A study in the velocity structure of December 5, 1997, M w = 7.8 Kronotskii rupture zone, Kamchatka

The object of the present study was to obtain and investigate the 3D velocity structure of the rupture zone of a large earthquake, to be specific, the great ( _Mw = 7.8) Kronotskii earthquake that occurred in Kamchatka on December 5, 1997. The event was preceded by a foreshock swarm (December 3–5, 1997) and followed by a long aftershock sequence. We investigated the V _P velocity distribution for different time periods: December 3–7, 1997 (when the chief events occurred, viz., the main shock and the larger aftershocks) and for subsequent periods of decaying aftershock activity until December 1998. The velocity distribution in the rupture zone proved to be inhomogeneous. Three regions have been identified: the northeastern (the main shock and foreshocks), the central, and the southwestern, which differ both in the character of seismicity and in velocity. The V _P distribution was found to be time-dependent. The velocity was below the standard values in the foreshock-aftershock area in December 1997, subsequently the velocity increased. These results may indicate the absence of a continuous rupture zone, with the main shock and the two largest aftershocks that occurred in the southwest probably being independent events rupturing a transverse fault during the stress rearrangement following the main shock.     

Spatial relation between source regions of strong earthquakes (M ≥ 7) on Southern Kamchatka and the distribution of velocities and elastic moduli in the Benioff zone

The presence of a phenomenological relationship between high velocity regions in the Benioff zone and sources of relatively strong earthquakes (M ≥ 6) was established for the first time from the comparison of such earthquakes with the velocity structure of central Kamchatka in the early 1970s. It was found that, in the region with P wave velocities of 8.1–8.5 km/s, the number of M ≥ 6 earthquakes over 1926–1965 was 2.5 times greater than their number in the region with velocities of 7.5–8.0 km/s. Later (in 1979), within the southern Kurile area, Sakhalin seismologists established that regions with V _P = 7.3–7.7 km/s are associated with source zones of M = 7.0–7.6 earthquakes and regions with V _P = 8.1–8.4 km/s are associated with M = 7.9–8.4 earthquakes. In light of these facts, we compared the positions of M = 7.0–7.4 earthquake sources in the Benioff zone of southern Kamchatka over the period 1907–1993 with the distribution of regions of high P velocities (8.0–8.5 to 8.5–9.0 km/s) derived from the interpretation of arrival time residuals at the Shipunskii station from numerous weak earthquakes in this zone (more than 2200 events of M = 2.3–4.9) over the period 1983–1995. This comparison is possible only in the case of long-term stability of the velocity field within the Benioff zone. This stability is confirmed by the relationship between velocity parameters and tectonics in the southern part of the Kurile arc, where island blocks are confined to high velocity regions in the Benioff zone and the straits between islands are confined to low velocity regions. The sources of southern Kamchatka earthquakes with M = 7.0–7.4, which are not the strongest events, are located predominantly within high velocity regions and at their boundaries with low velocity regions; i.e., the tendency previously established for the strongest earthquakes of the southern Kuriles and central Kamchatka is confirmed. However, to demonstrate more definitely their association with regions of high P wave velocities, a larger statistics of such earthquakes is required. On the basis of a direct correlation between P wave velocities and densities, the distributions of density, bulk modulus K, and shear modulus μ in the upper mantle of the Benioff zone of southern Kamchatka are obtained for the first time. Estimated densities vary from 3.6–3.9 g/cm³ in regions of high V _P values to 3.0–3.2 g/cm³ for regions of low V _P values. The bulk modulus K in the same velocity regions varies from (1.4–1.8) × 10¹² to (0.8–1.1) × 10¹² dyn/cm², respectively, and the shear modulus μ varies from (0.8–1.0) × 10¹² to (0.5–0.7) × 10¹² dyn/cm², respectively. Examination of the spatial correlation of the source areas of southern Kamchatka M = 7.0–7.4 earthquakes with the distribution of elastic moduli in the Benioff zone failed to reveal any relationship between their magnitudes and the moduli because of the insufficient statistics of the earthquakes used.     

