Regional centroid MT inversion of small to moderate earthquakes in the Alps using the dense AlpArray seismic network: challenges and seismotectonic insights

Abstract. The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating different tectonic units and blocks in close proximity to sedimentary foreland basins. Centroid moment tensor inversion provides insight into the faulting mechanisms of earthquakes and related tectonic processes, but is significantly aggravated in such an environment. Thanks to the dense AlpArray seismic network and our flexible bootstrap-based inversion tool Grond we are able to test different set-ups with respect to the uncertainties of the obtained moment tensors and centroid locations. We evaluate the influence of frequency bands, azimuthal gaps, input data types and distance ranges and study the occurrence and reliability of non-DC components. We infer that for most earthquakes (Mw ≥ 3.3) a combination of time domain full waveforms and frequency domain amplitude spectra in a frequency band of 0.02–0.07 Hz is suitable. Relying on the results of our methodological tests, we perform deviatoric MT inversions for events with Mw > 3.0. We present here 75 solutions and analyse our results in the seismo-tectonic context of historic earthquakes, seismic activity of the last three decades and GNSS deformation data. We study regions of high seismic activity, namely the western Alps, the region around Lake Garda, the SE Alps, besides clusters further from the study region, in the northern Dinarides and the Apennines. Seismicity is particularly low in the eastern Alps and in parts of the central Alps. We apply a clustering algorithm to focal mechanisms, considering additional focal mechanisms from existing catalogs. Related to the NS compressional regime, E-W to ENE-WSW striking thrust faulting is mainly observed in the Friuli area in the SE Alps. Strike-slip faulting with a similarly oriented pressure axis is observed along the northern margin of the central Alps and in the northern Dinarides. NW-SE striking normal faulting is observed in the NW Alps showing a similar strike direction as normal faulting earthquakes in the Apennines. Both, our centroid depths as well as hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow; about 80 % of the studied events have depths shallower than 10 km.

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Regional centroid moment tensor inversion of small to moderate earthquakes in the Alps using the dense AlpArray seismic network: challenges and seismotectonic insights

Solid Earth ◽

10.5194/se-12-1233-2021 ◽

2021 ◽

Vol 12 (6) ◽

pp. 1233-1257

Author(s):

Gesa Maria Petersen ◽

Simone Cesca ◽

Sebastian Heimann ◽

Peter Niemz ◽

Torsten Dahm ◽

...

Keyword(s):

Seismic Activity ◽

Moment Tensor ◽

Southern Alps ◽

Moment Tensor Inversion ◽

Seismic Network ◽

Central Alps ◽

Normal Faulting ◽

Centroid Moment Tensor ◽

Study Region ◽

Northern Dinarides

Abstract. The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating different tectonic units and blocks in close proximity to sedimentary foreland basins. Centroid moment tensor inversion provides insight into the faulting mechanisms of earthquakes and related tectonic processes but is significantly aggravated in such an environment. Thanks to the dense AlpArray seismic network and our flexible bootstrap-based inversion tool Grond, we are able to test different setups with respect to the uncertainties of the obtained moment tensors and centroid locations. We evaluate the influence of frequency bands, azimuthal gaps, input data types, and distance ranges and study the occurrence and reliability of non-double-couple (DC) components. We infer that for most earthquakes (Mw≥3.3) a combination of time domain full waveforms and frequency domain amplitude spectra in a frequency band of 0.02–0.07 Hz is suitable. Relying on the results of our methodological tests, we perform deviatoric moment tensor (MT) inversions for events with Mw>3.0. Here, we present 75 solutions for earthquakes between January 2016 and December 2019 and analyze our results in the seismotectonic context of historical earthquakes, seismic activity of the last 3 decades, and GNSS deformation data. We study regions of comparably high seismic activity during the last decades, namely the Western Alps, the region around Lake Garda, and the eastern Southern Alps, as well as clusters further from the study region, i.e., in the northern Dinarides and the Apennines. Seismicity is particularly low in the Eastern Alps and in parts of the Central Alps. We apply a clustering algorithm to focal mechanisms, considering additional mechanisms from existing catalogs. Related to the N–S compressional regime, E–W-to-ENE–WSW-striking thrust faulting is mainly observed in the Friuli area in the eastern Southern Alps. Strike-slip faulting with a similarly oriented pressure axis is observed along the northern margin of the Central Alps and in the northern Dinarides. NW–SE-striking normal faulting is observed in the NW Alps, showing a similar strike direction to normal faulting earthquakes in the Apennines. Both our centroid depths and hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow; about 80 % of the studied events have depths shallower than 10 km.

