Revisiting the Fault Locking of the Central Himalayan Thrust with a Viscoelastic Earthquake-Cycle Deformation Model

Based on a viscoelastic earthquake-cycle deformation model, we revisit the fault locking of the central Himalayan thrust using geodetic data acquired in the past three decades. By incorporating the viscoelastic relaxation effect induced by stress buildup and release, our viscoelastic model is capable of explaining the far-field observation with similar fault locking width obtained in previous studies. Elastic models underestimate the far-field deformation and consequently underestimate the fault slip rate by attributing the far-field deformation to stable intraplate deformation. A steady-state viscosity of ∼1019 Pa·s is required for the lower crust beneath south Tibet to best fit the crustal velocity. The optimal slip rate and locking width of the central Main Himalayan Thrust are estimated to 18.8 ± 1.6 mm/a and 85 ± 2.1 km, respectively. The inferred fault locking width, along with the down-dip rupture extension of the 2015 Gorkha earthquake, agrees well with the identified mid-crustal ramp, which leads to an interpretation that the fault geometry of the central Himalayan thrust plays an important role on fault kinematics. Our results highlight that viscoelastic relaxation during the earthquake cycle should be incorporated for robust estimation of fault locking parameters and reasonable data fitting.

Kinematics of Crustal Deformation along the Central Himalaya

Using an updated set of GPS surface velocities, the present study provides fault locking behavior and slip rate distribution of the Main Himalayan Thrust (MHT) along the central Himalaya. The two-dimensional velocity field is inverted through Bayesian inversion to estimate fault geometry and kinematic parameters of the MHT along the central Himalaya. The modeling results reveal that: (1) MHT is fully locked in the upper flat (0-9 km), partially locked along the mid-crustal ramp (15-21 km), and it is creeping in the deeper flat (> 21 km); (2) there is an insignificant slip rate of MHT along the locked-to-creeping transition zone, indicating its partially coupled/locked behavior; (3) along the deeper flat of the MHT, the estimated creeping rate is ~16.3 mm/yr, ~14.7 mm/yr, and ~14.3 mm/yr along western, central, and eastern Nepal, respectively; and (4) along the MHT on the upper crust, the modeled locking width turns out to be 97 km, 106 km, and 129 km in the western, central, and eastern Nepal, respectively. In addition, the posterior probability distribution of the locking width exhibits a bimodal Gaussian distribution coinciding with the two ramp geometry of the MHT along the western Nepal. Along the foothills of the Higher Himalaya, the inferred locking line is also aligned to the estimated maximum shear strain concentration and observed seismicity along the central Himalaya. With a general agreement to the previous geodetic results, geological estimates, and background seismicity, our findings provide a promising avenue of the contemporary crustal deformation along the Nepal Himalaya. The estimated inversion results in a Bayesian framework exhibit updated fault kinematics of the MHT and hence provides valuable inputs for seismic hazard assessment along the central Himalaya.

Cascadia megathrust earthquake rupture model constrained by geodetic fault locking

Paleo-earthquakes along the Cascadia subduction zone inferred from offshore sediments and Japan coastal tsunami deposits approximated to M9+ and ruptured the entire margin. However, due to the lack of modern megathrust earthquake records and general quiescence of subduction fault seismicity, the potential megathrust rupture scenario and influence of downdip limit of the seismogenic zone are still obscure. In this study, we present a numerical simulation of Cascadia subduction zone earthquake sequences in the laboratory-derived rate-and-state friction framework to investigate the potential influence of the geodetic fault locking on the megathrust sequences. We consider the rate-state friction stability parameter constrained by geodetic fault locking models derived from decadal GPS records, tidal gauge and levelling-derived uplift rate data along the Cascadia margin. We incorporate historical coseismic subsidence inferred from coastal marine sediments to validate our coseismic rupture scenarios. Earthquake rupture pattern is strongly controlled by the downdip width of the seismogenic, velocity-weakening zone and by the earthquake nucleation zone size. In our model, along-strike heterogeneous characteristic slip distance is required to generate margin-wide ruptures that result in reasonable agreement between the synthetic and observed coastal subsidence for the AD 1700 Cascadia Mw∼9.0 megathrust rupture. Our results suggest the geodetically inferred fault locking model can provide a useful constraint on earthquake rupture scenarios in subduction zones. This article is part of the theme issue ‘Fracture dynamics of solid materials: from particles to the globe’.

Soil gas CO2 emissions and fault locking along the Anninghe fault and the Zemuhe fault, in China Seismic Experimental Site

<p>The Anninghe fault (ANHF) and the Zemuhe fault (ZMHF) with high level of seismic hazards in the China Seismic Experimental Site, located in southeastern of Tibet, are some of the most active faults in China. Measurement of the soil gas CO<sub>2</sub> has been conducted in three sites along the ANHF and the ZMHF for the first time. Totally, 394 sampling points along 15 profiles were measured. The fault locking degree of different segments of the ANHF and the ZMHF were inverted by the negative dislocation model using GPS velocity data  since 2013 to 2017. The measurements results show that the average and maximum value of CO<sub>2</sub> in the ZMHF is significantly higher than that in the ANHF. Soil gas CO<sub>2</sub> geochemistry yielded different spatial anomalous features, indicating the different properties and permeability of the faults. The inversion results reveal that the level of coupling including the locking depth and intensity along the southern segment of the ANHF was significantly larger than the northern segment of the ZMHF. Combining the CO<sub>2</sub> emission results, we concluded that the intensive locking of the segments reduced their permeability due to the self-sealing process, results in less gas to escape from the deep. Correspondingly, the creeping fault with low level of coupling can maintain high permeability which is more favorable to gas CO<sub>2</sub> migration.</p>