Double-difference Algorithm Research Articles

Relocation of mainshock and aftershock sequence of the <italic>M</italic><sub>s</sub>7.0 Sichuan Jiuzhaigou earthquake

An earthquake of M s7.0 struck Jiuzhaigou Country in Sichuan Province on 8 August 2017. It occurred at the northeastern boundary of the Bayan Har block. The Jiuzhaigou earthquake was one of the largest earthquakes in continental China since the 2013 M s7.0 Lushan earthquake, and it was widely felt across Sichuan and adjacent provinces. The earthquake left 25 dead and up to 500 injured. Because the Jiuzhaigou earthquake did not produce obvious surface ruptures, the causative fault of this event was much debated. The shape of the seismogenic fault, the focal depth of the mainshock and the characteristics of the aftershock sequence were also not very clear. The Sichuan earthquake administration deployed 6 temporary seismic stations around the source region within 4 d of the mainshock. In this study, we collected seismic waveform data and phase arrival data recorded by permanent and temporary stations. We present a systematic investigation about the location of the Jiuzhaigou earthquake sequence with high precision. These results provide new constraints for refining earthquake rupture process, constructing seismogenic models, and evaluating regional earthquake risks. The mainshock of the Jiuzhaigou M s7.0 earthquake was relocated using a 3D velocity model. The procedure takes station elevation, topography and the Earth’s ellipticity into consideration. The location of the mainshock was found to be 103.806°E, 33.201°N, and the focal depth to be 20.4 km. The depth of the initial rupture point of the mainshock was estimated to be below 14 km with S-P arrival time difference recorded at the nearest strong motion station. The locations of early aftershocks were corrected with latter aftershocks recorded using dense temporary seismic stations. A double difference algorithm was used to relocate 1-month aftershock sequence of the Jiuzhaigou earthquake. The locations of 3030 aftershocks were here determined. The average location error was found to be 0.16, 0.15, and 0.18 km in the E-W, N-S, and U-D directions, respectively. The relocation results showed that the aftershocks spread approximately 42 km, trending NWW. The aftershock zone was found to connect the Tazang fault to the north and to the Huya fault to the south. The mainshock is located at the center of the aftershock zone, with similar aftershock length at the both sides. There is a ~5 km sparse aftershock segment to the NW of the mainshock which is consistent with large coseismic slip area. This phenomenon can be explained due to the large amount of stress released during the mainshock rupture. The depth of the aftershock in the northwest end was relatively shallow, and the aftershock zone was about 6 km wide. The depth of aftershock in the southeast section was deeper and the area was narrower, about 4 km. The predominant distribution of the focal depth is 4–20 km. The dip angle of the seismogenic fault is steep, with an average value of about 84°, and changes obviously along the strike direction. The seismogenic fault inclined to the southwest in the shallow part and to the northeast in the deep part. The complex fault shape should be considered when constructing seismogenic model and inverting earthquake rupture process. The depth of the initial rupture point of the mainshock is deeper than the centroid depth and average depth of the aftershock sequence, indicating that the earthquake rupture spreads from deep space to shallow. We find that the aftershocks migrated in the along-strike direction with logarithmic time since the mainshock. The length of the aftershock area expanded from 25 to 42 km in 2 d. The rate of spread was approximately 1.3 km/log(s), consistent with aftershock expansion caused by propagating afterslip. There may be afterslip in the source area of the Jiuzhaigou earthquake.

Characteristics in hypocenters of microseismic events due to hydraulic fracturing and natural faults: a case study in the Horn River Basin, Canada

For two to three decades, microseismic monitoring has been popular in the development of unconventional resources, because the fracture network generated by hydraulic fracturing mainly controls the productivity, and microseismic monitoring enables direct measurements for imaging the fracture network. Nevertheless, some refinements are required to make this method more practical. One challenge is to quantify the effects of pre-existing natural fractures for generating microseismic events. We determine the hypocenters of microseismic events occurring in a shale gas play in the Horn River Basin, Canada, and report several interesting spatial and temporal features of the hypocenter distributions. Automatic phase-picking is applied to waveform data recorded at 98 shallow buried three-component geophones, and phases thought to be from the same event are associated. The initial hypocenters of events are determined by iterative linear inversion algorithm then relocated using a double-difference algorithm, where relative travel time measurements are obtained with the waveform cross-correlation. We group events into many clusters based on fracking stages and their hypocenters, and then define the best-fitting plane of hypocenters for each cluster. Most strikes of the best-fitting planes are consistent with the direction of local horizontal stress maximum, indicating that hydraulic fracturing induces most microseismic events. However, the best-fitting planes of several clusters have strikes similar to those of pre-existing faults or fractures, indicating that pre-existing natural faults or fractures can affect the generation of microseismic events. In addition, some observations suggest that natural fractures can affect the temporal evolution of the spatial occurrence pattern of microseismic events. We observed specific migration patterns of microseismic events around known faults in the study area. Although further work is required for complete understanding of this phenomenon, our observations help elucidate the nature of microseismic generation.

