Seismic Imaging of a Bimaterial Interface Along the Hayward Fault,CA, with Fault Zone Head Waves and Direct P Arrivals

We observe fault zone head waves (FZHW) that are generated by and propagate along a roughly 80 km section of the Hayward fault in the San Francisco Bay area. Moveout values between the arrival times of FZHW and direct P waves are used to obtain average P-wave velocity contrasts across different sections of the fault. The results are based on waveforms generated by more than 5,800 earthquakes and recorded at up to 12 stations of the Berkeley digital seismic network (BDSN) and the Northern California seismic network (NCSN). Robust identification of FZHW requires the combination of multiple techniques due to the diverse instrumentation of the BDSN and NCSN. For single-component short-period instruments, FZHW are identified by examining sets of waveforms from both sides of the fault, and finding on one (the slow) side emergent reversed-polarity arrivals before the direct P waves. For three-component broadband and strong-motion instruments, the FZHW are identified with polarization analysis that detects early arrivals from the fault direction before the regular body waves which have polarizations along the source-receiver backazimuth. The results indicate average velocity contrasts of 3–8 % along the Hayward fault, with the southwest side having faster P wave velocities in agreement with tomographic images. A systematic moveout between the FZHW and direct P waves for about a 80 km long fault section suggests a single continuous interface in the seismogenic zone over that distance. We observe some complexities near the junction with the Calaveras fault in the SE-most portion and near the city of Oakland. Regions giving rise to variable FZHW arrival times can be correlated to first order with the presence of lithological complexity such as slivers of high-velocity metamorphic serpentinized rocks and relatively distributed seismicity. The seismic velocity contrast and geological complexity have important implications for earthquake and rupture dynamics of the Hayward fault, including a statistically preferred propagation direction of earthquake ruptures to the SE.     

Delineation of Subtrappean Mesozoic Sediments in Deccan Syneclise, India, Using Travel-Time Inversion of Seismic Refraction and Wide-Angle Reflection Data

2-D shallow velocity structure is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the E–W trending Narayanpur–Nandurbar and N–S Kothar–Sakri profiles, located in the Narmada–Tapti region of the Deccan syneclise. Deccan volcanic (Trap) rocks are exposed along the two profiles. Inversion of seismic data reveals two layered velocity structures above the basement along the two profiles. The first layer with a P-wave velocity of 5.15–5.25 km s^?1 and thickness varying from 0.7–1.5 km represents the Deccan Trap formation along the Narayanpur–Nandurbar profile. The Trap layer velocity ranges from 4.5 to 5.20 km s^?1 and the thickness varies from 0.95 to 1.5 km along the Kothar–Sakri profile. The second layer represents the low velocity Mesozoic sediments with a P-wave velocity of 3.5 km s^?1 and thickness ranging from about 0.70 to 1.6 km and 0.55 to 1.1 km along the E–W and N–S profiles, respectively. Presence of a low-velocity zone (LVZ) below the volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data together with the prominent wide-angle reflection phase immediately after the first arrivals from the Deccan Trap formation. The basement with a P-wave velocity of 5.8–6.05 km s^?1 lies at a depth ranging from 1.5 to 2.45 km along the profiles. The velocity models of the profiles are similar to each other at the intersection point. The results indicate the existence of a Mesozoic basin in the Narmada–Tapti region of the Deccan syneclise.     

