Verification of the shallow seismic crustal structure of the western Krušné Hory crystalline unit, Czech Republic

We analyze refraction measurements along a short profile in western Kru?né hory crystalline unit. The profile passed close to the seismically active zone of Nový Kostel. The measurements were carried out to distances of about 15 km using quarry blasts near the village of Horní Rozmy?l, located at the eastern margin of the crystalline unit. Smoothed P-wave travel times were interpreted using the Wiechert-Herglotz method, which yielded a 1-D velocity model of the shallow crustal structure of the crystalline unit down to a depth of 1.7 km. The P-wave velocity of the model increases from about 4.0 km/s at the surface to 5.9 km/s at a depth of 1.7 km. The superficial velocities of our model are somewhat higher than the superficial velocities of the model that is routinely used for earthquake location in the region.     

The effect of the descending lithosphere beneath the Tonga island arc on P-wave travel-time residuals at the Warramunga Seismic Array

P-wave travel-time residuals at the Warramunga Seismic Array (WRA) in the Northern Territory, Australia, have been studied from 49 earthquakes with epicenters south of 19°S in the Fiji-Tonga region. Focal depths are between 42 and 679 km as determined from pP-P. Using the Jeffreys-Bullen and the Herrin travel-time tables the epicentral parameters have been redetermined by considering only “normal” seismic stations in the location procedure. These are those stations where P-wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the Tonga trench. Epicenters of deep earthquakes below 300 km have been relocated by using stations at Δ > 25° only. Epicenters from shallower-depth earthquakes have been recalculated without using stations between 35 < Δ < 75° epicentral distance. In both cases focal depths were determined from pP-P times. The resulting pattern of P-residuals at WRA does not show any significant change with depth below 350 km. The residuals become more negative for shallower earthquakes above about 250 km. P-waves to WRA are advanced by approximately 2 s compared with those from deep earthquakes. The results do not essentially differ for the two different travel-time tables used. The observations can be interpreted by P-wave velocities that are higher in the sinking slab down to 350–400 km by 5±2% than in both the Jeffreys-Bullen and Herrin models. Without considering possible elevations of phase boundaries this estimate yields a temperature contrast of 1000±450°C between slab and normal mantle material in this depth range.     

Saxothuringian-Moldanubian Suture and Predisposition of Seismicity In The Western Bohemian Massif

We modelled the thickness and seismic anisotropy of the subcrustal lithosphere from the variations of P-wave delay times and the shear-wave splitting observed at seismological observatories and portable stations in the western part of the Bohemian Massif. The Saxothuringian lithosphere is characterized by a total thickness between 90 and 120 km, the Moldanubian lithosphere is generally thicker –120-140 km, on the average. The subcrustal lithosphere of both units is characterised by divergently dipping anisotropic structures and the suture between them is marked by a lithosphere thinning to about 80km. Within the subcrustal lithosphere a complex structure of the transition of both units extends to about 150 km toward the south. We suggest that the Saxothuringian-Moldanubian suture has created a zone of mechanical predisposition for the Tertiary Ohe (Eger) Graben, as well as for the occurrence of earthquake swarms in the region. Most earthquakes occur within the brittle part of the upper crust above the crossing of the suture between the Saxothuringian in the north and the Moldanubian and the Tepl´-Barrandian in the south, with the tectonically active Mariánské Lázn fault.     

大理石板中心受压至断裂时P波速度变化的实验研究c

本文叙述无水注入情况下,中心受压的大理石板(60060020毫米)破裂前,P波走时变化的实验结果。 路径穿过未来裂缝、靠近受压中心时,走吋上升一回降;不靠近受压中心则走时只上升。路径近于平行且靠近未来裂缝,走时一般不变。路径不穿过未来裂缝和受压中心,发射点附近形变小时,走时不变;发射点附近形变大时,走时一般下降。路径穿过受压中心吋,有些走时先上升后恢复再下降。有些则下降后回升。 这说明在岩石破裂前,破裂孕育区附近同时存在P波速正负异常区和正常区。 同时还观测到,P波速度负异常区随中心压力增加逐步扩大。 由裂缝一侧波源所得的正常区正好是由另一侧波源所得的负异常区。      

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.     

