most such precursors frequently occur without being followed by an earthquake. This means that the forecast must be made in probabilistic terms. Estimates of such the probabilities seem to be no greater than one in three, to one in ten and hence the forecasts have low reliability. A succession of unreliable forecasts is likely to do more harm than good.

One might hope that before a major earthquake several such precursor would occur. This might increase the reliability of a forecast. An earthquake forecast was made in China several decades ago, based on small earthquakes and unusual animal activity. People were able to sleep outside of their homes and thus were spared when the main quake struck and caused widespread destruction.

However, unfortunately, many earthquakes are preceded by no precursory events whatsoever. The next large Chinese event was entirely unheralded and scores of thousands of Chinese died.

(contributed by Dave Perkins)

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48 Conterminous States [ 2008 | 2002 | 1996 ]

The U.S. Geological Survey (USGS) National Seismic Hazard Maps display earthquake ground motions for various probability levels across the United States and are applied in seismic provisions of building codes, insurance rate structures, risk assessments, and other public policy. This update of the maps incorporates new findings on earthquake ground shaking, faults, seismicity, and geodesy. The resulting maps are derived from seismic hazard curves calculated on a grid of sites across the United States that describe the frequency of exceeding a set of ground motions.

2008 Hazard Map (PGA, 2% in 50 years)
Download Formats : [ EPS | PDF | JPG ]
Alaska [ 2007 | 1999 ]

Probabilistic seismic hazard maps have been prepared for Alaska portraying ground motion values (peak ground acceleration and spectral amplitude at periods of 0.2, 0.3 and 1.0 seconds) at probabilities of exceedance of 2% and 10% in 50 years. Preparation of these maps followed the same general strategy as that followed for the U.S.G.S. seismic hazard maps of the contiguous United States, combining hazard derived from spatially-smoothed historic seismicity with hazard from fault-specific sources.
Hawaii [ 1998 ]

Probabilistic seismic hazard maps were prepared for Hawaii portraying peak horizontal ground acceleration and horizontal spectral response acceleration for 0.2, 0.3, and 1.0 second periods with probabilities of exceedance of 10% in 50 years and 2% in 50 years. This particular data set is for horizontal spectral response acceleration for 1.0 second period with a 10% probability of exceedance in 50 years. All of the maps were prepared by combining hazard derived from spatially- smoothed historic seismicity with hazard from fault-specific sources.
Puerto Rico & the U.S. Virgin Islands [ 2003 ]

We present results of a new probabilistic seismic hazard assessment for Puerto Rico and the U. S. Virgin Islands (PRVI). The study area is located along the boundary between the northeastern Caribbean Sea and the Atlantic Ocean, at the intersection of the Greater and Lesser Antilles Island chains. These islands demarcate the boundary between the North American and Caribbean tectonic plates, with the North American plate moving west-southwestward relative to the Caribbean plate.

USGS Urban Hazard Maps

Seattle Urban Seismic Hazard Maps [ 2007 ]

The USGS has produced a new series of earthquake hazard maps for the City of Seattle. These ‘urban seismic hazard’ maps provide a much higher-resolution view of the potential for strong earthquake shaking than previously available. This new view is particularly important for Seattle, which sits atop a sedimentary basin that strongly affects the patterns of earthquake ground shaking and therefore, of potential damage.
Memphis, Shelby County Seismic Hazard Maps [ 2004 ]

Memphis, Shelby County, Tennessee, is located where damaging earthquakes are only moderately likely, but the consequences of earthquakes, mainly from the New Madrid seismic zone, can be very high. This densely populated urban area is built on a 1-kilometer-thick sequence of sediments deposited in a trough known as the Mississippi embayment. This thick pile of sediments significantly affects earthquake ground motions. We, the authors, have generated a suite of seismic hazard maps for a six-quadrangle area in Memphis, Shelby County, Tennessee, that accounts for these effects. These maps and their derivative products represent the collaborative efforts of the U.S. Geological Survey (USGS) and its partners.
Liquefaction Hazard in Oakland, California [ 2002 ]

The liquefaction hazard map predicts the approximate percentage of each designated area that will have surface manifestations of liquefaction during an M7.1 earthquake on the Hayward fault. An earthquake of this magnitude is expected if the whole Hayward fault ruptures in a single event (Working Group on California Earthquake Probabilities, 1999). This event dominates the deaggregated hazard near the eastern shore of San Francisco Bay.