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.     

Transtensional deformation in the Lake Tahoe region, California and Nevada, USA

Dextral transtensional deformation is occurring along the Sierra Nevada–Great Basin boundary zone (SNGBBZ) at the eastern edge of the Sierra Nevada microplate. In the Lake Tahoe region of the SNGBBZ, transtension is partitioned spatially and temporally into domains of north–south striking normal faults and transitional domains with conjugate strike-slip faults. The normal fault domains, which have had large Holocene earthquakes but account only for background seismicity in the historic period, primarily accommodate east–west extension, while the transitional domains, which have had moderate Holocene and historic earthquakes and are currently seismically active, primarily record north–south shortening. Through partitioned slip, the upper crust in this region undergoes overall constrictional strain.Major fault zones within the Lake Tahoe basin include two normal fault zones: the northwest-trending Tahoe–Sierra frontal fault zone (TSFFZ) and the north-trending West Tahoe–Dollar Point fault zone. Most faults in these zones show eastside down displacements. Both of these fault zones show evidence of Holocene earthquakes but are relatively quiet seismically through the historic record. The northeast-trending North Tahoe–Incline Village fault zone is a major normal to sinistral-oblique fault zone. This fault zone shows evidence for large Holocene earthquakes and based on the historic record is seismically active at the microearthquake level. The zone forms the boundary between the Lake Tahoe normal fault domain to the south and the Truckee transition zone to the north.Several lines of evidence, including both geology and historic seismicity, indicate that the seismically active Truckee and Gardnerville transition zones, north and southeast of Lake Tahoe basin, respectively, are undergoing north–south shortening. In addition, the central Carson Range, a major north-trending range block between two large normal fault zones, shows internal fault patterns that suggest the range is undergoing north–south shortening in addition to east–west extension.A model capable of explaining the spatial and temporal partitioning of slip suggests that seismic behavior in the region alternates between two modes, one mode characterized by an east–west minimum principal stress and a north–south maximum principal stress as at present. In this mode, seismicity and small-scale faulting reflecting north–south shortening concentrate in mechanically weak transition zones with primarily strike-slip faulting in relatively small-magnitude events, and domains with major normal faults are relatively quiet. A second mode occurs after sufficient north–south shortening reduces the north–south S_hmax in magnitude until it is less than S_v, at which point S_v becomes the maximum principal stress. This second mode is then characterized by large earthquakes on major normal faults in the large normal fault domains, which dominate the overall moment release in the region, producing significant east–west extension.     

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.     

Earthquakes in the Eastern Great Lakes Basin from a regional perspective

This paper presents a summary of the seismicity and its relation to stress and geologic structures in the Eastern Great Lakes Basin (EGLB) and compares it with that of other regions in the central and eastern North America (CENA). The earthquakes scattered throughout the EGLB are occurring at a rate somewhat less than that of the Appalachians and along the Atlantic Seaboard. Paleoseismology studies suggest that the lower seismicity rate may be characteristic of the EGLB since the Late Wisconsin. North of the EGLB, earthquakes have primarily thrust mechanisms, while to the south of the EGLB, most earthquakes are strike-slip. Throughout the region, including the EGLB, the average P axes of the earthquakes are oriented NE–SW and are aligned with the direction of the current plate driving stress. On a regional basis, earthquakes are centered primarily in the Precambrian basement beneath the Paleozoic cover. Many of the earthquakes in the EGLB have occurred in areas of preexisting faults, at least some of which may have been active during past episodes of continental rifting. For individual faults that have been studied in some detail, however, it is not clear whether earthquakes represent reactivations of local preexisting structures or nucleation of new ruptures in or near the old fault zones.     

The mechanism of induced seismicity at mosul reservoir based on the first motion analysis

In 1986 shortly after the impounding of Mosul reservoir, shallow earthquakes began occurring in the immediate reservoir vicinity, with magnitudes up to ML 3.0, at rates of up to 3 events per week. These events were almost certainly reservoir-induced and coincided with steadily increasing water levels. Cluster of epicenters was observed in the area located within a complex fault zone called the Sinjar-Dohouk-Kuchuk fault system. The presence of such fault system considers a potential source of earthquakes. A composite fault plane solution, based on first p-wave motion analyses, indicates that the mechanisms of seismicity were right-lateral strike-slip faulting along N44°E plane dipping 58° NW, in conformity with the local tectonics.     

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.     

