Moment Tensors Of Earthquakes Research Articles

Apparent Non-Double-Couple Components as Artifacts of Moment Tensor Inversion

Compilations of earthquake moment tensors from global and regional catalogs find pervasive non-double-couple (NDC) componentswith a mean deviation from a double-couple (DC) source of around 20%. Their distributions vary only slightly with magnitude, faulting mechanism, or geologic environments. This consistency suggests thatfor most earthquakes, especially smaller ones whose rupture processes are expected to be simpler, the NDC components are largely artifacts of the moment tensor inversion procedure. This possibility is also supported by the fact that NDC components for individual earthquakes with Mw<6.5 are only weakly correlated betweencatalogs. We explore this possibility by generating synthetic seismograms for the double-couple components of earthquakes around theworld using one Earth model and inverting them with a different Earth model. To match the waveforms with a different Earth model, the inversion changes the mechanisms to include a substantial NDC component while largely preserving the fault geometry (DC component). The resulting NDC components have a size and distribution similar to those reported for the earthquakes in the Global Centroid Moment Tensor (GCMT) catalog. The fact that numerical experiments replicate general features of the pervasive NDC components reported in moment tensor catalogs implies that these components are largely artifacts of the inversions not adequately accounting for the effects of laterally varying Earth structure.

Seismic moment tensor classification using elliptical distribution functions on the hypersphere

SUMMARY Discrimination of underground explosions from naturally occurring earthquakes and other anthropogenic sources is one of the fundamental challenges of nuclear explosion monitoring. In an operational setting, the number of events that can be thoroughly investigated by analysts is limited by available resources. The capability to rapidly screen out events that can be robustly identified as not being explosions is, therefore, of great potential benefit. Nevertheless, possible mis-classification of explosions as earthquakes currently limits the use of screening methods for verification of test-ban treaties. Moment tensors provide a physics-based classification tool for the characterization of different seismic sources and have enabled the advent of new techniques for discriminating between earthquakes and explosions. Following normalization and projection of their six-degree vectors onto the hypersphere, existing screening approaches use spherically symmetric metrics to determine whether any new moment tensor may have been an explosion. Here, we show that populations of moment tensors for both earthquakes and explosions are anisotropically distributed on the hypersphere. Distributions possessing elliptical symmetry, such as the scaled von Mises–Fisher distribution, therefore provide a better description of these populations than the existing spherically symmetric models. We describe a method that uses these elliptical distributions in combination with a Bayesian classifier to achieve successful classification rates of 99 per cent for explosions and 98 per cent for earthquakes using existing catalogues of events from the western United States. The 1983 May 5 Crowdie underground nuclear test and 2018 July 20 DAG-1 deep-borehole chemical explosion are the only two explosions out of 140 that are incorrectly classified. Application of the method to the 2006–2017 nuclear tests in the Democratic People’s Republic of Korea yields 100 per cent identification rates and we provide a simple routine MTid for general usage. The approach provides a means to rapidly assess the likelihood of an event being an explosion and can be built into monitoring workflows that rely on simultaneously assessing multiple different discrimination metrics.

Insights on the state of stress in the mantle beneath Pahala, Hawai‘i

Magma supply rates from the mantle to Hawaiian volcanoes serve as an important control on eruptive behavior at the surface. The Pa ̄hala Sill Complex, a collection of magma-bearing, seismogenic structures at 40 km depth beneath Hawai‘i, presents an opportunity to elucidate interactions between stress and magma transport processes in the mantle. We invert for full moment tensors of sill earthquakes and identify predominantly shear mechanisms with persistent tensile faulting components. Slip occurs in-plane with the sill structures. Pressure axes are radially oriented about a point near Mauna Loa, consistent with a stress field generated by a flexural load. Together, these observations suggest that magma flux through the sill structures generates seismicity by increasing pore pressure and promoting slip. Our results suggest that stress changes in mantle structures may enable fluctuations in magma supply rates to the surface over short timescales.

