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Tsunamis are rare events involving several complex physical phenomena. Due to this complexity and the relative scarcity of observations, tsunami research makes extensive use of numerical simulations. For seismogenic tsunamis, the source is often modeled as an instantaneous sea‐floor displacement (IS), while the tsunami propagation and inundation is computed through a shallow water approximation (SW). Here, we investigate what is the best tsunami inundation modeling strategy for different realistic earthquake source size and duration. We use 1D earthquake‐tsunami coupled simulations of large M > 8 earthquakes in Tohoku‐like subduction zone to test for which conditions the IS and/or the SW approximations can simulate with enough accuracy the tsunami evolution. We use as a reference a time‐dependent (TD), multi‐layer, non‐hydrostatic (NH) 1D model. Source duration, and size, are based on 1D dynamic rupture simulations with realistic stress drop and rigidity. We show that slow ruptures, generating slip in the shallow part of subduction zones (e.g., tsunami earthquakes), and very large events, with an along‐dip extent comparable with the trench‐coast distance (as occurs for megathrust events) require a TD‐NH modeling, especially for regions with steep coastal bathymetry. Conversely, deeper, higher stress‐drop events can be modeled through an IS‐SW approximation. We finally show that: (a) steeper bathymetries generate larger runups and, (b) a resonant mechanism emerges with runup amplifications associated with larger source size on flatter bathymetries. These results, obtained with 1D modeling, can serve as a guide for the appropriate 2/3D simulation approach for applications ranging from fundamental tsunami science to computational‐intensive hazard assessments. Plain Language Summary: In the last two decades, tsunamis originated by large earthquakes have generated major damage and, according to the World Health Organization, more than 250 k casualties (more than 200 k due to the 2004 Indian Ocean tsunami). Strategies to quantify and mitigate the associated risk are based on numerical simulations of the physical processes regulating the generation, propagation of the waves and subsequent flooding on the coast. These simulations require significant computational resources. To make simulations more affordable, numerous approximations are introduced that need to be tested. In this work, we studied which earthquakes, depending on the speed at which they deform the sea bottom when they trigger a tsunami, and on how big they are, require a more detailed modeling approach, and which ones, instead, might be accurately simulated through approximated approaches. We also show how such findings are related to different bathymetric characteristics near the coast and inland, which may enhance or reduce the tsunami effects. Key Points: Slow and large ruptures (e.g., tsunami earthquakes and megathrust) require a time‐dependent, non‐hydrostatic modelingDeeper, high stress‐drop earthquakes might be modeled through an instantaneous source, shallow water approximationInundation depends on bathymetric features: larger inundations on steeper depth gradients and resonant runup amplifications are observed [ABSTRACT FROM AUTHOR]

In this paper, a new geometric inversion method is proposed for obtaining the elastic parameters of an anisotropic focal regions More precisely, the eigenvalues of a vertical transversely isotropic elasticity tensor of a focal region are inverted up to a constant for a given only one moment tensor with the accuracy depending on the strength of anisotropy. The reason for using only one moment tensor is that although there might occur several earthquakes in the same focal region, the orientation of the sources is similar to each other. Hence, one cannot obtain independent equations from each earthquake in order to use them in the inversion of elastic parameters of the focal region. Moreover, this method can be applied for real‐time inversion once the moment tensor of an earthquake is evaluated. The inversion method relies on the geometric fact that a moment tensor can be written as a linear combination of the eigenvectors of the anisotropic focal region's elasticity tensor. Then, in the inversion, we use the fact that each coefficient of this unique decomposition is proportional to the eigenvalues of the focal region's elasticity tensor. Two approximations are used in this inversion method, in particular for the potency and the source orientations. The strength of anisotropy of the focal region determines how accurate these approximations are, and, hence, it also determines the resolutions of the inverted eigenvalues. Because of the anisotropy of the focal region, the errors in the inversion do depend on orientations of the dip and rake angles but not on the strike angle since the focal region is vertically transversely isotropic. The accuracy of the inversion for the five parameters of vertical transversely isotropic is shown for every 10°‐gridded orientation of the dip and rake angles on the stereographic projection. The results are very promising along some orientations as shown in the figures. The last section of the paper deals with the inversion of eigenvalues, up to a constant, for a given set of moment tensors; not by using only one moment tensor. It turns out that the best fit corresponds to the average of inversions obtained for different orientations of the sources. [ABSTRACT FROM AUTHOR]