On the influence of earthquake energy range on the estimated seismicity parameters before the magnitude 7.9 Kronotskii earthquake of 1997

Results are reported from a detailed study of central Kamchatka seismicity for the period 1962–1997 based on a modification of the traditional approach. The approach involves (a) a detailed structure of the seismic region that recognizes the Kronotskii and Shipunskii geoblocks and two further blocks, the continental slope, and the offshore portion, (b) a study of variations in the rate of M = 3.0–7.2 earthquakes and the amount of seismic energy released at depths of 0–50 and 51–100 km, (c) a study of seismicity variability, and (d) separate estimates of the recurrence of crust-mantle earthquakes (depths 0–50 km) and mantle events (51–100 km). As a result, apart from corroborating the fact of a quiescence preceding the December 5, 1997 Kronotskii earthquake (M 7.9), we also found that a relationship exists between its beginning and the position of the earthquake-generating region relative to the mainshock epicenter. The quiescence dominates the seismic process during the pre-mainshock period and is characterized by a decreased rate of earthquakes (the first feature) and a decreased amount of seismic energy release (the second feature). Based on the first feature, we found that the quiescence started in 1987 throughout the entire depth range (0–100 km) in both parts of the Kronotskii geoblock close to the rupture zone of the eponymous earthquake. As to the Shipunskii geoblock, which is farther from the rupture zone, the quiescence began in the mantle of the inner area first (1988) and somewhat later at depths of 0–50 km within the continental slope (1989). By the second feature, the quiescence began at shallower depths in the inner area of the Kronotskii geoblock at the same time and later on (a year later) in the mantle (1988). Under the continental slope of the trench in the Shipunskii geoblock the shallower quiescence also began in 1987, while it was 3 years late in the inner zone (1990) and involved the earthquake-generating earth volume at depths of 0–100 km. These data are identical with or sufficiently close to the estimate for the beginning of this quiescence using a circular area of radius 150 km that combines the Kronotskii and Shipunskii geoblocks by the RTL method (1990).     

On the seismicity of central Kamchatka before the 1997, M = 7.9 Kronotskii earthquake

We report results from a detailed study of seismicity in central Kamchatka for the period from 1960 to 1997 using a modified traditional approach. The basic elements of this approach include (a) segmentation of the seismic region concerned (the Kronotskii and Shipunskii geoblocks, the continental slope and offshore blocks), (b) studying the variation in the rate of M = 4.5–7.0 earthquakes and in the amount of seismic energy release over time, (c) studying the seismicity variations, (d) separate estimates of earthquake recurrence for depths of 0–50 and 50–100 km. As a result, besides corroborating the fact that a quiescence occurred before the December 5, 1997, M = 7.9 Kronotskii earthquake, we also found a relationship between the start of the quiescence and the position of the seismic zone with respect to the rupture initiation. The earliest date of the quiescence (decreasing seismicity rate and seismic energy release) was due to the M = 4.5–7.0 earthquakes at depths of 0–100 km in the Kronotskii geoblock (8–9 years prior to the earthquake). The intermediate start of the quiescence was due to distant seismic zones of the Shipunskii geoblock and the circular zone using the RTL method, combining the Shipunskii and Kronotskii geoblocks (6 years). Based on the low magnitude seismicity (M≥2.6) at depths of 0–70 km in the southwestern part of the epicentral zone (50–100 km from the mainshock epicenter), the quiescence was inferred to have occurred a little over 3 years (40 months) before the mainshock time and a little over 2 years (25 months) in the immediate vicinity of the epicenter (0–50 km). These results enable a more reliable identification of other types of geophysical precursors during seismic quiescences before disastrous earthquakes.     

On a possible origin of the anomalous effects that were observed during the May 24, 2013 earthquake in the Sea of Okhotsk

The deep-focus Sea of Okhotsk earthquake that occurred on May 24, 2013 (h = 630 km, M _w = 8.3) was accompanied by anomalous effects that were unknown previously. A combined analysis of published data concerning the source rupture evolution and some features of the deep structure provided an explanation of some anomalous effects, such as the large number of aftershocks and the low level of ground shaking in the epicentral area. However, GPS observations revealed high coseismic vertical displacements in the area. The seafloor uplift in the Sea of Okhotsk and the adjacent coasts was 3–12 mm, peaking at the approximate center of the sea, while Kamchatka and the North Kuril Islands subsided by 3–18 mm, peaking at the Apacha station 190 km east of the earthquake epicenter. These maximum estimates are 1.2–1.8 times the analogous values (10 mm) for the Chile mega-earthquake of May 20, 1960 (M _w ~ 9.5). It is known that the large distances at which ground shaking is felt during deep-focus earthquakes are due to the fact that the body waves travel through the high-Q lower mantle. However, this does not explain the paradox of the present earthquake in the Sea of Okhotsk, viz., a constant intensity of shaking (two grades) in the range of epicentral distances between 1300 and 9500 km. The explanation requires consideration of the earth’s free oscillations excited by the earthquake.     

