Distribution Of Active Faults Research Articles

Spatial distribution of near-fault landslides along Litang fault zones, eastern Tibetan Plateau

Active fault zones are areas impacted by intense tectonic activity, where geohazards such as landslides occur and their distribution characteristics vary across segments. Using the Litang fault zone in the eastern Tibetan Plateau as an example, this study cataloged landslides within a 10 km range on both sides of the fault zone. The Litang fault zone was divided into four sections based on landform characteristics and the spatial distribution of active faults. A comprehensive analysis was conducted on the relationship between landslide distribution and topography, lithology, and active faults in different sections. Additionally, the study employed a random forest model to compare landslide susceptibility between the whole-region and its sub-regions. The results show that lithology and topography significantly influence the distribution of landslide types. The Upper Triassic sandstone, Slate exhibit more debris flows in areas with slopes ranging from 5° to 35°, while slopes >35° in the Upper Triassic sandstone, Slate are prone to unstable slopes. Rock avalanche occur in Upper Triassic sandstone, Slate, and Quaternary sand and gravel. Active faults play a key role in control ling landslides distribution through distance, hanging wall, locked segment, and direction effects. The number of landslides is positively correlated with the activity of faults. Additionally, the dip angle and motion characteristics of faults have significant influence on the distribution of landslides. The results of landslide susceptibility evaluation show that sub-region models have higher accuracy than the whole-region model. This study provides a new method for exploring the spatial distribution of landslides and can provide scientific reference for reducing landslide disasters in active fault zones.

Shallow structure and late quaternary slip rate of the Osaka Bay fault, western Japan

The Osaka Bay is situated at a seismically active region north of the Median Tectonic Line and east of Awaji Island in western Japan, known as part of the Kinki Triangle and the Niigata–Kobe Tectonic Zone. Dense distribution of active faults and high geodetic strain rates characterize the region, posing a major seismic hazard potential to the coastal and metropolitan areas of the Kansai region. To investigate the shallow structure and recent deformation history of active faults in the Osaka Bay, we acquired 15 high-resolution seismic profiles using a Mini-GI airgun and a Boomer as active sources, together with multi-beam bathymetry data across the Osaka Bay Fault. Our seismic sections image a ~ 0.1 to 3.7 km-wide asymmetric anticline forelimb above the Osaka Bay Fault at shallow depths, coupled with a ~ 2.6 km-wide syncline to the west, and a broad, ~ 11 km-wide syncline in the footwall to the east. The synclinal axial surface at shallow depths measured in this study ranges 75°–89°. We observe the vertical displacement of the Osaka Bay Fault increasing northwards along strike. The sediment thickness on the hanging wall, however, is variable, modified by non-tectonic processes such as by tidal currents, affecting the geometry of growth strata. The most recent deformation by the Osaka Bay Fault reaches to near the seafloor by active folding, with large vertical offsets of 8–14 m over the last ~ 11 ka, and 5–11 m over the last ~ 5 ka. By combining with previously reported borehole age data, the average uplift rate on the Osaka Bay Fault is estimated to be ~ 1.0 to 1.7 m/ka during the Latest Pleistocene to Holocene. The inferred slip of the Osaka Bay Fault during the Holocene is likely to account for > 5% of the regional geodetic strain accumulation within the Kinki Triangle. Further studies to evaluate the Holocene slip rates of regional faults are necessary to assess the seismic hazards and the internal strain budgets within the Kinki Triangle and the Niigata–Kobe Tectonic Zone.Graphical abstract

Application of Shallow Reflection Seismic Exploration in The Investigation of Active Faults in Guanzhong Basin

Active fault detection is an important part of site selection of major projects in urban construction. In order to find out the distribution of active faults in Xixian New District of Guanzhong Basin, high-resolution shallow reflection seismic detection is carried out in the area. The optimal observation system parameters and vibrator excitation parameters are determined through a number of technical tests and the processing parameters are selected in the process of data processing to ensure to obtain high-quality seismic profiles. According to the detection test results, the distribution of the main active faults and the distribution characteristics of underground space in the area are found out, the buried depth of the bottom interface of the Quaternary is inferred and interpreted and the internal strata of the Quaternary are divided in detail.

