Full Moment Tensor Research Articles

Seismic moment tensors from synthetic rotational and translational ground motion: Green’s functions in 1-D versus 3-D

SUMMARY Seismic moment tensors are an important tool and input variable for many studies in the geosciences. The theory behind the determination of moment tensors is well established. They are routinely and (semi-) automatically calculated on a global scale. However, on regional and local scales, there are still several difficulties hampering the reliable retrieval of the full seismic moment tensor. In an earlier study, we showed that the waveform inversion for seismic moment tensors can benefit significantly when incorporating rotational ground motion in addition to the commonly used translational ground motion. In this study, we test, what is the best processing strategy with respect to the resolvability of the seismic moment tensor components: inverting three-component data with Green’s functions (GFs) based on a 3-D structural model, six-component data with GFs based on a 1-D model, or unleashing the full force of six-component data and GFs based on a 3-D model? As a reference case, we use the inversion based on three-component data and 1-D structure, which has been the most common practice in waveform inversion for moment tensors so far. Building on the same Bayesian approach as in our previous study, we invert synthetic waveforms for two test cases from the Korean Peninsula: one is the 2013 nuclear test of the Democratic People’s Republic of Korea and the other is an Mw 5.4 tectonic event of 2016 in the Republic of Korea using waveform data recorded on stations in Korea, China and Japan. For the Korean Peninsula, a very detailed 3-D velocity model is available. We show that for the tectonic event both, the 3-D structural model and the rotational ground motion, contribute strongly to the improved resolution of the seismic moment tensor. The higher the frequencies used for inversion, the higher is the influence of rotational ground motions. This is an important effect to consider when inverting waveforms from smaller magnitude events. The explosive source benefits more from the 3-D structural model than from the rotational ground motion. Nevertheless, the rotational ground motion can help to better constraint the isotropic part of the source in the higher frequency range.

Strain microseismics: Radiation patterns, synthetics, and moment tensor resolvability with distributed acoustic sensing in isotropic media

We have derived analytical formulations for the strain field produced by a moment tensor source in homogeneous isotropic media. Such formulations are important for microseismic projects that increasingly are monitored with fiber-optic distributed acoustic sensing (DAS) systems. We find that the spatial derivative of displacement produces new terms in strain proportional to [Formula: see text] with [Formula: see text]. In viscoelastic media, the derivative also produces an additional far-field term that is scaled by a frequency-dependent factor. When comparing with full wavefield synthetic data, we observe that the new terms proportional to [Formula: see text] can be considered part of a near-field in strain, similar to the practice with the displacement formulation. Analyses of moment tensor resolvability show that full moment tensors are resolvable with P-wave information from two or more noncoplanar vertical DAS cable geometries if intermediate- and far-field terms are considered and that S-wave information alone cannot constrain full moment tensors using only vertical wells. These results mirror previous observations made with displacement measurements. Furthermore, the addition of the new terms proportional to [Formula: see text] in strain improves the moment tensor resolvability but only in the case of a single vertical array. In the case of a single deviated/horizontal well, we can, in theory, resolve a full moment tensor but a case-by-case analysis is necessary to identify regions of full constraint around the well and the necessary noise conditions to guarantee reliable solutions. Real DAS measurements also are affected by the gauge length and interrogator details. In the case of the gauge length, we observe that this operator does not change the resolvability of the problem but it does affect inversion stability. The results derived here represent theoretical limits or in some cases specific examples. Practical implementations require analyses of conditioning, noise, coupling, and the effect of gauge length on a case-by-case basis.

Application of the shear-tensile source model to acoustic emissions in Westerly granite

