The rupture process during the 1999 Düzce, Turkey, earthquake from joint inversion of teleseismic and strong-motion data

Teleseismic and strong-motion data are inverted to determine the rupture process during the November 1999 Düzce earthquake in NW Turkey. The fault geometry, rise time and rupture velocity are determined from the aftershock distribution and preliminary inversions of the teleseismic data. Joint inversion of the teleseismic and strong-motion data is then carried out for the slip distribution. We obtain the strike 264°, dip 64°, rake −172°, seismic moment 5.0×10¹⁹ N m (M_w 7.1), and average stress drop 7 MPa. This earthquake was characterized by bilateral fault rupture and asymmetric slip distribution. Two asperities (areas of large slip) are identified, the eastern one being 1.5 times larger than the western one. The derived slip distribution is consistent with the aftershock distribution, surface rupture and damage. The point of rupture initiation in this Düzce earthquake coincided with the eastern tip of the aftershock distribution of the August 1999 Izmit earthquake.     

Spatial variation of aftershock activity across the rupture zone of the 17 August 1999 Izmit earthquake, Turkey

We examined the spatial variation in the aftershock activity from the 17 August 1999 Izmit, Turkey earthquake. We found that this aftershock sequence is non-uniform both in space and time, aspects that need to be taken into account in any further statistical analysis. Other aspects of this aftershock sequence are similar to other aftershock sequences, namely low b-values and a high degree of spatial variation. We have detected three zones of relatively high b-values, two of which coincide with asperities revealed by previous slip inversion studies. The third zone with an anomalous b-value is located beyond the fault rupture and indicates a weakened fractured zone in the Yalova-Tuzla area. This b-value analysis provided no evidence for any significant difference that may exist between the two sides of the mainshock fault plane.     

Strong ground motion during the largest aftershock (Mw=5.8) of the 1999 Izmit earthquake, Turkey

We present a preliminary study of strong ground motion during the largest aftershock (M_w 5.8) of the 1999 Izmit earthquake (M_w 7.4), Turkey, at 11:55 on 13 September 1999. The peak ground acceleration observed near the epicentre of this aftershock was in agreement with that predicted by standard empirical prediction equations. Its spectral source parameters of the largest aftershock are also typical for a M_w 5.8 earthquake. At greater epicentral distances, there is an order-of-magnitude in scatter in peak ground acceleration values for this aftershock, which is attributed to site effects. The presence of thick layers of low-velocity sediments caused significant amplification of S-waves in the Avcılar district of Istanbul, at frequencies of 1 Hz, explaining the observed concentration of damage there as a result of the Izmit mainshock.     

Elastic and inelastic triggering of earthquakes in the North Anatolian Fault zone

Deformation models used to explain the triggering mechanism often assume pure elastic behaviour for the crust and upper mantle. In reality however, the mantle and possibly the lower crust behave viscoelastically, particularly over longer time scales. Consequently, the stress field of an earthquake is in general time-dependent. In addition, if the elastic stress increase were enough to trigger a later earthquake, this triggered event should occur instantaneously and not many years after the triggering event. Hence, it is appropriate to include inelastic behaviour when analysing stress transfer and earthquake interaction.In this work, we analyse a sequence of 10 magnitude M_s > 6.5 events along the North Anatolian Fault between 1939 and 1999 to study the evolution of the regional Coulomb stress field. We investigate the triggering of these events by stress transfer, taking viscoelastic relaxation into account. We evaluate the contribution of elastic stress changes, of post-seismic viscoelastic relaxation in the lower crust and mantle, and of steady tectonic loading to the total Coulomb stress field. We analyse the evolution of stress in the region under study, as well as on the rupture surfaces of the considered events and their epicentres. We study the state of the Coulomb stress field before the 1999 İzmit and Düzce earthquakes, as well as in the Marmara Sea region.In general, the Coulomb stress failure criterion offers a plausible explanation for the location of these events. However, we show that using a purely elastic model disregards an important part of the actual stress increase/decrease. In several cases, post-seismic relaxation effects are important and greater in magnitude than the stress changes due to steady tectonic loading. Consequently, viscoelastic relaxation should be considered in any study dealing with Coulomb stress changes.According to our study, and assuming that an important part of the rupture surface must be stressed for an earthquake to occur, the most likely value for the viscosity of the lower crust or mantle in this region is 5 · 10¹⁷–10¹⁸ Pa · s. Our results cannot rule out the possibility of other time-dependent processes involved in the triggering of the 1999 Düzce event. However, the stress increase due to viscoelastic relaxation brought 22% of the 1999 Düzce rupture area over the threshold value of Δσ_c ≥ 0.01 MPa (0.1 bar), and took the whole surface closer to failure by an average of 0.2 MPa. Finally, we argue that the Marmara Sea region is currently being loaded with positive Coulomb stresses at a much faster rate than would arise exclusively from steady tectonic loading on the North Anatolian Fault.     