Distribution of Active Faults and Lithospheric Discontinuities in the Himalayan-Tibetan Orogenic Zone Identified by Multiscale Gravity Analysis

The lithospheric structure of the Tibetan Plateau and its adjacent area is a hot topic in geodynamic research. It is important to reveal the mechanism of crustal deformation and tectonic evolution of the study area. In this study, the techniques of wavelet multiscale decomposition and field edge detection were used to study the discontinuities of the lithosphere revealed by multilevel Bouguer gravity anomalies. Specifically, we evaluated the depth characteristics of the major active faults in the study area and identified 15 deep major faults that cut through the lithosphere. They are Chaman fault, Shyok suture zone, Altyn-Tagh fault, Karakash fault, Karakoram fault, Talas-Fergana fault, Kashgarr-Yeshgar transfer system, Rushan-Pshart suture zone, Sangri-Nacuo fault, Main Frontal thrust, Burmese fold belt, Yadong-Gulu fault, Gaoligong fault, Sagaing fault and Nujiang fault. We have also elucidated the tectonic mechanisms of two famous geodynamic phenomena in the Pamir Plateau. The first is the intense intermediate depth seismicity beneath Pamir-Hindukush. It cannot simply be described as the rupture of a subducted residual plate, which could be divided into two distinct tectonic units. One belongs to the Indian plate, the other to the Eurasian plate. Secondly, the mechanism of intense seismicity confined to the western upper crust of the Pamir Plateau could be explained as significant fragmentation of crustal material. Finally, and most importantly, we summarized the coupling mechanism between deep geodynamics and horizontal deformation as observed by modern geodetic techniques. In the upper mantle, the leading edge of the subducting Indian plate reached the SW boundary of Tarim basin and forms a closed structure in western Himalaya. Then, the Tibetan Plateau underwent a tectonic escape towards the east under the continuous compression between the Indian and Eurasian plates. During the process of tectonic escape, the role of the N–S direction normal faults in the Himalayan tectonic zone is limited.

Preliminary Estimation of Geological Structure Based on Relocated Hypocenter of Microearthquake: Case Study at “X” Geothermal Field

Generally, analysis of geological structure in the geothermal field is carried out using remote sensing methods, geological mapping, and gravity analysis. These methods can provide the distribution of geological structure on the surface but cannot provide information on whether the structure is active or not. This study aims to utilize improved hypocenter location which can be used to delineate the distribution of active faults. This is useful for obtain a better interpretation of geological structures in geothermal areas. Hypocenter locations are determined by picking the arrival time of the P-wave and S-wave. This method sometimes leads to uncertainty of hypocenter location due to subjectivity of picking arrival time. The double-difference method is applied with additional input pairing events from waveform cross-correlation (WCC) to overcome the uncertainty of hypocenter location. The result shows 3 microearthquake clusters formed in the western, central, and eastern parts of the “X” Geothermal Field. The cluster formed in the western area shows that there are two orientations of conjugated hypocenter distribution with the orientation of NW-SE and NE-SW. These hypocenters reveal the presence of geological structures which provide permeability pathways for fluids rising in the western part of the geothermal field. Multiplet-clustering analysis in the middle cluster which coincides with the reservoir zone shows two different sub-clusters indicating the possibility of two different sources mechanism working in the area. By integrating all the analysis and hypocenter distribution, these methods provide a better interpretation of structure’s distribution in the “X” Geothermal Field.

