← Back to Sessions

Dynamics of Early Afterslip-Aftershock Coevolution Following the 2004 Parkfield Earthquake

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip I

Type: Oral

Date: 4/22/2021

Presentation Time: 10:00 AM Pacific

Description: 

Large earthquakes often lead to transient deformation and enhanced seismic activity, with their fastest evolution occurring at the early, ephemeral postseismic period. Our knowledge of this transitional phase remains limited, due to the scarcity of on-scale broadband observations, high instrumental noise and confounding signals of source processes during this phase. Here we use high-rate geodetic and seismological observations to study the dynamics of early afterslip and aftershock evolution following the 2004 M_w 6.0 Parkfield, California, earthquake. We image the continuous evolution of afterslip, along with aftershocks, on the San Andreas fault over a postseismic timespan from minutes to days. Our results reveal a multistage scenario, which includes immediate onset of afterslip following tens-of-seconds-long coseismic shaking, short-lived slip reversals within minutes, expanding afterslip within hours, and slip migration between sub-parallel fault strands within days. At shallower depths, the amplitude of evolving afterslip largely follows a logarithmic function of time, consistent with the prediction of velocity-strengthening rate-and-state friction using near-constant effective parameter, (a-b), over extended postseismic periods, whereas deeper afterslip has an apparent time delay, suggesting the dependence of effective (a-b) values on time and probably slip rates. The earliest afterslip episodes and associated stress changes appear synchronized with local aftershock rates, with aseismic moment rates larger than seismic moment rates by three orders of magnitude. In addition, growing afterslip precedes two M5 aftershocks to the northwest of the peak-slip region, further suggesting a control of aseismic slip on fine-scale aftershock behavior. We interpret the shallow afterslip-aftershock processes as dynamic signatures of a 3D fault-zone structure. These findings highlight important roles of aseismic source processes and structural factors in controlling seismicity evolution and offer potential prospects for improving aftershock forecasts.

Presenting Author: Junle Jiang

Student Presenter: No

Authors