Probabilities of occurrence of great earthquakes in the Himalaya

Long-term conditional probabilities of occurrence of great earthquakes along the Himalaya plate boundary seismic zone have been estimated. The chance of occurrence of at least one great earthquake along this seismic zone over a period of 100 years (beginning the year 1999) is estimated to be about 0.89. The 100-year probability of such an earthquake occurring in the Kashmir seismic gap is about 0.27, in the central seismic gap about 0.52 and in the Assam gap about 0.21. The 25-year probabilities of their occurrence in these gaps are 0.07, 0.17, and 0.05 respectively. These probability estimates may be used profitably to assess the seismic hazard in the Himalaya and the adjoining Ganga plains.     

Seismicity and source parameters of moderate earthquakes in Sikkim Himalaya

In this study, we accurately relocate 360 earthquakes in the Sikkim Himalaya through the application of the double-difference algorithm to 4?years of data accrued from a eleven-station broadband seismic network. The analysis brings out two major clusters of seismicity??one located in between the main central thrust (MCT) and the main boundary thrust (MBT) and the other in the northwest region of Sikkim that is site to the devastating Mw6.9 earthquake of September 18, 2011. Keeping in view the limitations imposed by the Nyquist frequency of our data (10?Hz), we select 9 moderate size earthquakes (5.3????Ml????4) for the estimation of source parameters. Analysis of shear wave spectra of these earthquakes yields seismic moments in the range of 7.95?×?10²¹ dyne-cm to 6.31?×?10²³ dyne-cm and corner frequencies in the range of 1.8?C6.25?Hz. Smaller seismic moments obtained in Sikkim when compared with the rest of the Himalaya vindicates the lower seismicity levels in the region. Interestingly, it is observed that most of the events having larger seismic moment occur between MBT and MCT lending credence to our observation that this is the most active portion of Sikkim Himalaya. The estimates of stress drop and source radius range from 48 to 389?bar and 0.225 to 0.781?km, respectively. Stress drops do not seem to correlate with the scalar seismic moments affirming the view that stress drop is independent over a wide moment range. While the continental collision scenario can be invoked as a reason to explain a predominance of low stress drops in the Himalayan region, those with relatively higher stress drops in Sikkim Himalaya could be attributed to their affinity with strike-slip source mechanisms. Least square regression of the scalar seismic moment (M ₀) and local magnitude (Ml) results in a relation LogM ₀?=?(1.56?±?0.05)Ml?+?(8.55?±?0.12) while that between moment magnitude (M _w ) and local magnitude as M _w ?=?(0.92?±?0.04)Ml?+?(0.14?±?0.06). These relations could serve as useful inputs for the assessment of earthquake hazard in this seismically active region of Himalaya.     

Multifractal analysis of earthquakes in Kumaun Himalaya and its surrounding region

Himalayan seismicity is related to continuing northward convergence of Indian plate against Eurasian plate. Earthquakes in this region are mainly caused due to release of elastic strain energy. The Himalayan region can be attributed to highly complex geodynamic process and therefore is best suited for multifractal seismicity analysis. Fractal analysis of earthquakes (mb ?? 3.5) occurred during 1973?C2008 led to the detection of a clustering pattern in the narrow time span. This clustering was identified in three windows of 50 events each having low spatial correlation fractal dimension (D _C ) value 0.836, 0.946 and 0.285 which were mainly during the span of 1998 to 2005. This clustering may be considered as an indication of a highly stressed region. The Guttenberg Richter b-value was determined for the same subsets considered for the D _C estimation. Based on the fractal clustering pattern of events, we conclude that the clustered events are indicative of a highly stressed region of weak zone from where the rupture propagation eventually may nucleate as a strong earthquake. Multifractal analysis gave some understanding of the heterogeneity of fractal structure of the seismicity and existence of complex interconnected structure of the Himalayan thrust systems. The present analysis indicates an impending strong earthquake, which might help in better hazard mitigation for the Kumaun Himalaya and its surrounding region.     