Maximum magnitudes in aftershock sequences in Taiwan

In this work, Båth’s Law, the b-value in Gutenberg–Richter Law (G–R Law) in the form of the 1/β relationship, and both the a- and b-values in the G–R Law were introduced in order to estimate maximum aftershock magnitudes of earthquake sequences in the Taiwan region. The averaged difference of magnitude between the mainshock and the maximum aftershock is 1.20, and is consistent with Båth’s Law, however, with a large uncertainty. The large uncertainty implies that the difference may result from a variable controlled by other factors, such as the aftershocks number of an earthquake sequence and magnitude threshold for mainshock. With 1/β, since 86% of the earthquake sequences with a M ⩾ 6.0 mainshock follow this relationship, the upper bound of the maximum magnitude can be estimated for an earthquake sequence with a large mainshock. The a- and b-values in the G–R Law was also considered by evaluating maximum aftershock magnitudes. As there are low residuals between the model and the observations, the results suggest that the G–R Law is a good index for maximum aftershock magnitude determinations. In order to evaluate the temporal decays of maximum aftershock magnitudes, modified Omori’s Law was introduced. Using the approaches mentioned above, the maximum magnitudes and the temporal evolution of an earthquake sequence could be modeled. Among them, the model of the G–R Law has the best fit with observations for most of earthquake sequences. It shows its feasibility. The results of this work may benefit seismic hazards mitigation in the form of rapid re-evaluations for short-term seismic hazards immediately following devastating earthquakes.     

The 10 June 2012 Fethiye Mw 6.0 aftershock sequence and its relation to the 24–25 April 1957 Ms 6.9–7.1 earthquakes in SW Anatolia,Turkey

The 10 June 2012 M_w 6.0 aftershock sequence in southwestern Anatolia is examined. Centroid moment tensors for 23 earthquakes with moment magnitudes (M_w) between 3.7 and 6.0 are determined by applying a waveform inversion method. The mainshock is a shallow focus strike-slip with reverse component event at a depth of 30 km. The seismic moment (M_o) of the mainshock is estimated as 1.28 × 10¹⁸ Nm and rupture duration of the Fethiye mainshock is 38 s. The focal mechanisms of the aftershocks are mainly strike-slip faulting with a reverse component. The geometry of the focal mechanisms reveals a strike-slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to obtain a more accurate picture of the Fethiye earthquake stress field. The stress tensor inversion results indicate a predominant strike-slip stress regime with a NW–SE oriented maximum horizontal compressive stress (S_H). According to variance of the stress tensor inversion, to first order, the Fethiye earthquake area is characterized by a homogeneous interplate stress field. The Coulomb stress change associated with the mainshock and the largest aftershock are also investigated to evaluate any significant enhancement of stresses along the Gulf of Fethiye and surrounding region. Positive lobes with stress more than 0.4 bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards NNW–SSE and E–W directions.     

