The distribution of the Late Mesozoic volcanic arc in the South China Sea (SCS) continental margin has long been a controversial topic due to its significance in understanding the transition mechanism of a margin from subduction to extension. Here a comprehensive analysis was conducted in the margin using reprocessed magnetic data, newly collected drilling/dredging samples, depositional environment, and deformation style inferred from multichannel seismic profiles to jointly constrain the possible distribution of the Mesozoic volcanic arc. From the map of reduced‐to‐the‐pole magnetic anomaly, several high‐positive magnetic anomaly belts can be discriminated, which cross the central Pearl River Mouth Basin, extend southwestward to the Zhongsha (Macclesfield Bank) and Xisha Islands (Paracel Islands), and distribute discontinuously around the southwest subbasin. Geophysical analysis shows that these belts have similar amplitudes and magnetic depths to known volcanic arcs, such as the Luzon, Cagayan, and Sulu arcs. Furthermore, the high‐amplitude positive magnetic anomaly belts coincide with the distribution of Late Mesozoic arc‐like granites, intermediate rocks, and agglomerates, suggesting that the belts possibly originated from the existence of Late Mesozoic volcanic arc. Accretion and compression environment located in front of the inferred arc provides independent supports to our interpretation. Results indicate that the southwest part of the arc is distributed on both sides of the southwest SCS subbasin, whereas the northeast part remains nearly in its original location, further suggesting that the breakup locations for the SCS margin might be the volcanic front/forearc in the northeast and the arc in the southwest during the opening of the SCS basin.

We have numerically simulated dynamic ruptures along a “slip‐weakening” megathrust fault with a subducted seamount of realistic geometry, demonstrating that seamounts can act as a barrier to earthquake ruptures. Such barrier effect is calculated to be stronger for increased seamount normal stress relative to the ambient level, for larger seamount height‐to‐width ratio, and for shorter seamount‐to‐nucleation distance. As the seamount height increases from 0 to 40% of its basal width, the required increase in the effective normal stress on the seamount to stop ruptures drops by as much as ~20%. We further demonstrate that when a seamount is subducted adjacent to the earthquake nucleation zone, coseismic ruptures can be stopped even if the seamount has a lower effective normal stress than the ambient level. These results indicate that subducted seamounts may stop earthquake ruptures for a wide range of seamount normal stress conditions, including the case of the thrust fault being lubricated by seamount‐top fluid‐rich sediments, as suggested from observations in the Japan and Sunda Trenches.

Crustal faults usually have a fault core and surrounding regions of brittle damage, forming a low-velocity zone (LVZ) in the immediate vicinity of the main slip interface. The LVZ may amplify ground motion, influence rupture propagation, and hold important information of earthquake physics. A number of geophysical and geodetic methods have been developed to derive high-resolution structure of the LVZ. Here, I review a few recent approaches, including ambient noise cross-correlation on dense across-fault arrays and GPS recordings of fault-zone trapped waves. Despite the past efforts, many questions concerning the LVZ structure remain unclear, such as the depth extent of the LVZ. High-quality data from larger and denser arrays and new seismic imaging technique using larger portion of recorded waveforms, which are currently under active development, may be able to better resolve the LVZ structure. In addition, effects of the along-strike segmentation and gradational velocity changes across the boundaries between the LVZ and the host rock on rupture propagation should be investigated by conducting comprehensive numerical experiments. Furthermore, high-quality active sources such as recently developed large-volume air-gun arrays provide a powerful tool to continuously monitor temporal changes of fault-zone properties, and thus can advance our understanding of fault zone evolution.

We derive high‐resolution information on low‐velocity fault zone (FZ) structures along the San Jacinto Fault Zone (SJFZ), Southern California, using waveforms of local earthquakes that are recorded at multiple linear cross‐fault arrays. We observe clear across‐fault delays of direct P and S waves, indicating damage zones at different segments of the SJFZ. We then compute synthetic traveltimes and waveforms using generalized ray theory and perform forward modeling to constrain the FZ parameters. At the southern section near the trifurcation area, the low‐velocity zone (LVZ) of the Clark branch has a width of ~200 m, 30–45% reduction in Vp, and ~50% reduction in Vs. From array data across the Anza seismic gap, we find a LVZ with ~200 m width and ~50% reduction in both Vp and Vs, nearly as prominent as that on the southern section. We only find prominent LVZs beneath three out of the five arrays, indicating along‐strike variations of the fault damage. FZ‐reflected phases are considerably less clear than those observed above the rupture zone of the 1992 Landers earthquake shortly after the event. This may reflect partially healed LVZs with less sharp boundaries at the SJFZ, given the relatively long lapse time from the last large surface‐rupturing event. Alternatively, the lack of observed FZ‐reflected phases could be partially due to the relatively small aperture of the arrays. Nevertheless, the clear signatures of damage zones at Anza and other locations indicate very slow healing process, at least in the top few kilometers of the crust.