The 2002 Molise earthquake sequence: What can we learn about the tectonics of southern Italy?

On October 31, 2002 a M_L = 5.4 earthquake occurred in southern Italy, at the margin between the Apenninic thrust belt (to the west) and the Adriatic plate (to the east). In this area, neither historical event nor seismogenic fault is reported in the literature. In spite of its moderate magnitude, the earthquake caused severe damage in cities close to the epicenter and 27 people, out of a total of 29 casualties, were killed by the collapse of a primary school in S. Giuliano di Puglia. By inverting broadband regional waveforms, we computed moment tensor solutions for 15 events, as small as M_L = 3.5 (M_w = 3.7). The obtained focal mechanisms show pure strike-slip geometry, mainly with focal planes oriented to NS (sinistral) and EW (dextral). In several solutions focal planes are rotated counterclockwise, in particular for later events, occurring west of the mainshock. From the relocated aftershock distribution, we found that the mainshock ruptured along an EW plane, and the fault mechanisms of some aftershocks were not consistent with the mainshock fault plane. The observed stress field, resulting from the stress tensor inversion, shows a maximum principal stress axis with an east–west trend (N83°W), whereas the minimum stress direction is almost N–S. Considering both the aftershock distribution and moment tensor solutions, it appears that several pre-existing faults were activated rather than a single planar fault associated with the mainshock. The finite fault analysis shows a very simple slip distribution with a slow rupture velocity of 1.1 km/s, that could explain the occurrence of a second mainshock about 30 h after. Finally, we attempt to interpret how the Molise sequence is related to the normal faulting system to the west (along the Apennines) and the dextral strike-slip Mattinata fault to the east.     

2022年1月8日青海门源MS 6.9地震地表破裂考察的初步结果及对冷龙岭断裂活动行为和区域强震危险性的启示

2022年1月8日青海门源M_S 6.9地震发生在青藏高原东北缘的祁连山断块内部,仪器震中位于海原活动断裂系西段的冷龙岭断裂带上,是该断裂系自1920年海原8.5级大地震后再次发生M>6.5的强震。考察结果的初步总结表明,此次门源地震产生了呈左阶斜列分布、总长度近23 km的南北两条破裂,在两者之间存在长约3.2 km、宽近2 km的地表破裂空区。南支破裂(F₁)出现在托来山断裂的东段,走向91°,长约2.4 km,以兼具向南逆冲的左旋走滑变形为主,最大走滑位移近0.4 m。北支主破裂(F₂)出现在冷龙岭断裂的西段,总长度近20 km,以左旋走滑变形为主,呈整体微凸向北东的弧形展布,包含了走向分别为102°、109°和118°的西、中、东三段,最大走滑位移出现在中段,为3.0±0.2 m。此外,在北支主破裂中—东段的北侧新发现一条累计长度约7.6 km、以右旋正断为主的北支次级破裂(F₃),累计最大走滑量约0.8 m,最大正断位移约1.5 m。综合分析认为,整个同震破裂以左旋走滑变形为主,具有双侧破裂特点,宏观震中位于北支主破裂的中段,其地表走滑位移很大可能与震源破裂深度浅有关,其中的右旋正断次级破裂可能是南侧主动盘向东运移过程中拖曳北侧块体发生差异运动所引起的特殊变形现象。印度与欧亚板块近南北向强烈碰撞挤压导致南祁连断块沿海原左旋走滑断裂系向东挤出,从而引发该断裂系中的托来山断裂与冷龙岭断裂同时发生破裂,成为导致此次强震的主要动力机制。在此大陆动力学背景下,以海原左旋走滑断裂系为主边界的祁连山断块及其周边的未来强震危险性需得到进一步重视。      

