Microseismicity in the region of Tehran

We record and analyze small earthquakes around Tehran, Iran, using the permanent seismological network of the Institute of Geophysics of the University of Tehran and two temporary dense seismological networks installed for several weeks during 1999 and 2000. Regional seismicity is distributed throughout Central Alborz, extending from the Caspian Sea to the Iran Plain, with the highest level of activity east of Tehran related to the Mosha and to the Garmsar faults, which both dip northward and have had strong historical earthquakes. Our focal mechanisms, the first for these faults, confirm a left lateral strike slip motion.     

Time independent seismic hazard analysis in Alborz and surrounding area

The Bayesian probability estimation seems to have efficiencies that make it suitable for calculating different parameters of seismicity. Generally this method is able to combine prior information on seismicity while at the same time including statistical uncertainty associated with the estimation of the parameters used to quantify seismicity, in addition to the probabilistic uncertainties associated with the inherent randomness of earthquake occurrence. In this article a time-independent Bayesian approach, which yields the probability that a certain cut-off magnitude will be exceeded at certain time intervals is examined for the region of Alborz, Iran, in order to consider the following consequences for the city of Tehran. This area is located within the Alpine-Himalayan active mountain belt. Many active faults affect the Alborz, most of which are parallel to the range and accommodate the present day oblique convergence across it. Tehran, the capital of Iran, with millions of inhabitants is located near the foothills of the southern Central Alborz. This region has been affected several times by historical and recent earthquakes that confirm the importance of seismic hazard assessment through it. As the first step in this study an updated earthquake catalog is compiled for the Alborz. Then, by assuming a Poisson distribution for the number of earthquakes which occur at a certain time interval, the probabilistic earthquake occurrence is computed by the Bayesian approach. The highest probabilities are found for zone AA and the lowest probabilities for zones KD and CA, meanwhile the overall probability is high.     

Seismic potential of the North Qazvin zone,Alborz

The North Qazvin region is a part of the Central Alborz Mountains in Iran and has experienced destructive earthquakes. This region is a popular and industrial zone near Tehran, capital of Iran. To identify the highest and lowest seismic hazard location and consequently the seismic zonation of this region, different parameters, such as topography, geology, tectonics and seismicity, have been focused. Accordingly, the north of Qazvin region can be divided into three subzones: western, eastern and southern. Seismic activity of the western zone is higher than the other ones and seismic potential of the eastern zone is higher than the other two zones. This zoning is also necessary for all seismic active areas to find the most dangerous zone.     

Active faults in the Zagros and central Iran

Active tectonic movements in the northwestern Zagros include right lateral slip at the rate of about 10 mm/a along the Main Recent Fault, which inherits the position of the Main Thrust, now inactive, and active thrusting and accompanying folding distributed between several zones southwest of the Main Recent Fault. In the southeastern Zagros (the Fars Province), there are several right lateral faults that extend N–S obliquely to the overall trend of the Zagros fault-and-fold belt. These may be either branches of the Main Recent Fault, or faults accommodating relative broadening of the outer Zagros in its southeastern segment. The Main Thrust in the southeastern Zagros also remains inactive.

The Ipak, North Tehran, and Mosha fault zones and several minor structures in the eastern Alborz form the E–W-trending active fault system with combined reverse and left lateral slip. On the Ipak and Mosha zones, lateral movements with the late Quaternary mean rate exceeding 1 mm/a dominate over vertical fault movements. Together with right lateral faults stretching northeast of Zagros, the faults of the Alborz may accommodate east-directed motion of the Iranian microplate.     

Left-lateral active deformation along the Mosha–North Tehran fault system (Iran): Morphotectonics and paleoseismological investigations

The Mosha and North Tehran faults correspond to the nearest seismic sources for the northern part of the Tehran megacity. The present-day structural relationships and the kinematics of these two faults, especially at their junction in Lavasanat region, is still a matter of debate. In this paper, we present the results of a morphotectonic analysis (aerial photos and field investigations) within the central part of the Mosha and eastern part of the North Tehran faults between the Mosha valley and Tehran City. Our investigations show that, generally, the traces of activity do not follow the older traces corresponding to previous long-term dip–slip thrusting movements. The recent faulting mainly occurs on new traces trending E–W to ENE–WSW affecting Quaternary features (streams, ridges, risers, and young glacial markers) and cutting straight through the topography. Often defining en-echelon patterns (right- and left-stepping), these new traces correspond to steep faults with either north- or south-dipping directions, along which clear evidences for left-lateral strike–slip motion are found. At their junction zone, the two sinistral faults display a left-stepping en-echelon pattern defining a positive flower structure system clearly visible near Ira village. Further west, the left-lateral strike–slip motion is transferred along the ENE–WSW trending Niavaran fault and other faults. The cumulative offsets associated with this left-lateral deformation is small compared with the topography associated with the previous Late Tertiary thrusting motion, showing that it corresponds to a recent change of kinematics.     