Traditionally in seismology and in acoustic emission (AE), full moment tensor (MT) is applied as a default model of the mechanism. We present an alternative - an application of shear tensile crack (STC) source model to AEs generated by uniaxial compression loading of Westerly granite. The advantages of STC over the conventional MT are as follows: (i) contrary to the MT, the STC is physical source since it describes straight and simple fracture modes anticipated inside a loaded sample, namely the shear-slip and both of opening and closing tensile cracks; and (ii) the STC is simpler because it is described by fewer parameters (five instead of six required for an unconstrained MT), an essential feature for stabilizing the inverse problem.Better suitability of STC over MT is demonstrated by three exemplary AEs (tensile, shear, and combined). The obtained results were confirmed using a statistical analysis of 1630 reliably determined source mechanisms. The STC, as compared to the MT, provides smaller confidence regions for orientation and even smaller regions for decomposition parameters. Thus, the STC solution appeared to be substantially more useful than the MT, namely for mechanisms with a high content of non-double-couple (non-DC) component, as it allowed better distinction between tensile and shear AEs.Grain scale cracks of tension and combined source type, localized within the middle-height circumferential portion of the specimen, dominated fracturing. Azimuthal distribution of fault planes was found to be approximately uniform for all three source types. The fault dip increased with increasing content of the non-DC component. The average values were 16°, 21°, and 26° for the tensile, combined and shear source types, respectively. The specimen failed by flaking in areas of a high AE activity. AE locations and failure mechanisms indicate a perfect confinement end-boundary conditions between the tested specimen and the loading platens.

The Bayesian Earthquake Analysis Tool

AbstractThe Bayesian earthquake analysis tool (BEAT) is an open-source Python software to conduct source-parameter estimation studies for crustal deformation events, such as earthquakes and magma intrusions, by employing a Bayesian framework with a flexible problem definition. The software features functionality to calculate Green’s functions for a homogeneous or a layered elastic half-space. Furthermore, algorithm(s) that explore the solution space may be selected from a suite of implemented samplers. If desired, BEAT’s modular architecture allows for easy implementation of additional features, for example, alternative sampling algorithms. We demonstrate the functionality and performance of the package using five earthquake source estimation examples: a full moment-tensor estimation; a double-couple moment-tensor estimation; an estimation for a rectangular finite source; a static finite-fault estimation with variable slip; and a full kinematic finite-fault estimation with variable hypocenter location, rupture velocity, and rupture duration. This software integrates many aspects of source studies and provides an extensive framework for joint use of geodetic and seismic data for nonlinear source- and noise-covariance estimation within layered elastic half-spaces. Furthermore, the software also provides an open platform for further methodological development and for reproducible source studies in the geophysical community.

A Reappraisal of Seismicity Recorded During the 1996 Gjálp Eruption, Iceland, in Light of the 2014–2015 Bárðarbunga–Holuhraun Lateral Dike Intrusion

This study performs a reanalysis of the seismicity recorded during the 1996 Gjalp eruption that occurred at NW Vatnajokull, Iceland. The seismicity was recorded by the temporary HOTSPOT network consisting of 30 three-component broadband stations. In total 301 events were identified between 29 September and 12 October and their phases were manually picked. A velocity model was estimated from P-phase travel times by using VELEST. Events were first located using the algorithm NONLINLOC in order to obtain absolute locations. Precise relative locations were obtained with HYPODD by utilizing catalog and cross-correlation differential travel times. Results show that events clustered first along the SW rim of the Barðarbunga caldera and later along the Gjalp fissure, with most hypocentral depths located between 3 and 8 km. Waveforms of the 10 largest events that followed the Barðarbunga earthquake were inverted in order to obtain moment tensors. For all events we found that the deviatoric moment tensor fits the data better than pure double-couple or full moment tensor solutions. Events along the Barðarbunga caldera exhibited reverse focal mechanisms, while those at the Gjalp fissure exhibited mostly strike-slip faulting. Seismic velocity variations calculated using ambient noise interferometry, point to the possibility that a small subglacial eruption occurred at Barðarbunga before the main earthquake. This removed melt from the magma chamber causing its roof to collapse, and also resulted in the lateral migration of magma towards the Gjalp fissure. The 2014–2015 Barðarbunga–Holuhraun eruption shares common characteristics with the 1996 Gjalp eruption, although the size of the latter was much smaller.

The 2013 and 2017 Ms 5 Seismic Swarms in Jilin, NEChina: Fluid‐Triggered Earthquakes?

