Fault Lengths During the Patras 1993 Earthquake Sequence as Estimated from the Pulse Width of Initial P Wave

—The pulse width of the initial P wave was measured for three clusters of the aftershock sequence of the Patras (5.4 M _S ?) July 1993 earthquake. The data set consists of pulse width measure ments of 250 microearthquakes recorded at a low-noise station of the University of Patras Seismological Network between 4-7-1993 and 1-8-1993. The statistical relation between the fault length and the earthquake magnitude was established for each one of the three clusters and is compared with other published relations. The relation also, between seismic moment and fault length was examined and showed a dependence upon the prevailing tectonic regime.     

Recent seismicity in Northeast India and its adjoining region

Recent seismicity in the northeast India and its adjoining region exhibits different earthquake mechanisms – predominantly thrust faulting on the eastern boundary, normal faulting in the upper Himalaya, and strike slip in the remaining areas. A homogenized catalogue in moment magnitude, M _W, covering a period from 1906 to 2006 is derived from International Seismological Center (ISC) catalogue, and Global Centroid Moment Tensor (GCMT) database. Owing to significant and stable earthquake recordings as seen from 1964 onwards, the seismicity in the region is analyzed for the period with spatial distribution of magnitude of completeness m _t, b value, a value, and correlation fractal dimension D _C. The estimated value of m _t is found to vary between 4.0 and 4.8. The a value is seen to vary from 4.47 to 8.59 while b value ranges from 0.61 to 1.36. Thrust zones are seen to exhibit predominantly lower b value distribution while strike-slip and normal faulting regimes are associated with moderate to higher b value distribution. D _C is found to vary from 0.70 to 1.66. Although the correlation between spatial distribution of b value and D _C is seen predominantly negative, positive correlations can also be observed in some parts of this territory. A major observation is the strikingly negative correlation with low b value in the eastern boundary thrust region implying a possible case of extending asperity. Incidentally, application of box counting method on fault segments of the study region indicates comparatively higher fractal dimension, D, suggesting an inclination towards a planar geometrical coverage in the 2D spatial extent. Finally, four broad seismic source zones are demarcated based on the estimated spatial seismicity patterns in collaboration with the underlying active fault networks. The present work appraises the seismicity scenario in fulfillment of a basic groundwork for seismic hazard assessment in this earthquake province of the country.     

Gorkha earthquake (MW7.8) and aftershock sequence: A fractal approach

On April 25, 2015, Nepal was struck by the M_W7.8 Gorkha earthquake followed by an intense aftershock sequence. It was one of the most destructive earthquakes in the Himalayan arc, causing more than 8900 fatalities. In this study, we analyzed the dataset (429 events, magnitude of completeness (M_c) ≥ 4.2 local magnitude) of the first 45 days after the Gorkha earthquake to estimate the seismicity parameters b-value, D-value, and p-value. We used the maximum likelihood method to estimate the b-value and Omori-Utsu parameters, whereas the correlation integral method was applied to estimate the fractal dimension (D-value). The analysis was carried out using running and sliding window techniques. The lowest b-value (0.57 ± 0.04) and the highest D-value (1.65 ± 0.02) were computed at the time of the Gorkha earthquake, after which the b-value significantly increased to a maximum of 1.57. It again dropped to 0.93 at the time of the major aftershock on May 12, 2015. The D-value showed an initial quick drop and then decreased in a wavy pattern until the end of the study period, indicating the clustering and scattering of earthquakes in a fault region. The b-value contour map identified the eastern part of the study area as a high stress region (b = ～0.8), implying that the stress shifted to that region. The D-value contour map reveals that the seismogenic structure shifted from linear to planar in the region. The rate of aftershock decay (p = 0.86 ± 0.04) for a short period reflects that the level of stress decreased rapidly. This study helps to understand the level of stress and seismicity pattern of a region, which could be useful for aftershock studies.     