Seismic observations of underground nuclear explosions provide crucial data on source yield and depth that cannot easily be estimated from other geophysical methods. However, it is difficult to obtain reliable yield estimates for test sites for which we do not have direct seismic calibration experiments. To obtain source information from uncalibrated sites and paths, local and regional seismic records of six, proximal, declared underground nuclear explosions in North Korea are used to compute spectral ratios of narrow‐band waveform envelopes of body‐wave coda that remove path and site effects to reveal precise, relative source moment. The yields of these explosions are obtained from the observed source ratios by simultaneously fitting the classical source model of Mueller and Murphy (1971), https://doi.org/10.1785/bssa0610061675 to all event pairs. The source model provides an impressive fit to the observations considering that the P phase coda derived source spectral ratios did not require a prior knowledge of the source or regionally calibrated corrections to be applied to the data. However, the observed corner frequencies from S wave coda spectral ratios are lower than the source model predictions, but are well fit by models calculated using the corner frequency consistent with the Fisk conjecture. The results presented here provide novel constraints on the spectral distributions of the energy radiated by the sources of the DPRK test series, and allows for an independent evaluation of existing estimated source model parameters. Plain Language Summary: It is difficult to determine the size and depth of an underground nuclear explosion only using observations of the ground shaking caused by its detonation. Optimally the test site is first calibrated using a "ground truth" explosion with a known depth and size. However, for many sites of interest, such as the location of the six declared North Korean nuclear tests occurring between 2006 and 2017, this information is not available. In order to characterize these uncalibrated underground explosions, we use ground shaking measured by sensors located within ∼1,000 km of the test site to determine "coda spectral ratios." These coda spectral ratios use the measurements of two explosions which were recorded at the same seismic sensor, and both occurred at roughly the same location, to remove the changes in the wave caused by the rock, as the wave traveled through the Earth's crust from the site of the explosion to the location of the sensor. The resulting measurement is an estimate of the amount of energy created by the explosion for each frequency of vibrational wave that radiates out from the explosions. The size of these nuclear tests is then determined by fitting physical models of explosions to the derived data. Key Points: The P wave coda spectral ratios match the predicted source spectral ratios calculated using the Mueller and Murphy (1971), https://doi.org/10.1785/bssa0610061675 source model (MM71)The observed S wave corner frequencies are consistently lower than those predicted by MM71, and provide support for the Fisk conjectureThe coda spectral ratios obtained in this study are used to estimate new independent yield estimates of the North Korean tests [ABSTRACT FROM AUTHOR]

The invariants of the moment tensor such as its norm, eigenvalues and trace are closely related to the physical properties of the seismic source and the focal region, for example, seismic moment, radiation pattern, non‐double‐couple components. In this study, we investigate the relationship between these invariants and the eigenvalues and eigenvectors of the transversely isotropic elasticity tensor of the focal region. More specifically, we study how these invariants change as the source orientations vary with respect to the symmetry axes of the transversely isotropic elasticity tensor, by plotting these invariants on the stereographic net. Fortunately, one can plot them since they are independent of the strike of the fault when the focal region is a vertical transversely isotropic medium. Eigenvalues of the elasticity tensor control the invariants of the moment tensor; for instance, the ratio of the maximum and minimum norms achieved for some orientations of source is equal to the ratio of the two specific eigenvalues of the elasticity tensor. Moreover, the separation of the eigenvectors of the moment tensor from the eigenvectors of the source tensor is related to the deviation of the eigenvalues of the transversely isotropic elasticity tensor from the eigenvalues of the closest isotropic elasticity tensor. It is also found that this deviation is responsible for the percentages of non‐double‐couple components of the resulting moment tensor. This linear algebra point of view makes it easier to understand why and how the structure of the moment tensor changes for different orientations of sources. [ABSTRACT FROM AUTHOR]

