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It is well known that the use of different methods (e.g., spectral fitting, empirical Green’s functions) for compiling catalogs of source parameters (e.g., seismic moment, stress drop) can results in significant inconsistencies (Baltay et al., 2022). In this study, we present the application of coda-wave source parameters estimation by the Coda Calibration Tool (CCT) to different tectonic settings for closer analysis of the regional variations. CCT implements the empirical methodology outlined in Mayeda et al., (2003), which provides stable source spectra and source parameters even for events recorded by sparse local and regional seismic networks (e.g., Morasca et al., 2022). The main strength of the method is the use of narrowband coda waves measurements, which show low sensitivity to source and path heterogeneity. Additionally, we use independent ground-truth (GT) reference spectra for which apparent stresses are independently calculated through the coda spectral ratio (Mayeda et al., 2007), to break the path and site trade-off. The use of GT spectra eliminates the need to assume source scaling for the region, reducing the impact of a-priori model assumptions on the interpretation of scaling laws of source parameters and their variability. The CCT is a freely available Java-based code (https://github.com/LLNL/coda-calibration-tool) that significantly reduces the coda calibration effort and provides calibration parameters for future use in the same region for routine processing.Recently, several studies applied CCT in very different tectonic contexts, including (1) earthquakes in tectonically active regions (e.g., central Italy, Puerto Rico, southern California, Utah); (2) induced earthquakes in southern Kansas and northern Oklahoma; and (3) moderate-sized earthquakes in stable continental regions such as in Eastern Canada and the United Kingdom. There is excellent agreement between coda-derived Mw in all regions and available Mw from waveform modelling. In some cases, such as central Italy and Ridgecrest, the validation process also involved the comparison with estimates from different empirical techniques, such as spectral decomposition approaches applied to data sets sharing common events with CCT. Overall, there is a general consistency in the scaling laws obtained for different source parameters (e.g., seismic moment, corner frequency, radiated energy and apparent stress), with earthquakes in the UK and Canada having similar and higher apparent stresses than Utah, central Italy, Puerto Rico and southern California, while the induced regions are characterized by the lowest values. In conclusion, the application of a consistent methodological framework and the robustness demonstrated by the results of the seismic coda analysis allow comparison of source scaling relationships for different tectonic settings over a wide range of magnitudes.

The coda magnitude method of Mayeda and Walter (1996) provides stable source spectra and moment magnitudes (Mw) 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 Mw over a broad range of event sizes rather than relying on empirical magnitude relationships that attempt to tie various narrowband relative magnitudes (e.g., ML, MD, mb, etc.) to absolute Mw derived from long-period waveform modeling. The use of S-coda and P-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 (e.g., Mw, 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 (e.g., 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.

We have performed 3D simulations of underground chemical explosions conducted recently in granitic outcrop as part of the Source Physics Experiment (SPE) campaign. The main goal of these simulations is to understand the nature of the shear motions recorded in the near field considering uncertainties in a) the geological characterization of the joints, such as density, orientation and persistency and b) the geomechanical material properties, such as, friction angle, bulk sonic speed, poroelasticity etc. The approach is probabilistic; joints are depicted using a Boolean stochastic representation of inclusions conditional to observations and their probability density functions inferred from borehole data. Then, using a novel continuum approach, joints and faults are painted into the continuum host material, granite. To ensure the fidelity of the painted joints we have conducted a sensitivity study of continuum vs. discrete representation of joints. Simulating wave propagation in heterogeneous discontinuous rock mass is a highly non-linear problem and uncertainty propagation via intrusive methods is practically forbidden. Therefore, using a series of nested Monte Carlo simulations, we have explored and propagated both the geological and the geomechanical uncertainty parameters. We have probabilistically shown that significant shear motions can be generated by sliding on the joints caused by spherical wave propagation. Polarity of the shear motion may change during unloading when the stress state may favor joint sliding on a different joint set. Although this study focuses on understanding shear wave generation in the near field, the overall goal of our investigation is to understand the far field seismic signatures associated with shear waves generated in the immediate vicinity of an underground explosion. Therefore, we have abstracted the near field behavior into a probabilistic source-zone model which is used in the far field wave propagation.This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

Previous studies have shown that explosion sources produce fewer aftershocks and that they are generally smaller in magnitude compared to aftershocks of similarly sized earthquake sources (Jarpe et al., 1994, Ford and Walter, 2010). It has also been suggested that the explosion-induced aftershocks have smaller Gutenberg-Richter b-values (Ryall and Savage, 1969, Ford and Labak, 2016) and that their rates decay faster than a typical Omori-like sequence (Gross, 1996). Recent chemical explosion experiments at the Nevada National Security Site (NNSS) were observed to generate vigorous aftershock activity and allow for further comparison between earthquake- and explosion-triggered aftershocks. Of the four recent chemical explosion experiments conducted between July 2018 and June 2019, the two largest explosions (i.e. 10-ton and 50-ton) generated hundreds to thousands of aftershocks. Preliminary analysis indicates that these aftershock sequences have similar statistical characteristics to traditional tectonically driven aftershocks in the region. The physical mechanisms that contribute to differences in aftershock behavior following earthquake and explosion sources are poorly understood. Possible mechanisms may be related to weak material properties in the shallow subsurface that do not give rise to stress concentrations large enough to support brittle failure. Additionally, minimal changes in the shear component of the stress tensor for explosion sources may also contribute to differences in aftershock distributions. Here, we compare aftershock statistics and productivity of the explosion-related aftershocks at the NNSS site to synthetic catalogs of aftershocks triggered by explosion sources. These synthetic catalogs are built by coupling strains that result from modeling the explosion source process with the SW4 wave propagation code with the 3D physics-based earthquake simulation code, RSQSim. We compare statistical properties of the aftershock sequence (e.g. productivity, maximum aftershock magnitude, Omori decay rate) and the spatiotemporal relationship between stress changes and event locations of the synthetic and observed aftershocks to understand the primary mechanisms that control them.Prepared by LLNL under Contract DE-AC52-07NA27344.