A Non-Extensive Statistical Physics View in the Spatiotemporal Properties of the 2003 (Mw6.2) Lefkada,Ionian Island Greece,Aftershock Sequence

Investigation of the spatiotemporal properties of the 2003 Lefkada seismic sequence is performed through non-extensive statistical physics. Information on highly accurate aftershock source parameters became feasible from the recordings of a portable digital seismological network that was installed and operated in the study area, during the evolution of the seismic sequence. Thus, the spatiotemporal distribution of aftershocks onto the main and neighboring fault segments was investigated in detail, enabling the recognition of four distinctive seismicity clusters separated by less active patches. The aftershock spatiotemporal properties are studied here, using the ideas of non-extensive statistical physics (NESP). The cumulative distribution functions of the inter-event times and the inter-event distances are presented using the data set in each seismicity cluster, and the analysis results in values for the statistical thermodynamic q _T and q _D parameters for each cluster, where q _T varies from 1.16 to 1.47 and q _D from 0.42 to 0.77 for the inter-event times and distances distributions, respectively. These values confirm the complexity and non-additivity of the spatiotemporal evolution of seismicity, and the applicability of the NESP approach in investigating aftershocks sequence. The temporal pattern is discussed using the closely connected to NESP approach of superstatistics, which is based on a superposition of ordinary local equilibrium statistical mechanics. The result indicates that the temporal evolution of the Lefkada aftershock sequence in clusters A, B and C is governed by very low number of degrees of freedom, while D is a less organized seismicity structure with a much higher number of degrees of freedom.     

Properties of the Aftershock Sequences of the 2003 Bingöl, M D = 6.4, (Turkey) Earthquake

Aftershock sequences of the magnitude M _{W =}6.4 Bingöl earthquake of 1 May, 2003 (Turkey) are studied to analyze the spatial and temporal variability of seismicity parameters of the b value of the frequency-magnitude distribution and the p value describing the temporal decay rate of aftershocks. The catalog taken from the KOERI contains 516 events and one month’s time interval. The b value is found as 1.49 ± 0.07 with Mc =3.2. Considering the error limits, b value is very close to the maximum b value stated in the literature. This larger value may be caused by the paucity of the larger aftershocks with magnitude M _D ≥ 5.0. Also, the aftershock area is divided into four parts in order to detect the differences in b value and the changes illustrate the heterogeneity of the aftershock region. The p value is calculated as 0.86 ± 0.11, relatively small. This small p value may be a result of the slow decay rate of the aftershock activity and the small number of aftershocks. For the fitting of a suitable model and estimation of correct values of decay parameters, the sequence is also modeled as a background seismicty rate model. Constant background activity does not appear to be important during the first month of the Bingöl aftershock sequences and this result is coherent with an average estimation of pre-existing seismicity. The results show that usage of simple modified Omori law is reasonable for the analysis. The spatial variability in b value is between 1.2 and 1.8 and p value varies from 0.6 to 1.2. Although the physical interpretation of the spatial variability of these seismicity parameters is not straightforward, the variation of b and p values can be related to the stress and slip distribution after the mainshock, respectively. The lower b values are observed in the high stress regions and to a certain extent, the largest b values are related to Holocene alluvium. The larger p values are found in some part of the aftershock area although no slip occurred after the main shock and it is interpreted that this situation may be caused by the alluvium structure of the region. These results indicate that the spatial distribution in b and p values are generally related to the rupture mechanism and material properties of an aftershock area.     