Faulting and folding in quaternary deposits of Tehran’s piedmont (Iran)

Tehran lies on the southern flank of the Central Alborz, an active mountain belt characterized by many historical earthquakes, some of which have affected Tehran itself. The border between the Alborz Mountain and the Tehran’s piedmont (northern part of Tehran City) is marked by the North Tehran Fault (NTF), dividing the Eocene rock formation from the alluvial units of different ages (Early Pleistocene to the recent alluvium). A detail mapping of the piedmont, combined with structural study reveal that two active thrust faults (situated south of the NTF) are of importance for hazard assessment of the City. The geomorphological evidences along the NTF are not in agreement with an active fault, indicating that the fault activity may have been shifted southward. Furthermore differentiation of newly recognized alluvial units and their inferred ages, together with the mapped fault pattern permit us to characterize the Quaternary deformation. The Late Pleistocene alluvial deposits consist of three alluvial fans among them the youngest one together with the modern alluvial fan defines the Holocene deposit. The present deformation in the piedmont is accommodated along vertically left-lateral strike-slip faults and low-angle thrust faults trending in range from N070 to N110E.     

Estimating magnitudes of prehistoric earthquakes and seismic capability of fault from landslide data in Noor valley (central Alborz,Iran)

Detecting the paleoseismological specifications as well as seismic capability of faults has specific importance in estimating the earthquake hazard in any region. The geomorphic indices are used as indirect procedures in the mountainous area. They are appropriate and applicable methods in recognizing the specifications of active tectonics and evaluating fault seismicity in the mountainous areas. In this regard, giant landslides can be pointed out as proper indices. These landslides are usually related to tectonics and triggered by earthquakes in many cases. In this research, giant landslides existed in Noor valley (central Alborz) have been considered as geomorphological indices for recognizing the seismicity of the region and the seismic capability of its faults. There are four giant landslides in this region (Baladeh, Razan, Vakamar, and Iva) used for the mentioned purpose. No historical earthquake has been reported around Noor valley. However, the existence of giant and old landslides, related to earthquake, indicates the occurrence of numerous prehistoric earthquakes. In this research, three different age classes have been determined (Late Holocene, Early Holocene, and Late Pleistocene) for landslides. By the way, the possibility of identifying multiple earthquakes is provided in this area. The magnitudes of earthquakes are estimated as 7.7 ± 0.49 to 7.9 ± 0.49 based on their relations with maximum volume of displaced material. Regarding the distribution of landslides and other evidences, the eastern segment of Baladeh fault has probably been the main cause of the earthquakes.     

Influence of the Chuxiong Yao’an Earthquake on the Mineralization of Hot Springs in the Tengchong Geothermal Area, Southwestern China

<正>The Tengchong geothermal area,an active tectonic region with frequent earthquakes,is located in Yunnan Province of southwestern China.This area contains abundant active hot springs, which often display high metal concentrations and obvious mineralization phenomena.At 19:19 on 9 July 2009,an earthquake occurred in Yao'an,Chuxiong,Yunnan Province,which is 300 km to the northeast of the Tengchong geothermal area.We sampled water in the hot springs in the Tengchong area from 4 July to 9 July 2009 and from 10 July to 15 July 2009 to study the changes of elemental concentrations before and after the earthquake and discuss the influence of the earthquake on the mineralization of the hot springs.The concentrations of most trace elements increased slightly,but the concentration of REE(rare earth elements) decreased by 50%after the earthquake in the hot springs around a NS-trending fault.The elemental concentrations remained unchanged in Longtan and Suanshuigou,which are related to an inactive crater.The elemental concentrations in other springs controlled by superficial and small-scale faults decreased after the earthquake.The earthquake can stimulate the activity of deep faults and magma chambers,as is responsible for the increase of metal concentrations in the hot springs along the NS-trending deep fault;whereas it can decrease the porosity of permeable rocks,resulting in the decline of the flux of ore-bearing fluids and the corresponding mineralization in the hot springs related to superficial faults.     

