Across‐Fault Velocity Gradients and Slip Behavior of the San Andreas Fault Near Parkfield

• Published in: Geophysical research letters 2020-01, Vol.47 (1), p.e2019GL084480-n/a
• Main Authors: Piana Agostinetti, N., Giacomuzzi, G., Chiarabba, C.
• Format: Article
• Language: English
• Subjects: acrosso fault velocity gradients; Attitudes; Bayesian analysis; Bridges; Correlation; Creep (materials); Damping; Decay; Earthquake dampers; Earthquake Dynamics; Earthquakes; Elastic properties; Elastic waves; Fault lines; Fault rheology and slip behavior; Fluid pressure; fully non‐linear tomography; Geodesy and Gravity; Geological faults; Kinematics of Crustal and Mantle Deformation; Material properties; P waves; Physics; Probability theory; Properties; Questions; Radio Science; Research Letter; Research Letters; Rheological properties; Rheology; Rheology and Friction of Fault Zones; Role of Fluids; Rupture; Rupturing; Seismic activity; Seismic Cycle Related Deformations; Seismic tomography; Seismic velocities; Seismic wave velocities; Seismology; Slip; Solid Earth; Solifluction; Structural Geology; Tectonophysics; Tomography; Tomography and Imaging; Velocity; Velocity gradient; Velocity gradients; Wave velocity
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL084480

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. Plain Language Summary Slip behavior of faults is a long‐lasting question in earthquake physics. The San Andreas Fault (SAF) near Parkfield is the perfect place for investigating this issue because of the different slip attitude of the SAF and the wide abundance of data. We applied a new fully nonlinear seismic tomography technique to get rid of all the subjective assumptions associated with classical tomography approaches, which strongly influenced past results obtained with such data. We obtained high‐resolution images of the fault, achieving structural details useful to addressing physical processes on the fault. Our main finding is that velocity gradients across the SAF (differences in Vp and Vp/Vs between the two sides of the fault) correlate with the slip attitude. Strong gradients are observed in the fault section where velocity weakening is the principle slip mode. This correlation indicates processes governing the unstable slip of the fault. This finding has great impact on understanding natural processes and earthquake physics and represents one good example of creating bridges over different disciplines and scales (nature to labs). A further significance is that cautious monitoring of elastic properties in time can be useful to predict eventual changes at seismogenic depths and recognize weakening mechanisms. Key Points Velocity gradients across the SAF (differences in