Validation of Fault Displacements from Dynamic Rupture Simulations against the Observations from the 1992 Landers Earthquake

ABSTRACT Coseismic fault displacements in large earthquakes have caused significant damage to structures and lifelines on and near fault lines. Coseismic displacements represent a real threat, especially to distributed infrastructure systems. For infrastructure systems that can not avoid active faults, engineering displacement demands are defined using probabilistic fault-displacement hazard analyses (PFDHA). However, PFDHA models are sparse and poorly constrained partly due to the scarcity of detailed fault-displacement observations. Advancements in dynamic rupture simulation methods make them an attractive approach to address this important issue. Because fault displacements can be simulated for various geologic conditions as constrained by current knowledge about earthquake processes, they can be used to supplement the observation datasets. In addition to providing on-fault displacements, when used with appropriate constitutive models for the bulk medium, they can capture off-fault distributed inelastic deformations as well. For viable extrapolation, simulations must first be validated against data. In this article, we summarize the calibration and validation of the dynamic rupture model against the observations of the well-documented 1992 Landers earthquake. We defined a preferred model that reproduces several first-order fault-displacement metrics such as the on-fault partition of the total displacement, the mean fault-zone width, and the location of the peak displacement. Simulated ground motions consistent with the observations ensure that all physics important to modeling have been properly parameterized. For the extrapolation, we generated a suite of dynamic rupture models to quantify expected fault-displacement metrics, their intercorrelations, and magnitude dependencies, which are in part supported by the Landers and other recent earthquakes. Our validation and extrapolation exercise paves the way for using dynamic rupture modeling to quantitatively address fault-displacement hazard on a broader scale. The results are promising and are expected to be useful to inform PFDHA model development.

The 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquake sequence in Eastern California: rupture on a conjugate fault structure revealed by GPS and InSAR measurements

SUMMARY On 4 and 6 July 2019, an Mw 6.4 foreshock and an Mw 7.1 main shock successively struck the city of Ridgecrest in eastern California. These two events are the most significant earthquake sequences to strike in this part of California for the past 20 yr. We used both continuous global positioning system (GPS) measurements and interferometric synthetic aperture radar (InSAR) images taken by the Sentinel-1 and ALOS-2 satellites in four different viewing geometries to fully map the coseismic surface displacements associated with these two earthquakes. Using these GPS and InSAR measurements both separately and jointly, we inverted data to find the coseismic slip distributions and fault dips caused by the two earthquakes. The GPS-constrained slip models indicate that the Mw 7.1 main shock was predominately controlled by right-lateral motions on a series of northwest-trending faults, while the Mw 6.4 foreshock involved both right-lateral slipping on a northwest-trending fault and left-lateral slipping on a northeast-trending fault. The two earthquakes both generate significant surface slip, with the maximum inferred slip of 5.54 m at the surface. We estimate the cumulative geodetic moment of the two earthquakes to have been 4.93 × 1019 Nm, equivalent to Mw 7.1. Furthermore, our calculations of the changes in static Coulomb stress suggest that the Mw 7.1 main shock was promoted significantly by the Mw 6.4 foreshock. This latest Ridgecrest earthquake sequence ruptured only the northern part of the seismic gap between the 1992 Mw 7.3 Landers earthquake and the 1872 M 7.4–7.9 Owens Valley earthquake. The earthquake risk in this area, therefore, remains very high, considering the significant accumulation of strain in the Eastern California Shear Zone, especially in the southern part of the seismic gap.

Cascading elastic perturbation in Japan due to the 2012 Mw 8.6 Indian Ocean earthquake

Since the discovery of extensive earthquake triggering occurring in response to the 1992 Mw (moment magnitude) 7.3 Landers earthquake, it is now well established that seismic waves from earthquakes can trigger other earthquakes, tremor, slow slip, and pore pressure changes. Our contention is that earthquake triggering is one manifestation of a more widespread elastic disturbance that reveals information about Earth’s stress state. Earth’s stress state is central to our understanding of both natural and anthropogenic-induced crustal processes. We show that seismic waves from distant earthquakes may perturb stresses and frictional properties on faults and elastic moduli of the crust in cascading fashion. Transient dynamic stresses place crustal material into a metastable state during which the material recovers through a process termed slow dynamics. This observation of widespread, dynamically induced elastic perturbation, including systematic migration of offshore seismicity, strain transients, and velocity transients, presents a new characterization of Earth’s elastic system that will advance our understanding of plate tectonics, seismicity, and seismic hazards.