On comprehensive application of seismic reflection and MASW method for investigation of urban subsurface active faults and underneath structures: A case study in Haikou city of Hainan province, China

Tectonically, northeast of Hainan Island belongs to the south part of Lei-Qiong sag, and experienced a great subsidence process during the Cenozoic. It is likewise the strongest earthquake activity area on Hainan Island. In 1605, an M7.5 earthquake occurred in the northeast of Hainan Island, which caused a large area of land subsidence into the sea, and initially formed the present pattern of Dongzhaigang Bay in northeast Hainan Island. Previous studies have shown that the seismogenic fault of this earthquake was the Maniao-Puqian fault. About this fault, there has been a lack of in-depth research on the fault activity, the controlled subsidence process, and the state of the underground structure in this area since the late Cenozoic. To investigate the latest active age and the specific location distribution of the Maniao-Puqian fault, and the influence of its normal fault on the subsidence of the Dongzhaigang area, detailed investigation work has been done on this fault area, through the near-surface seismic reflection and Multichannel Analysis of Surface Wave (MASW) method. The results of near-surface seismic exploration showed that the Maniao-Puqian Fault belt is composed of a group of north-dipping normal faults with a near east-west distribution, which was named Maniao-Luodou fault, Guilingyang-Qiaotoucun fault, and Xixiu-Puqian fault. This differs from previous research that the Maniao-Puqian Fault belt is an en echelon type fault. This work redefines the nature and distribution of active faults in Haikou City, which can provide important technical support for the site selection of critical infrastructure construction in the Jiangdong New district of Haikou City.

Seismostratigraphic analysis of Lake Cahuilla sedimentation cycles and fault displacement history beneath the Salton Sea, California, USA

Abstract The Salton Trough (southeastern California, USA) is the northernmost transtensional stepover of the Gulf of California oblique-divergent plate boundary and is also where the southern terminus of the San Andreas fault occurs. Until recently, the distribution of active faults in and around the Salton Sea and their displacement histories were largely unknown. Subbottom CHIRP (compressed high-intensity radar pulse) surveys in the Salton Sea are used to develop a seismic facies model for ancient Lake Cahuilla deposits, a detailed map of submerged active faults, and reconstructed fault displacement histories during the late Holocene. We observe as many as fourteen Lake Cahuilla sequences in the Salton Sea (last ~3 k.y.) and develop a chronostratigraphic framework for the last six sequences (last ~1200 yr) by integrating CHIRP data and cone penetrometer logs with radiocarbon-dated stratigraphy at an onshore paleoseismic site. The Salton Sea contains northern and southern subbasins that appear to be separated by a tectonic hinge zone, and a subsidence signal across hinge-zone faults of 6–9 mm/yr (since ca. A.D. 940) increases toward the south to >15 mm/yr. The faults mapped to the south of the hinge zone appear to accommodate transtension within the San Andreas–Imperial fault stepover. We identify 8–15 distinct growth events across hinge-zone faults, meaning growth occurred at least once every 100 yr since Lake Cahuilla sedimentation began. Several faults offset the top of the most recent Lake Cahuilla highstand deposits, and at least two faults have offset the Salton Sea flood deposits. Active faults and folds were also mapped to a limited extent within the northern subbasin and display growth, but their kinematics and rupture histories require further study. The broad distribution of active faulting suggests that strain between the San Andreas, San Jacinto, and Imperial faults is highly distributed, thus discrepancies between geologic and geodetic slip-rate estimates from these major fault systems are to be expected.

Relationship between seismicity and active faults in Thanh Hoa province detected by local seismic network

Thanh Hoa province belongs to the southwest part of Northwest Vietnam, which is considered a tectonically active region. In the area of Thanh Hoa province, there are three deep-seated tectonic faults, namely Son La-Bim Son, Song Ma, and Sop Cop. As predicted by scientists, these faults are capable of producing credible earthquakes that might be the strongest in the territory of Vietnam. Besides the three main seismogenic sources, in the province, there are other smaller active faults such as Thuong Xuan-Ba Thuoc and Thuong Xuan-Vinh Loc but the relationship of these faults with seismic activity is still rather blurred. This may due to the sparseness of the Vietnamese National Seismic Network which can not record adequately small earthquakes in the area. This paper presents new results of additional monitoring from a local seismic network using 12 Guralp - 6TD broadband seismometers that have been deployed in Thanh Hoa province since November 2009. We found that the Thanh Hoa area is not seismically quiet. The average number of earthquakes recorded by the network has reached 80 - 90 events per year and some of them have magnitude from ML 3.0 to 4.0.By integration of the earthquake epicenters derived from the local network and distribution of active faults, we can detect several earthquakes locating near the three active faults, not only along the main faults but also along its subsidiary faults. We focused on the active faults of Thuong Xuan-Ba Thuoc and Thuong Xuan-Vinh Loc by using the recent results of the gravity, seismic, and magnetotelluric data analyses. Several recorded earthquakes distribute along the two small faults and some of them reach magnitude 3.0 or greater on the ML scale. In this study, the Thuong Xuan-Vinh Loc is recognized as a seismogenic source. To identify seismic hazard potential caused by earthquakes generated from the active faults, segmentation of the Thuong Xuan - Ba Thuoc fault had been done based on geological and geomorphological indications and seismic activity, and then the peak ground acceleration was determined for each fault segment. Besides, a large number of earthquake epicenters do not have a good correlation with a specific fault, especially in the area of Thanh Hoa coastal plain, which is covered by thick layers of Neogene - Quaternary sediment. This shows that there may be hidden active faults in the area which are needed to study further.