利用断裂带首波分析甘孜—玉树断裂带拉张盆地结构特征

对断裂带及附近区域地层精细结构的描述是理解地震孕育和发生过程的基础.不同板块或块体边界在构造或区域应力作用下,常常会形成速度间断面和低速的断裂带,间断面和低速带的存在会产生特殊的断裂带地震波,比如断裂带首波和围陷波,并会影响地震的物理过程、破裂速度和破裂方向等.本文主要利用2010年4月14日M_W6.9玉树地震后布设的流动地震台站,对沿着甘孜—玉树断裂带传播的断裂带首波进行识别和分析.分析结果表明,在甘孜—玉树断裂带的不同区域均观测到了断裂带首波,在西段的结隆拉张盆地附近的3个台站沿断裂带界面的平均纵波速度差异值在5%～8%,而其他区域的平均速度差异为1%～3%.通过台站位置分布和断裂带首波特征关系,初步分析了断裂带拉张盆地的结构,结果显示结隆盆地的长度为～40km,宽度为5.35～5.97km,深度不超过5km,在地表浅层形成了一个低速区,分别与巴颜喀拉块体(NE)和羌塘块体(SW)产生了两个物性差异界面,但没有延伸到主震和余震区震源深度.我们的结果表明结合密集台阵资料,通过断裂带首波特征分析可以为断裂带精细结构及几何特征提供一种新的技术方法和途径.     

Velocity Structure of the West-Bengal Sedimentary Basin, India along the Palashi-Kandi Profile Using a Travel-time Inversion of Wide-angle Seismic data and Gravity Modeling – An Update

The 2-D shallow velocity structure along the north-south Palashi-Kandi profile in the West Bengal sedimentary basin has been updated by travel-time inversion of seismic refraction, wide-angle reflection and gravity data. A six-layer shallow model up to a depth of about 7 km has been derived. The first layer, which has an average velocity of 2.0 kms^?1, represents the alluvium deposit, which rests over the shale formation with average velocity of 3.0 kms^?1. The thin (200 m) Sylhet limestone, observed at a nearby Palashi well, remains hidden in the present data set. Hence a 200-m thin layer with a velocity of 3.7 kms^?1, corresponding to the Sylhet limestone, has been assumed to be present throughout the profile. The fourth layer with a velocity of 4.5–4.7 kms^?1 at a depth of 1.7–2.4 km represents the Rajmahal traps. The ‘skip’ phenomenon and rapid amplitude decay of first arrivals indicate a low-velocity zone (LVZ) in the study area. Using the ‘skip’ phenomena and wide-angle reflection data, identified on seismograms, the LVZ with a velocity of 4.0 kms^?1, indicating the Gondwana sediments, has been delineated below the Rajmahal traps. The next layer with a velocity 5.4–5.6 kms^?1 overlying the crystalline basement (5.8–6.25 kms^?1) may be associated with the Singhbhum group of meta volcanic rock that has been exposed in the western part of the basin. The basement lies at a variable depth of 4.9 to 6.8 km. The overall uncertainties of various velocity and boundary nodes are ± 0.15 kms^?1 and ± 0.5 km, respectively. The elevated basement feature in the north might have acted as a structural barrier for the deposition of Sylhet limestone during the Eocene epoch. The seismically derived shallow structure correctly explains the observed Bouguer gravity anomaly along the profile. The addition of reflections in the present analysis provides a stronger control on the depths and velocities of basement and overlying sedimentary formations, compared to the earlier model derived mainly by the first arrival seismic data.     

Earthquake Source Zones in Northeast India: Seismic Tomography, Fractal Dimension and b Value Mapping

We have imaged earthquake source zones beneath the northeast India region by seismic tomography, fractal dimension and b value mapping. 3D P-wave velocity (Vp) structure is imaged by the Local Earthquake Tomography (LET) method. High precision P-wave (3,494) and S-wave (3,064) travel times of 980 selected earthquakes, m _d ≥ 2.5, are used. The events were recorded by 77 temporary/permanent seismic stations in the region during 1993–1999. By the LET method simultaneous inversion is made for precise location of the events as well as for 3D seismic imaging of the velocity structure. Fractal dimension and seismic b value has been estimated using the 980 LET relocated epicenters. A prominent northwest–southeast low Vp structure is imaged between the Shillong Plateau and Mikir hills; that reflects the Kopili fault. At the fault end, a high-Vp structure is imaged at a depth of 40 km; this is inferred to be the source zone for high seismic activity along this fault. A similar high Vp seismic source zone is imaged beneath the Shillong Plateau at 30 km depth. Both of the source zones have high fractal dimension, from 1.80 to 1.90, indicating that most of the earthquake associated fractures are approaching a 2D space. The spatial fractal dimension variation map has revealed the seismogenic structures and the crustal heterogeneities in the region. The seismic b value in northeast India is found to vary from 0.6 to 1.0. Higher b value contours are obtained along the Kopili fault (~1.0), and in the Shillong Plateau (~0.9) The correlation coefficient between the fractal dimension and b value is found to be 0.79, indicating that the correlation is positive and significant. To the south of Shillong Plateau, a low Vp structure is interpreted as thick (~20 km) sediments in the Bengal basin, with almost no seismic activity in the basin.     

