Regional Moment Tensor Inversion Research Articles

Complex Martinique Intermediate‐Depth Earthquake Reactivates Early Atlantic Break‐Up Structures

AbstractEarthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth.

2023 Earthquake Doublet in Türkiye Reveals the Complexities of the East Anatolian Fault Zone: Insights from Aftershock Patterns and Moment Tensor Solutions

Abstract The East Anatolian Fault Zone (EAFZ) is a 700-km-long left-lateral transform fault system along the boundary between the Anatolian and Arabian plates. In the interseismic period, the eastern segments of the EAFZ display relatively uniform seismic activity, whereas the western segments exhibit seismic gaps, localized clusters, and extensive diffuse zones. Hence, our understanding of the geometry and kinematics of the western and northern segments remain limited. The occurrences of the 6 February 2023 Mw 7.8 Kahramanmaraş on the main branch and Mw 7.6 Elbistan earthquakes on the northern branch have led to complex aftershock activity shedding light on the nature of these relatively silent segments. In this study, to better understand the complexities of the fault, we constructed a comprehensive catalog of ∼32,000 earthquakes that occurred between 6 February 2023 and 30 March 2023, using a deep-neural-network-based picker. In addition, 170 earthquake source mechanisms with Mw 3.5+ were obtained from regional moment tensor inversion. The spatial distribution of the aftershocks shows that most of the activity clusters around the fault bends and major depressions. Previously unmapped and inactive secondary faults of varying lengths are identified within these geometrical complexities. The new seismological observations provide compelling evidence of extension along the Karasu valley, compression occurring along the Erkenek segment, reactivation of basin faults near the Narlı fault zone and the persistent shallow seismic creep of the Pütürge segment. The analysis of seismicity and earthquake source mechanisms along the northern branch reveals the structures of previously inactive faults, both near the extensional Göksun bend in the west and the compressional Nurhak fault complex in the east. In summary, we illustrate the intricacies of previously quiet segments of the EAFZ and aim to gain a deeper understanding of how secondary faults and geometrical discontinuities along the EAFZ played a role in shaping the 2023 Türkiye doublet earthquakes.

Active seismotectonics of the East Anatolian Fault

SUMMARYThe East Anatolian Fault (EAF) is a 700-km-long left-lateral transform fault located between the Anatolian and Arabian plates. The proximity of the Euler Pole to the Arabia–Anatolia Plate boundary leads to rapid changes in plate velocity along the boundary, which is manifested by the decreasing slip rates from east (10 mm yr–1) to west (∼1–4 mm yr–1). The EAF displays heterogeneous seismicity patterns with seismic gaps, localized clusters and broad diffuse zones. In this study, in order to understand the origin of these complexities and quantify the seismic hazard along the EAF, we present an improved seismicity catalogue with more than 26 000 earthquakes and 160 focal mechanisms from regional moment tensor inversion between 2007 and 2020. The focal mechanisms and seismicity show that the EAF dips towards north and forms a well-defined plate boundary in the east between Palu and Çelikhan with almost pure left-lateral motion. Further west, the boundary becomes broader with activity along subparallel faults. Focal mechanisms show heterogeneous stress orientations in consistence with geodetically determined strain rate field. The stress orientations show a transition from strike-slip to extension towards the west of Çelikhan. Amongst all segments of EAF, the Pütürge segment, which holds the near-repeating earthquakes in the vicinity of the nucleation of the 2020 Mw 6.8 earthquake, is distinguished with its steady and high rate of seismicity. Further east, the neighbouring Palu segment is characterized by several distinct moderate earthquakes. We do not observe any change in the seismicity rate on these segments of the EAF following large earthquakes. In order to quantify the seismic hazard along the EAF, we calculate the recurrence time and maximum magnitude for each segment by using an extended seismicity catalogue of 150 yr including the large historical earthquakes and the geodetic strain rate. The results show ∼150 yr recurrence time with Mmax∼6.7–7.0 along the seismically active Palu and Pütürge segments on the east, while relatively silent western segments yield longer recurrence times; 237–772 for Pazarcık and 414–917 for Amanos segments with slightly larger magnitudes (Mmax ∼7–7.4). We infer that the seismicity patterns and strain-rate field along the EAF are shaped by several factors such as strong geometrical irregularities, heterogeneous coupling and complex plate motion leading to rapid change of fault slip rate.