A Probabilistic Assessment of Earthquake Hazard Parameters in NW Himalaya and the Adjoining Regions

The maximum likelihood estimation method is applied to study the geographical distribution of earthquake hazard parameters and seismicity in 28 seismogenic source zones of NW Himalaya and the adjoining regions. For this purpose, we have prepared a reliable, homogeneous and complete earthquake catalogue during the period 1500–2010. The technique used here allows the data to contain either historical or instrumental era or even a combination of the both. In this study, the earthquake hazard parameters, which include maximum regional magnitude (M _max), mean seismic activity rate (λ), the parameter b (or β?=?b/log e) of Gutenberg–Richter (G–R) frequency-magnitude relationship, the return periods of earthquakes with a certain threshold magnitude along with their probabilities of occurrences have been calculated using only instrumental earthquake data during the period 1900–2010. The uncertainties in magnitude have been also taken into consideration during the calculation of hazard parameters. The earthquake hazard in the whole NW Himalaya region has been calculated in 28 seismogenic source zones delineated on the basis of seismicity level, tectonics and focal mechanism. The annual probability of exceedance of earthquake (activity rate) of certain magnitude is also calculated for all seismogenic source zones. The obtained earthquake hazard parameters were geographically distributed in all 28 seismogenic source zones to analyze the spatial variation of localized seismicity parameters. It is observed that seismic hazard level is high in Quetta-Kirthar-Sulaiman region in Pakistan, Hindukush-Pamir Himalaya region and Uttarkashi-Chamoli region in Himalayan Frontal Thrust belt. The source zones that are expected to have maximum regional magnitude (M _max) of more than 8.0 are Quetta, southern Pamir, Caucasus and Kashmir-Himanchal Pradesh which have experienced such magnitude of earthquakes in the past. It is observed that seismic hazard level varies spatially from one zone to another which suggests that the examined regions have high crustal heterogeneity and seismotectonic complexity.     

Seismicity and seismotectonic deformations of the crust in the Southern Baikal basin

This paper addresses the seismicity of the Southern Baikal basin, where the M _w = 6.0 earthquake of 1999 was the strongest over the period of instrumental observations in this region. Focal mechanisms of background earthquakes and aftershocks are analyzed in relation to faults mapped on flanks of and within the basin. Based on a supplemented catalog of focal mechanisms, the value and direction of seismotectonic strain are evaluated. The results show that the territory to the west of the transverse Angara fault (the Mishikhinskaya depression) experiences deformation of pure extension, while the E-W basin segment west of the fault is subjected to deformation of extension with shear (the transtension regime). The crustal deformation directions as determined from GPS measurements and seismological observations are found to agree well. The average seismotectonic strain rate of the crust amounts to 2.95 × 10^?9 yr^?1, which is about an order of magnitude smaller than the value obtained from geodetic observations.     

Study of seismicity in the NW Himalaya and adjoining regions using IMS network

The Reviewed Event Bulletin (REB) of the International Data Center (IDC) has been used in order to investigate the seismicity of the Northwest Himalaya and its neighboring region for the time period June 1999 to March 2015 within the geographical coordinates 25–40° N latitude and 65–85° E longitude. We have used a very precisely located earthquake dataset recorded by the International Monitoring System (IMS) Network containing 7,583 events with body wave magnitudes from 2.5 to 6.3. The study area has been subdivided into six regions based on the Flinn-Engdahl (F-E) seismic and geographical regionalization scheme, which was used as the region classifications of the International Data Center catalog. The examined region includes NW India, Pakistan, Nepal, Xizang, Kashmir, and Hindukush. For each region, Magnitudes of completeness (Mc) and Gutenberg-Richter (GR) recurrence parameters (a and b values) have been estimated. The Gutenberg-Richter analysis is preceded by an overview of the seismotectonics of the study area. The obtained Mc values vary from 3.5 to 3.9. The lower value of Mc was found mainly in Xizang region whereas the higher Mc threshold is evident in Pakistan region. However, the b values vary from 1.19 to 1.48. The lowest b value is recorded in Xizang region, which is mostly related to the Main Karakoram Thrust (MKT) fault, whereas the highest b values are recorded in NW India and Kashmir regions, which are mostly related to the Main Frontal Thrust (MFT) fault. The REB for the selected period has been compared to the most renowned bulletin of global seismicity, namely that issued by the National Earthquake Information Center (NEIC) of the United States Geological Survey (USGS). A study of 4,821 events recorded by USGS in the study region indicates that about 36 % of seismic events were missed and the catalog is considered as complete for events with magnitudes ≥4.0. However, both a and b values are obviously higher than those of IMS catalog. The a and b parameters in the Gutenberg-Richter magnitude–frequency relationship have been utilized to forecast the probability of future earthquakes of different magnitudes and returned periods (recurrence intervals).     