2017年8月8日四川九寨沟7.0级地震发震构造浅析

2017年8月8日21时19分,四川阿坝州九寨沟县发生7.0级地震,震中位于巴颜喀拉块体东边界虎牙断裂和东昆仑断裂带东段塔藏断裂交汇区域,地震构造背景较为复杂。地震导致了房屋和道路破坏、滑坡崩塌。根据高分辨率卫星影像解译、阶地坎变形的测量和测年数据得到：塔藏断裂东段晚第四纪以来以左旋走滑为主,兼逆分量,水平滑动速率为2.7~4.1 mm/yr,垂直滑动速率为0.56~0.6 mm/yr。结合此次地震的主余震分布、主震震源机制解等综合结果,初步建立了三维发震构造模型,分析认为此次地震属于走滑型地震,主破裂倾角57°~77°,发震断层可能是塔藏断裂的一条分支,是青藏高原块体向东推挤的一次地震事件。基于历史地震、活动断裂和形变观测方面的研究,巴颜喀拉块体具备显著的强震构造背景,对于该块体边界带周缘的强震活动和变形需要继续关注。     

2017年四川九寨沟Ms7.0地震InSAR同震形变场及发震构造探讨

2017年8月8日四川省九寨沟县发生M_s7.0级地震,构造部位处于青藏高原东缘的巴颜喀拉地块东北角,震中位置是岷江断裂、塔藏断裂、虎牙断裂和雪山梁子断裂围闭的空震区。哪条断裂发震,如何界定其与周边活动断裂的关系,与青藏高原东缘近年来发生的大地震是否有成因联系等问题对于理解该区域现今构造活动模式、预判地震发展趋势和部署地震地质灾害防控等工作具有重要意义。利用地震前后两期Sentinel-1合成孔径雷达数据对地表同震形变场进行了InSAR测量,获取了极震区约2000 km²范围内的雷达视线向变形（-13~28 cm）和运动方向,呈现为主动盘单侧走滑兼逆冲的变形模式,结合震源机制、断裂展布、构造背景和近年地震迁移的分析,揭示了控震构造是巴颜喀拉地块北缘边界断裂弧形旋转体系的尾端构造,发震断层是该断裂系中塔藏断裂的南段,并有与虎牙断裂贯通的趋势,因此,应重视本次地震与虎牙断裂之间的空震区未来的强震危险性问题;从区域上看,此次九寨沟地震可能与汶川地震具有一定的时空成因联系,因在巴颜喀拉地块南北边界断裂破裂基本贯通的条件下,2008年汶川地震诱发的东缘中部锁固破裂导致块体加速向东挤出,2013年鲁甸地震又释放了东缘南段挤压构造应力,从而进一步加剧了东北角的应力集中,促使九寨沟地震的发生。     

Seismotectonics of the 26 November 2005 Jiujiang-Ruichang, Jiangxi, Ms?5.7 Earthquake

The 26 November 2005 Jiujiang-Ruichang, Jiangxi, Ms?5.7 earthquake occurred in a seismotectonic setting of moderate earthquake. The northwest-trending Xiangfan-Guangji fault (XFG) does not enter into the epicenter vicinity, but the northeast-trending Ruichang-Wuning fault (RWF) as a regional fault extends to the epicenter nearby, appearing as the Ruichang basin and its marginal faults. Tilting of the Ruichang Basin (RCB) in the Quaternary was controlled by the RCB southeast-marginal, buried fault (RSMBF). Shallow geophysical survey reveals that the RSMBF caused an offset of the reflection layers. Drill hole columnar section demonstrates that there are about 10–12?m displacement in the lower section of the middle-Pleistocene Series along the RSMBF, but no disruption is found in the upper section of the middle-Pleistocene Series. The RSMBF not only has activity in the Quaternary, but also coincides with the nodal plane I from the focal mechanism of the Jiujiang-Ruichang Ms?5.7 earthquake. This evidence, including aftershock distribution and isoseismic lines, strongly suggests that the RSMBF might be the seismogenic tectonics. The RWF is discontinuous at the surface, and consists of three en echelon Quaternary basins, which are the Ruichang, Fanzhen and Wuning basins. Three moderate earthquakes, the Fanzhen ML?4.9 earthquake, the Yejiapu ML?4.1 earthquake and the Jiujiang-Ruichang Ms?5.7 earthquake, have happened in the basins since 1995. The seismogenic tectonics of the Jiujiang-Ruichang Ms?5.7 earthquake is not isolated, but may be controlled by the RWF at depth, the slip of which causes the accumulation of energy for earthquake occurrence.     

