Coseismic steps recorded on creep meters along the San Andreas Fault

Creep meters spanning an active trace of the San Andreas fault in central California recorded many apparent fault-slip steps at times of moderate local earthquakes of magnitudes 4–5. These steps are small, (less than 1 mm) but are recorded over long fault segments (tens of kilometers). The corresponding strain steps, ranging from 10−5 to 10−4, are about 2 orders of magnitude larger than those calculated from the seismically determined source parameters on the assumption of uniform crustal materials. These anomalously large coseismic steps may be due to low effective rigidity of the fault gouge zone or to seismically triggered secondary surface breaks that occurred along the fault.