Aplicación de la metodología de relocalización de sismos con el algoritmo de doble diferencia usando HYPODD en cuatro secuencias sísmicas en Costa Rica

Varias secuencias sísmicas ocurren durante un año en Costa Rica. Cuando estas son registradas por la Red Sismológica Nacional, cada evento es localizado para el catálogo nacional. El análisis y la caracterización del conjunto se hace luego con solo la localización manual. Este estudio incluye el post proceso de cuatro secuencias sísmicas con localización manual, correlación cruzada de eventos y relocalización con un algoritmo de doble diferencia para ubicar de una manera más precisa la fuente de estos eventos. Las secuencias agrupan desde 21 hasta 64 sismos, y ocurrieron en 3 a 21 días. Cada evento fue registrado en al menos 27 estaciones, con una cobertura mínima de 105º alrededor del evento. La distancia hipocentral máxima entre eventos es de 11 km. Luego de la localización manual, los eventos que cumplieron con las características necesarias fueron seleccionados para la correlación cruzada entre eventos y el análisis de doble diferencia. Los resultados muestran una reducción de 2 hasta 20 veces la distancia hipocentral, y se delimita la ubicación de la fuente de cada secuencia sísmica.

EARTHQUAKE RELOCATION IN GREECE USING A UNIFIED AND HOMOGENIZED SEISMOLOGICAL CATALOGUE

In this study phase data from the Cornet seismological network installed by the Geophysics-Geothermics Department (University of Athens) and the permanent networks of the Geodynamic Institute of the National Observatory of Athens and of the Geophysics Department of Thessaloniki (Aristotle University), for the decade 1996-2006, were merged. The data were jointly used to relocate the earthquakes that occurred in the broader area of Greece. Initially, single-event algorithms were applied by minimizing spatiotemporal residuals. Following, the hypocenters were relocated using a doubledifference algorithm where rms, as well as the horizontal and vertical location errors (erh, erz) were minimized and a consequence of that better locations were achieved. After that process the spatial distribution of the epicenters outlined major local tectonic features in Greece. Moreover, the same methodology was applied to the aftershock sequences of large earthquakes and the results were successfully compared with those obtained by available local networks that were deployed.

Current microseismicity and generating faults in the Gyeongju area, southeastern Korea

A study of microseismicity in a 15×20km2 subregion of Gyeongju, southeastern Korea, establishes a direct link between minor earthquakes and known fault structures. The study area has a complex history of tectonic deformation and has experienced large historic earthquakes, with small earthquakes recorded since the beginning of modern instrumental monitoring. From 5years of continuously recorded local seismic data, 311 previously unidentified microearthquakes can be reliably located using the double-difference algorithm. These newly discovered events occur in linear streaks that can be spatially correlated with active faults, which could pose a serious hazard to nearby communities. At-risk infrastructure includes the largest industrial park in South Korea, nuclear power plants, and disposal facilities for radioactive waste. The current work suggests that the southern segment of the Yeonil Tectonic Line and segments of the Seokup and Waup Basin boundary faults are active. For areas with high rates of microseismic activity, reliably located hypocenters are spatially correlated with mapped faults; in less active areas, earthquake clusters tend to occur at fault intersections. Microearthquakes in stable continental regions are known to exist, but have been largely ignored in assessments of seismic hazard because their magnitudes are well below the detection thresholds of seismic networks. The total number of locatable microearthquakes could be dramatically increased by lowering the triggering thresholds of network detection algorithms. The present work offers an example of how microearthquakes can be reliably detected and located with advanced techniques. This could make it possible to create a new database to identify subsurface fault geometries and modes of fault movement, which could then be considered in the assessments of seismic hazard in regions where major earthquakes are rare.