One-Dimensional qP-Wave Velocity Model of the Upper Crust for the West Bohemia/Vogtland Earthquake Swarm Region

The western part of the Bohemian Massif (West Bohemia/Vogtland region) is characteristic in the relatively frequent recurrence of intraplate earthquake swarms and in other manifestations of past-to-recent geodynamic activity. In this study we derived 1D anisotropic qP-wave model of the upper crust in the seismogenic West Bohemia/Vogtland region by means of joint inversion of two independent data sets - travel times from controlled shots and arrival times from local earthquakes extracted from the WEBNET seismograms. We derived also simple 1-D P-wave and S-wave isotropic models. Reasons for deriving these models were: (a) only simplified crustal velocity models, homogeneous half-space or 1D isotropic layered models of this region, have been derived up to now and (b) a significant effective anisotropy of the upper crust in the region which was indicated recently by S-wave splitting. Both our anisotropic qP-wave and isotropic P-and S-wave velocity models are constrained by four layers with the constant velocity gradient. Weak anisotropy for P-waves is assumed. The isotropic model is represented by 9 parameters and the anisotropic one is represented by 24 parameters. A new robust and effective optimization algorithm - isometric algorithm - was used for the joint inversion. A two-step inversion algorithm was used. During the first step the isotropic P- and S-wave velocity model was derived. In the second step, it was used as a background model and the parameters of anisotropy were sought. Our 1D models are adequate for the upper crust in the West Bohemia/Vogtland swarm region up to a depth of 15 km. The qP-wave velocity model shows 5% anisotropy, the minimum velocity in the horizontal direction corresponds to an azimuth of 170°. The isotropic model indicates the V_P/V_S ratio variation with depth. The difference between the hypocentre locations based on the derived isotropic and anisotropic models was found to be several hundreds of meters.     

地幔上涌对鄂尔多斯西缘岩石圈的改造:来自远震多尺度层析成像的证据

本文利用喜马拉雅二期科学探测台阵的678个地震台站及26个固定台站记录到的9,641个地震共约160000条远震P波走时数据,采用基于稀疏约束的多尺度层析成像方法,获得了鄂尔多斯西缘及邻区上地幔800 km深度范围内P波速度结构.结果显示,在东经104°附近阿拉善地块与鄂尔多斯盆地间存在岩石圈深度的构造边界,这表明阿拉善地块与鄂尔多斯可能分别从属于不同的大地构造单元.以北纬38°线为界,鄂尔多斯地块西缘在岩石圈范围内南北存在明显的速度差异,鄂尔多斯南部上地幔200~300 km深度范围显示为高速异常,而鄂尔多斯北部上地幔显示大面积的低速异常.这一现象表明,鄂尔多斯地块南北两部分经历了不同的构造演化过程.根据本文的结果可以进一步推断,由于青藏高原、阿拉善地块向东北方向推挤以及岩石圈的拆离引起的上地幔扰动导致了地幔上涌,上涌的热物质改造了鄂尔多斯西北缘地区的岩石圈,并使该区的岩石圈减薄.地幔上涌也可能是东经104°边界带和北纬38°构造带形成的深部动力学因素.     

P-wave velocity anomalies in the earth's mantle from the Uppsala array observations

Summary Distribution of compressional-wave velocities in the mantle is determined fromdT/d measurements using the Uppsala seismograph array station (UPSAS). Short-period vertical-component seismograms from 181 events in the epicentral distance range 16°–100° have been used. The velocity distribution shows anomalous variations at depths of 750, 1500, 1800, 2300 and 2550 km. Evidence of lateral heterogeneity beneath the northern part of the Asian continent, in the depth range 1700–2300 km, is discussed. Computed travel times, based on this velocity-depth relation, are tested by an examination of travel-time residuals from the Long Shot and Milrow explosions on Amchitka, Aleutian Islands.     

京津唐地区地壳三维P波速度结构与地震活动性分析

本文利用华北遥测地震台网和首都圈数字地震台网112个台站记录到的1993~2004年发生在首都圈地区3983次地震的P波绝对到时资料和相对到时资料,采用双差地震层析成像方法联合反演了京津唐地区地壳三维P波速度结构和震源参数.京津唐地区的三维P波速度结构图像在浅层上很好地反映了地表地质、地形的特征.在平原和凹陷的盆地处呈现P波低速速度异常,而在隆起的山区或基岩出露区显示为P波高速速度异常.在研究区域内震级M≥6.0历史地震和经过重新定位后的震级M_L≥3.0的地震的震源位置在10 km深度和15 km深度处的P波相对速度扰动图上的投影都显示出相似的特点,即:绝大部分的地震的震源位置在P波相对速度扰动图上的投影分布在低、高速异常的交界地带,且偏高速体一侧,只有极少数的地震分布在P波速度异常体内部.     

