Seismotectonics of Portugal and its adjacent area in the Atlantic

New elements on the seismicity of Portugal and new focal-mechanism solutions of earthquakes with epicentres situated off the coast of the Portuguese mainland and in the Azores region are presented. Historical seismicity data show that in the territory of the Portuguese mainland there are active faults that are responsible for earthquakes that have caused important damage and many casualties. However, most of the intraplate earthquakes with epicentres situated in the Portuguese mainland or near the shore are normally of small magnitude and this renders difficult their interpretation in the light of focal mechanisms. A solution for one earthquake, with magnitude 5 and epicentre at the Nazaré submarine canyon, is presented.Southwestwards of Cape St. Vincent there is an important seismic zone responsible for high-magnitude earthquakes such as that of 1 November 1755. This zone is situated in the region where the extension of the Messejana fault into the ocean joins with the Azores-Gibraltar fault.The seismicity of the area situated between the western coast of the Portuguese mainland and the Azores increases approximately along the 15°W meridian, from the latitude of the Azores-Gibraltar fault up to 44°N. Focal mechanisms of earthquakes with epicentres situated along this line show very similar solutions.The interpretation of the focal mechanism solutions of the earthquakes with epicentres situated in the studied area shows that the stress field trends approximately NW-SE. It is assumed that this stress field results from the interaction of the Eurasian and African plates; however, this direction is not maintained in the Azores region.     

从断层中心论向块体中心论转变--论活动块体在地震活动中的作用

讨论活动块体在地震活动中的作用。板缘地震在空间上呈线性分布,而中国大陆地震在空间上呈片状分布。大陆强震往往涉及两个以上方向断层的活动,且强震往往沿块体边界迁移或在其两侧断层上对迁。一些强震前后由中小地震震源机制解反映的Ｐ轴方向往往发生近９０°的转向,这种现象难以用区域应力方向变化来解释,但可从块体活动角度出发,用块体两个边界断层的先后错动来解释。地震前异常的远程效应、震后烈度异常分布图像以及地震序列特点等也显示了活动块体的作用。不同地区由地震活动性推测的块体活动方式有所差别,其原因可能和区域主压应力轴与块体两个边界断层走向的夹角不同有关。据此认为,在分析中国地震活动时要把视角从以活动断层为中心转变为以活动块体为中心     

The large-scale distribution of great shallow intraplate earthquakes in mainland China and adjacent areas and seismic coupling along plate margins

In mainland China and its surroundings the large-scale distribution of great, shallow, intraplate earthquakes shows that there are four main areas of high intraplate seismicity which are (a) the Northern China Seismic Area in east China (30°–42°N); (b) the Southeast-coast Seismic Area in eastern China (19°–25°N); (c) the North-south trending Seismic Area in western China and its surroundings (Burma–China–Mongolia); (d) the Central Asian Seismic Area in west China and its surroundings (Pamirs–Tianshan Mts–Baikal). These four intraplate seismic areas are approximately perpendicular to those sections of the Eurasian plate boundary where the Eurasian plate has a strong seismic coupling with the North American–Pacific Ocean–Philippine Sea plates, and with the Indian plate. The large-scale uneven distribution of intraplate seismicity in China and its surroundings may be controlled by heterogeneity in the stress state on different sections of the plate boundary.     

Time-dependent analysis of aftershock events and structural impacts on intraplate crustal seismicity of the Van earthquake (Mw 7.1, 23 October 2011), E-Anatolia