Relationship between seismicity and active faults in Thanh Hoa province detected by local seismic network

Thanh Hoa province belongs to the southwest part of Northwest Vietnam, which is considered a tectonically active region. In the area of Thanh Hoa province, there are three deep-seated tectonic faults, namely Son La-Bim Son, Song Ma, and Sop Cop. As predicted by scientists, these faults are capable of producing credible earthquakes that might be the strongest in the territory of Vietnam. Besides the three main seismogenic sources, in the province, there are other smaller active faults such as Thuong Xuan-Ba Thuoc and Thuong Xuan-Vinh Loc but the relationship of these faults with seismic activity is still rather blurred. This may due to the sparseness of the Vietnamese National Seismic Network which can not record adequately small earthquakes in the area. This paper presents new results of additional monitoring from a local seismic network using 12 Guralp - 6TD broadband seismometers that have been deployed in Thanh Hoa province since November 2009. We found that the Thanh Hoa area is not seismically quiet. The average number of earthquakes recorded by the network has reached 80 - 90 events per year and some of them have magnitude from ML 3.0 to 4.0.By integration of the earthquake epicenters derived from the local network and distribution of active faults, we can detect several earthquakes locating near the three active faults, not only along the main faults but also along its subsidiary faults. We focused on the active faults of Thuong Xuan-Ba Thuoc and Thuong Xuan-Vinh Loc by using the recent results of the gravity, seismic, and magnetotelluric data analyses. Several recorded earthquakes distribute along the two small faults and some of them reach magnitude 3.0 or greater on the ML scale. In this study, the Thuong Xuan-Vinh Loc is recognized as a seismogenic source. To identify seismic hazard potential caused by earthquakes generated from the active faults, segmentation of the Thuong Xuan - Ba Thuoc fault had been done based on geological and geomorphological indications and seismic activity, and then the peak ground acceleration was determined for each fault segment. Besides, a large number of earthquake epicenters do not have a good correlation with a specific fault, especially in the area of Thanh Hoa coastal plain, which is covered by thick layers of Neogene - Quaternary sediment. This shows that there may be hidden active faults in the area which are needed to study further.

Quaternary Tectonics as the Neotectonics in Turkey

The base of the Quaternary was formally redefined in 2009 by the International Commision on Stratigraphy at 2.6 Ma by shifting a 800 ka, formerly part of the Pliocene epoch, into the Pleistocene. It has made tremendous impression in literature. The Quaternary period is unique because of its climatic characteristics; however, it is a geochronological unit, and a redefinition of its position would affect many areas, such as the definition of neotectonic period in Turkey. The major tectonic elements of the Eastern Mediterranean region are located in there. Recent studies related to neotectonics in Turkey present a consensus on the driving mechanism and age of this tectonic period: the late Pliocene (before the formal adding of Gelasian stage into the Quaternary period) westward escape of the Anatolian plate. Following the collision of the Arabian and Eurasian plates at ~11 Ma BP, the North Anatolian Fault Zone created in the east and the escape tectonics could not occur until the propagation of the North Anatolian Fault Zone to the Aegean Arc at ~2.5 Ma BP and gaining a transform fault character with a similar slip rate as in today. The spatial distribution of active faults in Turkey also supports such an approach. The great majority of them have no relations with the pre-Quaternary formations, on the other hand are bounding the Quaternary basins. In this study, the neotectonics of Turkey discussed within the view of the redefined Quaternary. The comparison suggests the using of the term Quaternary tectonics substantially coincides with neotectonics in literature for Turkey, because the base level of this period in this region overlaps with the redefined base level of the Quaternary period.