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The SWATH-D Seismological Network in the Eastern Alps

Seismological Research Letters ◽

10.1785/0220200377 ◽

2021 ◽

Vol 92 (3) ◽

pp. 1592-1609 ◽

Cited By ~ 2

Author(s):

Benjamin Heit ◽

Luigia Cristiano ◽

Christian Haberland ◽

Frederik Tilmann ◽

Damiano Pesaresi ◽

...

Keyword(s):

Eastern Alps ◽

Focal Mechanisms ◽

Seismic Network ◽

Moho Depth ◽

Dense Network ◽

Study Region ◽

The North ◽

Complex Part ◽

Broadband Network ◽

The Alps

Abstract The SWATH-D experiment involved the deployment of a dense temporary broadband seismic network in the Eastern Alps. Its primary purpose was enhanced seismic imaging of the crust and crust–mantle transition, as well as improved constraints on local event locations and focal mechanisms in a complex part of the Alpine orogen. The study region is a key area of the Alps, where European crust in the north is juxtaposed and partially interwoven with Adriatic crust in the south, and a significant jump in the Moho depth was observed by the 2002 TRANSALP north–south profile. Here, a flip in subduction polarity has been suggested to occur. This dense network encompasses 163 stations and complements the larger-scale sparser AlpArray seismic network. The nominal station spacing in SWATH-D is 15 km in a high alpine, yet densely populated and industrialized region. We present here the challenges resulting from operating a large broadband network under these conditions and summarize how we addressed them, including the way we planned, deployed, maintained, and operated the stations in the field. Finally, we present some recommendations based on our experiences.

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Method for Determination of Focal Mechanisms of Magnitude 2.5–4.0 Earthquakes Recorded by a Sparse Regional Seismic Network

Bulletin of the Seismological Society of America ◽

10.1785/0120190170 ◽

2020 ◽

Vol 110 (2) ◽

pp. 715-726

Author(s):

Nawa R. Dahal ◽

John E. Ebel

Keyword(s):

Focal Mechanism ◽

Signal To Noise Ratio ◽

Moment Tensor ◽

Focal Depth ◽

Focal Mechanisms ◽

Seismic Network ◽

Seismic Waveforms ◽

Virginia Earthquake ◽

Using Data ◽

Good Signal

ABSTRACT Focal mechanisms of earthquakes with magnitudes Mw 4.0 and less recorded by a sparse seismic network are usually poorly constrained due to the lack of an appropriate method applicable to finding these parameters with a sparse set of observations. We present a new method that can accurately determine focal mechanisms of earthquakes with Mw (3.70–3.04) using data from a few regional seismic stations. We filter the observed seismograms as well as synthetic seismograms through a frequency band of 1.5–2.5 Hz, which has a good signal-to-noise ratio for small earthquakes of the magnitudes with which we are working. The waveforms are processed to their envelopes to make the waveforms relatively simple for modeling. To find the optimal focal mechanism for an event, a nonlinear moment tensor inversion in addition to a coarse grid search over the possible dip, rake, and strike angles at a fixed value of focal depth and a fixed value of scalar moment is performed. We tested the method on 18 aftershocks of Mw (3.70–2.60) of the 2011 Mw 5.7 Mineral, Virginia, earthquake and on five aftershocks of Mw (3.62–2.63) of the 2013 Mw 4.5 Ladysmith, Quebec, earthquake. Our method obtains accurate focal mechanisms for 16 out of the 21 events that have previously reported focal mechanisms. Tests of our method for different crustal models show that event focal mechanism determinations vary with an average Kagan angle of 30° with the different crustal models. This means that the event focal mechanism determinations are only somewhat sensitive to the uncertainties in the crustal models tested. This study confirms that our method of modeling envelopes of seismic waveforms can be used to extract accurate focal mechanisms of earthquakes with short-time functions (Mw<4.0) using at least three regional seismic network stations at epicentral distances of 60–350 km.

