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The lives of millions will be changed after it breaks, and yet so few people understand it, or even realize it runs through their backyard. Dvorak reveals the San Andreas Fault's fascinating history and its volatile future.

We provide high‐resolution seismic imaging of the central Garlock fault using data recorded by two dense seismic arrays that cross the Ridgecrest rupture zone (B4) and the Garlock fault (A5). Analyses of fault zone head waves and P‐wave delay times at array A5 show that the Garlock fault is a sharp bimaterial interface with P waves traveling ∼5% faster in the northern crustal block. The across‐fault velocity contrast agrees with regional tomography models and generates clear P‐wave reflections in waveforms recorded by array B4. Kirchhoff migration of the reflected waves indicates a near‐vertical fault between 2 and 6 km depth. The P‐wave delay times imply a ∼300‐m‐wide transition zone near the Garlock fault surface trace beneath array A5, offset to the side with faster velocities. The results provide important constraints for derivations of earthquake properties, simulations of ruptures and ground motion, and future imaging studies associated with the Garlock fault. Plain Language Summary Along the northern edge of the Mojave Desert, the Garlock fault intersects the San Andreas fault and is the second largest (∼300 km long) fault in Southern California. It can host M > 7 earthquakes that pose significant seismic hazard to densely populated communities. However, the subsurface structure of the Garlock fault is not well understood due to the sparse seismic network and lack of seismic activity nearby. The 2019 Ridgecrest earthquake sequence in the Eastern California Shear Zone led to a rapid deployment of several dense linear arrays with ∼100 m spacing and apertures of a few kilometers, which cross the Ridgecrest rupture zone and also the Garlock fault. The recorded seismic data is used here to illuminate the internal structure of the central Garlock fault. Analyses of P‐wave delay times and waves refracted along and reflected by the fault interface indicate a near‐vertical Garlock fault that separates two distinctive crustal blocks with different wave speeds. The resolved high‐resolution fault zone image can have important implications for multiple studies associated with the Garlock fault. Key Points We image the central Garlock fault using data of aftershocks of the 2019 Ridgecrest earthquake recorded by two dense linear arrays A P‐wave velocity contrast across the fault (∼5% faster in the north) generates clear fault zone head and reflected waves Kirchhoff migration of P waves reflected by the fault indicates a near‐vertical interface with a sharp impedance contrast

The historical and instrumental seismicity records along the continental transform faults in the Eastern Mediterranean region represent periodicities within centennial scales. Half a century ago, it was suggested that the spatiotemporal sequences of millennial-scale historical seismicity along the interplate structures represent seismicity switching between the North Anatolian Fault Zone and the East Anatolian Fault Zone. However, many individual historical seismicity catalogs published in the last decade indicate a comparable number of earthquakes in the North Anatolian and Dead Sea fault zones, but not in the East Anatolian Fault Zone. Seismicity records of the instrumental period indicate that the North Anatolian Fault Zone is currently at its peak level of activity. On the other hand, it is well known from the historical records that the seismically quiescent instrumental period of the Dead Sea Fault Zone is not representative of its potential. The comparison of various individual historical earthquake catalogs implies a successive activity for the North Anatolian and Dead Sea fault zones, with a discernible time delay. This observation lends supports to elastic coupling between these continental transform faults, resulting from the direct interaction between the movements of the Arabian and Anatolian plates. The evaluation of both historical and instrumental periods together reveals a possibility to speculate that the Dead Sea Fault Zone could enter a more active phase in the near future, potentially exhibiting a time lag in relation to the activity observed in the North Anatolian Fault Zone.