Rupture Process of the 1995 Antofagasta Subduction Earthquake (Mw = 8.1)

—A finite-source rupture model of the July 30, 1995, M _w = 8.1 Antofagasta (Northern Chile) subduction earthquake is developed using body and surface waves that span periods from 20 to 290s. A long-period (150–290s) surface-wave spectral inversion technique is applied to estimate the average finite-fault source properties. Deconvolutions of broadband body waves using theoretical Green’s functions, and deconvolutions of broadband fundamental mode surface waves using empirical Green’s functions provided by a large aftershock, yield effective source time functions containing periods from 20 to 200s for many directivity parameters. The source time functions are used in an inverse radon transform to image a one-dimensional spatial model of the moment rate history. The event produced a predominantly unilateral southward rupture, yielding strong directivity effects on all seismic waves with periods less than a few hundred seconds. The aftershock information, spectral analysis, and moment rate distribution indicate a rupture length of 180–200km, with the largest slip concentrated in the first 120km, a rupture azimuth of 205°± 10° along the Chilean coastline, and a rupture duration of 60–68s with a corresponding average rupture velocity of 3.0–3.2km/s. The overall rupture character is quite smooth, accentuating the directivity effects and reducing the shaking intensity, however there are three regions with enhanced moment rate distributed along the rupture zone near the epicenter, 50 to 80km south of the epicenter, and 110 to 140km south of the epicenter.     

Parkfield,California, Earthquake of June 1966: Interpretation of the strong motion records

A detailed seismological interpretation of the strong motion records was attempted for the 1966 Parkfield earthquake, California. Velocity and displacement traces integrated from the corresponding recorded accelerograms were found most valuable in studying the earthquake mechanism and wave forms. A double-couple right-lateral strike — slip mechanism (along the San Andreas fault) is consistent with the recorded direct S-waves originating from the hypocentre. High energy arrivals observed on the velocity traces are interpreted as S-waves (‘stopping phases’) that originated at the termination of the rupture towards the south-east of the San Andreas fault. A double-couple left-lateral strike–slip mechanism is suggested as the cause of this rupture termination. From particle velocity diagrams of the stopping phases in the horizontal plane, the rupture length was found to be between 20 and 28 km. Corresponding rupture velocities are estimated to be 2·5 ± 0·1 and 3·1 ± 0·5 km/s. The inference from the strong motion records is that Love waves were more excited at the south-western than the north-eastern side of the fault, whereas the Rayleigh waves were more energetic at the north-eastern than the south-western side of the fault.     

福建漳州地热田及其邻近地区的地壳深部构造特征

本文根据1985-1986年在福建省穿过漳州热田地区完成的一条NNE向地震折射剖面资料,采用一维与二维射线追踪和理论地震图拟合、反演等方法,获得了该地区比较详细的地壳与上地幔顶部结构及速度分布图像.结果表明:漳州热田地区正好位于一个地壳厚度突变带上,从漳州市往南向海岸方向地壳上隆,厚度只有29.5km,往北迅速加大到32.0 km左右;在这个突变带上地壳内所有速度界面均发生扭曲甚至错动,它可能是一个断裂带;地壳中部有一个横向延伸有限的低速区,其最低速度小于5.80km/s,顶部深度大于10.2km,这个低速区正好位于漳州市下方的地壳突变带上.作者推断它很可能就是漳州热田的壳内热源体,并可能是一个部分熔融体.     