Aftershock activity of Bhuj earthquake of january 26th, 2001

Following a large-sized Bhuj earthquake (M _s = 7.6) of January 26th, 2001, a small aperture 4-station temporary local network was deployed, in the epicentral area, for a period of about three weeks and resulted in the recording of more than 1800 aftershocks (-0.07 ≤M _L <5.0). Preliminary locations of epicenters of 297 aftershocks (2.0 ≤M _L <5.0) have brought out a dense cluster of aftershock activity, the center of which falls 20 km NW of Bhachau. Epicentral locations of after-shocks encompass a surface area of about 50 × 40 km² that seems to indicate the surface projection of the rupture area associated with the earthquake. The distribution of aftershock activity above magnitude 3, shows that aftershocks are nonuniformly distributed and are aligned in the north, northwest and northeast directions. The epicenter of the mainshock falls on the southern edge of the delineated zone of aftershock activity and the maximum clustering of activity occurs in close proximity of the mainshock. Well-constrained focal depths of 122 aftershocks show that 89% of the aftershocks occurred at depths ranging between 6 and 25 km and only 7% and 4% aftershocks occur at depths less than 5 and more than 25 km respectively. The Gutenberg-Richter (GR) relationship, logN = 4.52 - 0.89M_L, is fitted to the aftershock data (1.0<-M _L<5.0) and theb-value of 0.89 has been estimated for the aftershock activity.     

The 2002 Molise earthquake sequence: What can we learn about the tectonics of southern Italy?

On October 31, 2002 a M_L = 5.4 earthquake occurred in southern Italy, at the margin between the Apenninic thrust belt (to the west) and the Adriatic plate (to the east). In this area, neither historical event nor seismogenic fault is reported in the literature. In spite of its moderate magnitude, the earthquake caused severe damage in cities close to the epicenter and 27 people, out of a total of 29 casualties, were killed by the collapse of a primary school in S. Giuliano di Puglia. By inverting broadband regional waveforms, we computed moment tensor solutions for 15 events, as small as M_L = 3.5 (M_w = 3.7). The obtained focal mechanisms show pure strike-slip geometry, mainly with focal planes oriented to NS (sinistral) and EW (dextral). In several solutions focal planes are rotated counterclockwise, in particular for later events, occurring west of the mainshock. From the relocated aftershock distribution, we found that the mainshock ruptured along an EW plane, and the fault mechanisms of some aftershocks were not consistent with the mainshock fault plane. The observed stress field, resulting from the stress tensor inversion, shows a maximum principal stress axis with an east–west trend (N83°W), whereas the minimum stress direction is almost N–S. Considering both the aftershock distribution and moment tensor solutions, it appears that several pre-existing faults were activated rather than a single planar fault associated with the mainshock. The finite fault analysis shows a very simple slip distribution with a slow rupture velocity of 1.1 km/s, that could explain the occurrence of a second mainshock about 30 h after. Finally, we attempt to interpret how the Molise sequence is related to the normal faulting system to the west (along the Apennines) and the dextral strike-slip Mattinata fault to the east.     

Seismic source characteristics in Kachchh and Saurashtra regions of Western India: b-value and fractal dimension mapping of aftershock sequences

Seismic source characteristics in the Kachchh rift basin and Saurashtra horst tectonic blocks in the stable continental region (SCR) of western peninsular India are studied using the earthquake catalog data for the period 2006–2011 recorded by a 52-station broadband seismic network known as Gujarat State Network (GSNet) running by Institute of Seismological Research (ISR), Gujarat. These data are mainly the aftershock sequences of three mainshocks, the 2001 Bhuj earthquake (M _w 7.7) in the Kachchh rift basin, and the 2007 and 2011 Talala earthquakes (M _w ≥ 5.0) in the Saurashtra horst. Two important seismological parameters, the frequency–magnitude relation (b-value) and the fractal correlation dimension (D _c) of the hypocenters, are estimated. The b-value and the D _c maps indicate a difference in seismic characteristics of these two tectonic regions. The average b-value in Kachchh region is 1.2 ± 0.05 and that in the Saurashtra region 0.7 ± 0.04. The average D _c in Kachchh is 2.64 ± 0.01 and in Saurashtra 2.46 ± 0.01. The hypocenters in Kachchh rift basin cluster at a depth range 20–35 km and that in Saurashtra at 5–10 km. The b-value and D _c cross sections image the seismogenic structures that shed new light on seismotectonics of these two tectonic regions. The mainshock sources at depth are identified as lower b-value or stressed zones at the fault end. Crustal heterogeneities are well reflected in the maps as well as in the cross sections. We also find a positive correlation between b- and D _c-values in both the tectonic regions.