Lateral structure variations and transient swarm revealed by seismicity along the Main Himalayan Thrust north of Kathmandu

The midcrustal seismicity along the Main Himalayan Thrust in Nepal presents lateral variations along the rupture of the 2015 Gorkha earthquake. In order to resolve these variations, we relocate the seismicity north of Kathmandu, during a period well covered by the Nepal National Seismological Network, using a double-difference algorithm. The 550 relocated events highlight a complex pattern of clustered seismicity within the unstable-stable transition zone. Part of the seismicity is densely clustered on a southward dipping plane which ruptured on January 31st 1997 (ML=5.8), activating a backthrust with a geometry consistent with the centroid moment tensor of this event calculated in this study. At its eastern end, the midcrustal cluster is offset by 20km to the south suggesting the presence of a tear fault. The analysis of the time sequence allows constraining a scenario involving stress transfer between these local midcrustal structures, beginning more than one month before the 1997 main shock. The temporal evolution of the seismicity is strikingly similar for two other transient seismic swarm episodes which developed hundreds of kilometers apart along the Main Himalayan Thrust at the same time. The local stress field appears responsible for the higher sensitivity of these regions to subtle strain transients developing along the Main Himalayan Thrust.

Analysis of Aftershocks Distribution of the June 16, 2010 Yapen Earthquake Based on Stress Changes on the Fault Plane.

On June 16, 2010, an earthquake (Mw 7.0) occurred on the north of the Papua island close to Yapen island. The area of earthquake source has been well identified due to the activity of faults which are formed by collision of four minor plates. The source mechanism of the June 16, 2010 Yapen earthquake was strike-slip fault type with two nodal planes. Determination of the actual fault plane of nodal planes is important for a detailed study of seismotectonics and earthquake risk assessment.In this study, we relocated the mainshock and aftershocks by using the Double-Difference algorithm, and analyzed the stress changes by using the Coulomb stress method on the study area. Furthermore, the determination of the actual fault plane was obtained by using the H-C method (Hypocenter-Centroid method). We used earthquake data from the Meteorology Climatology and Geophysics Agency (BMKG) and the Global Harvard Centroid Moment Tensor (CMT Global Harvard). Study results show that the relocated aftershock distribution lies close to the increase in stress coseismic area. The increase in stress coseismic area occurred at the nodal plane II as the actual fault plane. This results proved that the aftershock distribution was correlated to the actual fault plane.

Double Difference Motion Detection and Its Application for Madura Batik Virtual Fitting Room

Madura Batik Virtual Fitting Room using double difference algorithms motion detection is proposed in this research. This virtual fitting room consists of three main stages, i.e. motion detection, determination of region of interest of the detected motion, superimposed the virtual clothes into the region of interest. The double difference algorithm is used for the motion detection stage, since in this algorithm, the empty frame as the reference frame is not required. The double difference algorithm uses the previous and next frame to detect the motion in the current frame. Perception Test Images Sequences Dataset are used as the data of the experiment to measure the performance accuracy of this algorithm before the algorithm is used for the Madura batik virtual fitting room. The accuracy is 57.31%, 99.71%, and 78.52% for the sensitivity, specificity, and balanced accuracy, respectively. The build Madura batik virtual fitting room in this research can be used as the added feature of the Madura batik online stores, hence the consumer is able to see whether the clothes is fitted to them or not, and this virtual fitting room is also can be used as the promotion of Madura batik broadly.

The 2013 earthquake swarm in Helike, Greece: seismic activity at the root of old normal faults

