Seismotectonics and large earthquake generation in the Himalayan region

Bounded by the western and eastern syntaxes, the Himalayan region has experienced at least five M ~ 8 earthquakes during a seismically very active phase from 1897 through 1952. However, there has been a paucity of M ~ 8 earthquakes since 1952. Examining of various catalogues and seismograms from the Gottingen Observatory, it is established that this quiescence of M ~ 8 earthquakes is real. While it has not been possible to forecast earthquakes, there has been a success in making a medium term forecast of an M 7.3 earthquake in the adjoining Indo-Burmese arc. Similarly we find that in the central Himalayan region, earthquakes of M > 6.5 have been preceded by seismic swarms and quiescences. In the recent past, based on GPS data, estimates have been made of the accumulated strains and it is postulated that a number of M ~ 8 earthquakes are imminent in the Himalayan region. We examine these estimates and find that while earthquakes of M ~ 8 may occur in the region, however, the available GPS data and their interpretation do not necessarily suggest their size and time of occurrence and whether an earthquake in a particular segment will occur sooner in comparison to that in the neighboring segment. We also comment on the inference of occurrence of M ~ 8 earthquakes based on M8 algorithm for the region. We conclude that while an M ~ 8 earthquake could occur any time anywhere in the Himalayan region, there is no indication as of now as to where and when it would occur. We impress on the need for preparedness to mitigate the pending earthquake disaster in the region.     

Case study of reservoir triggered seismicity, Latian Dam, Tehran, Iran

Latian dam is located in the North East of Tehran in Elburz Mountain. It falls in the category of large dams according to the International Committee on Large Dams (ICOLD). It was constructed in 1967 for agricultural purposes, drinking water, and power generation. Producing triggered earthquakes may be a consequent result of dam construction. In this paper, the complete seismic statistics of the region from 1996 onwards has been studied to understand the seismic condition of Latain region. For this purpose, frequency of earthquakes within a radius of 30/60 km around the dam is studied considering its relationship with the reservoir volume variation. Using Gutenberg-Richter rule, parameter b of the region was determined within the same region. The results of this study show the existence of triggered seismicity around the reservoir of Latian dam. Considering the tectonic-geological condition of the region, the existence of triggered earthquakes may create landslides in the reservoir and around it.     

Features of seismicity of the Euro-Arctic region

New results from seismic monitoring in the Euro-Arctic region, including the seismicity of Gakkel Ridge and the Barents–Kara Sea shelf, are presented. The data used were obtained from the Arkhan-gelsk seismic network. The role of island-based seismic stations, in particular, those in Franz Josef Land, in the monitoring network is discussed. The possibility of specifying the nature of seismicity by waveform spectral-temporal analysis, even in the case of a single station, is considered.     

Seismotectonics of the Papua New Guinea—Solomon Islands region

Earthquakes for the period 1964–1973 are relocated by the method of Joint Hypocenter Determination in order better to resolve the configuration and the structure of the New Guinea—New Britain—Solomon Islands region. Focal mechanism solutions are integrated with the seismicity and interpreted closely with it. A zone of subduction exists beneath New Britain and the Solomon Islands, a zone of left-lateral strike-slip movement extends from New Ireland to New Guinea. The zone of seismicity in northern New Guinea has developed as a result of a continent—island-arc collision in late Oligocene—Miocene times and does not exhibit a well-developed inclined seismic zone. It is proposed that plate tectonics theory does not apply rigorously, but slip-line field theory allows the presentation of a new geodynamic model for this region.     

Long-term seismicity of Istanbul and of the Marmara Sea region

We examine the long-term seismicity of the Marmara Sea region in Turkey over a period of twenty centuries to attempt to answer two questions: (1) how well recorded in history is the earthquake activity of this region? and (2) does seismicity over this long period of time differ from that over the present century? We study this densely-populated and fast-developing region because it is one of the most tectonically active regions on the continents that appears today to be relatively quiescent.     

