Geodynamic Processes Preceding the Sea of Okhotsk Deep Earthquake of May 24, 2013, with Magnitude Mw = 8.3

Doklady Earth Sciences - This study analyzes the set of parameters of geophysical fields and geodynamic events before the very strong Sea of Okhotsk earthquake of May 24, 2013, with Mw = 8.3 and a...     

Source zones of the catastrophic Simushir earthquakes on November 15, 2006 (M w = 8.3) and January 13, 2007 (M w = 8.1) and the deep crust structure beneath the Middle Kuril segment

The data on catastrophic earthquakes with magnitudes of 8.3 and 8.1 that occurred in the Simushir Island area on November 15, 2006, and January 13, 2007, respectively, were compared with the results of land-sea deep seismic studies by different methods (deep seismic sounding, the correlation method of refracted waves, the earthquake converted-wave method, the common mid-point) in the Central Kuril segment. The structure of the Earth’s crust and the hypocentral zones of these earthquakes were analyzed. It was established that the hypocenter of the main shock of the first earthquake was located at the bend of the seismofocal zone under the island slope of the trench on the outer side of the subsiding lithospheric plate in the rapidly rising granulite-basite (ìbasalticî) crustal layer, which, at depths of 7–15 km, replaced the granulite-gneiss layer. This was accompanied by an increase of the seismic wave velocity from 6.4 to 7.1 km/s. The focus of the second earthquake was located beneath the axis of the deep-sea trench. The aftershocks were concentrated in two bands 60–120 km wide that extend along the trench, as well as in the third zone orthogonal to the island arc. It was shown that the epicenters of the earthquakes are linked with regional faults. The main shock of the first earthquake (November 15, 2006) was interpreted as a thrust fault and the second one (January 13, 2007) was attributed to a normal fault.     

Seismoacoustic effects of the Hovsgol earthquake (M w = 4.9) of December 5, 2014

Doklady Earth Sciences - The results of study of the Hovsgol earthquake with M w = 4.9, which occurred on December 5, 2014, in the northern part of Hovsgol Lake in Mongolia, are presented. An...     

Nature of the macroseismic paradox of the deep-focus earthquake in the Sea of Okhotsk on May 24, 2013 (<Emphasis Type="Italic">M</Emphasis> <Subscript> <Emphasis Type="Italic">w</Emphasis> </Subscript> = 8.3)

We analyzed macroseismic data and considered the effect of extremely long range propagation of sensible shocks during the deep-focus earthquake in the Sea of Okhotsk on May 24, 2013 (M_w = 8.3). In order to explain this effect, we formulated and qualitatively solved the problem of superposition of P-waves over the radial mode ₀S₀ of the natural oscillations of the Earth during this earthquake. Our results confirmed the possibility of such an interpretation of the observed macroseismic effect and also allowed us to explain the fact of anomalously low decay of seismic disturbances with distance.     

The Nature of Coseismic Ground Vibrations in the Epicentral Zone of the Sea of Okhotsk Earthquake,May 24, 2013, and Estimation of P-Wave Amplitudes at Teleseismic Distances

Doklady Earth Sciences - The GPS-based coseismic observations in the epicentral zone of the earthquake on the Sea of Okhotsk of May 24, 2013, are analyzed. The peculiarities of the changes in...     

2018年5月28日中国吉林松原M5.7级地震引起的液化构造*

2018年5月28日,吉林松原市宁江区毛都站镇牙木吐村发生M5.7级地震(45°16'12″N,124°42'35″E),震源深度13 km,震中位于郯庐断裂带西北侧的扶余/松原—肇东断裂带、第二松花江断裂带和扶余北断裂带交汇处。地震诱发震中距3 km范围内普遍的液化和地表裂缝,给当地居民带来严重灾害。可见液化构造以砂火山为主,其次为液化砂堆、液化砂脉和液化砂席等。液化砂火山又可分为有火山口型砂火山、无火山口型砂火山和无砂型(水)火山。地震液化伴生软沉积物变形构造有变形层理、负载构造和火焰构造、滑塌褶皱、碟状构造和包卷层理等。地震诱发液化砂火山形成过程包括液化层内超孔隙流体压力形成、上覆低渗透层破裂和水、砂喷出地表后砂涌3个阶段。液化和流化砂体在上涌过程中会注入低渗透黏土层形成各种形态的砂脉、砂席和多种类型的变形构造。垂向上地震液化结构可划分为底部松散可液化层、下部液化变形层、上部液化变形层和地表砂火山4层结构。液化层埋深2~5 m,液化层厚度2 m。松原M5.7级地震发震机制为NE-SW(35°~215°)方向挤压应力使断层活跃,推测扶余/松原—肇东断裂是主要的发震断层。松原地震液化构造研究为现代地震活动区和灾害易发区预测提供依据,为地震引发的现代软沉积物变形构造研究提供丰富的素材,兼具将今论古意义,为揭示本世纪以来郯庐断裂带北段进入了一个强断裂和地震活跃阶段提供了最新的实际资料。     