The geophysical significance of oceanic plagiogranite

Rocks ranging in composition from trondhjemite to diorite (plagiogranites) have been recovered from ocean ridges and are common constituents of ophiolites. Velocities and densities of diorite and trondhjemite from the Mid-Atlantic Ridge are shown to differ significantly from similar properties of metadolerite and gabbro. Compressional (V_p) and shear (V_s) velocities of plagiogranites are relatively low (V_p = 4.78–5.91km/s at1kbar,V_s = 2.81–3.37km/s at1kbar), as are densities (2.57–2.64 g/cm³) and Poisson's ratios (0.24–0.27). These data lend strong support to the probable existence of a low-velocity/density zone within layer 3 of the oceanic crust. Based on observations in ophiolites, it is postulated that this zone can be up to 1 km in thickness and is laterally discontinuous.     

Marine seismological observations in the Central Kurile segment before catastrophic earthquakes on November, 2006 (<Emphasis Type="Italic">M</Emphasis> = 8.3) and January, 2007 (<Emphasis Type="Italic">M</Emphasis> = 8.1)

The results of detailed seismological observations with bottom seismographs in the Central Kurile segment in August-September, 2006 are discussed. The system of six bottom seismographs was placed on the island slope of the Kurile deep-sea trench southeast of Urup Island and southwest of the Bussol Strait. Over 230 earthquakes with M _LH = 0.5–5.5 were registered in the area with a radius of 150 km around the center of the observation system at depths up to 300 km during 16 days. Records of 80 earthquakes with hypocenters in the earth crust (h = 0–30 km) beneath the island slope of the Kurile deep-sea trench were first obtained by bottom seismographs. These data are inconsistent with previous concepts of aseismicity of this zone. The discovery of the unique morphological structure of the Benioff zone beneath the central Kurile Arc represents the most important result of detailed seismological observations. The zone consists of an inner seismoactive subzone, which is located beneath the island slope of the arc at depths of 15–210 km, being characterized by an angle of incline of 50° under the latter and crosses the ocean bottom approximately 80 km away from the trench axis, and outer low-activity subzone. The latter is traceable beyond the trench almost parallel to the inner zone beginning from a depth of 50 km below the sea bottom up to a depth of approximately 300 km. Due to the slightly lower incline (∼45°) of the outer subzone, both subzones gradually converge downward. The integral thickness of the Benioff zone varies from 150 km in its upper part to 125 km at depths of 210–260 km. The medium sandwiched between these subzones is practically aseismic. The reality of this defined structure is confirmed by the distribution of aftershocks of the earthquake that occurred on November 15, 2006 (M = 8.3). These seismic events served as foreshocks for the subsequent strong earthquake of January 13, 2007 (M = 8.1) with the hypocenter located beyond the trench under the ocean bottom. Such a structure of this zone within the central Kurile Arc segment is unique, having no analogues either in the flanks of the Kurile-Kamchatka Arc or other arcs. The results of detailed seismological observations obtained two months before the first of the catastrophic Central Kurile earthquakes appeared to be typical for the period of foreshocks (the lower seismic activity of the Simushir block, which hosted the hypocenter of the earthquake that occurred on November 15, 2006, particularly at depths of 0–50 km, the gentler incline of the recurrence plot, and other features).     