The Corinth Rift in Central Greece has been studied extensively during the past decades, as it is one of the most seismically active regions in Europe. It is characterized by normal faulting and extension rates between 6 and 15 mm yr−1 in an approximately N10E° direction. On 2013 May 21, an earthquake swarm was initiated with a series of small events 4 km southeast of Aigion city. In the next days, the seismic activity became more intense, with outbursts of several stronger events of magnitude between 3.3 and 3.7. The seismicity migrated towards the east during June, followed by a sudden activation of the western part of the swarm on July 15th. More than 1500 events have been detected and manually analysed during the period between 2013 May 21 and August 31, using over 15 local stations in epicentral distances up to 30 km and a local velocity model determined by an error minimization method. Waveform similarity-based analysis was performed, revealing several distinct multiplets within the earthquake swarm. High-resolution relocation was applied using the double-difference algorithm HypoDD, incorporating both catalogue and cross-correlation differential traveltime data, which managed to separate the initial seismic cloud into several smaller, densely concentrated spatial clusters of strongly correlated events. Focal mechanism solutions for over 170 events were determined using P-wave first motion polarities, while regional waveform modelling was applied for the calculation of moment tensors for the 18 largest events of the sequence. Selected events belonging to common spatial groups were considered for the calculation of composite mechanisms to characterize different parts of the swarm. The solutions are mainly in agreement with the regional NNE–SSW extension, representing typical normal faulting on 30–50° north-dipping planes, while a few exhibit slip in an NNE–SSW direction, on a roughly subhorizontal plane. Moment magnitudes were calculated by spectral analysis of S waves, yielding b-values between 1.1 and 1.2 in their frequency–magnitude distribution. The seismic moment release history indicates swarm-like activity during the first phase, which could have acted as a preparatory stage for the second phase (after 12 July) that presented a more typical main-shock–aftershock behaviour. The spatiotemporal analysis reveals that the swarm has occurred in a volume that is likely related with the extension at depth of the NNE-dipping Pirgaki normal fault, outcropping ∼8 km to the south. The slow velocity of eastward migration of the epicentres during June implies triggering by fluids. The situation appears different in the second phase of the sequence, which was probably triggered by a build-up of stress during the first one. The relatively deep hypocentres of the 2013 swarm, compared to the shallower seismic layer within the rift, and their coincidence with the steeply dipping Pirgaki fault, favour an immature rift detachment model. Previous results from instrumental data indicate that approximately the same region had been activated during July–August 1991. The availability of the dense permanent seismological network data thus allowed for a detailed analysis of this crisis, a better understanding of its mechanical context and of the earlier events.

An active extensional deformation example: 19 May 2011 Simav earthquake (Mw= 5.8), Western Anatolia, Turkey

The Simav Earthquake that occurred on 19 May 2011 in western Turkey was investigated on the basis of seismological data and geological observations. Approximately WNW?ESE trending surface ruptures were observed on the Simav Fault. The focal mechanism parameters of the earthquake (Mw = 5.8) and its aftershocks (Mw??>??3.5) were estimated using time-domain moment tensor inversion. A total of 2245 events were located with Geiger?s conventional absolute location method then relocated using the double difference (DD) algorithm. The calculated locations at a depths between 2 and 16?km were found to be consistent with Coulomb stress variation in the area. Average variance reduction (VR) of the solutions was calculated as ~70%. The focal parameters of strike dip and slip of the main shock, occurring at a depth of 11?km dipping towards the NNE, were estimated at 277, 62 and ?92, respectively. The most striking indication of the study is that the area is dominated by normal faults with mainly WNW?ESE trends. It is also concluded that earthquakes in the region are caused by an active and regional NNE?SSW (N 12? E) trending (?3 axis) extension regime. The mean stress ratio is 0.80, indicating a triaxial stress state. This extension is probably associated with a slab?pull force and /or roll-back due to the complex subduction process of the African Plate beneath Anatolian block along both the Hellenic and Cyprus arcs in the eastern Mediterranean region.

How Did the 2013 Lushan Earthquake (Ms = 7.0) Trigger its Aftershocks? Insights from Static Coulomb Stress Change Calculations

The Lushan earthquake Ms7.0 occurred at 08:02 Beijing Time (00:02 UTC) on 20 April 2013, located on the eastern margin of the Tibetan Plateau, where the 2008 Ms8.0 Wenchuan earthquake took place. The other strong shock following the Wenchuan earthquake occurred on the Longmen Shan thrust fault. A large number of aftershocks were recorded after the mainshock. To investigate how the aftershocks were triggered, we firstly relocated the 6-month aftershock sequence by the double-difference algorithm. Next, we calculated Coulomb failure stress changes imparted by the Lushan mainshock both on the optimally oriented plane and on the aftershock nodal planes. Then we examined the correspondence between the Coulomb failure stress changes and the spatial distribution of the aftershocks. The computed results show that most of the aftershock hypocenters did not occur on the region with the positive Coulomb stress changes caused by the Lushan mainshock. Moreover, the majority of aftershock nodal planes were brought away from failure by coseismic Coulomb stresses. This implies that coseismic static Coulomb stress changes may not be the governing process for aftershock genesis. In contrast, postseismic stress transfer by processes of afterslip and pore fluid flow are assumed to play an important part in aftershock triggering, based upon the spatiotemporal distribution of the aftershocks.

