Depth Constraints on Coseismic Velocity Changes From Frequency-Dependent Measurements of Repeating Earthquake Waveforms

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.