The rupture process and asperity distribution of three great earthquakes from long-period diffracted P-waves

Maximum earthquake size varies considerably amongst the subduction zones. This has been interpreted as a variation in the seismic coupling, which is presumably related to the mechanical conditions of the fault zone. The rupture process of a great earthquake indicates the distribution of strong (asperities) and weak regions of the fault. The rupture process of three great earthquakes (1963 Kurile Islands, M_W = 8.5; 1965 Rat Islands, M_W = 8.7; 1964 Alaska, M_W = 9.2) are studied by using WWSSN stations in the core shadow zone. Diffraction around the core attenuates the P-wave amplitudes such that on-scale long-period P-waves are recorded. There are striking differences between the seismograms of the great earthquakes; the Alaskan earthquake has the largest amplitude and a very long-period nature, while the Kurile Islands earthquake appears to be a sequence of magnitude 7.5 events.The source time functions are deconvolved from the observed records. The Kurile Islands rupture process is characterized by the breaking of asperities with a length scale of 40–60 km, and for the Alaskan earthquake the dominant length scale in the epicentral region is 140–200 km. The variation of length scale and M_W suggests that larger asperities cause larger earthquakes. The source time function of the 1979 Colombia earthquake (M_W = 8.3) is also deconvolved. This earthquake is characterized by a single asperity of length scale 100–120 km, which is consistent with the above pattern, as the Colombia subduction zone was previously ruptured by a great (M_W = 8.8) earthquake in 1906.The main result is that maximum earthquake size is related to the asperity distribution on the fault. The subduction zones with the largest earthquakes have very large asperities (e.g. the Alaskan earthquake), while the zones with the smaller great earthquakes (e.g. Kurile Islands) have smaller scattered asperities.     

Foreshocks and aftershocks of the M W = 7.1, 1992, earthquake in the Atrato region, Colombia

We study the October 18, M _W = 7.1, 1992 Atrato earthquake, and its foreshocks and aftershocks, which occurred in the Atrato valley, northwestern Colombia. The main shock was preceded by several foreshocksof which the M _W = 6.6, October 17 earthquacke was the largest. Inparticular, we examine foreshocks and aftershocks performing joint-hypocenter relocations using high quality Pn and Sn wave readingsfrom permanent regional networks. We observed a few hours prior to the main shock a sudden increase of foreshocks. Maybe this could be used as a predictor since foreshocks have been known for other major events in the region. Our locations align for 90 km with a trend of 5^° ±4^° in agreement with the Harvard CMT solution showing the faultplane trending 9^° to be the plane of rupture. In relation to theepicenter of the main shock, maximum intensities were located to thesouth, consistent with a rupture that traveled from north to south witha larger energy release in the south as suggested by an empirical Green'sfunction study (Li and Toksöz, 1993; Ammon et al., 1994). The boundarybetween the Panama and North Andes blocks has been placed close to thePanama-Colombia border as either a sharp boundary or a diffuse zone. TheAtrato earthquake, however, shows that the plate boundary between thePanama and North Andes microblocks is a diffuse deformation zone. Thiszone has a width of at least 2^° stretching from 78^°W to 76^°W. Quantification of earthquake moment release (during the past30 years) in this zone shows a similar amount of moment release in thewestern and eastern parts of this zone.     

A new fault rupture scenario for the 2003 Mw 6.6 Bam earthquake,SE Iran: Insights from the high-resolution QuickBird imagery and field observations

The December 26, 2003 M_w 6.6 Bam earthquake is one of the most disastrous earthquakes in Iran. QuickBird panchromatic and multispectral satellite imagery with 61 cm and 2.4 m ground resolution, respectively provide new insights into the surface rupturing process associated with this earthquake. The results indicate that this earthquake produced a 2–5 km-wide surface rupture zone with a complex geometric pattern. A 10-km-long surface rupture zone developed along the pre-existing Bam fault trace. Two additional surface rupture zones, each 2–5 km long, are oblique to the pre-existing Bam fault in angles of 20–35°. An analysis of geometric and geomorphic features also shows that movement on the Bam fault is mainly right-lateral motion with some compressional component. This interpretation is consistent with field investigations, analysis of aftershocks as well as teleseismic inversion. Therefore, we suggest that the 2003 Bam earthquake occurred on the Bam fault, and that the surface ruptures oblique to the Bam fault are caused by secondary faulting such as synthetic shears (Reidel shears). Our fault model for the Bam earthquake provides a new tectonic scenario for explaining complex surface deformations associated with the Bam earthquake.     

