Compensated Linear Vector Dipole Research Articles

Time-Varying Moment-Tensor Analysis with Application to Buried Chemical Explosions

Abstract The Source Physics Experiment (SPE) Phase I consisted of a series of over-buried, horizontally colocated chemical explosions at the Nevada National Security Site. Seismic waveforms from these explosions recorded at near-source accelerometers, local geophone arrays, and regional seismic stations provided a rich suite of observations suitable for resolving fine source details. To investigate the time-varying source history of the explosions, we used the frequency-domain moment-tensor inversion method described in Yang et al. (2018) with added regularization and reconstruction to suppress the nonuniqueness evident in unconstrained inversion results. The inverted moment-rate spectra are accurate within the response band of the local geophones and, in all cases, display predominately isotropic characteristics. For SPE-4Prime, SPE-5, and SPE-6, we resolve predominately isotropic moment release followed by double couple and compensated linear vector dipole (CLVD) release later in the time-varying source history. We interpret these results both in terms of absolute depth and scaled depth of burial. The apparent non-isotropic release from SPE-4Prime and SPE-5 may simply reflect increased resolving power related to improved Earth model accuracy at greater absolute depths, whereas the non-isotropic release from SPE-6 likely reflects the larger damage associated with an event at a shallower scaled depth. These results provide insight into the time-varying source characteristics of shallow explosions and motivation to study shear-wave generation by inverting for fracture, spallation, induced slip, and other temporally delayed source processes through time-varying methods.

Seismic moment tensor inversion with theory errors from 2-D Earth structure: implications for the 2009–2017 DPRK nuclear blasts

SUMMARY Determining the seismic moment tensor (MT) from the observed waveforms with available Earth's structure models is known as seismic waveform MT inversion. It remains challenging for small to moderate-size earthquakes at regional scales. First, because shallow isotropic (ISO) and compensated linear vector dipole (CLVD) components of MT radiate similar long-period waveforms at regional distances, an intrinsic ISO-CVLD ambiguity impedes resolving seismic sources at shallow depths within the Earth's crust. Secondly, regional scales usually bear 3-D structures; thus, inaccurate Earth's structure models can cause unreliable MT solutions but are rarely considered a theory error in the MT inversion. So far, only the error of the 1-D earth model (1-D structural error), apart from data errors, has been explicitly modelled in the source studies because of relatively inexpensive computation. Here, we utilize a hierarchical Bayesian MT inversion to address the above problems. Our approach takes advantage of affine-invariant ensemble samplers to explore the ISO-CLVD trade-off space thoroughly and effectively. Station-specific time-shifts are also searched for as free parameters to treat the structural errors along specific source–station paths (2-D structural errors). Synthetic experiments demonstrate the method's advantage in resolving the dominating ISO components. The explosive events conducted by the Democratic People's Republic of Korea (DPRK) are well-studied, and we use them to demonstrate highly similar source mechanisms, including dominating ISO and significant CLVD components. The recovered station-specific time-shifts from the blasts present a consistent pattern, which provides a better understanding of the azimuthal variation of Earth's 2-D structures surrounding the events’ location.

Bayesian regional moment tensor from ocean bottom seismograms recorded in the Lesser Antilles: implications for regional stress field

