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.     

An insight into crack density,saturation rate,and porosity model of the 2001 Bhuj earthquake in the stable continental region of western India

The 2001 Bhuj earthquake (Mw 7.6) source zone is examined in the light of crack density (ε), saturation rate (ξ) and porosity parameter (ψ) using new data set derived from a large aftershock sequence recorded by the Gujarat seismic network (GSNet) during November, 2006–December, 2009. Processes of rupture initiations of the mainshock and its aftershock sequence are better understood by synthesizing the dynamic snapshots of the source zone using the new dataset. Pattern of crustal heterogeneities associated with high-ε, high-ξ and high-ψ anomalies at depths varying from 20 km to 25 km is similar to those of earlier study by Mishra and Zhao (2003). The anomalous zone is found extended distinctly by 50–60 km in the lateral direction, indicating the reinforcement of cracks and fractured volume of rock matrix due to long aftershock sequence since 2001 Bhuj earthquake in the source area. It is inferred that the presence of a fluid-filled fractured rock matrix with super saturation may have affected the structural and seismogenic strengths of the source zone and is still contributing significantly to the geneses of earthquakes in and around the source zone. Anomalous pattern of high-ε with wider distribution of high-ξ indicates the existence of micro-cracks in the lower crust, while high-ψ suggests the cementation of cracks through permeation of residual magma/metamorphic fluids into the hypocenter zone. The results suggest that the existence of residual fluids in the fractured rock matrix in the mid to lower crust might have played a key role in triggering the 2001 mainshock and is still responsible for its continued long aftershock sequences.     

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.     

Properties of the aftershocks sequences of the 2000 and 2002 earthquakes in Aksehir-Afyon graben,west-central Anatolia,Turkey

The b-value of the Gutenberg–Richter’s frequency–magnitude relation and the p-value of the modified Omori law, which describes the decay rate of aftershock activity, were investigated for more than 500 aftershocks in the Aksehir-Afyon graben (AAG) following the 15 December 2000 Sultandagi–Aksehir and the 3 February 2002 Çay–Eber and Çobanlar earthquakes. We used the Kandilli Observatory’s catalog, which contains records of aftershocks with magnitudes ≥2.5. For the Çobanlar earthquake, the estimated b-values for three aftershock sequences are in the range 0.34 ≤  b ≤ 2.85, with the exception of the one that occurred during the first hour (4.77), while the obtained p-values are in the range 0.44 ≤ p ≤ 1.77. The aftershocks of the Sultandagi earthquake have a high p-value, indicating fast decay of the aftershock activity. A regular increase of b can be observed, with b < 1.0 after 0.208 days for the Çay–Eber earthquake. A systematic and similar increase and decrease pattern exists for the b- and p-values of the Çobanlar earthquakes during the first 5 days.     

The intracratonic Caraíbas–Itacarambi earthquake of December 09, 2007 (4.9 mb), Minas Gerais State,Brazil

On December 9, 2007, a 4.9 m_b earthquake occurred in the middle of the São Francisco Craton, in a region with no known previous activity larger than 4 m_b. This event reached intensity VII MM (Modified Mercalli) causing the first fatal victim in Brazil. The activity had started in May 25, 2007 with a 3.5 magnitude event and continued for several months, motivating the deployment of a local 6-station network. A three week seismic quiescence was observed before the mainshock. Initial absolute hypocenters were calculated with best fitting velocity models and then relative locations were determined with hypoDD. The aftershock distribution indicates a 3 km long rupture for the mainshock. The fault plane solution, based on P-wave polarities and hypocentral trend, indicates a reverse faulting mechanism on a N30°Ε striking plane dipping about 40° to the SE. The rupture depth extends from about 0.3 to 1.2 km only. Despite the shallow depth of the mainshock, no surface feature could be correlated with the fault plane. Aeromagnetic data in the epicentral area show short-wavelength lineaments trending NNE–SSW to NE–SW which we interpret as faults and fractures in the craton basement beneath the surface limestone layer. We propose that the Caraíbas–Itacarambi seismicity is probably associated with reactivation of these basement fractures and faults under the present E–W compressional stress field in this region of the South American Plate.     