The mechanism of induced seismicity at mosul reservoir based on the first motion analysis

In 1986 shortly after the impounding of Mosul reservoir, shallow earthquakes began occurring in the immediate reservoir vicinity, with magnitudes up to ML 3.0, at rates of up to 3 events per week. These events were almost certainly reservoir-induced and coincided with steadily increasing water levels. Cluster of epicenters was observed in the area located within a complex fault zone called the Sinjar-Dohouk-Kuchuk fault system. The presence of such fault system considers a potential source of earthquakes. A composite fault plane solution, based on first p-wave motion analyses, indicates that the mechanisms of seismicity were right-lateral strike-slip faulting along N44°E plane dipping 58° NW, in conformity with the local tectonics.     

Seismotectonics and seismicity of the Silakhor region, Iran

This paper deals with seismotectonic and seismicity of the Silakhor region that shows high seismic activity in western Iran. Silakhor is a vast plain with several villages and cities of Dorud and Borujerd and a small town of Chalanchulan that were destroyed and/or damaged many times by large earthquakes. This paper addresses the historical and instrumental earthquakes and their causative faults, seismotectonic provinces and seismotectonic zones of the region. Available seismic data were normalized by means of time normalization technique that resulted in the magnitude-frequency relation for the Silakhor area and estimation of the return period of earthquakes with different magnitudes. Some active faults in this region include the Dorud fault, the main Zagros thrust, the Galehhatam fault, the Sahneh fault and others. Among them, the Dorud fault is an earthquake fault and is the cause for most of the large and intermediate earthquakes in the region. The return period of large earthquakes with magnitudes greater than 7.0 (Ms) is very low, however, the occurrence of destructive earthquakes is greater in the region than in the neighboring provinces. The study proves the high seismicity of this zone and it is required to develop an accurate national plan for future building and reinforcement of the existing buildings in this region.     

Body-waveforms and source parameters of some moderate-sized earthquakes near North Island, New Zealand

P-wave first motion and synthetic seismogram analysis of P- and SH-waveforms recorded at teleseismic distances on the WWSSN are used to estimate source parameters of seven of the largest earthquakes (6.1 ≤ m_b ≤ 6.3) that occurred in the vicinity of North Island, New Zealand since 1965. The source parameters of three other (m_b ≥ 6.1) events determined outside of this study are included and considered in the final analysis. Four of the earthquakes occurred at shallow depths (< 20 km), of which three were located within and to the north of North Island. Two of the shallow events show strike-slip and normal focal mechanisms with T-axes oriented in a manner consistent with their location in an area of known back-arc extension. One of the shallow events occurred in northern South Island and shows a reverse-type mechanism indicating horizontal contraction of the crust in an easterly azimuth. Six events occurred at intermediate depths (h = 39 to 195 km) of which five exhibit thrust mechanisms with T-axes consistently oriented near vertical. In the light of previously published plate tectonic models, the near vertical orientation of T-axes of the intermediate-depth events may be used to infer that the southern Kermadec plate boundary immediately north of North Island is not strongly coupled, and hence, not likely capable of producing great earthquakes. A similar inference cannot be made for the section of the Hikurangi Margin adjacent to North Island since the intermediate-depth events considered in this study lie to the north of this segment of the plate boundary.     

A seismotectonic study of the Southeastern Alaska Region

We compare relocations of recent (1973–2005) and historic (1919–1972) earthquakes to geologic and geophysical (gravity, aeromagnetic, and uplift) information to determine the relationship of seismicity to crustal deformation in southeastern Alaska. Our results suggest that along strike changes in the structure of the Pacific plate may control the location of the ends of rupture zones for large earthquakes along the offshore Queen Charlotte fault system in the southern portion of the study area. There is a marked increase in background seismicity in the northern portion of the study area where the Fairweather fault begins to bend toward the northwest and crustal uplift due to glacial unloading exceeds 20 mm/year. Focal mechanisms indicate that thrust and reverse mechanisms predominate in the region of maximum uplift, as might be expected by the decrease in ice sheet thickness. The diffuse nature of seismicity between the Fairweather and Denali faults in the northern study area suggests a complex interaction between plate/microplate interactions and glacial unloading, making it difficult to determine the optimal fault orientation for failure in moderate magnitude (5.5 to 6.5) earthquakes within this region.     

Seismotectonics of the Hindukush and Baluchistan arc

A seismicity map of that part of the Pakistan-Afghanistan region lying between the latitudes 28° to 38°N and longitudes 66° to 75°E is given using all available data for the period 1890–1970. The earthquakes of magnitude 4.5 and above were considered in the preparation of this map. On the basis of this map, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent, mutually orthogonal, seismicity lineaments, namely the northvestern and the north-eastern trends, characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well-defined zones of intermontane seismicity exhibiting north-south alignment.Thirty-two new focal-mechanism solutions for the above-mentioned region have been determined. These, together with the results obtained by earlier workers, suggest the pre-dominance of strike-slip faulting in the area. The Hazara Mountains, the Sulaiman wrench zone and the Kirthar wrench zone, as well as the supposed extension of the Murray ridge up to the Karachi coast, appear to be mostly undergoing strike-slip movements.In the Hindukush region, thrust and strike-slip faulting are found to be equally prevalent. Almost all the thrust-type mechanisms belonging to the Hindukush area have both the nodal planes in the NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of P-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of the seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast of Uzbekistan on the northwest through Hindukush. Almost orthogonal to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting.The above results are discussed from the point of view of convergence of the Indian and Eurasian plates in the light of plate tectonics theory.     

