Centroid Moment Tensor Solutions Research Articles

Seismotectonics and stress perspective of the Kumaon Himalaya: A geophysical evidence of a Lesser Himalayan duplex

The Kumaon Himalaya lies in the central seismic gap and therefore bears the potential to host a great Himalayan earthquake. The seismicity and seismotectonics of the Kumaon Himalaya and the adjacent region have been investigated based on local earthquake data recorded at 18 seismological stations. The seismically active zone of the eastern segment of the Kumaon Himalaya deviates from the usual pattern of seismicity in the Himalayan Seismic Belt of the NW Himalaya. Shallow-focus earthquakes in this region largely concentrate in the Chiplakot Crystalline Belt (CCB) immediately south of the Vaikrita Thrust. The Focal Mechanism Solutions of 41 earthquakes computed through waveform inversion technique along with the solutions of 12 earthquakes obtained from the Global Centroid Moment Tensor solution catalog are used to investigate the kinematics of the region. The study reveals a complex faulting pattern in the Inner Lesser Himalaya. The stress inversion results show a widely distributed stress pattern and low frictional coefficient which are attributed as one of the major causes of clustered seismicity observed in the CCB. Careful examination of the fault orientations indicate the presence of a hinterland dipping Lesser Himalayan Duplex over the ramp structure on the Main Himalayan Thrust. The high compressive stress and deformation rate in the CCB are partially accommodated by this duplex structure. The large concentration of shallow-focus earthquakes in the CCB is the result of the presence of fluid-rich zone as well as strain localization and large stress build-up due to locking in the ramp structure on the Main Himalayan Thrust beneath the CCB.

Towards a homogeneous moment magnitude determination for earthquakes in South Africa: Reduction of associated uncertainties

New MW–ML magnitude relations valid for the periods January 1971–March 1997, April 1997–September 2012 and October 2012–present are presented for South Africa. Three MW–ML relations were derived for the respective periods by applying a general orthogonal regression using 85 mining-related and tectonic events. Moment magnitudes (MW) were calculated using spectral analysis of the vertical component seismograms. A Brune model for source radiation was assumed. Attenuation functions previously derived from local seismic events were used. For each event, the final MW was computed by taking the average of the values determined at four or more stations. These MW values were used to compute new MW –ML regression equations for three previous ML relations that had been used to compile the South African earthquake catalogue. The new MW–ML relations were then compared with the MW determined from the Harvard Centroid Moment Tensor (CMT) project. The MW computed using the relation derived for the most recent period (October 2012–present) produces MW values that are similar to those reported by the Harvard CMT solutions. The relations can be used in the homogenization of the South African earthquake catalogue. Application of the new relations for the three periods will reduce uncertainty in the homogenized catalogues, which are important when calculating seismic hazard parameters (i.e. when estimating activity rates, b-values and Mmax).

Regional Earthquake Magnitude Conversion Relations for the Himalayan Seismic Belt

AbstractWe present regional earthquake magnitude conversion relations among different magnitude scales (Mw, Ms, mb, ML, and MD) for the Himalayan seismic belt developed from data of local, regional, and international seismological agencies (International Seismological Centre [ISC], National Earthquake Information Centre [NEIC], Global Centroid Moment Tensor Solution [CMT], International Data Centre [IDC], China Earthquake Administration [BJI], and National Centre for Seismology [NDI]). The intra- (within the same magnitude scale) and inter- (with different magnitude scales) magnitude regression relations have been established using the general orthogonal regression and orthogonal distance regression techniques. Results show that the intra-magnitude relations for Mw, Ms, and mb reported by the Global CMT, ISC, and NEIC exhibit 1:1 relationships, whereas ML reported by the IDC, BJI, and NDI deviates from this relationship. The IDC underestimates Ms and mb compared with the ISC, NEIC, and Global CMT; this may be due to different measurement procedures adopted by the IDC agency. The inter-magnitude relations are established between Mw,Global CMT and Ms, mb, and ML reported by the ISC, NEIC, IDC, and NDI, and compared with the previously developed regional and global regression relations. The duration (MD) and local (ML) magnitudes reported by NDI exhibit a 1:1 relationship. The derived magnitude regression relations are expected to support the homogenization of the earthquake catalogs and to improve seismic hazard assessment in this region.