The Van earthquake (M _W 7.1, 23 October 2011) in E-Anatolia is typical representative of intraplate earthquakes. Its thrust focal character and aftershock seismicity pattern indicate the most prominent type of compound earthquakes due to its multifractal dynamic complexity and uneven compressional nature, ever seen all over Turkey. Seismicity pattern of aftershocks appears to be invariably complex in its overall characteristics of aligned clustering events. The population and distribution of the aftershock events clearly exhibit spatial variability, clustering-declustering and intermittency, consistent with multifractal scaling. The sequential growth of events during time scale shows multifractal behavior of seismicity in the focal zone. The results indicate that the extensive heterogeneity and time-dependent strength are considered to generate distinct aftershock events. These factors have structural impacts on intraplate seismicity, suggesting multifractal and unstable nature of the Van event. Multifractal seismicity is controlled by complex evolution of crustal-scale faulting, mechanical heterogeneity and seismic deformation anisotropy. Overall seismicity pattern of aftershocks provides the mechanism for strain softening process to explain the principal thrusting event in the Van earthquake. Strain localization with fault weakening controls the seismic characterization of Van earthquake and contributes to explain the anomalous occurrence of aftershocks and intraplate nature of the Van earthquake.     

Earthquake focal mechanism studies in Koyna-Warna region in the last five decades — Current understanding on tectonics and seismogenesis

On 10th December 1967, the world’s largest reservoir triggered seismic (RTS) event of magnitude 6.3 shook the Koyna region, the prime site of RTS globally. Ever since, several studies have attempted to infer the seismotectonics and to comprehend the actual causative mechanism of triggered seismicity in this region. Initial studies, including those of the 1967 Koyna main shock and its aftershocks, were based on the conventional P wave polarity or the first motion approach. These studies provided the first ever understanding of a predominantly strike-slip environment in the Koyna region, concurrent with the direction of ambient stress field due to the Indian plate motion. Subsequent studies pointed to a normal faulting environment in theWarna region further south, subsequent to impoundment later in 1985. A few studies did report solutions based on composite focal mechanisms, which however, only represent the average picture of the region. More recent studies based on modelling of seismic broadband waveform data provided more accurate focal mechanisms with unprecedented location accuracies including focal depths. A catalog of 50 focal mechanism solutions is now available for the earthquakes of magnitude ～4 and larger that occurred during the last 50 years, which has paved way for a clear understanding of the stress field and the causative model of seismogenesis in this active intra-plate seismic RTS zone in western India. Based on stress inversion using this catalog, a new tectonic model depicting a periodically varying stress field and hence faulting mechanism has been inferred.     

Seismicity changes associated with reservoir loading

Changes in seismic activity have been related to the filling of large reservoirs in over thirty cases. These changes range from variations in the level of micro-earthquake activity detectable only with instruments of high sensitivity to destructive earthquakes with magnitudes greater than 6. On the other hand, the filling of many other large reservoirs has not been accompanied by increased seismicity.

A number of factors may contribute to the generation or absence of post-impounding seismicity. Increased vertical stress due to the load of the reservoir and decreased effective stress due to increased pore pressure can modify the stress regime in the reservoir region. Whether or not these stress changes are sufficient to generate earthquake activity will depend on a complex interaction of the induced stress with the state of pre-existing stress near the reservoir, and on the geologic and hydrologic conditions at the site. The combined effect of increased vertical load and increased pore pressure will have the greatest tendency to increase activity in regions where the maximum compressive stress is vertical (normal faulting). In regions where the minimum compressive stress is vertical (thrust faulting) increased stress due to a vertical load should have a minimum effect. For all of the larger reservoir-induced earthquakes the stress system determined from fault plane solutions is in agreement with the pre-existing stress field in the region of the reservoir. These earthquakes are all of strike-slip or normal type, there being no reported cases of large induced earthquakes with thrusting mechanisms.

The potential for major changes in seismicity may be highest in regions of moderate strain accumulation (low to moderate natural seismicity). In areas of high strain accumulation and high levels of natural seismicity, the stress changes induced by the reservoir will be small compared to natural variations. In aseismic areas, with low strain accumulation, the reservoir-induced stresses may be insufficient to raise the stress level to a state of failure.     