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Recent Seismicity in the Area of the Major, 1908 Messina Straits Earthquake, South Italy

Frontiers in Earth Science ◽

10.3389/feart.2021.667501 ◽

2021 ◽

Vol 9 ◽

Author(s):

Giancarlo Neri ◽

Barbara Orecchio ◽

Debora Presti ◽

Silvia Scolaro ◽

Cristina Totaro

Keyword(s):

Focal Mechanisms ◽

Current Debate ◽

Tyrrhenian Sea ◽

Primary Role ◽

Normal Faulting ◽

Study Region ◽

Northern Border ◽

Fractured Medium ◽

Horizontal Strain ◽

Subducting Slab

High-quality non-linear hypocenter locations and waveform inversion focal mechanisms of recent, shallow earthquakes of the Messina Straits have allowed us to obtain the following main results: 1) seismicity has occurred below the east-dipping north-striking fault proposed by most investigators as the source of the 1908, magnitude 7.1 Messina earthquake, while it has been substantially absent in correspondence of the fault and above it; 2) earthquake locations and related strain space distributions do not exhibit well defined trends reflecting specific faults but they mark the existence of seismogenic rock volumes below the 1908 fault representing primary weakness zones of a quite fractured medium; 3) focal mechanisms reveal normal and right-lateral faulting in the Straits, reverse faulting at the southern border of it (Ionian sea south of the Ionian fault), and normal faulting at the northern border (southeastern Tyrrhenian sea offshore southern Calabria); 4) these faulting regimes are compatible with the transitional character of the Messina Straits between the zone of rollback of the in-depth continuous Ionian subducting slab (southern Calabria) and the collisional zone where the subduction slab did already undergo detachment (southwest of the Ionian fault); 5) the whole seismicity of the study area, including also the less recent earthquakes analyzed by previous workers, is compared to patterns of geodetic horizontal strain and uplift rates available from the literature. We believe that the joint action of Africa-Europe plate convergence and rollback of the Ionian subducting slab plays a primary role as regard to the local dynamics and seismicity of the Messina Straits area. At the same time, low horizontal strain rates and large spatial variations of uplift rate observed in this area of strong normal-faulting earthquakes lead us to include a new preliminary hypothesis of deep-seated sources concurring to local vertical dynamics into the current debate on the geodynamics of the study region.

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Design of an optimal seismic network for monitoring geothermal fields

10.5194/egusphere-egu2020-12259 ◽

2020 ◽

Author(s):

Leonarda Isabel Esquivel Mendiola ◽

Marco Calò ◽

Anna Tramelli

Keyword(s):

Seismic Activity ◽

Regional Scale ◽

Three Dimensional ◽

Geothermal Field ◽

Seismic Network ◽

Topographic Effects ◽

Noise Levels ◽

Study Region ◽

Injection Test ◽

Seismic Models

The reliable monitoring and location of the seismic activity at a local and regional scale is a key factor for hazard assessment. The exploitation of a geothermal field can be affected by natural and induced seismicity, hence optimal planning of a seismic network is of great interest for geothermal development. Seismic monitoring depends on two main aspects: i) sensitivity of the seismic network and ii) the effectiveness of detection and location methods. In this study, we focus on the first aspect proposing and improvement of an algorithm for the optimization of seismic networks designed for monitoring the seismic activity related to the injection test that will be performed in a geothermal well. The algorithm is based on the method proposed by Tramelli et al. (2013) that tries to find the optimal station positions minimizing the volume of the error ellipsoid of the location for synthetic events using the D-criterion (Rabinowitz and Steinberg, 2000). In this version of the program we improve the algorithm to find an optimal seismic network considering several prior information such as: 1) maps of seismic noise levels at different frequency bands, 2) three-dimensional seismic models and 3) topographic gradient of the study region. This information is usually produced during the exploration stage of a geothermal site and available prior an injection test. We applied the methodology to the Acoculco geothermal field (Mexico) where an injection test is planned. In this work, we show a comparison between the standard approach that uses 1D seismic models, constant values of noise levels, and no topographic effects, with the new one showing how important is to consider these parameters for a more suitable optimization of the seismic network. This work is performed in the framework of the Mexican European consortium GeMex (Cooperation in Geothermal energy research Europe-Mexico, PT5.2 N: 267084 funded by CONACyT-SENER : S0019, 2015-04, and of the joint agreement between UNAM and INGV on the development of seismological research of volcanic and geothermal field (N:44753-1023-22-IV-16/1).

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Seismicity, focal mechanism, and stress tensor analysis of the Simav region, western Turkey

Open Geosciences ◽

10.1515/geo-2020-0010 ◽

2020 ◽

Vol 12 (1) ◽

pp. 479-490

Author(s):

Ahu Kömeç Mutlu

Keyword(s):

