Migration Of Seismic Activity Research Articles

Unraveling the Preparatory Processes of the 2023 Mw 7.8–7.6 Kahramanmaraş Earthquake Doublet

Abstract Within a span of 9 hr on 6 February 2023, two significant earthquakes, with magnitudes of Mw 7.8 and 7.6, struck the southeastern part of Türkiye and the northern region of Syria, resulting in significant casualties and widespread economic losses. The occurrence of such intense earthquakes in rapid succession on adjacent faults, especially within a highly complex intraplate region with a multifault network, poses a rare phenomenon, presenting new challenges for seismic hazard analysis in such areas. To investigate whether the preparatory processes for the Mw 7.8–7.6 earthquake doublet could be identified on a large spatial scale prior to the seismic events, we employed a data-driven approach for b-value calculation. The difference in b-values from the background values (Δb) in a reference period were used as inputs, and the cumulative migration pattern (CMP) method, quantitatively describing the migration of seismic activity, was utilized to calculate the corresponding probability distributions. The results indicate a widespread phenomenon of decreasing b-values in the study area over a decade before the occurrence of the earthquake doublet, revealing a significant enhancement of differential crustal stress over a large region. In addition, despite not being the region with the most pronounced decrease in b-values, there is a distinct high probability distribution of CMP near the nucleation points of the earthquake doublet, indicating a spatial and temporal “focus” of increased crustal differential stress in the study area, unveiling the preparatory process of the earthquake doublet. This study reveals quantifiable migration patterns over a long time scale and a large spatial extent, providing new insights into the evolution and occurrence processes of the 2023 Mw 7.8–7.6 Kahramanmaraş earthquake doublet. Moreover, it offers potential clues for seismic hazard analysis in such intraplate regions with multiple fault systems.

Seismicity during and after stimulation of a 6.1 km deep enhanced geothermal system in Helsinki, Finland

Abstract. In this study, we present a high-resolution dataset of seismicity framing the stimulation campaign of a 6.1 km deep enhanced geothermal system (EGS) in the Helsinki suburban area and discuss the complexity of fracture network development. Within the St1 Deep Heat project, 18 160 m3 of water was injected over 49 d in summer 2018. The seismicity was monitored by a seismic network of near-surface borehole sensors framing the EGS site in combination with a multi-level geophone array located at ≥ 2 km of depth. We expand the original catalog of Kwiatek et al. (2019), including detected seismic events and earthquakes that occurred 2 months after the end of injection, totaling 61 163 events. We relocated events of the catalog with moment magnitudes between Mw −0.5 and Mw 1.9 using the double-difference technique and a new velocity model derived from a post-stimulation vertical seismic profiling (VSP) campaign. The analysis of the fault network development at a reservoir depth of 4.5–7 km is one primary focus of this study. To achieve this, we investigate 191 focal mechanisms of the induced seismicity using a cross-correlation-based technique. Our results indicate that seismicity occurred in three spatially separated clusters centered around the injection well. We observe a spatiotemporal migration of the seismicity during the stimulation starting from the injection well in the northwest–southeast (NW–SE) direction and in the northeast (NE) direction towards greater depth. The spatial evolution of the cumulative seismic moment, the distribution of events with Mw≥1, and the fault plane orientations of focal mechanisms indicate an active network of at least three NW–SE- to NNW–SSE-oriented permeable zones, which is interpreted to be responsible for the migration of seismic activity away from the injection well. Fault plane solutions of the best-constrained focal mechanisms and results for the local stress field orientation indicate a reverse faulting regime and suggest that seismic slip occurred on a sub-parallel network of pre-existing weak fractures favorably oriented with the stress field, striking NNW–SEE with a dip of 45∘ ENE parallel to the injection well.

