A document by Xiaoyu Zou. Click on the document to view its contents.

Numerical simulations of Sequences of Earthquakes and Aseismic Slip (SEAS) have rapidly progressed to address fundamental problems in fault mechanics and provide self-consistent, physics-based frameworks to interpret and predict geophysical observations across spatial and temporal scales. To advance SEAS simulations with rigor and reproducibility, we pursue community efforts to verify numerical codes in an expanding suite of benchmarks. Here we present code comparison results from a new set of benchmark problems BP6-QD-A/S/C that consider a single aseismic slip transient induced by changes in pore fluid pressure consistent with fluid injection and diffusion in fault models with different treatments of fault friction. Ten modeling groups participated in problems BP6-QD-A and BP6-QD-S considering rate-and-state fault models using the aging and slip law formulations for frictional state evolution, respectively, allowing us to explore these ingredients across multiple codes and better understand how various computational factors affect the simulated evolution of pore pressure and aseismic slip. Comparisons of problems using the aging versus slip law illustrate how models of aseismic slip can differ in the timing and amount of slip achieved with different treatments of fault friction given the same perturbations in pore fluid pressure. We achieve excellent quantitative agreement across participating codes, with further agreement being found by ensuring sufficiently fine time-stepping and consistent treatment of remote boundary conditions. Our benchmark efforts offer a community-based example to reveal sensitivities of numerical modeling results, which is essential for advancing multi-physics SEAS models to better understand and construct reliable predictive models of fault dynamics.

The Superstition Hills Fault (SHF) exhibits a rich spectrum of slip modes, including M 6+ earthquakes, afterslip, quasi-steady creep, and both triggered and spontaneous slow slip events (SSEs). Following 13 years of quiescence, creepmeters recorded 25 mm of slip during 16-19 May 2023. Additional sub-events brought the total slip to 41 mm. The event nucleated on the northern SHF in early-May and propagated bi-laterally at rates on the order of kilometers per day. Surface offsets reveal a bi-modal slip distribution, with slip on the northern section of the fault being less localized and lower amplitude compared to the southern section. Kinematic slip models confirm systematic variations in the slip distribution along-strike and with depth and suggest that slip is largely confined to the shallow sedimentary layer. Observations and models of the 2023 SSE bear a strong similarity to previous slip episodes in 1999, 2006, and 2010, suggesting a characteristic behavior.

Orientations of active antithetic faults can provide useful constraints on in situ strength of the seismogenic crust. We use LINSCAN, a new unsupervised learning algorithm for identifying quasi-linear clusters of earthquakes, to map small-scale strike-slip faults in the Anza-Borrego shear zone, Southern California. We identify 332 right- and left-lateral faults having lengths between 0.1-3 km. The dihedral angles between all possible pairs of conjugate faults are nearly normally distributed around 70 degrees, with a standard deviation of 30 degrees. The observed dihedral angles are larger than those expected assuming optimal fault orientations and the coefficient of friction of 0.6-0.8, but similar to the distribution previously reported for the Ridgecrest area in the Eastern California Shear Zone. We show that the observed fault orientations can be explained by fault rotation away from the principal shortening axis due to a cumulated tectonic strain. 

The 2021 M7.4 Maduo (China) earthquake ruptured a 170 km-long left-lateral fault within the Bayan Har tectonic block in the northeast Tibetan Plateau. We use Sentinel-1 and ALOS-2 Interferometric Synthetic Aperture Radar, and Global Navigation Satellite System data to investigate the mechanisms of coseismic and postseismic deformation due to the Maduo earthquake. We present a refined coseismic slip model that features variations in both strike and dip angles, constrained by the rupture trace and precisely located aftershocks. The postseismic displacements are discontinuous along the fault trace, indicating shallow afterslip and velocity-strengthening friction in the top 2-3 km of the upper crust. Postseismic displacements that have the same sense as the coseismic ones are also observed at larger (> 50 km) distances away from the fault trace. The observed surface deformation is qualitatively consistent with either deep afterslip or viscoelastic relaxation, but does not exhibit obvious features that could be attributed to poroelastic effects. We developed a fully coupled model that accounts for both stress-driven creep on a deep localized shear zone and viscoelastic relaxation in the bulk of the lower crust. The mid- to near-field data can be reasonably well explained by either deep afterslip or non-Maxwellian visco-elasticity. However, a good fit to both the near and far-field (> 150 km) GNSS data cannot be achieved assuming the bulk viscoelastic relaxation alone, and requires a contribution of deep afterlip and/or a localized shear zone extending through much of the lower crust.