SUMMARY Seismic activity in the Lesser Antilles (LA) is characterized by strong regional variability along the arc reflecting the complex subduction setting and history. Although routine seismicity monitoring can rely on an increasing number of island stations, the island-arc setting means that high-resolution monitoring and detailed studies of fault structures require a network of ocean bottom seismometers (OBS). As part of the 2016–2017 Volatile recycling at the Lesser Antilles arc (VoiLA) project, we deployed 34 OBS stations in the forearc and backarc. During the deployment time, 381 events were recorded within the subduction zone. In this paper, we perform full-waveform regional moment tensor (RMT) inversions, to gain insight into the stress distribution along the arc and at depth. We developed a novel inversion approach, AmΦB—‘Amphibious Bayesian’, taking into account uncertainties associated with OBS deployments. Particularly, the orientation of horizontal components (alignment uncertainty) and the high noise level on them due to ocean microseisms are accounted for using AmΦB. The inversion is conducted using a direct, uniform importance sampling of the fault parameters within a multidimensional tree structure: the uniXtree-sampling algorithm. We show that the alignment of the horizontal OBS components, particularly in high noise level marine environments, influences the obtained source mechanism when using standard least-squares (L2) RMT inversion schemes, resulting in systematic errors in the recovered focal mechanisms including high artificial compensated linear vector dipole (CLVD) contributions. Our Bayesian formulation in AmΦB reduces these CLVD components by nearly 60 per cent and the aberration of the focal geometry as measured by the Kagan angle by around 40 per cent relative to a standard L2 inversion. Subsequently, we use AmΦB-RMT to obtain 45 (Mw > 3.8) regional MT solutions, out of which 39 are new to any existing database. Combining our new results with existing solutions, we subsequently analyse a total of 151 solutions in a focal mechanism classification (FMC) diagram and map them to the regional tectonic setting. We also use our newly compiled RMT database to perform stress tensor inversions along the LA subduction zone. On the plate interface, we observe the typical compressional stress regime of a subduction zone and find evidence for upper-plate strike slip and normal fault behaviour in the north that becomes a near arc-perpendicular extensional stress regime towards the south. A dominant slab perpendicular extensional stress regime is found in the slab at 100–200 km beneath the central part of the arc. We interpret this stress condition to be a result of slab pull varying along the arc due to partial slab detachment along previously hypothesized lateral slab tear near Grenada, at the southern end of the LA arc, leading to reactivation of pre-existing structures around the subducted Proto-Caribbean ridge.

Seismic Sources in Stress‐Induced Anisotropic Media

Rocks in the upper mantle are extensively subjected to initial stress. The rock exhibits elastic anisotropy under the deviatoric stress, which affects the seismic response of underground structures. This paper aims to study the effect of stress-induced anisotropy on the seismic source. The seismic moment tensor generated by a shear faulting in a homogeneous isotropic medium with initial stress is derived by using the motion description in the intermediate configuration, the constitutive relation of third-order elasticity, and the boundary condition considering the effect of initial stress. Then the seismic moment tensor is decomposed to investigate seismic source characters quantitatively. Our results show that shear faulting in a stress-induced anisotropic medium can produce significant non-double-couple (non-DC) mechanisms, including compensated linear vector dipole (CLVD) and isotropic (ISO) components. ISO and CLVD components vary linearly with the initial shear stress on faults. In addition, stress-induced anisotropy causes the double-couple (DC) to deviate from the plane defined by the fault normal and slip direction. The initial stress not only affects the source intensity but also affects the propagation of seismic waves. The radiation patterns of longitudinal waves have non-uniform lobes, which shows the characteristic of anisotropy. Stress-induced anisotropic parameters are smaller than intrinsic anisotropy at high-stress levels and a nearly linear function of the increased initial stress. These results underscore the importance of considering stress-induced anisotropy in the inversion of focal mechanisms and provide a new perspective for monitoring underground stress.

Numerical simulation of seismic waves in 3-D orthorhombic poroelastic medium with microseismic source implementation

SUMMARY We have carried out microseismic forward modelling in orthorhombic poroelastic medium. The seismic response to isotropic (ISO) source, double couple (DC) source and compensated linear vector dipole (CLVD) source was solved by finite-difference algorithm in time domain, respectively. We used analytical solution to test our numerical algorithm and find that the match between the analytical solutions and numerical solutions is sufficiently close. We then designed several different models to calculate the seismic response and analyse the effect of medium parameters and source mechanism on the propagation of seismic waves. We observed from the modelling results that ISO source excites two kinds of waves in isotropic medium, which are fast longitudinal wave and slow longitudinal wave, while the DC and CLVD sources excite three kinds of waves in isotropic medium, including a transverse wave in addition to the fast and slow longitudinal waves. All of these three kinds of sources generate four kinds of seismic waves in orthorhombic poroelastic medium. These are two separable transverse waves in addition to the fast and slow longitudinal waves. The fluid viscosity and medium tortuosity has an effect on the propagation of the slow longitudinal wave. The slow longitudinal wave appears in propagating mode under the effect of low fluid viscosity and appears at the source location in a static mode under the effect of high fluid viscosity. The wavefield snapshot of the slow longitudinal wave has a circular shape when the tortuosity is isotropic, whereas it has an oval shape when the tortuosity is anisotropic. As far as the anisotropic parameters are concerned, the anisotropy of the fast longitudinal wave is more sensitive to the value of $\varepsilon ( {{\varepsilon _1},{\varepsilon _2}} )$, while the value of ${\rm{\delta }}( {{{\rm{\delta }}_1},{{\rm{\delta }}_2},{{\rm{\delta }}_3}} )$ has more effect on the anisotropic behaviour of the two separable transverse waves.