Influence of the 2008 Wenchuan earthquake (Mw 7.9) on the occurrence probability of future earthquakes on neighboring faults

The Longquan–Shan fault and the Huya fault are two major neighboring faults of the Longmen–Shan fault zone where the 12 May 2008 Wenchuan earthquake (M_w 7.9) occurred. To study the influence of the Wenchuan event on these two active faults, we calculate changes of Coulomb stress on the Longquan–Shan fault and the Huya fault caused by the Wenchuan mainshock. Our results indicate that the Coulomb stress in the northern section (Zone A) of the Longquan–Shan fault is increased by 0.07–0.10 bars, that in the middle section (Zone B) by 0.04–0.11 bars, and that in the southern section (Zone C) shows almost no change. For the Huya fault, the Coulomb stress is decreased by 0.01–0.03 bars in the northern section (Zone A), 0.10–0.35 bars in the middle section (Zone B), and nearly 0.5 bars in the southern section (Zone C). The epicenter distribution of small earthquakes (M_L ⩾ 1.5) on the Longquan–Shan fault and the Huya fault after the Wenchuan earthquake is consistent with the distribution of the Coulomb stress change. This implies that the Wenchuan earthquake may have triggered small events on the Longquan–Shan fault, but inhibited those on the Huya fault. We then use the rate/state friction law to calculate the occurrence probability of future earthquakes in the study region for the next decade. They include the distribution of b-values, magnitude of completeness (M_c), the background seismicity rate, a value of Aσ_n and the duration for the transient effect (t_a) in the study region. We also estimate the earthquake occurrence probabilities on the neighboring faults after the Wenchuan earthquake. Our results show that, the occurrence probability of future earthquakes in the Longquan–Shan has a slight increase, being 7% for M ⩾ 5.0 shocks during the next decade, but the earthquake probability in the Huya region is reduced obviously, being 5–20%, 7–26% and 3–9% for M ⩾ 5.0 shocks during the next decade in sections A, B and C of the Huya fault, respectively.     

Geodetic and seismological investigation of crustal deformation near Izmir (Western Anatolia)

The Aegean region including western Turkey, mainland Greece, and the Hellenic Arc is the most seismological and geodynamical active domain in the Alpine Himalayan Belt. In this study, we processed 3 years of survey-mode GPS data and present the analysis of a combination of geodetic and seismological data around Izmir, which is the third most populated city in Turkey. The velocities obtained from 15 sites vary between 25 mm/yr and 28 mm/yr relative to the Eurasian plate. The power law exponent of earthquake size distribution (b-value) ranges from 0.8 to 2.8 in the Izmir region between 26.2°E and 27.2°E. The lowest b-value zones are found along Karaburun Fault (b = 0.8) and, between Seferihisar and Tuzla Faults (b = 0.8). A localized stress concentration is expected from numerical models of seismicity along geometrical locked fault patches. Therefore, areas with lowest b-values are considered to be the most likely location for a strong earthquake, a prediction that is confirmed by the 2005 M_w = 5.9 Seferihisar earthquake sequences, with epicentres located to the south of the Karaburun Fault. The north–south extension of the Izmir area is corroborated by extension rates up to 140 nanostrain/yr as obtained from our GPS data. We combined the 3-year GPS velocity field with the published velocity field to determine the strain rate pattern in the area. The spatial distribution of b-value reflects the normal background due to the tectonic framework and is corroborated by the geodetic data. b-Values correlate with strain pattern. This relationship suggests that decrease of b-values signifies accumulating strain.     

Low b-value prior to the Indo-Myanmar subduction zone earthquakes and precursory swarm before the May 1995 M 6.3 earthquake

Some 455 events (m_b ⩾ 4.5) in the Indo-Myanmar subduction zone are compiled using the ISC/EHB/NEIC catalogues (1964–2011) for a systematic study of seismic precursors, b-value and swarm activity. Temporal variation of b-value is studied using the maximum likelihood method beside CUSUM algorithm. The b-values vary from 0.95 to 1.4 for the deeper (depth ⩾60 km) earthquakes, and from 0.85 to 1.3 for the shallower (depth <60 km) earthquakes. A sudden drop in the b-value, from 1.4 to 0.9, prior to the occurrence of larger earthquake(s) at the deeper depth is observed. It is also noted that the CUSUM gradient reversed before the occurrence of larger earthquakes. We further examined the seismicity pattern for the period 1988–1995 within a radius of 150 km around the epicentre (latitude: 24.96°N; longitude: 95.30°E) of a deeper event M 6.3 of May 6, 1995 in this subduction zone. A precursory swarm during January 1989 to July 1992 and quiescence during August 1992 to April 1995 are identified before this large earthquake. These observations are encouraging to monitor seismic precursors for the deeper events in this subduction zone.     

