2020年云南巧家M5.0地震震源机制解与震源区应力场研究

基于国家测震台网数据中心提供的波形资料, 采用gCAP方法反演2020年5月18日云南巧家M5.0地震及研究区域51次地震震源机制解, 并收集研究区域震源机制解50个。 采用网格搜索法反演区域构造应力场, 并对研究区域采用不同划分进行应力场反演。 获得以下结论: ① 主震震源机制解节面Ⅰ的走向、 倾角和滑动角分别为175°、 67°和-19°, 节面Ⅱ的走向、 倾角和滑动角分别为273°、 73°和-156°, 矩震级为4.97, 矩心深度为8.8 km。 表明主震属于兼具逆冲分量的走滑型地震; ② 震后区域应力场主压应力轴方位为NWW, 倾角接近水平, 主张应力轴方位为NNE, 倾角接近水平, 属于走滑型应力状态, 与周边地质构造运动状态相吻合; ③ 对研究区域采用不同划分所得应力场结果相差不大, 表明该区域应力场比较稳定, 受深大断裂带和震源机制解类型影响较小。     

Stress Rotation in the Kachchh Rift Zone, Gujarat, India

Double difference relocations of the 1402 Kachchh events (2001–2006) clearly delineate two fault zones viz. south-dipping North Wagad fault (NWF) and almost vertical Gedi fault (GF). The relocated focal depths delineate a marked variation of 4 and 7 km in the brittle-ductile transition depths beneath GF and NWF, respectively. The focal mechanism solutions of 464 aftershocks (using 8–12 first motions) show that the focal mechanisms ranged between pure reverse and pure strike-slip except for a few pure dip-slip solutions. The stress inversions performed for five rectangular zones across the Kachchh rift reveal both clockwise and anticlockwise rotation (7–32°) in the σ₁ orientation within the rupture zone, favoring a heterogeneous stress regime with an average N-S fault normal compression. This rotation may be attributed to the presence of crustal mafic intrusives (5–35 km depth) in the rupture zone of the 2001 Bhuj main shock. Results suggest a relatively homogeneous stress regime in the GF zone favoring strike-slip motion, with a fault normal N-S compression.     

Shallow Seismic Velocity Structure of the Betic Cordillera (Southern Spain) from Modelling of Rayleigh Wave Dispersion

A detailed dispersion analysis of Rayleigh waves generated by local earthquakes and occasionally by blasts that occurred in southern Spain, was undertaken to obtain the shear-wave velocity structure of the region at shallow depth. Our database includes seismograms generated by 35 seismic events that were recorded by 15 single-component short-period stations from 1990 to 1995. All these events have focal depths less than 10 km and body-wave magnitudes between 3.0 and 4.0, and they were all recorded at distances between 40 and 300 km from the epicentre. We analysed a total of 90 source-station Rayleigh-wave paths. The collected data were processed by standard digital filtering techniques to obtain Rayleigh-wave group-velocity dispersion measurements. The path-averaged group velocities vary from 1.12 to 2.25 km/s within the 1.0-6.0 s period interval. Then, using a stochastic inversion approach we obtained 1-D shear-wave velocity–depth models across the study area, which were resolved to a depth of circa 5 km. The inverted shear-wave velocities range approximately between 1.0 and 3.8 km/s with a standard deviation range of 0.05–0.16 km/s, and show significant variations from region to region. These results were combined to produce 3-D images via volumetric modelling and data visualization. We present images that show different shear velocity patterns for the Betic Cordillera. Looking at the velocity distribution at various depths and at vertical sections, we discuss of the study area in terms of subsurface structure and S-wave velocity distribution (low velocity channels, basement depth, etc.) at very shallow depths (0–5 km). Our results characterize the region sufficiently and lead to a correlation of shear-wave velocity with the different geological units features.     