GCAPjoint, A Software Package for Full Moment Tensor Inversion of Moderately Strong Earthquakes with Local and Teleseismic Waveforms

AbstractEarthquake moment tensors and focal depths are crucial to assessing seismic hazards and studying active tectonic and volcanic processes. Although less powerful than strong earthquakes (M 7+), moderately strong earthquakes (M 5–6.5) occur more frequently and extensively, which can cause severe damages in populated areas. The inversion of moment tensors is usually affected by insufficient local waveform data (epicentral distance <5°) in sparse seismic networks. It would be necessary to combine local and teleseismic data (epicentral distance 30°–90°) for a joint inversion. In this study, we present the generalized cut-and-paste joint (gCAPjoint) algorithm to jointly invert full moment tensor and centroid depth with local and teleseismic broadband waveforms. To demonstrate the effectiveness and explore the limitations of this algorithm, we perform case studies on three earthquakes with different tectonic settings and source properties. Comparison of our results with global centroid moment tensor and other catalog solutions illustrates that both non-double-couple compositions of the focal mechanisms and centroid depths can be reliably recovered for very shallow (<10 km) earthquakes and intermediate-depth events with this software package.

Source Mechanisms and stress field of the 2017 Ayvacık/Çanakkale earthquake sequence in NW Turkey

Ayvacik district of Canakkale province was hit by four moderate size earthquakes on 6 February 2017 03:51 M w 5.2, 6 February 2017 10:58 M w 5.1, 7 February 2017 02:24 M w 5.2, 12 February 2017 13:48 M w 5.3. Centroid moment tensor solutions for 60 events with moment magnitudes (M w ) between 3.4 and 5.3 are computed by applying a waveform inversion method on data from the Turkish and Greek broadband seismic networks. The time span of data covers the period between 1 January and 28 February 2017. The mainshock is a shallow focus normal event with strike-slip component at a depth of 10 km. Focal depths of aftershocks range from 3 to 24 km. The seismic moment (M o ) of the mainshock is estimated as 7.62 × 10 16 Nm. The calculated rupture duration of the Ayvacik mainshock is 3 s. The focal mechanisms of the aftershocks are mainly normal faulting with a minor strike-slip component. The geometry of focal mechanisms reveals a normal faulting regime with NE-SW trending direction of T -axis in the entire activated region. We perform stress tensor inversion to acquire more accurate picture for the regional stress field. The stress tensor inversion results indicate a predominant normal stress regime with a WNW-ESE oriented maximum horizontal compressive stress (S H ). With respect the newly determined focal mechanisms, the effect of the propagation of the North Anatolian Fault into Aegean Sea is very clearly pronounced. According to the double-difference relocation algorithm for the 2017 Ayvacik seismic sequence, ten clusters are revealed. In order to find these clusters, we use HypoDD software. According to the relative relocations from HypoDD, we define 10 clusters along the five profiles. The aftershock activity in the observation period between 1 January and 28 February 2017 extends from W to E direction. Seismic cross-sections indicate that a complex pattern of the hypocenter distribution with the ruptured segments. The western cluster is associated with a fault plane trending mainly N- S and dipping vertical, while the eastern is related to a fault plane trending NE-SW and dipping towards NW. The best constrained focal depths operate in the approximate depth range from 3 to 24 km. Coulomb stress analysis is performed to calculate the cumulative effect of all the earthquakes of the swarm and correlate it with the activated region. Positive lobes with stress more than 3 bars are obtained, these values are large enough to trigger failure towards NW-SE of the swarm activity.

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.

Teleseismic body wave inversion in determining rupture process of Sumbawa doublet earthquake, November 25th, 2007