Time variations in parameters of split S waves from weak local earthquakes beneath eastern Hokkaido

Parameters of seismic waves from clusters of local weak earthquakes that occurred at the upper boundary of the seismofocal zone in depth intervals of 40–60 and 70–90 km along eastern Hokkaido are investigated for the period 1998–2003, including the strong (M = 8.0) Tokachi-Oki earthquake of September 26, 2003. Analysis of data indicates that the distribution of anisotropic properties along Hokkaido is inhomogeneous and parameters of split waves (the azimuth of the fast S wave and the time delay between split S waves) are sensitive to variations in the stress-strain state of the medium. Unstable behavior of split wave parameters and increased values of the ratio V _P /V _S for clusters of events in the areas of the Hidaka Mountains and Nemuro Peninsula imply that the medium is in a mechanically weakened state (in the regime of intense dilatant deformation). On the contrary, the regions beneath the Tokachi and Kushiro plains are more rigid and are characterized by lower V _P /V _S values and comparatively stable behavior of wave parameters. Anomalous parameters of split waves from events of clusters in areas of different stations correlate with each other and are related to occurrence times of large earthquakes around Hokkaido, which may point to a redistribution of stresses and strains and fluid migration in the subduction zone.     

2014年11月22日康定M6.3级地震序列发震构造分析

2014年11月22日在NW向鲜水河断裂带中南段四川康定县发生M6.3级地震,11月25日在该地震震中东南约10km处再次发生M5.8级地震.基于中国国家数字地震台网和四川区域数字地震台网资料,采用多阶段定位方法对本次康定M6.3级地震序列进行了重新定位;利用gCAP(generalized Cut And Paste)矩张量反演方法获得了M6.3和M5.8级地震的震源机制解与矩心深度,分析了本次地震序列的发震构造,并结合历史强震破裂时空分布和2001年以来小震重新定位结果,对鲜水河断裂带中段强震危险性进行了初步探讨.获得的主要结果如下:(1)M6.3级主震震中位于101.69°E、30.27°N,震源初始破裂深度约10km,矩心深度9km;M5.8级地震震中位于101.73°E、30.18°N,初始破裂深度约11km,矩心深度9km.gCAP矩张量反演结果揭示这两次地震双力偶分量占主导,M6.3级地震的最佳双力偶解节面Ⅰ走向143°/倾角82°/滑动角-9°,节面Ⅱ走向234°/倾角81°/滑动角-172°.M5.8级地震最佳双力偶解节面Ⅰ走向151°/倾角83°/滑动角-6°,节面Ⅱ走向242°/倾角84°/滑动角-173°.依据余震分布长轴展布与断裂走向,判定节面Ⅰ为发震断层面,M6.3和M5.8级地震均为带有微小正断分量的左旋走滑型地震.(2)序列中重新定位的459个地震平均震源深度约9km,地震主要集中分布在6~11km深度区间,余震基本发生在M6.3和M5.8级地震震源上部.依据余震密集区展布范围,推测本次康定地震的震源体尺度长约30km、宽约4km、深度范围约6km.M6.3级主震震源附近的余震稀疏区可能是一个较大的凹凸体(asperity),在主震中能量得以充分释放.(3)最初3天的余震主要分布在M6.3级地震NW侧;而M5.8级地震之后的余震主要集中在其震中附近.M6.3级地震以及最初3天的绝大部分余震发生在倾角约82°近直立的NW走向色拉哈断裂上;M5.8级地震与其后的多数余震发生在倾角约83°近直立的NW走向折多塘断裂北端走向向北偏转部位,M5.8级地震可能是M6.3级地震触发相邻的折多塘断裂活动所致.(4)康定M6.3与M5.8级地震发生在鲜水河断裂带乾宁与康定之间的色拉哈强震破裂空段,本次地震破裂尺度较小,尚不足以填补该强震空段.色拉哈段以及相邻的乾宁段7级地震平静时间均已超过其平均复发周期估值,未来几年存在发生7级地震的危险.康定M6.3级地震序列基本填补了震前存在于塔公与康定之间的深部小震空区,未来强震发生在塔公至松林口段深部小震稀疏区内的可能性很大.     