Imaging the ramp–décollement geometry of the Chelungpu fault using coseismic GPS displacements from the 1999 Chi-Chi, Taiwan earthquake

We use coseismic GPS data from the 1999 Chi-Chi, Taiwan earthquake to estimate the subsurface shape of the Chelungpu fault that ruptured during the earthquake. Studies prior to the earthquake suggest a ramp–décollement geometry for the Chelungpu fault, yet many finite source inversions using GPS and seismic data assume slip occurred on the down-dip extension of the Chelungpu ramp, rather than on a sub-horizontal décollement. We test whether slip occurred on the décollement or the down-dip extension of the ramp using well-established methods of inverting GPS data for geometry and slip on faults represented as elastic dislocations. We find that a significant portion of the coseismic slip did indeed occur on a sub-horizontal décollement located at 8 km depth. The slip on the décollement contributes 21% of the total modeled moment release. We estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth: homogeneous, layered, and layered with lateral material contrast across the fault. It is shown, however, that heterogeneity has little influence on our estimated fault geometry. We also investigate several competing interpretations of deformation within the E/W trending rupture zone at the northern end of the 1999 ground ruptures. We demonstrate that the GPS data require a 22- to 35-km-long lateral ramp at the northern end, contradicting other investigations that propose deformation is concentrated within 10 km of the Chelungpu fault. Lastly, we propose a simple tectonic model for the development of the lateral ramp.     

Interpretation of earthquake-induced landslides triggered by the 12 May 2008, M7.9 Wenchuan earthquake in the Beichuan area, Sichuan Province, China using satellite imagery and Google Earth

The 12 May 2008 M7.9 Wenchuan earthquake in the People’s Republic of China represented a unique opportunity for the international community to use commonly available GIS (Geographic Information System) tools, like Google Earth (GE), to rapidly evaluate and assess landslide hazards triggered by the destructive earthquake and its aftershocks. In order to map earthquake-triggered landslides, we provide details on the applicability and limitations of publicly available 3-day-post- and pre-earthquake imagery provided by GE from the FORMOSAT-2 (formerly ROCSAT-2; Republic of China Satellite 2). We interpreted landslides on the 8-m-resolution FORMOSAT-2 image by GE; as a result, 257 large landslides were mapped with the highest concentration along the Beichuan fault. An estimated density of 0.3 landslides/km² represents a minimum bound on density given the resolution of available imagery; higher resolution data would have identified more landslides. This is a preliminary study, and further study is needed to understand the landslide characteristics in detail. Although it is best to obtain landslide locations and measurements from satellite imagery having high resolution, it was found that GE is an effective and rapid reconnaissance tool.     

Activity and distribution of geohazards induced by the Lushan earthquake,April 20, 2013

An Ms7.0 earthquake, focal depth 13 km, struck Lushan on April 20, 2013, caused 196 deaths and 21 missing, 13,484 injuries, and affected more than two million people. A field investigation was taken immediately after the quake, and the induced hazards were analyzed in comparison with the Wenchuan earthquake. We have identified 1,460 landslides and avalanches and four dammed lakes, which were generally small and concentrated on high elevation. Avalanches and rockfalls developed in cliffs and steep slopes of hard rocks, including Jinjixia of Baosheng Town and Dayanxia of Shuangshi Town, Lushan, and the K317 section the Xiaoguanzi section north to Lingguan Town along the provincial highway S210. Landslides were relatively less, mainly in moderate and small scales, developing in sandstone, shale, and loose colluviums. Only one single large landslide was observed to turn into debris slide-flow. Dammed lakes were formed by avalanches and landslides, all in small size and of low danger degree. The earthquake-induced hazards distributed in belt on the hanging wall along the faults, and their major controlling factors include tectonics, lithology, structure surface, and landform. More than 99 % landslides were within 30 km to the epicenter, and 678 within 10 km, accounting for 46 % of the total; about 50 % landslides were distributed on slopes between 35° and 55°, and 11 % on slope exceeding 75°; 60 % on slopes at the altitudes between 1,000 and 1,500 m, 77 % on slopes between 900 and 1,500 m; and 24 and 62 % in hard rocks and section between hard and soft rocks, respectively. Compared with the case of Wenchuan earthquake, both the number and extension of landslides and avalanches in Lushan earthquake-affected area are much smaller, only 5.53 % in number and 0.57 % in area. The earthquake has increased the instability of slope and potentiality of landslide and debris flow. Accordingly, the active period is expected to be relatively short comparing with that in Wenchuan earthquake-hit area. However, the insidious and concealed hazards bring difficulty for risk investigation.     