Kamchatka earthquake of 17 August 1983: A large intermediate event at the junction of the Aleutian and Kuril-Kamchatka arcs

This paper gives macroseismic and instrumental data on the 17 August 1983 Kamchatka earthquake which occurred in the center of the Kamchatka Gulf bend at a depth of 98 km with an epicentral intensity of VI–VII (MSK-64 scale), energy class 15, and magnitude MLH = 6.9. The focal mechanism represents a thrust along the inclined surface across the strike of the Kamchatka Gulf coastal line. The Primary-wave seismic moment M ₀ is 6.3 × 10¹⁹ Nm, the Rayleigh wave M ₀ is 1.6 × 10¹⁹ Nm, and the stress drop is 2.5 MPa. Copies of displacement and acceleration records are presented and the temporal and spatial distribution of the aftershocks is analyzed.     

Tsunami magnitudes in Taiwan, the Philippines and Indonesia

The regional characteristics of tsunami magnitudes in the SE Asia region are discussed in relation to earthquake magnitudes during the period from 1960 to 1994. Tsunami magnitudes on the Imamura-Iida scale are investigated by the author's method (Hatori 1979, 1986) using the data of inundation heights near the source area and tide-gauge records observed in Japan. The magnitude values of the Taiwan tsunamis showed relatively to be small. On the contrary, the magnitudes of tsunamis in the vicinities of the Philippines and Indonesia exceed more than 1–2 grade (tsunami heights: 2–5 times) compared to earthquakes with similar size on the circum-Pacific zone. The relation between tsunami magnitude, m, and earthquake magnitude, M _s, is expressed as m = 2.66 M _s– 17.5 for these regions. For example, the magnitudes for the 1976 Mindanao tsunami (M _s= 7.8, 3702 deaths) and the 1992 Flores tsunami (M _s= 7.5, 1713 deaths) were determined to be m = 3 and m = 2.5, respectively. The focal depth of tsunamigenic earthquakes is shallower thand< 36 km, and the detectively of tsunamis is small for deep earthquakes being d > 40 km. For future tsunamis, it is indispensable to take precautions against shallow earthquakes having the magnitudes M _s> 6.5.     

2013年云南香格里拉、德钦-四川得荣交界5.9级地震灾害及发震构造

2013年8月28日、31日, 云南迪庆藏族自治州香格里拉县、德钦县、四川省甘孜藏族自治州得荣县交界地区连续发生5.1级、5.9级地震.为了查明此次地震的影响破坏程度, 进行了地震现场建筑物震害考察并对震中附近断裂进行了野外构造地质剖面调查.两次地震在短时间内并在相近位置连续发生, 造成了此次云南香格里拉、德钦-四川得荣交界地震比以往同级地震的破坏程度要高, 地震烈度最高为Ⅷ级, 有感范围大, 5.9级地震宏观震中大致处在整个灾区破坏最严重的奔子栏镇争古村一带(28.20°N, 99.36°E), 距离地震微观震中约5.1km.等震线沿德钦-中甸断裂呈北西向分布, 近似为椭球状, 结合此次地震震中附近区域现场断裂调查、震源机制解数据以及地震余震空间分布特征, 初步推断此次地震的发震构造为德钦-中甸断层, 其主要表现为一次以正断为主兼有左旋走滑错动的地震事件.      

Local source tomography: applications to Italian areas

We describe three study cases in which we used local earthquake and shot travel-time residuals to investigate the upper crustal structure of three regions in Italy. We inverted for velocity and hypocentral parameters using a damped least-squares technique making use of parameter (velocity and hypocentre) separation. The three studied regions are in Italy, namely (a) the Vulsinian Volcanic Complex (Latium), where there is an active geothermal field; (b) the Irpinia (Campania–Lucania) region, in the Southern Appennines, site of the strongest earthquake in Italy for at least 65 years (November 1980, M_s= 6.9); (c) the Friuli region, in Northeastern Italy, where another strong earthquake (M_s= 6.5) occurred in 1976. The computed shallow velocity models generally correspond with surface geological structures. For the three studied areas, the main results are, respectively: (a) A low-velocity anomaly detected in the centre of the Vulsinian Volcanic Complex at a depth of 5–8 km, probably due to anomalous heat flow caused by a partially molten or cooling intrusive body; (b) the identification of a deep (10 km) discontinuity in the crust beneath the Irpinia fault zone, approximately corresponding with the fault extension at depth; (c) the detection of a wedge of high-velocity, high density material at seismogenic depth (5–10 km) beneath the Friuli region, interpreted as a buried thrust of the metamorphic basement.     