The Sichuan-Yunnan area is located at the southeastern margin of the Tibetan Plateau, where tectonic movement is strong with deep and large faults distributed in a staggered manner, which results in strong seismic activities and severe earthquake hazards. Since the 21st century, several earthquakes of magnitude 7.0 or above occurred in this region, which have caused huge casualties and economic losses, especially the 2008 M s 8.0 Wenchuan earthquake. At present, earthquake monitoring and source parameter inversion, strong earthquake hazard analysis and disaster assessment are still the focus of seismological researches in the Sichuan-Yunnan region. Regional high-precision 3D community velocity models are fundamental for these studies. In this paper, by assembling seismic observations at permanent seismic stations and several temporary dense seismic arrays in this region, we obtained about 7.06 million body wave travel time data (including absolute and differential travel times) using a newly developed artificial intelligence body wave arrival time picking method and about 100,000 Rayleigh wave phase velocity dispersion data in the period range of 5–50 s from ambient noise cross-correlation technique. Based on this abundant dataset, we obtained the three-dimensional high resolution V p and V s model in the crust and uppermost mantle of southwest (SW) China by adopting the joint body and surface wave travel time tomography method considering the topography effect starting from the first version of community velocity model in SW China (SWChinaCVM-1.0). Compared to SWChinaCVM-1.0, this newly determined velocity model has higher resolution and better data fitness. It is accepted by the China Seismic Experimental Site as the second version of the community velocity model in SW China (SWChinaCVM-2.0). The new model shows strong lateral heterogeneities in the shallow crust. Two disconnected low velocity zones are observed in the middle to lower crust, which is located in the Songpan-Ganzi block and the northern Chuandian block to the west of the Longmenshan-Lijiang-Xiaojinhe fault, and beneath the Xiaojiang fault zone, respectively. The inner zone of the Emeishan large igneous province (ELIP) exhibits a high velocity anomaly, which separates the two aforementioned low velocity anomalies. Low velocity anomaly is also shown beneath the Tengchong volcano. The velocity structures in the vicinity of the 2008 M s 8.0 Wenchuan earthquake, the 2013 M s 7.0 Lushan earthquake and the 2017 M s 7.0 Jiuzhaigou earthquake mainly show high V p and V s anomalies and the mainshocks are basically located at the transition zone between the high and low velocity anomalies. Along with the segmentation characteristics of seismic activity, we suggest that areas with significant changes in velocity structures, especially in active fault zones, might have a greater potential to generate moderate to strong earthquakes.

The NE- to NNE-striking Tan-Lu Fault Zone (TLFZ) is the largest fault zone in East China, and a typical representative for the circum-Pacific tectonics. Its late Mesozoic evolution resulted from subduction of the Paleo-Pacific Plate, and can be used for indication to the subduction history. The TLFZ reactivated at the end of Middle Jurassic since its origination in Middle Triassic. This phase of sinistral motion can only be recognized along the eastern edge of the Dabie-Sulu orogenis, and indicates initiation of the Paleo-Pacific (Izanagi) Plate subduction beneath the East China continent. After the Late Jurassic standstill, the fault zone experienced intense sinistral faulting again at the beginning of Early Cretaceous under N-S compression that resulted from the NNW-ward, low-angle, high-speed subduction of the Izanagi Plate. It turned into normal faulting in the rest of Early Cretaceous, which was simultaneous with the peak destruction of the North China Craton caused by backarc extension that resulted from rollback of the subducting Izanagi Plate. The TLFZ was subjected to sinistral, transpressive displacement again at the end of Early Cretaceous. This shortening event led to termination of the North China Craton destruction. The fault zone suffered local normal faulting in Late Cretaceous due to the far-field, weak backarc extension. The late Mesozoic evolution of the TLFZ show repeated alternation between the transpressive strike-slip motion and normal faulting. Each of the sinistral faulting event took place in a relatively short period whereas every normal faulting event lasted in a longer period, which are related to the subduction way and history of the Paleo-Pacific Plates.

