Energy Partitioning, Dynamic Fragmentation, and Off‐Fault Damage in the Earthquake Source Volume

Abstract

During an earthquake, released elastic strain energy rel E U is converted to the energy conv E U required to advance the rupture and to overcome the frictional resistance to slip on the trailing fault (see Table 1 for notation used in this paper). The rest radiates away as elastodynamic waves rad E U that produce ground shaking. rad E U is the only quantity that can be directly measured and generally comprises less than 15%-20% of rel E U (e.g., Lachenbruch &

High Stress Deformation and Short‐Term Thermal Pulse Preserved in Pyroxene Microstructures From Exhumed Lower Crustal Seismogenic Faults (Lofoten, Norway)

“…The fine‐grained orthopyroxene at the pseudotachylyte fault vein margin (Figures 6 and 7) is interpreted to have formed via brittle grain size reduction, based on: (a) the fracture systems evident in BSE micrographs (Figure 6) and the jigsaw‐breccia type grain shapes preserved in some fine‐grained regions (Figure 6g), and (b) the absence of polygonal subgrains from the interior of the large orthopyroxene fragments, in accordance with the expected lack of recovery in orthopyroxene at the deformation temperature of 700–750°C (e.g., Kanagawa et al., 2008; Kohlstedt & Vander Sande, 1973). This jigsaw‐style geometry, preservation of the overall shape of the parent crystal (Johnson et al., 2021), and general lack of evidence for cataclastic processes such as grinding, fragment rotation and frictional slip, are also indicative of a lack of shear displacement (Figure 7d, see also Petley‐Ragan et al., 2019; Soda & Okudaira, 2018; B. R. Song et al., 2020). Together, these observations support a mostly tensile origin for this fragmentation within the orthopyroxene.…”

“…Such stress magnitudes prior to rupture have been implied by field characterization of pseudotachylyte‐bearing faults (Campbell et al., 2020) and, perhaps more commonly, are also associated with deformation linked to coseismic loading once rupture propagation has initiated (e.g., Anderson et al., 2021). This microstructural record of progressive and transient stress variation throughout the earthquake cycle (Anderson et al., 2021; Bestmann et al., 2012; Brückner & Trepmann, 2021; Campbell & Menegon, 2019; Johnson et al., 2021; Mancktelow et al., 2022; Petley‐Ragan et al., 2019) offers an under‐explored opportunity to further constrain deformation mechanisms and conditions associated with both prerupture stress amplification and coseismic rupture within the lower crust.…”

Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths

“…The synthetic microstructures and EBSD analysis do not contain microdefects such as microfractures and pores. Although most microdefects are closed at confining pressures greater than 100–400 MPa (e.g., Birch, 1960; Burlini & Fountain, 1993; Christensen, 1965; Ji et al., 2015; Kern et al., 2008, 2009; Kern & Wenk, 1990), a few microfractures and pores might survive depending on their shapes and the temperatures of deformation (e.g., Bazargan et al., 2021; Johnson, Song, Vel, et al., 2021; Meyer et al., 2021). In addition, if bulk stiffness via EBSD analysis is computed by analytical homogenization methods such as Hill average, grain boundaries can be considered as microdefects that affect petrophysical wave‐speed measurements, which might result in a small difference between EBSD‐derived and petrophysical AV P for the same mica content.…”

“…Johnson, Song, Vel, et al. (2021) have summarized these relations and their implications for energy expenditure in the earthquake source.…”

“…In the present study, we investigate potential effects of elastic contrast (represented by difference in seismic wave velocity) on rupture directivity and damage distribution in bimaterial faults/shear zones separating dissimilar and elastically anisotropic rocks. We calculate elastic properties and seismic wave velocities of two anisotropic rocks (quartzofeldspathic rock and mica‐rich schist) juxtaposed across the deeply exhumed, seismogenic Sandhill Corner shear zone, an ancient strike‐slip fault that exhibits strongly asymmetric damage distribution (B. R. Song et al., 2020; Johnson, Song, Vel, et al., 2021). We determine elastic contrast of the anisotropic rocks and conclude that the horizontally polarized shear wave propagating parallel to the fault may be the most relevant wave to consider when comparing our results to isotropic bimaterial rupture models.…”