Plain Language Summary
Earthquake stress drop (the change in shear stress before and after the
earthquake) reflects the properties of the fault where earthquakes
occur. Determining the scaling relationships between earthquake stress
drop and magnitude enables us to predict the behavior of infrequent
large earthquakes from the measurements of the more abundant small
earthquakes. Measurement of stress drop is challenging, especially for
small earthquakes. Different studies can obtain different stress drop
values for the same earthquakes, leading to different interpretations.
We use recordings from a high-resolution borehole network in Parkfield,
California to measure source parameters for small earthquakes. We
examine the influence of data limitation (e.g., frequency bandwidth) and
corrections for material properties on stress drop resolution. We find
that if the bandwidth is too narrow, the resulting source parameters can
be systematically underestimated with large scatter. Insufficient
corrections of material properties can lead to biased interpretation of
stress drop patterns and scaling relationships. Our final results show
that the stress drops of earthquakes in Parkfield do not depend on
magnitude or depth, but exhibit strong spatial variations that are
stable with time. The 2004 M6 earthquake caused temporal variations of
stress drop, and the temporal changes are different at different fault
patches.