Spatio-temporal variation and focal mechanism of the Wenchuan MS8.0 earthquake sequence

Based on abundant aftershock sequence data of the Wenchuan M_S8.0 earthquake on May 12, 2008, we studied the spatio-temporal variation process and segmentation rupture characteristic. Dense aftershocks distribute along Longmenshan central fault zone of NE direction and form a narrow strip with the length of 325 km and the depth between several and 40 km. The depth profile (section of NW direction) vertical to the strike of aftershock zone (NE direction) shows anisomerous wedgy distribution characteristic of aftershock concentrated regions; it is related to the force form of the Longmenshan nappe tectonic belt. The stronger aftershocks could be divided into northern segment and southern segment apparently and the focal depths of strong aftershocks in the 50 km area between northern segment and southern segment are shallower. It seems like 'to be going to rupture' segment. We also study focal mechanisms and segmentation of strong aftershocks. The principal compressive stress azimuth of aftershock area is WNW direction and the faulting types of aftershocks at southern and northern segment have the same proportion. Because aftershocks distribute on different secondary faults, their focal mechanisms present complex local tectonic stress field. The faulting of seven strong earthquakes on the Longmenshan central fault is mainly characterized by thrust with the component of right-lateral strike-slip. Meantime six strong aftershocks on the Longmenshan back-range fault and Qingchuan fault present strike-slip faulting. At last we discuss the complex segmentation rupture mechanism of the Wenchuan earthquake.     

Preliminary analysis on the source properties and seismogenic structure of the 2017 <Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript>7.0 Jiuzhaigou earthquake

At GMT time 13:19, August 8, 2017, an M_s7.0 earthquake struck the Jiuzhaigou region in Sichuan Province, China, causing severe damages and casualties. To investigate the source properties, seismogenic structures, and seismic hazards, we systematically analyzed the tectonic environment, crustal velocity structure in the source region, source parameters and rupture process, Coulomb failure stress changes, and 3-D features of the rupture plane of the Jiuzhaigou earthquake. Our results indicate the following: (1) The Jiuzhaigou earthquake occurred on an unmarked fault belonging to the transition zone of the east Kunlun fault system and is located northwest of the Huya fault. (2) Both the mainshock and aftershock rupture zones are located in a region where crustal seismic velocity changes dramatically. Southeast to the source region, shear wave velocity at the middle to lower crust is significantly low, but it rapidly increases northeastward and lies close to the background velocity across the rupture fault. (3) The aftershock zone is narrow and distributes along the northwest-southeast trend, and most aftershocks occur within a depth range of 5–20 km. (4) The focal mechanism of the Jiuzhaigou earthquake indicates a left-lateral strike-slip fault, with strike, dip, and rake angles of 152°, 74° and 8°, respectively. The hypocenter depth measures 20 km, whereas the centroid depth is about 6 km. The co-seismic rupture mainly concentrates at depths of 3–13 km, with a moment magnitude (M_w) of 6.5. (5) The co-seismic rupture also strengthens the Coulomb failure stress at the two ends of the rupture fault and the east segment of the Tazang fault. Aftershocks relocation results together with geological surveys indicate that the causative fault is a near vertical fault with notable spatial variations: dip angle varies within 66°–89° from northwest to southeast and the average dip angle measures ~84°. The results of this work are of fundamental importance for further studies on the source characteristics, tectonic environment, and seismic hazard evaluation of the Jiuzhaigou earthquake.     

2017年米林6.9级地震震源区速度结构与余震重定位

利用西藏自治区林芝地区的固定地震台站与南迦巴瓦流动测震台站在2017年11月18日至2017年11月24日记录到的430个余震的直达波走时数据反演得到了震源区的三维P波速度、S波速度结构,并利用三维速度结构对余震进行了重定位.成像结果显示,米林地震震源区在0~5km深度内存在低地震波速度异常;在5~15km深度内,存在高地震波速度异常,该高速异常致使震源区西南侧的地震波速度高于东北侧.重定位结果中,余震呈条带状以NW-SE走向展布,震源深度具有西南方向深、东北方向浅的特征.主震位于11km深度处、高地震波速异常体顶部,余震主要分布在高地震波速度与低地震波速度过渡的区域.对成像结果的分析表明,震源区浅部的低速异常具有低泊松比的特性,与富石英的沉积变质杂岩体-东久杂岩单元的岩性特征有关;深部的速度结构特征则可能反映了发震断层上盘地震波速度高,下盘地震波速度低的介质特性.余震重定位结果与成像结果联合表明:此次地震发震断层从11km深度处,东久杂岩体下方的高地震波速度异常顶部开始破裂,继而在5~15km深度内发生后续破裂,后续破裂的发生区域正处于喜马拉雅构造单元与冈底斯构造单元接触的形变区内.此外,根据地震波速度计算的泊松比反映了震源区持续的低泊松比特征,暗示此次地震与流体活动并无直接关系.     

