Aurélie Guilhem

Nonvolcanic Tremor Evolution and the San Simeon and Parkfield, California, Earthquakes

Parkfield Tremors Parkfield, California, sits on the San Andreas Fault near the end of a major historic rupture in 1857. Recent monitoring has resolved a change in the seismic tremor—small repeating earthquakes that have been occurring in certain parts of the fault. Nadeau and Guilhem (p. 191) now show that the tremors increased and periodic episodes began around the time of two moderate nearby earthquakes 4 years ago. Surprisingly, the tremor episodes have persisted, rather than decaying after the quakes, which may imply that there has been a step change in the state of stress on this part of the San Andreas Fault. Small repeating earthquakes increased and have become periodic on the San Andreas fault near one end of a major historic rupture. Nonvolcanic tremors occur adjacent to locked faults and may be closely related to the generation of earthquakes. Monitoring of the San Andreas Fault in the Parkfield, California, region revealed that after two strong earthquakes, tremor activity increased in a nearly dormant tremor zone, increased and became periodic in a previously active zone, and has remained elevated and periodic for over 4 years. Static shear- and Coulomb-stress increases of 6 to 14 kilopascals from these two earthquakes are coincident with sudden increases in tremor rates. The persistent changes in tremor suggest that stress is now accumulating more rapidly beneath this part of the San Andreas Fault, which ruptured in the moment magnitude 7.8 Ft. Tejon earthquake of 1857.