Surface deformation associated with fractures near the 2019 Ridgecrest earthquake sequence

Abstract

Contemporary earthquake hazard models hinge on an understanding of how strain is distributed in the crust and the ability to precisely detect millimeter-scale deformation over broad regions of active faulting. Satellite radar observations revealed hundreds of previously unmapped linear strain concentrations (or fractures) surrounding the 2019 Ridgecrest earthquake sequence. We documented and analyzed displacements and widths of 169 of these fractures. Although most fractures are displaced in the direction of the prevailing tectonic stress (prograde), a large number of them are displaced in the opposite (retrograde) direction. We developed a model to explain the existence and behavior of these displacements. A major implication is that much of the prograde tectonic strain is accommodated by frictional slip on many preexisting faults.

Small Fractures Caused by the 2019 Ridgecrest Earthquake Sequence: Insights From 3D Coseismic Displacement and Uniaxial Loading Rock Experiments

“…High-resolution optical remote-sensing images are also used to study the coseismic displacement and strain along the surface ruptures, showing detailed strain tensors along ruptures of the foreshock and mainshock (e.g., Barnhart et al, 2020;Milliner and Donnellan, 2020). In addition to the right-lateral slip along the NW-SE Mw 7.1 rupture and the left-lateral slip along the NE-SW Mw 6.4 rupture, the sequence activated a series of structures with lengths ranging from 1 to 10 km, which are well revealed by phase gradient maps of SAR interferograms (Xu et al, 2020a;Xu et al, 2020b). Small fractures were also identified in the 1992 Landers and 1999 Hector Mine earthquakes (Sandwell et al, 2000;Fialko et al, 2002;Fialko et al, 2004), both occurred in the ECSZ.…”

“…However, we are not able to reveal small fractures from these interferograms because the deformation across those small fractures is too small to generate a significant phase pattern. The phase gradient is sensitive to detect discontinuities by differential phase of adjacent positions in the north-south direction and the east-west direction (Supplementary Figure S3) (Xu et al, 2020a;Xu et al, 2020b). We identify 461 fractures (Figure 2) from phase gradient maps derived from Sentinel-1 ascending and descending interferograms acquired on the same dates, facilitated by coseismic interferograms and pixel offsets.…”

“…We analyze the east-west deformation across small fractures using ascending and descending interferograms acquired on exactly the same dates (July 4-16, 2019). By using such interferograms, we can largely ensure the measured LOS deformations are projected from the same surface deformation for ascending and descending data, preventing the systematic errors that may be introduced by stacking interferograms acquired on different dates for deformation decomposition (e.g., Xu et al, 2020b). The coseismic deformation is a result of the main rupture superimposed over the deformation produced by small fractures, while we are interested in the local deformation across those small fractures at the surface.…”

“…We prepare two marble samples, one with a prefabricated fault and the other with two prefabricated orthogonal faults for uniaxial loading experiments (Supplementary Figure S4). The parameters of prefabricated faults are scaled to 1:1,000,000 based on previous studies (Liu et al, 2019;Xu et al, 2020b;Wang and Zhan, 2020). The first marble sample is with only one prefabricated fault that is similar to the main rupture of the 2019 Ridgecrest sequence.…”

“…Small fractures were also identified in the 1992 Landers and 1999 Hector Mine earthquakes (Sandwell et al, 2000;Fialko et al, 2002;Fialko et al, 2004), both occurred in the ECSZ. Xu et al (2020b) propose models to explain the mechanisms of shear deformation across these fractures. Specifically, the triggered frictional slip model is applied to explain the deformation prograde to the direction of tectonic stress.…”