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M. M. Clark

Publications and source records attributed to M. M. Clark.

8 recordsLinked to original sources
USGS1993

Sand boils without earthquakes

Sedimentary deformation caused by liquefaction has become a popular means for inferring prehistoric strong earthquakes. In this report, we describe a new mechanism for generating such features in the absence of earthquakes. Sand boils and a 180-m-long sand dike formed in Fremont Valley, California, when sediment-laden surface runoff was intercepted along the upslope part of a 500-m-long preexisting ground crack, flowed subhorizonally in the crack, and then flowed upward in the downslope part of the crack where it discharged as sand boils on the land surface. If the sand boils and their feeder dike were stratigraphically preserved, they could be misinterpreted as evidence for earthquake-induced liquefaction.

Geology↗
USGS1984

The 1984 Morgan Hill, California, earthquake

The Morgan Hill, California, earthquake (magnitude 6.1) of 24 April 1984 ruptured a 30-kilometer-long segment of the Calaveras fault zone to the east of San Jose. Although it was recognized in 1980 that an earthquake of magnitude 6 occurred on this segment in 1911 and that a repeat of this event might reasonably be expected, no short-term precursors were noted and so the time of the 1984 earthquake was not predicted. Unilateral rupture propagation toward the south-southeast and an energetic late source of seismic radiation located near the southeast end of the rupture zone contributed to the highly focused pattern of strong motion, including an exceptionally large horizontal acceleration of 1.29g at a site on a dam abutment near the southeast end of the rupture zone.

California↗
USGS1982

Magmatic resurgence in Long Valley caldera, California: Possible cause of the 1980 Mammoth Lakes earthquakes

Changes in elevation between 1975 and October 1980 along a leveling line across the Long Valley caldera indicate a broad (half-width, 15 kilometers) uplift (maximum, 0.25 meter) centered on the old resurgent dome. This uplift is consistent with reinflation of a magma reservoir at a depth of about 10 kilometers. Stresses generated by this magmatic resurgence may have caused the sequence of four magnitude 6 earthquakes near Mammoth Lakes in May 1980.

California↗