Imaging Strong Lateral Heterogeneities Across the Contiguous US Using Body‐To‐Surface Wave Scattering

Abstract

Seismic scattering originated from structural heterogeneity covers a wide range of scales within the Earth's interior. Conventionally, stochastic approaches are employed to study high-frequency scattering process from random heterogeneity (Sato et al., 2012). One typical example is the characterization of P and S coda waves from local and regional earthquakes (Aki, 1969). On the other hand, deterministic imaging of subsurface structures has long been undertaken using either backward or forward scattering. For example, in earthquake seismology, receiver functions rely on the forward P-to-S scattering to image seismic discontinuities in the crust and mantle (Langston, 1979). Seismic reflection surveys utilize backward reflected waves to characterize petroleum reservoirs in exploration seismology and the Earth's crust in controlled-source crustal seismology (Prodehl & Mooney, 2012; Sheriff & Geldart, 1995). Unlike subhorizontal structures, strong lateral heterogeneity associated with near-vertical structure poses a significant challenge in deterministic scattered-wave imaging. In exploration seismology, complex structures can be imaged using densely distributed sources and receivers, in combination with sophisticated imaging or inversion techniques, such as reverse time migration or full-waveform inversion (Sheriff & Geldart, 1995; Virieux & Operto, 2009). However, such frameworks do not generally apply to earthquake seismology. The sparse distribution of earthquake sources and seismic stations greatly limits the detection ability of subsurface lateral heterogeneities, such as basin edges, fault zones, and Moho offsets, despite them having strong effects on seismic waveforms. Furthermore, many seismic imaging methods, such as common conversion point stacking of conventional receiver functions, assume subhorizontal structures, which cannot readily be applied to image strong lateral heterogeneities. Body-to-surface wave conversion is a special case of seismic scattering originated from strong lateral heterogeneity. Compared to body-to-body wave scattering, scattered surface waves propagate horizontally along

Very Low Frequency Earthquakes in Between the Seismogenic and Tremor Zones in Cascadia?

“…Here we considered the possibility that the detected VLFEs are actually scattered energy from the Canal de Ballenas event seismic waves rather than a unique local source. For example, the observations could have been from S‐wave to surface‐wave conversions that have been observed from the US west coast (Buehler et al., 2018; Yu et al., 2021). However, we found that this hypothesis violates the observations in a number of ways.…”

“…In addition to triggered seismic events, heterogeneous subsurface structure can cause a secondary coherence surface wavefield by reflecting or converting the incoming waves (Buehler et al, 2018;Maeda et al, 2014;Obara & Matsumura, 2010;Yu et al, 2017Yu et al, , 2021. Here we considered the possibility that the detected VLFEs are actually scattered energy from the Canal de Ballenas event seismic waves rather than a unique local source.…”