Measurements of Wave‐Induced Attenuation in Saturated Metapelite and the Band‐Limitation of Low‐Frequency Earthquakes

Abstract

The elastic waves produced by "typical" small magnitude earthquakes (M ω between 1 and 3) have a recorded frequency range between 1 and 30 Hz (Shelly et al., 2007). In contrast, the families of earthquakes known as very low-frequency earthquakes (VLFEs) (Ide et al., 2007;Obara & Kato, 2016) and low-frequency earthquakes (LFEs) (Obara, 2002;Rogers & Dragert, 2003;Schwartz & Rokosky, 2007) are depleted in high frequencies when compared to small "typical" earthquakes of similar magnitudes (Figure 1a). The frequencies depleted in VLFEs and LFEs are generally observed for the S-waves and occur above approximately 0.1 and 2-8 Hz, respectively (Bostock et al., 2015;Farge et al., 2020;Supino et al., 2020). These events have lower corner frequencies than typical earthquakes of the same size, which marks the onset of a rapid decrease in the amplitude spectra and indicates a depletion of high frequencies (Figure 1a). The corner frequency is inversely proportional to rupture duration. Given this relationship, it is hypothesized that rupture and slip are slower for VLFEs and LFEs than for typical earthquakes of the same size, because their lower corner frequencies imply longer duration. The most common explanation for this observation is that the sources of VLFEs and LFEs are governed by different rupture and slip physics than typical earthquakes (

What makes low-frequency earthquakes low frequency

“…Assuming that rays from deep earthquakes pass through the region where low-frequency events occur in the orange column, the observed difference in spectra between regular earthquakes and low-frequency earthquakes cannot be explained by the greatest possible attenuation below the subduction interface where low-frequency earthquakes occur, as estimated from intraslab earthquakes. This contrasts with recent experimental work on lithologies suspected to host low-frequency earthquakes (36). Our results indicate that the absence of high-frequency components in low-frequency seismic events is not primarily controlled by abnormally high attenuation.…”

“…The presence of fluids, supported here by our local estimates of 1D seismic velocity and attenuation, can not only affect the dynamic rupture propagation by reducing dynamic friction ( 37 ) but also slow down rupture speeds through dilatancy strengthening ( 38 ). Compared to dry conditions, fluids can cause frequency-dependent attenuation to dominate under saturated conditions ( 36 ) that is difficult to model. We sidestep this challenge by correcting low-frequency earthquake spectra with empirical estimates of attenuation that do not rely on a given model ( Fig.…”

Evidence for faulting and fluid-driven earthquake processes from seismic attenuation variations beneath Los Angeles

“…However, the proposed models are frequency-independent and do not distinguish between scattering and intrinsic attenuation. Laboratory measurements on samples of the Orocopia schist which constitutes part of the crust in Southern California show increases in intrinsic attenuation as pore pressure increases 41 confirming that measurable changes in seismic attenuation can reflect changes in rock-fluid properties. Here, instead of using the regional seismic network only, we take advantage of the densely distributed seismic nodes of the temporary BASIN network (Fig.…”

Duplex Underplating, Sediment Dehydration and Quartz Vein Mineralization in the Deep Tremor Source Region

“…While the band‐limited nature of LFEs due to high‐frequency depletion has been assumed to represent slower fault rupture and slip, it can also occur due to near‐source attenuation associated with high fluid pressure in the LVZ (Bostock et al., 2017; Gomberg et al., 2012). Recently, laboratory experiments have demonstrated that water‐saturated metapelite at low effective pressure can cause high‐frequency attenuation (Fliedner & French, 2023). Shear and tensile failures in quartz veins produce high‐frequency components, but may be attenuated in metapelite and metabasite, which form a high fluid pressure zone tens to hundreds of meters thick.…”