The Storfjorden, Svalbard, Earthquake Sequence 2008–2020: Transtensional Tectonics in an Arctic Intraplate Region

AbstractAn earthquake sequence in the Storfjorden offshore area southwest of Spitsbergen in the Svalbard archipelago initiated with a 21 February 2008 magnitude Mw 6.1 event. This area had previously not produced any significant earthquakes, but between 2008 and 2020, a total of ∼2800 earthquakes were detected, with ∼16 of them being of moderate size (ML≥4.0). Applying double-difference relocation to improve relative locations reveals that the activity is linked to several subparallel faults striking southwest–northeast that extend across the entire crust. The southwest–northeast trend is also found as a possible fault plane from regional moment tensor inversion. The solutions range from oblique normal in the center of the cluster to pure strike slip farther away and are consistent with the compressional σ1 axis roughly in the east–west direction and plunging 57°, and the extensional σ3 axis subhorizontal trending north–south. The mainshock fault is steeply dipping to the southeast, but several other faults appear to be near vertical. The existence of oblique, right-lateral strike-slip motion on southwest–northeast-trending faults with a normal component and pure normal faulting events in between suggests transtensional tectonics that in and around Storfjorden result in activation of a complex fault system.

Regional Moment Tensor Inversion Using Rotational Observations

AbstractAbstractThere are benefits from the addition of rotational motions to translational displacements for moment tensor (MT) inversions. The rotational radiation pattern is orthogonal to the shear radiation pattern, thus incorporating rotations is equivalent to gaining another observation point on the focal sphere. We demonstrated this by simulating curl and displacement wavefields for a regional distance station. Thus, one 6‐C station (3‐Component translational + 3‐Component rotational) gathers the same information on radiation pattern as two 3‐C stations at 90° azimuth from one another along the focal plane axis, which is sometimes difficult to obtain when restricted to surface sensors. We added rotational Green's functions to a regional MT inversion scheme (long‐period, time‐domain, and linear inversion) by computing spatial gradients from f‐κ reflectivity synthetics. For rotational data, we used Array Derived Rotations (ADR) from Piñon Flats Observatory Array and Golay array deployed during IRIS Community Wavefield Demonstration Experiment. The hope is to ultimately use compact and field deployable broadband rotational seismometers instead of ADRs. Rotational motions were predicted from well‐constrained deviatoric MT solutions of nine earthquakes recorded by the arrays and other seismic networks. We formed three station sparse datasets with (3‐C and 6‐C) and without rotational ground motions (3‐C only) for eight earthquakes testing the benefits of including rotational waveforms in MT inversions when they are equally weighted with translational waveforms. Adding rotational motions improved the double‐couple components and reduced compensated‐linear‐vector‐dipole and isotropic components in full‐MT solutions using sparse datasets with poor coverage.

Towards a regional, automated full moment tensor inversion for medium to large magnitude events in the Iranian plateau

Considering the high seismicity rate of the Iranian plateau, the development and implementation of an automated routine for near-real-time determination of earthquake point source parameters would be extremely beneficial, e.g., for dedicated seismological studies, constraining the regional stress field, and shakemap assessment. We discuss the implementation of a fast, automated regional moment tensor inversion routine, based on SeisComP3 and KIWI tool packages. To assess the routine efficiency, we used a dataset including 90 earthquakes spatially distributed within all seismotectonic provinces of Iran, recorded at local to regional distances, with magnitudes in the range MN 4.5–7.3. Resolved moment tensor solutions and source parameters are in most cases in good agreement with reference ones from global and regional catalogs. However, we identify consistent differences in magnitudes and depths, with respect to global catalogs, which can be attributed to a higher resolution of our approach, using local and regional data and considering regional velocity models. The depth estimations are shallower than in reference global catalogs but compatible with the tectonic settings of the Iranian region. The analysis of the selected dataset is accompanied by a test run of automated processing for 8 months of online operation, from July 2019 to February 2020. The processing time in online routine, with an average of ~ 23 min, mostly depends on the magnitude, number of stations, azimuthal coverage, data quality, and complexity of the crustal model, so that computation times may vary significantly for earthquakes located in different Iranian provinces.