A prototype electromagnetic vibrator, referred to here as E‐Vib, was upgraded and developed for broadband hardrock and mineral exploration seismic surveys. We selected the iron oxide mine in Blötberget, central Sweden, for a test site in 2019 for the newly developed E‐Vib because of the availability of earlier seismic datasets (from 2015 to 2016) for verification of its performance for hardrock imaging purposes. The two‐dimensional data acquisition consisted of a fixed geometry with 550 receiver locations spaced at every 5 m, employing both cabled and wireless seismic recorders, along an approximately 2.7 km long profile. The E‐Vib operated at every second receiver station (i.e. 10 m spacing) with a linear sweep of 2–180 Hz and with a peak force of 7 kN. The processing workflow took advantage of the broadband signal generated by the E‐Vib in this challenging hardrock environment with varying ground conditions. The processed seismic section shows a set of reflections associated with the known iron oxide mineralization and a major crosscutting reflection interpreted to be from a fault system likely to be crosscutting the mineralization. The broadband source data acquisition and subsequent processing helped to improve signal quality and resolution in comparison with the earlier workflows and data where a drophammer seismic source was used as the seismic source. These results suggest new possibilities for the E‐Vib source for improved targeting in hardrock geological settings. [ABSTRACT FROM AUTHOR]

The number of gigaton‐sized iceberg‐calving events occurring annually at Greenland glaciers is increasing, part of a larger trend of accelerating mass loss from the Greenland Ice Sheet. Though visual observation of large calving events is rare, ∼60 glacial earthquakes generated by these calving events are currently recorded each year by regional and global seismic stations. An empirical relationship between iceberg size and MCSF, a summary measure of glacial‐earthquake size, was recently demonstrated by Olsen and Nettles (2019), https://doi.org/10.1029/2019JF005054. However, MCSF is known to be sensitive to choices made in modeling the seismic source. We incorporate constraints on the seismic source from laboratory studies of calving and test multiple source time functions using synthetic and observed glacial‐earthquake waveforms. We find that a simple, fixed time function with a shape informed by laboratory results greatly improves estimates of earthquake size. The average ratio of estimated to true peak force values is 1.03 for experiments using our preferred source model, compared with an average of 0.3 for models used in previous studies. We find that maximum‐force values estimated from waveform modeling depend far less on model choices than does MCSF, and therefore prefer maximum force as a measure of glacial‐earthquake size. Using both synthetic and real data, we confirm a correlation between maximum force and iceberg mass. Our results support the possibility of developing useful scaling relationships between seismic observables and physical parameters controlling glacier calving. Plain Language Summary: The Greenland Ice Sheet is losing ice mass. About half of that ice is lost when large icebergs break off, or calve, from the fronts of glaciers into the ocean. Knowing the sizes of these icebergs would be valuable, but iceberg calving is rarely captured on camera. However, the largest icebergs produce seismic signals when they calve, referred to as glacial earthquakes. We investigate the relationship between the size of an iceberg and the magnitude of the glacial earthquake it produces, building new models to describe the forces that generate a glacial earthquake. Previously, most details of the force evolution during iceberg calving were unknown. We use observations from laboratory experiments conducted using a plastic block in a tank of water, built to mimic the glacier‐ocean setting. We find that incorporating information from these laboratory experiments into our seismic model greatly improves estimates of earthquake size. Using our new models, we confirm a correlation between glacial‐earthquake magnitude and iceberg size, and show that our improved estimates are likely to be more realistic. Our results suggest that using seismic information to estimate iceberg size and related quantities is a promising path forward. Key Points: A physics‐based source model for glacial‐earthquake modeling improves recovery of seismic‐magnitude valuesMaximum force is less sensitive to model choices than MCSF and is preferred for describing glacial‐earthquake sizeA rapid force reversal during iceberg calving is the most important feature to capture in a glacial‐earthquake source model [ABSTRACT FROM AUTHOR]