AbstractUnderstanding the mechanism of moderate earthquake swarm is important for the time‐dependent hazard assessment. Five Ms ≥ 5.0 earthquakes occurred between October and November 2013 in Qianguo, Jilin, a region with low seismicity in northeast China. After 4 years, two more Ms > 5.0 earthquakes struck Ningjiang, Jilin. To investigate the swarm mechanism and the relationship between these two swarm activities, we determine the full moment tensor solutions for 12 Ms > 3.5 earthquakes, including the seven Ms ≥ 5.0 events. The focal mechanisms show that the 2013 Ms ≥ 5.0 events mostly exhibit thrust compression with significant volume closure components, whereas the 2017 Ms ≥ 5.0 earthquakes illustrate a dominated strike‐slip type. In order to explain such differences, we conduct double‐difference tomography in the study region. Our tomographic results reveal that both swarms are located in the low‐velocity zones and the fault materials exhibit structural variations along the strike, possibly indicating that the structural contrast could be critical for the variations in the focal mechanism and hypocenter migration. The faults provide pathways for the upward intrusion of fluids that control the distribution and migration of the swarm sequences. A long‐term fluid‐rock interaction could lead to the fault materials intensely fractured in the 2013 swarm area but relatively less damaged in the 2017 swarm area. We can conclude that fluids play an important role in triggering the two seismic swarms.

Discriminating a Nuclear Blast from A Natural Seismic Event Using A Full Moment-Tensor Inversion Method

Identification of seismic signals if they are generated by tectonic or volcanic origin or underground nuclear tests is of primary importance for impacts on environmental issues. The recent blast on September 3, 2017 has been in question due to its potential destruction on the global environment. Here, we applied a method of full moment-tensor inversions to analyse broadband waveforms generated by such an explosion. The signals were recorded by a network of 4 regional seismic stations (MAJO, INCN, MDJ, BJT), situated between 360 and 1,100 km away from the explosion epicenter. The results showed that a variance reduction of 72.6%, providing high confidence for all values obtained from the method used: a seismic moment of Mo = 6.25 · 1024 dyne cm, corresponding to a moment magnitude of Mw = 5.8, with a dominant volumetric component (ISO) of 50% relatively compared to a 6.8% double-couple (DC) part and a 43.2% compensated linear vector dipole (CLVD) part, and a centroid-depth of 1.0 km. The relative dominance of the ISO seismic component over the other two components, along with the shallow source extracted from the cross-correlation and source mechanism analysis, suggests that the September 3, 2017 explosion was induced by a suspected nuclear test.

1-D velocity model for the North Korean Peninsula from Rayleigh wave dispersion of ambient noise cross-correlations

Monitoring seismic activity in the north Korean Peninsula (NKP) is important not only for understanding the characteristics of tectonic earthquakes but also for monitoring anthropogenic seismic events. To more effectively investigate seismic properties, reliable seismic velocity models are essential. However, the seismic velocity structures of the region have not been well constrained due to a lack of available seismic data. This study presents 1-D velocity models for both the inland and offshore (western East Sea) of the NKP. We constrained the models based on the results of a Bayesian inversion process using Rayleigh wave dispersion data, which were measured from ambient noise cross-correlations between stations in the southern Korean Peninsula and northeast China. The proposed models were evaluated by performing full moment tensor inversion for the 2013 Democratic People’s Republic of Korea (DPRK) nuclear test. Using the composite model consisting of both inland and offshore models resulted in consistently higher goodness of fit to observed waveforms than previous models. This indicates that seismic monitoring can be improved by using the proposed models, which resolve propagation effects along different paths in the NKP region.

Background seismicity and seismic monitoring in the Lai Chau reservoir area

Reservoir-triggered seismic activity depends not only on the technical characteristics of the future reservoir (filling volume, the height of water column) but also on the seismo-tectonics and the natural seismic processes occurring in the area before construction of an artificial reservoir. Passive seismic monitoring was realised near Lai Chau (Vietnam) before the impoundment started. It allowed exploration of the natural seismicity in the area of the future dam. Locations of seismic events several months prior to the reservoir impoundment were observed with ten stations installed in the reservoir vicinity. Events were mainly located near the dam along the Da river headwaters fault. However, only four stations were available for the entire period before the impoundment. Despite the network limitations, completeness of seismic catalogue and b value were determined and may be used as a baseline for analysis of the seismicity in this area after impoundment. The magnitude completeness level is significantly smaller than in the broader seismogenic zones covering an area of the dam. The b value differs from the results obtained for regional seismicity of Northeastern Vietnam. The capability of the local network for moment tensor inversion was estimated with the use of synthetic data tests. Test results provided the requirements for the station number according to azimuthal coverage of the network to obtain the reliable full moment tensor (MT) solution. Preliminary analysis of the seismic activity after Lai Chau reservoir impoundment indicates some changes in activity related to the impoundment and reservoir exploitation.