An attempt to reconstruct some features of the May 6, 1976, Friuli earthquake by means of aftershocks

The goal of the present work is to reconstruct some features of the May 6, 1976, Friuli earthquake in different sites of the area involved. The starting data for the elaboration are: seismograms of aftershocks at various sites; very detailed geological knowledge of the sites; accelerographs of the main shocks at Tolmezzo. Only aftershocks having magnitudes 2.5–4 and epicentres located within the most damaged area were considered. Fourier and response spectra for the events considered at the sites were computed. The problem is to establish the differences between, on the one hand, the Fourier and response spectra of the main shock, and on the other hand the aftershocks. Since one of these sites (Peonis) has about the same conditions (topography, geology, epicentral distance for the May 6 shock) as Tolmezzo, similarity in shaking and the shape of the spectra were assumed at the two sites for the main shock. On the basis of the known mean spectral ratios (computed on the aftershocks) between Peonis and the other sites, and the same ratio between the main shock and the aftershocks at Peonis, the spectral features of the May 6 shock at all the other sites were computed.     

The characteristics and failure mechanism of the largest landslide triggered by the Wenchuan earthquake, May 12, 2008, China

Strong earthquakes are among the prime triggering factors of landslides. The 2008 Wenchuan earthquake (M _w = 7.9) triggered tens of thousands of landslides. Among them, the Daguangbao landslide is the largest one, which covered an area of 7.8 km² with a maximum width of 2.2 km and an estimated volume of 7.5 × 10⁸ m³. The landslide is located on the hanging wall of the seismogenic fault, the Yingxiu–Beichuan fault in Anxian town, Sichuan Province. The sliding mass travelled about 4.5 km and blocked the Huangdongzi valley, forming a landslide dam nearly 600 m high. Compared to other coseismic landslides in the study area, the Daguangbao landslide attained phenomenal kinetic energy, intense cracking, and deformation, exposing a 1-km long head scarp in the rear of the landslide. Based on the field investigation, we conclude that the occurrence of the landslide is controlled mainly by the seismic, terrain, and geological factors. The special location of the landslide and the possible topographic amplification of ground motions due to the terrain features governed the landslide failure. The effects of earthquakes on the stability of slopes were considered in two aspects: First, the ground shaking may reduce the frictional strength of the substrate by shattering of rock mass. Second, the seismic acceleration may result in short-lived and episodic changes of the normal (tensile) and shear stresses in the hillshopes during earthquakes. According to the failure mechanism, the dynamic process of the landslide might contain four stages: (a) the cracking of rock mass in the rear of the slope mainly due to the tensile stress generated by the ground shaking; (b) the shattering of the substrate due to the ground shaking, which reduced the frictional strength of the substrate; (c) the shearing failure of the toe of the landslide due to the large shear stress caused by the landslide gravity; and (d) the deposition stage.     

Imaging the heterogeneous source area of the 2003 M6.4 northern Miyagi earthquake, NE Japan, by double-difference tomography

A shallow M6.4 inland earthquake occurred on 26 July 2003 in the northern part of Miyagi Prefecture, northeastern Japan. This earthquake was a typical inland thrust earthquake, a type that is common in NE Japan. We obtained a detailed seismic velocity structure in the focal area of this earthquake by the double-difference tomography method. Arrival-time data came from temporary seismic stations deployed above the mainshock fault plane. Both the P-wave and S-wave velocities in the hanging wall were lower than those in the footwall. Aftershocks were aligned along a zone where the seismic velocity changes rapidly. This is consistent with the interpretation that the 2003 northern Miyagi earthquake occurred along a fault that acted as a normal fault in the Miocene and has been reactivated as a reverse fault under the present compressional stress regime. The large slip area by the main shock rupture (asperity) corresponds to an area with relatively high P- and S-wave velocities. A zone with low V_p/V_s was detected along the aftershock area. One of the possible causes of this low-V_p/V_s zone is the existence of high-aspect-ratio pores that contain water. Hypocenters of the main shock, largest foreshock, and largest aftershock are also located within the low-V_p/V_s zone.     

