New perspectives on self‐similarity for shallow thrust earthquakes

Abstract

Scaling of dynamic rupture processes from small to large earthquakes is critical to seismic hazard assessment. Large subduction earthquakes are typically remote, and we mostly rely on teleseismic body waves to extract information on their slip rate functions. We estimate the P wave source spectra of 942 thrust earthquakes of magnitude Mw 5.5 and above by carefully removing wave propagation effects (geometrical spreading, attenuation, and free surface effects). The conventional spectral model of a single‐corner frequency and high‐frequency falloff rate does not explain our data, and we instead introduce a double‐corner‐frequency model, modified from the Haskell propagating source model, with an intermediate falloff of f−1. The first corner frequency f1 relates closely to the source duration T1, its scaling follows M0∝T13 for Mw<7.5, and changes to M0∝T12 for larger earthquakes. An elliptical rupture geometry better explains the observed scaling than circular crack models. The second time scale T2 varies more weakly with moment, M0∝T25, varies weakly with depth, and can be interpreted either as expressions of starting and stopping phases, as a pulse‐like rupture, or a dynamic weakening process. Estimated stress drops and scaled energy (ratio of radiated energy over seismic moment) are both invariant with seismic moment. However, the observed earthquakes are not self‐similar because their source geometry and spectral shapes vary with earthquake size. We find and map global variations of these source parameters.

Subduction zone megathrust earthquakes

“…They also found along-strike variations, in particular, very low values along portions of the Central America subduction zone and near the hypocenter of the 2004 Sumatra earthquake. Similar to the depth dependence of source duration discussed in the "Source Duration" subsection herein, Denolle and Shearer (2016) found very weak depth dependence for stress drop and scaled energy using their global catalog. They observed regional variations, with lower stress drop in general in western Pacific subduction zones as compared to eastern Pacific zones, and very low stress drops for earthquakes along the Alaska-Aleutian subduction zone.…”

“…Denolle and Shearer (2016) examined spectral ratios for over 900 thrust earthquakes (M > 5.5) and found weak dependence of scaled source duration with increasing depth on the fault. Similarly, Ye et al (2016c) found longer-duration earthquakes in the shallowest (<18 km) portion of the subduction megathrust using finite fault determinations for over 100 M w > 7 earthquakes.…”

“…Point source time functions are routinely determined by the SCARDEC method (e.g., Vallée and Douet, 2016). Efforts are under way to synthesize the accumulated data sets (e.g., Kanamori, 2014;Lay, 2015;Ye et al, 2016bYe et al, , 2016cDenolle and Shearer, 2016;Meier et al, 2017;Melgar and Hayes, 2017;Hayes, 2017), and we will not attempt to summarize the multitude of studies. Large shallow coseismic slip occurred in the 2015 Illapel (e.g., Li et al, 2016;Melgar et al, 2016), 2011 Tohoku (e.g., Lay et al, 2011b;Iinuma et al, 2012;Ozawa et al, 2012;Satake et al, 2013;Romano et al, 2014;Bletery et al, 2014;Melgar and Bock, 2015;Lay, 2017), 2010 Maule (e.g., Vigny et al, 2011;Yue et al, 2014b;Yoshimoto et al, 2016;Maksymowicz, et al, 2017), and 2004 Sumatra (e.g., Ammon et al, 2005;Rhie et al, 2007;Fujii and Satake, 2007) events, accompanying slip on the downdip portions of the megathrusts.…”

Global and Interregion Characterization of Subduction Interface Earthquakes Derived From Source Time Functions Properties

“…72 earthquakes (over 90) correspond to ruptures with impulsivity and scaled energy below the depth‐varying averages (Figures and ). Based on teleseismic source spectra analysis, Denolle and Shearer () also report low values of stress drop and radiated energy in this subduction zone, with a similar increase when approaching the Solomon Islands. values are lower than expected by values, which indicates that ruptures have simple time history ( C ind ≤2 for 68 earthquakes over 90).…”

“…The most classical one is the corner frequency f c of the seismic spectrum, which can be related to the stress drop Δ σ when assuming a simple source model with constant rupture velocity (e.g., Brune, ; Madariaga, ). Systematic analyses of the corner frequency have been done both at the local scale (Prejean & Ellsworth, ; Uchide et al, , for instance) and at the global scale (e.g., Allmann & Shearer, , and more recently Denolle & Shearer, ).…”