Resolution and uncertainties in estimates of earthquake stress drop and energy release

Abstract

Our models and understanding of the dynamics of earthquake rupture are based largely on estimates of earthquake source parameters, such as stress drop and radiated seismic energy. Unfortunately, the measurements, especially those of small and moderate-sized earthquakes (magnitude less than about 5 or 6), are not well resolved, containing significant random and potentially systematic uncertainties. The aim of this review is to provide a context in which to understand the challenges involved in estimating these measurements, and to assess the quality and reliability of reported measurements of earthquake source parameters. I also discuss some of the ways progress is being made towards more reliable parameter measurements. At present, whether the earthquake source is entirely self-similar, or not, and which factors and processes control the physics of the rupture remains, at least in the author's opinion, largely unconstrained. Detailed analysis of the best recorded earthquakes, using the increasing quantity and quality of data available, and methods less dependent on simplistic source models is one approach that may help provide better constraints. This article is part of the theme issue ‘Fracture dynamics of solid materials: from particles to the globe’.

Friction and slip measured at the bed of an Antarctic ice stream

“…Even the smallest magnitude icequakes (𝑀 7 ≈ −1.5) have high signal-to-noise-ratios, and so could elucidate any lower limits on the fundamental size of earthquake nucleation for given fault properties [54][55][56] . Additionally, icequakes in this study have stress-drops that vary with magnitude, contrary to magnitude-invariant stress-drops observed for larger earthquakes 57 .…”

Does Earthquake Stress Drop Increase With Depth in the Crust?

“…Unfortunately, both the rupture dimension and stress drop of small earthquakes are very hard to measure, largely because of the difficulty of removing the effects of propagation through the earth from the recorded seismograms (see Abercrombie, 2021 for a review). The attenuation structure of the earth is not sufficiently well known at the relatively high frequencies needed to resolve small earthquake source parameters, and so empirical Green's function (EGF) approaches have become popular for isolating the source radiation (e.g., Abercrombie et al., 2017; Mori & Frankel, 1990; Ross et al., 2017).…”

Fracture dynamics of solid materials: from particles to the globe

“…With the main keyword ‘fracture dynamics', this theme issue covers diverse subjects, extending from theoretical yet significantly important model analyses, such as J. D. Eshelby-based [14] collective treatment of damage evolution in brittle materials [3], to practical safety and environmental problems associated with mining or rock excavation engineering, including the fundamental study on the brand-new contactless rock drilling technique using thermal shock [4]. As stated earlier, the scale treated is also diverse, ranging from a fracture at the atomic or microscopic level [2] to largely macroscopic rock/strain bursts or sudden collapse of stopes in deep underground gold mines [9], field observation-based reliable physical estimation of earthquake source parameters [8] or even to megathrust rupture (fracture) near tectonic boundaries of the earth [11]. The theme issue also handles front-line topics like fracture asperity development during the change between stick-slip and stable sliding [5], fracture-induced gravitational perturbations that propagate at the speed of light [12] and seismic events induced by human activity like a fluid injection that can become concerns of the residents nearby [6].…”

“…In the latter part, Abercrombie [8] comprehensively summarizes and points out the difficulties in estimating the fracture properties of seismic sources from seismological datasets, in particular, those of small and moderate-sized earthquakes (magnitude less than about 5 or 6) that are not well resolved and may contain significant random and potentially systematic uncertainties. She also discusses the recent progress made towards more reliable estimates of seismic source parameters.…”

“…The former part [2–7] handles the fundamental mechanisms related to physical processes of dynamic fracture, basically at the laboratory scale. By knowing the essentials, the readers proceed to the latter part [8–12] where more practical and geophysical issues at larger scales are discussed. Finally, unsolved problems of dynamic fracture are introduced for further consideration [13].…”