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

Abstract

In many ways, the progression of our understanding of the seismic radiation pattern of earthquakes has mirrored the evolution of earthquake science itself. By the early 1900s (Reid, 1911), it was well understood that earthquakes represented shear slip on faults at depth within the Earth's crust. However, connecting this heuristic understanding with more sophisticated theoretical models of the earthquake source proved surprisingly difficult. In the 1920s, Nakano (1923) suggested that the recorded ground motions from earthquakes may be explained by a double-couple representation of the earthquake source. This hypothesis remained controversial for nearly 40 yr, until a series of theoretical and observational studies (Balakina et al., 1961;Burridge & Knopoff, 1964;Honda, 1962;Vvedenskaya, 1956), combined with routine estimation of earthquake mechanisms made possible through the advent of the World-Wide Network of Seismograph Stations (Oliver & Murphy, 1971) definitively demonstrated the double-couple model's validity. To this day, knowledge of the radiation pattern of a double-couple source is fundamental to many problems in seismology and is thus a cornerstone of many introductory classes in earthquake science.While it is widely acknowledged that real earthquake sources may deviate from a theoretical double couple in important ways (Frohlich, 1994;Hayashida et al., 2020;Julian et al., 1998), the double-couple model has proven remarkably successful in explaining the radiation pattern of earthquakes at long periods (frequencies < ∼2 Hz). At higher frequencies, however, the situation becomes more complicated and the model notably less satisfactory (Castro et al., 2006). The effects of scattering become more pronounced at higher

The High‐Frequency Signature of Slow and Fast Laboratory Earthquakes

“…trend of dense nodal array deployments with excellent station coverage in the near-source region may permit a more robust and comprehensive analysis of high-frequency energy characteristics for small and large earthquakes (Catchings et al, 2020;Fan & McGuire, 2018;Kemna et al, 2020;Trugman et al, 2021). Nevertheless, our laboratory data indicate that measurable high-frequency energy exists well-beyond the corner frequency and could have important implications for understanding the mode of faulting.…”

“…To our knowledge, there are no field‐based studies that have quantified high‐frequency radiation that extends this far above the measured corner frequency, namely due to the bandlimited nature of the data and/or recording instrument (e.g., Baltay et al., 2010 ). However, the emerging trend of dense nodal array deployments with excellent station coverage in the near‐source region may permit a more robust and comprehensive analysis of high‐frequency energy characteristics for small and large earthquakes (Catchings et al., 2020 ; Fan & McGuire, 2018 ; Kemna et al., 2020 ; Trugman et al., 2021 ). Nevertheless, our laboratory data indicate that measurable high‐frequency energy exists well‐beyond the corner frequency and could have important implications for understanding the mode of faulting.…”

High-frequency S and S-coda waves at ocean-bottom seismometers

“…Owing to the superposition of randomly scattered waves, path effects at later coda parts are considered as common at all stations. Additionally, the effects of the source radiation pattern are expected to be large for S waves at near-source stations even for higher (> 2 Hz) frequencies but weak for later coda portions (e.g., Sato et al 1997;Takemura et al 2016;Trugman et al 2021). Thus, the relative amplitudes of coda amplitudes at a certain reference station are mainly dependent on the source energy and the local site beneath a certain seismic station.…”

Empirical Evidence of Frequency‐Dependent Directivity Effects From Small‐To‐Moderate Normal Fault Earthquakes in Central Italy

“…While the scatter between 𝐴𝐴 𝐴𝐴min and 𝐴𝐴 𝐴𝐴max is relatively large (red and green symbols in Figure 9), binned median values (red and green solid lines in Figure 9) exhibit a decaying trend with increasing magnitude. The decay rate is roughly similar to that of the corner frequency, suggesting a relation of the directivity bandwidth on the source properties (Trugman et al, 2021). Frequencies 𝐴𝐴 𝐴𝐴min and 𝐴𝐴 𝐴𝐴𝑐𝑐 attain approximately the same values for events with magnitude greater or equal than 4 (although they are only a small proportion of the total events), while for smaller magnitudes the relationship between 𝐴𝐴 log 2 𝑓𝑓min and 𝐴𝐴 log 2 𝑓𝑓𝑐𝑐 seems constant and equal to about 𝐴𝐴 1.6 log 2 units.…”