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disasters10.16notes - 1 6.1 Ch 6 Some Earthquakes in...

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6.1: Ch. 6 – Some Earthquakes in Western North America (Continued) Date:__________ How Faults Work : the fault movement begins at a hypocenter and then propagates outward for a certain distance and length of time . How much stored energy is released during an EQ depends on the number of seconds the fault moves . (It was previously thought that an EQ would release all the elastic strain and the buildup would begin anew.) (Fig. 6.33) Southern San Andreas Fault : neotectonic analyses indicate that the last four major EQs on the Coachella Valley segment occurred about 250 years apart, and the most recent one happening ~ 1690 , over 315 years ago! In 1988, this fault was given a 40 % probability of a 7.5 M W EQ by 2018 . The Coachella Valley segment has about 20 ft of unreleased movement. If that EQ triggers the San Bernardino segment (last EQ in 1812, ~14 ft of unreleased movement), the combined magnitude could be about 7.8 , and if it also triggers the Mojave segment (last EQ in 1857, ~15 ft of unreleased movement), the combined magnitude could reach 8.0 ! Thrust Faultss in Southern California : the constriction at the “Big Bend” of the San Andreas fault has caused shredding and slicing, which have split Southern California into a complex maze of east-west oriented thrust faults (reverse faults) , narrowing valleys and uplifting mountains. Many of these faults do not reach the surface and are called blind thrusts . 1
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The Los Angeles area is being compressed 10-15 mm/year, which could generate an EQ in the M W mid-6s every 6 years, plus an EQ of an M W 7 every 10 years!
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