The 2019 Ridgecrest, California, earthquake sequence included Mw 6.4 and 7.1 earthquakes that occurred on successive days beginning on 4 July 2019. These two largest earthquakes of the sequence occurred on orthogonal faults that ruptured the Earth’s surface. To better evaluate the 3D subsurface fault structure, (P- and S-wave) velocity, 3D and temporal variations in seismicity, and other important aspects of the earthquake sequence, we recorded aftershocks and ambient noise using up to 461 three-component nodal seismographs for about two months, beginning about one day after the Mw 7.1 mainshock. The ∼30,000Mw≥1 earthquakes that were recorded on the dense arrays provide an unusually large volume of data with which to evaluate the earthquake sequence. This report describes the recording arrays and is intended to provide metadata for researchers interested in evaluating various aspects of the 2019 Ridgecrest earthquake sequence using the nodal data set.

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The development of Urban Underground Space (UUS) has become an effective way to solve the problem of urban space. Seismic methods have achieved good results in underground engineering construction, active faults detection, urban geological structure detection and other fields. Active source seismic methods in UUS include reflection methods, refraction methods, surface wave methods and vertical seismic profile (VSP) methods and so on. Ambient noise method is also rising gradually, which becomes an important method of urban underground space detection in the future, because it overcomes the problem that the active source method is interfered by urban noise. We briefly summarize these methods and introduce the application examples of UUS.

Worldwide, slip on earthquake faults causes numerous disasters, resulting in large losses in human life and built structures. To minimize future losses associated with earthquakes along such faults, it is important to precisely locate the faults relative to the built environment and to determine the subsurface geometry of the faults. In Beijing, China, we used shallow-depth geophysical methods to evaluate the location and subsurface geometry of the Huangzhuang-Gaoliying fault (HGF), one of the principal tectonic faults of Beijing area. We used seismic reflection and refraction tomography, multi-channel analysis of surface waves (MASW), and paleoseismic trenching to characterize the north section of HGF near the Gaoliying section of Beijing. Our seismic images indicated that there are at least two strands of the HGF that are distributed over an approximately 200-m-wide zone. We identified a principal fault strand (F1) that is observed in all the seismic images, as well as in a paleoseismic trench. The F1 strikes approximately N49°E and dips southeastward at 70° to 75°. Over the past few years, surface ruptures have occurred along the HGF in several locations, but it is unclear if the surface ruptures were the result of tectonic slip on the HGF or were related to land subsidence along the fault.