ScienceDaily (Feb. 2, 2009) — New hazard maps for communities from San Jose to Palo Alto in Northern California delineate the probability of earthquake-induced liquefaction, based on three scenarios: a magnitude 7.8 on the San Andreas Fault comparable to the 1906 event, a magnitude 6.7 on the Hayward Fault comparable to the 1868 event, and a magnitude 6.9 on the Calavaras Calaveras Fault.

The probability of liquefaction is highest at approximately 33 to 37 percent in some areas along major creeks for the San Andreas Fault scenario when the water table is relatively shallow, according to the report published by the February issue of the Bulletin of the Seismological Society of America (BSSA).

Until now, scientists have offered only qualitative maps of the liquefaction hazard in the Santa Clara Valley. Liquefaction is a physical process that takes place during some earthquakes, causing loose soil to act like water rather than a solid. The liquefied sand or soil may flow and the ground may move and crack, causing damage to surface structures and underground utilities, as it did in the valley during both the 1868 and 1906 earthquakes.

These probabilistic maps detail the degree of hazard within broader hazard zones and thereby provide a perspective on the actual risk to the user.

Story Source:

Adapted from materials provided by Seismological Society of America, via EurekAlert!, a service of AAAS.

Journal Reference:

1. Holzer et al. Scenario Liquefaction Hazard Maps of Santa Clara Valley, Northern California. Bulletin of the Seismological Society of America, 2009; 99 (1): 367 DOI: 10.1785/0120080227

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This map shows the likelihood of liquefaction in Northern Santa Clara County during a magnitude 7.8 earthquake on the northernmost segments of the San Andreas Fault. This earthquake is similar to the 1906 San Francisco earthquake. At each location, the map predicts the approximate probability that shallow wet sands will liquefy and cause surface manifestations of liquefaction such as sand boils and ground cracking. Liquefaction is a phenomenon that is caused by earthquake shaking. Wet sand can become liquid-like when strongly shaken. The liquefied sand may flow and the ground may move and crack, causing damage to surface structures and underground utilities.The map depicts the hazard at a regional scale and should not be used for site-specific design and consideration. Subsurface conditions can vary abruptly and borings are required to address the hazard at a given location. The map assumes the historically shallowest water table conditions and does not reflect current ground-water conditions. If the current water table is deeper, the probability of liquefaction is reduced. The map includes the communities of San Jose, Campbell, Cupertino, Los Altos, Los Gatos, Milpitas, Mountain View, Palo Alto, Santa Clara, Saratoga, and Sunnyvale. (Credit: Image courtesy of Seismological Society of America)

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