Moment Tensor ◽

Aftershock Sequence ◽

Movement Direction ◽

Normal Faults ◽

Normal Faulting ◽

Stress Inversion ◽

Western Turkey ◽

Stress Orientations ◽

Extension Direction ◽

Inversion Analysis

AbstractThis study focuses on the seismicity and stress inversion analysis of the Simav region in western Turkey. The latest moderate-size earthquake was recorded on May 19, 2011 (Mw 5.9), with a dense aftershock sequence of more than 5,000 earthquakes in 6 months. Between 2004 and 2018, data from earthquake events with magnitudes greater than 0.7 were compiled from 86 seismic stations. The source mechanism of 54 earthquakes with moment magnitudes greater than 3.5 was derived by using a moment tensor inversion. Normal faults with oblique-slip motions are dominant being compatible with the NE-SW extension direction of western Turkey. The regional stress field is assessed from focal mechanisms. Vertically oriented maximum compressional stress (σ1) is consistent with the extensional regime in the region. The σ1 and σ3 stress axes suggest the WNW-ESE compression and the NNE-SSW dilatation. The principal stress orientations support the movement direction of the NE-SW extension consistent with the mainly observed normal faulting motions.

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Seismic doublets and a complex seismic sequence controlled by the rotation of the Juan Fernández microplate

10.5194/egusphere-egu21-1119 ◽

2021 ◽

Author(s):

Simone Cesca ◽

Carla Valenzuela Malebrán ◽

José Ángel López-Comino ◽

Timothy Davis ◽

Carlos Tassara ◽

...

Keyword(s):

Source Parameters ◽

Strike Slip ◽

Focal Mechanisms ◽

Joint Analysis ◽

Seismic Sequence ◽

Local Data ◽

Study Region ◽

Coulomb Stress Changes ◽

Earthquake Source Parameters ◽

Stress Changes

A complex seismic sequence took place in 2014 at the Juan Fern&#225;ndez microplate, a small microplate located between Pacific, Nazca and Antarctica plates. Despite the remoteness of the study region and the lack of local data, we were able to resolve earthquake source parameters and to reconstruct the complex seismic sequence, by using modern waveform-based seismological techniques. The sequence started with an exceptional Mw 7.1-6.7 thrust &#8211; strike slip earthquake doublet, the first subevent being the largest earthquake ever recorded in the region and one of the few rare thrust earthquakes in a region otherwise characterized by normal faulting and strike slip earthquakes. The joint analysis of seismicity and focal mechanisms suggest the activation of E-W and NE-SW faults or of an internal curved pseudofault, which is formed in response to the microplate rotation, with alternation of thrust and strike-slip earthquakes. Seismicity migrated Northward in its final phase, towards the microplate edge, where a second doublet with uneven focal mechanisms occurred. The sequence rupture kinematics is well explained by Coulomb stress changes imparted by the first subevent. Our analysis show that compressional stresses, which have been mapped at the northern boundary of the microplate, but never accompanied by large thrust earthquakes, can be accommodated by the rare occurrence of large, impulsive, shallow thrust earthquakes, with a considerable tsunamigenic potential.

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Short-term seismic crustal deformation of Iran

Annals of Geophysics ◽

10.4401/ag-6413 ◽

2014 ◽

Vol 57 (2) ◽

Author(s):

Shoja Ansari ◽

Ahmad Zamani

Keyword(s):

Seismic Moment ◽

Deformation Rate ◽

Moment Tensor ◽

Focal Mechanisms ◽

Average Strain ◽

Short Term ◽

Maximum Shear Strain ◽

Eastern Iran ◽

Seismic Deformation ◽

Orogenic Belts

In this paper the short-term seismic deformation of Iran is determined by the earthquake moment tensor summation. The study areas include the Alborz, Kopeh-Dagh, eastern Iran, Makran and Zagros orogenic belts. The spatial distribution and focal mechanisms of the earthquakes delineate the deformation zones. The mean directions of the P and T axes are determined by the equal area projection of the seismic moment tensors. The orientations of the P-axes are dominantly correlated with the NE crustal motion of Iran relative to Eurasia. The average strain rates are calculated in all of the regions. The maximum shear strain and dilatation rates are defined by the eigenvalues of the average strain rate tensors. The dilatation rate indicates that not only the dominant compression but also the subsidiary tension affects the Alborz and Makran orogenic belts. The velocity tensor components discriminate the vertical thickening and thinning of the crust in some regions of Iran. The seismic deformation rates, which are determined by the velocity tensors, are smaller than the geodetic deformation rates. In the high seismic deformation zones, such as the eastern Iran and Alborz, the geodetic deformation rate is comparable with the seismic deformation rate. Our results indicate that the NW Zagros and Kopeh-Dagh have the lowest seismic deformation rates. The seismic shortening rate increases from NW to SE in the Zagros orogenic belt. The seismic deformation orientations are different from the P-axes, probably due to the lateral translation. The maximum percentage of the seismic deformation in the study areas is related to the eastern Iran and the minimum one is related to the Makran orgenic belt. The average shape tensors indicate that the focal mechanisms in the Kopeh-Dagh have the highest internal similarity. The eastern Iran has the largest seismic moment rate, while the central Zagros has the lowest one.