Topography of the Moho and Conrad discontinuities in the Kyushu district,Southwest Japan

The first P-arrival time data from local earthquakes are inverted for two-dimensional variation of the depths to the Conrad and Moho discontinuities in the Kyushu district, southwest Japan. At the same time, earthquake hypocenters and station corrections are determined from the data. The depths to the discontinuities are estimated by minimizing the travel time residuals of first P-arrival phases for 608 earthquakes observed at 57 seismic stations. In the land area of Kyushu, the Conrad and Moho discontinuities are located within the depth ranges of 16–18 and 34–40 km, respectively. The Conrad discontinuity is not as largely undulated as the Moho discontinuity. The depth to the Moho is deep along the east coast of Kyushu, and the deepest Moho is closely related to markedly low velocity of P wave. We regard the deepest Moho as reflecting the Kyushu–Palau ridge subducting beneath the Kyushu district, together with the Philippine Sea slab. In western Kyushu, the shallow Moho is spreading in the north–northeast–south–southwest direction in the Okinawa trough region. Based on the presence of low-velocity anomaly in three-dimensional velocity structure and seismogenic stress field of shallow crustal earthquakes, the shallow Moho is interpreted as being due to lower crustal erosion associated with a small-scale mantle upwelling in the Okinawa trough region. The velocity discontinuity undulation basically has insignificant effect on hypocenter determination of the local earthquakes, but the Moho topography makes changes in focal depths of some upper mantle earthquakes. The depth variation of the Moho discontinuity has a good correlation with the Bouguer gravity anomaly map; i.e., the shallow Moho of western Kyushu and the deep Moho of eastern Kyushu closely correlate with the positive and negative Bouguer gravity anomalies, respectively.     

地形起伏对基于地震波形的浅源地震深度反演影响——以2017年9月3日朝鲜M6.3事件为例

震源深度是核试验以及塌陷等浅源地震研究中的关键参数,可以为事件成因分析提供关键信息.然而朝鲜核试验区域地形起伏较大,地形效应可能对震源深度反演的结果造成影响.本文基于理论地震图进行测试,研究了地形起伏对震源深度反演的影响.发现震源深度小于2km时,不考虑地形影响,反演得到的震源深度会系统偏浅0.2km左右.然后利用MDJ2速度结构模型,我们反演了2017年9月3日朝鲜M6.3事件的震源参数,结果显示震源深度约为0.8km.进一步基于带地形的格林函数重新反演了该事件的震源深度,发现在1km处波形拟合结果较好.不同速度模型测试结果显示该事件的震源深度反演误差约为1km.案例研究表明,基于层状均匀速度模型,利用区域地震波形资料反演的震源深度可以为浅源事件成因分析提供关键约束.     

New data on the Indus Kohistan seismic zone and its extension into the Hazara–Kashmir Syntaxis, NW Himalayas of Pakistan

This paper deals with the data obtained from local networks in northern Pakistan for 251 earthquakes of magnitude ≥4.0 for October 8, 2005 to December 31, 2006 period. The study presents focal mechanism solutions (FMS) of 12 pre- (1904–2005) and 17 post- (October 8, 2005–December, 2005) Muzaffarabad Earthquake, their detailed tectonic interpretation, and correlation with surface evidence of co-seismic rupture with published synthetic aperture radar data. Distribution of landslides obtained from National Engineering Services of Pakistan and the earthquake damages are also discussed. Aftershock distribution, which is more prominent in the crystalline zone (northwest of Muzaffarabad), defines a 50-km-wide NW–SE trending zone that extends for 200 km from the main mantle thrust to the center of the Hazara–Kashmir Syntaxis. The FMS of the main shock and 16 aftershocks having magnitude ≥4.0 indicate thrusting to be the dominant mechanism with rupture planes having NW–SE trend and NE dip. In addition, 12 FMS of pre-Muzaffarabad Earthquake (1904–2004) from the same area have been determined and results are compared. This leads to the conclusion that the wedge-shaped NW–SE trending blind zone, referred to by earlier workers as the Indus Kohistan Seismic Zone (IKSZ), has been activated during the Muzaffarabad earthquake. The right-lateral component in all FMS, supported by the surface evidences, suggests the involvement of Balakot–Bagh Fault (BBF). We propose that the IKSZ is the source of the October 8, 2005 Muzaffarabad earthquake that reactivated the BBF. Furthermore, the IKSZ does not end at the nose of the syntaxis but extends further southeast of it. Tectonic complexity seems to be due to a variety of factors. Also, thrust and reverse solutions near the northern collisional boundary (main mantle thrust) have mostly NE/SW-directed P-axis orientations. From the detailed FMS analysis, three conclusions have been drawn: (1) Shallow events (depth ≤10 km) with prominent strike slip solutions (earlier earthquakes) are associated with the surface strike slip faults (e.g., Muzaffarabad Fault) and/or the Besham domal structure; (2) moderate depth events (depth 10–25 km) with thrust/reverse solutions but having minor right-lateral strike slip component (all Muzaffarabad earthquakes and two earlier) are associated with the IKSZ; (3) deeper earthquakes (depth below IKSZ) with pure thrust/reverse solutions may be related to the under-thrusting of the Indian plate beneath the IKSZ, which represents a major thrust zone. Imbricate thrusting and breaking and thickening of the crust are considered to be caused by steep bending of the under-thrusting plate at the collisional boundary. An erratum to this article can be found at     