Distribution of Active Faults in Japan Sea and Future Issues

Distribution of Active Faults in Japan Sea and Future Issues

Risk assessment of seismic hazards in hydraulic fracturing areas based on fuzzy comprehensive evaluation and AHP method (FAHP): A case analysis of Shangluo area in Yibin City, Sichuan Province, China

In this paper, a fuzzy comprehensive evaluation (FCE) based on the analytic hierarchy process (AHP) model (FAHP) has been established to quantitatively evaluate the seismic risk of the region with covers 3080.25 km2 including hydraulic fracturing operation areas. Taking into account the combined effect exerted by influencing factors such as geological structures, the seismic history and the stress accumulation, etc., six major factors are identified, namely, seismicity rates (Z value), magnitude-frequency coefficients (b value), salt mines, reservoirs, regional geotectonic (distribution of active faults), stress fields and the spatial distribution of fracturing platforms. In addition to using massive raw data as objective information, this model takes experts' assessments as the subjective judgment of expertise. According to the maximum membership principle, the most favorable risk evaluation have been made in 625 grids with optimized results through methods of the weighted average and the interpolation. Comprehensive evaluation maps have been drawn based on the seismic data from 2008 to 2016 used in modeling and have been validated by the seismic data in 2017. Results show that most of the seismic events in 2017 have fit into the high-risk area, especially those felt earthquakes larger than magnitude 3 have entirely fit into the medium-high risk area after the interpolation, indicating that the assessment results have certain objectivity and accuracy.

Geometric-kinematic characteristics of the main faults in the W-SW of the Lut Block (SE Iran)

The area to the W-SW of the Lut Block in Iran has experienced numerous historical and recent destructive earthquakes. We examined a number of faults in this area that have high potential for generating destructive earthquakes. In this study a number of faults are introduced and named for the first time. These new faults are Takdar, Dehno, Suru, Hojat Abad, North Faryab, North Kahnoj, Heydarabad, Khatun Abad and South Faryab. For a group of previously known faults, their mechanism and geological offsets are investigated for the first time. This group of faults include East Nayband, West Nayband, Sardueiyeh, Dalfard, Khordum, South Jabal-e-Barez, and North Jabal-e-Barez. The N-S fault systems of Sabzevaran, Gowk, and Nayband induce slip on the E-W, NE-SW and NW-SE fault systems. The faulting patterns appear to preserve different stages of fault development. We investigated the distribution of active faults and the role that they play in accommodating tectonic strain in the SW-Lut. In the study area, the fault systems with en-echelon arrangement create structures such as restraining and releasing stepover, fault bend and pullapart basin. The main mechanism for fault growth in the region seems to be ‘segment linkage of preexisting weaknesses’ and also for a limited area through ‘process zone’. Estimations are made for the likely magnitudes of separate or combined failure of the fault segments. Such magnitudes are used in hazard analysis of the region.

Nests of red wood ants (Formica rufa-group) are positively associated with tectonic faults: a double-blind test.

Ecological studies often are subjected to unintentional biases, suggesting that improved research designs for hypothesis testing should be used. Double-blind ecological studies are rare but necessary to minimize sampling biases and omission errors, and improve the reliability of research. We used a double-blind design to evaluate associations between nests of red wood ants (Formica rufa, RWA) and the distribution of tectonic faults. We randomly sampled two regions in western Denmark to map the spatial distribution of RWA nests. We then calculated nest proximity to the nearest active tectonic faults. Red wood ant nests were eight times more likely to be found within 60 m of known tectonic faults than were random points in the same region but without nests. This pattern paralleled the directionality of the fault system, with NNE–SSW faults having the strongest associations with RWA nests. The nest locations were collected without knowledge of the spatial distribution of active faults thus we are confident that the results are neither biased nor artefactual. This example highlights the benefits of double-blind designs in reducing sampling biases, testing controversial hypotheses, and increasing the reliability of the conclusions of research.

