Seismological and SAR signature of unrest at Nisyros caldera, Greece

Nisyros island, a Quaternary volcanic center located at the SE of the Aegean Volcanic Arc, has been in the past characterized by periods of intense seismic activity accompanied sometimes by hydrothermal explosions, the last one being in 1887. The recent long lasting episode of unrest (1995–1998) in the area is the first instrumentally documented providing information on the behavior of the volcano. Evidence from seismicity and SAR interferometry suggests that the presently active part of the Kos–Nisyros volcano-tectonic complex is located at the NW coast of Nisyros island defining an area much smaller than the whole volcano-tectonic area. Seismicity patterns vary both temporally and spatially consistently with different rates of vertical ground deformation inferred from SAR interferometry. These observations help us to discuss the different elements controlling the behavior of the volcanic system such as: the existence, location and timing of magma chamber inflation, the occurrence of tensile failure at the boundaries of the chamber and the possibility of magmatic fluids being expelled to form a shallow magmatic intrusion, the seismic failure and migration of hypocenters indicating shallow magma transport.     

Natural tracers for identifying the origin of the thermal fluids emerging along the Aegean Volcanic arc (Greece): Evidence of Arc-Type Magmatic Water (ATMW) participation

The Aegean volcanic arc is the result of a lithosphere subduction process during the Quaternary time. Starting from the Soussaki area, from west to east, the arc proceeds through the islands of Egina, Methana, Milos, Santorini, the Columbus Bank, Kos and Nisyros. Volcano-tectonic activities are still pronounced at Santorini and Nisyros in form of seismic activity, craters of hydrothermal explosions, hot fumaroles and thermal springs. A significant number of cold water springs emerge in the vicinity of hot waters on these islands.Chemical and isotopic analyses were applied on water and fumaroles samples collected in different areas of the volcanic arc in order to attempt the assessment of these fluids. Stable isotopes of water and carbon have been used to evaluate the origin of cold and thermal water and CO₂.Chemical solute concentrations and isotopic contents of waters show that the fluids emerging in Egina, Soussaki, Methana and Kos areas represent geothermal systems in their waning stage, while the fluids from Milos, Santorini and Nisyros proceed from active geothermal systems.The δ²H–δ¹⁸O–Cl^? relationships suggest that the parent hydrothermal liquids of Nisyros and Milos are produced through mixing of seawater and Arc-Type Magmatic Water (ATMW), with negligible to nil contribution of local ground waters and with very high participation of the magmatic component, which is close to 70% in both sites. A very high magmatic contribution to the deep geothermal system could occur at Santorini as well, perhaps with a percentage similar to Nisyros and Milos, but it cannot be calculated because of steam condensation heavily affecting the fumarolic fluids of Nea Kameni before the surface discharge.The parent hydrothermal liquid at Methana originates through mixing of local groundwaters, seawater and ATMW, with a magmatic participation close to 19%. All in all, the contribution of ATMW is higher in the central–eastern part of the Aegean volcanic arc than in the western sector. This difference, which is spotted in the variable isotopic composition of the sampled fluids from west to east along the arc, is probably due to several causes, including the tectonic regime, the depth of the deep reservoir below sea level, the age of volcanic activity and in general the geomorphologic state of each island.     

河北平原地震带b值时空变化特征

利用华北地区近44年地震资料,在区域地震序列完整性分析的基础上,用最小二乘法进行b值时间扫描计算,用最大似然法进行b值空间扫描计算。时间扫描中的b值为研究区内每个扫描窗口的平均b值,因此其变化幅度不大,基本保持在0.62~1.05之间。研究区b值空间分布范围基本维系在0.5~1.4,低b值区域为昌平—宝坻断裂段和唐山—迁安断裂段,变化范围为0.5~0.7,表明该区域地壳介质正处于相对高应力或闭锁状态,存在未来可能发生中强以上地震的潜在危险。     

川东南长宁地区地震活动及b值演化特征

利用全国统一目录和流动台站目录,研究了四川盆地东南部长宁地区的地震活动特征和b值的空间分布特征.研究结果显示,长宁地区的地震活动在时间上呈现明显的分段特征,地震活动在2015年后明显增强;在空间上,长宁地区的地震活动主要集中在以28.3°N为界限的南、北2个地区,对于这2个区域的b值演化,计算结果显示出不同的分段特征....     