Passive tomography to image stress redistribution prior to failure on granite

Understanding stress redistribution within rock helps improve the safety and efficiency of mining projects. In order to better understand rock failure mechanism, a granite sample is prepared in a manner that facilitates shear failure along preconceived cracks. The testing process of this method employed several steps. First, the sample is loaded and then monitored in order to study the stress field associated with shear failure. Passive acoustic emissions systems are then used to monitor seismic events during the loading process. The data obtained from the test is processed by using the double-difference location algorithm. The velocity pattern of the sample is then calculated. The distribution of velocity is projected using double-difference tomography. It is shown that the sample failed along the plane where two parallel cracks were prepared. Most of the relocated events were located within or around the plane. The tomograms created from the recorded data accurately image the areas of interest within the rock samples. It is also shown that passive acoustic emissions monitoring systems coupled with double-difference tomography are capable of monitoring and forecasting the failure of rock.

The April–June 2007 Trichonis Lake earthquake swarm (W. Greece): New implications toward the causative fault zone

On 10 April 2007, three moderate earthquakes with Mw=4.9–5.1 occurred in the vicinity of Trichonis Lake (W. Greece). A local network composed of 12 three-component digital seismographs was installed in the epicentral area and recorded more than 1600 events. The double-difference algorithm HYPODD, incorporating both catalog and waveform cross-correlation differential travel-time data, was applied for the successful relocation of 1490 earthquakes. The latter led to the distinction of a main NW-SE trending and NE-dipping zone, as well as of three neighboring faults; a conjugate NW-SE striking and SW-dipping marginal fault mapped along the northeastern flanks of the lake; a E-W trending and south-dipping low-angle normal fault, possibly related to the major Agrinio Fault Zone (AFZ), parallel to the northern bank of the lake; a NE-SW striking and NW-dipping normal fault, likely related to a segment of the active Evinos fault, located south of the lake. Calculation of the Coulomb stress induced by the combination of the 1975 Mw=6.0 event and the three largest events of 10 April 2007 on the inferred structures, reveals that most of the seismicity lies within the “stress-loaded” region, except for the westernmost activity, which probably belongs to the deep part of the AFZ. A total of 178 reliable focal mechanisms were determined by regional and local body-wave modeling (5 largest events) and P-wave first motion polarity data. The types of the obtained focal mechanisms are predominantly normal and strike-slip, however, numerous earthquakes were found to exhibit reverse faulting. Inversion of focal mechanism data showed that the prevailing principal horizontal component σ3 is quite homogeneous throughout the activated area with a roughly NW-SE trend, parallel to the strike of the Hellenides. On the contrary, the compressional field σ1 appears in two patterns: NE-SW trending onshore and NW-SE trending beneath the lake. This apparent rotation of σ1 by 90° reveals a complex system enclosed by the suggested NW-SE trending antithetic faults in depths between 7 and 9km. The calculated stress ratios beneath the lake imply that vertical forces are close to the overburden pressure. The overall inferred stress pattern is rather linked to topographic variations, locally imposing increase or decrease of the vertical forces. The presence of the water in the lake possibly plays an additional important role, penetrating through the bedrock, reducing the friction coefficient, while the pore pressure and, consequently, the effective stress increase. Thus, shearing along mature fractures is enhanced, likely yielding the observed diversity.

The 2011 Lorca seismic series: Temporal evolution, faulting parameters and hypocentral relocation