Methodical approach to isolation of seismic activity migration episodes of the northeastern Baikal rift system (Russia)

Methodical approach to isolation of seismic activity migration episodes of the northeastern Baikal rift system (Russia)

Statistical Monitoring and Early Forecasting of the Earthquake Sequence: Case Studies after the 2019 M 6.4 Searles Valley Earthquake, California

ABSTRACTThis study considers the possible implementation of the operational short-term forecasting, and analysis of earthquake occurrences using a real-time hypocenter catalog of ongoing seismic activity, by reviewing case studies of the aftershocks of the Mw 6.4 Searles Valley earthquake that occurred before the Mw 7.1 Ridgecrest earthquake. First, the short-term prediction of spatiotemporal activity is required in real time along with the background seismic activity over a wide region to obtain practical probabilities of large earthquakes; snapshots from the continuous forecasts during the Searles Valley and Ridgecrest earthquake sequence are included to monitor the growth and migration of seismic activity over time. We found that the area in and around the rupture zone in southern California had a very high background rate. Second, we need to evaluate whether a first strong earthquake may be the foreshock for a further large earthquake; the rupture region in southern California had one of the highest such probabilities. Third, short-term probability forecast of early aftershocks are much desired despite the difficulties with data acquisition. The aftershock sequence of the Mw 6.4 Searles Valley event was found to significantly increase the probability of a larger earthquake, as seen in the foreshock sequence of the 2016 MJMA 7.4 Kumamoto, Japan, earthquake. Finally, detrending the temporal activity of all the aftershocks by stretching and shrinking the ordinary time scale according to the rate given by the Omori–Utsu formula or the epidemic-type aftershock sequence model, we observe the spatiotemporal occurrences in which seismicity patterns may be abnormal, such as relative quiescence, relative activation, or migrating activity. Such anomalies should be recorded and listed for the future evaluation of the probability of a possible precursor for a large aftershock or a new rupture nearby. An example of such anomalies in the aftershocks before the Mw 7.1 Ridgecrest earthquake is considered.

Migration of seismic activity associated with phreatic eruption at Merapi volcano, Indonesia

Phreatic activity of Merapi started after nearly 2 years of quiescence following the October–November 2010 eruption which was the largest in more than 100 years. A dozen eruptions identified by visual and/or seismic observations took place between August 2012 and April 2014. We present in this work the results of a detailed analysis of the April 20, 2014 phreatic eruption. We attempted to reconstruct the eruptive process, which lasted for over 30 min. To this end, we determined the wavefield composition by polarization analysis, located high-frequency earthquakes occurring in the initial part of the eruption process and then determined the seismic source migration of low-frequency part of the tremor-like signal [0.3–3 Hz] over time. Source depth of low-frequency signal was obtained by comparing the slowness vector calculated using 3 stations of the seismic antenna with a slowness vector model obtained by ray tracing in the structure, taking into account the topography and a 1D velocity model obtained by spatial auto-correlation analysis. The results allow to distinguish 3 different phases: 1) High-frequency transients interpreted as the result of a sudden decompression caused by the transition of the volcanic fluid to a gaseous phase that occurred approximately 1.5 km deep. This decompression process in the hydrothermal system generated a migration of the low-frequency seismic source from 900 m to 1800 m above sea level; 2) A second decompression process revealed by high-frequency micro-seismicity and associated to the migration of the low-frequency tremor source which is marked first by a descent phase, followed by a sharp ascent until reaching the surface. The evolution of the back-azimuth during the migration process indicates a slight inclination of the conduit, presumably in the orientation of the dome fracture, in the NW-SE direction. This direction is consistent with the alignment of regional tectonic structures and with the directivity of eruption deposits. 3) The seismic source then remains positioned at the altitude of the dome for over 10 min. This phase probably corresponds to the ash emission process. The average migration speed of the low-frequency seismic source from the starting eruptive process to ash emission is about 5 m/s.

