Trenching Evidence of Active Faulting
Exploratory trenches provide a valuable means to precisely locate and establish the recency of movement of particular fault traces. Locations of trenches are shown on the maps as thin blue lines and each is labeled using codes (T) and abbreviated descriptions of the evidence found in the trench. ABBREVIATED REFERENCES are used on the map. Full references are in the PAMPHLET.
Most trenches across the Hayward fault were excavated to meet the requirements of the Alquist-Priolo Earthquake Fault Zoning Act, to insure that structures are not built over active fault traces. Other trenches have been used by scientists to learn about the fault's capability as a source of earthquakes. Earthquake geologists probe in to the fault's past behavior using the tools of paleoseismology.
PALEOSEISMOLOGY: Paleoseismologists, geologists who trench across faults, document evidence of paleoearthquakes, prehistoric earthquakes large enough to rupture the fault at the ground surface. Using radiocarbon analysis to date these past earthquakes, scientists have shown that these large earthquakes occur roughly every 100 to 200 years on the Hayward fault.
Recurrence interval, time between large earthquakes (click image to download 0.3 Mb PDF )
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