Spatial Distribution of Time-independent Seismicity in China

—The seismicity in the territory of China, a seismotectonically complicated region, has been examined by using three complete samples of earthquakes which occurred during the last two centuries (1800–1995). The b value of the Gutenberg-Richter relation was estimated by using this data sample. Taking into account the fact that the b value is spatially more stable than the a value, the b values were calculated at the nodes of a normal grid superposing on the entire area studied, and their distribution was examined. The results showed that the b value increases smoothly from 0.4 in inner-Mongolia to 0.8 in the east, south and southwest of China with higher values (b>0.8) in the Taiwan region. Furthermore, keeping fixed the obtained b values, the a value distribution was also examined. In order to display more detailed information about the seismicity, smaller cell surface (10,000 km²) for the calculation of the a values was chosen. The mean return periods for different cutoff magnitudes were also calculated for each of these small cells. It was observed that the mean return periods are the shortest ones in China, which are 10 and 50 years for the magnitude larger than or equal to 6.0 and 7.0, respectively.     

Seismic source characterization for the Shillong Plateau in Northeast India

The present study aims at understanding the seismotectonic province of the Shillong Plateau (SP) by identifying the potential seismic source zones within a radius of 500 km from the centre of the SP. From existing literature and earthquake (EQ) data, the seismotectonic region is found to vary in terms of seismicity, tectonic features, geology, thickness of overburden, rupture characteristics and rate of movement. Thus, entire 500-km-radius seismotectonic region is divided into four seismic source zones: namely (1) the Shillong Plateau–Assam Valley Zone (SP-AVZ), (2) the Indo-Burma Ranges Zone (IBRZ), (3) the Bengal Basin Zone (BBZ) and (4) the Eastern Himalaya Zone (EHZ). EQ catalogues for each source zone is analysed for completeness of magnitude and time. Seismic parameter b estimated using a maximum likelihood method is found to be 0.91 ± 0.03, 0.94 ± 0.02, 0.80 ± 0.03 and 0.89 ± 0.03 for the SP-AVZ, IBRZ, BBZ and EHZ, respectively. In addition, the maximum likelihood method is used to estimate the mean annual activity rate, maximum possible magnitude (m _max), return period and probability of exceedance for the four zones. The b values estimated suggest that the BBZ is seismically more active; however, the rate of occurrence of EQs is highest in the IBRZ. Findings from this study are an indication of the relative contribution from each of the four seismic source zones towards a seismic hazard of the SP.     

Seismicity and stress field in Okinawa Trough and Ryukyu regions

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A first-order seismotectonic regionalization of Mexico for seismic hazard and risk estimation

The purpose of this work is to define a seismic regionalization of Mexico for seismic hazard and risk analyses. This seismic regionalization is based on seismic, geologic, and tectonic characteristics. To this end, a seismic catalog was compiled using the more reliable sources available. The catalog was made homogeneous in magnitude in order to avoid the differences in the way this parameter is reported by various agencies. Instead of using a linear regression to converts from m _b and M _d to M _s or M _w , using only events for which estimates of both magnitudes are available (i.e., paired data), we used the frequency-magnitude relations relying on the a and b values of the Gutenberg-Richter relation. The seismic regions are divided into three main categories: seismicity associated with the subduction process along the Pacific coast of Mexico, in-slab events within the down-going COC and RIV plates, and crustal seismicity associated to various geologic and tectonic regions. In total, 18 seismic regions were identified and delimited. For each, the a and b values of the Gutenberg-Richter relation were determined using a maximum likelihood estimation. The a and b parameters were repeatedly estimated as a function of time for each region, in order to confirm their reliability and stability. The recurrence times predicted by the resulting Gutenberg-Richter relations obtained are compared with the observed recurrence times of the larger events in each region of both historical and instrumental earthquakes.     