FOCAL MECHANISM SOLUTIONS AND STRESS FIELD OF THE 2019 CHANGNING,SICHUAN MAINSHOCK AND ITS MODERATE-STRONG AFTERSHOCKS(MS≥4.0)

A M_S6.0 earthquake with shallow focal depth of 16km struck Changning County, Yibin City, Sichuan Province at 22:55: 43(Beijing Time)on 17 June 2019. Although the magnitude of the earthquake is moderate, it caused heavy casualties and property losses to Changning County and its surrounding areas. In the following week, a series of aftershocks with M_S≥4.0 occurred in the epicentral area successively. In order to better understand and analyze the seismotectonic structure and generation mechanism of these earthquakes, in this paper, absolute earthquake location by HYPOINVERSE 2000 method is conducted to relocate the main shock of M_S6.0 in Changning using the seismic phase observation data provided by Sichuan Earthquake Administration, and focal mechanism solutions for Changning M_S6.0 main shock and M_S≥4.0 aftershocks are inferred using the gCAP method with the local and regional broadband station waveforms recorded by the regional seismic networks of Sichuan Province, Yunnan Province, Chongqing Municipality, and Guizhou Province. The absolute relocation results show that the epicenter of the main shock is located at 28.35°N, 104.88°E, and it occurred at an unusual shallow depth about only 6.98km, which could be one of the most significant reasons for the heavier damage in the Changning and adjoining areas. The focal plane solution of the Changning M_S6.0 earthquake indicates that the main shock occurred at a thrust fault with a left-lateral strike-slip component. The full moment tensor solution provided by gCAP shows that it contains a certain percentage of non-double couple components. After the occurrence of the main shock, a series of medium and strong aftershocks with M_S≥4.0 occurred continuously along the northwestern direction, the fault plane solutions for those aftershocks show mostly strike-slip and thrust fault-type. It is found that the mode of focal mechanism has an obvious characteristic of segmentation in space, which reflects the complexity of the dislocation process of the seismogenic fault. It also shows that the Changning earthquake sequences occurred in the shallow part of the upper crust. Combining with the results from the seismic sounding profile in Changning anticline, which is the main structure in the focal area, this study finds that the existence of several steep secondary faults in the core of Changning anticline is an important reason for the diversity of focal mechanism of aftershock sequences. The characteristics of regional stress field is estimated using the STRESSINVERSE method by the information of focal mechanism solutions from our study, and the results show that the Changning area is subject to a NEE oriented maximum principal stress field with a very shallow dipping and near-vertical minimum principal stress, which is not associated with the results derived from other stress indicators. Compared with the direction of the maximum principal compressive stress axis in the whole region, the direction of the stress field in the focal area rotates from the NWW direction to the NEE direction. The Changning M_S6.0 earthquake locates in the area with complex geological structure, where there are a large number of small staggered fault zones with unstable geological structure. Combining with the direction of aftershocks distribution in Changning area, we infer that the Changning M_S6.0 earthquake is generated by rupturing of the pre-existing fault in the Changning anticline under the action of the overall large stress field, and the seismogenic fault is a high dip-angle thrust fault with left-lateral strike-slip component, trending NW.     

四川木里小震群活动特征研究

本文通过对四川木里地区地震震源位置的重新确定,反演了较大地震震源机制解,结果显示：①重定位后的小震群震中分布成带状,地震震源深度分布在0—12km范围内;②经过对震群空间分布进行仔细分析,认为其发震构造是小金河断裂西侧的一条NWW向分支断裂;③3次4.0级以上地震震源参数存在明显差异,浅源逆冲事件表现有受垂直方向应力（重力）作用的特征,走滑事件表现出与区域应力构造活动有关。     

长宁地区复杂地震活动的力学成因分析

长宁地区地震活动复杂,发生在长宁背斜轴部的长宁Ms6.0地震及其后续中强余震的震源机制与相距仅十几公里的南部向斜区内的中强震具有显著差异;这种差异仅仅是由于发震断层构造不同所造成的,还是孕育地震的应力场本身也存在局部变化?为了回答这一问题,本文在通过双差层析成像反演修正小震定位和波速结构的基础上,利用小震综合震源机制解...     

