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U.S. Geological Survey, Menlo Park, California 94025
Abstract
Sandblows formed in man-made lake deposits near Watsonville, California, resulted from the October 1989 M 7.1 Loma Prieta earthquake and its April 1990 M 5.5 aftershock and a March 1991 M 4.6 aftershock. Excavations in the dry bed of Soda Lake expose the subsurface structure of sandblows induced by the October and April events. We also made surface observations three days after the March 1991 event. Lateral-spread fissures developed in the lake sediment during the mainshock, and the host deposits were vertically offset 3 to 6.5 cm. Liquefied sand erupted through the fissures onto the dry lake bed and formed sandblow cone deposits. These deposits range from low-angle, conical structures 30 to 50-cm thick and 2 to 5 m in diameter to elongate deposits up to 35-m long. Sandblows associated with the April 1990 aftershock commonly reoccupied preexisting fissures but also formed new solitary vents. Sandblows developed during the March 1991 aftershock erupted only through preexisting vents. Sandblow vents reactivated in April 1990 contain sediment clasts of sandblow deposits formed in the October 1989 mainshock. Clasts of host sediment were also present in the sand dikes.
Lamination in the sandblow cones and sand dikes shows that they were not formed in a single episode. Surface cones and feeder dikes, composed of multiple laminated subunits, represent the three generations of sandblows. The subunits suggest that the cone deposits were formed by pulses of water and sediment expelled from the vent. The pulses result from cyclic phases of local excess pore pressure and its release in the liquefied bed at depth.
The three generations of structures at Soda Lake, formed six months to a year apart, have distinct tonal and textural differences that may mimic liquefaction structures formed decades or centuries apart. This study provides modern analogs for comparison to and interpretation of prehistoric liquefaction structures that are frequently partly eroded or poorly exposed.
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