Exploratory Data Analysis of Earthquake Moment Tensor Catalog in Japan Using Non-linear Graph-Based Dimensionality Reduction

The catalog of moment tensor solutions, which contains the information on spatial location, origin time and fault mechanism type of earthquakes, has been interpreted by researchers based on a wealth of past knowledge. However, the long-term routine analyses have led to the accumulation of a huge amount of data in the moment tensor catalog, and it is worth considering moving away from the artisanal approach. In this study, using dimensionality reduction of unsupervised machine learning, we performed exploratory data analysis of the moment tensor catalog in Japan to objectively obtain comprehensive images of seismic activity and to acquire knowledge on the spatial and temporal characteristics of the earthquake mechanism. Source parameters of the moment tensor catalog in Japan, spatial location (latitude, longitude, and focal depth) and source-mechanism diagram information were embedded in two-dimensional space via a non-linear graph-based dimensionality-reduction method, Uniform Manifold Approximation and Projection. On the embedding map, earthquakes in eastern and western Japan are distributed separately and are further embedded to reflect their characteristic fault mechanism and focal depth in each region. The similarity degree of the earthquakes can be obtained as the distance on the embedding map. This study demonstrates that the data visualization using dimensionality reduction is useful for intuitively and objectively understanding the regional characteristics of earthquake mechanisms. The embedding map can also be employed to visualize temporal changes in regional seismic activity and to perform a similarity search with a past event.

When are Non-Double-Couple Components of Seismic Moment Tensors Reliable?

There has been considerable discussion as to how to assess when non-double-couple (NDC) components of seismic moment tensors represent real source processes. We explore this question by comparing moment tensors (MTs) of earthquakes in three global catalogs, which use different inversion procedures. Their NDC components are only weakly correlated between catalogs, suggesting that they are largely artifacts of the inversion. A monotonic decrease in the NDC components' standard deviation with magnitude indicates increased reliability of the NDC components for larger earthquakes. The standard deviation begins to decrease for large NDC components exceeding 60%, suggesting that they represent real source processes. Randomly generated NDC components with the same mean and standard deviation as in the MT catalogs only reproduce some of this decrease. Thus NDC components of large earthquakes and NDC components that exceed 60% are likely to represent real source processes.

The 2015–2017 Pamir earthquake sequence: foreshocks, main shocks and aftershocks, seismotectonics, fault interaction and fluid processes

SUMMARYA sequence of three strong (MW7.2, 6.4, 6.6) earthquakes struck the Pamir of Central Asia in 2015–2017. With a local seismic network, we recorded the succession of the foreshock, main shock and aftershock sequences at local distances with good azimuthal coverage. We located 11 784 seismic events and determined 33 earthquake moment tensors. The seismicity delineates the tectonic structures of the Pamir in unprecedented detail, that is the thrusts that absorb shortening along the Pamir’s thrust front, and the strike-slip and normal faults that dissect the Pamir Plateau into a westward extruding block and a northward advancing block. Ruptures on the kinematically dissimilar faults were activated subsequently from the initial MW 7.2 Sarez event at times and distances that follow a diffusion equation. All main shock areas but the initial one exhibited foreshock activity, which was not modulated by the occurrence of the earlier earthquakes. Modelling of the static Coulomb stress changes indicates that aftershock triggering occurred over distances of ≤90 km on favourably oriented faults. The third event in the sequence, the MW 6.6 Muji earthquake, ruptured despite its repeated stabilization through stress transfer in the order of –10 kPa. To explain the accumulation of MW > 6 earthquakes, we reason that the initial main shock may have increased nearby fault permeability, and facilitated fluid migration into the mature fault zones, eventually triggering the later large earthquakes.