Slip Partitioning in the 2016 Alboran Sea Earthquake Sequence (Western Mediterranean)

A MW=5.1 earthquake on January 21st 2016 marked the beginning of a significant seismic sequence in the southern Alboran Sea, culminating in a MW=6.3 earthquake on January 25th, and continuing with further moderate magnitude earthquakes until March. We use data from 35 seismic broadband stations in Spain, Morocco and Portugal to relocate the seismicity, estimate seismic moment tensors, and isolate regional apparent source time functions for the main earthquake. Relocation and regional moment tensor inversion consistently yield very shallow depths for the majority of events. We obtain 50 moment tensors for the sequence, showing a mixture of strike-slip faulting for the foreshock and the main event and reverse faulting for the major aftershocks. The leading role of reverse focal mechanisms among the aftershocks may be explained by the geometry of the fault network. The mainshock nucleates at a bend along the left-lateral Al-Idrisi fault, introducing local transpression within the transtensional Alboran Basin. The shallow depths of the 2016 Alboran Sea earthquakes may favour slip-partitioning on the involved faults. Apparent source durations for the main event suggest a ~21 km long, asymmetric rupture that propagates primarily towards NE into the restraining fault segment, with fast rupture speed of ~3.0 km/s. Consistently, the inversion for laterally variable fault displacement situates the main slip in the restraining segment. The partitioning into strike-slip rupture and dip-slip aftershocks confirms a non-optimal orientation of this segment, and suggests that the 2016 event settled a slip deficit from previous ruptures that could not propagate into the stronger restraining segment.

An analysis of a moderate earthquake, eastern flank of the Red Sea, Saudi Arabia

The present study presents the analysis of a moderate earthquake (Mw 4.0) and its largest aftershocks located along the Red Sea coast, southwestern Saudi Arabia, with the aim to understand the enigma of peculiar seismicity in the Arabian Shield. We analyzed a high-quality waveform dataset collected from ten well-recorded earthquakes of moment magnitude ranging from 2.0 to 4.0 in order to determine the hypocenter locations and focal mechanisms. The focal mechanisms were retrieved from the regional moment-tensor inversion for the mainshock and using the P-wave polarities for the corresponding aftershocks of Mw ≥ 2.0, respectively. The focal mechanism solutions were inverted to retrieve the seismogenic stress using the stress tensor inversion. The combination of the nearby fault trends and the obtained results from hypocentral relocations, focal mechanism solutions, and stress tensor inversion emphasizes that the NE fault trend is likely to be the causative fault resulting in the 2017 Namas earthquake sequence, implying that the local tectonic setting is incompatible with the large-scale regional tectonics of the Red Sea opening. On the contrary, estimates of low-stress drops exhibit typical values compatible with those reported for the shallow plate-boundary earthquakes that occurred in the Red Sea, suggesting that the existing weak zones in the southernmost part of the Arabian Shield may be attributed to the large-scale regional tectonics of the Red Sea opening.

ИЛЬПЫРСКОЕ ЗЕМЛЕТРЯСЕНИЕ 13 марта 2013 г. с Mwрег=5.8, ML=6.2, КS=13.9, I0p=8 (Камчатский перешеек)

The strong (Mwreg=5.8, ML=6.2) near-surface seismic event (Ilpyrskoye earthquake) occurred at 03h12m on 13 March, in the Kamchatka Isthmus. It was the strongest earthquake between 1962 and 2013 for this area. The greatest macroseismic effect was observed at a distance of ~30 km, I=6–7 on the scale MSK-64. We used two independent methods for determining its regional focal mechanism: 1) regional moment tensor in-version using broadband waveforms; 2) solution based on polarities of the P waves. The results are similar: the focal mechanism of Ilpyrskoye earthquake is thrust faulting with strike-slip component; the compression axis is subhorizontal and is oriented in the north-east – south-west direction. The mechanisms for the two strongest aftershocks were also identified, as a result, a change in focal movements during the aftershock process was revealed.The analysis of the aftershock process which consists of two stages with different de-cay character was performed. The process lasted ~ 75 days. About 200 aftershocks ML=3.0–5.7 (КS=7.5–12.9) were recorded, hypocenter depth estimations vary from 0 to 10 km for about 80 % of them. The strongest aftershock was on May 6, 2013 with ML=5.7, Mwreg=4.8, at which the change in focal movements occurred. According to the results of near real time processing, aftershock cloud of Ilpyrskoye earthquake had a pronounced linearity and a great length, which was an artifact. The main cause of the artifact is the minimum number of stations involved in determining the hypocenters of most aftershocks and their quasi-linear disposition. The confidence areas within which solutions are equivalent are shown. We concluded that Ilpyrskoye earthquake is a serious argument that the area of compression between the Okhotsk and North American plates is extended further to the east and the border passes through the Kamchatka Isthmus