Seismic Moment Tensor Inversion of an Induced Microseismic Event, Offshore Norway: An Insight into the Possible Cause of Wellbore Liner Failure during a Drilling Operation

Abstract A microseismic event with Mw∼0.8 was recorded at the Grane oilfield, offshore Norway, in June 2015. This event is believed to be associated with a failure of the wellbore liner in well 25/11-G-8 A. The failure mechanism has been difficult to explain from drilling parameters and operational logs alone. In this study, we analyzed the detected microseismic event to shed light on the possible cause of this event. We inverted for the seismic moment tensor, analyzed the S/P amplitude ratio and radiation pattern of seismic waves, and then correlated the microseismic data with the drilling reports. The inverted seismic moment indicates a shear-tensile (dislocation) event with a strong positive isotropic component (67% of total energy) accompanied by a positive compensated linear vector dipole (CLVD) and a reverse double-couple (DC) component. Drilling logs show a strong correlation between high pump pressure and the occurrence of several microseismic events during the drilling of the well. The strongest microseismic event (Mw∼0.8) occurred during peak pump pressure of 277 bar. The application of high pump pressure was associated with an attempt to release the liner hanger running tool (RT) in the well, which had been obstructed. Improper setting of the liner hanger could have caused the forces from the RT release to be transferred to the liner and might have resulted in ripping and parting of the pipe. The possible direct impact of the ripped liner with the formation or the likely sudden hydraulic pressure exposure to the formation caused by the liner ripping may explain the estimated isotropic component in the moment tensor inversion in the well. This impact can promote slip along the pre-existing fractures (the DC component). The presence of gas in the formation or the funneled fluid to the formation caused by the liner ripping may explain the CLVD component.

The 2014–2015 complex collapse of the Bárðarbunga caldera, Iceland, revealed by seismic moment tensors

Between August 2014 and February 2015, a caldera collapse process in Bárðarbunga volcano accompanied a fissure eruption at the Holuhraun lava field after magma had propagated laterally 45 km from the Bárðarbunga magma chamber to the lava field. The Icelandic Meteorological Office (IMO) reported around 30,000 earthquakes at the caldera, including more than 70 earthquakes with moment magnitude MW ≥ 5.0. We built a moment tensor catalog of around 230 seismic events at the caldera with MW between 3.7 and 5.5 using a waveform inversion method. We identified five event families based on focal mechanisms, double couple (DC) and compensated linear vector dipole (CLVD) components, epicentral distribution relative to the caldera center, and time of occurrence. The main characteristics of these families are (1) DC to CLVD normal faulting events with steep (50°–70°) to near-vertical (>70°) dip angles on the northern side of the caldera; (2) DC to CLVD normal events with predominant steep (50°–70°) dip angles on the southern side of the caldera; (3) intra-caldera oblique strike-slip events; and (4) thrust events at the eastern side of the caldera with a horizontal tension axis compensated linear vector dipole (T-CLVD) component. Each family provided new insights about the Bárðarbunga collapse mechanism such as (1) a systematic increase in the CLVD component with magnitude, corroborating that the curvature in the ring fault is the likely cause for the vertical CLVD moment tensors observed for the largest events; (2) seismic evidence of near-vertical dip angles in agreement with the vertical faults observed in many calderas worldwide, and numerical and analog models of caldera collapse structures; (3) intra-caldera oblique and strike-slip events on the western side of the caldera occurred by asymmetric collapse; (4) thrust horizontal T-CLVD events observed at the last stage of the eruption, possibly related to a viscoelastic response.

