Many ruptures across many scales: The period of seismic quiescence in Southern California was rudely interrupted by the Ridgecrest earthquake sequence in July 2019. Ross et al. mapped the slip sequences during the magnitude 6.4 and 7.1 earthquakes that shook the region. They found that ruptures of a few larger, but many smaller, faults occurred during both earthquakes. The Ridgecrest sequence calls for rethinking seismic hazard, as multifault ruptures are not usually considered when assessing seismic risk. Science , this issue p. 346

Exploring the subsurface structure and stratification of Mars advances our understanding of Martian geology, hydrological evolution and palaeoclimatic changes, and has been a main task for past and continuing Mars exploration missions^1–10. Utopia Planitia, the smooth plains of volcanic and sedimentary strata that infilled the Utopia impact crater, has been a prime target for such exploration as it is inferred to have hosted an ancient ocean on Mars^11–13. However, 45 years have passed since Viking-2 provided ground-based detection results. Here we report an in situ ground-penetrating radar survey of Martian subsurface structure in a southern marginal area of Utopia Planitia conducted by the Zhurong rover of the Tianwen-1 mission. A detailed subsurface image profile is constructed along the roughly 1,171 m traverse of the rover, showing an approximately 70-m-thick, multi-layered structure below a less than 10-m-thick regolith. Although alternative models deserve further scrutiny, the new radar image suggests the occurrence of episodic hydraulic flooding sedimentation that is interpreted to represent the basin infilling of Utopia Planitia during the Late Hesperian to Amazonian. While no direct evidence for the existence of liquid water was found within the radar detection depth range, we cannot rule out the presence of saline ice in the subsurface of the landing area.

Rapid seismic deployments after major earthquakes often produce critical data for characterizing postseismic processes. Taking advantage of pre‐existing optical fibers, the recently emerging distributed acoustic sensing (DAS) technology can quickly establish ultra‐dense seismic arrays after the mainshocks. Here we present the first example of such a rapid‐response experiment using four telecommunication fiber optic cables near the 2019 M 7.1 Ridgecrest earthquake in California. By applying template matching to the Ridgecrest DAS array, we detected 6 times more aftershocks than the standard catalog within the three‐month period. The enhanced catalog reveals abundant aftershocks on multiple crosscutting faults near the epicenters of the mainshock and the M 6.4 foreshock. Given the widespread fiber optic networks around the world, DAS has the potential to deliver fast and high‐resolution aftershock monitoring and promote better understanding of earthquake physics.

Seismic sensor orientation is one of the most critical parameters for modern three‐component seismological observation. However, this parameter is easily subject to error imposed by strong magnetic anomalies near the station or by human error in declination calibration. It is therefore very important to inspect and correct for sensor misorientation before utilizing three‐component waveform data. In this study, we measured the epoch‐dependent sensor misorientation for our temporary seismic array (NorthEast China Seismic Array to Investigate Deep Subduction, or NECsaids) by analyzing P ‐wave particle motions. We applied principal component analysis and the minimizing transverse energy method to study earthquakes with epicentral distance between 5° and 90° to estimate the sensor misorientation. Our results show high consistency with the direct gyrocompass measurements, with a correlation coefficient of 0.95. Our statistical analysis suggests that we can estimate robust sensor misorientation utilizing 10 earthquakes with high signal‐to‐noise ratio records and highly linear P ‐wave polarizations. We also find that the influence of anisotropy or a dipping interface produces a periodical pattern with back azimuth and is relatively small for our misorientation estimation. By analyzing the amplitude change of synthetic seismograms due to misorientation and taking into account the influence of anisotropy and dipping interfaces, as well as the measurement errors, we expect engineers to be able to orient seismic sensors with error smaller than 3°.

Myanmar is located at the eastern margin of the ongoing Indo‐Eurasian collision system, has experienced a complex tectonic history and is threatened by a high level of seismic hazard. Here we develop a crustal scale 3‐D seismic velocity model of Myanmar, which is not only critical for understanding the regional tectonic setting and its evolution but can also provide the foundation for a variety of seismological studies, including earthquake location determinations, earthquake focal mechanism inversions, and ground motion simulations. We use the newly deployed Earth Observatory of Singapore‐Myanmar broadband seismic network and other seismic stations in and around Myanmar to study the station‐based 1‐D velocity structure through a joint inversion of receiver functions, H/V amplitude ratio of Rayleigh waves, and surface wave dispersion measurements. Our results reveal a highly variable crustal structure across Myanmar region, characterized by a series of N‐S trending sedimentary basins, with thicknesses up to ~15 km in central Myanmar and an ~5‐km step in the depth of the Moho across the Sagaing‐Shan Scarp fault system. We interpolate our station‐based 1‐D velocity profiles to obtain an integrated 3‐D velocity model from southern Bangladesh to Myanmar. Using three regional earthquakes located to the south, within, and north of the seismic network, we show that our proposed model performs systematically better than the CRUST 1.0 model for both Pnl waves and surface waves. Our study provides a preliminary community velocity model for the region, with further refinements and interpretations anticipated in the near future.