Source characteristics of the March 16, 2014 Mw 6.7 earthquake and its implications for the Mw 8.2 Pisagua mainshock

The March 16, 2014 Mw 6.7 earthquake played an important role in the complex seismic sequence leading to the nucleation of the April 1st, 2014 Mw 8.2 Pisagua earthquake in northern Chile. The Mw 6.7, an upper plate reverse faulting event with nodal planes highly rotated counterclockwise with respect to the strike of the megathrust at its location, is analyzed. For each nodal plane of the regional W-phase centroid moment tensor solution, the spatiotemporal slip distribution is inverted using near-field records. The optimal space and time smoothing constraints are determined objectively based on the Akaike's Bayesian Information Criterion. We analyze seismicity on the region affected by the Mw 6.7 and Mw 8.2 Pisagua mainshock, with accurate hypocenter locations obtained using a 3D heterogeneous medium. We also performed regional moment tensor inversion of events (M≥4.0) occurred before the Mw 8.2. Our results suggest the sub-vertical fault plane dipping southward (dip∼70°) as being the causative fault of the March 16 Mw 6.7 earthquake. The rupture propagated to the northwest, lasted 15s, and yielded a total seismic moment of 1.36×1019Nm (Mw 6.7). Estimated slip is characterized by total rupture length of ∼22km, and suggest shallow slip distributed between ∼ 3km, down to 17km depth. A comparison between coastal tide gauge records and forward tsunami modeling show similarity of the time series, thus supporting the plausibility of our estimated slip models. Our interpretation suggests that the rupture of the Mw 6.7 intraplate could be explained by stress accommodation between two segments along-dip with different mechanical properties at the interplate boundary, and inside the upper plate.

Normal Faulting in the 1923 Berdún Earthquake and Postorogenic Extension in the Pyrenees

AbstractThe 10 July 1923 earthquake near Berdún (Spain) is the largest instrumentally recorded event in the Pyrenees. We recover old analog seismograms and use 20 hand‐digitized waveforms for regional moment tensor inversion. We estimate moment magnitude Mw 5.4, centroid depth of 8 km, and a pure normal faulting source with strike parallel to the mountain chain (N292°E), dip of 66° and rake of −88°. The new mechanism fits into the general predominance of normal faulting in the Pyrenees and extension inferred from Global Positioning System data. The unique location of the 1923 earthquake, near the south Pyrenean thrust front, shows that the extensional regime is not confined to the axial zone where high topography and the crustal root are located. Together with seismicity near the northern mountain front, this indicates that gravitational potential energy in the western Pyrenees is not extracted locally but induces a wide distribution of postorogenic deformation.

A New Automatic Full‐Waveform Regional Moment Tensor Inversion Algorithm and Its Applications in the Taiwan Area

A new algorithm is developed in this study to automatically determine regional moment tensor (MT) and its centroid depth with real-time waveforms in 2–4 min after the Central Weather Bureau (CWB) issues an earthquake notice. The program selects 3–7 BATS (Broadband Array for Taiwan Seismology) stations under three criteria: best azimuthal distribution, highest signal-to-noise ratio, and shortest distances. It then inverts MT solutions in parallel with various settings including Moho depth of velocity models, frequency bands, and isotropic constraints. The optimal solution is determined by searching for the best waveform-fit with an acceptable non-double-couple component from the results of combinations of these inversion settings. Our new rapid MT report system greatly reduces computational resources and avoids human judgments. By applying this full-scanning approach on BATS (named AutoBATS), we re-determine the MTs for over 3000 regional earthquakes from 1996 to 2016 to provide the most up-to-date MT catalog for the Taiwan area. The AutoBATS MTs are overall consistent with the Global Centroid Moment Tensors with a mean difference in Kagan angle of 22.0 ± 16.6° and MW of − 0.08 ± 0.10. Those focal mechanisms better illuminate the tectonic structures owing to the much improved resolving ability for shallow ( 140 km) earthquakes. With the new regional MT catalog, we refine the relationship between moment and local magnitudes: MW = 0.87 ML + 0.23 for the Taiwan region.