The Lorca 2011 seismic series was recorded by an unprecedented set of high quality on scale broadband seismograms and strong motion accelerograms. The waveforms from permanent and temporary broadband seismic networks deployed in the region by different institutions allowed to invert regional moment tensor for the fore, main and largest aftershock of the complete seismic sequence. Using double-difference algorithm we have performed a precise relocation of the seismic series, where body wave travel times from strong ground motion accelerograms were included in the data set. Regional moment tensor inversion for the three main events show similar oblique-reverse faulting regime with a northeast-southwest fault orientation. The scalar seismic moment, moment magnitude and focal depth retrieved from the inversion yield the following values for each event: \(\hbox {Mo}=6.5\times 10^{16}\) Nm (Mw = 5.2) for the mainshock, \(\hbox {Mo}= 9.6 \times 10^{15}\) Nm (Mw = 4.6) for the foreshock and \(\hbox {Mo}=7.3\times 10^{14}\) Nm (Mw = 3.9) for the large aftershock. The centroid depths range between 4 and 6 km. The double-difference relocation of the seismic series shows significant epicentral differences with the preliminary routine location. The epicentral solutions given by this relocation show a seismic sequence distributed following a NE–SW strike, subparallel to the Alhama de Murcia fault and compatible with the faulting parameters inverted from the moment tensor analysis. The hypocenters of the series generate a subvertical trend in depth distribution, being concentrated between 2 and 6 km. The depth distribution of the main events, which range from 4.6 to 5.5 km, is in good relationship with the faulting and depth parameters deduced from the moment tensor inversion technique. The regional moment tensor solutions for the three largest earthquakes, the epicentral distribution and the focal depths show good relationship with the surface geometry and tectonic regime of the Alhama de Murcia fault. The stress drop deduced for the mainshock gives a value ranging between 58 and 85 bars, which does not support the idea of a high stress drop release as a main factor contributing to the high ground acceleration recorded at Lorca. The PGA values observed at Lorca, which contributed to the high damage independently of structural deficiencies, could be generated mainly by shallowness and proximity to the seismic source together with a directivity effect in the seismic radiation.

Relocation of the mainshock and aftershock sequences of M S7.0 Sichuan Lushan earthquake

The mainshock of April 20, 2013 Sichuan Lushan M S7.0 earthquake was relocated using a 3-D velocity model. Double difference algorithm was applied to relocate aftershock sequences of Lushan earthquake. The locations of 2405 aftershocks were determined. The location errors in E-W, N-S and U-D direction were 0.30, 0.29 and 0.59 km on average, respectively. The location of the mainshock is 102.983°E, 30.291°N and the focal depth is 17.6 km. The relocation results show that the aftershocks spread approximately 35 km in length and 16 km in width. The dominant distribution of the focal depth ranges from 10 to 20 km. A few earthquakes occurred in the shallow crust. Focal depth profiles show fault planes dip to the northwest, manifested itself as a listric thrust fault. The dip angle is steep in the shallow crust and gentle in the deep crust. Although the epicenters of aftershocks distributed mainly along both sides of the Shuangshi-Dachuan fault, the seismogenic fault may be a blind thrust fault on the eastern side of the Shuangshi-Dachuan fault. Earthquake relocation results reveal that there is a southeastward tilt aftershock belt intersecting with the seismogenic fault with y-shape. We speculate it is a back thrust fault that often appears in a thrust fault system. Lushan earthquake triggered the seismic activity of the back thrust fault.

Accurate Location of the Lushan, Sichuan M7.0 Earthquake on 20 April 2013 and Its Aftershocks and Analysis of the Seismogenic Structure

AbstractThe M7.0 Lushan earthquake and its aftershocks in 9 days were relocated by the double difference algorithm and a layered velocity model, using the seismic phases recorded at stations within 150 km including the fixed stations of the Sichuan earthquake network center, temporary stations, and reservoir stations. The events analyzed include the main shock and its 3324 aftershocks. The results show that the origin time of the main shock is 8:02:46.8 on 20 April 2013, hypocenter location 30.278°N, 102.989°E and the focal depth 16.67 km. The main rupture is about 40 km long, 20 km wide with an apparent area 800 km2. It strikes in southwest and dips about 40°. Most aftershocks occurred at depths 10∼22 km, concentrating on the hanging wall of the Dayi‐Mingshan fault, forming a southwest‐trending belt on the surface. Based on these and other data, this paper makes a preliminary analysis of the seismogenic structure of the Lushan event.

The Earthquake’s Relative Location Algorithms Effected by Data Recorded at Nearby Station

Based on the primary theory of location, it is found that the data recorded at nearby stations are applicable in double difference relocation algorithm, but not in other relative location algorithms. Equation derivation and simulated calculation demonstrate that double difference algorithm allows to get the absolute locations of earthquakes with certain accuracy by using the nearby stations records. The errors of relative location become larger when using the nearby station records alone than that using the distant station records. Combining the nearby and distant station records, the best locations are obtained. In the case that hypocenter depth is far less than the epicenter distance without depth phases available, only the relative epicenter distribution could be obtained accurately by double difference method, the depth determined would be not stable. The relocation test of a part of aftershocks in Jiashi earthquake sequence verify the effect of nearby station records on the double difference algorithm mentioned above.