A study of the 2 December 2002 (M5.5) Vartholomio (western Peloponnese, Greece) earthquake and of its largest aftershocks

Broadband data from the Greek National Seismological Network are used to study the moderate size (M5.5) earthquake, which occurred on 2 December 2002 near the town of Vartholomio, in western Peloponnese (Greece). Time domain moment tensor inversion applied to retrieve the focal mechanism of the mainshock and of three of the larger aftershocks of the sequence, revealed almost pure strike-slip faulting along NW–SE or NE–SW trending nodal planes. The relative source time functions for the mainshock, obtained from an empirical Green's function analysis, do not reveal any clear directivity to any of the stations. A careful observer might suggest directivity towards NW, if any. Optimum values are 0.4 s for the rise time and 2.7 km/s for the rupture velocity. The spatial and temporal distribution of fault slip showed that the major part of the resolved slip occurred beneath the mainshock's epicenter, 20 km underneath the western coast of Peloponnese. This probably accounts for the considerable damage observed to the nearby towns. The resolution between the two nodal planes does not permit an identification of the fault plane; however the statistics on the slip distribution model, the preliminary analysis of aftershock locations and macroseismic data favour the NW–SE trending plane as the fault plane, which is connected with sinistral strike-slip motions. These are the first implications for sinistral strike-slip motions in this area and more data are needed in the future to get more reliable resolution of the motions.     

2012年缅甸德贝金M7.0地震及其地表破裂特征

文章以地质地貌与地震遗迹野外调查获得的第一手资料为基础,重点介绍了实皆断裂的活动习性、2012年地震产生的建筑物破坏与地震地表破裂带特征.实皆断裂是一条规模宏大,以右旋走滑为主的全新世活动断裂,其水平滑动速率为18～20 mm/a.历史上沿实皆断裂曾发生10余次7级以上强震,迄今保留有1839年曼德勒因瓦M 8、193...     

The 1905 Chamonix earthquakes: active tectonics in the Mont Blanc and Aiguilles Rouges massifs

Linking earthquakes of moderate size to known tectonic sources is a challenge for seismic hazard studies in northwestern Europe because of overall low strain rates. Here we present a combined study of macroseismic information, tectonic observations, and seismic waveform modelling to document the largest instrumentally known event in the French northern Alps, the April 29, 1905, Chamonix earthquake. The moment magnitude of this event is estimated at M_w 5.3 ± 0.3 from records in Göttingen (Germany) and Uppsala (Sweden). The event of April 29 was followed by several afterschocks and in particular a second broadly felt earthquake on August 13, 1905. Macroseismic investigations allow us to favour a location of the epicentres 5–10 km N–NE of Chamonix. Tectonic analysis shows that potentially one amongst several faults might have been activated in 1905. Among them the right lateral strike-slip fault responsible for the recent 2005 M_w = 4.4 Vallorcine earthquake and a quasi-normal fault northeast of the Aiguilles Rouges massif are the most likely candidates. Discussion of tectonic, macroseismic, and instrumental data favour the normal fault hypothesis for the 1905 Chamonix earthquake sequence.     

Analysis of macroseismic and instrumental data for the study of the 19 November 1923 earthquake in the Aran Valley (Central Pyrenees)

The 19 November 1923 earthquake in the Aran Valley (Central Pyrenees), with observed maximum intensityI _max = VIII (MSK), has been studied through the compiling and reviewing of macroseismic information and collecting and processing early seismograms. Analysis of macroseismic data gives a focal depth ofh = 5 km and an anelastic attenuation coefficient 10^–3 km^–1. Analysis of early instrumental records allows the computation of estimate of magnitude (M _L = 5.6) and seismic moment (M _o = 1.1 × 10¹⁷ N × m) which are consistent with the values ofI _o andh obtained from macroseismic data.     