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.

Source mechanism of a lower crust earthquake beneath the Himalayas and its possible relation to metamorphism

The nature of deep-crustal, intermediate and deep-focus earthquakes and their relation to metamorphic reactions is a topic of debate. Here we seek to better understand a possible link between the earthquake process and metamorphism by analyzing the mechanism of ongoing deep-crustal earthquakes. We focus on a region in the Himalayas with observed seismicity at depths expected to experience active eclogite-facies metamorphism and dehydration reactions. There are few permanent seismic stations in the region, therefore we use waveform data from a temporary seismic array deployment. We find two earthquakes with magnitude and station coverage adequate for moment tensor inversion. For a given earthquake we estimate its seismic full moment tensors (and magnitude) together with uncertainties using all available waveforms. For the largest earthquake (Mw 3.7) we obtain a best-fitting moment tensor and uncertainties that show a double-couple with a tensional crack component. In the context of geological records that document similar processes, and of laboratory experiments conducted at spatial scales that are 5–6 orders of magnitude smaller, this mechanism may be related to dehydration-driven stress changes triggering slight crack opening, and ambient stresses favoring slip along a fault.

Source Analysis of Post-Blasting Events Recorded in Deep Copper Mine, Poland

Seismic and rock burst hazard is a very important factor that has to be considered in seismically active mining areas. Several different methods are used to decrease seismic activity in mines. Among others, an active prevention method known as destress blasting, is considered the most effective technique. The aim of this work is to find possible influences of such active prevention on observed seismic activity recorded in the established waiting time after blasts. Using technological knowledge and seismic data recorded by an underground seismic network, we estimated several parameters that characterize sources provoked by blasting works and compared them with the parameters obtained for spontaneous, non-provoked mining tremors. According to our studies the source mechanisms of post-blasting seismicity are characterized by the similar non-DC part of the full moment tensor whereas the source mechanisms of spontaneous events are not characterized by any specific features. On the other hand non-provoked seismicity presents lowest values of apparent stress. Also ES to EP ratio suggest some differences between events occurred immediately after blasting (up to 30 s) and the rest dataset. We believe that these parameters could be considered valuable tools supporting established safety time limitations after active prevention.

Focal Mechanism of Mining-Induced Seismicity in Fault Zones: A Case Study of Yongshaba Mine in China

It is essential to investigate focal mechanisms of induced seismicity for understanding the rock fracturing, the failure mode, and the hazard evolution in underground mines. But the conventional methods using empiricism to infer the source mechanisms usually lead to ambiguous results for individual events. An optimized moment tensor inversion method using full waveforms was employed to quantitatively determine the rock fracturing orientation and the type of rupture process. Source parameters including the scalar moment, the moment magnitude, the full moment tensor, and the fault plane solutions were resolved of a seismic sequence in fault zones. Results show that the shear failure events in the fault F1 vicinities have similar focal mechanisms and suggest that the fault F1 is a reverse fault. The resolved strikes and dips are basically constant with the orientation of the fault. The events in the fault F2 vicinities are mainly dominated by shear-tensional failure. But the shear events experienced shear rupture and crack opening simultaneously, resulting in slippages not along the actual fault plane. There are more events in the fault F3 area which are characterized by complicated focal mechanisms. Three of the non-shear events are dominated by compressional failure and related to rock collapse, while the other non-shear events are dominated by tensional failure and related to crack opening. The shear dominated events experienced dual effects of shear failure and compression failure. The resolved fault plane solutions cannot reflect the actual geometry of the fault. It is proved that the moment tensor inversion is able to quantitatively analyze the focal mechanism of mining-induced seismicity in fault zones and it provides beneficial understandings of mining-induced fault slips.

