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Recent observations show that certain rupture phase can propagate backward relative to the earlier one during a single earthquake event. Such back‐propagating rupture (BPR) was not well considered by the conventional earthquake source studies and remains a mystery to the seismological community. Here we present a comprehensive analysis of BPR, by combining theoretical considerations, numerical simulations, and observational evidences. First, we argue that BPR in terms of back‐propagating stress wave is an intrinsic feature during dynamic ruptures; however, its signature can be easily masked by the destructive interference behind the primary rupture front. Then, we propose an idea that perturbation to an otherwise smooth rupture process may make some phases of BPR observable. We test and verify this idea by numerically simulating rupture propagation under a variety of perturbations, including a sudden change of stress, bulk or interfacial property and fault geometry along rupture propagation path. We further cross‐validate the numerical results by available observations from laboratory and natural earthquakes, and confirm that rupture "reflection" at free surface, rupture coalescence and breakage of prominent asperity are very efficient for exciting observable BPR. Based on the simulated and observed results, we classify BPR into two general types: interface wave and high‐order re‐rupture, depending on the stress recovery and drop before and after the arrival of BPR, respectively. Our work clarifies the nature and excitation of BPR, and can help improve the understanding of earthquake physics, the inference of fault property distribution and evolution, and the assessment of earthquake hazard. Plain Language Summary: Under the traditional view on earthquakes, rupture is typically considered to propagate away from where it starts, a process called forward propagation. However, recent studies show that sometimes rupture can reverse its propagation direction relative to the earlier one, which is referred to as back‐propagating rupture (BPR). The lack of a comprehensive understanding of BPR motivates us to explore why and how BPR could occur, by combining theoretical analyses, computer simulations, and experimental or natural observations. Our theoretical analyses suggest that BPR in terms of back‐propagating stress wave should almost always exist during dynamic ruptures, but can also be masked by the canceling effect of interfering waves. Nonetheless, introducing perturbations during a relatively smooth rupture process may highlight certain phases of BPR. We verify the above idea by computer simulations and available observations, showing that perturbed rupture propagation, for example, by the encountering of free surface, another rupture, and fault asperity, indeed can excite observable BPR. We further classify BPR into interface wave and high‐order re‐rupture, when there is a negligible and finite offset to the baseline of shear stress, respectively. Our work provides new insights into earthquake source process and can help improve earthquake hazard assessment. Key Points: Back‐propagating rupture (BPR) (as stress wave) is an intrinsic feature during dynamic ruptures but can be masked by destructive interferenceObservable BPR can be excited by introducing a variety of perturbations to an otherwise smooth rupture processBPR represents interface wave or high‐order re‐rupture, and can help infer the evolution behind the primary rupture [ABSTRACT FROM AUTHOR]

Understanding the in situ stress state is crucial in many engineering problems and earth science research. The present article presents new insights into the interaction mechanism between the stress state and faults. In situ stresses can be influenced by various factors, one of the most important being the existence of faults. A fault could significantly affect the value and direction of the stress components. Reorientation and magnitude changes in stresses exist adjacent to faults and stress jumps/discontinuities across the fault. By contrast, the change in the stress state may lead to the transformation of faulting type and potential fault reactivation. Qualitative fault reactivation assessment using characteristic parameters under the current stress environment provides a method to assess the slip tendency of faults. The correlation between in situ stresses and fault properties enhances the ability to predict the fault slip tendency via stress measurements, which can be used to further refine the assessment of the fault reactivation risk. In the future, stress measurements at greater depths and long-term continuous real-time stress monitoring near/on key parts of faults will be essential. In addition, much attention needs to be paid to distinguishing the genetic mechanisms of abnormal stress states and the type and scale of stress variations and exploring the mechanisms of pre-faulting anomaly and fault reactivation. [ABSTRACT FROM AUTHOR]

The Finite-Fault Rupture Detector (FinDer) algorithm computes rapid line-source rupture models from high-frequency seismic acceleration amplitudes (PGA). In this paper, we propose two extensions to FinDer, called FinDerS and FinDerS+, which have the advantage of taking into account a geological property of the source fault, its structural maturity, as well as its relation to the earthquake slip distribution. These two new algorithms calculate real-time earthquake slip profiles by backprojecting seismic and/or geodetic displacement amplitudes onto the FinDer line-source. This backprojection is based on a general empirical equation established in previous work that relates dynamic peak ground displacement (PGD) at the stations to on-fault coseismic slip. While FinDerS projects PGD onto the current FinDer line-source, FinDerS+ allows the rupture to grow beyond the current model extent to predict future rupture evolution. For an informed interpolation and smoothing of the estimated slip values, FinDerS and FinDerS+ both employ a generic empirical function that has been shown to relate the along-strike gradient of structural maturity of the ruptured fault, the earthquake slip distribution, and the rupture length. Therefore, while FinDer derives magnitudes from a relatively uncertain and general empirical rupture length-magnitude relations, FinDerS and FinDerS+ provide alternate and better informed magnitude estimates using the mean slip of the profiles derived from the integration of fault source maturity. The two new algorithms can incorporate both seismic strong-motion and geodetic displacement data. In order to recover PGD from strong-motion instruments, we double-integrate and high-pass filter (> 0.075 Hz) the seismic acceleration records. Together, the three algorithms exploit the full spectrum of ground-motions, including high frequencies to derive a source fault model (FinDer) and low frequencies to determine the static offsets along this model (FinDerS and FinDerS+). We test the three algorithms for the 2019 MW 7.1 Ridgecrest (California), 2016 MW 7.0 Kumamoto (Japan), and 2008 MW 7.9 Wenchuan (China) earthquakes. Conclusively, low-frequency PGD data and integration of the fault maturity gradient do not speed-up calculations for these events, but provide additional information on slip distribution and final rupture length, as well as alternative estimates of magnitudes that can be useful to check for consistency across the algorithm suite. The FinDer algorithms systematically outperform previously established real-time PGD-based magnitude estimates in terms of speed and accuracy. The resulting slip distributions can be useful for improved ground-motion prediction given the observed relationship between seismic radiation and fault maturity.