The impact of faulting complexity and type on earthquake rupture dynamics

The statistical properties of seismicity are known to be affected by several factors such as the rheological parameters of rocks. We analysed the earthquake double-couple as a function of the faulting type. Here we show that it impacts the moment tensors of earthquakes: thrust-faulting events are characterized by higher double-couple components with respect to strike-slip- and normal-faulting earthquakes. Our results are coherent with the stress dependence of the scaling exponent of the Gutenberg-Richter law, which is anticorrelated to the double-couple. We suggest that the structural and tectonic control of seismicity may have its origin in the complexity of the seismogenic source marked by the width of the cataclastic damage zone and by the slip of different fault planes during the same seismic event; the sharper and concentrated the slip as along faults, the higher the double-couple. This phenomenon may introduce bias in magnitude estimation, with possible impact on seismic forecasting.

Possible origin of the spatiotemporal evolution of the 2008 earthquake swarm in Northwest-Bohemia, Czech Republic

The phenomenon ‘earthquake swarm’ is well known from numerous locations on Earth but its mechanism and trigger process are still an enigma. The influence of fluids on fluid-driven seismicity is generally well accepted but the temporal behaviour and the migration of activated phases is still an open question. We attempt to answer these open questions by combining the results of research on induced-seismicity with new reliable analyses of full moment tensors (FMTs) of the swarm earthquakes of 2008 to 2018 in Northwest-Bohemia, Czech Republic by Vavryčuk et al. (2017, 2021). The FMTs reveal evidence of compressive fracturing as the prevailing focal mechanism during the swarms. Our proposed hypotheses considered these earthquakes as the origin of subsequent pore pressure pulses in the respective active swarm phase. The diffusion of these pore pressure pulses, which could trigger the nearby earthquakes as well as the successive swarm phase in the vicinity after a respective diffusion process in space and time, are the target of this studies. Missing information regarding the real diffusion distances are compiled by a strict statistical approach of a one-to-one analysis of the swarm earthquakes. The analyses suggest that parts of the swarm earthquakes could be triggered by pore pressure diffusion with an estimated diffusivity range of approximately D = 0.01 to 3.0 m2/s. In exceptional cases, which make about 5% of all cases, we have to consider also diffusivities up to 15 m2/s. Examples of hydraulic diffusivities observed in our region of interest confirm this range. Such a process can also be assumed during further swarms in the years 2011 to 2018 because of the continuous and predominant occurrence of compressive fracturing as a focal mechanism beside pure shear and tensile fracturing. Our results could explain for the first time the missing link regarding the temporal and spatial migration of the subsequent swarm phases in our studied region at Nový Kostel (Northwest-Bohemia, Czech Republic).

Seismic Analysis of the 2020–2021 Santa Fe Seismic Sequence in the Granada Basin, Spain: Relocations and Focal Mechanisms

Abstract The 2020–2021 Santa Fe seismic sequence is the first well-instrumented and widely felt seismic series occurred in the Granada basin during the Spanish instrumental period since the 1979 Fuente Vaqueros seismic series and the 1955–1956 Armilla and Purchil destructive earthquakes. It began in December 2020 with an Mw 3.7 earthquake followed by five Mw>4 earthquakes between 23 and 28 January 2021 and a long sequence of aftershocks during the next four months. Over six months later, on 12 August, another large earthquake of magnitude 4.5 stroke the area to the southwest of Santa Fe city where the previous shocks occurred. This seismic sequence offers a unique opportunity to study the seismic activity in this region—one of the most seismically active zones and with the highest seismic hazard in the Iberian Peninsula. The wide station coverage and good-quality data available allows us to perform high-precision absolute and relative relocations and to obtain the seismic moment tensor (MT) and focal mechanism of the largest earthquakes. Relocation results reveal a clustered distribution of the seismicity between Santa Fe and Pinos Puente faults, focused on a depth range between 2 and 5 km. Hypocenters follow a near-vertical pattern, and Mw>4 events display a northeast–southwest-dipping trend that seems to fit Pinos Puente fault plane. However, regional MT solutions show predominant normal fault mechanisms, with minor oblique component but with strike and dip orientations more compatible to Santa Fe fault. To explain this apparent discrepancy with the known mapped active faults, we propose a change in dip of Santa Fe fault plane from near vertical at shallow depths, bended below the first kilometers, to a possible fault splay in depth compatible with the known geologic structure of the area.