The 12th June 2017 Mw = 6.3 Lesvos earthquake from detailed seismological observations

A major earthquake (Mwö=ö6.3) occurred on the 12th of June 2017 (12:28 GMT) offshore, south of the SE coast of Lesvos Island, at a depth of 13ökm, in an area characterized by normal faulting with an important strike-slip component in certain cases. Over 900 events of the sequence between 12 and 30 June 2017 were manually analyzed and located, employing an optimized local velocity model. Double-difference relocation revealed seven spatially separated groups of events, forming two linear branches, roughly aligned N130°E, compatible with the strike of known mapped faults along the southern coast of Lesvos Island. Spatiotemporal analysis indicated gradual migration of seismicity towards NW and SE from the margins of the main rupture, while a strong secondary sequence at a separate fault patch SE of the mainshock, oriented NW-SE, was triggered by the largest aftershock (Mwö=ö5.2) that occurred on 17 June. The focal mechanisms of the mainshock (φö=ö122°, δö=ö40° and λö=ö−83°) and of the major aftershocks were determined using regional moment tensor inversion. In most cases normal faulting was revealed with the fault plane oriented in a NW-SE direction, dipping SW, with the exception of the largest aftershock that was characterized by strike-slip faulting. Stress inversion revealed a complex stress field south of Lesvos, related both to normal, in an approximate E-W direction, and strike-slip faulting. All aftershocks outside the main rupture, where gradual seismicity migration was observed, are located within the positive lobes of static stress transfer determined by applying the Coulomb criterion for the mainshock. Stress loading on optimal faults under a strike-slip regime explains the occurrence of the largest aftershock and the seismicity that was triggered at the eastern patch of the rupture zone.

The Chiloé Mw 7.6 earthquake of 2016 December 25 in Southern Chile and its relation to the Mw 9.5 1960 Valdivia earthquake

On 25 December 2016, a Mw 7.6 earthquake broke a portion of the Southern Chilean subduction zone south of Chiloe Island, located in the central part of the Mw 9.5 1960 Valdivia earthquake. This region is characterized by repeated earthquakes in 1960 and historical times with very sparse interseismic activity due to the subduction of a young (~15 Ma), and therefore hot, oceanic plate. We estimate the co-seismic slip distribution based on a kinematic finite fault source model, and through joint inversion of teleseismic body waves and strong motion data. The coseismic slip model yields a total seismic moment of 3.94×1020 Nm that occurred over ~30 s, with the rupture propagating mainly downdip, reaching a peak-slip of ~4.2 m. Regional moment tensor inversion of stronger aftershocks reveals thrust type faulting at depths of the plate interface. The fore- and aftershock seismicity is mostly related to the subduction interface with sparse seismicity in the overriding crust. The 2016 Chiloe event broke a region with increased locking and most likely broke an asperity of the 1960 earthquake. The updip limit of the main event, aftershocks, foreshocks and interseismic activity are spatially similar, located ~15 km offshore and parallel to Chiloe Islands west coast. The coseismic slip model of the 2016 Chiloe earthquake suggests a peak slip of 4.2 m that locally exceeds the 3.38 m slip deficit that has accumulated since 1960. Therefore, the 2016 Chiloe earthquake possibly released strain that has built up prior to the 1960 Valdivia earthquake.

Aftershocks of the 2012Mw 7.6 Nicoya, Costa Rica, Earthquake and Mechanics of the Plate Interface