First evidence for large earthquakes on the Deshir Fault,Central Iran Plateau

Although sliced by several strike slip faults, a large part of Central Iran remained aseismic during the period of time covered by the instrumental and historical seismic records. Stating the existence of earthquakes in the Holocene is therefore important for the assessment of the regional seismic hazard. A palaeoseismic study of the Deshir fault demonstrates that Central Iran hosted large earthquakes during latest Pleistocene and Holocene. The last event corresponds to 1 m‐deep fissures, which sandy infilling yielded an optically stimulated luminescence (OSL) age of 2.8 ± 1.4 ka. At least two previous events, outlined by older fissures and/or colluvial wedges, have been recorded over the last 10–30 ka. The magnitudes are difficult to assess because the actual slips per event are unknown. The size of the fissures and the significant vertical displacement associated with a colluvial wedge are nevertheless compatible with M ≈ 7 events along a primary strike‐slip surface break.     

The occurrence of large earthquakes in south Italy

An analysis of the data in the catalogues of Italian earthquakes indicates that large earthquakes which occur in the area of radius of about 140 km centered in the Straits of Messina occur in sequences. Each sequence is generally formed by two events and covers an average time window of 10 years.The last four sequences occurred in the time windows 1783–1891, 1818–1823, 1865–1870, 1905–1908 and are separated by about 40 years indicating that in that area there is now a gap in the time domain.The analysis of the data in the Catalogue for the region between the latitudes 39°N and 41°50′N indicates that in that region the large earthquakes occurred in 13 sequences. Each sequence is formed by 3 events in average and covers an average time window of 7 years. This indicates that, after the earthquake of Nov. 1980, which occurred after a gap of 67 years, other moderately large earthquakes may be expected in that area in the next few years.     

Seismicity and source parameters of local earthquakes in Bilaspur region of Himachal Lesser Himalaya

The pattern of local seismicity (110 events) and the source parameters of 26 local events (1.0?≤?Mw?≤?2.5) that occurred during May 2008 to April 2009 in Bilaspur region of Himachal Lesser Himalaya were determined. The digital records available from one station have been used to compute the source parameters and f _max based on the Brune source model (1970) and a high-frequency diminution factor (Boore 1983) above f _max. The epicentral distribution of events within 30 km of local network is broadly divided into three clusters of seismic activity: (1) a cluster located to the south of the Jamthal (JAMT) station and falls to the north of the Main Boundary Thrust (MBT) which seems to reflect the contemporary local seismicity of the segment of the MBT, (2) an elongated zone of local seismicity NE–SW trending, delineated NE of JAMT station that falls in the Lesser Himalaya between the MBT and the Main Central Thrust, and (3) NE–SW trending zone of local seismic activity located at about 10 km east of NHRI station and about 15 km northeast of NERI station and extending over a distance of about 20 km. Majority of events occur at shallow depths up to 20 km, and the maximum number of events occurs in the focal depth range between 10 and 15 km. The entire seismic activity is confined to the crust between 5 and 45 km. The average values of these source parameters range from 3.29?×?10¹⁷ to 3.73?×?10¹⁹?dyne-cm for seismic moment, 0.1 to 9.7 bars for stress drops, and 111.78 to 558.92 m for source radii. The average value of f _max for these events varies from 7 to 18 Hz and seems to be source dependent.     

Seismicity and the state of stress from investigations of local earthquakes in the Kumaon Himalaya

Results of investigations of local earthquakes in the region of the Main Central Thrust (MCT) in the Kumaon Himalaya, between and adjacent to the valleys of the Bhagirathi and Yamuna rivers, are presented. Records of over 250 earthquakes were analysed and the following facts emerged:

1. (1) Earthquakes in the Himalayas occur in specific areas and belts. One such belt has been identified in the region under investigation, hypocentral estimates being more reliable for earthquakes occurring in the middle segment approximately 70 km long of this belt, crossing the Yamuna river between the villages of Barkot and Syanachatti.

2. (2) All but a few epicentres in this middle segment, lie to the southwest of the surface trace of the MCT in a zone with a width of 10–30 km.

3. (3) Most of the earthquakes in this segment occur at depths of less than 10 km below the ground surface, the maximum estimated depth being 32 km.