Implications of north pacific plate tectonics in central Alaska: Focal mechanisms of earthquakes

The focal mechanisms for 86 selected earthquakes (3.0 m_b 5.5) located in central Alaska have been investigated from P-wave first motions; the data were gathered by local seismic networks. The results show a depth-dependent characteristic to the fault-plane solutions. For earthquakes having focal depths shallower than 60–70 km, the focal mechanisms indicate either strike-slip or normal faults, while for earthquakes with foci at intermediate depths the focal mechanisms correspond to thrust faults. The nature of the seismicity indicates the hinge line of the Pacific lithospheric plate under the study area to be striking N17°E from Cook Inlet towards interior Alaska. The comparison of the focal mechanisms with the seismicity shows that the strike-slip and normal faults are the predominant processes of stress release along the shallow section of the plate. The earthquakes with intermediate foci systematically occur along the inclined section of the plate. If the gently dipping nodal planes for these earthquakes are chosen as the fault planes, the focal mechanisms correspond to underthrust motions at the foci. In these, the slip vectors are oriented either to the west or north with the resultant being in the N30°W direction. The tension axes for the underthrust solutions are also found to be parallel to the local dip of the plate, indicating that the subducted plate in interior Alaska is undergoing gravitational sinking.     

Focal mechanism solutions and dynamic parameters of earthquakes in the eastern Himalayas and northern Burma

Focal-mechanism solutions of four earthquakes in the eastern Himalayas and northern Burma are determined using the first motion of compressional waves. Two possible solutions thus obtained for each event reveal steeply dipping fault with predominantly strike-slip motion. The stress directions inferred from the focal mechanism solutions are interpreted in the light of predictions of the plate tectonics theory, viz., the underthrusting of the Indian plate in the Burma region in an easterly direction.Dynamic parameters (seismic moment, apparent stress and average dislocations) are obtained using the corrected spectra of Love waves. The earthquakes are found to possess low seismic moment and apparent stress values. A comparison of these estimates with values for intraplate earthquakes is given. It is suggested that these earthquake might be a consequence of a nonhomogeneous rupture process.     

Seismicity and earthquake focal mechanisms in the Baikal rift zone

The seismicity of the Baikal rift zone is considered on the basis of instrumental and field observations. The spatial pattern of epicentres, the frequency of earthquakes and the relations between seismicity and the elements of fault tectonics are analyzed. The regional and local stress states in the crust of the Pribaikalye region, obtained from studying earthquake focal mechanisms for various energies are summarized.     

美国北加州吉塞尔斯诱发地震与热汽田特征的关系

吉塞尔斯地震活动可能是因蒸汽开发引起的.水在一个承受很高构造剪应力和应变的大范围破裂体中汽化为蒸汽。 汽田地震震源机制解与区域构造应变场几乎一致,并且在该区域范围内汽田地震与别的构造地震不易区分。观测中注意到地震活动与注液历史无关,这表明孔隙水压力增高与注液不可能是吉塞尔斯诱发地震的成因。 相反,所有证据都表明诱发地震与孔隙水压力及温度降低有关。形成机制有两种最大可能:其一,是裂隙排水(汽)导致局部剪应力增加所致,其二,是由稳定滑动转化为不稳定滑动(粘滑)。没有其它记载的诱发地震机制与吉塞尔斯汽田条件相符。     

Double-sided subduction with contrasting polarities beneath the Pamir-Hindu Kush: Evidence from focal mechanism solutions and stress field inversion

The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward subduction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.     

Intraplate earthquakes in Northern Svalbard

A temporary network of microearthquake seismographs deployed in the northern part of Svalbard recorded numerous earthquakes during the summer of 1982. Most of the earthquakes occurred in concentrated zones in restricted portions of three fault complexes in central Nordaustlandet, but minor activity also occurred to the east and west of these complexes. Within the uncertainty of the epicentral locations, most earthquakes which occurred on land in regions not covered by glaciers could be associated with segments of mapped faults. There are numerous fault segments and even entire faults, however, along which no recorded earthquake occurred. The concentrated spatial locations of these earthquakes along preferred portions of mapped faults, and the absence of throughgoing lines of activity, suggest that these are intraplate earthquakes occurring along existing faults in response to an applied stress field. Composite fault-plane solutions for the three most active regions each include one nodal-plane which can be associated, at least roughly, with a trend of seismicity and trend of a mapped fault. The directions of greatest principle stress for the three fault plane solutions vary between N90°W and N55°W, directions which agree approximately with maximum stress directions determined for a region about 200 km to the south. The consistency of the directions of maximum principle stress obtained for earthquakes throughout a broad region of Svalbard suggests that the earthquakes there occur in response to a stress field which is of plate tectonic origin.     