Moment tensor catalogue of earthquakes in West Bohemia from 2008 to 2018

Abstract. We present a unique catalogue of full moment tensors (MTs) of earthquakes with ML between 0.5 and 4.4 that occurred in West Bohemia, Czech Republic, in the period from 2008 to 2018 (Vavryčuk et al., 2022a, b). The MTs were calculated from vertical components of P-wave amplitudes. The MT inversion was based on principal component analysis applied to optimally filtered velocity records of local seismic stations deployed in the West Bohemia area. The minimum number of inverted stations is 15, and the rms between theoretical and observed amplitudes is lower than 0.5. The catalogue is exceptional in several aspects: (1) it represents an extraordinary, extensive dataset of more than 5100 MTs; (2) it covers a long period of seismicity in the studied area, during which several prominent earthquake swarms took place; (3) the locations and retrieved MTs of earthquakes are of a high accuracy. Additionally, we provide three-component records at the West Bohemia (WEBNET) seismic stations, the velocity model in the region, and the technical specification of the stations. The dataset is ideal for being utilized by a large community of researchers for various seismological purposes, e.g. for studies of (1) the migration of foci and the spatiotemporal evolution of seismicity, (2) redistribution of stress during periods of intense seismicity, (3) the interaction of faults, (4) the Coulomb stress along the faults and local stress anomalies connected to fault irregularities, (5) diffusivity of fluids along the activated faults, or (6) the time-dependent seismic risk due to the migration of seismicity in the region. In addition, the dataset is optimum for developing and testing new inversions for MTs and for tectonic stress. Since most of the earthquakes are non-shear, the dataset can contribute to studies of non-double-couple components of MTs and their relation to shear–tensile fracturing and/or seismic anisotropy in the focal zone.

Toward Improving Point‐Source Moment‐Tensor Inference by Incorporating 1D Earth Model's Uncertainty: Implications for the Long Valley Caldera Earthquakes

AbstractTo infer seismic moment tensor (MT) of moderate earthquakes at regional scales, seismologists typically simulate waveforms using available structural models of the Earth to match the observed seismograms. This procedure is known as waveform seismic MT inversion. However, the seismic data are noisy, and the Earth model is inevitably different from the actual structure of the Earth; hence, there is a discrepancy between the predicted and the observed waveforms. This discrepancy arises from the noise in the data and imperfections in theoretical predictions, stemming most significantly from the Earth model. This study introduces structural uncertainty, estimated empirically, and referred to as “theory uncertainty,” as part of the combined covariance matrix. In the synthetic setting, we first show through a series of synthetic experiments that the structural uncertainty plays a critical role in retrieving MT solutions, especially for short‐period waveforms. The method is then benchmarked against the waveforms of non‐double‐couple earthquakes in Long Valley Caldera, California. We confirm the highly isotropic nature of the source in a pilot event but find a non‐negligible CLVD component that was overlooked in past studies ignoring the uncertainty in Earth model. Thus, careful consideration of the Earth model's uncertainty as part of the MT inversion schemes will be necessary for future applications to better understand the complicated physics of earthquake sources.

Estimation of Seismic Moment Tensors Using Variational Inference Machine Learning

AbstractWe present an approach for rapidly estimating full moment tensors of earthquakes and their parameter uncertainties based on short time windows of recorded seismic waveform data by considering deep learning of Bayesian Neural Networks (BNNs). The individual neural networks are trained on synthetic seismic waveform data and corresponding known earthquake moment‐tensor parameters. A monitoring volume has been predefined to form a three‐dimensional grid of locations and to train a BNN for each grid point. Variational inference on several of these networks allows us to consider several sources of error and how they affect the estimated full moment‐tensor parameters and their uncertainties. In particular, we demonstrate how estimated parameter distributions are affected by uncertainties in the earthquake centroid location in space and time as well as in the assumed Earth structure model. We apply our approach as a proof of concept on seismic waveform recordings of aftershocks of the Ridgecrest 2019 earthquake with moment magnitudes ranging from Mw 2.7 to Mw 5.5. Overall, good agreement has been achieved between inferred parameter ensembles and independently estimated parameters using classical methods. Our developed approach is fast and robust, and therefore, suitable for down‐stream analyses that need rapid estimates of the source mechanism for a large number of earthquakes.