Mantle Attenuation Estimated from Regional and Teleseismic P-waves of Deep Earthquakes and Surface Explosions

We estimated the network-averaged mantle attenuation t*(total) of 0.5 s beneath the North Korea test site (NKTS) by use of P-wave spectra and normalized spectral stacks from the 25 May 2009 declared nuclear test (mb 4.5; IDC). This value was checked using P-waves from seven deep (580–600 km) earthquakes (4.8 < M _w < 5.5) in the Jilin-Heilongjiang, China region that borders with Russia and North Korea. These earthquakes are 200–300 km from the NKTS, within 200 km of the Global Seismic Network seismic station in Mudanjiang, China (MDJ) and the International Monitoring System primary arrays at Ussuriysk, Russia (USRK) and Wonju, Republic of Korea (KSRS). With the deep earthquakes, we split the t*(total) ray path into two segments: a t*(u), that represents the attenuation of the up-going ray from the deep hypocenters to the local-regional receivers, and t*(d), that represents the attenuation along the down-going ray to teleseismic receivers. The sum of t*(u) and t*(d) should be equal to t*(total), because they both share coincident ray paths. We estimated the upper-mantle attenuation t*(u) of 0.1 s at stations MDJ, USRK, and KSRS from individual and stacks of normalized P-wave spectra. We then estimated the average lower-mantle attenuation t*(d) of 0.4 s using stacked teleseismic P-wave spectra. We finally estimated a network average t*(total) of 0.5 s from the stacked teleseismic P-wave spectra from the 2009 nuclear test, which confirms the equality with the sum of t*(u) and t*(d). We included constraints on seismic moment, depth, and radiation pattern by using results from a moment tensor analysis and corner frequencies from modeling of P-wave spectra recorded at local distances. We also avoided finite-faulting effects by excluding earthquakes with complex source time functions. We assumed ω² source models for earthquakes and explosions. The mantle attenuation beneath the NKTS is clearly different when compared with the network-averaged t* of 0.75 s for the western US and is similar to values of approximately 0.5 s for the Semipalatinsk test site within the 0.5–2 Hz range.     

Seismic detections of the 15 February 2013 Chelyabinsk meteor from the dense ChinArray

Chin Array is a dense portable broadband seismic network to cover the entire continental China, and the Phase I is deployed along the north-south seismic belt in southwest China. In this study, we analyze seismic data recorded on the Chin Array following the February 15,2013 Chelyabinsk(Russia) meteor. This was the largest known object entering the Earth's atmosphere since the1908 Tunguska meteor. The seismic energy radiated from this event was recorded by seismic stations worldwide including the dense Chin Array that are more than 4000 km away. The weak signal from the meteor event was contaminated by a magnitude 5.8 Tonga earthquake occurred *20 min earlier. To test the feasibility of detecting the weak seismic signals from the meteor event, we compute vespagram and perform F-K analysis to the surface-wave data. We identify a seismic phase with back azimuth(BAZ) of 329.7° and slowness of 34.73 s/deg, corresponding to the surface wave from the Russian meteor event(BAZ *325.97°). The surface magnitude(MS) of the meteor event is 3.94 ± 0.18. We also perform similar analysis on the data from the broadband array F-net in Japan, and find the BAZ of the surface waves to be316.61°. With the different BAZs of Chin Array and F-net,we locate the Russian meteor event at 58.80°N, 58.72°E.The relatively large mislocation(*438 km as compared with 55.15°N, 61.41°E by others) may be a result of thebending propagation path of surface waves, which deviates from the great circle path. Our results suggest that the dense Chin Array and its subarrays could be used to detect weak signals at teleseismic distances.     