Multi-parameter imaging in the 2016 Mw 7.0 Kumamoto earthquake, Kyushu, Japan and its implications for fluids in seismogenesis

Fluids associated with structural heterogeneities in a seismogenic layer play a crucial role in triggering earthquakes and controlling their rupture processes, but the mechanism of fluid intrusion and the relationship with earthquake initiation are unclear. This study presents new seismic evidence of an earthquake mostly triggered by fluid intrusion associated with magmatism in Kyushu, SW Japan. Seismic imaging revealed two zones with slow seismic velocities beneath the Unzen and Aso Volcanoes with high Poisson's ratio, extending into the rupture belt of the 2016 Mw 7.0 Kumamoto earthquake. The following features, particularly the high Poisson's ratio anomalies, suggest the presence of fluids in the source area ascending from nearby magmatism. Anomalous crack density and saturation rate imply that the fractures/pores are not fully saturated with fluids in the crust of the Beppu-Shimabara Graben, possibly providing a path for fluids to penetrate into the source area, either through highly fracture-damaged faults or via incompletely saturated pores/cracks. Fluid penetrating into the hypocenter areas from deep magma increases the pore pressure and decreases the Coulomb failure function. We therefore speculate that a system such as this, with high pore pressure and fluids in the host faults might lower the threshold for fault failure, thus increasing the likelihood of failure of the mainshock fault. The results of this study suggest that the presence of fractures/pores within extensional rift zones around the world might be critical for fluid migration, and contribute to the initiation, development and migration of seismic activity.

Multivariate Poisson hidden Markov models with a case study of modelling seismicity

SummaryWhile the literature on multivariate models for continuous data flourishes, there is a lack of models for multivariate counts. We aim to contribute to this framework by extending the well known class of univariate hidden Markov models to the multidimensional case, by introducing multivariate Poisson hidden Markov models. Each state of the extended model is associated with a different multivariate discrete distribution. We consider different distributions with Poisson marginals, starting from the multivariate Poisson distribution and then extending to copula based distributions to allow flexible dependence structures. An EM type algorithm is developed for maximum likelihood estimation. A real data application is presented to illustrate the usefulness of the proposed models. In particular, we apply the models to the occurrence of strong earthquakes (surface wave magnitude ≥5), in three seismogenic subregions in the broad region of the North Aegean Sea for the time period from 1 January 1981 to 31 December 2008. Earthquakes occurring in one subregion may trigger events in adjacent ones and hence the observed time series of events are cross‐correlated. It is evident from the results that the three subregions interact with each other at times differing by up to a few months. This migration of seismic activity is captured by the model as a transition to a state of higher seismicity.