新疆北天山中东段呼图壁地区震源深度的重新测定

联合Hyposat法、PTD法和gCAP矩张量反演法,重新测定新疆北天山中东段呼图壁地区2010—2017年502个地震的震源深度,并对震源深度剖面进行初步分析。结果表明,重新测定的震源深度优势分布为15—20km,平均震源深度为16km,呼图壁M_S 6.2地震的震源深度为20km;研究区南部和中部的震源深度集中分布在20km左右,与北天山壳内低速体的层位相当,可能是上地壳和下地壳之间的韧性剪切带存在的部位,起到滑脱层的作用,研究区北部的震源深度则向浅部扩展;呼图壁M_S 6.2地震的发震断裂可能在清水河子断裂下方的1条隐伏反冲断层上,可能是霍尔果斯断裂向前沿断坡冲断受阻而在相反方向上发育分支反冲断层的结果。     

2010年玉树地震序列ML≥4.0事件震源机制解特征分析

采用CAP方法反演2010年玉树7.1级地震序列前震、主震及余震19个M_L≥4.0事件的震源机制解,19个结果以走滑类型为主,前震、主震的震源机制解十分接近,反映出前震、主震之间密切的联系;震源深度集中在7~12 km,震源最浅（4.5 km）与最深（34 km）的两个余震事件具有明显的逆冲性质,表现出明显的边界特征;19个事件的震中分布在甘孜-玉树断裂北支玉树-隆宝断裂上,目前已经证明该断裂即为玉树地震的发震构造。自SE-NW沿玉树-隆宝断裂走向拉一剖面,观察震源深度沿剖面的变化情况,可看出玉树-隆宝断裂西北段震源深度要大于东南段,该段主要是余震活动的中后期,因此在地震活动的中后期,余震向地壳深部扩展,断裂累积的应变能得到更进一步的释放;P轴方位角优势分布集中在220°~230°,T轴方位优势分布集中在310°~320°,两个优势分布互相垂直性与单个事件的沙滩球应力轴一样,说明玉树地震的震源机制解类型较为简单;玉树周边地区应力场分布比较均匀,并不像汶川周边地区那么复杂,本次玉树地震为巴颜喀拉地块与羌塘块体边界处甘孜-玉树断裂应变能量的正常释放。     

四川长宁MS6.0地震区域构造应力场特征分析

2019年6月17日在四川宜宾市长宁县（28.34°N,104.90°E）发生M_S6.0地震,余震发育。本文利用区域测震台网的地震观测数据基于CAP方法计算了28°~29°N,104°~105°E范围内的14个M_S>3.0以上地震的震源机制解,结合全球矩心矩张量目录和部分前人研究结果中该区域的共27个震源机制解数据,应用MSATSI软件反演了研究区域的应力场。将研究区域按0.1°×0.1°划分成25个应力网格,最终得到9个网格的应力分布结果,大多数应力场方向稳定,根据主震所在应力网格点得到主震的断层类型为主逆冲型。本文研究成果为四川长宁地区的孕震机理、活动构造以及地震趋势判定提供了可靠的参考依据。     

Active tectonics of the outer northern Apennines: Adriatic vs. Po Plain seismicity and stress fields

The recent earthquake sequences of 2012 (northern Italy) and 2013 (Marche offshore) provided new, fundamental constraints to the active tectonic setting of the outer northern Apennines. In contrast to the Po Plain area, where the 2012 northern Italy earthquakes confirmed active frontal thrusting, the new focal mechanisms obtained in this study for the 2013 Marche offshore earthquakes indicate that only minor thrust fault reactivation occurs in the Adriatic domain, even for a theoretically favourably oriented maximum horizontal compression. Recent seismicity in this domain appears to be mainly controlled by transcurrent crustal faults dissecting the Apennine thrust belt. The along-strike stress field variation from the Po Plain to the Adriatic area has been quantitatively investigated by applying the multiple inverse method (MIM) to the analysis of the entire seismicity recorded from January 1976 to August 2014, from the top 12 km of the crust (fault plane solutions from 127 earthquakes with M_W ≥ 4), allowing us to obtain a comprehensive picture of the state of stress over the outer zone of the fold and thrust belt. The present-day stress field has been defined for 39 cells of 1.5° × 1.5° surface area and 12 km depth. The obtained stress field maps point out that, although the entire outer northern Apennines belt is characterized by a sub-horizontal maximum compressive axis (σ₁), the minimum compression (σ₃) is sub-vertical only in the Po Plain area, becoming sub-horizontal in the Adriatic sector, thus confirming that the latter region is dominated by an active tectonic regime of strike-slip type.     

