Change of apparent segmentation of the San Andreas fault around Parkfield from space geodetic observations across multiple periods

Abstract

Sequences of earthquakes are commonly represented as a succession of periods of interseismic stress accumulation followed by coseismic and postseismic phases of stress release. Because the recurrence time of large earthquakes is often greater than the available span of space geodetic data, it has been challenging to monitor the evolution of interseismic loading in its entire duration. Here we analyze large data sets of surface deformation at different key episodes around the Cholame, Parkfield and creeping segments of the San Andreas Fault that show evidence of significant deceleration of fault slip during the interseismic period. We compare the average fault slip rates before and after the 2004 Mw6 Parkfield earthquake, in the 1986–2004 and 2006–2012 periods, respectively, avoiding 2 years of postseismic deformation after 2004. Using a combination of GPS data from the Plate Boundary Observatory, the Southern California Earthquake Center Crustal Motion Map and the Bay Area Velocity Unification networks and interferometric synthetic aperture radar from the Advanced Land Observing Satellite (ALOS) and Envisat satellites, we show that the area of coupling at the transition between the Parkfield and Cholame segments appears larger later in the interseismic period than it does earlier on. While strong plate coupling is uniform across the Parkfield and Cholame segments in the 1986–2004 period, creep occurs south of the 2004 epicenter after 2006, making segmentation of the San Andreas Fault south of Parkfield more clearly apparent. These observations indicate that analyses of surface deformation late in the earthquake cycle may overestimate the area of plate coupling. A fault surface creeping much below plate rate may in some case be a region that does not promote earthquake nucleation but rather just be at a slower stage of its evolution. Our analysis also shows signs of large variation of slip velocity above and below plate rate in the creeping segment indicating that cycles of weakening and hardening can also be at play in dominantly aseismic areas.

Off-fault long-term damage: A condition to account for generic, triangular earthquake slip profiles

“…Paradoxically, the theory of elasticity lays the foundation for nearly every aspect of our understanding of earthquake source mechanics [e.g., Aki and Richards , ; Ma and Andrews , ]. Our results, along with others before [ Barbot et al ., ; Cochran et al ., ; Kaneko and Fialko , ], therefore motivate a reinterpretation and/or reformulation of the available earthquake source inversion models since most of them do not include inelastic deformation occurring in the volume around the earthquake ruptures (inversion models including damage have only been developed recently to model interseismic slip [see Barbot et al ., ]).…”

Aseismic slip and seismogenic coupling in the Marmara Sea: What can we learn from onland geodesy?

“…Strain modeling along the eastern part of the NAF indicates nonnegligible variations of locking depth and slip rate [e.g., Tatar et al, 2012;Aktug et al, 2015;Hussain et al, 2016]. Such spatial variability of interseismic coupling is visible on other major continental strike-slip faults (e.g., the San Andreas fault) [Jolivet et al, 2015], and locking is often correlated with the location of earthquake ruptures [Maurer and Johnson, 2014;Barbot et al, 2013]. In the Marmara Sea, the first months of seafloor acoustic measurements suggest that the KS is coupled and accumulating stress [Sakic et al, 2016].…”

Gamra: Simple meshing for complex earthquakes

“…This is important for evaluating stress and other dynamic variables. All of these features make the proposed approach optimal for generating stress and displacements kernels for inversions for fault slip [5], investigation of the surrounding elastic structure [8,14], and building stress and displacement kernels for simulations using the boundary-integral method.…”

“…The axis of the cylinder is angled 18 degrees from the x axis. The model covers (−5, 1, 0) to (5,11,10). The offset is to avoid the singularity at the origin.…”