SLOW DEFORMATION WAVES IN THE SEISMIC REGIME AND GEOPHYSICAL FIELDS AT THE NORTHERN MARGIN OF THE AMUR PLATE

The interaction between the Amur, Pacific and Eurasian tectonic plates initiates seismic activity at the plate margins as well as in the plate periphery, as evidenced by intracontinental earthquakes. In the Amur plate, the dynamics of intercontinental seismicity is controlled by deformation wave fronts comprising a regular pattern of equidistant zones [Sherman, 2013]. According to [Trofimenko et al., 2015a, 2015b, 2016], maximum values of seismic activity in the range of magnitudes 2≤M≤4 also form a sequence of spatial cells in the form of seismic clusters from the east (Sakhalin – Sakh) to the west (the western boundary of the Baikal rift zone – BRZ ) (Fig. 1). One of the main characteristics of the seismic process is seismic activity migration given as sequential activation of seismogenic structures within the seismically active zones and on the global scale [Vikulin et al., 2012; Khain, Khalilov, 2008]. Direct observations show that crust deformation migrates from the Japan-Kuril-Kamchatka subduction zone towards the continent, and the estimated migration rates range from 10 to 140 km per year (e.g. [Ishii et al., 1978; Kasahara, 1979; Harada et al., 2003; Yoshioka et al., 2015]). In the Baikal and Amur regions (107–140°E), the fronts of deformation waves migrate at a rate of 5–20 km per year [Sherman, 2007, 2013]. Considering the order of magnitude, this rate is comparable to the rates of crust deformation migration from the Japan-Kuril-Kamchatka zone (10–100 km per year). Our studies show that the sequential activation of the seismic clusters in the northeastern segment of the Amur plate (Sakh – TanLu – Al-St) occurs at a rate of 1000 km per year [Trofimenko et al., 2015a] (Fig. 1). In the meridional tectonic structures, the shifting chains of maximum seismicity values are sequentially replaced by minimum values (i.e. inversion zones). Based on the spatial cycles with the phase shift of the maximum seismic activity values at the rate of 1000 km per year, it is possible to represent the dynamics of seismicity in the form of a process initiated by long-period stress waves/deformations. According to [Mogi, 1968; Kasahara, 1979; Malamud, Nikolaevskii, 1989; Saprygin et al., 1997; Harada et al., 2003; Bykov, 2005, 2014; Sherman, 2007, 2013, 2014; Milyukov et al., 2013], slow deformation waves of the global and regional scale are generated at the margins of lithospheric plates. Under this concept, the migration rate of seismic activity and the spatial extent of seismic cycles can be identified as the velocity and length of deformation waves. Using the data on seismicity of the most active region of the Baikal rift zone – the northwestern segment of the Amur plate, we have studied the periodic components of seismicity along the entire northern boundary of the Amur plate. An indirect evidence of the existence of deformation waves is the migration of anomalies of geophysical fields and its correlation with the migration of seismic activity. The space-time anomalies of the magnetic and gravity fields were studied in the South Yakutian geodynamic polygon [Trofimenko, 1990; Trofimenko, Grib, 2003, 2016], and the indicators of deformation waves were revealed in the seismic regime and the geophysical fields at the northern margin of the Amur plate. The sequential manifestation of anomalies in the magnetic and gravity fields is associated with the activation of latitudinal tectonic structures. Our estimations show that the geophysical anomalies migrate at different rates, from 100 to 1000 km per year. Based on the results obtained in our study and their comparison with other available data, the dynamics of seismicity along the northern margin of the Amur plate is identified as a wave process.

MIGRATIONS OF RELEASED SEISMIC ENERGY IN VARIOUS GEODYNAMIC CONDITIONS

The properties of slow seismic activity migration have been revealed by the space-time analysis of the total earthquake energy (LgE sum ). Our study of seismic activity covers the fragments of the Central Asian, Pacific and Alpine seismic belts: the Baikal rift system (BRS, Russia), the San Andreas fault zone (California, USA), the Christchurch fault (New Zealand), the North and East Anatolian faults (Turkey), the Philippine subduction zone, and the central fragment of the Mid-Atlantic oceanic ridge. The chains of LgE sum clusters mark the propagation of the maximum stresses front in the weaker crust areas, the zones of fault dynamic influence, and the regions of conjugated tectonic structures. The migration process is characterized by a periodicity, changes in direction, and similar modular values of the migration rates within a single fault segment (or a fault zone), which is probably related to the mechanical and rheological crust and upper mantle properties. The data analysis shows that a strong earthquake source may occur at a location wherein the front of seismic activity propagates with periodical changes in direction, and such a source can develop within a period that is multiple of the migration fluctuations, probably associated with the influence of external periodic factors. The main periods of migration fluctuations (2–4 years, and 9–13 years, in different ratios) are present in the seismic regimes of different seismic belts. The migration rate, as well as the propagation velocity of the maximum stresses front, directly depends on the velocity of movements between the plates in the region.

Rushan earthquake swarm in eastern China and its indications of fluid-triggered rupture