The role of crustal models in the dynamics of the India–Eurasia collision zone

SUMMARY We investigate how different crustal models can affect the stress field, velocities and associated deformation in the India–Eurasia collision zone. We calculate deviatoric stresses, which act as deformation indicators, from topographic load distribution and crustal heterogeneities coupled with density driven mantle convection constrained by tomography models. We use three different crustal models, CRUST2.0, CRUST1.0 and LITHO1.0 and observe that these models have different crustal thickness and densities. As a result, gravitational potential energy (GPE) calculated based on these densities and crustal thicknesses differ between these models and so do the associated deviatoric stresses. For GPE only models, LITHO1.0 provides a better constraint on deformation as it yields the least misfit (both orientation and relative magnitude) with the surface observations of strain rates, lithospheric stress, plate motions and earthquake moment tensors. However, when the stresses from GPE are added to those associated with mantle tractions arising from density-driven mantle convection, the coupled models in all cases provide a better fit to surface observations. The N–S tensional stresses predicted by CRUST2.0 in this area get reduced significantly due to addition of large N–S compressional stresses predicted by the tomography models S40RTS and SAW642AN leading to an overall strike-slip regime. On the other hand, the hybrid models, SINGH_S40RTS and SINGH_SAW that are obtained by embedding a regional P-wave model, Singh et al., in global models of S40RTS and SAW642AN, predict much lower compression within this area. These hybrid models provide a better constraint on surface observations when coupled with CRUST1.0 in central Tibet, whereas the combined LITHO1.0 plus mantle traction model provides a better fit in some other areas, but with a degradation of fit in central Tibet.

Features of the Focal Mechanisms of the Strongest Mediterranean Earthquakes

The focal mechanisms of earthquakes in the Mediterranean region that do not match the double couple focal model and that describe the complex source rupture are considered. Analysis is performed by decomposition of the observed seismic moment tensor of the strongest earthquakes into isotropic (ISO), double couple (DC), and compensated linear vector dipole (CLVD) components. There were 121 earth- quakes with magnitudes over 5.5 in the region according to the observations for 2000–2019. Of these, 57 events do not match the double couple model. The CLVD parameters are obtained for these earthquakes, and the physical interpretation of the results of the seismic moment tensor decomposition are proposed. This is the first time such an analysis is conducted. Conclusions about the features of the observed destruction processes and the structure of the medium in the focal zones are made based on the results.

Non‐Double‐Couple Moment Tensors of Earthquakes Calculated Using Empirical Green’s Functions

AbstractWe present a joint inversion for empirical Green’s functions (EGFs) and high‐resolution non‐double‐couple (non‐DC) moment tensors. First, the EGFs are constructed using known moment tensors of earthquakes occurring in a small focal zone. Second, the estimated EGFs are applied to refine the original moment tensors used for constructing the EGFs. Because the EGFs describe the velocity model better than the standard GFs, the refined moment tensors are more accurate. The method is applied to real observations of earthquakes of the 2008 swarm in West Bohemia, Czech Republic, where tiny details in fracturing in the focal zone are revealed. Refined moment tensors indicate fault closing caused by compaction of fault gouge during fracturing process related to fault weakening by fluids in the focal zone. The application of the proposed inversion can improve moment tensors reported in existing local, regional, or global catalogs for areas with a concentrated seismicity.