松潘—甘孜地区百年地震构造和现今动力学

研究区位于青藏高原的东北隅(96°～107°E,30°～35°N)。基于该地区长度大于2km的4 781条1∶20万数字化实测断裂、1900年以来的5 220条数字地震记录,以及野外地质观测数据,识别出993条不同属性的地震断层,构建了该地区百年地震构造格局。1970年以来十年期地震断层跃迁图像表明,自20世纪80年代中期白马—虎牙强烈震群爆发之后,地震活动在沿各主要走滑断层带自西(北西)向东(南东)迁移的同时,逐渐向中部贡玛—达曲断裂带和南部鲜水河断裂带的东南段集中。地震活动的断裂构造联系主要表现为挤压剪切转换机制和典型的楔顶效应。研究区165个GPS速度矢量展现了与3个地块和以鲜水河断裂带为主的速度域、速度梯度带和速度扰动区。跨研究区南缘鲜水河断裂带的位移速率因贡玛—达曲断裂带汇聚而达到了6.5～8.6mm/a,而跨北缘东昆仑断裂带的位移速度只有1.8～2.2mm/a。因鲜水河断裂走向在其中南段发生向南的急剧偏转,垂直断层面的位移矢量分量不断增强,形成了汶川8.0级地震成核及NE向单边破裂的动力学条件。     

Seismotectonics of Taiwan

High-quality seismicity data and focal mechanism solutions obtained during 1973–1983 by the permanent Taiwan Telemetered Seismographic Network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the Taiwan area. Seismicity distribution in southern Taiwan clearly reveals an east-dipping Benioff zone which has a thickness of about 30 km and begins to deepen along 121°E at a dip angle of 55°–60°. The leading edge of this Benioff zone reaches a depth of about 180 km between 21°N and 22°N, but tapers off to a shallower depth of about 100 km from 22°N to 23°N. The presence of this seismic zone implies that subduction of the South China Sea plate under the Philippine Sea plate extends from Luzon northward to about 23°N. The position of the northern boundary of the South China Sea plate, as tentatively defined according to the seismicity distribution, passes through southern Taiwan from the offshore area in the Taiwan Strait west of Kaohsiung in an east-northeast direction to the Taitung area where a triple junction probably lies. Seismicity is found to disappear abruptly below a certain depth in many parts of Taiwan. This phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle. Comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely-developed faults and fractures. However, the converse is not necessarily true. This may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas. Finally, focal mechanism solutions for west central Taiwan and the Kuangfu-Fuli area in eastern Taiwan predominantly show a maximum horizontal compression in the SE-NW direction which can be related to collision between the Eurasian and Philippine Sea plates. However, focal mechanism solutions for both the Hualien area in eastern Taiwan and the Tainan area in southwestern Taiwan show remarkable irregularities which may result from local tectonic complexities.     

Seismotectonics and Earth tides

The results of the processing and analysis of the global earthquake distribution (more than 250000 events based on the ISC catalog) and the study of moonquakes distribution (about 900 events based on the published materials) are presented. It was found that the number of events and the energy for both cases show a bimodal distribution with maximums in the middle latitudes, zero values at the polarcaps, and a local minimum in the vicinity of the equator. The probable influence of tectonic processes on the revealed character of the seismic event distribution is analyzed, and the role of Earth tides in the activation of the seismicity in the symmetric zones on both sides of the equator is shown.     

Recent tectonics and seismicity of the western Altai-Sayan mountainous region,Junggar basin,and Chinese Tien Shan

According to GPS monitoring, recent tectonic process between Tarim and West Siberia in the band within 80°–95° E is generated by the northward movement of the Tarim block. During the accompanying horizontal compression of the area, orogeny takes place within linear mobile zones when blocks are squeezed into the upper half-space. When the orientation of the mobile zones is transverse to the compression direction, the leading orogenic process is reverse faulting. When these directions intersect at an acute angle, the principal features of the mountain relief are formed by oblique-slip and strike-slip faults.The spatial distribution of seismic activity A10 over a 40-year period of instrumental observations within the mobile zones of the study area is extremely nonuniform. Seismic activity increases to the south, toward the source of deformations—the Indo-Eurasian collision. The maximum activity is observed at the reverse-fault boundaries of the eastern Tien Shan (~ 40). The seismic activity of the strike-slip fault boundaries of the Great Altai is considerably lower (0.11–0.16).     

Hydrogeochemical investigation in an arid region of Iran (Tabas, Central Iran)

Groundwater is the most important source of water supply in Iran and understanding the geochemical evolution of groundwater is important for sustainable development of the water resources in Tabas area. A total of 29 samples of groundwater in Tabas area have been analyzed for ions and major elements. Groundwater of the study area is characterized by the dominance of Na–Cl water type. Groundwater was generally acidic to high alkaline with pH ranging from 5.42 to 10.75. The TDS as a function of mineralization characteristics of the groundwater ranged from 479 to 10,957 mg/l, with a mean value of 2,759 mg/l. The Ca²⁺, Mg²⁺, SO₄ ^2? and HCO₃ ^? were mainly derived from the dissolution of calcite, dolomite and gypsum. The Cu, Pb and Zn ions are not mobile in recent pH–Eh, but these conditions controlled dissolved Se, V and Mo in groundwater. The As is released in groundwater as a result of the weathering of sulfide minerals like arsenopyrite.     