The rupture patterns of the Sumbawa doublet earthquake that occurred on November 25th, 2007 was estimated using teleseismic body wave inversion. The first event occurred at 16:02:18 UTC with epicenter at 8.292°N and 118.37°E, short after this, the second event occurred at 19:53:08 UTC with epicenter at -8.224°N and 118.467°E. For each event, teleseismic body wave data were retrieved from the Incorporated Research Institutions for Seismology – Data Management Center (IRIS-DMC). The waves were windowed for 70 s which started at 20 s before the P-wave arrival, band-pass filtered between 0. 01 and 0.1 Hz and then integrated into a displacement with a sampling time of 0.5 s. The Green’s function is calculated using the Kikuchi and Kanamori method. The fault geometry is based on the earthquake parameters of the Global Centroid Moment Tensor solution. Strike and dip for first event were 88° and 23°, respectively and those for the second event were 87° and 25°, respectively. The seismic moment (Mo) estimated by the inversion for the first event is 0.617×1019 Nm or equivalent to moment magnitude (Mw) 6.46 with source duration of approximately 55 s. While for the second event, the seismic moment is estimated to be 0.644×1019 Nm (Mw 6.47), with slightly longer source duration. The inversion for both events provided a similar slip pattern. The rupture propagated along the dip direction, and the maximum slip occurred near the hypocenter region. The maximum slip amount for first event and second event were 1.43 m and 1.48 m, respectively.

Earth's deepest earthquake swarms track fluid ascent beneath nascent arc volcanoes

Most of the world's explosive volcanoes are located in volcanic arcs, formed by fluid-fluxed melting of upper mantle rocks. The fluids that facilitate melting are released from subducted tectonic plates as they sink into the mantle. Yet, we have sparse knowledge of the migration pathways of melts through the upper mantle (i.e., between the surface of the subducted plate and arc volcanoes). We are also uncertain of the time required for this migration to occur. Here, we show evidence of two earthquake swarms that occur in the upper mantle beneath the Mariana and Izu-Bonin arc systems. The best-resolved swarm occurs beneath the Mariana arc, where the earthquakes define a sub-vertical pipe-like structure with a diameter of ∼50 km and occurs between depths of ∼10–250 km. To test the robustness of depth locations, we used a fully non-linear grid search algorithm, double-difference relocation, as well as an analysis of pP-P arrival times and depth sensitive phases. In addition, we calculated centroid moment tensor solutions using a 3D Earth model to understand the mechanism of failure within the swarms. These data demonstrate that the sub-vertical earthquake swarm occurs within the upper mantle between the subducted slab and the overriding volcanic arc, with seismicity concentrated within discrete day- to month-long swarms of activity over a single two-year period. The Izu-Bonin example shares a similar sub-vertical pipe-like geometry with seismic activity bracketed within a two-year period. We infer that these rare earthquake swarms record the ascent of hydrous melt and/or fluid, from dehydration of the subducting plate. This implies that hydrous minerals within subducted slabs continue to dehydrate to depths of at least 200–250 km. Also, the short duration of earthquake swarms implies that fluids/melts can be rapidly transported through the sub-arc mantle at rates in the order of km/hr. This is consistent with rapid melt ascent rates inferred from geospeedometry and experimental petrology and is reminiscent of patterns seen during episodic tremor and slip events.

Estimation of the Seismic Moment Rate from an Incomplete Seismicity Catalog, in the Context of the InSight Mission to Mars

We evaluate methods to estimate the global seismic moment rate of a planet from the k >= 1 largest events observed during a limited and possibly short-time span, as can be expected, for example, for lander missions to Mars. The feasibility of the approach is demonstrated with a temporary broadband seismometer that was recording in the Mojave desert, California, for 86 days in 2014, and by application to the Global Centroid Moment Tensor (CMT) catalog, subsets thereof, and a catalog of stable continental regions seismicity. From the largest event observed at Goldstone alone (M-w approximate to 7.9), we estimate the Earth's global moment rate to be 1.03 x 10(22) N . m/yr, whereas an estimation based on the 10 strongest events yields a rate of 5.79 x 10(21) N . m/yr. Summation of 42 yr of Global CMT solutions results in an average of 7.61 x 10(21) N . m/yr. In general, a 2 yr interval of Global CMT solutions is sufficient to estimate the Earth's annual moment rate to within a factor of 5 or better. A series of numerical experiments with more than 560 million synthetic catalogs based on the tapered Gutenberg-Richter distribution shows that the estimation is rather insensitive against an unknown slope of the distribution, and that bias and variance of the estimator depend on the ratio between moment rate and corner moment of the size frequency distribution. Moment rates of published Mars models differ by a factor of 1000 or more. Tests with simulated catalogs show that it will be possible to reject some of these models with data returned by NASA's InSight mission after two years of nominal mission life time.

