Paleoseismology

Key to assessing the likelihood of such an earthquake is knowing how often they have occurred in the past, and when and how large the last one was. These questions underly the science of paleoseismology. Paleoseismologists, geologists who trench across faults, document evidence of paleoearthquakes, prehistoric earthquakes large enough to rupture the fault at the ground surface. By exhuming the top few meters of an active fault, paleoseismologists may find disturbed ancient soil layers or other traces left by past earthquakes. Exploratory trenches provide a valuable means to precisely locate and establish the recency of movement of particular fault traces. When dates of the past earthquakes can be estimated through the use of radiocarbon and other dating techniques, they provide a basis for estimating the probability of the next earthquake.

We are currently exhuming several faults in the San Francisco Bay region in search of their ancient earthquake (paleoseismic) histories. The process involves digging a trench across (or along side) the fault in order to map, date and interpret disturbances in the soil layers (geologic strata) seen in the facing walls of the trench.