On the problem of destructive Iranian earthquakes and their causative faults

This article is devoted to evaluating destructive earthquakes (magnitude >6) of Iran and determining properties of their source parameters. First of all, a database of documented earthquakes has been prepared via reliable references and causative faults of each event have been determined. Then, geometric parameters of each fault have been presented completely. Critical parameters such as Maximum Credible Rupture, MCR, and Maximum Credible Earthquake, MCE, have been compiled based on the geometrical parameters of the earthquake faults. The calculated parameters have been compared to the maximum earthquake and the surface rupture which have been recorded for the earthquake faults. Also, the distance between the epicenter of documented earthquake events and their causative faults has been calculated (the distance was less than 20 km for 90% of the data). Then, the distance between destructive earthquakes (with the magnitude more than 6) and the nearest active fault has been calculated. If the estimated distance is less than 20 km and the mechanism of the active fault and the event are reported the same, the active fault will be introduced as a probable causative fault of that earthquake. In the process, all of the available geological, tectonic, seismotectonic maps, aerial geophysical data as well as remote sensing images have been evaluated. Based on the quality and importance of earthquake data, the events have been classified into three categories: (1) the earthquakes which have their causative faults documented, (2) the events with magnitude higher than 7, and (3) the events with the magnitude between 6 and 7. For each category, related maps and tables have been compiled and presented. Some important faults and events have been also described throughout the paper. As mentioned in this paper, these faults are likely to be in high seismic regions with potential for large-magnitude events as they are long, deep and bound sectors of the margins characterized by different deformation and coupling rates on the plate interface.     

长江中游湖南、湖北地区主要活动断裂及地震地质特征

通过收集整理前人成果资料,结合湖南、湖北地区地震地质特征、历史近代地震数据等,全面梳理分析该区主要活动断裂及历史地震,总结该区主要活动断裂系（带）及控震特征、地震活动性及时空分布特征。研究结果表明,该区主要活动断裂以北东、北北东、北西向为主,主要活动断裂系有6个,自北向南分别为秦昆北西向断裂系、鄂东北东向断裂系、江汉-洞庭盆地断裂系、鄂西-湘西北东向断裂系、湘中南北东向断裂系、湘东北东向断裂系,其中第四纪活动较为显著且影响程度大的是江汉-洞庭盆地断裂系及秦昆北西向断裂系西段。“两湖”地区地震活动水平相对较低,正处于第三活动期的相对平静期。结合近代中强震资料及中国地震烈度区划特征分析认为,江汉-洞庭盆地南部的东、西边界、鄂州-黄冈-武汉一带以及鄂西北断块隆起区地壳较不稳定,具有发震潜力,应在城市群规划建设、护江大堤设防和重大工程建设中予以特别关注。      

Statistic and genetic investigation of faults in North Tehran tectonic wedge (South Central Alborz)

In this research, we have focused on the geometrical characteristics of young faults in North Tehran tectonic wedge which is confined with the Mosha and North Tehran faults, the most outstanding active faults in Alborz fold-thrust belt. The statistical, genetic, and kinematic relationships between internal faults, boundary faults, and the stress regime in the area (at the finite state of deformation path) are considered in detail with the help of rose diagrams and Riedel??s model. On this basis, all faults with diverse mechanisms have been classified into different Riedel fractures and their orders of formation are identified. Pattern of faults implies a more or less N?CS compression at the period of faulting. Consideration of geometry and tectonic setting of abundant normal faults have led to propose folding and listric faulting model to explain the origin of normal faults in a compressional tectonic region. These structural models represent the mechanism of normal faulting in response to compression in crustal and upper crustal scales, respectively.     

渤海海峡跨海通道工程区断裂活动性及地震分布特征

利用近年来采集的高分辨率地震剖面资料,编制了渤海海峡跨海通道工程区主要活动断裂分布图,并对其中各断裂的垂直活动速率进行计算,发现渤海海峡跨海通道工程区内NE—NNE向断裂晚更新世以来的平均垂直活动速率为0103 mm/a,NW向断裂的平均垂直活动速率为0080 mm/a,其中NE—NNE向断裂和NW向断裂的活动速率呈由南到北逐渐增强的趋势,NW向断裂还表现出明显的自西向东活动速率逐渐增强的特点。另外,通过与现代小震资料和历史地震资料进行对比,发现研究区内地震分布具有不均匀性,地震活动性随着与断裂之间距离的增加而减弱,且在断裂交点和端点处活动性较强。研究区内地震的这些分布特征能够用弹性回跳学说解释。此外,研究区内地震活动性与断裂的水平位移速率关系可能更为密切,其与断裂垂直活动速率的关系还需要进一步研究。     