龙滩水库蓄水后库区中小地震震源机制

本文在对龙滩库区2006年9月30日至2013年5月26日发生的3682次地震进行精定位的基础上,利用FOCMEC方法和改进的格点尝试法反演了ML2.0以上地震震源机制及区域构造应力场,并综合龙滩库区地层岩性、断裂构造和渗透条件等资料,探讨了地震活动类型与库区蓄水过程的关系,获得以下认识:(1)龙滩水库蓄水后的地震活动主要丛集在罗妥、八茂、拉浪、坝首和布柳河5个深水区,地震类型以逆断为主,正断和走滑也占有一定比例.蓄水初期,库区地震类型呈现多样性,蓄水约4年3个月后,地震主要发生在浅部地层中,并大多为逆断型地震.(2)5个地震丛构造应力场最大主应力方向以NW—SE为主,倾角均较小,中等和最小主应力分布较凌乱,在此构造应力环境中,龙滩库区主要断裂整体上呈现走滑运动的性质,局部伴生不同程度的逆倾滑或正倾滑运动分量.(3)库区深、浅部地震活动水平和地震性质之所以会随蓄水过程发生变化,可能与深、浅部构造应力环境、岩体力学性质和渗透性能的差异有关.     

An Assessment of the Seismicity of the Bursa Region from a Temporary Seismic Network

A temporary earthquake station network of 11 seismological recorders was operated in the Bursa region, south of the Marmara Sea in the northwest of Turkey, which is located at the southern strand of the North Anatolian Fault Zone (NAFZ). We located 384 earthquakes out of a total of 582 recorded events that span the study area between 28.50–30.00°E longitudes and 39.75–40.75°N latitudes. The depth of most events was found to be less than 29 km, and the magnitude interval ranges were between 0.3 ≤ M_L ≤ 5.4, with RMS less than or equal to 0.2. Seismic activities were concentrated southeast of Uludag Mountain (UM), in the Kestel-Igdir area and along the Gemlik Fault (GF). In the study, we computed 10 focal mechanisms from temporary and permanents networks. The predominant feature of the computed focal mechanisms is the relatively widespread near horizontal northwest-southeast (NW–SE) T-axis orientation. These fault planes have been used to obtain the orientation and shape factor (R, magnitude stress ratio) of the principal stress tensors (σ₁, σ₂, σ₃). The resulting stress tensors reveal σ₁ closer to the vertical (oriented NE–SW) and σ₂, σ₃ horizontal with R = 0.5. These results confirm that Bursa and its vicinity could be defined by an extensional regime showing a primarily normal to oblique-slip motion character. It differs from what might be expected from the stress tensor inversion for the NAFZ. Different fault patterns related to structural heterogeneity from the north to the south in the study area caused a change in the stress regime from strike-slip to normal faulting.     

汶川Mw7.9和集集Mw7.6地震前应力场转换现象及其可能的前兆意义

2008年四川汶川Mw7.9地震和1999年台湾集集Mw7.6地震均为挤压推覆构造环境下发生的板内逆断层型地震.通过对比分析2次地震前的CMT解、震源区附近的中小地震震源机制解及其反演的应力场可知,集集地震主震震源机制解与用台湾内陆中西部的CMT解反演得到的逆断层类型构造应力场吻合,而在主震前震源区附近中小地震震源机制...     

