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The seismic potential of active low-angle normal faults (LANFs, <30° dip) remains enigmatic under Andersonian faulting theory, which predicts that normal faults dipping less than 30° should be inactive. The Alto Tiberina fault (ATF) in the northern Apennines, a partly creeping 17°-dipping LANF, has not been associated with any historical earthquakes but could potentially generate earthquakes up to Mw~7. We investigate the mechanical preconditions and dynamic plausibility of large ATF earthquakes using 3D dynamic rupture and seismic wave propagation simulations constrained by multidisciplinary data from the Alto Tiberina Near Fault Observatory (TABOO-NFO). Our models incorporate the complex non-planar ATF fault geometry, including hanging wall secondary faults and a recent geodetic coupling model. We show that potential large earthquakes (up to Mw~7.4) are mechanically viable under Andersonian extensional stress conditions if the ATF is statically relatively weak (μs=0.37). Large earthquakes only nucleate on favorably oriented, steeper fault sections (dip ≥30°), and remain confined to the coupled portion, limiting earthquake magnitude. These ruptures may dynamically trigger an intersecting synthetic branch but are unlikely to affect more distant antithetic faults. Jointly integrating fault geometry and geodetic coupling is crucial for forecasting dynamic rupture nucleation and propagation. 

Abstract Understanding the dynamics of microearthquakes is a timely challenge with the potential to address current paradoxes in earthquake mechanics, and to better understand earthquake ruptures induced by fluid injection. We perform fully 3D dynamic rupture simulations caused by fluid injection on a target fault for Fault Activation and Earthquake Ruptures experiments generatingM_w ≤ 1 earthquakes. We investigate the dynamics of rupture propagation with spatially variable stress drop caused by pore pressure changes and assuming different slip‐weakening constitutive parameters. We show that the spontaneous arrest of propagating ruptures is possible by assuming a high fault strength parameter S, that is, a high ratio between strength excess and dynamic stress drop. In faults with high S values (low rupturing potential), even minor variations inD_c(from 0.45 to 0.6 mm) have a substantial effect on the rupture propagation and the ultimate earthquake size. Modest spatial variations of dynamic stress drop determine the rupture mode, distinguishing self‐arresting from run‐away ruptures. Our results suggest that several characteristics inferred for accelerating dynamic ruptures differ from those observed during rupture deceleration of a self‐arresting earthquake. During deceleration, a decrease of peak slip velocity is associated with a nearly constant cohesive zone size. Moreover, the residual slip velocity value (asymptotic value for a crack‐like rupture) decreases to nearly zero. This means that an initially crack‐like rupture becomes a pulse‐like rupture during spontaneous arrest. These findings highlight the complex dynamics of small induced earthquakes, which differ from solutions obtained from conventional crack‐like models of earthquake rupture. 

Abstract Earthquakes are rupture-like processes that propagate along tectonic faults and cause seismic waves. The propagation speed and final area of the rupture, which determine an earthquake’s potential impact, are directly related to the nature and quantity of the energy dissipation involved in the rupture process. Here, we present the challenges associated with defining and measuring the energy dissipation in laboratory and natural earthquakes across many scales. We discuss the importance and implications of distinguishing between energy dissipation that occurs close to and far behind the rupture tip, and we identify open scientific questions related to a consistent modeling framework for earthquake physics that extends beyond classical Linear Elastic Fracture Mechanics. 

Understanding the dynamics of microearthquakes is a timely challengewith the potential to address current paradoxes in earthquake mechanics,and to better understand earthquake ruptures induced by fluid injection.We perform fully 3D dynamic rupture simulations caused by fluidinjection on a target fault for FEAR experiments generating Mw ≤ 1earthquakes. We investigate the dynamics of rupture propagation withspatially variable stress drop caused by pore pressure changes andassuming different constitutive parameters. We show that the spontaneousarrest of propagating ruptures is possible by assuming a high faultstrength parameter S, that is, a high ratio between strength excess anddynamic stress drop. In faults with high S values (low rupturingpotential), even minor variations in Dc (from 0.45 to 0.6 mm) have asubstantial effect on the rupture propagation and the ultimateearthquake size. Our results show that modest spatial variations ofdynamic stress drop determine the rupture mode, distinguishingself-arresting from run-away ruptures. Our results suggest that severalcharacteristics inferred for accelerating dynamic ruptures differ fromthose observed during rupture deceleration of a self-arrestingearthquake. During deceleration, a decrease of peak slip velocity isassociated with a nearly constant cohesive zone size. Moreover, theresidual slip velocity value (asymptotic value for a crack-like rupture)decreases to nearly zero. This means that an initially crack-likerupture becomes a pulse-like rupture during spontaneous arrest. Insummary, our findings highlight the complex dynamics of smallearthquakes, which are partially contrasting with established crack-likemodels of earthquake rupture. 

ABSTRACT The 2019 Ridgecrest earthquake sequence culminated in the largest seismic event in California since the 1999 Mw 7.1 Hector Mine earthquake. Here, we combine geodetic and seismic data to study the rupture process of both the 4 July Mw 6.4 foreshock and the 6 July Mw 7.1 mainshock. The results show that the Mw 6.4 foreshock rupture started on a northwest-striking right-lateral fault, and then continued on a southwest-striking fault with mainly left-lateral slip. Although most moment release during the Mw 6.4 foreshock was along the southwest-striking fault, slip on the northwest-striking fault seems to have played a more important role in triggering the Mw 7.1 mainshock that happened ∼34  hr later. Rupture of the Mw 7.1 mainshock was characterized by dominantly right-lateral slip on a series of overall northwest-striking fault strands, including the one that had already been activated during the nucleation of the Mw 6.4 foreshock. The maximum slip of the 2019 Ridgecrest earthquake was ∼5  m, located at a depth range of 3–8 km near the Mw 7.1 epicenter, corresponding to a shallow slip deficit of ∼20%–30%. Both the foreshock and mainshock had a relatively low-rupture velocity of ∼2  km/s, which is possibly related to the geometric complexity and immaturity of the eastern California shear zone faults. The 2019 Ridgecrest earthquake produced significant stress perturbations on nearby fault networks, especially along the Garlock fault segment immediately southwest of the 2019 Ridgecrest rupture, in which the coulomb stress increase was up to ∼0.5  MPa. Despite the good coverage of both geodetic and seismic observations, published coseismic slip models of the 2019 Ridgecrest earthquake sequence show large variations, which highlight the uncertainty of routinely performed earthquake rupture inversions and their interpretation for underlying rupture processes.