Using observations of first motion for a composite focal mechanism solution, the nodal planes were observed to be near vertical and the compression axis near horizontal and normal to the local strike of the seismic belt and of the MCT.We conclude that although the Main Central Thrust itself is not seismically active in this region, there is considerable activity immediately to the southwest of it. Furthermore, the mode of faulting as inferred from the records of these earthquakes, is strike slip.     

Rupture length and velocity for earthquakes in the Mid-Atlantic Ridge from directivity effect in body and surface waves

The dimensions and rupture velocities of four earthquakes, two in the Mid-Atlantic Ridge and two in Iceland with strike–slip mechanisms and magnitudes (M_w) between 6.2 and 6.8 were studied using the directivity effects of Rayleigh and body waves. For Rayleigh waves we used the directivity function for different pairs of stations and for body waves the waveforms of P and SH waves corresponding to a simple extended line source. We have found that three have very shallow depths about 3 km and one 8 km, fault lengths between 12 km and 21 km, and a low rupture velocity of about 1.5 km/s to 2.0 km/s which supports the idea of the presence of slow earthquakes in transform faults.     

Ground motion simulations of the SW Iberia margin: rupture directivity and earth structure effects

In this study, we focus on the region between Gorringe Bank and the Horseshoe Fault located in the SW Iberia margin, which is believed to be the site of the great 1755 earthquake. We model ground motions using an extended source located near the Horseshoe scarp to generate synthetic waveforms using a wave propagation code, based on the finite-difference method. We compare the simulated waveforms, for the Algarve Basin and the Lower Tagus Valley Basin (Portugal), using a 3-D velocity model down to the Moho discontinuity with a simple 1-D layered model. The radiated wave field is very sensitive to the velocity model and a small number of source parameters, in particular, the rupture directivity. The rupture directivity, the strike direction and the fault dimensions are critical to the azimuthal distribution of the maximum amplitude oscillations. We show that the use of a stratified 1-D model is inappropriate in SW Iberia, where sources are located in the oceanic domain and receivers in the continental domain. The crustal structure varies dramatically along the ray paths, with large-scale heterogeneities of low or high velocities. Moreover, combined with the geometric limitations inherent to the region, a strong trade-off between several parameters is often observed; this is particularly critical when studying moderate magnitude earthquakes (M < 6), which constitute the bulk of the seismic catalogue in SW Iberia.     

巴颜喀喇断块边界断裂强震活动分析

断裂带上的地震活动,尤其是现代强震活动,一直是活动构造研究的重要内容.随着研究的深入,研究的对象也从一条断裂带上的强震活动,发展到多条断裂带所构成的空间内的强震活动和断块的强震活动.汶川地震和玉树地震后,巴颜喀喇断块的活动引起了广泛的关注.本文从巴颜喀喇断块整体运动的角度出发,根据断块周边各条断裂带上1900年以来的历史强震记录(Ms≥7)和现代强震活动,分析了断块的地震活动及其准周期性、震源机制、最大震级以及边界走滑断裂与强震活动的关系,主要的认识有:①1900年以来,地震活动呈现出3个地震系列,即:1923～1937年叠溪—花石峡地震系列、1947～1976年达日—炉霍地震系列和1997年开始的昆仑汶川地震系列;②地震系列之间的时间间隔和每一次地震系列持续时间有较大的差异,达日—炉霍地震系列与前后两次地震系列差别较大,而昆仑—汶川地震系列尚未结束;③经过2次8级和8级以上大地震之后,昆仑汶川地震系列虽然尚未结束,今后还可能发生较大地震,但是巴颜喀喇断块很可能已经历了本地震系列的大释放阶段,在该地震系列时间范围内大释放阶段之后的地震震级将小于大释放阶段的震级;④走滑断裂是控制巴颜喀喇断块运动的主要边界断裂,走滑型强震是地震系列的主体;⑤印度板块向北存在一个强烈的推挤过程,是使这些边界断裂带从孕震到真正发生强震的重要条件.     