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.     

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 seismic b-value and its correlation with Bouguer gravity anomaly over the Shillong Plateau area: Tectonic implications

Clues to the understanding of intra- and inter-plate variations in strength or stress state of the crust can be achieved through different lines of evidence and their mutual relationships. Among these parameters Bouguer gravity anomalies and seismic b-values have been widely accepted over several decades for evaluating the crustal character and stress regime. The present study attempts a multivariate analysis for the Shillong Plateau using the Bouguer gravity anomaly and the earthquake database, and establishes a causal relationship between these parameters. Four seismic zones (Zones I–IV), with widely varying b-values, are delineated and an excellent correlation between the seismic b-value and the Bouguer gravity anomaly has been established for the plateau. Low b-values characterize the southwestern part (Zone IV) and a zone (Zone III) of intermediate b-values separates the eastern and western parts of the plateau (Zones I and II) which have high b-values. Positive Bouguer anomaly values as high as +40 mgal, a steep gradient in the Bouguer anomaly map and low b-values in the southwestern part of the plateau are interpreted as indicating a thinner crustal root, uplifted Moho and higher concentration of stress. In comparison, the negative Bouguer anomaly values, flat regional gradient in the Bouguer anomaly map and intermediate to high b-values in the northern part of the plateau are consistent with a comparatively thicker crustal root and lower concentration of stress, with intermittent dissipation of energy through earthquake shocks. Further, depth wise variation in the b-value for different seismic zones, delineated under this study, allowed an appreciation of intra-plateau variation in crustal thickness from ∼30 km in its southern part to ∼38 km in the northern part. The high b-values associated with the depth, coinciding with lower crust, indicate that the Shillong Plateau is supported by a strong lithosphere.     

Crustal heterogeneities beneath the 2011 Talala,Saurashtra earthquake,Gujarat, India source zone: Seismological evidence for neo-tectonics

During the 1st decade of the 21st century, the study area of Talala, Saurashtra of western India witnessed three damaging earthquakes of moderate magnitude, year 2007 [Mw 5.0; Mw 4.8] and in the year 2011 [Mw 5.1] that generated public panic in the region. The last damaging moderate earthquake of the 20th October 2011 in Talala region (21.09°N;70.45°E), located at about 200 km south to the devastating 2001 Bhuj (23.412°N, 70.232°E) mainshock (Mw 7.6), jolted the entire Saurashtra region of Gujarat. A long series of aftershocks followed hereafter, recorded at nine seismograph/accelerograph stations. Hypocenters of aftershocks were relocated accurately using absolute and relative travel time (double-difference) method. In this study, we, for the first time, determined 3-D tomographic images of the upper crust beneath the 2011 Talala earthquake source zone by inverting about 1135 P and 1125 S wave arrival time data. Estimates of seismic velocities (Vp, Vs) and Poisson’s ratio (σ) structures offer a reliable interpretation of crustal heterogeneities and their bearing on geneses of moderate earthquakes and their aftershock sequences beneath the source zone. It is found that the 2011 Talala mainshock hypocenter depth (6 km) is located near the boundary of the low and high velocity (Vp, Vs) and the source zone is associated with low-σ anomalies guarded by the prominent high-σ anomalies along the active fault zone having strike-slip motion beneath the earthquake source zone. The pattern of distribution of (Vp, Vs, σ) and its association with occurrences of aftershocks provide seismological evidence for the neo-tectonics in the region having left lateral strike-slip motion of the fault.     

Seismicity of the Baikal rift system from regional network observations

In the paper we report the state-of-the-art of seismicity study in the Baikal rift system and the general results obtained. At present, the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185,000 events. The spatial distribution of the epicenters, which either gather along well-delineated belts or in discrete swarms is considered in detail for different areas of the rift system. At the same time, the hypocenters are poorly constrained making it difficult to identify the fault geometry. Clustered events like aftershock sequences or earthquake swarms are typical patterns in the region; moreover, aftershocks of M ⩾ 4.7 earthquakes make up a quarter of the whole catalog. The maximum magnitude of earthquakes recorded instrumentally is M_LH7.6 for a strike-slip event in the NE part of the Baikal rift system and M_LH6.8 for a normal fault earthquake in the central part of the rift system (Lake Baikal basin). Predominant movement type is normal faulting on NE striking faults with a left lateral strike-slip component on W–E planes. In conclusion, some shortcomings of the seismic network and data processing are pointed out.     