     

Spatial analysis of the frequency–magnitude distribution of aftershock activity of 2003 Bam earthquake: southeast Iran

The Bam earthquake (2003 December 26, M _W = 6.6) was one of the largest earthquakes that occurred in southeast of Iran during last century. It took place along an N–S trending right-lateral strike-slip fault, almost near the southern end of Nyband–Gowk fault. In this study, we mapped the frequency–magnitude distribution of aftershock events spatially across the Bam aftershock zone. The b-value varies between 0.6 and 1.1 across the Bam rupture zone. The overall depth distribution of b-value in Bam aftershock zone reveals two distinct increases in b-value: (1) at depths of 8–10 km and (2) shallower than 4 km beneath the Bam city. There is no correlation between high b- value anomalies found in this study and the region of largest slip, whereas the spatial correlation between high b-value anomalies and the zone of low V _s and high σ (in earlier tomography study) is obvious. This correlation reveals that material properties and increasing heterogeneity are more important in controlling b-value distribution in Bam earthquake rupture zone. The high b-value anomaly near the surface of northern part of rupture zone may be related to unconsolidated and water-rich quaternary alluvial sediments and probable low-strength rocks beneath them. The high b-value anomaly at depth range 8–10 km can be correlated with fractured and fluid-filled mass, which may result from the movement of magma during Eocene volcanism in the Bam area. In this study, the induced changes in pore fluid pressure due to main shock are suggested as a mechanism for aftershock generation.     

Evaluation of earthquake potential along the Northern Anatolian Fault Zone in the Marmara Sea using comparisons of GPS strain and seismotectonic parameters

Seismotectonic parameters including the Gutenberg-Richter b-value and multifractal dimensions D₂ and D₁₅ of seismicity patterns (both spatial and temporal) were compared to GPS-derived maximum shear and dilatation strains measured in the Marmara Sea region of western Turkey along the Northern Anatolian Fault Zone (NAFZ). Comparisons of seismotectonic parameters and GPS-derived maximum shear and dilatation strain along the NAFZ in the vicinity of the 1999 M7.4 Izmit earthquake reveal a positive correlation (r = 0.5, p = 0.05) between average dilatation and the Gutenberg-Richter b-value. Significant negative correlation (r = − 0.56, p = 0.03 and r = − 0.56, p = 0.02) was also observed between the spatial fractal dimension D₂ and GPS-derived maximum geodetic and shear strain. This relationship suggests that, as maximum geodetic and shear strains increase, seismicity becomes increasingly clustered.Anomalous interrelationships are observed in the Marmara Sea region prior to the Izmit event along a bend in the NAFZ near the eastern end of the Marmara Sea known as the Northern Boundary Fault (NBF). An asperity is located near the northwest end of the NBF. Along the 50-km length of the NBF, GPS strains become slightly compressive. The correlation between b-value and GPS-derived dilatation suggests that regions in compression have increased probability of larger magnitude rupture. The NBF appears to serve as an impediment to the transfer of strain from east to west along the NAFZ. Recurrence times for large earthquakes along the NBF are larger than in surrounding areas. Temporal clustering of seismicity in the vicinity of the NBF may represent foreshocks of an impending rupture.     

3-D seismic structure of the source area of the 1993 Latur, India, earthquake and its implications for rupture nucleations

The Latur earthquake (M_w 6.1) of 29 September 1993 is a rare stable continental region (SCR) earthquake that occurred on a previously unknown blind fault. In this study, we determined detailed three-dimensional (3-D) P- and S-wave velocity (V_p, V_s) and Poisson's ratio (σ) structures by inverting the first P- and S-wave high-quality arrival time data from 142 aftershocks that were recorded by a network of temporary seismic stations. The source zone of the Latur earthquake shows strong lateral heterogeneities in V_p, V_s and σ structures, extending in a volume of about 90 × 90 × 15 km³. The mainshock occurred within, but near the boundary, of a low-V_p, high-V_s and low-σ zone. This suggests that the structural asperities at the mainshock hypocenter are associated with a partially fluid-saturated fractured rock in a previously unknown source zone with intersecting fault surfaces. This might have triggered the 1993 Latur mainshock and its aftershock sequence. Our results are in good agreement with other geophysical studies that suggest high conductivity and high concentration of radiogenic helium gas beneath the source zone of the Latur earthquake. Our study provides an additional evidence for the presence of fluid related anomaly at the hidden source zone of the Latur earthquake in the SCR and helps us understand the genesis of damaging earthquakes in the SCR of the world.     

