Abstract

Earthquake stress drop is an important source parameter that directly links to strong ground motion and fundamental questions in earthquake physics. Stress drop estimations may contain significant uncertainties due to factors such as variations in material properties and data limitations, which limits the applications of stress drop interpretations. Using a high-resolution borehole network, we analyze 4537 earthquakes in the Parkfield area in Northern California between 2001 and 2016 with spectral decomposition and an improved stacking method. To evaluate the influence of spatiotemporal variations of material properties on stress drop estimations, we apply six different strategies to account for spatial variations of velocity and attenuation changes, and divide earthquakes into three separate time periods to correct temporal variations of attenuation. These results show that appropriate corrections can significantly reduce the scatter in stress drop estimations, and decrease apparent depth and magnitude dependence. We further investigate the influence of data limitations on stress drop estimations, and show that insufficient bandwidth may cause systematic underestimation and increased stress drop scatter. The stress drop measurements from the high-frequency borehole recordings exhibit complex stable spatial patterns with no clear correlation with the nature of fault slip, or the slip distribution of the 2004 M6 earthquake. In some regions with the largest numbers of earthquakes, we can resolve temporal variations that indicate stress drop decrease following the 2004 earthquake, and gradual recovery. These temporal variations do not affect the long-term stress drop spatial variations, suggesting local material properties may control the spatial heterogeneity of stress drop.