Seismic radiation analyses in anisotropic media based on general dislocation source model

AbstractAnisotropy affects the focal mechanism and makes it complicated. A shear motion generates a pure double-couple (DC) source in isotropic media. While in anisotropic media, it will produce non-DC components, which contain isotropic (ISO) and compensated linear vector dipole (CLVD) components. Besides, coupled with the diversity of fault motion, the source may become extremely complicated. In this paper, the seismic moment tensor is obtained based on the dislocation model, and then a variety of analyses are performed with the moment tensor, including moment tensor decomposition, radiation pattern, radiated energy ratio and seismic propagation characteristics. Since the anisotropy of the medium also influences seismic wave propagation, a hypothesis is made that the source region is minimal and anisotropic, but the propagation path is isotropic. The research gives some interesting conclusions. It is found that the anisotropy mainly affects the focal mechanism under low slope angle while high slope angle has little effect on the polarity. In terms of the moment tensor decomposition, if only one of ISO or CLVD exists, it can be asserted that the source region is anisotropic because ISO components are accompanied by CLVD components in isotropy media. As for the DC component, the results indicate it is one of the most important factors for determining the ratio of radiant energy. This paper presents some valuable findings of the focal mechanism of the general dislocation source under anisotropy, which helps to recognise the source characteristics of the earthquake and build solid foundations for the subsequent inversion of the focal mechanism.

Quantitative Acoustic Emissions Source Mechanisms Analysis of Soft and Competent Rocks through Micromechanics-Seismicity Coupled Modeling

This research studied seismic source mechanisms of acoustic emission (AE) events generated during the failure of intact and jointed rock samples using a microscale mechanical-seismicity coupled microscale model. During rock failure, forces and displacements around microcracks were measured in the model to determine seismic moment tensor. Interpretation of AE data was carried out by decomposing the moment tensor into a double couple (DC), a compensated linear vector dipole (CLVD), and isotropic component (ISO). In this study, microscale numerical models of Lac du Bonnet (LDB) granite (intact and jointed) and a Berea sandstone sample were built to represent the mechanical behavior of hard and soft rocks. Subsequently, several numerical triaxial tests under different confining pressures were conducted to analyze source mechanisms of AE events during rock failure. Numerical analysis shows the significant contribution of positive non-DC component during the LDB granite rock failure and significant DC component during the Berea rock failure. The source mechanisms of AE move to the negative non-DC component at higher confining for both granite and sandstone samples. Joint failure causes non-DC component toward crack opening at low confining pressure and a non-DC component toward crack closure events under higher confining pressure. Simulation results, presented in this work, show how micromechanical properties of rocks and joints as well as stress conditions give rise to different types of AE source mechanisms during rock failure.

Effects of Secondary Sources of Underground Nuclear Explosions on the mb : Ms Criterion and Implications for Discrimination of the DPRK’s Nuclear Tests

ABSTRACTThe mb : Ms (mb vs. Ms) relationship is an important criterion for screening explosions from earthquakes and has been widely adopted in seismological monitoring by the Comprehensive Nuclear-Test-Ban Treaty Organization. In general, the earthquakes have larger Ms than the underground explosions with equivalent mb. However, it has been reported that this recognition criterion failed to identify some explosions at the North Korea nuclear test site. In this study, we investigate the potential effects of secondary source components, including the compensated linear vector dipole (CLVD) and double-couple (DC) sources, on mb and Ms magnitude measurements and the physical mechanism of the mb : Ms recognition criterion by calculating synthetic seismograms. The results show an apparent critical body-wave magnitude of 5 when using the mb : Ms method to discriminate North Korean underground nuclear explosions. The Ms measurements decrease as the CLVD components increase, whereas the effects from the DC source can be neglected. Small events, such as the first five North Korean nuclear tests, generate weak CLVD components, leading to the failure of mb : Ms-based discrimination, whereas the last event, with a larger magnitude, caused extensive damage and hence can be successfully discriminated. In addition, the large difference between the source spectrum of explosions and those of earthquakes might be another important factor in the successful mb : Ms-based discrimination of the sixth North Korean nuclear test.

Generalized orthonormal moment tensor decomposition and its source-type diagram

Moment tensor decomposition is a method for deriving the isotropic (ISO), double-couple (DC), and compensated linear vector dipole (CLVD) components from a seismic moment tensor. Currently, there are two families of methods, namely, standard moment tensor decomposition and Euclidean moment tensor decomposition. Although both methods can usually provide workable solutions, there are some minor inconsistencies between the two methods: an equality inconsistency that occurs in standard moment tensor decomposition and the pure CLVD unity and flip basis inconsistency encountered in Euclidean moment tensor decomposition. Moreover, there is a sign problem when disentangling the CLVD component from a DC-dominated case. To address these minor inconsistencies, we propose a new moment tensor decomposition method inspired by both previous methods. The new method can not only avoid all these minor inconsistencies but also withstand deviations in ISO- or CLVD-dominated cases when using source-type diagrams.