On the aftershock sequence of a 4.6 mb earthquake of the Garhwal Himalaya

Locally recorded data for eighteen aftershocks of a magnitude(m_b) 4.6 earthquake occurring near Ukhimath in the Garhwal Himalaya were analysed. A master event technique was adopted to locate seventeen individual aftershock hypocentres relative to the hypocentre of the eighteenth aftershock chosen as the master event. The aftershock epicentres define an approximately 30 km² rupture zone commensurate with the magnitude of the earthquake. The distribution of epicentres within this zone and the limited amount of first motion data support the view that a group of parallel, sub-vertical, sinistral strike-slip faults oriented N46°, transverse to the regional NW-SE trend of the Garhwal Himalaya, was involved in this seismic episode. Since the estimated focal depth range for aftershocks of this sequence is 3–14 km, we infer that this transverse fault zone extends through the upper crustal layer to a depth of 14 km at least.     

Coseismic elastic models of folds above blind thrusts in the Betic Cordilleras (Spain) and evaluation of seismic hazard

Although it is generally considered that near-surface earthquakes result from movements along faults that cut through the surface, several recent large earthquakes have been partly attributed to blind thrusts. Movements along blind thrusts lead to the formation of surface folds, which are highly dependent upon fault geometry at depth and often not considered in seismic hazard evaluation. Several authors have studied the relationship between surface folding and thrusting for geological situations in which fault geometries are quite simple. However, active fault geometries can be quite complex e.g., segmented thrust faults associated with strike-slip faults. The aim of this contribution is to reconstruct the fault kinematics at depth for a relatively complex geological structure located in the Eastern Betic Cordilleras (Orihuela-Guardamar-Torrevieja region) using the patterns of kilometre-scale folds observed in the field. In order to model surface deformation, the assumption is made that surface km-scale folds have been created by coseismic deformation associated with movement along blind thrusts. By means of a coseismic deformation model, movements at depth have been calculated for three possible hypotheses. Hypothesis 1 assumes that each superficial fold is created by an independent fault. Hypotheses 2 and 3 assume that a sequence of two superficial folds can be created by movement along a single fault displaying a flat and ramp geometry. In Hypothesis 2, the flat is a superficial décollement level between the sedimentary cover and the Betic basement; in Hypothesis 3, it is a deeper décollement level within the Betic basement.

Knowing the approximate age of surface deformation, rough estimates of fault slip-rates and recurrence periods for two possible earthquake magnitudes (7 Ms and 6.7 Ms) have been made, from calculated dislocations at depth. Slip-rates and recurrence periods for flat and ramp fault geometries are in the range of 0.75–1 mm/yr and 1000–2000 yr, respectively. These values are close to those calculated by direct methods in similar seismotectonic contexts.     

Tectonic control on exhumation and seismicity in the Garhwal‐Kumaun Himalaya,NW‐India

Despite similar geological and tectonic setting along the Himalayan orogen, distinct thermochronological/exhumational and seismicity variability exists between the Kumaun and the Garhwal regions of the NW‐ Himalaya. The processes responsible for such variability are still debated. To understand this, published thermochronological ages from several traverses across the Higher Himalayan Crystalline (HHC) and Lesser Himalayan Crystalline (LHC) have been correlated with the seismicity pattern in both Garhwal and Kumaun segments. The seismicity pattern coincides with the zone of rapid uplift and exhumation. The profiles of seismicity across the Kumaun and the Garhwal regions agree with the existence of the Main Himalayan Thrust (MHT) underlying the regions and reflect its geometry and architecture. Slip along the MHT is responsible for occurrence of seismicity on decade time‐scale and exhumation pattern on Myr time‐scale of the Himalaya.     

The influence of the Nurek Reservoir on local earthquake activity

The Nurek Reservoir is located in an area of high seismicity. An average of 5–6 earthquakes of (Russian energy class)K = 10 and 1–2 earthquakes ofK = 11 per year occurred in the vicinity of the reservoir. The largest local earthquake recorded fell in the energy classK = 14 (1956).