Moving from 1-D to 3-D velocity model: automated waveform-based earthquake moment tensor inversion in the Los Angeles region

SUMMARYEarthquake focal mechanisms put primary control on the distribution of ground motion, and also bear on the stress state of the crust. Most routine focal mechanism catalogues still use 1-D velocity models in inversions, which may introduce large uncertainties in regions with strong lateral velocity heterogeneities. In this study, we develop an automated waveform-based inversion approach to determine the moment tensors of small-to-medium-sized earthquakes using 3-D velocity models. We apply our approach in the Los Angeles region to produce a new moment tensor catalogue with a completeness of ML ≥ 3.5. The inversions using the Southern California Earthquake Center Community Velocity Model (3D CVM-S4.26) significantly reduces the moment tensor uncertainties, mainly owing to the accuracy of the 3-D velocity model in predicting both the phases and the amplitudes of the observed seismograms. By comparing the full moment tensor solutions obtained using 1-D and 3-D velocity models, we show that the percentages of non-double-couple components decrease dramatically with the usage of 3-D velocity model, suggesting that large fractions of non-double-couple components from 1-D inversions are artifacts caused by unmodelled 3-D velocity structures. The new catalogue also features more accurate focal depths and moment magnitudes. Our highly accurate, efficient and automatic inversion approach can be expanded in other regions, and can be easily implemented in near real-time system.

Filling the gap in a double seismic zone: Intraslab seismicity in Northern Chile

Double seismic zones (DSZs) of intermediate-depth intraslab seismicity are observed in many subduction zones around the globe, and have been related to dehydration reactions in the downgoing crust and mantle lithosphere. These reactions occur at, to first order, constant temperatures, which explains the observed linear arrangements of seismicity that appear to follow isotherms of thermal models.Intermediate-depth seismicity in Northern Chile, however, exhibits a pattern of intraslab seismicity that substantially deviates from a classical DSZ. Whereas two parallel seismicity planes are present in the updip part of the slab, these abruptly change into a 25–30 km thick, homogeneously seismogenic volume at a depth of ∼80–100 km. Seismicity rate and moment release significantly increase in this depth interval. In order to understand which processes evoke this configuration and what distinguishes the Northern Chile subduction zone from more conventional subduction zone settings (e.g. Japan), we performed a detailed seismological investigation of slab seismicity in Northern Chile using data from the IPOC permanent network. We determined >600 moment tensors of intraslab earthquakes, processed and evaluated location uncertainties for 8 years of high-resolution earthquake hypocenter data, and performed statistical analysis of the different seismicity populations.We observe that earthquakes both in the highly active cluster and the DSZ above exhibit consistently downdip extensive source mechanisms that align with the dip angle and direction of the slab. This implies strong slab pull, which is also evident from slab steepening outlined by hypocenters towards the downdip termination of the highly active cluster. Moreover, events in the cluster show a very weak aftershock productivity and a high background event rate, which leads to a temporal distribution of seismicity that is close to a purely random process. We find that the position of the highly seismogenic volume spatially coincides with: 1) the disappearance of the velocity contrast between oceanic crust and the underlying mantle in receiver function images, 2) the transition from the “cold nose” (i.e. the stagnant part) to the hot part of the mantle wedge, as evidenced by seismic attenuation images, and 3) with an increase of the slab dip angle. Based on these different pieces of evidence, we speculate that high tensile stresses and heat input from above could lead to a sudden burst of kinetically delayed metamorphic reactions there that then enables the observed increased seismicity rates. Since these reactions have overall a negative volume change that leads to slab densification and hence further increases slab pull, the spatial pattern of seismicity we observe could result from a runaway-type process, which would explain its abrupt start and high moment release rates.