The crustal structure under Sanjiang and its dynamic implications: Revealed by seismic reflection/refraction profile between Zhefang and Binchuan,Yunnan

The Sanjiang area in southwest China is considered as a tectonic intersection belt between the Tethys-Alps and the western Pacific, and has endured three-phase evolution processes: Proto-Tethys,Paleo-Tethys and Meso-Tethys[1―4]. In this area, its tectonics and struc- ture are extremely complicated, and intensively extru-sive deformation and faults are widely developed[1―3]. For that, the area is considered as the ideal na- ture-laboratory to study the evolution of Paleo-Tethys and also …     

新丰江水库中上地壳P波三维速度结构特征及库水的渗透影响

利用震源位置和速度结构的联合反演得到2007年6月—2014年7月新丰江水库地区地震序列的震源位置及中上地壳P波三维速度结构模型,并进一步研究库区序列分布及速度结构特征.结果显示:库区中上地壳不同深度P波速度存在显著横向不均匀性,浅部库区速度高于周缘,在5~10 km上地壳从库坝下游白田至库尾锡场NW方向存在高速异常体以及2个低速间断区域,低速间断区分别位于人字石断裂与南山—坳头断裂交汇处以及1962年6.1级地震震源区,库水可能沿低速间断区的人字石断裂、石角—新港—白田断裂下渗至13~14 km的地壳.在10~14 km地壳以NE走向的大坪—岩前断裂为界,NW侧为最高速度6.2 km·s~(-1)的高速区域,SE侧从库区中部回龙至库坝下游白田为显著低速异常区域,是可能的库水渗透影响区域,亦是库区中强地震集中区.库区地震多发生在高速体内部、高低速过渡带或低速的渗水通道两侧.     

The P and S wave velocity structures of the crust and upper mantle beneath Tibetan Plateau

Using the P-and S-wave arrivals from the 150 earthquakes distributed in Tibetan Plateau and its neighboring areas, recorded by Tibetan seismic network, Sichuan seismic network, WWSSN and the mobile network situated in Tibetan Plateau, we have obtained the average P-and S-wave velocity models of the crust and upper mantle for this region:

The 15 August 2007 Peru Earthquake and Tsunami: Influence of the Source Characteristics on the Tsunami Heights

The tsunami caused by the 2007 Peru earthquake (M_w 8.0) provoked less damage than by the seismic shaking itself (numerous casualties due to the earthquake in the vicinity of Pisco). However, it propagated across the Pacific Ocean and small waves were observed on one tide gauge in Taiohae Bay (Nuku Hiva, Marquesas, French Polynesia). We invert seismological data to recover the rupture pattern in two steps. The first step uses surface waves to find a solution for the moment tensor, and the second step uses body waves to compute the slip distribution in the source area. We find the slip distribution to consist of two main slip patches in the source area. The inversion of surface waves yields a scalar moment of 8.9 10²⁰ Nm, and body-wave inversion gives 1.4 10²¹ Nm. The inversion of tsunami data recorded on a single deep ocean sensor also can be used to compute a fault slip pattern (yielding a scalar moment of 1.1 10²¹ Nm). We then use these different sources to model the tsunami propagation across the Pacific Ocean, especially towards Nuku Hiva. While the source model taken from the body-wave inversion yields computed tsunami waves systematically too low with respect to observations (on the central Pacific Ocean DART buoy as on the Polynesian tide gauge), the source model established from the surface-wave inversion is more efficient to fit the observations, confirming that the tsunami is sensitive to the low frequency component of the source. Finally we also discuss the modeling of the late tsunami arrivals in Taiohae Bay using several friction coefficients for the sea bottom.     

