The role of pore fluids in supershear earthquake ruptures

The intensity and damage potential of earthquakes are linked to the speed at which rupture propagates along sliding crustal faults. Most earthquakes are sub-Rayleigh, with ruptures that are slower than the surface Rayleigh waves. In supershear earthquakes, ruptures are faster than the shear waves, leading to sharp pressure concentrations and larger intensities compared with the more common sub-Rayleigh ones. Despite significant theoretical and experimental advances over the past two decades, the geological and geomechanical controls on rupture speed transitions remain poorly understood. Here we propose that pore fluids play an important role in explaining earthquake rupture speed: the pore pressure may increase sharply at the compressional front during rupture propagation, promoting shear failure ahead of the rupture front and accelerating its propagation into the supershear range. We characterize the transition from sub-Rayleigh to supershear rupture in fluid-saturated rock, and show that the proposed poroelastic weakening mechanism may be a controlling factor for intersonic earthquake ruptures.

Crustal Electrical Structure of the Ganzi Fault on the Eastern Tibetan Plateau: Implications for the Role of Fluids in Earthquakes

The initiation and evolution of seismic activity in intraplate regions are controlled by heterogeneous stress and highly fractured rocks within the rock mass triggered by fluid migration. In this study, we imaged the electrical structure of the crust beneath the Ganzi fault using a three-dimensional magnetotelluric inversion technique, which is host to an assemblage of resistive and conductive features extending into the lower crust. It presents a near-vertical low-resistance zone that cuts through the brittle ductile transition zone, extends to the lower crust, and acts as a pathway for fluid migration from the crustal flow to the upper crustal depths. Conductors in the upper and lower crust are associated with saline fluids and 7% to 16% partial melting, respectively. The relationship between the earthquake epicenter and the surrounding electrical structure suggests that the intraplate seismicity is triggered by overpressure fluids, which are dependent on fluid volume changes generated by the decompression dehydration of partially molten material during upwelling and native fluid within the crustal flow.

Dilation of subglacial sediment governs incipient surge motion in glaciers with deformable beds

Glacier surges are quasi-periodic episodes of rapid ice flow that arise from increases in slip rate at the ice–bed interface. The mechanisms that trigger and sustain surges are not well understood. Here, we develop a new model of incipient surge motion for glaciers underlain by sediments to explore how surges may arise from slip instabilities within a thin layer of saturated, deforming subglacial till. Our model represents the evolution of internal friction, porosity and pore water pressure within the till as functions of the rate and history of shear deformation, and couples the till mechanics to a simple ice-flow model. Changes in pore water pressure govern incipient surge motion, with less permeable till facilitating surging because dilation-driven reductions in pore water pressure slow the rate at which till tends towards a new steady state, thereby allowing time for the glacier to thin dynamically. The reduction of overburden (and thus effective) pressure at the bed caused by dynamic thinning of the glacier sustains surge acceleration in our model. The need for changes in both the hydromechanical properties of the till and the thickness of the glacier creates restrictive conditions for surge motion that are consistent with the rarity of surge-type glaciers and their geographical clustering.

Causal mechanism of injection-induced earthquakes through the Mw 5.5 Pohang earthquake case study

Causal mechanisms for fluid injection-induced earthquakes remain a challenge to identify. Past studies largely established spatiotemporal correlations. Here, we propose a multi-process causal mechanism for injection-induced earthquakes through a case study of the 2017 M_w 5.5 induced earthquake near Pohang Enhanced Geothermal System, Korea, where detailed hydraulic stimulation and on-site seismicity monitoring data provide an unprecedented opportunity. Pore pressure modeling reveals that pore pressure changes initiate seismicity on critically stressed faults and Coulomb static stress transfer modeling reveals that earthquake interactions promote continued seismicity, leading to larger events. On the basis of these results, we propose the following causal mechanism for induced seismicity: pore pressure increase and earthquake interactions lead to fault weakening and ultimately triggering larger earthquakes later in the process. We suggest that it is prudent that pore pressure change, initial seismicity locations, and Coulomb static stress transfer from seismicity earlier in the sequence are assessed in real-time.

The authors here suggest a causal mechanism for injection-induced earthquakes. They further suggest pore pressure modeling as a practical alternative to direct in-situ pore pressure observation which can then be used for stress build-up monitoring.

Land Surface Temperature Variation Following the 2017 Mw 7.3 Iran Earthquake

During an earthquake, crustal deformation, fluid flow, and temperature variation are coupled; however, earthquake-related land surface temperature (LST) variations remain unclear. To determine whether post-seismic fluid migration can cause changes in LST, and taking the Mw 7.3 2017 Iran earthquake as an example, we modeled surface cooling (CA) and warming (WA) areas induced by co-seismic slip and fluid migration using a thermo-hydro-mechanical (THM) coupled numerical simulation. Moreover, using nighttime LST data with 15-min resolution, the daily attenuation coefficient k of nighttime LST was extracted by attenuation function fitting, and the trend of the k time series was analyzed using the Mann–Kendall and Sen’s methods. Based on the comparison of k trends between the post-seismic and 2010–2016 periods, we obtained cooling and warming trends for the modeled CA and WA. The numerical simulation and observational data show good consistency, and both indicate that fluid migration caused by crustal deformation can lead to changes in LST. The numerical simulations show that after the Iran earthquake, the surface projection area of co-seismic slip correlated with a cooling area (CA), while the surrounding area correlated with a warming area (WA). For the LST observational data, the post-seismic k trends of the calculated CA and WA are positive and negative, indicating sustained cooling and warming processes, respectively. This study provides evidence that LST variation is caused by co-seismic crustal deformation and fluid migration and reveals the coupled evolution of deformation, fluid, and temperature fields. The results provide new insights into the mechanisms of seismic thermal anomalies.

Localization and instability in sheared granular materials: Role of friction and vibration

Shear banding and stick-slip instabilities have been long observed in sheared granular materials. Yet, their microscopic underpinnings, interdependencies, and variability under different loading conditions have not been fully explored. Here we use a nonequilibrium thermodynamics model, the Shear Transformation Zone theory, to investigate the dynamics of strain localization and its connection to stability of sliding in sheared, dry, granular materials. We consider frictional and frictionless grains as well as the presence and absence of acoustic vibrations. Our results suggest that at low and intermediate strain rates, persistent shear bands develop only in the absence of vibrations. Vibrations tend to fluidize the granular network and delocalize slip at these rates. Stick-slip is observed only for frictional grains, and it is confined to the shear band. At high strain rates, stick-slip disappears and the different systems exhibit similar stress-slip response. Changing the vibration intensity, duration or time of application alters the system response and may cause long-lasting rheological changes. We analyze these observations in terms of possible transitions between rate strengthening and rate weakening response facilitated by a competition between shear-induced dilation and vibration-induced compaction. We discuss the implications of our results on dynamic triggering, quiescence, and strength evolution in gouge-filled fault zones.