High-frequency earthquakes at White Island volcano, New Zealand: insights into the shallow structure of a volcano-hydrothermal system

Volcano-tectonic earthquakes at White Island are concentrated in a single seismically active zone, southeast of the active vents and at depths of less than 1 km. A few deeper earthquakes also occur beneath the active vents. A composite focal mechanism indicates that the stress regime in the shallow seismic zone is N-S extensional. Shallow seismicity occurs within the main volume of the volcano-hydrothermal system that underlies the Main Crater floor, and we interpret this as a region where the rocks have been weakened by past magmatic intrusions, elevated pore fluid pressure and physico-chemical effects of acid volcanic fluids, thereby allowing preferential seismic failure. Brittle seismic failure within this region requires a temperature less than about 400 °C, and implies high horizontal temperature gradients close to the active craters and fumaroles. Spasmodic bursts events are also a result of brittle failure, but occur close to zones of significant permeability in response to changes in local fluid pressure.     

四川芦山MS7.0地震震源区及其周边区域P波三维速度结构研究

利用四川数字地震台网和流动地震台站在芦山M_S7.0地震震后(2013年4月20日—6月23日)记录到的2026次区域地震事件的28188条P波到时资料,采用地震层析成像方法反演得到了芦山地震震源区及其周边区域中上地壳P波三维速度结构. 结果表明,浅部地壳的P波速度异常分布特征与地表地质构造、 地形和岩性密切相关,即成都断陷盆地表现出与第四纪沉积有关的低速异常区;犍为、 乐山一带的川中微升区和川青块体龙门山以西的邻近地带均表现为与构造抬升有关的高速异常;宝兴、 康定附近分布的基性火山岩及火山碎屑岩均呈局部高速异常分布. 芦山地震震源位于高低速异常分界线附近且偏向高速体一侧,其下方存在明显的低速异常分布,可能与流体的存在有关. 流体的作用导致中上地壳内部发震层的弱化,使孕震断层易于破裂,可能对芦山地震起到了触发作用. 芦山地震与汶川地震两次地震的余震密集区相距50 km,这50 km地震空区震源体的深度范围附近目前正处于高速异常区内,加之龙门山断裂带西南段又具有比较典型的断错地貌发育,使得该段地震空区(大邑—邛崃活动断裂破裂空段)现在所处的深浅部构造环境变得复杂,其潜在的地震危险性仍值得进一步关注.      

The thermal structure of subduction zones constrained by seismic imaging: Implications for slab dehydration and wedge flow

Thermal models of subduction zones often base their slab–wedge geometry from seismicity at mantle depths and, consequently, cannot be used to evaluate the relationship between seismicity and structure. Here, high-resolution seismic observations from the recent Broadband Experiment Across the Alaska Range (BEAAR) constrain, in a rare instance, the subducting slab geometry and mantle wedge temperature independent of seismicity. Receiver functions reveal that the subducting crust descends less steeply than the Wadati-Benioff Zone. Attenuation tomography of the mantle wedge reveals a high Q and presumably cold region where the slab is less than 80 km deep. To understand these two observations, we generate thermal models that use the improved wedge geometry from receiver functions and that incorporate temperature- and strain-rate-dependent olivine rheology. These calculations show that seismicity within the subducting crust falls in a narrow belt of pressure–temperature conditions, illuminating an effective Clapeyron slope of 0.1 K/MPa at temperatures of 450–750 °C. These conditions typify the breakdown of high-pressure hydrous minerals such as lawsonite and suggest that a single set of dehydration reactions may trigger intermediate-depth seismicity. The models also require that the upper, cold nose of the mantle wedge be isolated from the main flow in the mantle wedge in order to sustain the cold temperatures inferred from the Q tomography. Possibly, sufficient mechanical decoupling occurs at the top of the downgoing slab along a localized shear zone to 80 km depth, considerably deeper than inferred from thrust zone seismicity.     