华北克拉通北缘(怀来-苏尼特右旗)地壳结构

2009年,中国地质科学院地质研究所与美国俄克拉荷马大学合作实施了一条长453 km的深地震反射、宽角反射与折射、三分量反射地震联合探测剖面. 剖面南起怀来盆地,向北依次穿过燕山造山带西缘、内蒙地轴、白乃庙弧带、温都尔庙杂岩带,到达索伦缝合带. 其中,宽角反射与折射剖面采用8个0.5~1.5 t炸药震源激发,使用300套Texan单分量数字检波器接收,获得了高质量的地震资料. 通过资料分析和处理,识别出沉积层及结晶基底的折射波（Pg）、来自上地壳底界面的反射波（Pcp）,中地壳底界面的反射波（Plp）,莫霍界面的反射波（Pmp）及上地幔顶部的折射波（Pn）等5个震相. 分别采用Hole有限差分层析成像和Rayinvr算法对华北克拉通北缘及中亚造山带南部进行了上地壳P波速度结构成像和全地壳二维射线追踪反演成像. 结果显示：（1）中亚造山带地壳厚度~40 km,变化平缓,低于全球平均造山带地壳平均厚度,可能为造山后区域伸展的结果. 阴山-燕山带附近莫霍明显加深,推测其为燕山期造山过程形成的山根,但该山根很可能在后期被改造. （2）测线中部地壳上部速度较高,对应地表大面积花岗岩出露,而下地壳速度较低,速度梯度低,呈通道状,推测其可能曾为古亚洲洋向南俯冲消亡的主动陆缘,并在碰撞后演变为伸展环境下岩浆侵入的通道. （3）华北克拉通北缘与中亚造山带显示出不同速度变化特征,前者变化相对缓而后者则变化剧烈,二者的分界出现在赤峰-白云鄂博断裂附近.     

Spatial-temporal characterization of the San Andreas Fault by fault-zone trapped waves at seismic experiment site,Parkfield, California

In this article, we review our previous research for spatial and temporal characterizations of the San Andreas Fault (SAF) at Parkfield, using the fault-zone trapped wave (FZTW) since the middle 1980s. Parkfield, California has been taken as a scientific seismic experimental site in the USA since the 1970s, and the SAF is the target fault to investigate earthquake physics and forecasting. More than ten types of field experiments (including seismic, geophysical, geochemical, geodetic and so on) have been carried out at this experimental site since then. In the fall of 2003, a pair of scientific wells were drilled at the San Andreas Fault Observatory at Depth (SAFOD) site; the main-hole (MH) passed a ～200-m-wide low-velocity zone (LVZ) with highly fractured rocks of the SAF at a depth of ～3.2 km below the wellhead on the ground level (Hickman et al., 2005; Zoback, 2007; Lockner et al., 2011). Borehole seismographs were installed in the SAFOD MH in 2004, which were located within the LVZ of the fault at ～3-km depth to probe the internal structure and physical properties of the SAF. On September 282004, a M6 earthquake occurred ～15 km southeast of the town of Parkfield. The data recorded in the field experiments before and after the 2004 M6 earthquake provided a unique opportunity to monitor the co-mainshock damage and post-seismic heal of the SAF associated with this strong earthquake. This retrospective review of the results from a sequence of our previous experiments at the Parkfield SAF, California, will be valuable for other researchers who are carrying out seismic experiments at the active faults to develop the community seismic wave velocity models, the fault models and the earthquake forecasting models in global seismogenic regions.     

Crustal Architecture of the Inverted Central Lapland Rift Along the HUKKA 2007 Profile

We have studied the lateral velocity variations along a partly buried inverted paleo–rift in Central Lapland, Northern Europe with a 2D wide-angle reflection and refraction experiment, HUKKA 2007. The experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy. The shots were recorded by 102 stations with an average spacing of 3.45 km. Two-dimensional crustal models of variations in P-wave velocity and Vp/Vs-ratio were calculated using the ray tracing forward modeling technique. The HUKKA 2007 experiment comprises a 455 km long profile that runs NNW–SSE parallel to the Kittilä Shear Zone, a major deformation zone hosting gold deposits in the area. The profile crosses Paleoproterozoic and reactivated Archean terranes of Central Lapland. The velocity model shows a significant difference in crustal velocity structure between the northern (distances 0–120 km) and southern parts of the profile. The difference in P-wave velocities and Vp/Vs ratio can be followed through the whole crust down to the Moho boundary indicating major tectonic boundaries. Upper crustal velocities seem to vary with the terranes/compositional differences mapped at the surface. The lower layer of the upper crust displays velocities of 6.0–6.1 km/s. Both Paleoproterozoic and Archean terranes are associated with high velocity bodies (6.30–6.35 km/s) at 100 and 200–350 km distances. The Central Lapland greenstone belt and Central Lapland Granitoid complex are associated with a 4 km-thick zone of unusually low velocities (<6.0 km/s) at distances between 120 and 220 km. We interpret the HUKKA 2007 profile to image an old, partly buried, inverted continental rift zone that has been closed and modified by younger tectonic events. It has structural features typical of rifts: inward dipping rift shoulders, undulating thickness of the middle crust, high velocity lower crust and a rather uniform crustal thickness of 48 km.     

