Local seismotectonic analysis of the July 2019 Molucca Sea earthquake sequence based on moment tensor solutions

We investigate the local seismotectonic of the Molucca Sea area using moment tensor calculations for the earthquakes that occurred in July 2019 at a depth of 10–55 km. The mainshock of Mw 6.8 occurred on July 7, followed by aftershocks until July 18, with magnitudes ranging from Mw 4.6 to Mw 5.8. Moment tensor solutions are calculated by applying Isolated Asperities (ISOLA) software using the full waveform data recorded at regional seismic stations. The analyzed frequency bands used in this study are 0.01–0.03 Hz and 0.04–0.05 Hz for the event with Mw ≥ 5 and Mw < 5, respectively. We provide validations of new moment tensor solutions for Mw < 5 events in the Molucca Sea region for the period during the earthquake sequence. The results show that thrust and oblique faults are dominant during this event, which indicate a compressional stress of divergent double subduction (DDS) of the Sangihe and Halmahera arcs. Only one full moment tensor solution reveals the normal fault mechanism, which may indicate the manifestation of strain release of compressional stress in the surrounding area. Furthermore, these results also support the previous studies suggesting that the Talaud-Mayu Ridge located in the middle of the Molucca Sea has developed as a consequence of the transpressional tectonic activity.

Full Moment Tensor Solutions of U.S. Underground Nuclear Tests for Event Screening and Yield Estimation

ABSTRACT Moment tensor (MT) solutions are proving increasingly valuable in explosion monitoring, especially now that they are more routinely calculated for the unconstrained, full (six component) MT. In this study, we have calculated MTs for U.S. underground nuclear tests conducted at the Nevada National Security Site using seismic recordings primarily from the Livermore Nevada Network. We are able to determine them for 130 nuclear explosions from 1970 to 1996 for a range of yields and under a variety of material conditions, which we have supplemented with 10 additional chemical explosions at the test site. The result is an extensive database of MTs that can be used to assess the performance of important monitoring tasks such as event identification and yield determination. We test the explosion event screening on the fundamental lune of the MT eigensphere and find MT screening to be a robust discriminant between earthquakes and explosions. We then explore the estimation of moment-derived yield, in which we find that material properties are the largest contributor to differences in the estimated moment-to-yield ratio. Further research conducted on this dataset can be used to develop, test, and improve various explosion monitoring methodologies.

gCAPjoint, A Software Package for Full Moment Tensor Inversion of Moderately Strong Earthquakes with Local and Teleseismic Waveforms

Earthquake moment tensors and focal depths are crucial to assessing seismic hazards and studying active tectonic and volcanic processes. Although less powerful than strong earthquakes (M 7+), moderately strong earthquakes (M 5–6.5) occur more frequently and extensively, which can cause severe damages in populated areas. The inversion of moment tensors is usually affected by insufficient local waveform data (epicentral distance &lt;5°) in sparse seismic networks. It would be necessary to combine local and teleseismic data (epicentral distance 30°–90°) for a joint inversion. In this study, we present the generalized cut-and-paste joint (gCAPjoint) algorithm to jointly invert full moment tensor and centroid depth with local and teleseismic broadband waveforms. To demonstrate the effectiveness and explore the limitations of this algorithm, we perform case studies on three earthquakes with different tectonic settings and source properties. Comparison of our results with global centroid moment tensor and other catalog solutions illustrates that both non-double-couple compositions of the focal mechanisms and centroid depths can be reliably recovered for very shallow (&lt;10 km) earthquakes and intermediate-depth events with this software package.

Seismic source inversion using Hamiltonian Monte Carlo and a 3-D Earth model in the Japanese Islands

&lt;p&gt;We present a proof-of-concept catalogue of full-waveform seismic source solutions for the Japanese Islands area. Our method is based on the Bayesian inference of source parameters and a tomographically derived heterogeneous Earth model, used to compute Green&amp;#8217;s strain tensors. We infer the full moment tensor, location and centroid time of the seismic events in the study area.&lt;/p&gt;&lt;p&gt;To compute spatial derivatives of Green&amp;#8217;s functions, we use a previously derived regional Earth model (Simut&amp;#279; et al., 2016). The model is radially anisotropic, visco-elastic, and fully heterogeneous. It was constructed using full waveforms in the period band of 15&amp;#8211;80 s.&lt;/p&gt;&lt;p&gt;Green&amp;#8217;s strains are computed numerically with the spectral-element solver SES3D (Gokhberg &amp; Fichtner, 2016). We exploit reciprocity, and by treating seismic stations as virtual sources we compute and store the wavefield across the domain. This gives us a strain database for all potential source-receiver pairs. We store the wavefield for more than 50 F-net broadband stations (www.fnet.bosai.go.jp). By assuming an impulse response as the source time function, the displacements are then promptly obtained by linear combination of the pre-computed strains scaled by the moment tensor elements.&lt;/p&gt;&lt;p&gt;With a feasible number of model parameters and the fast forward problem we infer the unknowns in a Bayesian framework. The fully probabilistic approach allows us to obtain uncertainty information as well as inter-parameter trade-offs. The sampling is performed with a variant of the Hamiltonian Monte Carlo algorithm, which we developed previously (Fichtner and Simut&amp;#279;, 2017). We apply an L2 misfit on waveform data, and we work in the period band of 15&amp;#8211;80 s.&lt;/p&gt;&lt;p&gt;We jointly infer three location parameters, timing and moment tensor components. We present two sets of source solutions: 1) full moment tensor solutions, where the trace is free to vary away from zero, and 2) moment tensor solutions with the isotropic part constrained to be zero. In particular, we study events with significant non-double-couple component. Preliminary results of ~Mw 5 shallow to intermediate depth events indicate that proper incorporation of 3-D Earth structure results in solutions becoming more double-couple like. We also find that improving the Global CMT solutions in terms of waveform fit requires a very good 3-D Earth model and is not trivial.&lt;/p&gt;

An MCMC Bayesian full moment tensor inversion constrained by first-motion polarities and double couple percent

Monte Carlo Markov chain (MCMC) samplings can obtain a set of samples by directed random walk, mapping the posterior probability density of the model parameters in Bayesian framework. We perform earthquake waveform inversion to retrieve focal angles or the elements of moment tensor and source location using a Bayesian MCMC method with the constraints of first-motion polarities and double couple percentage using full Green functions and data covariance matrix. The algorithm tests the compatibility with polarities and also checks the double couple percentage of every site before the time-consuming synthetic seismogram computation for every sample of moment tensor of every trial source position. Other than large earthquakes, the method is especially suitable for weak events (M