Insights Into Fault‐Magma Interactions in an Early‐Stage Continental Rift From Source Mechanisms and Correlated Volcano‐Tectonic Earthquakes

AbstractStrain in magmatic rifts is accommodated by both faulting and dike intrusion, but little is known of the frequency of dike intrusions in early‐stage rifts. We use a new earthquake data set from a dense temporary seismic array (2013–2014) in the ~7‐Myr‐old Magadi‐Natron‐Manyara section of the East African Rift, which includes the carbonatitic Oldoinyo Lengai volcano that erupted explosively in 2007–2008. Full moment tensor analyses were performed on M > 3.4 earthquakes (0.03‐ to 0.10‐Hz band) that occurred during the intereruptive cycle. We find two opening crack‐type and various non‐double‐couple earthquake source mechanisms and interpret these as fluid‐involved fault rupture. From waveform analysis on the nearest permanent seismic station, we conclude that similar rupture processes probably occur over eruptive and intereruptive cycles. The repeated and dynamically similar fluid‐involved seismicity, along with intrabasinal localization of active deformation, suggests that significant and persistent strain is accommodated by magmatic processes, modulated by tectonic cycles.

A multi-technology analysis of the 2017 North Korean nuclear test

Abstract. On 3 September 2017 official channels of the Democratic People's Republic of Korea announced the successful test of a thermonuclear device. Only seconds to minutes after the alleged nuclear explosion at the Punggye-ri nuclear test site in the mountainous region in the country's northeast at 03:30:02 (UTC), hundreds of seismic stations distributed all around the globe picked up strong and distinct signals associated with an explosion. Different seismological agencies reported body wave magnitudes of well above 6.0, consequently estimating the explosive yield of the device on the order of hundreds of kT TNT equivalent. The 2017 event can therefore be assessed as being multiple times larger in energy than the two preceding North Korean events in January and September 2016. This study provides a multi-technology analysis of the 2017 North Korean event and its aftermath using a wide array of geophysical methods. Seismological investigations locate the event within the test site at a depth of approximately 0.6 km below the surface. The radiation and generation of P- and S-wave energy in the source region are significantly influenced by the topography of the Mt. Mantap massif. Inversions for the full moment tensor of the main event reveal a dominant isotropic component accompanied by significant amounts of double couple and compensated linear vector dipole terms, confirming the explosive character of the event. The analysis of the source mechanism of an aftershock that occurred around 8 min after the test in the direct vicinity suggest a cavity collapse. Measurements at seismic stations of the International Monitoring System result in a body wave magnitude of 6.2, which translates to an yield estimate of around 400 kT TNT equivalent. The explosive yield is possibly overestimated, since topography and depth phases both tend to enhance the peak amplitudes of teleseismic P waves. Interferometric synthetic aperture radar analysis using data from the ALOS-2 satellite reveal strong surface deformations in the epicenter region. Additional multispectral optical data from the Pleiades satellite show clear landslide activity at the test site. The strong surface deformations generated large acoustic pressure peaks, which were observed as infrasound signals with distinctive waveforms even at distances of 401 km. In the aftermath of the 2017 event, atmospheric traces of the fission product 133Xe were detected at various locations in the wider region. While for 133Xe measurements in September 2017, the Punggye-ri test site is disfavored as a source by means of atmospheric transport modeling, detections in October 2017 at the International Monitoring System station RN58 in Russia indicate a potential delayed leakage of 133Xe at the test site from the 2017 North Korean nuclear test.