Explaining the physical origin of Båth's law

Båth's law is one of the three well-known scaling laws for earthquakes. It states that the difference in magnitudes of the mainshock and its largest aftershock is approximately constant, independent of the magnitude of the mainshock. Despite the progress in understanding the nature of Båth's law, the question of whether this law has a physical basis, or is simply a consequence of basic statistical features of aftershock sequences, has remained controversial. In this article we show that Båth's law can be derived within the Cosserat continuum theory from equations describing fault interaction. Our equations can describe both (1) the interacting mainshocks and aftershocks, and (2) the interacting foreshocks and mainshocks. We also derive (1) spatial extension of Båth's law to the normalized distance between the locations of the interacting mainshocks and aftershocks (or foreshocks and mainshocks), and (2) temporal extension of Båth's law to the difference between the time of the interacting mainshocks and aftershocks (or foreshocks and mainshocks).     

Analysis of Izmit aftershocks 25 days before the November 12th 1999 Düzce earthquake, Turkey

We investigate spatial clustering of 2414 aftershocks along the Izmit M_w = 7.4 August 17, 1999 earthquake rupture zone. 25 days prior to the Düzce earthquake M_w = 7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere–Düzce (KDC). We determine the earthquake frequency–magnitude distribution (b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Düzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Düzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Düzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Düzce fault segment was active before the 12 November 1999 Düzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Düzce Basin. This high density aftershock activity presumably helped to trigger the Düzce event (M_w = 7.2) after the Izmit M_w 7.4 mainshock.     

Spatial mapping of earthquake hazard parameters in the Hindukush–Pamir Himalaya and adjacent regions: Implication for future seismic hazard

The study deals spatial mapping of earthquake hazard parameters like annual and 100-years mode along with their 90% probability of not being exceeded (NBE) in the Hindukush–Pamir Himalaya and adjoining regions. For this purpose, we applied a straightforward and most robust method known as Gumbel’s third asymptotic distribution of extreme values (GIII). A homogeneous and complete earthquake catalogue during the period 1900–2010 with magnitude M_W ⩾ 4.0 is utilized to estimate these earthquake hazard parameters. An equal grid point mesh, of 1° longitude X 1° latitude, is chosen to produce detailed earthquake hazard maps. This performance allows analysis of the localized seismicity parameters and representation of their regional variations as contour maps. The estimated result of annual mode with 90% probability of NBE is expected to exceed the values of M_W 6.0 in the Sulaiman–Kirthar ranges of Pakistan and northwestern part of the Nepal and surroundings in the examined region. The 100-years mode with 90% probability of NBE is expected to exceed the value of M_W 8.0 in the Hindukush–Pamir Himalaya with Caucasus mountain belt, the Sulaiman–Kirthar ranges of Pakistan, northwestern part of the Nepal and surroundings, the Kangra–Himanchal Pradesh and Kashmir of India. The estimated high values of earthquake hazard parameters are mostly correlated with the main tectonic regimes of the examined region. The spatial variations of earthquake hazard parameters reveal that the examined region exhibits more complexity and has high crustal heterogeneity. The spatial maps provide a brief atlas of the earthquake hazard in the region.     

断层旋性与地震危险性

文中以“平行同旋走滑断层减震”的观点论证了兰州、北京、昆明这些位于强震活动区的大城市今后百年内不会发生 6 .5级以上地震。以“平行异旋走滑断层加震”的观点解释了西南棱形块体北边界和南边界在发生大震方面相互促进的现象。对于由构造分段求震级来说 ,在遇到不同的横交断层作为分段点时 ,还需考虑将来发震时始破裂点的位置以及发震断层的旋性 ,不然就会造成对震级估计不足 ,继而成为抗震建设的潜在不安全因素。在主震后为了预报余震的强度 ,可应用物理学中的科里奥利力理论 ,应用时必须知道断层类型和旋性。对于走滑断层来说 ,左旋余震弱 ,右旋余震强。例如 1997年藏北玛尼 7.5级地震 ,余震仅为 5 .3级 ,震级偏小 ,因主震为左旋的缘故 ;1976年唐山 7.8级地震 ,余震可达 7.1级 ,因主震是右旋。对于逆断层来说 ,上盘错动方向在当地子午面左侧者余震强度大 ,在右侧者余震强度弱。据此讨论了 1999年台湾南投 7.6级大震余震强度达 7.1级是因为主震为逆断层 ,上盘向西错动。     

