Fault zone structures at many scales largely dictate earthquake ruptures and are controlled by the geologic setting and slip history. Characterizations of these structures at diverse scales inform better understandings of earthquake hazards and earthquake phenomenology. However, characterizing fault zones at sub-kilometer scales has historically been challenging, and these challenges are exacerbated in urban areas, where locating and characterizing faults is critical for hazard assessment. We present a new procedure for characterizing fault zones at sub-kilometer scales using distributed acoustic sensing (DAS). This technique involves the backprojection of the DAS-measured scattered wavefield generated by natural earthquakes. This framework provides a measure of the strength of scattering along a DAS array and thus constrains the positions and properties of local scatterers. The high spatial sampling of DAS arrays makes possible the resolution of these scatterers at the scale of tens of meters over distances of kilometers. We test this methodology using a DAS array in Ridgecrest, CA which recorded much of the 2019 Mw7.1 Ridgecrest earthquake aftershock sequence. We show that peaks in scattering along the DAS array are spatially correlated with mapped faults in the region and that the strength of scattering is frequency-dependent. We present a model of these scatterers as shallow, low-velocity zones that is consistent with how we may expect faults to perturb the local velocity structure. We show that the fault zone geometry can be constrained by comparing our observations with synthetic tests.

The mechanical coupling between a planet and its atmosphere enables the conversion of seismic waves into infrasound waves, i.e. low-frequency pressure perturbations (< 20Hz), which propagate to the upper atmosphere. Since the characteristics of the seismically-induced pressure perturbations are connected to their seismic counterparts, they provide a unique opportunity to investigate the atmospheric and interior structures of a planet or to constrain source properties. However, in Earth’s remote regions, deploying seismic or infrasound networks at the surface can be a difficult task. Stratospheric balloon platforms equipped with pressure sensors have therefore gained interest since they provide a unique and inexpensive way to record pressure signals in the atmosphere with a low noise level. Yet, infrasound observations of Earthquakes on balloon platforms have never been reported in the literature. In this study, we investigate the seismo-acoustic wavefield generated by the aftershocks of the 2019 Ridgecrest sequence and other regional low-magnitude Earthquakes on July 22 and August 9, 2019 using four free-flying balloons equipped with pressure sensors. We observed a strong signal coherence after the largest event between seismic motions at the surface and balloon pressure variations which matches our numerical simulations. A first atmospheric earthquake detection is crucial to demonstrate the viability of this novel technique to monitor infrasound from natural and artificial seismicity on Earth, and the study of seismic activity on planets such as Venus.

Fault zone complexities contain important information about earthquake physics. High-resolution fault zone imaging requires high-quality data from dense arrays and new seismic imaging techniques that can utilize large portions of recorded waveforms. Recently, the emerging Distributed Acoustic Sensing (DAS) technique has enabled near-surface imaging by utilizing existing telecommunication infrastructure and anthropogenic noise sources. With dense sensors at several meters’ spacing, the unaliased wavefield can provide unprecedented details for fault zones. In this work, we use a DAS array converted from a 10-km underground fiber-optic cable across Ridgecrest City, California. We report clear spurious arrivals and coda waves in ambient noise cross-correlations caused by surface-to-surface wave scattering. We use these scattering-related waves to locate and characterize potential faults. The mapped fault locations are generally consistent with those in the USGS Quaternary Fault database of the United States but are more precise. We also use waveform modeling to infer that a 35-m wide, 90-m deep fault with 30% velocity reduction can best fit the observed scattered coda waves for one of the identified fault zones. These findings demonstrate the potential of DAS for passive imaging of fine-scale faults in an urban environment.