Abstract Subduction of the Cocos plate beneath the Nicoya Peninsula, Costa Rica, generates large underthrusting earthquakes with a recurrence interval of about 50 yrs. The most recent of these events occurred on 5 September 2012 ( M w 7.6). A vigorous sequence of more than 6400 aftershocks was recorded by a local seismic network within the first four months after the mainshock. We determine locations and focal mechanisms for as many aftershocks as possible with M ≥1.5 occurring within the first nine days of the mainshock, all aftershocks with M ≥3 through the end of 2012, and all events with M ≥4 through the end of 2015. We determine faulting geometries using regional full waveform moment tensor (MT) inversion for the largest events ( M ≥4) and P ‐wave first‐motion polarities for smaller events, producing a mechanism catalog with 347 earthquakes. Sixty percent of these events are identified as underthrusting, and their locations are compared with spatial distributions of mainshock slip, afterslip, prior interplate seismicity, and slow‐slip phenomena to better understand the mechanical behavior of the plate interface. Most of the aftershocks on the megathrust occur up‐dip of the coseismic slip, where afterslip is large, and between coseismic slip and shallow slow‐slip patches. The pattern of interplate seismicity during the interseismic period is similar to that for the aftershocks but does not extend to as great a depth. The coseismic slip extends even deeper than the interplate aftershocks, suggesting that the mainshock ruptured a strongly locked patch driving down‐dip slip into the conditionally stable part of the deep plate interface that also hosts slow slip. About 80% of the aftershocks have one nodal plane oriented favorably to promote failure from static stress changes following the mainshock and early afterslip, whereas most others occur in regions of large afterslip. Electronic Supplement: Tables of hypocenter location and focal mechanisms and selection criteria for underthrusting events and figures showing the Gutenberg–Richter distribution, regional moment tensor inversion, spatial distribution of earthquake activity, and temporal distribution of underthrusting events.

SOURCE PARAMETERS OF MODERATE AND STRONG EARTHQUAKES IN THE BROADER AREA OF ZAKYNTHOS ISLAND (W. GREECE) FROM REGIONAL AND TELESEISMIC DIGITAL RECORDINGS

The source parameters of all the moderate and strong events that occurred in the broader area of Zakynthos Island for the period 1997–2009 are determined using two different techniques depending on epicentral distance. For the strong events we employed teleseismic body-wave modeling between 30° and 90°, while for moderate events a regional moment tensor inversion approach was used. In both cases we used broadband waveform data with a good signal-to-noise ratio. The calculated focal mechanisms are representative of the displacement and strain fields of the broader area. Those associated with the Cephalonia transform fault are consistent with dextral strike-slip motion. The area between the island of Zakynthos and the Western coast of Peloponnese is also characterized by strike–slip faulting, while reverse faulting is mainly observed south of Zakynthos Island. Using both techniques, we also retrieved the source parameters of the biggest events of the seismic sequence of April 2006 near the southern coast of Zakynthos Island. The depths of the located events of the sequence vary mainly between 10 and 25 km. The fault plane solutions revealed thrust type faulting in all cases, something which can be related with upward motions in the southern part of the Island observed by DGPS measurements.

The 2015 Canada Day,Mw 3.8, Earthquake in Nova Scotia

ABSTRACT On 1 July 2015, a magnitude ( M N , M w ) 3.8 earthquake occurred north of Yarmouth in southwestern Nova Scotia, a region of extremely low seismicity. Prior to this event, only five earthquakes had been recorded within 50 km of its epicenter since 2000. The earthquake caused no damage but was widely felt in southwestern Nova Scotia with nearly 600 people filling out the Geological Survey of Canada’s online “Did You Feel It?” form. Good‐quality regional seismograms enabled us to determine the source parameters of the earthquake. The focal mechanism determined by a regional centroid moment tensor inversion is indicative of oblique‐thrust faulting. We believe this to be the first focal mechanism solution for a Nova Scotian earthquake. It is consistent with those of earthquakes in the neighboring regions of New Brunswick and Maine and with the regional stress field. Depths were obtained by the moment tensor inversion and by regional depth phase modeling, with consistent results indicative of a focal depth of 9–10 km. The M w 3.8 is larger than average based on the M N value but within the range of previous observations for eastern Canada. A magnitude ( M N ) 2.2 aftershock occurred ∼7 min after the mainshock but was not reported as felt. A focal depth of 8 km was obtained via the regional depth phase method. The focal mechanism of the aftershock was not determined directly, but a comparison of the waveforms to those of the mainshock suggests a similar source.