REGIONAL CHARACTERISTICS OF STRESS STATE OF MAIN SEISMIC ACTIVE FAULTS IN MID‐NORTHERN PART OF SICHUAN‐YUNNAN BLOCK

AbstractWe restore the seismic source spectrums of 1012 earthquakes (2.0 ≤ ML ≤ 5.0) in the mid‐northern part of Sichuan‐Yunnan seismic block between January 1, 2009 and December 31, 2015, then calculate the source parameters (e.g., seismic moments M0, focal scale r and stress drop Δσ) and fit the calibration relationship between these parameters. Based on the regional seismic tectonic background, the distribution of active faults and seismicity, the study area is divided into four statistical units. For each unit the stress distribution characteristics, change of stress drop with location, correlation between the stress‐strain loading and the dynamic process of regional deformation, are discussed respectively. The results show that seismic moments M0 are consistent with the magnitude‐moment relation that lgM0 = 0.92ML + 10.46. The relationship between stress drop and magnitude is consistent with the result gained by Nuttli that intraplate earthquake follows the ISD model, with a statistical relationship lgΔσ = 0.31 lgM0 – 3.92.Seismic source stress drop results show the following, (1) The stress at the end of the Jinshajiang fault is low, the overall sliding rate of the fault unit is high, and strong earthquake activity is very rare. In the fault belt consisting of three secondary faults, stress‐strain loading deceases gradually from northwest to southeast along Litang fault, the northwest section which is relatively locked is more likely to accumulate strain than southeast section. (2) Stress drop of Xianshuihe fault zone is divided by Kangding, the southern section is low and northern section is high. Southern section (Kangding‐Shimian) is difficult to accumulate higher strain in the short term, but in northern section (Garzê‐Kangding), moderate and strong earthquakes have not filled the gaps of seismic moment release, there is still a high stress accumulation in partial section. (3) High stress‐drop events were concentrated on Anninghe‐Zemuhe fault zone, strain accumulation of this unit is strong, and stress level is the highest, earthquake risk is high. (4) On Lijiang‐Xiaojinhe fault zone, stress drop characteristics of different magnitude earthquakes are not the same, which is related to complex tectonic setting, the specific reasons still need to be discussed deeply. Stress background in the Muli area is low and may be affected by the local tectonic environment. The study also shows that, (1) Stress drops display a systematic change with different faults and locations, high stress‐drop events occur mostly in the fault intersection area. Faults without locking condition and mainly creeping are mainly characterized by low stress drop. (2) Contrasting to what is commonly thought that “strike‐slip faults are not easy to accumulate stress”, Xianshuihe fault zone and Anninghe‐Zemuhe fault zone all exhibit high stress levels, which may be due to that the magnitude and intensity of medium‐strong earthquakes are not enough to release the accumulated energy. On the other hand, in the complex dynamics of regional deformation, when the tectonic force blocking fault movement and its contribution to accumulation of stress plays a key role, the earthquake of same magnitude will release higher stress drop.

Detection of the Shuangshi-Dachuan Fault using Shallow Seismic Reflection in the Southern Section of the Longmenshan Fault Zone

A significant number of Chinese and international earthquake disaster surveys indicate active faults as the root cause of earthquakes. Areas along fault lines experience the most severe damage and a greater number of personal injuries compared to regions on either side of the fault. Therefore, by adopting the shallow seismic reflection method to accurately detect the location and distribution of active faults, and developing effective measures to evade active faults, we can reduce earthquake disaster damage and economic losses. On April 20, 2013, the Lushan Ms7.0 earthquake occurred near the Shuangshi-Dachuan Fault (SDF) in the southern section of the Longmenshan fault zone. Because of its relatively high rate of earthquake activity, complex seismo-tectonic environment, and unique geological conditions, achieving usable data with seismic reflection is not easy. For that reason, available shallow seismic data of the SDF is extremely scarce. In our work, we carefully designed a survey to meet the constraints of the project and we have identified the precise location and near-surface structural form of the NE-trending SDF in the disaster area of the earthquake. The seismic surveys were located in Shuangshi Town of Lushan County and Daxi of Baoxing County. The detection of the SDF provided a scientific basis for selecting sites for post-disaster recovery and reconstruction to avoid these active fault zones, and reduce possible disastrous losses that could be caused by future earthquakes.