Aftershock sequences in the mid-ocean ridge environment: an analysis using hydroacoustic data

Hydroacoustic data from autonomous arrays and the U.S. Navy's Sound Surveillance System (SOSUS) provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow-spreading Mid-Atlantic Ridge (MAR), intermediate-spreading Juan de Fuca Ridge (JdFR) and fast-spreading East Pacific Rise (EPR). Aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single-link cluster (SLC) analysis, with a combined space–time metric. In the regions examined, hydroacoustic data improve the completeness level of the earthquake catalog by 1.5–2.0 orders of magnitude, allowing the decay constant, p, of the modified Omori law (MOL) to be determined for individual sequences. A non-parametric goodness-of-fit test indicates six of the seven sequences examined are described well by a MOL model. The p-values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data. For three sequences along the Siqueiros, Discovery and western Blanco Transforms, p-values are estimated to be 0.94–1.29. The spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra-transform spreading centers at these locations. An aftershock sequence following a 7.1M_s thrust event near the northern edge of the Easter Microplate exhibits p=1.02±0.11. Within the sequence, aftershocks are located to the north of a large topographic ridge, which may represent the surface expression of the shallow-dipping fault that ruptured during the mainshock. Two aftershock sequences near 24°25′N and 16°35′N on the MAR exhibit higher p-values, 1.74±0.23 and 2.37±1.65, although the latter estimate is not well constrained because of the small number of aftershocks. Larger p-values along the ridge crest might reflect a hotter thermal regime in this setting. Additional monitoring, however, will be needed to determine if p-value differences between the ridge and transform sequences are robust. A 1999 sequence on the Endeavour segment of the JdFR, which has been correlated with changes in the hydrothermal system, is described poorly by the MOL model. The failure of the MOL model, the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence.     

Aftershock sequences of two great Sumatran earthquakes of 2004 and 2005 and simulation of the minor tsunami generated on September 12, 2007 in the Indian Ocean and its effect

We present the seismic energy, strain energy, frequency–magnitude relation (b-value) and decay rate of aftershocks (p-value) for the aftershock sequences of the Andaman–Sumatra earthquakes of December 26, 2004 (M _w 9.3) and March 28, 2005 (M _w 8.7). The energy released in aftershocks of 2004 and 2005 earthquake was 0.135 and 0.365% of the energy of the respective mainshocks, while the strain release in aftershocks was 39 and 71% for the two earthquakes, respectively. The b-value and p-value indicate normal value of about 1. All these parameters are in normal range and indicate normal stress patterns and mechanical properties of the medium. Only the strain release in aftershocks was considerable. The fourth largest earthquake in this region since 2004 occurred in September 2007 off the southern coast of Island of Sumatra, generating a relatively minor tsunami as indicated by sea level gauges. The maximum wave amplitude as registered by the Padang, tide gauge, north of the earthquake epicenter was about 60 cm. TUNAMI-N2 model was used to investigate ability of the model to capture the minor tsunami and its effect on the eastern Indian Coast. A close comparison of the observed and simulated tsunami generation, propagation and wave height at tide gauge locations showed that the model was able to capture the minor tsunami phases. The directivity map shows that the maximum tsunami energy was in the southwest direction from the strike of the fault. Since the path of the tsunami for Indian coastlines is oblique, there were no impacts along the Indian coastlines except near the coast of epicentral region.     

The 2007 Talala,Saurashtra, western India earthquake sequence: Tectonic implications and seismicity triggering