The transport of water in subduction zones

The transport of water from subducting crust into the mantle is mainly dictated by the stability of hydrous minerals in subduction zones. The thermal structure of subduction zones is a key to dehydration of the subducting crust at different depths. Oceanic subduction zones show a large variation in the geotherm, but seismicity and arc volcanism are only prominent in cold subduction zones where geothermal gradients are low. In contrast, continental subduction zones have low geothermal gradients, resulting in metamorphism in cold subduction zones and the absence of arc volcanism during subduction. In very cold subduction zone where the geothermal gradient is very low(?5?C/km), lawsonite may carry water into great depths of ?300 km. In the hot subduction zone where the geothermal gradient is high(25?C/km), the subducting crust dehydrates significantly at shallow depths and may partially melt at depths of 80 km to form felsic melts, into which water is highly dissolved. In this case, only a minor amount of water can be transported into great depths. A number of intermediate modes are present between these two end-member dehydration modes, making subduction-zone dehydration various. Low-T/low-P hydrous minerals are not stable in warm subduction zones with increasing subduction depths and thus break down at forearc depths of ?60–80 km to release large amounts of water. In contrast, the low-T/low-P hydrous minerals are replaced by low-T/high-P hydrous minerals in cold subduction zones with increasing subduction depths, allowing the water to be transported to subarc depths of 80–160 km. In either case, dehydration reactions not only trigger seismicity in the subducting crust but also cause hydration of the mantle wedge. Nevertheless, there are still minor amounts of water to be transported by ultrahigh-pressure hydrous minerals and nominally anhydrous minerals into the deeper mantle. The mantle wedge overlying the subducting slab does not partially melt upon water influx for volcanic arc magmatism, but it is hydrated at first with the lowest temperature at the slab-mantle interface, several hundreds of degree lower than the wet solidus of hydrated peridotites. The hydrated peridotites may undergo partial melting upon heating at a later time. Therefore, the water flux from the subducting crust into the overlying mantle wedge does not trigger the volcanic arc magmatism immediately.     

Machine learning-based automatic construction of earthquake catalog for reservoir areas in multiple river basins of Guizhou province,China

Large reservoirs have the risk of reservoir induced seismicity. Accurately detecting and locating microseismic events are crucial when studying reservoir earthquakes. Automatic earthquake monitoring in reservoir areas is one of the effective measures for earthquake disaster prevention and mitigation. In this study, we first applied the automatic location workflow (named LOC-FLOW) to process 14-day continuous waveform data from several reservoir areas in different river basins of Guizhou province. Compared with the manual seismic catalog, the recall rate of seismic event detection using the workflow was 83.9%. Of the detected earthquakes, 88.9% had an onset time difference below 1 s, 81.8% has a deviation in epicenter location within 5 km, and 77.8% had a focal depth difference of less than 5 km, indicating that the workflow has good generalization capacity in reservoir areas. We further applied the workflow to retrospectively process continuous waveform data recorded from 2020 to the first half of 2021 in reservoir areas in multiple river basins of western Guizhou province and identified five times the number of seismic events obtained through manual processing. Compared with manual processing of seismic catalog, the completeness magnitude had decreased from 1.3 to 0.8, and a b-value of 1.25 was calculated for seismicity in western Guizhou province, consistent with the b-values obtained for the reservoir area in previous studies. Our results show that seismicity levels were relatively low around large reservoirs that were impounded over 15 years ago, and there is no significant correlation between the seismicity in these areas and reservoir impoundment. Seismicity patterns were notably different around two large reservoirs that were only impounded about 12 years ago, which may be explained by differences in reservoir storage capacity, the geologic and tectonic settings, hydrogeological characteristics, and active fault the reservoir areas. Prominent seismicity persisted around two large reservoirs that have been impounded for less than 10 years. These events were clustered and had relatively shallow focal depths. The impoundment of the Jiayan Reservoir had not officially begun during this study period, but earthquake location results suggested a high seismicity level in this reservoir area. Therefore, any seismicity in this reservoir area after the official impoundment deserves special attention.     

