Impact Versus Frictional Earthquake Models for High‐Frequency Radiation in Complex Fault Zones

Abstract

We compare the predicted earthquake ground motions from elastic impacts in a complex fault zone with predictions from frictional models • Ground motions from elastic impacts are caused by the size and shape of fault-zone structures instead of by off-fault viscoplastic parameters • High-frequency ground motions from elastic impacts are more isotropic and have higher P/S radiated energies

Earthquake Source Complexity Controls the Frequency Dependence of Near‐Source Radiation Patterns

“…The fact that we do not observe substantial differences between magnitude bins despite their presumably different corner frequencies suggests that coherence in the radiation pattern is not controlled by the corner frequency of the source spectrum. Instead, our results may be more consistent with a rupture process in which the length scale of fault zone complexity, rather than event size, controls the high frequency spectral content and radiation pattern (Tsai & Hirth, 2020), which would have important repercussions for the measurement and interpretation of earthquake source spectra (Tsai et al, 2021). In the near-source region, we begin to observe significant distortion of the P-wave radiation pattern only at frequencies above ∼15 Hz.…”

“…This may be because directivity effects, while prominent at low frequencies, are often not preserved in the frequency bands we consider in this study (Pacor et al, 2016;Somerville et al, 1997). In summary, earthquake rupture dynamics can be sensitive to various forms of fault zone complexity (Huang & Ampuero, 2011;Huang et al, 2014;Ma & Elbanna, 2019), which in turn can influence the details of the recorded seismic wavefield (Ben-Zion & Ampuero, 2009;Trugman et al, 2020;Tsai & Hirth, 2020;Tsai et al, 2021).…”

“…In addition, the mixture and coupling of SV and SH waves can generate additional frequency-dependent distortion of the radiation pattern (Satoh, 2002;Takenaka et al, 2003). While these two mechanisms are essentially path effects, complexity in the earthquake source may also drive frequency-dependent distortions in the radiation pattern (Liu & Helmberger, 1985), for example, through heterogeneity in the rupture process (Graves & Pitarka, 2016) or through fault zone complexity (Chester & Chester, 1998;Faulkner et al, 2003;Tsai et al, 2021). Understanding this distinction between source and path effects is not just of scientific interest but also of practical importance, as quantitative constraints on the spatial distribution of high-frequency ground motions is fundamental to earthquake hazard analysis.…”

Fault Interactions Enhance High‐Frequency Earthquake Radiation

“…Other aspects of faults, such as damage zone width, may also be considered as “complexity” but again we are motivated by the desire to quantify the potential for fault interaction. Specifically, alignment and density of faults are key aspects of the model of Tsai and Hirth (2020) in which discrete fault structures collide to produce high‐frequency radiation, requiring fault‐fault interactions, and which cannot be explained in the context of single rough faults (Tsai et al., 2021).…”

“…We use surface fault maps to define two new metrics characterizing fault complexity, aiming to describe the geometry and scale of potential fault interactions, motivated by the recent suggestion that these properties may be important for high‐frequency radiation (Tsai et al., 2021; Tsai & Hirth, 2020). We apply these metrics to regions in Southern California with well‐mapped faults and large catalogs of documented seismicity for which stress drop catalogs obtained from source spectral measurements are available.…”