CHARACTERISTICS OF SEISMICITY OF THE XIAOWAN RESERVOIR IN AN AREA OF ACTIVE TECTONICS FROM DOUBLE‐DIFFERENCE RELOCATION ANALYSIS

AbstractWe simultaneously relocate the M ≥ 1.0 earthquakes occurred from July 2005 to December 2014 in Xiaowan reservoir area, which is located in an active tectonic region in the middle and lower section of Lancang Jiang in western Yunnan province of China, using cross‐correlation (CC) and double‐difference (DD) method. The relocated epicenters are distinctly clustered in 8 subareas (subarea A to H), and the earthquakes located inside the Xiaowan reservoir area are generally shallower than that outside of the reservoir area. The relocated focal depths, the relation between the seismicity and water level, and the calculated b values in different subareas indicate that the characteristics of seismicity within Xiaowan reservoir area are quite complicated. The earthquakes occurred outside of the reservoir areas are likely to be tectonic earthquakes irrelevant to water filling of Xiaowan reservoir, and there are both reservoir‐triggered seismicity (RTS) and tectonic earthquake activities in Xiaowan reservoir area. The reservoir‐triggered earthquakes (RTE) are mainly located along the Heihui Jiang (subarea A) and Lancang Jiang (subarea B and C), and could be classified into rapid response RTS. In subarea A, a significant “migration” of hypocenters is observed in the first filling cycle along the Heihui valley and Qiaoxi‐Weihou fault zone, which probably indicates the water permeation process in this area. According to the distribution of active faults, rock properties and relocated focal depths in the reservoir area, we suggest that the karst action and fluid permeation caused by water filling, and fault movements are the key factors of RTS in Xiaowan reservoir. Further studies are needed to better understand the physical process of RTS and tectonic seismicity in some subareas in the reservoir.

Active Faults in Iberia

The distribution of active faults in the Iberian Peninsula is not homogeneous, being mainly present, but not only, in areas affected by the Alpine Orogeny. They are located in several regions: (1) The Iberian Massif, including Portugal, and Galicia and Cantabrian Mountains in the North, (2) The Pyrenees, (3) The NE of Iberia, (4) The Iberian Cordillera, and (5) The Betic Cordillera. The Pyrenees, the Iberian Cordillera, and particularly the Betic Cordillera are the most active regions. These faults have being considered active in a broad sense, that is to say, corresponding to faults that, at least, present movements during the Quaternary, not restricting them only to those presenting displacements during the late 10,000 years. Although important, faults situated offshore have not being included in the present review.

Distribution of active faults and large earthquakes along Japan trench and Nankai trough

Distribution of active faults and large earthquakes along Japan trench and Nankai trough

鳥海山南西麗における活断層の分布と断層変位地形

庄内平野東縁に分布する活断層群のうち鳥海山南西麓（日向川以北）を調査対象とし, 当該地域における活断層の分布と変位地形の特徴を明らかにした．鳥海山南西麓付近では, 平野との境界付近に断続的に分布する小丘陵の西麓に比高の大きい断層崖が位置し, 東麓付近では地形面が逆傾斜したり逆向き低断層崖が存在したりする場合が多い．これらのことから, 小丘陵西麓には主断層である東傾斜の低角な逆断層が存在し, 東麓には副次的な西傾斜のバックスラスト（共役断層）が存在すると考えられる．一方, 酒田衝上断層帯の西側に隣接する連続性の良い丘陵（丸森丘陵）では, 東麓に比高の大きい断層崖が形成されていると推定される．平野沿いの断層群の第四紀後期の平均鉛直変位速度は, 北部では 0.5 mm/yr以下, 南部では0.7～0.8 mm/yr以上である．