阿尔泰山活动断裂

文中介绍了位于亚洲腹地阿尔泰山地区的活动断裂。中国阿尔泰山 (阿尔泰山西南麓 )和蒙古阿尔泰山 (阿尔泰山的东麓 )以NNW向大型走滑断裂为主 ,科布多断裂是阿尔泰山东麓的一条主要NNW向走滑断裂 ,长度近 70 0km。第四纪中晚期右旋走滑速率可达 6 10mm/a ,其上发现有长逾2 0 0km的古地震形变带。富蕴断裂则是阿尔泰山西南麓的一条主要NNW向断裂 ,中晚第四纪的走滑运动速率为 (4± 2 )mm/a ,在中国阿尔泰山的西端还发育规模相对较小的NNW向右旋走滑断裂 ,中晚第四纪走滑速率为 (2± 1)mm/a。中国阿尔泰山 (阿尔泰山的西南麓 )还发育NWW向右旋走滑逆断裂 ,其规模相对较小 ,至中国阿尔泰山西端NWW向的额尔齐斯断裂具有明显的右旋走滑性质。蒙古阿尔泰山的南端则发育近东西向的左旋走滑逆断裂。在与戈壁阿尔泰山交汇部位 ,左旋走滑运动具主导作用。戈壁阿尔泰山发育的戈壁阿尔泰断裂带断续延伸可达 10 0 0km以上 ,目前的研究认为 ,其滑动速率为 12mm/a。其中的博格德断裂上 195 7年发生了戈壁阿尔泰 8.3级地震 ,形变带长约 2 5 0km。阿尔泰山活动断裂的规模、运动强度和强地震活动表明这里不仅受到遥远的印度板块北向推挤作用的影响 ,而且受到较近的地球动力学过程的影响或控制。     

Space—time correlations between inland earthquakes in central Japan and great offshore earthquakes along the Nankai trough: Implication for destructive earthquake prediction

Based on the tectonic framework of central Japan, including the surrounding submarine areas, the space-time relationship between destructive inland earthquakes of magnitudesM 6.4 or greater and great offshore earthquakes along the Nankai trough was examined. From east to west, four tectonic lines are defined as lines linking active faults: the Itoigawa-Shizuoka tectonic line (ISTL), the Tsurugawan-Isewan tectonic line (TITL), the Hanaore-Kongo fault line (HKFL), and the Arima-Takatsuki tectonic line (ATTL). The TITL divides central Japan into the Chubu and Kinki districts, and probably extends southward to the Nankai trough. The Chubu district is subdivided into four blocks by boundary lines linking NW-SE trending active faults having left-lateral strike slip. In the Kinki district, N-S trending, active reverse, steep-dip faults are dominant in the triangular region north of the Median Tectonic line, between the TITL and HKFL, forming a basin-and-range province.

Starting from 1586 A.D., a seismic space-time sequence of high seismic activity in the Chubu district in which earthquake occurrence migrates from the eastern to western tectonic lines of central Japan was identified. The sequence also revealed that inland earthquakes preceded great offshore earthquakes which occurred along the Nankai trough. It was also found that a destructive earthquake tends to occur on the HKFL within 30 years after the occurrence on the TITL, and that the western Nankai trough generated great earthquakes ofM≥7.0 at intervals ranging from 8 to 49 years after the HKFL earthquakes. If the eastern Nankai trough is coupled with the western Nankai trough, a forthcoming greater earthquake measuringM 8.5 may be expected. Since such great earthquakes are always accompanied by large tsunamis, much attention should be focussed on possible tsunami disasters along the Pacific coast of central Japan.

Based on its tectonic structure, a tectonic model of central Japan is proposed. The seismic space-time sequence, which attempts to explain the cause of the sequential earthquake generation, is also discussed.     

Space-time distribution patterns of destructive earthquakes in the inner belt of central Japan: Activity intervals and locations of earthquakes

Based on a block structure model of the inner belt of central Japan, an examination was conducted of the space-time distribution patterns of destructiv magnitudes M 6.4 or greater (M =Japan Meteorological Agency Scale). The distribution patterns revealed a periodicity in earthquake activit seismic gaps. Major NW—SE trending left-lateral active faults divide the inner belt of central Japan into four blocks, 20–80 km wide. The occurrenc A.D. with M ≥ 6.4, which have caused significant damage, were documented in the inner belt of central Japan. The epicenters of these earthquakes close to the block boundaries.

Using the relationship between the magnitude of earthquakes which occurred in the Japanese Islands and the active length of faults that generated them, movement is calculated for each historical earthquake. Space—time distributions of earthquakes were obtained from the calculated lengths, the latitud of generation. When an active period begins, a portion or segment of the block boundary creates an earthquake, which in turn appears on the ground surf active period ends when the block boundary generates earthquakes over the entire length of the block boundary without overlapping.