Preliminary report of the September 5, 2022 MS 6.8 Luding earthquake,Sichuan, China

The 2022 M_S 6.8 Luding earthquake is the strongest earthquake in Sichuan Province, Western China, since the 2017 M_S 7.0 Jiuzhaigou earthquake. It occurred on the Moxi fault in the southeastern segment of the Xianshuihe fault, a tectonically active and mountainous region with severe secondary earthquake disasters. To better understand the seismogenic mechanism and provide scientific support for future hazard mitigation, we summarize the preliminary results of the Luding earthquake, including seismotectonic background, seismicity and mainshock source characteristics and aftershock properties, and direct and secondary damage associated with the mainshock. The peak ground displacements in the NS and EW directions observed by the nearest GNSS station SCCM are ～35 mm and ～55 mm, respectively, resulting in the maximum coseismic dislocation of 20 mm along the NWW direction, which is consistent with the sinistral slip on the Xianshuihe fault. Back-projection of teleseismic P waves suggest that the mainshock rupture propagated toward south-southeast. The seismic intensity of the mainshock estimated from the back-projection results indicates a Mercalli scale of VIII or above near the ruptured area, consistent with the results from instrumental measurements and field surveys. Numerous aftershocks were reported, with the largest being M_S 4.5. Aftershock locations (up to September 18, 2022) exhibit 3 clusters spanning an area of 100 km long and 30 km wide. The magnitude and rate of aftershocks decreased as expected, and the depths became shallower with time. The mainshock and two aftershocks show left-lateral strike-slip focal mechanisms. For the aftershock sequence, the b-value from the Gutenberg-Richter frequency-magnitude relationship, h-value, and p-value for Omori’s law for aftershock decay are 0.81, 1.4, and 1.21, respectively, indicating that this is a typical mainshock-aftershock sequence. The low b-value implies high background stress in the hypocenter region. Analysis from remote sensing satellite images and UAV data shows that the distribution of earthquake-triggered landslides was consistent with the aftershock area. Numerous small-size landslides with limited volumes were revealed, which damaged or buried the roads and severely hindered the rescue process.     

b-value and fractal dimension variations in Iran

Iran sits on a region with a high intrinsic level of seismic activity due to its tectonic setting. Through statistical examination of the earthquakes listed in the catalogue from International Institute of Earthquake Engineering and Seismology (IIEES), this research attempted to calculate some seismicity factors and find correlation between them. A preliminary analysis indicated changes in the b-value of the Gutenberg-Richter relationship over the study region. Thus, the study area was divided into five zones (Alborz, Zagros, Azerbaijan, Central and East) and b-value was computed for each zone. Considering faulting mechanism styles and the b-values in the region, it was found that the lowest b-values belong to the thrust events and strike-slip faulting earthquakes have intermediate values. These findings support previous studies. Furthermore, results of b-value calculation were used for the estimation of accumulated differential stresses (σ₁–σ₂) over each zone. Overall, the b-value for Iran is averagely low which signifies the high stress tectonic regime in this region. Also, by having calculated fractal dimension (D) in each zone, a correlation obtained showing that in Iran region, the b-value correlates to fractal dimension by D = 4.2b–2 relation which does not support Aki's (1981) speculation of D = 3b/c.     

川东南长宁地区地震活动及b值演化特征

利用全国统一目录和流动台站目录,研究了四川盆地东南部长宁地区的地震活动特征和b值的空间分布特征.研究结果显示,长宁地区的地震活动在时间上呈现明显的分段特征,地震活动在2015年后明显增强;在空间上,长宁地区的地震活动主要集中在以28.3°N为界限的南、北2个地区,对于这2个区域的b值演化,计算结果显示出不同的分段特征....     