Fault movements and stress accumulation before the 1976 Tangshan earthquake in North China

This paper analyses geodetic data including the results of short baseline and short levelling surveys across active faults, and of relevellings over a wide area collected at Tangshan and in its vicinity during the several years before the 1976 Tangshan earthquake of magnitude 7.8. Using a theoretical model for slip on a fault plane with an arbitrary dip in a viscoelastic half-space, the parameters of the aseismic fault slip prior to the shock are obtained, and the stress changes caused in the area of Tangshan by such slip are estimated. The results are comparable with the seismic activity and the changes in time and space of the b-value in the relation N = exp(a - bM) observed in the same period. It is demonstrated that during 1968–1975 the Cangdong fault, the main NNE-trending active fault in the southwest of the seismic region, had gradually started aseismic right-lateral strike-slip and that the occurrence of the Tangshan earthquake was related to the stress field produced by the slip. Finally, two sequences of periodic earthquake migration that took place in North China during 1966–1976 are discussed in connection with the Tangshan earthquake.     

Active faulting in the western Pyrénées (France): Paleoseismic evidence for late Holocene ruptures

We have identified a 50-km-long active fault scarp, called herewith the Lourdes Fault, between the city of Lourdes and Arette village in the French Pyrénées. This region was affected by large and moderate earthquakes in 1660 (Io = VIII–IX, MSK 64,), in 1750 (Io = VIII, MSK 64) and in 1967 (M_d = 5.3, Io = VIII, MSK 64). Most earthquakes in this area are shallow and the few available focal mechanism solutions do not indicate a consistent pattern of active deformation. Field investigations in active tectonics indicate an East–West trending and up to 50-m-high fault scarp, in average, made of 3 contiguous linear fault sub-segments. To the north, the fault controls Quaternary basins and shows uplifted and tilted alluvial terraces. Deviated and abandoned stream channels of the southern block are likely due to the successive uplift of the northern block of the fault. Paleoseismic investigations coupled with geomorphic studies, georadar prospecting and trenching along the fault scarp illustrate the cumulative fault movements during the late Holocene. Trenches exhibit shear contacts with flexural slip faulting and thrust ruptures showing deformed alluvial units in buried channels. ¹⁴C dating of alluvial and colluvial units indicates a consistent age bracket from two different trenches and shows that the most recent fault movements occurred between 4221 BC and 2918 BC. Fault parameters and paleoseismic results imply that the Lourdes Fault and related sub-segments may produce a M_W 6.5 to 7.1 earthquake. Fault parameters imply that the Lourdes Fault segment corresponds to a major seismic source in the western Pyrénées that may generate earthquakes possibly larger than the 1660 historical event.     

基于分裂节点法的地震同震和震后形变数值模拟及其在汶川大地震中的应用

大地震导致的同震及震后效应,对于分析不同地震之间的相互影响及区域地震危险性等有着重要的作用.文中开发了模拟地震同震及震后效应的三维黏弹性有限元程序,通过计算走滑断层震例(概念性模型)引起的同震及震后效应,并与解析/半解析解进行对比,验证了程序的可靠性.同时基于概念性模型,分析了不同介质参数对同震及震后的地表变形的影响....     

Recent activity and seismotectonics of the Eastern Pyrenees

Results from a recent earthquake in the Eastern Pyrenees are presented and the seismotectonics of the region is analyzed from the presently available data. On 26 September 1984 an earthquake (M_L = 4.4) took place in the area of the historical destructive earthquake of 1428. Several portable stations installed in the epicentral area to record aftershocks permitted of defining a precise location at 42°19.2′N, 2°10.2′E and 5 km depth. A maximum felt intensity of V (MSK) is obtained from macroseismic data. The epicentral location lies within a block bounded by E-W-trending structures and the focal solution shows right-lateral shearing with a NW-SE pressure axis.The seismicity in the Eastern Pyrenees shows a complex pattern which can be associated with both E-W fractures and NE-SW fault systems. Focal solutions of another two recent earthquakes of M_L ~ 4, with differences in horizontal pressure axis, are also discussed.     