South Australian earthquakes, 1980–92

The seismicity of South Australia over the period 1980–92 is presented as a follow‐up to earlier studies. The South Australian seismic network has undergone a significant expansion in the last decade, and with it an increase in the number and precision of located earthquakes. The distribution of recent seismic activity is similar to the historical pattern of earthquakes and the previous instrumental seismicity maps, all of which show the three main areas as being the Flinders‐Mt Lofty Ranges, Eyre Peninsula, and the southeast. The one notable exception in the recent study is the presence of earthquake activity in the Musgrave Block, a previously aseismic region. Intensity characteristics are reported for earthquakes that were sufficiently widely felt. Fault plane solutions for three Flinders Ranges earthquakes (previously unpublished) are also presented; the focal mechanisms are consistent with predominant northeast‐southwest compression. The seismic moment method was used to estimate the seismic risk for the major population centres in terms of probability of exceedance of seismic intensity within a given period. These estimates are based on the recurrence parameters and intensity attenuation function for the region. The results place Adelaide close to the AS2121 ‐ 1979 Earthquake Code Zone I/Zone 2 boundary.     

Seismicity of Gujarat

Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M _w7.8), 1956 (M _w6.0) and 2001 (M _w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.     

Synthesis of seismicity studies for western Alaska

An examination of the Alaskan earthquake catalogs revealed that from 1928 through 1965, twelve earthquakes of magnitudes (M) in the range 5.6 to 7.3 were located in and around Seward Peninsula region of western Alaska. Moreover, a number of earthquakes of M < 5.0 were found to distribute over the same area. Further investigation of the seismicity employing a local seismographic network in the above area showed a higher level of onshore and offshore seismic activity than had been previously recognized. A number of clusters of earthquakes have been identified. Some of them are located in the epicentral areas of past strong earthquakes (M > 5.5) and some are located in areas traversed by mapped faults. The nature of the seismicity as identified with the local network data is primarily crustal over the entire study area. Investigation of focal mechanisms of isolated strong earthquakes or clusters of small earthquakes show normal faulting as the dominant mode of strain energy release in the western part of Alaska. Moreover, in areas lying, approximately, south and north of Kotzebue Sound, the principal components of horizontal stresses tend to orient in the NW-SE and WNW-ESE directions, respectively.     

Precursory activity in seismic belts containing the focal region of forthcoming large intraplate earthquake

Seismic activity in the region surrounding the foci is investigated for three severe earthquakes (two with a magnitude of 6.1 and one with a magnitude of 5.3) which have occurred in Japan in recent years. The most conspicuous feature commonly noticed is precursory activation of seismic belts which include the focal regions of main shocks. The repetition of the same pattern in the space-time distribution of earthquake occurrence along the seismic belt is also observed for each case. The precursory activity of seismic belts terminates in rather a short period and, after that, the area around the focus of the forthcoming large earthquake becomes quiescent, which demonstrates the appearance of the seismic gap of the second kind (Mogi, 1979). The periods of seismic quiescence for the cases investigated in this paper are longer than those which are given by the regression relationship between earthquake magnitude and precursor time proposed for example, by Sekiya (1977). However, our definition of anomalous seismic activity is clear, and it is possible to give a physical meaning to it as an increase in the local stress field in the seismic belt. We propose that a kind of coupling between intraplate tectonic blocks, analogous to interplate coupling in the subduction region, is responsible for the formation of the stress field relevant to these earthquakes. Although this is at present only one of the possible viewpoints on the formation of the focal region of large intraplate earthquakes, it may be worthwhile to study various precursory phenomena in-connection with this hypothesis.     