The Dinar Earthquake (Mw = 6.2; October 1, 1995; Afyon-Turkey) and Earthquake Hazard of the Dinar-Çivril Fault

—A moderately strong earthquake (M _w = 6.2) occurred in the town of Dinar at 17.57 UT on October 1, 1995, taking the lives of 90 people and damaging about 4500 buildings. Its epicenter is located near the Dinar-Çivril fault and its focal mechanism is linked to a northeast-southwesterly tensional stress field arising from the interaction between the subducting African plate and the overriding Aegean-Anatolian plate in the eastern Mediterranean.¶Surface cracks of the October 1 earthquake have been observed 10 km continuously along the Dinar-Çivril fault. The cracks have displayed a mode of dip-slip; however, some have also indicated lateral slip. The different modes of slip are generally in agreement with the fault plane solution and are indicators of the complex nature of the rupture process.¶In investigating the earthquake hazard of the Dinar-Çivril fault and proximity, the maximum likelihood method was used to estimate seismic hazard parameters of b-value, seismicity activity rate λ _m and the expected maximum magnitude M _max?. The data consisted of the historical data covering the period between 1800–1900 and instrumental data between 1900 and 1992. This method, allowing use of the mixed earthquake catalogue containing both historical and instrumental earthquake data, yielded values of 0.70, 1.92 and 7.14 for b, λ _m and M _max?, respectively. The recurrence time estimated for an earthquake of a magnitude of M _w = 6.2 is 123 years. The non-occurrence probabilities of such an earthquake in 1 and 50 years are 0.21 and 0.04, respectively.     

Temporal variation of b value associated with M ��4 earthquakes in the reservoir-triggered seismic environment of the Koyna�CWarna region, Western India

It is generally found that the b values associated with reservoir-triggered seismicity (RTS) are higher than the regional b values in the frequency magnitude relation of earthquakes. In the present study, temporal and spatial variation of b value is investigated using a catalog of 3,000 earthquakes from August 2005 through December 2010 for the Koyna?CWarna region in Western India, which is a classical site of RTS globally. It is an isolated (30?×?20?km²) zone of seismicity where earthquakes of up to M ??5 are found to occur during phases of loading and unloading of the Koyna and Warna reservoirs situated 25?km apart. For the Warna region, it is found that low b values of 0.6?C0.9 are associated with earthquakes of M ??4 during the loading phase. The percentage correlation of the occurrence of an M????4 earthquake with a low b value outside the 1?? or 2?? level is as high as 78?%. A drastic drop in the b value of about 50?% being reported for an RTS site may be an important precursory parameter for short-term earthquake forecast in the future.     

Application of the dynamic calibration method to international monitoring system stations in Central Asia using natural seismicity data

The dynamic calibration method (DCM), using natural seismicity data and initially elaborated in [Kedrov, 2001; Kedrov et al., 2001; Kedrov and Kedrov, 2003], is applied to International Monitoring System (IMS) stations in Central Asia. The algorithm of the method is refined and a program is designed for calibrating diagnostic parameters (discriminants) that characterize a seismic source on the source-station traces. The DCM calibration of stations in relation to the region under study is performed by the choice of attenuation coefficients that adapt the diagnostic parameters to the conditions in a reference region. In this method, the stable Eurasia region is used as the latter. The calibration used numerical data samples taken from the archive of the International Data Centre (IDC) for the IMS stations MKAR, BVAR, EIL, ASF, and CMAR. In this paper, we used discriminants in the spectral and time domains that have the form