滇缅活动地块构造应力场研究

本文收集了1976—2017年滇缅活动地块98个M_W 4.8—7.0地震的震源机制解,分析震源机制解和震源深度的空间分布特征,探讨了其构造动力学背景。结果表明:①滇缅活动地块震源深度优势分布范围为10—30km,90%以上的地震震源深度小于30km,结合研究区统计时段内地震震源深度、优势度、众数等参数,推断滇缅活动地块及周边震源深度的下界为30km,脆性多震层位于10—30km,且主要位于15km附近;②滇缅活动块体不同断裂带、块体内部各次级块体之间、块体内外表现出不同的震源机制解,在空间上存在着明显的分区性特征,揭示出位于青藏高原东南缘的滇缅活动块体及周边地区应力场的非均匀性;③滇缅活动地块区域构造应力场明显受周边板块作用的控制,活动地块内部由于构造格局及其运动的差异,应力状态具有明显的区域特征。根据研究区各主要断裂带所反映的与构造背景作用一致的震源机制分布特征,可以将滇缅活动地块初步分为3个应力区。     

2004年东乌珠穆沁旗地震震源参数研究

利用波形反演方法, 研究并确定了2004年3月24日发生在内蒙古东乌珠穆沁旗与西乌珠穆沁旗交界处地震的震源机制及深度。 近震波形对震源机制有很好的约束, 而远震体波对于震源深度分辨率很高, 因此综合利用了近震和远震波形数据。 对于近震宽频带波形记录, 用CAP方法反演震源机制解。 利用远震直达P波、 pP波和sP波的到时能较好的确定震源深度。 综合利用近震宽频带波形和远震体波波形记录, 反演得出2004年03月24日地震为逆冲型, 震级为M_W5.3。 其两个节面分别为: 节面I的走向、 倾角、 滑动角分别为147°、 22°、 87°, 节面Ⅱ的走向、 倾角、 滑动角分别为 330°、 68°、 91°; 震源深度为(12±2) km, 是一个典型的浅源上地壳地震。 与前震序列对比, 节面Ⅰ是可能的发震构造。     

2019年四川长宁MS6.0地震序列重定位和震源特征分析

采用双差定位方法对2019年1月1日至2019年10月20日期间四川区域台网记录到的地震进行重定位,得到7 030个重定位事件,并获得了四川长宁M_S6.0地震序列较准确的空间分布,并据此计算了震后长宁震源区的平均b值,分析了地震序列的活动性;利用近震全波形拟合方法获得了主震及4次M_S≥5.0地震的震源机制解和矩心深度,初步分析了本次地震序列的发震构造,获得如下主要结果:① 四川长宁余震序列呈NW?SE向分布,余震深度分布整体呈现出西深东浅的趋势,且西部地区地震的频度远远高于东部地区;② b值空间分布显示,震后长宁地区呈现出明显的挤压构造环境;③ 主震和4次震级较大余震的矩心深度均较浅,尽管均为逆冲型为主的地震事件,但破裂面走向有所差异;④ 推测主震及中强余震是长宁背斜地区既有断裂或者同震过程中所产生的新生断层长期受到外力挤压而错断所致。      

Seismic swarms, fault plane solutions, and stress tensors for S?o Miguel Island central region (Azores)

The central region of S?o Miguel Island is one of the most seismically active areas of the Azores archipelago. A revised analysis of the seismicity distribution at this region has, for the first time, shown that the seismicity is clustered in two distinct areas: the area around Fogo Volcano (Fogo) and the area around Congro maar (Congro), with each area having a highly localized swarm activity. From a total of about 15,000 events in the period from 2002 to 2010, 78 best located events were selected to make fault plane solutions using P-wave polarities and amplitude ratios. This set of fault plane solutions, and another six subsets derived from it, were inverted for the best fitting stress tensor. The stress tensor using all the 78 fault plane solutions is characterized by a subhorizontal ??1 striking WNW-ESE and a ??3 striking NNE-SSW, consistent with the regional stress field for this region. A similar result, using only the fault plane solutions located in the Fogo area, was obtained. On the other hand, for the Congro area, a local stress field seems to be superimposed on the regional field: subhorizontal ??3, striking NNE-SSW, and a near-vertical ??1. The same stress regime persists in the first 5?km depth, probably related to the upwelling of thermal fluids. The rising fluids generate horizontal extension at shallow depths, which favour the opening of cracks and the circulation and ascension of hydrothermal fluids. The stress regime deeper than 5?km is more uncertain; however, it is indicative of a compressional regime. Thus, it can be conclude that the smaller Fogo area appears to be dominated by the normal regional stress field while the high active Congro area seems to have a different, highly heterogeneous stress field dominated by local conditions.