Observed correlations between aftershock spatial distribution and earthquake fault lengths

We observe the spatial distributions of the magnitude of aftershocks following the six earthquakes of focal depth shallower than 20 km with magnitude more than 5.0 from 1983 to 1987 in Japan. The upper limit of the aftershock magnitude is examined as a function of the distance from mainshock hypocentre. The observed spatial distributions of the upper limit are bimodal, with a tendency of the upper limit to decrease as the distance from mainshock hypocentre increases. Moreover, we observe the correlations between the aftershock spatial distribution and earthquake fault length. We focus on the largest aftershocks in each of two aftershock sequences constituting the bimodal distribution. The distances of the two largest aftershocks from the mainshock hypocentre are equal to the fault lengths of shallow earthquakes in Japan and to the maximum earthquake fault lengths.     

Source complexity of the 4 March 2010 Jiashian,Taiwan, Earthquake determined by joint inversion of teleseismic and near field data

The Jiashian earthquake (M_L 6.4) occurred on 4 March 2010. It was the largest inland event in southern Taiwan of 2010. The mainshock location was unexpected since it occurred in an area with relatively low background seismicity. In addition, reports of earthquake focal mechanisms do not fit with any known active fault geometry. In order to understand the origin of this earthquake, especially its rupture process, we perform a joint source inversion by using teleseismic body wave, GPS coseismic displacements and near field ground motion data. In this study, we considered a northwest–southeast trending fault with a northeast dip retrieved from GPS coseismic data and aftershocks distribution. To analyze the detailed slip distribution in space and time, we used near field 3D Green’s functions provided by spectral-element method and a full time–space inversion technique. We find a complex rupture process with several slip patches distributed inside two main asperities. The slip map reveals a mean slip of 12.9 cm for a maximum slip of 27.3 cm leading to a M_w 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20 km depth, and propagated upward up to 2 km depth to form the two asperities. The source time function of this event revealed two pulses corresponding to the two asperities, for a total duration time of about 16 s. Most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one. We infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake.     

Geologic hazards associated with seismogenic faulting in southern Siberia and Mongolia: forms and location patterns

The forms and location patterns of geologic hazards induced by earthquakes in southern Siberia, Mongolia, and northern Kazakhstan in1950 through 2008 have been investigated statistically, using a database of coseismic effects created as a GIS MapInfo application, with a handy input box for large data arrays. The database includes 689 cases of macroseismic effects from M_S = 4.1–8.1 events at 398 sites. Statistical analysis of the data has revealed regional relationships between the magnitude of an earthquake and the maximum distance of its environmental effects (soil liquefaction and subsidence, secondary surface rupturing, and slope instability) to the epicenter and to the causative fault. Thus estimated limit distances to the fault for the M_S = 8.1 largest event are 40 km for soil subsidence (sinkholes), 80 km for surface rupture, 100 km for slope instability (landslides etc.), and 130 km for soil liquefaction. These distances are 3.5–5.6 times as short as those to the epicenter, which are 150, 450, 350, and 450 km, respectively. Analysis of geohazard locations relative to nearest faults in southern East Siberia shows the distances to be within 2 km for sinkholes (60% within 1.5 km), 4.5 km for landslides (90% within 1.5 km), 8 km for liquefaction (69% within 1 km), and 35.5 km for surface rupture (86% within 2 km). The frequency of hazardous effects decreases exponentially away from both seismogenic and nearest faults. Cases of soil liquefaction and subsidence are analyzed in more detail in relation to rupture patterns. Equations have been suggested to relate the maximum sizes of secondary structures (sinkholes, dikes, etc.) with the earthquake magnitude and shaking intensity at the site. As a result, a predictive model has been created for locations of geohazard associated with reactivation of seismogenic faults, assuming an arbitrary fault pattern. The obtained results make basis for modeling the distribution of geohazards for the purposes of prediction and estimation of earthquake parameters from secondary deformation.