The source characteristics of slow and fast earthquakes provide a window into the mechanical properties of faults. In particular, the average stress drop controls the evolution of friction, fault slip, and event magnitude. However, this important source property is typically inferred from the analysis of seismic waves and is subject to many epistemic uncertainties. Here, we investigate the source properties of 53 earthquakes and 17 slow‐slip events on thrust and strike‐slip faults in various tectonic settings using slip distributions constrained by geodesy in combination with other data. We determine the width, potency, and potency density of slow and fast earthquake sources based on static slip distributions. The potency density, defined conceptually as the ratio of average slip to rupture radius, is a measure of anelastic deformation with limited bias from rigidity differences across depths and tectonic settings. Strike‐slip earthquakes have the highest potency density, varying from 20 to 500 microstrain. The potency density is on average lower on continental thrust faults and megathrusts, from 10 to 200 microstrain, with an algebraic decrease with centroid depth, indicative of systematic changes in dominant rupture processes with depth. Slow slip events represent an end‐member style of rupture with low potency density and large rupture width. Significant variability in potency density of slow‐slip events affects their moment‐duration scaling. The variations of source properties across tectonic settings, depth, and rupture styles can be used to better constrain numerical simulations of seismicity and to assess the source characteristics of future earthquakes and slow slip events. Plain Language Summary: Natural earthquakes reduce the stress that accumulates on faults due to plate tectonics. To better understand the variability of seismic hazards around active faults, we survey the properties of slow and fast earthquakes around the world. The potential of faults to concentrate large slip in the rupture area differs depending on the geological setting, the depth of the source, and the type of rupture. Earthquakes in a continental setting condense more slip in a given rupture area, particularly in transform faults like the San Andreas fault. Subduction zone earthquakes, although some of the largest events on Earth, generally distribute less slip over a wider area, but this varies as a function of depth. Slow earthquakes represent an extreme case of little slip distributed over a large area. The propensity of rupture characteristics to vary with fault type and depth may help forecast the hazards posed by future seismicity. Key Points: We compile finite slip distributions for slow and fast earthquakes to quantify static source propertiesThe potency density varies systematically with rupture style, tectonic setting, and centroid depthThe moment‐duration scaling of slow‐slip events is affected by large variability in potency density [ABSTRACT FROM AUTHOR]

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The timing of petroleum charge, especially in structurally complex basins, is often poorly defined, with few data to constrain petroleum systems models. In this study we reconstruct in detail the evolution of the >6 Tcf Maui Field, a mature gas-condensate field in the southern Taranaki Basin, northwestern New Zealand. Structural restoration suggests that closure developed between 6 and 5.3 Ma. Maturation modelling shows that charge of the main gas reservoir is only feasible from the nearby Maui sub-basin kitchen, across a major fault zone, the Cape Egmont Fault Zone (CEFZ). We apply petroleum systems modelling to predict timing and amount of charge using a reconstructed distribution of shaly fault rock as an approximation of fault permeability. Models reproducing the variable oil and gas accumulations in different areas of the field suggest that the field was charged within 4 Myr. This supports the notion that connected permeability exists even across large fault zones, allowing charge in geologically short times frames. Models using a range of fault and carrier bed properties suggest that presence of connected sandy carrier beds linking the fault zone with reservoirs is essential for petroleum migration. Predicted migration pathways occur through outer parts of the CEFZ characterized by greater architectural complexity as well as through the central area of the CEFZ with the largest offset. This suggests that the impact of shale-rich fault rock on petroleum migration across large fault zones is smaller than commonly expected. In particular, in deeper parts of the basin, a major fault zone can provide sufficient permeability to be the preferred pathway for vertical migration. These results illustrate that despite the complexity of migration pathways, petroleum migration can occur in geologically short time. • The giant Maui gas-condensate field in New Zealand was charged within less than 4 Myrs. • Charge occurred across a major fault zone. • A variable shale gouge ratio model best reproduces petroleum migration. • Major fault zones can provide migration pathways in deep strata while sealing in shallow strata. [ABSTRACT FROM AUTHOR]