$D_i = X_i - a_m m_b - b_\Delta \log \Delta $

and are independent of the magnitude m _b and the epicentral distance Δ; these discriminants were elaborated in [Kedrov et al., 1990; Kedrov and Lyuke, 1999] on the basis of a method used for identification of events at regional distances in Eurasia. Prerequisites of the DCM are the assumptions that the coefficient a _m is regionindependent and the coefficient b _Δ depends only on the geotectonic characteristics of the medium and does not depend on the source type. Thus, b _Δ can be evaluated only from a sample of earthquakes in the region studied; it is used for adapting the discriminants D(X _i ) in the region studied to the reference region. The algorithm is constructed in such a way that corrected values of D(X i) are calculated from the found values of the calibration coefficients b _Δ, after which natural events in the region under study are selected by filtering. Empirical estimates of the filtering efficiency as a function of a station vary in a range of 95–100%. The DCM was independently tested using records obtained at the IRIS (Incorporated Research Institutions for Seismology) stations BRVK and MAKZ from explosions detonated in India on May 11, 1998, and Pakistan on May 28, 1998; these stations are similar in location and recording instrumentation characteristics to the IMS stations BVAR and MKAR. This test resulted in correct recognition of the source type and thereby directly confirmed the validity of the proposed calibration method of stations with the use of natural seismicity data. It is shown that the calibration coefficients b _Δ for traces similar in the conditions of signal propagation (e.g., the traces from Iran to the stations EIL and ASF) are comparable for nearly all diagnostic parameters. We arrive at the conclusion that the method of dynamic calibration of stations using natural seismicity data in a region where no explosions were detonated can be significant for a rapid and inexpensive calibration of IMS stations. The DCM can also be used for recognition of industrial chemical explosions that are sometimes erroneously classified in regional catalogs as earthquakes.

     

Temporal variations of seismicity parameters in the Central Alborz,Iran

Temporal changes of b-value, fractal (correlation) dimensions of epicenters (D _e ) and occurrence time of earthquakes (D _t ) and relations between these parameters were calculated to investigate precursory changes before 28 May 2004, Baladeh-Kojour earthquake (M _w = 6.3) of Central Alborz, Iran. 2086 events with M _N ≥ 1.7 were selected for our analyses. A wide range of variation was seen in these parameters: b-value ～ 0.6–1.11, D _e ～ 0.97–1.64, and D _t ～ 0.13–0.93. The results showed decreases in all fractal parameters several months before the main shock. This decrease, which might have arisen due to clusters of events occurred between 2002–2003, was followed by a systematic increase, corresponding to the increased level of low-magnitude seismicity. It seems that changes in fractal parameters may be precursors of Baladeh-Kojour earthquake which was caused by seismic activation and quiescence. Furthermore, a positive correlation between b-value and D _e was detected before the main shock (D _e = 0.87 + 0.7b) and during aftershock sequences (D _e = 2b ± 0.09), which was further on changed to a negative one (D _e = 2.56–1.32b).     

Seismological Studies of a Fluid Injection in Sedimentary Rocks, East Texas

—?Induced microseismicity data from a large volume fluid injection into sedimentary rock was analyzed to study the fracture system, fluid pathways, and state of stress in the lower Frio formation in east Texas. Seismicity data are from two arrays of 25 3-component geophone packages sited in two monitoring boreholes. From a total of 2,894 event triggers, a subset of 54 microearthquakes was chosen for their high quality seismograms and clear P and S arrivals. Arrival times were picked with a precision of 0.5 to 1.0?ms, and microearthquakes were located with hypocentral uncertainties estimated as less than 10–20?m. Hypocenters farthest from the injection well define a nearly horizontal tube of seismicity approximately aligned in the direction of the injection well. A simultaneous inversion of arrival times for transverse isotropic velocity structure and hypocenters yielded P-wave anisotropy of ?14% and S-wave anisotropy of ?2%. Thus, velocities along vertical ray paths are higher than those along horizontal paths, probably because of lithologic differences. Single-event focal mechanisms were determined for 47 events, and many of them are normal fault type. The minimum principal stress derived from the focal mechanisms is nearly horizontal and trends approximately north-south, consistent with the regional stress state. An imaging analysis of the seismograms shows the presence of strong seismic scatterers at positions that correlate with boundaries seen in the hypocenters; both features probably result from a similar set of heterogeneities. This study demonstrates the abundance of information that can be extracted from induced seismicity data and underscores the value of induced seismicity monitoring for studying the fluid and fracture systems created by fluid injections.