Abstract
A long-lasting question in earthquake physics is why slip on faults occurs as creep or dynamic rupture. We compute passive measurements of the seismic P wave velocity gradient across the San Andreas Fault near Parkfield, where this transition of slip mode occurs at a scale of a few kilometers. Unbiased measurements are obtained through the application of a new Bayesian local earthquake tomographic code that avoids the imposition of any user-defined, initial velocity-contrast across the fault, or any damping scheme that may cause biased amplitude in retrieved seismic velocities. We observe that across-fault velocity gradients correlate with the slip behavior of the fault. The P wave velocity contrast decays from 20% in the fault section that experience dynamic rupture to 4% in the creeping section, suggesting that rapid change of material properties and attitude to sustain supra-hydrostatic fluid pressure are conditions for development of dynamic rupture. Low Vp and high Vp/Vs suggest that fault rheology at shallow depth is conversely controlled by low frictional strength material.