The 2008 Kultuk earthquake with M w = 6.3 in the south of Baikal: Spatial-temporal analysis of seismic activation

The results of investigating the data of stationary and field observations in the epicentral zone of the Kultuk earthquake of August 27, 2008 with M _w = 6.3 localized south of Baikal are presented. The seismic activation amounting to 1790 aftershocks with K _p ?? 4 (M _w ?? 0.9) affected a part of the general fault bounding the southwestern coast of the lake and shallower intrabasin structures. It was established through the cluster analysis that the main shock was located at the periphery of the cloud of the concentrated seismicity component, and three main clusters reflecting a complex character of rupture in the earthquake source were identified in the zone of aftershocks. Owing to a high accuracy of aftershock hypocenters determinations (ERZ ?? 1.2 km), the local character of the seismisity-generating zones was revealed. Based on the method of mapping seismic regime parameters, it was established that zones of reduced fractionality are recognizable in the central part of the source area; in this case, the entire periphery, except for the southeastern direction (where the main shock epicenter is located), is characterized by a high fractionality, which also points to the complex character of the rupture in the source. On the whole, the aftershock sequence under consideration showed the activation of the southwestern flank of the Southern Baikal region, which remained passive for more than one century, and demonstrated destructive features of the Earth??s crust.     

Space-time variations in the parameters of split S waves excited by local earthquakes in southern Kamchatka

An analysis of the distribution of split S-wave parameters due to small local earthquakes occurring down to a depth of 200 km beneath the PET IRIS station (in the town of Petropavlovsk-Kamchatskii) for the period 1993–2002 revealed an inhomogeneous distribution of anisotropic properties and time-dependent changes in the state of stress and strain in the sinking slab. Three time intervals have been identified from the behavior of the split-wave parameters: 1993–1995, 1996–1998, and 1999–2002. The most orderly orientation of fast-wave polarization azimuths at all depths and lower stresses in the earth was observed in 1996–1998. Regions of relatively stable and unstable parameter behavior consistent with increased and decreased P and S velocities have been identified. The central block down to a depth of 120 km and the lower part of the Benioff zone are classified as stable regions dominated by E-SE fast-wave azimuths. Unstable behavior of fast-wave azimuths was noticed for depths of 60 to 90 km and around the top of the Benioff zone. The highest values of S-wave delay time and regions where the delay times were migrating mostly occur at contacts between more rigid and weakened blocks.     

Tectonic Features of the M_S8.0 Wenchuan Earthquake and Its Aftershocks

The M8.0 Wenchuan earthquake occurred on the Longmenshan fault zone. Based on field investigation of the surface rupture and focal mechanism study of the aftershocks, we discuss the geological relationship of the main, secondary and triggered ruptures. The main rupture is about 200km long and can be divided into the south part and the north part. The south part consists of two parallel fault zones characterized by reverse faulting, with several parallel secondary ruptures on the hanging wall of the main fault, and the north part is a single main fault zone characterized by lateral strike-slip and reverse faulting. Compared to a 300km long aftershock distribution, the surface rupture only occupies 200km, and the remaining 100km on the northeast of the main rupture was triggered by aftershocks. Study on the ruptures of this earthquake will be useful for studying the earthquake risk evolution on the Longmenshan fault system.     