Revisiting The 2018 Kalibening Earthquake Sequence In Central Java: Call for the Revision of Earthquake Hazard

As moderate devastating earthquake that impacted the economic loss was about $ 1.68 million US, the 2018 Kalibening earthquake quite shocked where the faulting mechanism still unconfirmed. There was no reference indicating the existence of the faulting zone exactly on the earthquake sequence. We take a benefit from seismic temporary network with the density ±2 km, which deployed while a macroseismic survey in purposing to evaluate the 2018 Kalibening earthquake sequence. Earthquake relocation using double difference combined with cross-correlation and determining earthquake moment tensor inversion solution using near source seismograms were applied. We suggested the deformation scheme of this earthquake with a thrust faulting with 307.5 / 28.8 / 118.5 (Strike / Dip / Rake) as a result from mainshock and supported by aftershock moment tensor solutions. This parameter consistent to aftershocks relocation results which formed a lineation trending NW-SE appropriate with Strike = 307.5. The cross-section exhibits aftershocks pattern which elongated deeper and formed a slope from SW to NE approximately fit to Dip = 28.8. The results from investigating the background seismicity in Banjarnegara Region using combined catalog (ISC-USGS-BMKG) compared to BJI (single station) showed the sparse and the lacking of InaTEWS seismic network configurations. Finally impacted to losses in earthquake cataloging, leads the low area coverages around Banjarnegara region. Together with our results, we called for the revision of earthquake hazard assessment for Central Java province, especially in Banjarnegara region.

Role of Large‐Scale Tectonic Forces in Intraplate Earthquakes of Central and Eastern North America

AbstractCentral and eastern United States (CEUS) have experienced large intraplate earthquakes. Yet, at present there is no comprehensive model to explain stresses, strain, and seismicity in this intraplate setting. Models to explain the intraplate stresses in CEUS include glacio‐isostatic adjustment, ridge push effects, local stresses along preexisting fracture zones, and large‐scale convection. In this paper, we present a self‐consistent model of the dynamics of CEUS that explains the stress field responsible for these intraplate earthquakes. The earthquakes represent slow, ongoing deformation associated with forces arising from a combination of lithosphere topography and structure, together with the effects of density‐driven mantle flow. Using GPS data, we calculate strain rates that are likely to arise from tectonic effects and conclude that intraplate strain rates associated with tectonic effects are unlikely to exceed 1 × 10−9 year−1. We test several models of lateral viscosity variations by comparing model stress orientation output with earthquake moment tensors, directions from stress inversion, and P axes of earthquakes. A model that satisfies stress and earthquake constraints and also strain rate magnitude constraints requires high viscosity (1025 Pa·s) craton and old oceanic lithosphere of the western Atlantic block and weaker (5 × 1024 Pa·s) accreted Appalachian terrane. Other strength contrasts within the lithosphere are likely present. Incorporation of these into future models using constraints from seismology, along with refined geodetic measurements and improved estimates of crust and upper mantle densities, is needed to further refine long‐term dynamic models and better evaluate the hazards associated with this very slow, ongoing permanent deformation within the eastern and central United States.

Resolving Horizontal Rupture Directivity of Moderate Crustal Earthquake in Sparse Network With Ambient Noise Location

AbstractEarthquake rupture directivity can remove fault plane ambiguity in earthquake moment tensor solutions; thus, it is helpful to improve reliability of shake map and understanding the seismogenic processes. In this paper, we propose an algorithm to resolve the horizontal rupture directivity of moderate crustal earthquakes in two steps. The first step is to determine the two candidate fault planes from focal mechanism inversion. Then, rupture directivity is estimated via the difference between the hypocenter and the centroid location which is constrained on the candidate fault planes. Accurate centroid location is determined with ambient noise location method, in which the noise cross‐correlation functions (NCFs) provide a stable calibration for the path effects. This technique is applied to the 2011 Oklahoma earthquake, and the estimated centroid location is about 2–4 km southwest to the hypocenter, indicating that the mainshock ruptured along the 234° plane for about 4–8 km. The result is consistent with directivity estimation using a foreshock as reference event and also agrees with early aftershock distribution as well as finite fault inversion. More applications to the 2004 Parkfield earthquake and the 2014 Napa earthquake are performed to explore the effectiveness and limitation of this method. This method is helpful for determining rupture directivity of earthquakes in sparse network but requiring one or two nearby stations. Alternatively, temporary stations can be installed in the area of high seismic hazard, then a library of NCFs and path calibrations would be available for rapidly determining rupture directivity of future earthquakes.