An extraordinary earthquake swarm occurred at Rushan on the Jiaodong Peninsula from October 1, 2013, onwards, and more than 12,000 aftershocks had been detected by December 31, 2015. All the activities of the whole swarm were recorded at the nearest station, RSH, which is located about 12 km from the epicenter. We examine the statistical characteristics of the Rushan swarm in this paper using RSH station data to assess the arrival time difference, t_{{{text{S}} ,-, {text{P}}}}, of Pg and Sg phases. A temporary network comprising 18 seismometers was set up on May 6, 2014, within the area of the epicenter; based on the data from this network and use of the double difference method, we determine precise hypocenter locations. As the distribution of relocated sources reveals migration of seismic activity, we applied the mean-shift cluster method to perform clustering analysis on relocated catalogs. The results of this study show that there were at least 16 clusters of seismic activities between May 6, 2014, and June 30, 2014, and that each was characterized by a hypocenter spreading process. We estimated the hydraulic diffusivity, D, of each cluster using envelope curve fitting; the results show that D values range between 1.2 and 3.5 m2/d and that approximate values for clusters on the edge of the source area are lower than those within the central area. We utilize an epidemic-type aftershock sequence (ETAS) model to separate external triggered events from self-excited aftershocks within the Rushan swarm. The estimated parameters for this model suggest that α = 1.156, equivalent to sequences induced by fluid-injection, and that the forcing rate (μ) implies just 0.15 events per day. These estimates indicate that around 3% of the events within the swarm were externally triggered. The fact that variation in μ is synchronous with swarm activity implies that pulses in fluid pressure likely drove this series of earthquakes.

Recent movement changes of main fault zones in the Sichuan-Yunnan region and their relevance to seismic activity

The Wenchuan earthquake occurred near the “triple junction” linking the Bayan Har block, the South China block, and the Sichuan-Yunnan rhombic block, and its influences on the surrounding blocks and the main fault zones in the Sichuan-Yunnan region, i.e., the block boundary zone, cannot be ignored. In this paper, changes of movement and stress of the fault zones before and after a strong earthquake were simulated based on the GPS repetition survey results recently obtained during 1999–2007, 2009–2011, and 2011–2013 with a two-dimensional finite-element contact model and the “block-loading” method. The results show that, before the Wenchuan earthquake, the movement of the Longmenshan fault zone was very slow and its compressive stress accumulated rapidly; after the Wenchuan earthquake, movements toward the E-SSE direction of the Bayan Har, southwestern Yunnan, and rhombic blocks were enhanced, and the dextral and horizontal compressive speeds and annual accumulative compressive stress of the Longmenshan fault zone increased markedly by factors of 4.5, 2.1, and 2.5, respectively. The southern Xianshuihe, Anninghe, Zemuhe, Daliangshan, and Lijiang-Xiaojinhe fault zones accumulated compressive stress rapidly, forming enhanced compressive stress zones along a NE strike crossing the central part of the Sichuan-Yunnan region. The tensional movement of the Xianshuihe fault zone was enhanced and the slip movement in the central part of the zone was reversed in a short time. The changes are tightly related to the medium-intensity earthquakes that occurred during the same period in this region, revealing that the spatial migration of seismic activity is related to changes of movement of the blocks.

Migration of seismic activity in strike-slip zones: A case study of the boundary between the North American and pacific plates

The results of computer visualization of the spatiotemporal distribution of the total earthquake energy for the instrumental period are presented. This visualization was implemented to determine the parameters of slow earthquake migrations (of a few kilometers to a few hundreds of kilometers per year) in strike-slip zones. A case study of the San Andreas and Mendocino fault zones referring to the boundary between the North American and Pacific plates is considered. The obtained results are compared with the migration peculiarities revealed for the structures under other geodynamical conditions, in particular those in Trans-Bakalia. The spatiotemporal analysis is carried out with the use of strong, moderate, and weak events. Migration episodes are detected in the seismic data projection zones constructed for epicenter concentrations at the Mendocino triple junction, at the junction of the San Andreas and Calaveras faults, the Santa Monica and San Gabriel ones, and the Camp Rock Fault Zone; these areas host the strongest earthquakes of the instrumental period. Migration in these zones is manifested as a phenomena both preceding and following the seismic events and can reflect the dynamics of the stress redistribution in the zone of interaction between tectonic structures. The calculated migration rates vary from 7 ± 2 to 250 ± 50 km/yr. It is concluded that there is no dependence of the seismic activity migration on the geodynamical type of the fault zones, because similar rates (10–70 km/yr) are reported for the Mendocino and San Andreas faults and the Baikal Rift System. The migration rates for the Mendocino and San Andreas, exceeding 70 km/yr, are likely caused by a high rate of the interplate displacements, which is higher than that in the Baikal Rift System by more than an order. The extent of the seismoactive segments of faults zones along which migration episodes were detected is from 20 to 70 km. The results of the mutual correlation analysis of time series carried out for such parameters as the earthquake numbers and earthquake energy suggest that migrations are caused by local geodynamical conditions affecting the seismicity on the scale of zones where large segments of fault zone interact with each other.