Performance of uniform and heterogeneous slip distributions for the modeling of the November 2016 off Fukushima earthquake and tsunami, Japan

The Mw 6.9 earthquake off Fukushima Prefecture, Japan, of 22 November 2016 was followed by a tsunami that struck the Japanese coast from Hokkaido in northern Japan to Wakayama Prefecture in western Japan. We compared the performance of a seismologically deduced single-fault model, a seismologically deduced finite fault slip model (FFM), an optimized single-fault model based on tsunami data, the FFM with horizontal shift, and the tsunami waveform inversion models of the previous studies considered for this earthquake regarding reproduction of tsunami waves by tsunami computations. It is important to discuss how these models work well because it is sometimes desirable to obtain an earthquake source model to estimate tsunami waves with a simple process obtained with limited data from the viewpoint of tsunami prediction. The seismologically deduced FFM has an advantage in terms of the information of slip regions of fault plane and was superior to the seismologically deduced single-fault model, especially in predicting amplitudes of tsunami waves. This means that when only with seismic data, the FFM could narrow the range of forecast of tsunami amplitude. In the comparison of models optimized with tsunami data, the single-fault model showed the almost equivalent performance of the tsunami waveform inversion models of previous studies regarding the waveform coincidence with observations and the horizontal location at the negative peak of the initial sea surface displacement. In case the main generation region of the tsunami is concentrated in one place, the tsunamis can be expressed by a single-fault model by conducting the detailed grid search. We also confirmed that the centroid location of centroid moment tensor (CMT) solution and the absolute location of the FFM were not necessarily suitable to express tsunamis, while the moment magnitude, the focal mechanism, the centroid depth of CMT solution, and the relative slip distribution of the FFM were effective to represent tsunamis. Since this event occurred at the shallow depth, the speed of tsunami wave is particularly slow. Therefore, it would be advisable to pay attention to the horizontal uncertainty to apply seismologically obtained solution to tsunami forecast, especially when a tsunami occurs in shallow water.

Regional stress field in Yunnan revealed by the focal mechanisms of moderate and small earthquakes

We determined focal mechanism solutions of 627 earthquakes of magnitude M ≥ 3.0 in Yunnan from January 2008 to May 2018 by using broadband waveforms recorded by 287 permanent and temporary regional stations. The results clearly revealed predominantly strike-slip faulting characteristics for earthquakes in Yunnan, with focal depths concentrated in the top 10 km of the crust. The earthquake mechanisms obtained were combined with the global centroid moment tensor solutions of 80 additional earthquakes from 1976 to 2016 to invert for the regional variations of stress field orientation by using a damped regional-scale stress inversion scheme. Results of the stress field inversion confirmed that the Yunnan region is under a strike–slip stress regime, with both maximum and minimum stress axes being nearly horizontal. The maximum compressional axes are primarily oriented in a northwest-southeast direction, and they experience a clockwise rotation from north to south, whereas the maximum extensional axes are oriented largely northeast-southwest. The maximum compressional axes are in line with the global positioning system–inferred horizontal velocity field and the southeastward escape of the Sichuan–Yunnan Rhombic Block, whereas the maximum extensional axes are consistent with anisotropy derived from SKS splitting. Against the strike–slip background, normal faulting stress regimes can be seen in the Tengchong volcanic area as well as in other areas with complex crisscrossing fault zones.

Prediction of Tsunami Waves by Uniform Slip Models

AbstractConventional tsunami warning systems for local and far‐field areas utilize uniform slip models to predict tsunami waves. The reasons are twofold. First, it is challenging to develop accurate finite‐fault slip models in a short time after an earthquake. Second, tsunami waves are long waves, and hence, the main feature may be predicted without knowing earthquake rupture details. Still, there have been few studies that quantitatively analyze the errors caused by uniform slip models. In this paper, we evaluate if and how such models may be applied for tsunami warnings. For the 2011 Tohoku, 2014 Iquique, and 2015 Illapel tsunamis, we first construct optimum uniform slip models with minimum tsunami waveform misfit and then compare the synthetic tsunami waves with finite‐fault model predictions. Predictions from both type of models match the tsunami data very well, indicating that the prediction errors caused by neglecting slip heterogeneity are insignificant. Further, we derive a common relation between the rupture area and earthquake magnitude. Additionally, the optimum rupture length and width ratio in terms of predicting tsunami waves is determined to be 1 for the three earthquakes. Lastly, we find that moving the uniform slip model to the center of global Centroid Moment Tensor solution produces reasonably small errors in the predicted waveforms. Applying the methodology to three more historic tsunamis shows that uniform slip models can well recover the Deep‐ocean Assessment and Reporting of Tsunamis system recordings, but the rupture center can differ from the global Centroid Moment Tensor solution. Our findings can potentially prompt more reliable tsunami warning strategies for future events.

