Nonvolcanic Tremor Evolution and the San Simeon and Parkfield, California, Earthquakes
Robert M. Nadeau and
Aurélie Guilhem
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.