Analysis of Seismic Activity in the Middle Part of the North-South Seismic Belt—Joint Study on Deep Seismic Sounding Profile and Seismicity Parameters

Using the rich deep seismic sounding data recorded in the middle part of the North-South Seismic Belt in China, the horizontal and vertical profiles are constructed to obtain the seismic velocity structure, analyze the seismic distribution and calculate the seismic energy and the thickness of the seismogenic layer at the same time. On this basis, the seismicity parameters are calculated using the earthquake catalogue of the study area for the past 40 years, and the relationship between the b-value distribution and the velocity structure is analyzed. The results show an uneven b-value distribution in the study area and a segmented feature along the Longmenshan fault zone. Most of the earthquakes occur in the transition zone anomalies from the positive to the negative. In addition, the thickness of the crust drops from ~60 km to ~48 km from the Southeastern to the Northeastern Qinghai-Tibetan Plateau, but the thickness of the seismogenic layer increases gradually. It is speculated that the crustal composition of the Northeastern margin contains more felsic materials and has relatively stronger seismic activities than the Southeastern Qinghai-Tibetan Plateau, possibly associated with the subduction and compression of the Indian Ocean Plate.     

漳州盆地及其邻区地壳深部结构的探测与研究

漳州盆地及其邻区地处我国大陆东南沿海地震带中段。通过该地区高分辨率折射及宽角反射，折射地震探测剖面，获得了该区地壳几何结构与速度结构、地壳深浅部断裂的几何形态和构造关系等。结果表明，该区地壳分为上地壳和下地壳。上地壳的厚度为16．5～18．8km，下地壳厚度为12．0～13．0km。上地壳分为上下两部分。在上地壳下部有一个低速层，速度约为6．00km／s，低速层顶面深度为12．0km左右，厚度约为5．0km。下地壳也分为上下两部分。Moho界面的深度为29．0～31．8km。该区6条地壳浅部正断层大部分向地下延伸深度不超过4km，最大延伸深度达5km左右。据推测，浅部正断层下方有一条高倾角地壳深断裂带，该断裂带向下断至Moho面，向上断至上地壳下部低速层中。深浅部断裂构造不相连接。漳州盆地深浅部构造组合特征表明，九龙江断裂带是该区内一条特征明显、具有复杂深浅构造背景的深断裂带。这一深地震探测成果的获得，使得该地区深部资料解释的可靠性和探测精度比以往显著提高；对深浅部构造的组合可作统一解释，地壳的分层和结构特征更为确切和精细；首次发现上地壳的拉张性构造及铲式正断层组合特征，不仅有助于对漳州及其邻区地震危险性的综合判定，而且对深化东南沿海地震带深部动力学过程的认识具有重要意义。     

Deep melting of old subducted oceanic crust recorded by superchondritic Nb/Ta in modern island arc lavas

Niobium–tantalum systematics of slab-derived melts are powerful tracers that discriminate residual high-pressure rutile-bearing eclogite from low-pressure garnet-bearing amphibolite in subducting plates. Previously reported low Nb–Ta ratios in modern slab melts suggested a predominance of shallow melting in the presence of residual amphibole and that deep melting of rutile-bearing eclogitic slabs, devoid of residual amphibole, is volumetrically insignificant. This study evaluates Nb/Ta in combination with other trace element systematics of modern intra-oceanic and slab melt-related arc lavas from the south-western volcanic chain of the Solomon Islands that cover over 1000 km of the SW Pacific plate border. After a change of subduction polarity, an old subducted Pacific slab and a recently subducting Indian–Australian slab are both present beneath the arc. Solomon arc lavas show sub- to superchondritic Nb–Ta ratios (ca. 10 to 27) which is the largest range ever reported in modern island arc lavas. The large range of Nb/Ta likely results from enrichment of the depleted sub-arc mantle by two distinct slab-derived melts in addition to fluids. One minor slab melt component is derived from the shallow and recent subducting Indian–Australian plate where amphibole is still a significant residual phase. The second slab melt component is predominant in Solomon arc lavas and can be attributed to deep rutile–eclogite-controlled melting of old subducted Jurassic Pacific oceanic crust where residual amphibole is entirely absent or insignificant. The deep Pacific slab melt component is the most likely origin of the extremely high and superchondritic Nb/Ta signatures that produce the upper half of the observed range of Nb/Ta in Solomon arc lavas. The slab melt component that enriched the sub-arc mantle with an unusually high Nb/Ta signature is derived from an initially intact Pacific plate that was probably subject to a slab break-off event and subsequent melting at depths exceeding 100 km. The geochemical evidence presented here shows that old and cold subducted oceanic crust, which is initially not torn, may resist shallow melting but can melt at greater depths instead. The resulting slab melts are generated in the presence of residual rutile-bearing eclogite and significantly fractionate Nb–Ta ratios which may be of relevance at a global scale.     