Interaction Between Moderate Earthquakes and Volcanic Eruptions: Analyses of Global Data Catalog

AbstractThe interaction between earthquakes and volcanic eruptions is investigated by analyzing a modern seismic data catalog of centroid moment tensor solutions and eruption records summarized by the Smithsonian Institution. To determine the average features of the interactions, the numbers of earthquakes that occurred before and after the initiation of each eruption with a volcanic explosivity index of larger than or equal to 2 are analyzed. The results show that moderate earthquakes, which have moment magnitudes of 5 to 6 (Mw 5–6), are activated within a horizontal distance of 50 km from volcanoes for about 0.3 years after the initiation of an eruption. About 13% of volcanic eruptions are accompanied by moderate earthquakes. The occurrence possibility of earthquakes every 0.1 years for 0.3 years after an eruption is about 5 times larger than normal. No significant dependence on the tectonic settings and major rock types is recognized.

Contribution from Multiple Fault Ruptures to Tsunami Generation During the 2016 Kaikoura Earthquake

The 2016 Kaikoura, New Zealand, earthquake was one of the most complex ruptures ever recorded. The epicentre was located well inland, but the rupture area extended offshore and generated a modest tsunami which was recorded by tide gauges. Here, we present a detailed estimate of seafloor vertical displacement during the earthquake sequence by a joint inversion of tsunami waveforms and vertical displacement data observed at GPS stations and obtained by field surveys. The combined dataset provides a solution with good resolution, capable of resolving test sources of 20 km of characteristic diameter throughout the study area. We found two seafloor uplift regions which are located very close to the coast, one is located offshore of the Kaikoura peninsula and the other larger uplift region is located near the Kekerengu and Needles faults. To estimate crustal deformation with a complete spatial coverage of the event, the estimated seafloor vertical displacement was combined with the inland vertical displacement from InSAR and GPS datasets. This vertical displacement is then inverted for the fault slip distributions of the Needles, Jordan–Kekerengu, Papatea, Hundalee, Hump faults, and a newly identified fault beneath Kaikoura. We also found that the Needles fault is probably an offshore extension of the Kekerengu fault. The seismic moment calculated from the fault slip distributions by assuming a rigidity of 2.7 × 1010 N/m2, is 5.19 × 1020 Nm or equivalent to Mw 7.8. This total seismic moment estimate is consistent with that of the Global Centroid Moment Tensor solution. The tsunami potential energy calculated from the seafloor vertical displacement is 9.40 × 1012 J, of which about 70% is attributed to movement on the faults known to have ruptured, suggesting a secondary source for tsunami generation.

InSAR-Based Mapping to Support Decision-Making after an Earthquake

It has long been recognized that earthquakes change the stress in the upper crust around the fault rupture and can influence the behaviour of neighbouring faults and volcanoes. Rapid estimates of these stress changes can provide the authorities managing the post-disaster situation with valuable data to identify and monitor potential threads and to update the estimates of seismic and volcanic hazard in a region. Here we propose a methodology to evaluate the potential influence of an earthquake on nearby faults and volcanoes and create easy-to-understand maps for decision-making support after large earthquakes. We apply this methodology to the Mw 7.8, 2016 Ecuador earthquake. Using Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) and continuous GPS data, we measure the coseismic ground deformation and estimate the distribution of slip over the fault rupture. We also build an alternative source model using the Global Centroid Moment Tensor (CMT) solution. Then we use these models to evaluate changes of static stress on the surrounding faults and volcanoes and produce maps of potentially activated faults and volcanoes. We found, in general, good agreement between our maps and the seismic and volcanic events that occurred after the Pedernales earthquake. We discuss the potential and limitations of the methodology.