Estimation of Source Parameters of M w 6.9 Sikkim Earthquake and Modeling of Ground Motions to Determine Causative Fault

In this study, source parameters of the September 18, 2011 M _w 6.9, Sikkim earthquake were determined using acceleration records. These parameters were then used to generate strong motion at a number of sites using the stochastic finite fault modeling technique to constrain the causative fault plane for this earthquake. The average values of corner frequency, seismic moment, stress drop and source radius were 0.12 Hz, 3.07 × 10²⁶ dyne-cm, 115 bars and 9.68 km, respectively. The fault plane solution showed strike-slip movement with two nodal planes oriented along two prominent lineaments in the region, the NE-oriented Kanchendzonga and NW-oriented Tista lineaments. The ground motions were estimated considering both the nodal planes as causative faults and the results in terms of the peak ground accelerations (PGA) and Fourier spectra were then compared with the actual recordings. We found that the NW–SE striking nodal plane along the Tista lineament may have been the causative fault for the Sikkim earthquake, as PGA estimates are comparable with the observed recordings. We also observed that the Fourier spectrum is not a good parameter in deciding the causative fault plane.     

基于交叉梯度约束的地震初至纵波与瑞雷面波联合反演

本文提出了一种初至纵波(P波)与瑞雷面波的交叉梯度联合反演策略.通过对初至P波进行全波形反演可以获得近地表P波速度结构;通过对仅含瑞雷面波信息的地震数据转换到频率-波数域进行加窗振幅波形反演(Windowed-Amplitude Waveform Inversion,w-AWI)可获得近地表横波(S波)速度结构.在二者反演的目标函数中均加入P波速度和S波速度的交叉梯度作为正则化约束项,使得在反演过程中P波速度和S波速度相互制约,相互约束,从而实现对地震初至P波与瑞雷面波的联合反演.数值模拟结果表明交叉梯度联合反演可以提高S波速度反演分辨率,而P波速度反演结果并没有得到提高.实际资料的反演结果表明,交叉梯度联合反演能够获得更加可信的近地表速度结构.     

可可西里岩石圈向北俯冲到柴达木地幔的地震学证据

利用中美德INDEPTH IV合作项目2007—2009年间布置于青藏高原中、北部140个宽频地震台站记录到的天然地震数据,经过接收函数成像处理,获得了3条穿过昆仑—阿尼玛卿缝合带清晰的壳幔结构图像.结果显示柴达木南缘莫霍面位于约50 km深度,羌塘地块、可可西里地块、东昆仑造山带莫霍面位于约65 km深度,昆仑—阿尼玛卿缝合带以北约50 km存在莫霍面深度突变.在可可西里和柴达木岩石圈地幔之间观测到北倾界面,这可能是可可西里岩石圈向北俯冲到柴达木地幔之下的证据.可可西里地块地壳内宽缓的负转换震相带是低速带的反映,其向北挤入到东昆仑山下发生挤压增厚,可能是东昆仑山隆升的原因;由于刚性柴达木岩石圈的阻挡,物质向东改向,则可能是该地区向东旋转的构造应力场产生的原因.本文研究结果不支持亚洲岩石圈地幔在东昆仑—柴达木交界处向南俯冲,据此,我们提出了新的东昆仑造山模式.