Source Process for Two Enigmatic Repeating Vertical‐T CLVD Tsunami Earthquakes in the Kermadec Ridge

AbstractTwo repeating compensated linear vector dipole (CLVD) earthquakes occurred in 2009 and 2017 on the Kermadec Ridge near the Curtis submarine volcano. The tsunami and seismic waveforms of both events are almost identical. Simulated tsunami and seismic waveforms are compared with observations of the 2017 event to estimate the location and geometry of the source. A CLVD source model at 8–14 km depth is consistent with the global CMT solution but generates no tsunami. The tsunami waveforms can be well reproduced if the source model is about 1.5 km deep. However, a seismic source at this depth is not consistent with the observed seismic waves. This suggests that two sources were involved with different depths. The main, shallow, tsunami source could be due to hydrofracturing of heated water in a shallow sediment layer triggered by a deeper earthquake.

Source mechanism identification using regional waveform inversion approach, case study: July 7, 2019 Molucca Sea earthquake

Molucca Sea is a seismically active area in eastern Indonesia. An earthquake occurred near to Ternate City, Province of North Maluku (M6.8: depth 29 km) on July 7, 2019. To investigate the detail about the mechanism of the earthquake, we analyzed the moment tensor of the earthquake by applying the regional waveform inversion. We used three components waveform broadband data from 18 station of IA-net seismic network in this study. We carried out the deviatoric mode to determine the double couple and compensated linear vector dipole (CLVD) component of the earthquake. The position and origin time of the earthquake were calculated by a space-time grid search in vertical and lateral positions. The frequency band of 0.01 – 0.023 Hz is used in the inversion process to reduce the instrument low-frequency disturbance and the effect of inaccurate velocity model for the synthetic seismogram. The moment tensor inversion result shows that the source mechanism of the earthquake is transpressional fault. This result agrees well with the tectonic setting of the study area.

Non-Double-Couple Components of the Moment Tensor in a Transversely Isotropic Medium

ABSTRACTThe non-double-couple (non-DC) components of the moment tensor provide insight into the earthquake processes and anisotropy of the near-source region. We investigate the behavior of the isotropic (ISO) and compensated linear vector dipole (CLVD) components of the moment tensor for shear faulting in a transversely ISO medium with an arbitrarily oriented symmetry axis. Analytic formulas for ISO and CLVD depend on the orientation of the fault relative to the anisotropy symmetry axis as well as three anisotropic parameters, which describe deviations of the medium from isotropy. Numerical experiments are presented for the preliminary reference Earth model. Both ISO and CLVD components are zero when the axis of symmetry is within the fault plane or the auxiliary plane. For any orientation in which the ISO component is zero, the CLVD component is also zero, but the opposite is not always true (e.g., for strong anisotropy). The relative signs of the non-DC components of neighboring earthquakes may help distinguish source processes from source-region anisotropy. We prove that an inversion of ISO and CLVD components of a set of earthquakes with different focal mechanisms can uniquely determine the orientation and strength of anisotropy. This study highlights the importance of the ISO component for constraining deep slab anisotropy and demonstrates that it cannot be neglected.

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.

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.

Seismic detection of strong ground motions by MW5.6 North Korean nuclear explosion

The North Korean nuclear explosion test site in Punggye-ri is located in a seismically quiescent region on a stable Precambrian basement. The 3 September 2017 MW5.6 North Korean underground nuclear explosion (UNE) test produced unprecedented strong ground motions. The peak ground accelerations might reach tens to hundreds m/s2 on the surface of the UNE test site, decaying exponentially with distance. Ten shallow events with magnitudes greater than or equal to ML2.5 and source depths less than 3 km followed the 2017 UNE for 5 months in an area with a radius of 15 km from the UNE where strong ground shaking was experienced. The largest event with MW3.7 occurred 20 days after the 2017 UNE test at shallow depths less than 3 km. Its moment tensor solution indicates a combined source behavior with comparable strengths of double-couple and compensated linear vector dipole (CLVD) components, suggesting an unusual event different from typical natural earthquakes in the Korean Peninsula. The clustered shallow seismic events appeared to have occurred in damaged media that were effectively perturbed by the strong ground motions of the UNE.

