We revisit velocity changes caused by the 2004 Parkfield earthquake through the analysis of the coda of repeating earthquakes. Our results reveal the change to be strongly frequency dependent. Low-frequency components of the coda of repeating earthquakes are more affected by the velocity alteration than high-frequency components. We interpret this to indicate that the coseismic velocity reduction primarily occurs at a shallow depth, and is expressed in low-frequency energy resulting from reverberations near the Earth's surface. This can only be observed at low frequencies because the shallow crust is highly dissipative. The high-frequency component is relatively unaffected, as it is comprised of body waves scattered throughout the crust. We support this argument with direct observations of seismic wavefields across a wide range of depths in the shallow crust, supplemented by using downhole geophones and distributed acoustic sensing measurements.
All Science Journal Classification (ASJC) codes
- !!Geochemistry and Petrology
- !!Earth and Planetary Sciences (miscellaneous)
- !!Space and Planetary Science