A damaging and widely felt moderate (M_w 5.0) earthquake occurred in the Talala region of Saurashtra, Gujarat (western India) on November 6, 2007. The highly productive sequence comprised about 1300 micro earthquakes (M > 0.5) out of which 325 of M ? 1.5 that occurred during November 6, 2007–January 10, 2008 were precisely located. The spatial aftershock distribution revealed a NE–SW striking fault in accordance with the centroid moment tensor solution, which in turn implies left-lateral motion. The orientation and sense of shear are consistent with similarly orientated geological fault identified in the area from satellite imagery and field investigation.The aftershocks temporal decay, b-value of frequency–magnitude distribution, spatial fractal dimension, D, and slip ratio (ratio of the slip occurred on the primary fault to the total slip) were examined with the purpose to identify the properties of the sequence. The high b-value (1.18 ± 0.01) may be attributed to the paucity of the larger (M ? 4.0) aftershocks and reveals crustal heterogeneity and low stress regime. The high p-value (1.10 ± 0.39), implying fast decay rate of aftershocks, evidences high surface heat flux. A value of the spatial fractal dimension (D) equal to 2.21 ± 0.02 indicates random spatial distribution and source in a two-dimensional plane that is being filled-up by fractures. A slip ratio of 0.42 reveals that more slip occurred on secondary fault systems.The static Coulomb stress changes due to the coseismic slip of the main shock, enhanced off fault aftershock occurrence. The occurrence of a moderate earthquake (M_w 4.3) on October 5, 2008 inside a region of positive Coulomb stress changes supports the postulation on aftershock triggering. When the stress changes were resolved on a cross section including the stronger (M4.8) foreshock plane that is positioned adjacent to the main fault, it became evident that the activity continued there due to stress transfer from the main rupture.     

The 20 March 1992 South Aegean,Greece, earthquake (Ms= 5.3): possible anomalous effects

The earthquake (M_s= 5.3) of 20 March 1992 and its aftershocks, which occurred near the volcanic island complex of Milos, South Aegean, Greece, are studied on the basis of filed observations and instrumental data. The mainshock caused some building damage, the maximum intensity of VI+ (MM) being assigned to Triovasalos, Milos. Ground cracks, liquefaction in soil, landslides and rockfalls were observed in Milos. Liquefaction took place at an apparently anomalously long epicentral distance (D= 12 km) and is associated with unusually small earthquake magnitude. Abnormal animal behaviour was reported no longer than twelve hours before the mainshock. The b-value (= 1.02) of the G–R relation for the aftershock sequence, the exponentially decreasing number of aftershocks with time, and the difference (= 0.5) in magnitude between the mainshock and its largest aftershock imply that the origin of these earthquakes is tectonic and not associated with the volcanic field of Milos.     

The 1998 Hida Mountain, Central Honshu, Japan, earthquake swarm: Double-difference event relocation, frequency–magnitude distribution and Coulomb stress changes

By using the double-difference relocation technique, we have determined the fine structure of seismicity during the 1998 Hida Mountain earthquake swarm. The distribution of seismic activity defines two main directions (N–S and E–W) that probably correspond to the regional stress pattern. The detailed structure of seismicity reveals intense spatio-temporal clustering and earthquake lineations. Each cluster of events contains a mainshock and subsequent aftershock activity that decays according to the Omori law. The seismicity and the b-value temporal and spatial patterns reflect the evolution of the static stress changes during the earthquake swarm. About 80% of the swarm's best-relocated events occur in regions of increased ΔCFF. The smaller value of b found in the northern part of the swarm region and a larger b-value observed to the south, for the same period of time, could be well explained by the static stress changes caused by the larger events of the sequence. We argue that the state of stress in the crust is the main factor that controls the variation of b-value.     

Seismic source characteristics in Kachchh and Saurashtra regions of Western India: <Emphasis Type="Italic">b</Emphasis>-value and fractal dimension mapping of aftershock sequences

Seismic source characteristics in the Kachchh rift basin and Saurashtra horst tectonic blocks in the stable continental region (SCR) of western peninsular India are studied using the earthquake catalog data for the period 2006–2011 recorded by a 52-station broadband seismic network known as Gujarat State Network (GSNet) running by Institute of Seismological Research (ISR), Gujarat. These data are mainly the aftershock sequences of three mainshocks, the 2001 Bhuj earthquake (M _w 7.7) in the Kachchh rift basin, and the 2007 and 2011 Talala earthquakes (M _w ≥ 5.0) in the Saurashtra horst. Two important seismological parameters, the frequency–magnitude relation (b-value) and the fractal correlation dimension (D _c) of the hypocenters, are estimated. The b-value and the D _c maps indicate a difference in seismic characteristics of these two tectonic regions. The average b-value in Kachchh region is 1.2 ± 0.05 and that in the Saurashtra region 0.7 ± 0.04. The average D _c in Kachchh is 2.64 ± 0.01 and in Saurashtra 2.46 ± 0.01. The hypocenters in Kachchh rift basin cluster at a depth range 20–35 km and that in Saurashtra at 5–10 km. The b-value and D _c cross sections image the seismogenic structures that shed new light on seismotectonics of these two tectonic regions. The mainshock sources at depth are identified as lower b-value or stressed zones at the fault end. Crustal heterogeneities are well reflected in the maps as well as in the cross sections. We also find a positive correlation between b- and D _c-values in both the tectonic regions.     