The Xianshuihe fault zone (XSHF) in southwestern China accommodates both aseismic creep and seismic slip, yet geological evidence constraining these processes remains limited. Hence, we conducted microstructural, geochemical, and rock magnetic analyses of fault‐zone materials from the Cuoniulongba River outcrop. Fault breccias and relatively undeformed sandstones adjacent to the principal slip zone (PSZ) exhibit elevated magnetic susceptibility (MS), likely reflecting monoclinic pyrrhotite precipitated from post‐seismic low‐temperature hydrothermal fluids. Creep‐related fault gouge within the PSZ is characterized by low MS. Notably, the gouge sample S4 within the PSZ shows high MS, reflecting magnetite neoformation induced by coseismic frictional heating (>500°C), consistent with the observation of thermal decomposed kaolinite due to fluid drainage. This study presents the geological evidence associated with the XSHF linked to aseismic creep, coseismic frictional heating, and post‐seismic hydrothermal alteration, and provides new insights into the seismic behavior of continental strike‐slip faults.

Primarily due to the scarce direct field evidence along a same fault, understanding the relationship between the pre‐existing off‐fault damage and coseismic slip distribution remains challenging. This study offers the first field‐based quantitative assessments of fault damage zones along the surface rupture generated by the 2008 Mw 7.9 Wenchuan earthquake in China over eight profiles with 254 outcrops. The width and mean tectonic fracture intensity of the damage zones exhibit pronounced variations along the fault strike. We introduce the damage index as a proxy to quantify the extent of pre‐existing damage. We find a statistically significant anti‐correlation between the damage index and coseismic on‐fault slip. We thus infer that pre‐existing off‐fault damage plays a significant role in dissipating rupture energy, thereby reducing coseismic on‐fault slip. This study provides a natural case for linking the long‐term fault zone evolution and the short‐term earthquake rupture dynamics. Plain Language Summary Large earthquakes can cause significant surface ruptures, which often spatially coincide with severe seismic disasters. Therefore, understanding where large coseismic slip occurs is crucial for mitigating hazards. Off‐fault damage is an indispensable component of fault zone structure, and its impact on seismic behavior has been widely emphasized. However, unlike coseismic off‐fault damage, pre‐existing off‐fault damage cannot be easily observed through remote sensing. Investigating pre‐existing off‐fault damage and its influence on coseismic on‐fault slip is challenging yet essential for comprehending earthquake processes. To address this gap, we conducted a detailed field investigation along the surface rupture of the 2008 Mw 7.9 Wenchuan earthquake, China, characterizing the along‐strike distribution of fault damage zone. Comparing the extent of pre‐existing fault damage with the coseismic on‐fault slip profiles revealed an inverse relationship: a higher extent of pre‐existing off‐fault damage leads to smaller coseismic on‐fault slip. We propose that the dissipation of seismic energy occurring within densely fractured damage zones provides a possible explanation for this phenomenon. Our findings establish critical connections between long‐term fault zone evolution and short‐term earthquake rupture dynamics. From a disaster reduction perspective, identifying off‐fault damage along major active faults before future earthquakes may enhance fault displacement hazard assessments. Key Points Detailed measurements of the pre‐existing off‐fault damage along the surface ruptures associated with the 2008 Mw 7.9 Wenchuan Earthquake We provide compelling evidence that a higher extent of pre‐existing off‐fault damage leads to smaller coseismic on‐fault slip We discuss the seismic energy dissipation hypothesis for the mechanism of pre‐existing off‐fault damage impeding coseismic on‐fault slip

Fault zones are often surrounded by a damage zone that exhibits lower seismic velocities than the wall rock, influencing earthquake propagation and arrest. We present a one‐dimensional minimal model of frictional rupture that approximates the elastodynamics of a fault embedded within a damage zone. This model predicts two families of steady‐state rupture solutions: an overdamped regime, describing a crack‐like rupture, and an underdamped regime with oscillating slip‐rate in the wake of the rupture, which promotes pulse‐like dynamics. The minimal model contains two free parameters: the pre‐stress on the fault, and the seismic velocity reduction in the damage zone. We present how the one‐dimensional prediction is consistent with previously published two‐dimensional simulations and discuss the applicability of our results to natural observations, identifying the preferred rupture style as function of structure of the fault zone, and the geological consequences of oscillatory slip in the wake of pulse‐like ruptures.