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

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’.

HEX: Hyperbolic Event eXtractor, a Seismic Phase Associator for Highly Active Seismic Regions

The task of seismic phase association is to correlate the onsets of radiated seismic energy with an underlying source. Commonly applied within seismic monitoring networks for event detection, it forms a vital component of many seismic processing pipelines. With the complexity of this task naturally increasing with the number of phases to simultaneously correlate, rapid advancements in the number of sensors per seismic deployment, along with improved picking algorithms have greatly increased the volume of phases now recorded across seismic networks. Although traditional phase association methods work well for historic catalogs, they become unreliable when tasked with associating the frequent smaller events recorded in the latest seismic datasets. Accurately correlating such events is crucial if seismologists are to better understand the underlying physical processes. The phase association problem is, therefore, being revisited with novel techniques now being applied to improve performance. We present a new technique for associating seismic phases, Hyperbolic Event eXtractor (HEX). HEX adapts the logic of Random Sample Consensus, a model estimation approach widely used in the computer vision community and specifically designed to deal with high proportions of noise in the data distribution. We demonstrate the performance of HEX in associating phases over a synthetic dataset for a regional seismic network in northern Chile. Synthetic testing reveals that HEX can correlate seismic phases when events have up to a ∼15 s average spacing.

Seismoelectromagnetic effects associated with the 2017 February 15 Veracruz earthquake (Mw = 4.8)

SUMMARY In this study, we investigated correlations between electromagnetic and seismic signals of the 2017 February 15 Veracruz, Mexico, earthquake (Mw = 4.8). We carried out a time–frequency misfit analysis based on the continuous wavelet transform in order to compare electric, magnetic and seismic records accurately. This analysis was performed for horizontal and vertical components separately. Our results from time–frequency misfit and goodness-of-fit criteria confirm the general similarity between seismic and electromagnetic signals both in frequency and time. Additionally, we studied the behaviour of peak amplitudes of seismoelectromagenetic records as a function of magnitude and distance. Our observations are in good agreement with previous studies, confirming scaling with magnitude and attenuation with distance. Radiated seismic energy estimations were performed with two methods: integration of velocity records and empirical Green function, respectively. Estimated energy magnitudes (4.35 < Me < 4.98) are consistent with reported seismic magnitudes for this event. We propose a method for determining electric and magnetic coseismic energies based on the concept of energy flux as implemented in the frequency domain by the integration of electromagnetic records. The calculated energies showed that the radiated seismic energy is much higher than the electric and magnetic energies.

The Coda Calibration and Processing Tool: Java-Based Freeware for the Geophysical Community

<p>The coda magnitude method of <em>Mayeda and Walter</em> (1996) provides stable source spectra and moment magnitudes (<em>M</em><em><sub>w</sub></em>) for local to regional events from as few as one station that are virtually insensitive to source and path heterogeneity. The method allows for a consistent measure of <em>M</em><em><sub>w</sub></em> over a broad range of event sizes rather than relying on empirical magnitude relationships that attempt to tie various narrowband relative magnitudes (<em>e.g.,</em> <em>M</em><em><sub>L</sub>, M<sub>D</sub>, m<sub>b</sub></em>, etc.) to absolute <em>M</em><em><sub>w </sub></em>derived from long-period waveform modeling. The use of <em>S</em>-coda and <em>P</em>-coda envelopes has been well documented over the past several decades for stable source spectra, apparent stress scaling, and hazard studies. However, up until recently, the method requires extensive calibration effort and routine operational use was limited only to proprietary US NDC software. The Coda Calibration Tool (CCT) stems from a multi-year collaboration between the US NDC and LLNL scientists with the goal of developing a fast and easy Java-based, platform independent coda envelope calibration and processing tool. We present an overview of the tool and advantages of the method along with several calibration examples, all of which are freely available to the public via GitHub (https://github.com/LLNL/coda-calibration-tool). Once a region is calibrated, the tool can then be used in routine processing to obtain stable source spectra and associated source information (<em>e.g.</em>, <em>M</em><em><sub>w</sub></em>, radiated seismic energy, apparent stress, corner frequency, source discrimination on event type and/or depth). As more events are recorded or new stations added, simple updates to the calibration can be performed. All calibration and measurement information (<em>e.g.,</em> site and path correction terms, raw & measured amplitudes, errors, etc.) is stored within an internal database that can be queried for future use. We welcome future collaboration, testing and suggestions by the geophysical community.  </p>