Investigating Factors Influencing Moment Tensor Inversion of Induced Seismicity in Virtual IoT

The Internet of Things (IoT) for seismicity in underground mine consists of a combination of sensor network hardware and software, which allows the collection of seismic data and data processing for basic event parameters. A seismic moment tensor is one of the essential parameters in evaluating the seismic hazard. The reliability of the resolved seismic moment tensor depends on numerous factors, and it is important to understand the influence of these disturbances. We focused on influencing factors, including the azimuthal coverage, the accuracy of source coordinate, the mismodeling of velocity structure, and the noise contamination. A set of moment tensor inversions using synthetic data simulated for a double couple source and a superimposed source from virtual IoT was carried out to test the effects of different factors. We concluded that the sensitivity of the moment tensor inversion to the azimuthal coverage is closely related to the type of the source. For the shear event, the inversion does not require a good azimuthal coverage, but for the non-pure shear event, at least 90° azimuthal coverage is required to get the reliable solution. The full moment tensor is not sensitive to the location accuracy when the source error is less than 60 m, regardless of the type of source. But the double couple deviation increases with the growing error of the location, especially for the fault-slip-related or the shear rupture-dominated event. The erroneous velocity models show nearly no influence on the full moment tensor inversion results. The fit of amplitude spectra does not require a precise alignment of the observed and the synthetic waveforms and is less dependent on the chosen velocity model. The superimposed source is more sensitive to noise than the pure shear source. Although the noise does not necessarily affect the fault plane solution of the event, it is capable to lead an inaccurate seismic moment tensor and mislead the interpretation of the source type.

Source characteristics of the upper mantle 21 May, 2014 Bay of Bengal earthquake of $${{\varvec{M}}}_{\!{{\varvec{w}}}}$$ M w 5.9

We measure source parameters for the 21 May, 2014 Bay of Bengal earthquake through inversion modeling of S-wave displacement spectra from radial–transverse–vertical (RTZ) components recorded at ten broadband stations in the eastern Indian shield. The average source parameters are estimated using estimates from seven near stations (within epicentral distances \({\le }500\,\hbox {km}\)). The average seismic moment and source radius are determined to be \(1.0{\times }10^{18}\,\hbox {N-m}\) and 829 m, respectively, while average stress drop is found to be 76.5 MPa. The mean corner frequency and moment magnitude are calculated to be \(1.6\pm 0.1\) and \(5.9\pm 0.2\) Hz, respectively. We also estimated mean radiated energy and apparent stress, which are found to be \(6.1{\times }10^{13}\) joules and 1.8 MPa, respectively. We observe that mean \(E_{s}/M_{o}\) estimate of \(5.5{\times }10^{-5}\) is found to be larger than the global average for oceanic strike-slip events. This observation along with large stress drop and apparent stress estimates explains the observed remarkably felt intensity data of the 2014 event. The full waveform moment tensor inversion of the band-passed (0.03–0.12 Hz) broadband displacement data suggests the best fit for the multiple point sources on a plane located at 65 km depth, with a moment magnitude 6.4, and a focal mechanism with strike \(318^{\mathrm{o}}\), dip \(87^{\mathrm{o}}\), and rake \(34^{\mathrm{o}}\).

Magmatic or Not Magmatic? The 2015–2016 Seismic Swarm at the Long-Dormant Jailolo Volcano, West Halmahera, Indonesia

Seismic swarms close to volcanoes often signal the onset of unrest. Establishing whether magma is the culprit and the unrest can be flagged as magmatic may be challenging. Here we analyze the spatio-temporal pattern of a seismic swarm that occurred in November 2015 – February 2016 around Jailolo volcano, a long-dormant and poorly studied volcano located on Halmahera island, North Moluccas, Indonesia. The swarm included four Mw>5 earthquakes and hundreds of events were felt by the population. We relocate the earthquakes using both the Indonesian Seismic Network and single-station location techniques. We find that the earthquakes cluster in a narrow strip, stretching 5 km E–W and 20 km N–S, migrating southward away from Jailolo volcano at ~10 km/d. We investigate the source mechanisms of the largest earthquakes via full moment tensor inversion. The non-double-couple component is around 50%, such that the earthquakes, besides normal faulting, included a relatively large opening component. After a thorough examination of the possible causes of the Jailolo swarm we conclude that a laterally propagating dike of tens of millions of cubic meters is the triggering mechanism for the seismicity. The swarm marks the first documented magmatic unrest at Jailolo. We find that there is a probability >0.1 that the unrest will last for more than two years. This magmatic unrest calls for the classification of Jailolo volcano as active, and for an urgent assessment of the associated volcanic hazard.