Stress field estimation based on focal mechanisms and back projected imaging in the Eastern Llanos Basin (Colombia)

At least 156 earthquakes (Mw 2.8–4.4) were detected in Puerto Gaitán, Colombia (Eastern Llanos Basin) between April 2013 and December 2014. Out of context, this figure is not surprising. However, from its inception in 1993, the Colombian National Seismological Network (CNSN) found no evidence of significant seismic events in this region. In this study, we used CNSN data to model the rupture front and orientation of the highest-energy events. For these earthquakes, we relied on a joint inversion method to estimate focal mechanisms and, in turn, determine the area's fault trends and stress tensor. While the stress tensor defines maximum stress with normal tendency, focal mechanisms generally represent normal faults with NW orientation, an orientation which lines up with the tracking rupture achieved via Back Projection Imaging for the study area. We ought to bear in mind that this anomalous earthquake activity has taken place within oil fields. In short, the present paper argues that, based on the spatiotemporal distribution of seismic events, hydrocarbon operations may induce the study area's seismicity.     

The devastating Muzaffarabad earthquake of 8 October 2005: New insights into Himalayan seismicity and tectonics

The recent earthquake of 8 October 2005 in the Muzaffarabad region in western Himalaya destroyed several parts of Pakistan and the north Indian state of Jammu and Kashmir. The earthquake of magnitude 7.6 claimed more than 80,000 lives, clearly exposing the poor standards of building construction — a major challenge facing the highly populated, earthquake prone, third world nations today. In this paper, we examine variations in the stress field, seismicity patterns, seismic source character, tectonic setting, plate motion velocities, GPS results, and the geodynamic factors relating to the geometry of the underlying subsurface structure and its role in generation of very large earthquakes. Focal mechanism solutions of the Muzaffarabad earthquake and its aftershocks are found to have steep dip angles comparable to the Indian intra-plate shield earthquakes rather than the typical Himalayan earthquakes that are characterized by shallow angle northward dips. A low p-value of 0.9 is obtained for this earthquake from the decay pattern of 110 aftershocks, which is comparable to that of the 1993 Latur earthquake in the Indian shield — the deadliest Stable Continental Region (SCR) earthquake till date. Inversion of focal mechanisms of the Harvard CMT catalogue indicates distinct stress patterns in the Muzaffarabad region, seemingly governed by an overturned Himalayan thrust belt configuration that envelops this region, adjoined by the Pamir and Hindukush regions. Recent developments in application of seismological tools like the receiver function technique have enabled accurate mapping of the dipping trends of the Moho and Lithosphere–Asthenosphere Boundary (LAB) of Indian lithosphere beneath southern Tibet. These have significantly improved our understanding of the collision process, the mechanism of Himalayan orogeny and uplift of the Tibetan plateau, besides providing vital constraints on the seismic hazard threat posed by the Himalaya. New ideas have also emerged through GPS, macroseismic investigations, paleoseismology and numerical modeling approaches. While many researchers suggest that the Himalayan front is already overdue for several 8.0 magnitude earthquakes, some opine that most of the front may not really be capable of sustaining the stress accumulation required for generation of great earthquakes. We propose that the occurrence of great earthquakes like those of 1897 in Shillong and 1950 in Assam have a strong correlation with their proximity to multiple plate junctions conducive for enormous stress build up, like the eastern Himalayan syntaxis comprising the junction of the India, Eurasia plates, and the Burma, Sunda micro-plates.     

Seismic behavior in central Taiwan: Response to stress evolution following the 1999 Mw 7.6 Chi-Chi earthquake

Following the 1999 M_w 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight M_w ⩾ 5.8 earthquakes took place following the Chi-Chi earthquake, whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998. Since the seismicity rate during the Chi-Chi postseismic period has never returned to the background level, such seismicity activation cannot simply be attributed to modified Omori’s Law decay. In this work, we attempted to associate seismic activities with stress evolution. Based on our work, it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress. On the contrary, the stress changes imparted by the afterslip; lower crust–upper mantle viscoelastic relaxation; and sequent events resulted in a stress drop in most of the study region. Understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation.