Each station determines the time interval between the arrival of the first P wave and the first S wave at their location•A travel-time graph is used to determine each station’s distance to the epicenter
Locating the source of earthquakesLocating the epicenter of an earthquake•A circle with a radius equal to the distance to the epicenter is drawn around each station •The point where all three circles intersect is the earthquake epicenter
Finding an earthquake epicenter
Measuring the size of earthquakesTwo measurements that describe the size of an earthquake are •Intensity – a measure of the degree of earthquake shaking at a given locale based on the amount of damage •Magnitude – estimates the amount of energy released at the source of the earthquake
Measuring the size of earthquakesIntensity scales •Modified Mercalli Intensity Scale was developed using California buildings as its standard •The drawback of intensity scales is that destruction may not be a true measure of the earthquake’s actual severity
Measuring the size of earthquakesMagnitude scales •Richter magnitude - concept introduced by Charles Richter in 1935 •Richter scale•Based on the amplitude of the largest seismic wave recorded •Accounts for the decrease in wave amplitude with increased distance
Measuring the size of earthquakesMagnitude scales•Richter scale•Largest magnitude recorded on a Wood-Anderson seismograph was 8.9 •Magnitudes less than 2.0 are not felt by humans •Each unit of Richter magnitude increase corresponds to a tenfold increase in wave amplitude and a 32-fold energy increase
Measuring the size of earthquakesMagnitude scales•Other magnitude scales•Several “Richter-like” magnitude scales have been developed •Moment magnitude was developed because none of the “Richter-like” magnitude scales adequately estimates very large earthquakes •Derived from the amount of displacement that occurs along a fault
Measuring the size of earthquakes
Earthquake destructionAmount of structural damage attributable to earthquake vibrations depends on •Intensity and duration of the vibrations •Nature of the material upon which the structure rests •Design of the structure
Earthquake destructionDestruction from seismic vibrations •Ground shaking•Regions within 20 to 50 kilometers of the epicenter will experience about the same intensity of ground shaking•However, destruction varies considerably mainly due to the nature of the ground on which the structures are built
Important historical earthquakes•Shaanxi, China, 1556 - ~830,000 deaths M 8.0•Tangshan, China, 1976 - ~650,000 deaths M 8.2•Aleppo, Syria, 1138 - ~230,000 deaths M 8.5•Sumatra, Indonesia, 2004– 227,898 deaths M 9.1•Haiti, 2010– 222,570 deaths M 7.0•Gansu, China, 1920 - ~220,000 deaths M 7.8-8.5•Damghan, Iran, 856 - ~200,000 deaths M 8.0•Xining, China, 1927 - ~200,000 deaths M 7.9•Kanto, Japan, 1923 – 142,800 deaths M 7.9•Ashgabat, Turkmenistan, 1948 – 110,000 deaths M 7.3•Messina, Italy, 1908 – 100,00-200,000 deaths M 7.1
Largest earthquakes since 1900•Valdivia, Chile, 1960– M 9.5•Anchorage, Alaska, 1964 – M 9.2•Sumatra, Indonesia, 2004 – M 9.1
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- Spring '08