Parkfield is at the northern end of a locked segment of the San Andreas Fault (SAF) that, in 1857, ruptured south from Monarch Peak (MP) in the great 7.8 magnitude Ft. Tejon quake. As a result of nearby earthquakes in 2003 and 2004, tremors developed under Cholame and Monarch Peak. The black dots pinpoint 1250 well-located tremors. The square boxes are 30 kilometers (19 miles) on a side.
Color contours give regional shear-stress change at 20 km depth from the Parkfield earthquake (green segment) along the SAF. The thrust-type San Simeon earthquake rupture is represented by the gray rectangle and line with triangles labeled SS. The currently locked Cholame segment is about 63 km long (solid portion of the arrow) and is believed capable of rupturing on its own in a magnitude 7 earthquake. The gray lines within the Cholame box bound the west quadrant, where quasiperiodic episodes predominate.
BERKELEY — Increases in mysterious underground tremors observed in several active earthquake fault zones around the world could signal a build-up of stress at locked segments of the faults and presumably an increased likelihood of a major quake, according to a new University of California, Berkeley, study. Parkfield is at the northern end of a locked segment of the San Andreas Fault (SAF) that, in 1857, ruptured south from Monarch Peak (MP) in the great 7.8 magnitude Ft. Tejon quake. As a result of nearby earthquakes in 2003 and 2004, tremors developed under Cholame and Monarch Peak. The black dots pinpoint 1250 well-located tremors. The square boxes are 30 kilometers (19 miles) on a side. Color contours give regional shear-stress change at 20 km depth from the Parkfield earthquake (green segment) along the SAF.
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