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A functional tool to explore the reliability of micro-earthquake focal mechanism solution for seismotectonic purposes

10.5194/se-2021-88 ◽

2021 ◽

Author(s):

Guido Maria Adinolfi ◽

Raffaella De Matteis ◽

Rita De Nardis ◽

Aldo Zollo

Keyword(s):

Focal Mechanism ◽

Fault Plane ◽

Focal Mechanisms ◽

Seismic Network ◽

Fault System ◽

Focal Mechanism Solution ◽

Fault Plane Solutions ◽

Stress Regime ◽

Focal Mechanism Solutions ◽

Earthquake Focal Mechanism

Abstract. Improving the knowledge of seismogenic faults requires the integration of geological, seismological, and geophysical information. Among several analyses, the definition of earthquake focal mechanisms plays an essential role in providing information about the geometry of individual faults and the stress regime acting in a region. Fault plane solutions can be retrieved by several techniques operating in specific magnitude ranges, both in the time and frequency domain and using different data. For earthquakes of low magnitude, the limited number of available data and their uncertainties can compromise the stability of fault plane solutions. In this work, we propose a useful methodology to evaluate how well a seismic network used to monitor natural and/or induced micro-seismicity estimates focal mechanisms as function of magnitude, location, and kinematics of seismic source and consequently their reliability in defining seismotectonic models. To study the consistency of focal mechanism solutions, we use a Bayesian approach that jointly inverts the P/S long-period spectral-level ratios and the P polarities to infer the fault-plane solutions. We applied this methodology, by computing synthetic data, to the local seismic network operated in the Campania-Lucania Apennines (Southern Italy) to monitor the complex normal fault system activated during the Ms 6.9, 1980 earthquake. We demonstrate that the method we propose can have a double purpose. It can be a valid tool to design or to test the performance of local seismic networks and more generally it can be used to assign an absolute uncertainty to focal mechanism solutions fundamental for seismotectonic studies.

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Spatio-temporal modelling for non-stationary point referenced data

10.26686/wgtn.15147396 ◽

2021 ◽

Author(s):

Lindsay Morris

Keyword(s):

Random Forest ◽

Goodness Of Fit ◽

Clustering Algorithm ◽

Dependence Structure ◽

Geostatistical Model ◽

Study Region ◽

Global Estimate ◽

Covariance Functions ◽

Power Of The Test ◽

Spatio Temporal

Spatial and spatio-temporal phenomena are commonly modelled as Gaussian processes via the geostatistical model (Gelfand & Banerjee, 2017). In the geostatistical model the spatial dependence structure is modelled using covariance functions. Most commonly, the covariance functions impose an assumption of spatial stationarity on the process. That means the covariance between observations at particular locations depends only on the distance between the locations (Banerjee et al., 2014). It has been widely recognized that most, if not all, processes manifest spatially nonstationary covariance structure Sampson (2014). If the study domain is small in area or there is not enough data to justify more complicated nonstationary approaches, then stationarity may be assumed for the sake of mathematical convenience (Fouedjio, 2017). However, relationships between variables can vary significantly over space, and a ‘global’ estimate of the relationships may obscure interesting geographical phenomena (Brunsdon et al., 1996; Fouedjio, 2017; Sampson & Guttorp, 1992). In this thesis, we considered three non-parametric approaches to flexibly account for non-stationarity in both spatial and spatio-temporal processes. First, we proposed partitioning the spatial domain into sub-regions using the K-means clustering algorithm based on a set of appropriate geographic features. This allowed for fitting separate stationary covariance functions to the smaller sub-regions to account for local differences in covariance across the study region. Secondly, we extended the concept of covariance network regression to model the covariance matrix of both spatial and spatio-temporal processes. The resulting covariance estimates were found to be more flexible in accounting for spatial autocorrelation than standard stationary approaches. The third approach involved geographic random forest methodology using a neighbourhood structure for each location constructed through clustering. We found that clustering based on geographic measures such as longitude and latitude ensured that observations that were too far away to have any influence on the observations near the locations where a local random forest was fitted were not selected to form the neighbourhood. In addition to developing flexible methods to account for non-stationarity, we developed a pivotal discrepancy measure approach for goodness-of-fit testing of spatio-temporal geostatistical models. We found that partitioning the pivotal discrepancy measures increased the power of the test.

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