古登堡-里希特定律中的b值统计样本量研究

b值是研究地震活动的重要指标,其广泛应用于地震危险性分析和地震预测研究之中,与实际资料的完整性、样本量的大小、计算方法等因素有着重要的关系。常见的b值计算方法有最小二乘法和最大似然法,样本量的大小对这2种方法影响很大。本文利用蒙特卡罗模拟地震目录和汾渭地震带实际目录作为样本,从中抽取不同大小的样本量进行计算,研究不同样本量下这2种方法计算得到的b值与设定值或真实值之间的差别。结果表明,最小二乘法需要的最低样本量为1000,最大似然法为200;当样本量达不到要求时,计算出的b值是不可靠的;由于对样本量的要求不同,前者适用于计算区域的整体b值,而后者在研究某区域b值在时间轴上的变化方面更有优势。本研究为确定2种b值计算方法对样本量的最低要求提供了参考依据。     

Neotectonics and the Kos Plateau Tuff eruption of 161 ka,South Aegean arc

A high-resolution seismic-reflection survey of the area between Kos and Tilos islands is used to constrain the nature of the Kos Plateau Tuff (KPT) eruption and post-eruptive subsidence. A unique acoustically incoherent unit tens of metres thick at a subbottom depth of 15–35 m is recognised throughout the West Kos basin, which lies between Pachia, Yali and Kos. It commonly unconformably overlies folded, stratified sediment, but in places is concordant with stratified sediment more than 100 m thick. In places south of Kos, the acoustically incoherent unit is overlain by an unconformity and irregularly stratified sediment interpreted as terrestrial or shallow marine. Southeast of Nisyros, a correlative acoustically incoherent unit overlies a planar marine transgression erosion surface that extends almost to Tilos. The stratigraphic level of this unit is dated by comparison with the global eustatic sea-level record and the presence of major transgressive erosion surfaces on adjacent continental shelves, constrained by regional sedimentation rates, and indicates that it is of similar age to the Kos Plateau Tuff eruption. The relationship of this unit to coastal erosion surfaces, and its absence in many areas where seismic-reflection profiles show continuous marine sedimentation, suggests that it is a pyroclastic deposit of subaerial, or at most very shallow marine, origin from the Kos Plateau Tuff eruption. This presence of transgressive unconformities implies that a coastal plain or shallow sea extended southeast of Nisyros to Tilos and the Datça peninsula, and thus it is unlikely that pyroclastic flows crossed large stretches of deep water towards Tilos, as proposed by Allen and Cas (Allen, S.R., Cas, R.A.F., 2001. Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece. Bull. Volcanol. 62, 441-456). Late Pleistocene tectonic subsidence has taken place throughout the whole region between Kos and Tilos, in places at an average rate of at least 3 mm/a.The older dacites of Pyrgousa and Pachia and the post-KPT volcanics of Yali, Strongili and Nisyros lie on a NE–SW-trending lineament marked by major faults and abrupt changes in bathymetry. This NE–SW faulting was initiated in the early Pleistocene and parallels prominent lineaments in the east Cretan Sea. ENE–WSW sinistral strike-slip faulting initiated in the middle Pleistocene in the area from Santorini to Kos would have produced extension on this older lineament, playing a major role in channelling magma to the surface and permitting the ingress of water to the magma conduit.     