2021年5月21日漾濞MS6.4地震的发震断层及其破裂特征: 地震序列的重定位分析结果

据中国地震台网测定，2021年5月21日21时48分在云南省大理州漾濞县发生M_S6.4地震，及时查明此次地震的发震构造及震源破裂特征，可为认识该区孕震条件和判别未来强震危险性提供关键依据。采用双差定位方法对漾濞地震序列进行重新定位，得到3863次地震事件的精确震源位置。结果显示:漾濞地震序列整体呈北西—南东向分布，长约25 km；整体走向135°；M_S6.4主震震中位置为25.688°N，99.877°E；震源深度约9.6 km。综合地震序列深度剖面和震源机制解结果可知，发震断层应为北西走向、整体向西南方向陡倾的右旋走滑断层，倾角具有自北西向南东逐渐变缓的趋势。进一步分析地震序列的时空演化过程发现，该地震具有典型的"前震-主震-余震型"地震序列活动特点，其破裂过程主要包括3个阶段。破裂成核阶段:首先在发震断层10~12 km深度处相对脆弱部位产生小尺度破裂，之后失稳加速破裂，发生M_S5.6地震；主震破裂阶段:在构造应力场持续加载和周围小尺度破裂的共同影响下，促使浅部较高强度断层闭锁区破裂，形成M_S6.4主震；尾端拉张破裂阶段:主震破裂向东南扩展过程中，在东南端形成与之呈马尾状斜交的、具有正断性质的次级破裂，并产生M_S5.2余震。而且此次地震还在源区北东侧触发了北北东向的左旋走滑破裂。综合分析认为，漾濞地震是兰坪-思茅地块内部北西向草坪断裂在近南北向区域应力挤压作用下发生右旋走滑运动的结果，具有明显的新生断裂特征。近年来兰坪-思茅地块内部一系列中强地震的发生表明，青藏高原物质向东南持续挤出的过程中，遇到该地块的阻挡，正在导致地块内部早期断层贯通形成新的活动断裂。因此，川滇地块西南边界带上或相邻地块内部老断层的复活和新生断裂的产生是区域中强地震危险性分析评价中值得关注的重要课题，同时建议需重视未来该区中强地震进一步向东南和向北的迁移或扩展的可能性。      

Heterogeneous response of hydrogeological systems to the Izmit and Düzce (Turkey) earthquakes of 1999

At four sites in Turkey and Armenia the physico-chemical properties of thermal and mineral waters were monitored continuously during the Izmit and Düzce earthquakes that occurred along the North Anatolian fault in August and November 1999. The epicentral distances between the moment magnitude (M_w) 7.6 Izmit earthquake and the monitoring locations were 313, 488, 1,161, and 1,395 km. At the most distant site, the specific electrical conductivity of mineral water from a flowing artesian well dropped co-seismically and postseismically by 7%. No changes were observed at the other sites, although the estimated earthquake strains and peak ground accelerations are much higher. A similar pattern was observed after the Düzce earthquake, which happened three months after the Izmit event. The response of a hydrogeological system seems to depend on the site characteristics rather than on the nature of the earthquake. A hydrogeological model for the sensitive observation site farthest from the Izmit earthquake explains the observations in terms of a changed mixing ratio between two fluid components. Passing seismic waves may trigger a local pore-pressure increase according to the mechanism of advective overpressure. The preconditions for this mechanism, free gas bubbles in the aquifer in combination with a trap for rising bubbles, is probably not fulfilled by the other groundwater systems. Electronic Publication