Five seismic gaps with fault lengths of 20 km or longer can be found in the inner belt of central Japan. It is predicted that the gaps will generate ea magnitudes of 7.0. These data are of significance for estimating a regional earthquake risk over central Japan in the design of large earthquake resist

The time sequences of earthquakes on the block boundaries reveal a similar tendency, with alternating active periods with seismic activity and quiet pe activity. The inner belt of central Japan is now in the last stage of an active period. The next active period is predicted to occur around 2500 A.D.     

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。     

Soil-gas radon anomalies in three study areas of Central-Northern Calabria (Southern Italy)

Soil-gas radon concentrations and exhalation rates have generally been observed to be anomalously high along active faults in many parts of the world. The soil-gas method is based on the principle that faults and fractures in rocks are highly permeable pathways along which gases can migrate upward from deep crust and mantle to soil cover, retaining their source signatures. The present study summarizes the influence of fault zones on anomalous radon concentrations in soil by integrated geophysical and geo-structural analyses in three study areas of Central-Northern Calabria (Southern Italy). Soil-gas radon surveys have been carried out by means of an alpha scintillation counting system, at 12,509 locations between 2002 and 2004. A geostatistical approach has been used to estimate the spatial distribution of soil radon concentrations. Relations among soil-gas distribution and geo-structural features have been evaluated by ordinary multi-Gaussian kriging. Highest soil radon concentrations (ca. 90 kBq m^?3) have been measured in the Rossanese sector. In the three study areas, no appreciable differences can be noticed among lithotypes, with the highest concentration values (ca. 89 kBq m^?3) measured in alluvial deposit and in clay. Measurements of soil-gas radon reveal anomalies clearly connected to the tectonic structures. Increased signals are linearly distributed along regional WNW–ESE trending shear zones, with main pathways of concentration also recognizable along the E–W fault system in the Rossanese sector, the N–S fault system in the Crati Graben and the Catanzaro Trough, and the NE–SW fault system in the Catanzaro Trough. The distribution of epicentres of historical earthquakes occurred between 1184 and 2001 confirms the recent activity of the same fault systems. Soil-gas radon concentrations generally increase, as expected, with decreasing distance to the faults.     

Integrated geophysical methods for determining geometry of the Kahrizak Fault,Tehran, Iran

Historical studies and geologic investigations show that the Kahrizak fault located at the southern foot of the central Alborz Mountains poses a high seismic risk for the Tehran region. However, little is known about the geometry and mechanism of movement along this seismogenic fault. This paper uses three different geophysical methods namely, seismic refraction, electrical resistivity and magnetic techniques to investigate geometry and sense of motion across the Kahrizak fault in southern Tehran. Although the geoelectric measurements do not show clear anomaly across the fault deep down into the ground, it reveals an obvious anomaly in shallow depths. However, looking at the seismic refraction and magnetic profiles across the Kahrizak scarp, we identified a high angle fault dipping south. This result in conjunction with the local relief across the fault scarp would suggest that the southern block is downthrown with respect to the northern block across a normal fault. Such knowledge can contribute to better evaluate the seismic hazard potential of one of the main seismogenic faults in the Tehran area.     

Active faulting and continental slope instability in the Gulf of Patti (Tyrrhenian side of NE Sicily,Italy): a field,marine and seismological joint analysis

The Gulf of Patti and its onshore sector represent one of the most seismically active regions of the Italian Peninsula. Over the period 1984–2014, about 1800 earthquakes with small-to-moderate magnitude and a maximum hypocentral depth of 40 km occurred in this area. Historical catalogues reveal that the same area was affected by several strong earthquakes such as the Mw = 6.1 event in April 1978 and the Mw = 6.2 one in March 1786 which have caused severe damages in the surrounding localities. The main seismotectonic feature affecting this area is represented by a NNW–SSE trending right-lateral strike-slip fault system called “Aeolian–Tindari–Letojanni” (ATLFS) which has been interpreted as a lithospheric transfer zone extending from the Aeolian Islands to the Ionian coast of Sicily. Although the large-scale role of the ATLFS is widely accepted, several issues about its structural architecture (i.e. distribution, attitude and slip of fault segments) and the active deformation pattern are poorly constrained, particularly in the offshore. An integrated analysis of field structural geology with marine geophysical and seismological data has allowed to better understand the structural fabric of the ATLFS which, in the study area, is expressed by two major NW–SE trending, en-echelon arranged fault segments. Minor NNE–SSW oriented extensional structures mainly occur in the overlap region between major faults, forming a dilatational stepover. Most faults display evidence of active deformation and appear to control the main morphobathymetric features. This aspect, together with diffused continental slope instability, must be considered for the revaluation of the seismic and geomorphological hazard of this sector of southern Tyrrhenian Sea.