Seismic image of the Mount Spurr magmatic system

 The three-dimensional P-wave velocity structure of Mount Spurr is determined to depths of 10 km by tomographic inversion of 3,754 first-arriving P-wave times from local earthquakes recorded by a permanent network of 11 seismographs. Results show a prominent low-velocity zone extending from the surface to 3–4 km below sea level beneath the southeastern flank of Crater Peak, spatially coincident with a geothermal system. P-wave velocities in this low-velocity zone are approximately 20% slower than those in the shallow crystalline basement rocks. Beneath Crater Peak an approximately 3-km-wide zone of relative low velocities correlates with a near-vertical band of seismicity, suggestive of a magmatic conduit. No large low-velocity zone indicative of a magma chamber occurs within the upper 10 km of the crust. These observations are consistent with petrologic and geochemical studies suggesting that Crater Peak magmas originate in the lower crust or upper mantle and have a short residence time in the shallow crust. Earthquakes relocated using the three-dimensional velocity structure correlate well with surface geology and other geophysical observations; thus, they provide additional constraints on the kinematics of the Mount Spurr magmatic system. Received: 4 December 1997 / Accepted: 27 February 1998     

Variation in noble gas isotopic composition of gas samples from the Aegean arc, Greece

In contrast to most other arcs with oceanic plate subduction, the Aegean arc is characterized by continent–continent subduction. Noble gas abundances and isotopic compositions of 45 gas samples have been determined from 6 volcanoes along the arc, 2 islands in the back-arc region and 7 sites in the surrounding areas. The ³He/⁴He ratios of the samples ranged from 0.027R_A to 6.2R_A (R_A denotes the atmospheric ³He/⁴He ratio of 1.4×10⁻⁶), demonstrating that even the maximum ³He/⁴He ratio in the region is significantly lower than the maximum ratios of most oceanic subduction systems, which are equal to the MORB value of 8±1 R_A. Regional variations in the ³He/⁴He ratio were observed both along and across the arc. The maximum ³He/⁴He ratio was obtained from Nisyros volcano located in the eastern end of the arc, and the ratio decreased westward possibly reflecting the difference in potential degree of crustal assimilation or the present magmatic activity in each volcano. Across the volcanic arc, the ³He/⁴He ratio decreased with an increasing distance from the arc front, reaching a low ratio of 0.063R_A in Macedonia, which suggested a major contribution of radiogenic helium derived from the continental crust. At Nisyros, a temporal increase in ³He/⁴He ratio due to ascending subsurface magma was observed after the seismic crisis of 1995–1998 and mantle neon was possibly detected. The maximum ³He/⁴He ratio (6.2R_A) in the Aegean region, which is significantly lower than the MORB value, is not probably due to crustal assimilation at shallow depth or addition of slab-derived helium to MORB-like mantle wedge, but inherent characteristics of the subcontinental lithospheric mantle (SCLM) beneath the Aegean arc.     

马尼拉海沟北段俯冲带输入板块的不均一性

本文整合了横跨马尼拉海沟北段的21条多道地震层位信息、海底地形以及天然地震数据,分析了研究区内的输入板块性质差异及其对增生楔变形和地震活动性的影响.研究发现,沿马尼拉海沟北段的输入板块在地壳性质、基底起伏和沉积物厚度上存在明显的自北向南的差异:(1)最北段基底埋深大,上覆沉积物厚,地壳厚度较薄,地壳性质可能为初始南海洋壳或者圈闭的菲律宾海洋壳;(2)中段基底埋深浅,上覆沉积物薄,地壳厚度大,地壳属性表现为过渡壳性质,受到岩浆活动的影响,初始的地壳性质可能为华南陆块张裂分离出的微小陆壳块体,或者是南海洋壳;(3)南段基底埋深和沉积物厚度介于中间,存在明显的地磁条带,地壳性质为正常的南海洋壳.这一输入板块性质的不均一性可解释该区的特殊增生楔变形现象,如恒春弱变形带的出现,向海方向内凹的海沟形态以及上陆坡海底的大幅抬升等,同时也影响了研究区内的板片俯冲形态和发震构造的地震活动性.研究结果证实了沿马尼拉海沟北段存在南北向的地球物理性质的差异,但对于地壳属性的最终厘定还需要更多的地质与地球化学证据.     