Key Points: • We use Sentinel-1 Synthetic Aperture Radar (SAR) data to derive a finite fault model for the 2021 M7.4 Maduo (Qinghai, China) earthquake • The along-strike averaged coseismic slip has a maximum at depth of 3-4 km, with an amplitude of ∼2.5 m. • Up to ∼0.1 m of afterslip occurred on the fault trace in the first month following the earthquake. Abstract The 2021 Maduo earthquake ruptured a 150 km-long left-lateral fault in the northeast Tibet. We used Synthetic Aperture Radar data collected by the Sentinel-1A/B satellites within days of the earthquake to derive a finite fault model and investigate the details of slip distribution with depth. We generated coseismic interferograms and pixel offsets from different look directions corresponding to the ascending and descending satellite orbits. At the eastern end the rupture bifurcated into two sub-parallel strands, with larger slip on the northern strand. Inversions of coseismic displacements show maximum slip to the east of the epicenter. The averaged coseismic slip has a peak at depth of 3-4 km, similar to slip distributions of a number of shallow strike-slip earthquakes. Postseismic observations over several weeks following the Maduo earthquake reveal surface slip with amplitude up to 0.1 m that at least partially eliminated the coseismic slip deficit in the uppermost crust. Plain language summary A large earthquake occurred in a remote area of northeast Tibet (Qinghai Province, China) on May 21, 2021. The earthquake produced a 150 km-long rupture with surface offsets up to several meters. We used data collected by orbiting satellites to map motions of the Earth’s surface that occurred during and shortly after the earthquake. The measured surface displacements were used to constrain the rupture geometry and slip distribution at depth. Best-fitting models suggest that rupture occurred on a sub-vertical fault steeply dipping to the north, with most of slip occurring to the east of the earthquake epicenter. The maximum coseismic slip occurred in the uppermost crust, in the depth interval of 3-4 km below the Earth’s surface. A decrease in the fault offsets toward the Earth’s surface is likely caused by an increased frictional resistance of the shallow layer to rapid coseismic slip. Satellite observations made in the first month after the earthquake reveal that the shallow part of the fault is slowly catching up with a deeper part to make up for the difference in the amount of slip produced during the earthquake.

Key Points: • We present geodetic observations of coseismic and postseismic deformation due to the 2015 M w 7.2 Sarez (Pamir) earthquake. • Near-field postseismic deformation is dominated by shallow afterslip and possibly poroelastic relaxation at the NE end of the earthquake rupture. • Data do not show a clear signal expected of viscoelastic relaxation, indicating effective viscosity of the lower crust > 10 19 Pa s. • We investigate triggering relationships between the M7.2 earthquake and a pair of M6+ events that occurred within 1 year and 100 km of the mainshock.

Strength of the upper brittle part of the Earth’s lithosphere controls deformation styles in tectonically active regions, surface topography, seismicity, and the occurrence of plate tectonics, yet it remains one of the most debated quantities in geophysics. Direct measurements of stresses acting at seismogenic depths are largely lacking. Seismic data (in particular, earthquake focal mechanisms) have been used to infer orientation of the principal stress axes. I show that the focal mechanism data can be combined with information from precise earthquake locations to place constraints not only on the orientation, but also on the magnitude of absolute stress at depth. The proposed method uses relative attitudes of conjugate faults to evaluate the amplitude and spatial heterogeneity of the deviatoric stress and frictional strength in the seismogenic zone. Relative fault orientations (dihedral angles) and sense of slip are determined using quasi-planar clusters of seismicity and their composite focal mechanisms. The observed distribution of dihedral angles between active conjugate faults in the area of Ridgecrest (California, USA) that hosted a recent sequence of strong earthquakes suggests in situ coefficient of friction of 0.4-0.6, and depth-averaged shear stress on the order of 25-40 MPa, intermediate between predictions of the “strong” and “weak” fault theories.

The Mw 7.2 Sarez strike-slip earthquake occurred on Dec 7, 2015 in the Pamir region at the North-West of the Tibetan Plateau. We used Sentinel-1A/B Interferometric Synthetic Aperture Radar (InSAR) data to investigate postseismic deformation due to the Mw 7.2 Sarez earthquake. We analyzed 2.5 years of post-earthquake acquisitions from the ascending track 100 and the descending track 005. Despite challenging surface conditions (that include rugged topography and snow cover), we were able to derive time series of line of sight (LOS) displacements within 150 km from the earthquake rupture. We used stacks of interferograms covering snow-free months, as well as persistent scatterers to detect possible deformation transients. Preliminary results indicate no postseismic relaxation signals above the noise level (10-20 mm/yr for average LOS velocities). We do observe coseismic deformation due to two M>6 earthquakes that occurred ~100 km to the North-East from the epicenter of the Sarez earthquake. We use constraints on the maximum amplitude of surface displacements provided by the InSAR data to provide bounds on the effective rheologic structure of the lower crust and upper mantle beneath Pamir.