Migration of seismic and volcanic activity as display of wave geodynamic process

Publications about the earthquake foci migration have been reviewed. An important result of such studies is establishment of wave nature of seismic activity migration that is manifested by two types of rotational waves; such waves are responsible for interaction between earthquakes foci and propagate with different velocities. Waves determining long-range interaction of earthquake foci are classified as Type 1; their limiting velocities range from 1 to 10 cm/s. Waves determining short-range interaction of foreshocks and aftershocks of individual earthquakes are classified as Type 2; their velocities range from 1 to 10 km/s. According to the classification described in [Bykov, 2005], these two types of migration waves correspond to slow and fast tectonic waves. The most complete data on earthquakes (for a period over 4.1 million of years) and volcanic eruptions (for 12 thousand years) of the planet are consolidated in a unified systematic format and analyzed by methods developed by the authors. For the Pacific margin, Alpine-Himalayan belt and the Mid-Atlantic Ridge, which are the three most active zones of the Earth, new patterns of spatial and temporal distribution of seismic and volcanic activity are revealed; they correspond to Type 1 of rotational waves. The wave nature of the migration of seismic and volcanic activity is confirmed. A new approach to solving problems of geodynamics is proposed with application of the data on migration of seismic and volcanic activity, which are consolidated in this study, in combination with data on velocities of movement of tectonic plate boundaries. This approach is based on the concept of integration of seismic, volcanic and tectonic processes that develop in the block geomedium and interact with each other through rotating waves with a symmetric stress tensor. The data obtained in this study give grounds to suggest that a geodynamic value, that is mechanically analogous to an impulse, remains constant in such interactions. It is thus shown that the process of wave migration of geodynamic activity should be described by models with strongly nonlinear equations of motion.

Geometry and spatio-temporal evolution of the 2001 Agios Ioanis earthquake swarm (Corinth Rift, Greece)

The Agios Ioanis earthquake swarm occurred below the southern edge of the Corinth Rift (Greece). The swarm is of interest due to its location and especially because the activated structure is oriented at a high angle to the rift's major active normal faults. We present a detailed study of the swarm including a high-resolution relocation of multiplets, accompanied by an analysis of their geometry and of their spatio-temporal evolution. P- and S-wave traveltime delays, calculated in the spectral domain to within subsampling precision, are inverted considering all pairs of events. In a second step, the large-scale structure of the swarm is refined by inverting traveltime delays calculated from absolute wave arrival-times. The relocation image, in combination with the results of the multiplet geometrical analysis, reveals a complex structure suggesting seismic activity on parallel planes that strike southwest and dip to the northwest. We propose that this fault zone connects to the Kerinitis Valley, where geological observations suggest the presence of a transverse fault linking normal faults striking 100 degrees N. The spatio-temporal evolution of the relocated seismicity provides evidence of a approximately 20 m day (super -1) migration of seismic activity towards the surface. Assuming fluid-driven seismicity, we model the spatio-temporal evolution as the diffusion of a pore-pressure perturbation. The preferred model indicates a hydraulic diffusivity equal to 0.1 m (super 2) s (super -1) , suggesting that the spatio-temporal evolution of the seismicity is related to fluid flow at depth. Moreover, we estimate the hydraulic conductivity (1.15X10 (super -5) m s (super -1) ) and the rock permeability (7X10 (super -13) m (super 2) ), which compare well to available local observations.