Seismotectonics of the transitional region from the Baikal Rift Zone to orogenic rise of the Stanovoi range

This paper is based on the data obtained during the field study of active faults carried out in 2005–2006 in the Chita and Amur oblast and South Yakutia in connection with detailed seismic demarcation of the projected East Siberia-Pacific Ocean pipeline route. The comprehensive geomorphic and geophysical fieldwork was focused on paleoseismogeology and accompanied by trenching in the zones of reactivated faults. These works allowed us to specify the available information on the present-day structure, seismotectonic regime, and potential seismic hazard of the conjugation of the Baikal Rift Zone and the arched-block rise of the Stanovoi Ridge.     

Seismotectonics of the Calabrian arc

Relocation of intermediate and deep earthquakes of Tyrrhenian Sea area through joint hypocenter determination for the period 1962–1979, has allowed a more detailed definition of the geometry of this peculiar Benioff zone. Earthquakes dip along a quasi-vertical plane to 250 km depth; there is a 50° dip in the 250–340 km depth range, and a low dip angle to 480 km depth. The structure sketched from the hypocenters is almost continuous, but most energy has been released in the 230–340 km depth interval. An evaluation of fault plane solutions of intermediate earthquakes in this area indicates predominance of down-dip compressions in the central part of the slab. At the border, strike-slip motion occurs independent of depth. Some earthquakes that occurred at intermediate depth (less than 100 km) along the Ionian margin of Calabria show predominance of reverse faulting, with the P-axis oriented SE-NW. However, shallow earthquakes in the Calabria-Sicily region indicate a more complex motion, with predominance of normal faulting. A possible interpretation of these features according to the available geological history, which involves subduction of continental lithosphere, is discussed.     

On the seismicity and tectonics associated with the Sinkiang—Tibetan region

A plausible seismo-tectonic boundary of the Sinkiang—Tibetan region is defined on the basis of the trend of higher magnitude earthquakes (M7.0) and energy released by them for the period 1905–1965. In order to study the nature of forces at the northwestern and eastern sides of the region focal mechanisms for eleven shocks have been determined using P-wave first-motion directions reported in the Bulletin of the International Seismological Centre (Edinburgh). Of these, seven mechanisms show thrust faulting, three strike-slip and one normal faulting. The sense of motions of underthrusting blocks in thrust-faulting mechanisms for the two sides are directed towards the Sinkiang—Tibetan region. The slip vectors of strike-slip faulting are also in agreement with the direction of movement of thrust faulting. Thus, the seismicity, energy released, slip vectors and the orientation of T-axes reflect that the northwestern and eastern sides of the Sinkiang—Tibetan region are the plausible seismo-tectonic boundary and the major earthquakes and higher crustal thickness are the results of the movements of surrounding plates towards the region.     

Temporal multifractal pattern of seismicity in northwest Himalayan region

The variation of temporal fractal dimension D2 (t) value may be well applied for understanding the future large earthquakes. In this present analysis the time series of inter-occurrence of earthquakes of consecutive hundred events window in Himalayan region is analyzed. We observed the variation in D2 (t) values in the range of 0.19 to 0.68 indicating the variation of clustering of events with respect to time. The multifractal nature of earthquakes is also investigated by plotting Dq(t) versus q. The result shows the temporal distribution of earthquakes have multifractal structure of definitive pattern rather than random pattern.     

New seismicity map for the European sector of the Russian Arctic region

Based on seismological monitoring data specifying earthquake epicenters, a new map is made for the Western sector of the Russian Arctic region. The seismicity data is reworked in detail by specifying the epicentral positions of earthquakes and adding data on weak seismicity for areas which were earlier insufficiently studied, including those at the boundary of the Arctic Shelf of the Russian Federation. The fundamental possibility of applying seismological observations to construct a regional geodynamic map is discussed (in particular, the important role of island-based seismic stations). The possibility of specifying the nature of seismicity in terms of spectral-temporal analysis (STAN) of waveforms based on data from the new seismic station on the Franz Josef Land is considered. Possible application of the character of seismicity obtained from geophysical data to specify the geodynamic nature of events accompanying interaction of the ridge and shelf spreading is discussed.