Regional Moment Tensor Inversion for Earthquakes in Turkey and Its Surroundings: 2008–2015

ABSTRACT Turkey and its surrounding regions are tectonically active and have high‐seismicity rates. The seismicity in the region is observed by the national networks of the Kandilli Observatory and Earthquake Research Institute (KOERI) and the Disaster and Emergency Management Presidency Earthquake Department (AFAD). The quality and quantity of seismic stations improved significantly after the 17 August 1999 M w 7.4 Izmit earthquake. In the meantime, a number of focal mechanism catalogs were produced, covering different time periods. However, a homogeneous long‐term catalog of moment tensor (MT) solutions with increased data amount and quality was not available for the region. In this article, we perform a systematic MT inversion of 133 M ≥4.5 earthquakes within Turkey between 2008 and 2015, using the dense seismic networks of KOERI and AFAD. The use of combined networks and site‐specific velocity models for the computation of MT inversion results in high‐quality solutions of focal mechanisms. This homogeneous focal mechanism catalog provides important new information on the seismotectonics in this region.

The regional moment tensor of the 5 May 2014 Chiang Rai earthquake (Mw = 6.5), Northern Thailand, with its aftershocks and its implication to the stress and the instability of the Phayao Fault Zone

On 5 May 2014, the largest earthquake in Thailand modern history occurred in Northern Thailand with over a thousand aftershocks. Most of the epicenters are located within the transition area of the Mae Lao segment (north) and Pan segment (central) of the Phayao Fault Zone (PFZ). Good quality data from all events (ML>4) are only available for the seismic stations closer to the epicenters (<500km). The regional moment tensor (RMT) inversion was applied to derive a sequence of thirty focal mechanisms, moment magnitudes and source depths generated along the PFZ. Our studies reveal that 24 events are strike – slip with normal (transtensional), four are strike – slip with thrust (transpressional), and two are reverse. The main shock has an Mw of 6.5, slightly larger than previously estimated (ML 6.3) while Mw of the aftershocks is mostly lower than ML. This suggests that a regional magnitude calibration is necessary. The hypocenter depths of most events are around 11km, not as shallow as estimated earlier. In addition, a stress inversion was applied to these 30 focal mechanisms to determine the stresses of the region, the Mohr’s diagram, and the principal fault planes. The retrieved maximum stress direction (N18E) is in agreement with other studies. One of the derived principal fault plane with a strike of N48E is in good agreement with that of the Mae Lao segment. Both estimated shape ratio and plunges led us to conclude that this area has a uniaxial horizontal compression in NNE-SSW with small WNW-ESE extension, similar to the interpretation of Tingay et al. (2010). Based on the Mohr’s diagram of fault plane solutions, we provide geophysical evidence which reveals that the high shear stress Mae Lao segment is likely to slip first producing the main shock on 5 May 2014. The energy transfer between the segments has then led to many aftershocks with mixed mechanisms. At the end, we re-visited the analysis of the former largest earthquake in Northern Thailand in the past decades, the 11 September 1994 event. Its focal mechanism was re-calculated based on the available P-wave polarities. The strike – slip motion should be the mechanism of the earthquake, not the normal motion as originally believed.

Complex Rupture Process of the 19 March 2013, Rudna Mine (Poland) Induced Seismic Event and Collapse in the Light of Local and Regional Moment Tensor Inversion

On 19 March 2013, a strong, shallow, induced seismic event struck a mining panel in the room‐and‐pillar Rudna copper mine in southeastern Poland. The event caused important damage at the mining tunnel and trapped 19 miners, who were safely rescued a few hours later. Although mining‐induced seismicity is frequent at this mine, the 19 March event was unusual because of its larger magnitude, its occurrence far from the mining stopes, and because it was accompanied by a strong hazardous rockburst. The mining inspections following the event verified the occurrence of a rockfall with tunnel floor uplift but also recognized the presence of a faulting structure at the hypocentral location. The availability of three monitoring networks (including local and regional data, short‐period and broadband seismometers, and surface and in‐mine installation) presented an optimal setup to determine rupture parameters and to compare the performance and results from different installations. We performed waveform and spectral‐based analysis to infer source properties, with a particular interest to the determination of the rupture processes, using different moment tensor (MT) inversion techniques. Our results are surprisingly different, ranging from a dominant thrust mechanism, resolved at closest distances, to a collapse‐type rupture, resolved at regional distances. We demonstrate that a complex rupture model is needed to explain all observations and to justify these discrepancies. The final scenario indicates that the rupture nucleated as a weaker thrust mechanism along a pre‐existing weakened surface and then continued in a more energetic collapse event. The local LUMINEOS surface network has the potential to resolve both subevents but not using a standard MT decomposition. Here, we propose a new MT decomposition and an alternative MT fitting procedure that can be used to analyze the MT of collapse sources.