Abstract: We present a probabilistic seismic point source inversion, taking into account 3‐D heterogeneous Earth structure. Our method rests on (1) reciprocity and numerical wavefield simulations in complex media and (2) Hamiltonian Monte Carlo sampling that requires only a small amount of test models to provide reliable uncertainty information on the timing, location, and mechanism of the source. Using spectral element simulations of 3‐D, viscoelastic, anisotropic wave propagation, we precompute receiver side strain tensors in time and space. This enables the fast computation of synthetic seismograms for any hypothetical source within the volume of interest, and thus a Bayesian solution of the inverse problem. To improve efficiency, we developed a variant of Hamiltonian Monte Carlo sampling. Taking advantage of easily computable derivatives, numerical examples indicate that Hamiltonian Monte Carlo can converge to the posterior probability density with orders of magnitude less samples than the derivative‐free Metropolis‐Hastings algorithm, which we use for benchmarking. Exact numbers depend on observational errors and the quality of the prior. We apply our method to the Japanese Islands region where we previously constrained 3‐D structure of the crust and upper mantle using full‐waveform inversion with a minimum period of 15 s. [ABSTRACT FROM AUTHOR]

The Valparaiso 2017 sequence occurred in the Central Chile megathrust, an active zone where the last mega-earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 mainshock, a slow trenchward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater-type seismicity. To characterize the rupture process of the mainshock, we perform a dynamic inversion using the strong-motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semiaxis of 10 km and 5 km, with a subshear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m, and kappa value of 1.98). This earthquake could be the beginning of a long-term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a megathrust earthquake like 1730 is expected. [ABSTRACT FROM AUTHOR]

We investigate the rupture process of the 2016, Mw6.1 Amatrice earthquake, the first shock of a seismic sequence characterized by three damaging earthquakes occurred in central Italy between August and October. We jointly invert strong motion, High‐Rate GPS data, GPS, and DInSAR displacements and we adopt ad hoc velocity profiles of the crust below each station. The retrieved source model reveals a high degree of complexity, characterized by a prominent bilateral rupture with low slip at the hypocenter, two well‐separated slip patches and a rupture front accelerating when breaking the largest patch. The rupture of the main asperity features a slip‐velocity pulse that is impeded ahead of its current direction and splits into two pulses. In this fault section we find clues of structural and rheological control of the rupture propagation due to the fault system segmentation. Plain Language Summary: This work provides relevant information for Seismology community and the results may have important implications for earthquake hazard assessment. Indeed, indication of the role played by the fault segmentation in limiting the size of single earthquake in Central Italy is gained.In particular, we retrieve the source process of the 2016 Amatrice, central Italy, earthquake by jointly invert for a comprehensive dataset (strong motion, HRcGPS, GPS and DInSAR measurements).The main outcome of our study is that the rupture process has been characterized by a strong geometrical and rheological control due to the presence of important segmentation of the thrust fault system, distinctive of the central Apennines. Indeed, our results provide a complete description of the rupture propagation and inhibition due to the presence of inherited tectonic structures, which role in the seismic sequence evolution could be due to the favourable or unfavourable orientation within the active stress regime geometries and rheological variations between fault blocks. We believe that, in this context, our results are of potential impact for a broad readership because the interaction between new and/or inherited faults, strongly influences the fault segmentation and fault lengths, determining the maximum magnitude of individual earthquakes and consequently the seismic hazard. Key Points: Full kinematic rupture process of the 2016 Amatrice earthquake was retrieved from nonlinear joint inversion of Strong Motion, HRcGPS, GPS, and DInSAR dataThe complexity of the rupture history has been investigated by means of a rupture velocity and rupture mode propagation detailed analysisThe rupture process of the 2016 Amatrice earthquake has been characterized by a strong geometrical and rheological control due to the fault system segmentation in the central Apennines [ABSTRACT FROM AUTHOR]