Observation of abnormal thermal and infrasound signals prior to the earthquakes: a study on Bonin Island earthquake M7.8 (May 30, 2015)

Scientists from all over the world try to incorporate multi-disciplinary precursors to forecast the earthquake on a short-term basis. The authors here have analyzed outgoing longwave radiations acquired from polar-orbiting National Oceanic and Atmospheric Adminstration (NOAA) satellites and long-period infrasound waves recorded by the ground observatories in China prior to the recent Bonin Islands, Japan region earthquake which occurred on May 30, 2015 with the magnitude of 7.8. The anomalous outgoing longwave radiation (OLR) was observed on May 15, 2015, and was recorded by “NOAA 18” satellite during its “night pass.” Similarly, an abnormal infrasound spike was recorded at the Beijing station on May 17, 2015. The delay in observing anomalous infrasound waves compared to the OLR anomaly is due to these low-frequency waves traveling at low speed with the velocity range of 10–15 m/s. From the analysis of the results, it can be inferred that there is a substantial relation between parameters like OLR and infrasound waves; hence, the authors conclude that it is possible to forecast the earthquake on a short-term basis with reasonable accuracy.     

A rare case of grass flow induced by the M8.4 Arequipa earthquake, June 2001, in the Altiplano of Northern Chile

On June 23, 2001, an M8.4 earthquake that originated in southern Peru triggered the partial collapse of the Chislluma bofedal (water meadows) in the Altiplano (high-altitude plateau) of northern Chile. The seismic waves evidently produced the liquefaction of the bofedal and caused its partial collapse generating a flow. The flow deposit was mainly made of long-fiber grass and water, with minor amounts of clastic material. It traveled more than 14 km downstream at a peak velocity of 50 km/h. It destroyed the water meadows and killed more than 20 llamas. Slurry flows caused by meadow liquefaction are a previously unrecognized seismic-induced geological hazard for high-altitude plateau areas such as the Altiplano.     

5.12汶川8级地震次生地质灾害的基本特征及其形成机制浅析

5.12汶川8级大地震沿龙门山断裂带形成长350多km,宽约50 km的地表破裂带,触发了1万多处崩塌、滑坡、泥石流(碎屑流)地质灾害,其中巨型灾害体87处、大型灾害体606处,形成了136个较大规模的堰塞湖。地震地质灾害的链生特征显著,形成地震-崩塌、地震-滑坡-碎屑流-堰塞湖-堰塞坝溃决-泥石流等典型地质灾害链。地震次生地质灾害具有分布范围广、数量多、种类全、密度大、强度高、致灾重的特点。在部分地区,崩塌、滑坡和碎屑流的分布面积占地震极重灾区面积的30%~58％,甚至高达80％。据初步统计,崩塌、滑坡和碎屑流共导致大约2万人死亡,其中北川县老县城滑坡导致1 600多人死亡。地震次生地质灾害主要沿断裂带、河谷和交通线分布。崩塌、滑坡的破裂源主要位于河流拐弯处靠近侵蚀岸一侧、山脊两侧及坡肩部位,这与上述部位对地震动峰值加速度的放大作用直接相关。地震次生地质灾害主要受地震动峰值加速度和地形控制,其次为岩性、斜坡结构、活动断裂、人类工程活动。许多大型崩塌、滑坡还具有高速远程的特征,部分崩塌、滑坡 碎屑流位移达数km,速度高达100~300 m/s,其运动轨迹复杂多变,常常导致多处人员伤亡,是高山峡谷地区地质灾害风险评估和减灾防灾必须面临的新课题。根据上述情况,文中对汶川地震次生地质灾害的基本特征、分布规律和主要影响因素进行了初步总结,并对地震滑坡的形成机制和运动模式进行了初步探讨。首次提出高山峡谷地区单一斜坡上呈阶梯状多级滑动的群发性地震滑坡的形成模式:强烈地震往往引起剧烈的地面震动,而高陡的山脊及其坡肩部位对地震波具有明显的放大作用,因此,上述部位往往是地震滑坡的高易发地段,当地震动峰值加速度超过不稳定性斜坡的临界峰值加速度时,斜坡失稳破坏形成一系列的群发性滑坡,从上到下往往形成阶梯状多级滑动的滑坡群,此种模式适用于残坡积层、风化层地震滑坡和主滑面较缓的地震基岩滑坡。最后,指出了今后应重点研究的科学问题,并对防灾减灾措施提出了一些建议。