Seismicity of Morocco for the Period 1987–1994

— A seismic data file of 3,740 earthquakes from January 1987 to December 1994 has been elaborated for Morocco and the border regions, with 10 main events registering magnitudes from 5 to 5.6. Such seismicity is particularly important for Morocco as the released seismic energy constitutes a considerable part of the total energy radiated during the 20th century. Relative seismicity maps confirm the persistence of the major features of the seismicity of Morocco. An important seismic activity is observed in the Alboran region continental crust, which absorbs the maximum deformation resulting from the convergence of the African and Iberian plates. However, in the longitude window 3.5°– 6° W at depths of 25 to 50 km, a seismic gap zone seems to take place. An explanation of this phenomenon may be provided by the slab breakoff model. Even if the seismicity of Morocco remains moderate, heavy damage is observed when the magnitude of earthquakes exceeds 4.5, especially in the case of traditional buildings.     

Magma Intrusions and Earthquake Swarm Occurrence in the Western Part of the Bohemian Massif

We are proposing a hypothesis that earthquake swarms in the West Bohemia/Vogtland seismoactive region are generated by magmatic activity currently transported to the upper crustal layers. We assume that the injection of magma and/or related fluids and gases causes hydraulic fracturing which is manifested as an earthquake swarm at the surface. Our statements are supported by three spheres of evidence coming from the western part of the Bohemian Massif: characteristic manifestations of recent geodynamic activity, the information from the neighbouring KTB deep drilling project and from the 9HR seismic reflection profile, and the detailed analysis of local seismological data. (1) Recent manifestations of geodynamic activity include Quaternary volcanism, rich CO ₂ emissions, anomalies of mantle-derived ³ He, mineral springs, moffets, etc. (2) The fluid injection experiment in the neighbouring KTB deep borehole at a depth of 9 km induced hundreds of micro-earthquakes. This indicates that the Earth's crust is near frictional failure in the western part of the Bohemian Massif and an addition of a small amount of energy to the tectonic stress is enough to induce an earthquake. Some pronounced reflections in the closely passing 9HR seismic reflection profile are interpreted as being caused by recent magmatic sills in the crust. (3) The local broadband seismological network WEBNET provides high quality data that enable precise localization of seismic events. The events of the January 1997 earthquake swarm are confined to an extremely narrow volume at depths of about 9 km. Their seismograms display pronounced reflections of P- and S-waves in the upper crust. The analysis of the process of faulting has disclosed a considerable variability of the source mechanism during the swarm. We conclude that the mechanism of intraplate earthquake swarms generated by magma intrusions is similar to that of induced seismicity. As the recent tectonic processes and manifestations of geodynamic activity are similar in European areas with repeated earthquake swarm occurrence (Bohemian Massif, French Massif Central, Rhine Graben), we assume that magma intrusions and related fluid and gas release at depths of about 10 km are the universal cause of intraplate earthquake swarm generation     

The Study on Seismicity and Relocation of the July 3,2015 Pishan MS6.5 Earthquake

The July 3, 2015 Pishan M_S6.5 earthquake occurred in the intersection area of the Tarim block and West Kunlun block where the moderate-strong earthquakes have become active in recent years. This paper has studied the seismicity parameters of the earthquake sequences such as the b-value in the Pishan region and its vicinity. In addition, we also relocated the aftershocks of the Pishan M_S6.5 earthquake using the seismic phase report by the double-difference method. The temporal and spatial variation characteristics of the Pishan earthquake sequence in the rupture zone are analyzed. The study is of great significance in the seismic hazard assessment in this region.