Velocities of slow migration of seismic activity in Cis-Baikal region

Three-dimensional space-time diagrams of «logarithm of total energy released by earthquakes» parameter, lgEsum are constructed for regions with stable concentrations of earthquake epicenters in Cis-Baikal region for a period from 1964 to 2002. Based on analyses of such diagrams, areas of slow migration of seismic activity are defined. Estimated are distances, time and velocities of slow migration in the range of the first kilometers – first dozen of kilometers per year. Procedures of seismic data projection and construction of 3D diagrams are described in brief. A general scheme including contours of projection areas is proposed for the Pribaikalie (Fig. 1). Three space-time diagrams are presented as examples of application of the above mentioned procedures. They are constructed for the Middle and Southern Baikal basins and the western part of the NE flank of the Baikal rift system (Fig. 2). Integrated analytical results are presented for all the diagrams which record earthquake migration within the Baikal rift system. We also present a scheme of the zone of slow migrations ranked by dominating velocities (Fig. 3) and a diagram of the migration velocity range. We consider possible causes of slow migration of seismic activity at variable velocities: (1) slow deformation waves spreading in the crust, and (2) independent propagation of the deformation front along active faults. Regulations of migration of strong earthquakes can be useful for definition of timelines and locations of future strong seismic events.

Estimation of static stress changes after the 2001 Bhuj earthquake: Implications towards the northward spatial migration of the seismic activity in Kachchh, Gujarat

Spatial-temporal patterns of aftershocks of the 2001 Mw7.7 Bhuj earthquake during 2001-2008 reveal a northward spatial migration of seismic activity in the Kachchh seismic zone, which could be related with the loading stresses caused by the continued occurrences of aftershocks on the north Wagad fault (NWF), the causative fault of the 2001-mainshock. Aiming at explaining the observed northward migration of activity, we modelled the Coulomb failure stress change (DCFS) produced by the 2001-mainshock, the 2006 Mw5.6 Gedi fault (GF) and the 2007 Mw4.5 Allah bund fault (ABF) events on optimally oriented plane. A strong correlation between occurrences of earthquakes and regions of increased DCFS is obtained on the associated three faults i.e. NWF, ABF and GF. Predicted DCFS on the GF increased by 0.9 MPa at 3 km depth, where the 7 th March 2006 Mw5.6 event occurred, whereas predicted DCFS on the ABF increased by 0.07 MPa at 30 km depth, where the 15 th December 2007 Mw4.5 event occurred. Focal mechanism solutions of three events on the ABF have been estimated using the iterative inversion of broadband data from 5-10 stations, which are also constrained by the first P-motion data from 8-12 stations. These focal mechanism solutions for the ABF events reveal a dominant reverse movement with a strike-slip component along a preferred northwest or northeast dipping plane (∼50-70°). Focal mechanisms of the events on all the three fault zones reveal an N-S oriented P- axis or maximum principal stress in the region, which agrees with the prevailing N-S compression over the Indian plate. It is apparent that the northward migration of the static stress changes from the NWF, resulting from the occurrence 2001 Bhuj mainshock, might have caused the occurrence of the events on the GF and ABF during 2006-08.

Seismicity pattern in north Sumatra-Great Nicobar region: In search of precursor for the 26 December 2004 earthquake

We analyse the seismicity pattern including b-value in the north Sumatra-Great Nicobar region from 1976 to 2004. The analysis suggests that there were a number of significant, intermediate and short-term precursors before the magnitude 7.6 earthquake of 2 November 2002. However, they were not found to be so prominent prior to the magnitude 9.0 earthquake of 26 December 2004 though downward migration of activity and a 50-day short-term quiescence was observed before the event. The various precursors identified include post-seismic and intermediate-term quiescence of 13 and 10 years respectively, between the 1976 (magnitude 6.3) and 2002 earthquakes with two years (1990–1991) of increase in background seismicity; renewed seismicity, downward migration of seismic activity and foreshocks in 2002, just before the mainshock. Spatial variation in b-value with time indicates precursory changes in the form of high b-value zone near the epicenter preceding the mainshocks of 2004 and 2002 and temporal rise in b-value in the epicentral area before the 2002 earthquake.

