Rupture propagation on heterogeneous fault: Challenges for predicting earthquake magnitude

Abstract

<p indent="0mm">Predicting magnitudes and ground motion of future strong earthquakes on seismogenic faults holds critical implications on seismic hazard zonation and mitigation. Although present approaches to seismic hazard assessment can effectively identify potential regions to host strong earthquakes, it remains extremely challenging to accurately predict future earthquake magnitudes because of various heterogeneities of fault zones, such as fault geometry, medium structure, and frictional properties. In this work, we review a few aspects that play important roles in earthquake rupture propagation and extent, so as to determine the final magnitude of earthquakes. Under the condition of heterogeneous stress distribution on faults, the final magnitude of earthquakes as well as shallow slip distribution depends on where the rupture initiates. As shown in recent numerical models of dynamic rupture simulations, different hypocenters with similar stress level can lead to completely different rupture scenarios, some of which may significantly break the ground whereas others may be buried underground. Such findings shed critical light on seismic hazard preparation, because surface-breaching ruptures can lead to severe damage as vividly evidenced by the 2022 Menyuan <italic>M</italic>_w 6.7 earthquake. To date, we cannot predict future hypocenter locations according to our knowledge of earthquake nucleation and therefore future relevant investigations are highly demanded. Furthermore, the size of seismogenic zone on strike-slip faults controls whether ruptures may become “break-away” to form large earthquakes or stop spontaneously as “self-arresting” of moderate magnitude events. Observations show that aspect ratios between rupture length and down-dip width of dip-slip earthquakes are usually no more than 8. In contrast, the length/width ratios of strike-slip earthquakes may rise to 40 and have a drastic change around the rupture width of <sc>~10 km.</sc> Intrinsic mechanism of such variation in strike-slip earthquakes is attributed to the energy release rate of rupture fronts, which is controlled by the down-dip width. As such, high-resolution constraints on down-dip seismogenic width can be used to estimate the magnitude of future earthquakes on strike-slip faults. Moreover, fault zone structure significantly affects rupture directivity and slip extents. For instance, low-velocity fault zones may promote rupture propagation and thus enlarge the earthquake magnitude. Given the rapid development of seismic observations and seismic imaging techniques, high-resolution fault zone structure can be obtained from data recorded by dense arrays, including nodal network and the newly developed distributed acoustic sensing (DAS) arrays. Integrating with laboratory results of rheology from samples representing crustal rocks, the near-fault high-resolution seismic structure can be converted into rheological properties on faults, which can then be used to outline asperities for future earthquakes. This may provide a critical step to link seismic structure to earthquake potential, as the outlined asperities can be further used to derive rupture scenarios and earthquake magnitudes. To advance our understanding on this front, future work may integrate multiple sources of observations and modeling. Conducting rupture simulations with constraints from geodetic and seismic measurements, as well as laboratory frictional experiments, can be helpful in exploring potential earthquake magnitudes. By considering heterogeneities on faults with reasonable constraints, the numerical models can be useful for investigating the probable hypocenter locations of large earthquakes. Thus, the numerical results may support future near-field observations to improve monitoring of nucleation and development of future large earthquakes. Furthermore, the ground motions generated from the numerical models can also fill the data gap of near-field observations of large earthquakes and provide critical support for physics-based seismic hazard assessment.