Kaihua Ding

Supershear rupture of the 5 January 2013 Craig, Alaska (Mw 7.5) earthquake

Supershear rupture, in which a fractures crack tip expansion velocity exceeds the elastic shear wave velocity, has been extensively investigated theoretically and experimentally and previously inferred from seismic wave observations for six continental strike-slip earthquakes. We find extensive evidence of supershear rupture expansion of an oceanic interplate earthquake, the 5 January 2013 Mw = 7.5 Craig, Alaska earthquake. This asymmetric bilateral strike-slip rupture occurred on the Queen Charlotte Fault, offshore of southeastern Alaska. Observations of first-arriving Sn and Sg shear waves originating from positions on the fault closer than the hypocenter for several regional seismic stations, with path calibrations provided by an empirical Greens function approach, indicate a supershear rupture process. Several waveform inversion and modeling techniques were further applied to determine the rupture velocity and space-time distribution of slip using regional seismic and geodetic observations. Both theoretical and empirical Greens functions were used in the analyses, with all results being consistent with a rupture velocity of 5.5 to 6 km/s, exceeding the crustal and upper mantle S wave velocity and approaching the crustal P wave velocity. Supershear rupture occurred along ~100 km of the northern portion of the rupture zone but not along the shorter southern rupture extension. The direction in which supershear rupture developed may be related to the strong material contrast across the continental-oceanic plate boundary, as predicted theoretically and experimentally. The shear and surface wave Mach waves involve strongly enhanced ground motions at azimuths oblique to the rupture direction, emphasizing the enhanced hazard posed by supershear rupture of large strike-slip earthquakes.

Evaluating seasonal loading models and their impact on global and regional reference frame alignment

Seasonal variations are observed in GPS time series, but are not included in the International Terrestrial Reference Frame (ITRF) models. Unmodeled seasonal variations at sites used for reference frame alignment are aliased into the reference frame parameters and bias all coordinates in the transformed solution. We augment ITRF2008 with seasonal loading models based either on Gravity Recovery and Climate Experiment (GRACE) measurements or a suite of models for atmospheric pressure, continental hydrology, and nontidal ocean loading. We model the seasonal components using either annual and semiannual terms or a nonparametric approach. When we include a seasonal variation model, the weighted root-mean-square misfit after seven-parameter transformation decreases for 70–90% of the daily GPS solutions depending on the network and seasonal model used, relative to a baseline case using ITRF2008. When seasonal variations are included in the reference frame solution, the observed seasonal variations are more consistent with the GRACE-based model at 80–85% of the GPS sites that were not used in the frame alignment. The suite of forward models performs nearly as well as the GRACE-based model for North America, but substantially worse for other parts of the world. We interpret these findings to mean that the use of ITRF2008 without seasonal terms causes the amplitude of seasonal variations in the coordinate time series to be damped down relative to the true loading deformation and that the observed GPS time series are more consistent with a TRF model that includes seasonal variations. At present, a seasonal model derived from GRACE captures seasonal variations more faithfully than one based on hydrologic models.

Source model of the 2015 Mw 6.4 Pishan earthquake constrained by interferometric synthetic aperture radar and GPS: Insight into blind rupture in the western Kunlun Shan

The Pishan, Xinjiang, earthquake on 3 July 2015 is the one of largest events (Mw 6–7) that has occurred along the western Kunlun Shan, northwestern edge of the Tibetan Plateau in recent time. It involved blind thrusting at a shallow depth beneath the range front, providing a rare chance to gain insights into the interaction between the Tarim Basin and the Tibetan Plateau. Here we present coseismic ground displacements acquired by high-resolution ALOS-2 SAR imagery and derived from GPS resurveys on several near-field geodetic markers after the event. We observed a maximum displacement exceeding 10 cm in the epicentral region. Analysis of the data based on a finite fault model indicates that coseismic slip occurred on a subsurface plane of 22 km × 8 km in size with a dip of about 27° to the north and a strike of 114°, representing partial break of one ramp fault buried in Paleozoic strata at 8–16 km depths beneath the foothill of the western Kunlun Shan. This blind rupture is characterized largely by a compact thrusting patch with a peak slip of 0.63 m, resulting in a stress drop of 2.3 MPa. The source model yields a geodetic moment of 5.05 × 1018 N · m, corresponding to Mw 6.4. The Pishan earthquake suggests a northward migration of deformation front of the Tibetan Plateau onto the Tarim Basin. Our finding highlights slip along ramp-decollement faults to build up the western Kunlun Shan as the Tarim slab is subducting beneath western Tibet.

The 2016 Mw 6.7 Aketao earthquake in Muji range, northern Pamir: Rupture on a strike-slip fault constrained by Sentinel-1 radar interferometry and GPS

Abstract On 25 November 2016, the Aketao, Xinjiang earthquake occurred on the Muji fault, which is located at the northernmost end of the right-lateral Karakorum Fault (KF). This event provides a rare chance to gain insights into how the stress accumulates in Pamir margin as the Indian plate collides with the Eurasian plate. Space geodetic measurements including InSAR and GPS were used to obtain coseismic surface displacements associated with this earthquake. Based on a finite fault model, the coseismic slip distribution inverted by the combined datasets indicates that the 2016 Aketao event is caused by a primary shallow strike-slip with minor normal-slip at a steep-dipping angle. To explore the real structure of Muji fault, listric fault model inferred by relocated aftershocks as well as the planar fault model, were used in our slip distribution inversion. The results suggest that the optimal fault model should be a highly-dipping planar fault with two separated asperities. The large slip zone is beneath the surface near the epicenter with a maximum slip of 1.1 m, while the small one in the east breaks the surface, in a good agreement with the field seismic geological survey. The total geodetic moment is 1.35 × 10 19 N∙m , equivalent to Mw 6.7. The nearly pure dextral strike-slip Aketao earthquake, and the recent 2015 Mw 7.2 sinistral strike-slip Tajikstan earthquake in this region, to some extent, manifest the extension motion is dominated in northern Pamir Plateau, in response to the northward convergence between Indian and Eurasian collision.