Seismic Subduction of the Nazca Ridge as Shown by the 1996–97 Peru Earthquakes

—By rupturing more than half of the shallow subduction interface of the Nazca Ridge, the great November 12, 1996 Peruvian earthquake contradicts the hypothesis that oceanic ridges subduct aseismically. The mainshock’s rupture has a length of about 200 km and has an average slip of about 1.4 m. Its moment is 1.5 × 10²⁸ dyne-cm and the corresponding M _w is 8.0. The mainshock registered three major episodes of moment release as shown by a finite fault inversion of teleseismically recorded broadband body waves. About 55% of the mainshock’s total moment release occurred south of the Nazca Ridge, and the remaining moment release occurred at the southern half of the subduction interface of the Nazca Ridge. The rupture south of the Nazca Ridge was elongated parallel to the ridge axis and extended from a shallow depth to about 65 km depth. Because the axis of the Nazca Ridge is at a high angle to the plate convergence direction, the subducting Nazca Ridge has a large southwards component of motion, 5 cm/yr parallel to the coast. The 900–1200 m relief of the southwards sweeping Nazca Ridge is interpreted to act as a "rigid indenter," causing the greatest coupling south of the ridge’s leading edge and leading to the large observed slip. The mainshock and aftershock hypocenters were relocated using a new procedure that simultaneously inverts local and teleseismic data. Most aftershocks were within the outline of the Nazca Ridge. A three-month delayed aftershock cluster occurred at the northern part of the subducting Nazca Ridge. Aftershocks were notably lacking at the zone of greatest moment release, to the south of the Nazca Ridge. However, a lone foreshock at the southern end of this zone, some 140 km downstrike of the mainshock’s epicenter, implies that conditions existed for rupture into that zone. The 1996 earthquake ruptured much of the inferred source zone of the M _w 7.9–8.2 earthquake of 1942, although the latter was a slightly larger earthquake. The rupture zone of the 1996 earthquake is immediately north of the seismic gap left by the great earthquakes (M _w ～8.8–9.1) of 1868 and 1877. The M _w 8.0 Antofagasta earthquake of 1995 occurred at the southern end of this great seismic gap. The M _w 8.2 deep-focus Bolivian earthquake of 1994 occurred directly downdip of the 1868 portion of that gap. The recent occurrence of three significant earthquakes on the periphery of the great seismic gap of the 1868 and 1877 events, among other factors, may signal an increased seismic potential for that zone.     

Ometepec-Pinotepa Nacional,Mexico Earthquake of 20 March 2012 (Mw 7.5): A preliminary report

An analysis of local and regional data produced by the shallow, thrust Ometepec-Pinotepa Nacional earthquake (M_w 7.5) of 20 March 2012 shows that it nucleated at 16.254°N 98.531°W, about 5 km offshore at a depth of about 20 km. During the first 4 seconds the slip was relatively small. It was followed by rupture of two patches with large slip, one updip of the hypocenter to the SE and the other downdip to the north. Total rupture area, estimated from inversion of near-source strong-motion recordings, is ~25 km × 60 km. The earthquake was followed by an exceptionally large number of aftershocks. The aftershock area overlaps with that of the 1982 doublet (M_w 7.0, 6.9). However, the seismic moment of the 2012 earthquake is ~3 times the sum of the moments of the doublet, indicating that the gross rupture characteristics of the two earthquake episodes differ. The small-slip area near the hypocenter and large-slip areas of the two patches are characterized by relatively small aftershock activity. A striking, intense, linear NE alignment of the aftershocks is clearly seen. The radiated energy to seismic moment ratios, (E_s/M₀), of five earthquakes in the region reveal that they are an order of magnitude smaller for near-trench earthquakes than those that occur further downdip (e.g., 2012 and the 1995 Copala earthquakes). The near-trench earthquakes are known to produce low A_max. The available information suggests that the plate interface in the region can be divided in three domains. (1) From the trench to a distance of about 35 km downdip. In this domain M~6 to 7 earthquakes with low values of (E_s/M₀) occur. These events generate large number of aftershocks. It is not known whether the remaining area on this part of the interface slips aseismically (stable sliding) or is partially locked. (2) From 35 to 100 km from the trench. This domain is seismically coupled where stick-slip sliding occurs, generating large earthquakes. Part of the area is probably conditionally stable. (3) From 100 to 200 km from the trench. In this domain slow slip events (SSE) and nonvolcanic tremors (NVT) have been reported.The earthquake caused severe damage in and near the towns of Ometepec and Pinotepa Nacional. The PGA exceeded 1 g at a soft site in the epicentral region. Observed PGAs on hard sites as a function of distance are in reasonable agreement with the expected ones from ground motion prediction equations derived using data from Mexican interplate earthquakes. The earthquake was strongly felt in Mexico City. PGA at CU, a hard site in the city, was 12 gal. Strong-motion recordings in the city since 1985 demonstrate that PGAs during the 2012 earthquake were not exceptional, and that similar motion occurs about once in three years.