San Francisco Bay Region Geology and Geologic Hazards

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Fremont - Trench on the Hayward Fault

One of the techniques geologists use to get a better view of a fault is to dig a trench across its surface trace or traces. If the trench is excavated in an area where many layers of Quaternary sediment have accumulated, geologists can get a good idea of how the fault has behaved in Quaternary time. At Tule Pond (also called Tyson's Lagoon), just south of the BART station in Fremont, geologists have found just such a location. Trench studies there have revealed evidence for as many as ten earthquakes on the southern Hayward Fault in the past 1900 years.

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USGS geologists supervise trench digging across the Hayward Fault in Fremont.


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This diagram, overlain on a mosaic of photos taken in a trench across the Hayward Fault, shows the type of detailed geology revealed in trench studies. The yellow triangles show where carbon was collected for radiometric dating. Colored areas highlight various layers of Holocene sediment. Geologists have found evidence for 5 earthquakes in this trench.

 

Learn more about the Tule Pond trench in Fremont.

 

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