At the end of 1972, after the water level had risen to 100 m, earthquake activity increased sharply. Three events ofK = 12 were recorded during one month and the total number of earthquakes (K 7) exceeded three times the former mean level of seismicity.

Spatial and temporal properties in the epicentral distribution showed a migration from the southwest toward the reservoir.

Earthquake mechanisms of 215 events ofK 9 since 1960 were analysed. Fault plane solutions for some of the earthquakes which occurred after the impoundment were found to be different from the ordinary ones.     

湘东-赣西NNE向走滑断裂与地震、地热关系

该文重点研究了湘东、赣西地区ＮＮＥ向活动走滑断裂系与地震和温泉分布之间的密切联系。结果表明：①研究区地震和温泉主要集中在３条ＮＮＥ向主走滑断裂带及及伴生的Ｐ、Ｐ’断裂带;②高温热泉或强震震中区的基本构造样式为走滑断层左行、左阶雁列带和拉分盆地构造;③陆壳结构分层特征是影响该区地震作用强度的一个重要因素。     

辽宁地区主要断裂活动性和地震危险性评估

基于辽宁地区主要活动断裂的几何特征和空间展布，对1980年以来辽宁地区M_L≥2.0地震的累计频次和1900年以来M_s≥5.0地震的年发生率的空间分布及其与活动断裂构造背景关系进行研究，获得了基于地震学的辽宁省内主要断裂和构造区（带）的活动性与地震危险性的初步评估结果。辽宁地区主要断裂活动性较高的有海城河断裂、金州断裂九寨—盖州北段、朝阳—北票断裂等；辽宁地区未来3年发生M_s≥5.0地震危险性较高的断裂依次有海城河断裂、金州断裂、熊岳—庄河断裂、鸭绿江断裂及赤峰—开原断裂与柳河断裂交汇处等。在判定区域地震危险性和城市地震风险时，除了依据前兆异常的空间分布，还应充分考虑区内主要构造（断裂）的活动性与地震危险性。     

塔北隆起X型走滑断裂成因机制的新解释

塔里木盆地塔北隆起发育两组呈小角度相交(40°)的透入性X型走滑断裂,分别沿着NNE走向和NNW延伸。在对塔北哈拉哈塘地区三维地震资料解释的基础上,对走滑断层的几何展布特征以及断层的剖面变形特征进行研究;同时重点解析了RP6断裂和HA13断裂,分析比较NNW向与NNE向断层的变形及发育特征差异;结合盆地重磁资料以及周缘造山带的活动特征,对塔北隆起小角度的X型走滑断层的发育机制以及演化进行了分析。研究表明,塔北隆起走滑断层在垂向上具有明显的分层变形特征,分为三个构造层:震旦系-中寒武统下构造层(TH₃界面以下)、上寒武统-中奥陶统中构造层(TH₃-TO₃t界面)和上奥陶统-石炭系上构造层(TO₃t-TP界面)。断层在下构造层和中构造层中整体处于压扭环境,多发育正花状构造;上构造层中断层主要发育负花状构造或正断层,整体处于张扭环境。两组断裂比较,NNW向断裂活动性强,在各构造层中均有显著的断裂特征发育,垂向连通性强,发育先存基底断裂,而NE向断层主要发育在中构造层,在下构造层和上构造层中断层发育不明显。活动性分析表明,断层的形成与演化具有多期性,走滑断层的形成经历了三期主要活动:中寒武世末、中晚奥陶世和志留纪-石炭纪。塔北隆起X型走滑断裂的形成受到了NNW向基底断裂和薄弱带的控制,NNW向先存基底断裂带或薄弱带优先发育走滑断裂,基底断裂与主挤压应力方向的夹角小于45°-Φ/2,NNE断层的发育受NNW向先存断裂限制,最终形成小角度相交的X型断裂。     

佳伊断裂带晚白垩世走滑-逆冲事件的新证据

在四平市叶赫镇发现一系列走滑-逆冲断层,断层面平直、陡倾,走向集中在NNE15°~35°范围内,组成了佳木斯—伊通两条主干边界断裂之间的分支断裂带,分支断裂呈雁列式排布,与走向NE45°的主干边界断裂呈锐角相交,指示边界断裂具有右旋走滑特征。叶赫镇走滑-逆冲断裂带的发现为佳木斯—伊通断裂存在晚白垩世晚期—末期的走滑-逆冲事件提供了新证据。叶赫镇分支断裂带是石岭镇分支断裂带向南部的延伸,两者切割了相同的地层,具有相同的构造特征和构造属性,属于同一走滑-逆冲断裂系统,它们是晚白垩世晚期—末期同一地球动力学背景下的产物。