Effects of large historical earthquakes on archeological structures in Jordan

This study tackles a set of conclusions and involves an evaluation of presumptive historical earthquakes in the Eastern Mediterranean Region, which hit the region and strongly affected the archeological sites in Jordan. Actually, the core of the study was the ancient cities of Umm Qais (Gadara), Umm Al-Jimal, Deir Al-Kahf, Al-Azraq, and Pella in the North and Dhahil water reservoir and Humaima in the South. The archeological excavations made during the past 20 years helped a lot of the region’s seismicity re-evaluation and relocation of historical earthquakes, about which the researchers argued on determining their epicenters and magnitudes. The recent excavations at Umm Qais (Gadara) indicated that earthquake-impacted ruins have been buried under nearly 2 m of dust deposits for centuries. So, such important indicators that can be relied upon to determine the epicenter and magnitude evaluation of these historical earthquakes were unveiled. Based on the recent available excavations, it is obvious that Umm Qais was affected by two seismic events within a time interval of not less than 100 years. In the light of the results found in Umm Qais and the remote archeological sites of Umm Al-Jimal and Deir Al-Kahf, there was no azimuthal projection of neither collapsed nor tilted columns indicating that there was a major earthquake with the epicenter located in the Carmel rupture zone in the North of Palestine, and not in the rift zone as reported earlier. But, the second earthquake was at the northwest of Umm Qais. The earthquake reported here seems to coincide with the reported major earthquake in 748 AD while the second one that occurred earlier corresponds to the 551 AD. The eastward collapsed towers in the South with respect to the southern archeological site of Humaima and a seismic swarm in Sep. 20, 2003, suggested that the Wadi Araba fault may not be continuous but segmented, subject to confirmation by detailed sub-surface structural information.     

A simulation of upper crustal stresses for great and moderate thrust earthquakes of the Himalaya

We assume that great and moderate Himalayan earthquakes occur through reactivation of subhorizontal thrust faults by frictional failure under the action of stresses induced by Himalayan topography, isostasy related buoyancy forces, crustal overburden and plate tectonic causes. Estimates of stresses are based on two dimensional plane strain calculations using analytical formulae of elasticity theory and rock mechanics under suitable simplifying assumptions. Considerable attention is focussed on a point on the detachment at a depth of 17 km below mean sea level under the surface trace of the Main Central Thrust (MCT). According to recent views, great Himalayan earthquakes should nucleate in the detachment in the vicinity of such a point. Also many moderate earthquakes occur on the detachment similarly under the MCT. Vertical and horizontal normal stresses of 622 and 262 MPa and a corresponding shear stress of 26 MPa are estimated for this point due to topography, buoyancy and overburden. For fault friction coefficient varying between 0.3 to 1.0, estimates of plate tectonic stress required are in the range of 386 to 434 MPa, when the cumulative principal stresses are oriented favourably for reactivation of the detachment. Estimates of shear stress mobilized at the same point would be from 27 to 32 MPa for the identical range of fault friction coefficient. Our calculations suggest that presence of pore water in the fault zones is essential for reactivation. Pore pressure required is between 535 to 595 MPa for friction coefficient in the range of 0.3 to 1.0 and it is less than lithostatic stress of 603 MPa at the above point. For the specific nominal value of 0.65 for fault friction coefficient, the estimated values of plate tectonic stress, shear stress and pore pressure at the above point on the detachment are 410 MPa, 30 MPa and 580 MPa respectively. Similar estimates are obtained also for shallower points on the detachment up to the southern limit of the Outer Himalaya. Our estimates of the plate tectonic stress, shear stress and pore pressure for reactivation of upper crustal thrust faults compare favourably with those quoted in the literature.     

Tectonic implications of the large shioya-oki earthquakes of 1938

The tectonic processes taking place along the southern part of the Japan trench are discussed on the basis of the focal mechanism of the 1938 Shioya-Oki event which consists of the five large earthquakes of M_s = 7.4, 7.7, 7.8, 7.7 and 7.1. Detailed analyses of seismic waves and tsunamis are made for each of these earthquakes, and the dislocation parameters are obtained. The total seismic moment amounts to 2.3 · 10²⁸ dyn.cm. The five earthquakes are grouped into either a low-angle thrust type or a nearly vertical normal-fault type. These mechanisms are common with other great earthquakes of the northwestern Pacific belt, and can be explained in terms of the interaction between the oceanic and continental plates. The vertical displacement inferred from the seismic results is in approximate agreement with the precise level data over the period from 1939 and 1897. This agreement suggests that the rate of the strain accumulation at the preseismic time is very small in the epicentral area. Repeated levelings at the postseismic time reveal a large-scale recovery of the coseismic subsidence. The postseismic deformation is one-third to one-half of the coseismic displacement. The time constant of the recovery is estimated to be 5 years or less. This type of deformation may be a manifestation of viscoelasticity of a weak zone underlying the continent. The amount of dislocation, together with the longterm seismicity, suggests a seismic slip rate of about 0.4 cm/year, which is one order of magnitude smaller than that for the adjacent regions. This suggests that a large part of the plate motion is taking place aseismically in this region. The tectonic process now taking place in the southern Japan trench can be considered to represent a stage just prior to a complete detachment of the sinking portion of the oceanic plate.     