The critical behaviour of a power law between preseismic electric signals and earthquakes of different mechanism in Greece and Japan

The consistency of the critical exponent in the power law relation between the stress drop of the earthquake and the lead time of the precursory seismic electric signal is checked using new data from the recent M _w 4.9 earthquake of strike-slip mechanism that occurred on 12 November 2013 in northern Evia island, Greece and the megathrust M _w 9.0 Tohoku earthquake on 11 March 2011, in Japan. For the first case, the derived exponent is in excellent agreement with previous ones obtained from all non thrust events analysed by the author and matches the value of critical exponent for fracture. On the other hand, the megathrust Tohoku earthquake follows the behaviour of all thrust events studied by the author, and thus, the calculated exponent significantly deviates from this critical value. The different behaviour between non thrust and thrust-type events could be attributed to the fact that thrust mechanism earthquakes usually occur in collision or subduction zones which are characterised by high accumulation of strain. However, a larger number of thrust events are required in order to obtain reliable results and shed light in the above experimental findings.     

Insights on the small tsunami from January 28, 2020, Caribbean Sea MW7.7 earthquake by numerical simulation and spectral analysis

A major left-lateral strike-slip Mw7.7 earthquake occurred in the vicinity of the Caribbean Sea on January 28, 2020. As a result, a small-scale tsunami was generated. The properties of the seismogenic source were described using observational data gathered for the earthquake and tsunami, as well as information on the regional tectonic setting. The tsunami was simulated with the COMCOT model and Okada’s dislocation model from finite fault solutions for M_W7.7 Caribbean Sea earthquakes published by the United States Geological Survey. The simulation results were compared to tide gauge records to validate whether the seafloor’s vertical displacement generated by the strike-slip fault caused a small-scale tsunami. We conducted a spectral analysis of the tsunami to better understand the characteristics of tsunami records. The tsunami simulation results showed that the co-seismic vertical displacement caused by a strike-slip M_W7.7 earthquake could have contributed to the small-scale tsunami, but the anomalously large high-frequency tsunami waves recorded by the George Town tide gauge 11 min after the earthquake were unrelated to the earthquake-generated tsunami. According to the spectrum analysis, the predominant period of noticeable high-frequency tsunami waves recorded by the George Town tide gauge occurred only two minutes after the earthquake. This indicates that the source of the small-scale tsunami was close to the George Town station and the possible tsunami source was 150 km away from George Town station. These facts suggest that a submarine landslide was caused by the strike-slip earthquake. The comprehensive analysis showed that the small-scale tsunami was not caused solely by co-seismic seafloor deformation from the strike-slip event but that an earthquake-triggered submarine landslide was the primary cause. Therefore, the combined impact of two sources led to the small-scale tsunami.

     

Coseismic landslides triggered by the 8th August 2017 <Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript> 7.0 Jiuzhaigou earthquake (Sichuan,China): factors controlling their spatial distribution and implications for the seismogenic blind fault identification

On 8th August 2017, a magnitude M_s 7.0 earthquake struck the County of Jiuzhaigou, in Sichuan Province, China. It was the third M_s ≥?7.0 earthquake in the Longmenshan area in the last decade, after the 2008 M_s 8.0 Wenchuan earthquake and the 2013 M_s 7.0 Lushan earthquake. The event did not produce any evident surface rupture but triggered significant mass wasting. Based on a large set of pre- and post-earthquake high-resolution satellite images (SPOT-5, Gaofen-1 and Gaofen-2) as well as on 0.2-m-resolution UAV photographs, a polygon-based interpretation of the coseismic landslides was carried out. In total, 1883 landslides were identified, covering an area of 8.11 km², with an estimated total volume in the order of 25–30?×?10⁶ m³. The total landslide area was lower than that produced by other earthquakes of similar magnitude with strike-slip motion, possibly because of the limited surface rupture. The spatial distribution of the landslides was correlated statistically to a number of seismic, terrain and geological factors, to evaluate the landslide susceptibility at regional scale and to identify the most typical characteristics of the coseismic failures. The landslides, mainly small-scale rockfalls and rock/debris slides, occurred mostly along two NE-SW-oriented valleys near the epicentre. Comparatively, high landslide density was found at locations where the landform evolves from upper, broad valleys to lower, deep-cut gorges. The spatial distribution of the coseismic landslides did not seem correlated to the location of any known active faults. On the contrary, it revealed that a previously-unknown blind fault segment—which is possibly the north-western extension of the Huya fault—is the plausible seismogenic fault. This finding is consistent with what hypothesised on the basis of field observations and ground displacements.