The seismic sequence of 30th May–9th June 2016 in the geothermal site of Torre Alfina (central Italy) and related variations in soil gas emissions

In the framework of a medium-enthalpy geothermal exploitation project, seismicity and soil gas emissions have been monitored in the area of Castel Giorgio–Torre Alfina (central Italy) since 2014. A dedicated local seismic network, called ReMoTA, allows deepening of the knowledge of the natural local seismicity in terms of source mechanisms, high-quality event localization and magnitude estimation. From November 2014 to May 2016, ReMoTA recorded 846 seismic events with a magnitude range from Md 0.1 to ML 2.8 and with hypocentres between 4 and 8 km depth. Most of these events occurred in six short lasting clusters. On the 30th May 2016 a Mw 4.3 earthquake occurred near Castel Giorgio, followed by almost 1700 aftershocks. The moment tensor solution for the main shock depicts a WNW-ESE oriented normal fault with small right-lateral strike component. An overview of the epicentral distribution of the different clusters recorded since 2014 highlights that the active tectonic structures are orientated principally along the NE–SW and WNW-ESE directions. The relocation of the 1957 Me 4.9 earthquake suggests that this event occurred in the same fault system as of the 2016 seismic sequence. In the study area, there is only one natural emergence (Solfanare) emitting a CO2 dominated gas, having the same chemical and isotopic composition of the gas contained in a cap at the summit of the geothermal reservoir. Apart from small perturbations, no significant compositional variations were recorded during the 2016 seismic sequence in the gas of the Solfanare vents that was analyzed continuously by an automatic gas-chromatographic station. The diffuse soil CO2 flux is monitored since 2013 in six target areas located around the future production and reinjection wells, in order to assess the level of background natural degassing. In all target areas the maximum value of soil CO2 flux has been recorded during the 2016 seismic sequence. However, values remained relatively low (maximum 112 g ⋅m−2 ⋅d−1) and the values of δ13C of the emitted CO2 (−25.25 to −24.22‰ vs. PDB) indicated a shallow biological origin of the gas (by soil respiration). Only at Solfanare high values of diffuse soil CO2 flux were recorded up to a maximum of 20125 g ⋅m−2 ⋅d−1). All the seismicity of the May–June 2016 sequence is located above the ML4.1 main event and is distributed on small distinct faults (such as at San Lorenzo Nuovo and Acquapendente) and triggered by the main shock. The source mechanism provided by the full moment tensor indicates that rupture processes at depth probably deviate from a pure normal fault. The significant contribution of CLVD and isotropic components suggest a possible opening of fluid cracks below the geothermal reservoir hosted in fractured Mesozoic limestones. In spite of the increase of the CO2 flux, no significant changes have been observed in the chemical and isotopic composition of the Solfanare gas. The seismo-tectonic scenario indicates that the Solfanare fault was not activated. Kinematics and orientation of the activated faults suggest a relationship with the Bolsena caldera collapse.

Estimation of Full Moment Tensors, Including Uncertainties, for Nuclear Explosions, Volcanic Events, and Earthquakes

AbstractA seismic moment tensor is a 3 × 3 symmetric matrix that provides a compact representation of a seismic source. We develop an algorithm to estimate moment tensors and their uncertainties from observed seismic data. For a given event, the algorithm performs a grid search over the six‐dimensional space of moment tensors by generating synthetic waveforms for each moment tensor and then evaluating a misfit function between the observed and synthetic waveforms. “The” moment tensor M0 for the event is then the moment tensor with minimum misfit. To describe the uncertainty associated with M0, we first convert the misfit function to a probability function. The uncertainty, or rather the confidence, is then given by the “confidence curve” , where is the probability that the true moment tensor for the event lies within a certain neighborhood of M that has fractional volume V. The area under the confidence curve provides a single, abbreviated “confidence parameter” for M0. We apply the method to data from events in different regions and tectonic settings: 17 nuclear explosions and 12 earthquakes at the Nevada Test Site, 63 small (Mw<2.5) events at Uturuncu volcano in Bolivia, and 21 moderate (Mw>4) earthquakes in the southern Alaska subduction zone. Characterization of moment tensor uncertainties puts us in better position to discriminate among moment tensor source types and to assign physical processes to the events.