Evidence for triggering of the Vogtland swarms 2000 by pore pressure diffusion

Seismicity triggered by fluid injection in boreholes usually has certain typical spatiotemporal characteristics. These characteristics can be partly recognized as signatures, when pore pressure diffusion is the dominant triggering mechanism. Starting from these diffusion‐typical signatures of man‐made earthquakes, we seek analogous patterns for the earthquakes in Vogtland/NW Bohemia (VB) at the German/Czech border region in central Europe. VB is characterized by recurring intraplate earthquake swarms with magnitudes up to ML 4.5. There is strong geophysical evidence that the seismic events are correlated to fluid‐related processes in the crust. This study aims to investigate the possible role of fluids in VB. We test the hypothesis that ascending magmatic fluids trigger earthquakes by the mechanism of pore pressure diffusion (i.e., relaxation). This triggering process is mainly controlled by two physical fields, the hydraulic diffusivity and the seismic criticality (i.e., critical pore pressure value leading to failure; stable locations are characterized by higher critical pressures), both heterogeneously distributed in rocks. The results of the analysis of the year 2000 VB earthquake swarm data support this concept. We were able to recognize diffusive signatures and to obtain scalar estimates of hydraulic diffusivities with values between 0.3 and 10 m2/s for the seismically active region. Using a numerical model, where spatially correlated diffusivity and criticality patches (where patches with higher diffusivity are assumed to be less stable) are considered, we successfully simulate a general seismicity pattern of the swarms 2000, including the spatiotemporal clustering of events and the migration of seismic activity.

Migration of seismic activity during the 1998 volcanic unrest at Iwate volcano, northeastern Japan, with reference to P and S wave velocity anomaly and crustal deformation

We describe the seismicity at Iwate volcano, northeastern Japan, during the volcanic unrest of 1998 with reference to a three-dimensional P and S wave velocity model from tomographic analysis. The abnormal seismic activity beneath Iwate volcano started under the caldera in February, 1998 and migrated westward in the period February to August, 1998. Previous geodetic modeling [Sato and Hamaguchi, Chikyu Monthly 21 (1999) 312–317] suggested the growth of a dike in the time of the seismic activity. Comparing the seismicity and dike extension with the tomographic images of the P and S wave velocity structure, we find that the trace of the growing dike coincides with the region of the high V p and high V p/ V s ratio beneath the volcano. The seismic and geodetic data are consistent with an intrusion of magma or other fluid under the caldera in 1998. Another pressure source causing the predominant crustal deformation at Iwate volcano was detected from geodetic data, which was located in the region with high V p/ V s ratio under the western end of the volcano through the period from February to August. It is suggested that the activation of the point pressure source probably associated with the inflation of a hot fluid reservoir relate to a geothermal region adjacent to the western edge of the volcano.

Seismicity and geodynamics of the low- to high-grade transition zone of peninsular India

A new seismogenetic belt has been identified in the well-known transition zone of peninsular India (lat. 12°–14° N and long. 74°–80° E) based on a critical analysis of historical and recent seismic data. The diffused and weak seismicity in the area could be attributed to the reactivation of adjoining shear zones. A seismic energy release map has been prepared for southern peninsular India, which depicts the long-term seismicity in this region. Temporal variations in seismic activity in space indicate migration of seismic activity from east to west within the transition zone. This phenomenon is discussed in relation to the mobility of shear belts adjacent to the Dharwar Craton. Further, the maximum horizontal principal compressive stress directions on the Indian continent are also briefly discussed. These stresses may be mainly responsible for the mobility of the major shear zones adjacent to the Dharwar Craton. The analysis of the seismic data in space and time, thus, suggests two possible geodynamic models viz., (1) a stick-slip model and (2) a crustal shortening model.