The dense micro-earthquake activity at the boundary between the Anatolian and South Aegean microplates

The Aegean coast of Turkey is a transition zone between the Anatolian and the South Aegean microplates. The recent geophysical studies suggest the existence of additional microplates and suggest a modification of the block boundaries; the Karaburun Peninsula is most likely located on one of these boundaries. The earthquake activity in and around the peninsula was very low until the Mw 5.8 Sığacık Bay earthquake of 17 October 2005. The continuing seismicity in the Karaburun Peninsula has been monitored with the help of the dense seismology network. I re-located more than 5000 earthquakes using the double-difference algorithm and obtained fault plane solutions to understand the seismotectonic properties of the region. It is found that the micro-seismic activity has increased towards the NW of the Sığacık Bay. These observations imply the existence of a NW–SE-oriented sinistral strike-slip fault. This fault is possibly a conjugate of the dominant NE–SW-oriented dextral strike-slip faults in this transition region occupying the area between western Anatolia and the mainland of Greece, where a dominantly extensional regime prevails. Moreover, it is clear that the micro-earthquake activity is related to the Uzunkuyu Intrusive located in the middle of an approximately 40km strike-slip Karaburun Seismic Zone between the Sığacık and Gerence bays.

Seismicity and source parameters of moderate earthquakes in Sikkim Himalaya

In this study, we accurately relocate 360 earthquakes in the Sikkim Himalaya through the application of the double-difference algorithm to 4 years of data accrued from a eleven-station broadband seismic network. The analysis brings out two major clusters of seismicity—one located in between the main central thrust (MCT) and the main boundary thrust (MBT) and the other in the northwest region of Sikkim that is site to the devastating Mw6.9 earthquake of September 18, 2011. Keeping in view the limitations imposed by the Nyquist frequency of our data (10 Hz), we select 9 moderate size earthquakes (5.3 ≥ Ml ≥ 4) for the estimation of source parameters. Analysis of shear wave spectra of these earthquakes yields seismic moments in the range of 7.95 × 1021 dyne-cm to 6.31 × 1023 dyne-cm and corner frequencies in the range of 1.8–6.25 Hz. Smaller seismic moments obtained in Sikkim when compared with the rest of the Himalaya vindicates the lower seismicity levels in the region. Interestingly, it is observed that most of the events having larger seismic moment occur between MBT and MCT lending credence to our observation that this is the most active portion of Sikkim Himalaya. The estimates of stress drop and source radius range from 48 to 389 bar and 0.225 to 0.781 km, respectively. Stress drops do not seem to correlate with the scalar seismic moments affirming the view that stress drop is independent over a wide moment range. While the continental collision scenario can be invoked as a reason to explain a predominance of low stress drops in the Himalayan region, those with relatively higher stress drops in Sikkim Himalaya could be attributed to their affinity with strike-slip source mechanisms. Least square regression of the scalar seismic moment (M0) and local magnitude (Ml) results in a relation LogM0 = (1.56 ± 0.05)Ml + (8.55 ± 0.12) while that between moment magnitude (Mw) and local magnitude as Mw = (0.92 ± 0.04)Ml + (0.14 ± 0.06). These relations could serve as useful inputs for the assessment of earthquake hazard in this seismically active region of Himalaya.

Seismic relocation, focal mechanism and crustal seismic anisotropy associated with the 2010 Yushu MS7.1 earthquake and its aftershocks

The 2010 Yushu MS7.1 earthquake occurred in Ganzi-Yushu fault, which is the south boundary of Bayan Har block. In this study, by using double difference algorithm, the locations of mainshock (33.13°N, 96.59°E, focal depth 10.22 km) and more than 600 aftershocks were obtained. The focal mechanisms of the mainshock and some aftershocks with MS>3.5 were estimated by jointly using broadband velocity waveforms from Global Seismic Network (GSN) and Qinghai Seismic Network as well. The focal mechanisms and relocation show that the strike of the fault plane is about 125° (WNW-ESE), and the mainshock is left-laterally strikeslip. The parameters of shear-wave splitting were obtained at seismic stations of YUS and L6304 by systematic analysis method of shear-wave splitting (SAM) method. Based on the parameters of shear-wave splitting and focal mechanism, the characteristics of stress field in seismic source zone were analyzed. The directions of polarization at stations YUS and L6304 are different. It is concluded that after the mainshock and the MS6.3 aftershock on April 14, the stress-field was changed.