Estimation of Seismic Centroid Moment Tensor Using Ocean Bottom Pressure Gauges as Seismometers

AbstractWe examined the dynamic pressure change at the seafloor to estimate the centroid moment tensor solutions of the largest and second largest foreshocks (Mw 7.2 and 6.5) of the 2011 Tohoku‐Oki earthquake. Combination of onshore broadband seismograms and high‐frequency (~20–200 s) seafloor pressure records provided the resolution of the horizontal locations of the centroids, consistent with the results of tsunami inversion using the long‐period (≳10 min) seafloor pressure records although the depth was not constrained well, whereas the source locations were poorly constrained by the onshore seismic data alone. Also, the waveforms synthesized from the estimated CMT solution demonstrated the validity of the theoretical relationship between pressure change and vertical acceleration at the seafloor. The results of this study suggest that offshore pressure records can be utilized as offshore seismograms, which would be greatly useful for revealing the source process of offshore earthquakes.

Complex rupture process of the Mw 7.8, 2016, Kaikoura earthquake, New Zealand, and its aftershock sequence

The M7.8 Kaikoura Earthquake that struck the northeastern South Island, New Zealand, on November 14, 2016 (local time), is one of the largest ever instrumentally recorded earthquakes in New Zealand. It occurred at the southern termination of the Hikurangi subduction margin, where the subducting Pacific Plate transitions into the dextral Alpine transform fault. The earthquake produced significant distributed uplift along the north-eastern part of the South Island, reaching a peak amplitude of ∼8 m, which was accompanied by large (≥10 m) horizontal coseismic displacements at the ground surface along discrete active faults. The seismic waveforms' expression of the main shock indicate a complex rupture process. Early automated centroid moment tensor solutions indicated a strong non-double-couple term, which supports a complex rupture involving multiple faults. The hypocentral distribution of aftershocks, which appears diffuse over a broad region, clusters spatially along lineaments with different orientations. A key question of global interest is to shed light on the mechanism with which such a complex rupture occurred, and whether the underlying plate-interface was involved in the rupture. The consequences for seismic hazard of such a distributed, shallow faulting is important to be assessed. We perform a broad seismological analysis, combining regional and teleseismic seismograms, GPS and InSAR, to determine the rupture process of the main shock and moment tensors of 118 aftershocks down to Mw 4.2. The joint interpretation of the main rupture and aftershock sequence allow reconstruction of the geometry, and suggests sequential activation and slip distribution on at least three major active fault domains. We find that the rupture nucleated as a weak strike-slip event along the Humps Fault, which progressively propagated northward onto a shallow reverse fault, where most of the seismic moment was released, before it triggered slip on a second set of strike-slip faults at the northern end of the rupture. The northern and southern strike-slip fault domains have the same orientation but are spatially separated by >15 km. In our model, the low angle splay thrust fault is located above the slab and connects the strike-slip faults kinematically. During the aftershock phase, the entire fault system remained active.

The Mw6.5 17 November 2015 Lefkada (Greece) Earthquake: Structural Interpretation by Means of the Aftershock Analysis