Near real-time magnitude determination for large crustal earthquakes

We introduce an empirical method of near real-time, near-field magnitude determination for large (M>6.5) crustal earthquakes. Time integration over the strong shaking duration on the absolute values of the acceleration records is carried out for nearby stations surrounding many large earthquake sources in Taiwan. The integrated quantity, here denoted as total effective shaking, is used in a regression process to derive an empirical relationship for a quick Mw determination useful for a reliable real-time operation in earthquake rapid reporting and earthquake early warning systems (EWSs).     

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.     

Great earthquakes,seismicity gaps and potential for earthquake disaster along the Himalaya plate boundary

Analysis of the space-time patterns of seismicity in the Himalaya plate boundary has established the existence of three seismic gaps:

• 1.(1) The “Kashmir gap” lying west of the 1905 Kangra earthquake;
• 2.(2) the “Central gap”, situated between the 1905 Kangra and the 1934 Bihar earthquakes;
• 3.(3) the “Assam gap” between the 1897 and 1950 Assam earthquakes.

This study has shown that the above great earthquakes were preceded as well as followed by long periods (⩾ 19 years) of decreased levels of seismic activity in the epicentral regions. Remarkable decrease in the seismicity following the year 1970 has been observed in the western half of the Central gap as well as in the Assam gap. Local seismic investigation in the Assam gap confirms this feature and the seismicity suggests the existence there of an asperity.The local seismic investigations in Garhwal Himalaya have shown that the small earthquakes are confined to the upper 6–8 km of the crust and may have strike-slip motions. These earthquakes occur in a region where teleseismically recorded events were few.     

Nature and timing of large landslides in the Himalaya and Transhimalaya of northern India

Four large landslides, each with a debris volume >10⁶ m³, in the Himalaya and Transhimalaya of northern India were examined, mapped, and dated using ¹⁰Be terrestrial cosmogenic radionuclide surface exposure dating. The landslides date to 7.7±1.0 ka (Darcha), 7.9±0.8 ka (Patseo), 6.6±0.4 ka (Kelang Serai), and 8.5±0.5 ka (Chilam). Comparison of slip surface dips and physically reasonable angles of internal friction suggests that the landslides may have been triggered by increased pore water pressure, seismic shaking, or a combination of these two processes. However, the steepness of discontinuities in the Darcha rock-slope, suggests that it was more likely to have started as a consequence of gravitationally-induced buckling of planar slabs. Deglaciation of the region occurred more than 2000 years before the Darcha, Patseo, and Kelang Serai landslides; it is unlikely that glacial debuttressing was responsible for triggering the landslides. The four landslides, their causes, potential triggers and mechanisms, and their ages are compared to 12 previously dated large landslides in the region. Fourteen of the 16 dated landslides occurred during periods of intensified monsoons. Seismic shaking, however, cannot be ruled out as a mechanism for landslide initiation, because the Himalaya has experienced great earthquakes on centennial to millennial timescales. The average Holocene landscape lowering due to large landslides for the Lahul region, which contains the Darcha, Patseo, and Kelang Serai landslides, is ～0.12 mm/yr. Previously published large-landslide landscape-lowering rates for the Himalaya differ significantly. Furthermore, regional glacial and fluvial denudation rates for the Himalaya are more than an order of magnitude greater. This difference highlights the lack of large-landslide data, lack of chronology, problems associated with single catchment/large landslide-based calculations, and the need for regional landscape-lowering determinations over a standardized time period.