The rupture process of the Peru intermediate and deep earthquakes

<p>The seismicity of Peru is associated with the subduction process of the Nazca plate under South America and characterized by the occurrence of shallow, intermediate and deep earthquakes. In this study, we focus our attention on the rupture process of earthquakes (Mw>6.0 ) that occurred during the period 2018-2019 at intermediate depth (50<h<200 km) and deep depth (500<h<700 km). Focal mechanisms have been estimated from slip inversion of body waves at teleseismic distances (Kikuchi and Kanamori, 1991). We investigate possible differences in the moment rate functions at different focal depths using our results and those provided by SCARDEC database. Furthermore, an estimation of the radiated seismic energy (E<sub>R</sub>) was provided from the direct integration of the velocity P wave recorded at teleseismic and regional distances, getting values between 10<sup>15 </sup>to 10<sup>16</sup> J. The data were corrected by geometrical spreading, anelastic attenuation, and free surface effect. These results are interpreted in terms of the seismotectonics of the region</p>

Spatial Distribution of Radiated Seismic Energy of Three Aftershocks Sequences at Guerrero, Mexico, Subduction Zone

General characteristics of seismic energy release of thrust earthquakes in Mexico have been reviewed in the past; however, a detailed analysis can contribute to a better understanding of the mechanisms that control its distribution along the Guerrero, Mexico, subduction zone. To address it, we obtain the source spectra of the 2012 Mw 7.5 Ometepec‐Pinotepa Nacional, the 2014 Mw 7.2 Papanoa, and the 2018 Mw 7.2 Pinotepa Nacional earthquakes, as well as of their M≥4.0 aftershocks to estimate their seismic moment M0 and radiated seismic energy ES. The first and the last sequences occurred at the southern border of the Guerrero seismic gap, a region where no significant earthquake (M>7.0) has occurred at least in the last century; whereas the second sequence was located at the northern edge of the same seismic gap. The mean value of the log of radiated seismic energy scaled with the seismic moment, log(e˜)=log(ES/M0), for this set of earthquakes is −5.05±0.25. We classify the analyzed events into four regions, two in the southern edge of the gap and two in the northern one. At both ends, there is one region that shows regular values of log(e˜) (−4.64±0.25 and −4.62±0.25), whereas the other one shows low values of log(e˜) (−5.40±0.25 and −5.55±0.25) that could be related to a possible slow‐rupture behavior. These last regions are identified near the trench at southern Guerrero coast and immediately outside the northern end of the seismic gap. The distribution of log(e˜) is spatially heterogeneous along the trench, suggesting variations on the shear strength and coupling at the interface.

Characterizing large earthquakes before rupture is complete

Whether earthquakes of different sizes are distinguishable early in their rupture process is a subject of debate. Studies have shown that the frequency content of radiated seismic energy in the first seconds of earthquakes scales with magnitude, implying determinism. Other studies have shown that recordings of ground displacement from small to moderate-sized earthquakes are indistinguishable, implying a universal early rupture process. Regardless of how earthquakes start, events of different sizes must be distinguishable at some point. If that difference occurs before the rupture duration of the smaller event, this implies some level of determinism. We show through analysis of a database of source time functions and near-source displacement records that, after an initiation phase, ruptures of M7 to M9 earthquakes organize into a slip pulse, the kinematic properties of which scale with magnitude. Hence, early in the rupture process—after about 10 s—large and very large earthquakes can be distinguished.