The 2015 Mw6.5 Lefkada main shock occurred at the south western part of Lefkada Island (Greece), less than 2 years after the occurrence of a doublet along the western part of the nearby Kefalonia Island, Paliki peninsula (on 25/01/2014, with Mw6.1 and 03/02/2014 with Mw6.0) and 12 years after the 2003 Mw6.2 main shock that struck the northwestern part of Lefkada Island. The four failed dextral strike slip fault segments belong to the Kefalonia transform fault zone (KTFZ), the major active boundary that bounds from the west the area of central Ionian Islands, namely Lefkada and Kefalonia. It is associated with several known historical earthquakes and is considered the most hazardous area in the Greek territory. The KTFZ fault segments are characterized by high slip rates (of the order of tens of millimeters per year), with maximum earthquake magnitudes up to 6.7 for Lefkada and 7.2 for Kefalonia fault zone, respectively. The double difference location technique was employed for relocating the aftershocks revealing a seismogenic layer extending from 3 to 16 km depth and multiple activation on well-defined fault planes, with strikes that differ than the main rupture and dips either to east or to west. This implies that strain energy was not solely released on a main fault only, but on secondary and adjacent fault segments as well. The reliable definition of their geometry forms the basis for the structural interpretation of the local fault network. The aftershock spatial distribution indicates three main clusters of the seismic activity, along with activation of smaller faults to an extent of more than 50 km. A northeasterly striking cluster is observed to the north of the main shock epicenter, with a remarkable aftershock density. The central cluster is less dense than the previous one with an epicentral alignment in full accordance with the strike provided by the main shock centroid moment tensor solution, and is considered as the main rupture with a length of 17 km. The third cluster, encompassing a large number of aftershocks, is located in the offshore area between Lefkada and Kefalonia Islands with a NE–SW epicentral alignment, alike the first cluster. The northeast–southwest striking secondary faults positioned obliquely and in continuation of the main fault segment, reveal that the KTFZ is being deformed in a complex tectonic setting. The presence of faults with this geometry implies strain partitioning and sheds light to new components necessary to be taken into account in the seismic hazard assessment. Stress transfer models of the M ≥6.0 main shocks were investigated and the calculated static stress changes may well explain their sequential occurrence. Static stress changes due to the 2015 coseismic slip were also calculated with the main objective of exploring the aftershock occurrence pattern and it was found as the driving mechanism that triggered the vast majority of the off-fault aftershocks.

Coseismic changes of gravitational potential energy induced by global earthquakes based on spherical‐Earth elastic dislocation theory

AbstractWe compute the coseismic gravitational potential energy Eg change using the spherical‐Earth elastic dislocation theory and either the fault model treated as a point source or the finite fault model. The rate of the accumulative Eg loss produced by historical earthquakes from 1976 to 2016 (about 42,000 events) using the Global Centroid Moment Tensor Solution catalogue is estimated to be on the order of −2.1 × 1020 J/a, or −6.7 TW (1 TW = 1012 W), amounting to 15% in the total terrestrial heat flow. The energy loss is dominated by the thrust faulting, especially the megathrust earthquakes such as the 2004 Sumatra earthquake (Mw 9.0) and the 2011 Tohoku‐Oki earthquake (Mw 9.1). It is notable that the very deep focus events, the 1994 Bolivia earthquake (Mw 8.2) and the 2013 Okhotsk earthquake (Mw 8.3), produced significant overall coseismic Eg gain according to our calculation. The accumulative coseismic Eg is mainly lost in the mantle of the Earth and also lost in the core of the Earth but with a relatively smaller magnitude. By contrast, the crust of the Earth gains gravitational potential energy cumulatively because of the coseismic deformations. We further investigate the tectonic signature in the coseismic crustal Eg changes in some complex tectonic zone, such as Taiwan region and the northeastern margin of the Tibetan Plateau. We found that the coseismic Eg change is consistent with the regional tectonic character.

A Comprehensive Earthquake Catalog for Iraq in Terms of Moment Magnitude

A comprehensive earthquake catalog was compiled for Iraq and neighboring areas as part of a broader probabilistic seismic‐hazard assessment project. The Iraq Seismic Network (ISN) was established in 1976 and became operational in the early 1980s. However, recording and reporting of seismic data has been intermittent in Iraq. Hence, events were collected from various sources, including the ISN when available, International Seismological Centre (ISC), European‐Mediterranean Seismological Centre, U.S. Geological Survey Centennial Catalog, Global Centroid Moment Tensor solutions, and Ambraseys’ extensive work on cataloging of instrumental era earthquakes in the Middle East (e.g., Ambraseys, 1978, 2001, 2009). We supplement these with new direct moment magnitude calculations based on coda calibration technique. For many of the larger events in the catalog, more than one magnitude is available. Directly calculated moment magnitudes ( M w) were favored, followed by body‐wave magnitude ( m b) obtained from the ISC. Where no directly calculated M w was available, other magnitude scales were converted to M w using relationships compatible with the local catalog. The resulting earthquake catalog spans from the year 1900 until the end of 2009, covers the region bounded by 26°–40° N latitudes and 36°–51° E longitudes, and includes more than 18,000 earthquakes, of which roughly 4000 are M w 4.0 or larger.