Seismic Detection of a Magma Reservoir beneath Turtle Island of Taiwan by S-Wave Shadows and Reflections

Although surface geology, eruption information and clustering seismicity all suggest Turtle Island (Kueishantao) of northern Taiwan is an active volcano, there was no direct evidence to conclude that magma reservoirs exist beneath it. Even less evidence is available to determine their spatial configuration. If the magma reservoirs are filled by liquids and melt, S-waves are totally reflected and leave behind a shadow, like when passing through the Earth’s outer core. We detect both these S-wave shadows and strong reflections from the surface using earthquakes at different depths and azimuths. These observations identify a km-scale molten-filled volume located beneath Turtle Island. The magmatic nature of the reservoir is supported by the onset of non-double-couple earthquakes with strong CLVD (Compensated Linear Vector Dipole) and ISO (Isotropic) components, which show a tensor crack compatible with some volume changes within the reservoir. Combining these results with two independent 3-D velocity models and aeromagnetic anomalies recorded in Taiwan, a partially-molten ~19% low-velocity volume is estimated in the mid-crust (13–23 km), with spatial uncertainties of ~3 km. The elongated direction approximately follows the strike of the Okinawa trough, indicating that the source of the magma reservoir might be a back-arc opening.

Determination of source parameters of the 2017 Mount Agung volcanic earthquake from moment-tensor inversion method using local broadband seismic waveforms

Monitoring of volcanoes has been an important issue for many purposes, particularly hazard mitigation. With regard to this, the aims of the present work are to estimate and analyse source parameters of a volcanic earthquake driven by recent magmatic events of Mount Agung in Bali island that occurred on September 28, 2017. The broadband seismogram data consisting of 3 local component waveforms were recorded by the IA network of 5 seismic stations: SRBI, DNP, BYJI, JAGI, and TWSI (managed by BMKG). These land-based observatories covered a full 4-quadrant region surrounding the epicenter. The methods used in the present study were seismic moment-tensor inversions, where the data were all analyzed to extract the parameters, namely moment magnitude, type of a volcanic earthquake indicated by percentages of seismic components: compensated linear vector dipole (CLVD), isotropic (ISO), double-couple (DC), and source depth. The results are given in the forms of variance reduction of 65%, a magnitude of MW 3.6, a CLVD of 40%, an ISO of 33%, a DC of 27% and a centroid-depth of 9.7 km. These suggest that the unusual earthquake was dominated by a vertical CLVD component, implying the dominance of uplift motion of magmatic fluid flow inside the volcano.

Probabilistic waveform inversion for 22 earthquake moment tensors in Hungary: new constraints on the tectonic stress pattern inside the Pannonian basin

We have successfully estimated the full moment tensors of 22 local earthquakes with local magnitude ranging from 1.2 to 4.8 that occurred in the Hungarian part of the Pannonian basin between 1995 and 2014. We used a probabilistic waveform inversion procedure that takes into account the effects of the random noise contained in the seismograms, the uncertainty of the hypocentre determined from arrival times and the inaccurate knowledge of the velocity structure, while estimating the error affecting the derived focal parameters. The applied probabilistic approach maps the posterior probability density functions (PPDFs) for both the hypocentral coordinates and the moment tensor components. The final estimates are given by the maximum likelihood points of the PPDFs, while solution uncertainties are presented by histogram plots. The estimated uncertainties in the moment tensor components are plotted on the focal sphere in such a way, that the significance of the double couple (DC), the compensated linear vector dipole (CLVD) and the isotropic (ISO) parts of the source can be assessed. We have shown that the applied waveform inversion method is equally suitable to recover the source mechanism for low-magnitude events using short-period local waveforms as well as for moderate-size earthquakes using long-period seismograms. The non-DC components of the retrieved focal mechanisms are statistically insignificant for all the analysed earthquakes. The negligible amount of the ISO component implies the tectonic nature of the investigated events. The moment tensor solutions reported by other agencies for five of the ML > 4 earthquakes studied in this paper are very similar to those calculated by the applied waveform inversion algorithm. We have found only strike-slip and thrust faulting events, giving further support to the hypothesis that the Pannonian basin is currently experiencing a compressional regime of deformation. The orientations of the obtained focal mechanisms are in good agreement with the main stress pattern published for the Pannonian region. The azimuth of the subhorizontal P principal axis varies from about NNE-SSW in SW Hungary through NE-SW well inside the basin to around E-W in the NE part of the country. Most of the analysed earthquakes occurred on faults or subfaults differently oriented than the main fault system.