Fault Zone Imaging with Distributed Acoustic Sensing: Body-to-Surface
Wave Scattering
Abstract
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.