Inversion of travel times obtained during active seismic refraction experiments CELEBRATION 2000, ALP 2002 and SUDETES 2003

A series of kinematic inversions based on robust non-linear optimization approach were performed using travel time data from a series of seismic refraction experiments: CELEBRATION 2000, ALP 2002 and SUDETES 2003. These experiments were performed in Central Europe from 2000 to 2003. Data from 8 profiles (CEL09, CEL10, Alp01, S01, S02, S03, S04 and S05) were processed in this study. The goal of this work was to find seismic velocity models yielding travel times consistent with observed data. Optimum 2D inhomogeneous isotropic P-wave velocity models were computed. We have developed and used a specialized two-step inverse procedure. In the first “parametric” step, the velocity model contains interfaces whose shapes are defined by a number of parameters. The velocity along each interface is supposed to be constant but may be different along the upper and lower side of the interface. Linear vertical interpolation is used for points in between interfaces. All parameters are searched for using robust non-linear optimization (Differential Evolution algorithm). Rays are continuously traced by the bending technique. In the second “tomographic” step, small-scale velocity perturbations are introduced in a dense grid covering the currently obtained velocity model. Rays are fixed in this step. Final velocity models yield travel time residuals comparable to typical picking errors (RMS ∼ 0.1 s). As a result, depth-velocity cross-sections of P waves along all processed profiles are obtained. The depth range of the models is 35–50 km, the velocity varies in the range 3.5–8.2 km/s. Lowest velocities are detected in near-surface depth sections crossing sedimentary formations. The middle crust is generally more homogeneous and has typical P wave velocity around 6 km/s. Surprisingly the lower crust is less homogeneous and the computed velocity is in the range 6.5–7.5 km/s. The MOHO is detected in the depth ≈30–45 km.     

Azimuthal anisotropy of the P-wave velocity in the hypocentral volume of the Krn Mt. (Slovenia) earthquake sequence

Azimuthal anisotropy of P-wave velocity in the hypocentral volume of the Krn Mt. (Slovenia) earthquake sequence is measured using the differences of travel times and of travel paths of the Pg-phase towards the recording stations of the local and regional networks. The observed velocity varies between 6.0 km/s in the ENE–WSW direction and 6.4 km/s for waves propagating NNW–SSE. These directions closely match those of the mean regional principal stress components obtained from individual fault-plane solutions for events from the Krn Mt. sequence. A large part of observed anisotropy may be explained if hypocentral volume is assumed to be pervaded by a system of vertical/subvertical extensive-dilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field.     

Minimum 1D Velocity Model from Local Earthquake Data in the Provence Region,South-Eastern France

We computed a one-dimensional (1D) velocity model and station corrections refering to the Provence region (South-eastern France) by inverting P-wave arrival times recorded on an eight-station local seismic network. Using this velocity model and the program HYPOELLIPSE (Lahr, 1989), we relocated a set of 108 local events. The quality comparison between previous earthquake locations and new relocated shows a good improvement.The obtained Minimum 1D velocity model can be used in a better-quality routine for earthquake location and represent a first step towards more detailed seismic studies in the Provence region.     

Crustal and upper mantle structures of the brazilian coast

The Rayleigh wave phase and group velocities in the period range of 24–39 sec, obtained from two earthquakes which occurred in northeastern brazil and which were recorded by the Brazilian seismological station RDJ (Rio de Janeiro), have been used to study crustal and upper mantle structures of the Brazilian coastal region. Three crustal and upper mantle models have been tried out to explain crustal and upper mantle structures of the region. The upper crust has not been resolved, due basically to the narrow period range of the phase and group velocities data. The phase velocity inversions have exhibited good resolutions for both lower crust and upper mantle, with shear wave velocities characteristic of these regions. The group velocity data inversions for these models have showed good results only for the lower crust. The shear wave velocities of the lower crust (3.86 and 3.89 km/sec), obtained with phase velocity inversions, are similar to that (=3.89 km/sec) found byHwang (1985) to the eastern South American region, while group velocity inversions have presented shear velocity (=3.75 km/sec) similar to that (=3.78 km/sec) found byLazcano (1972) to the Brazilian shield. It was not possible to define sharply the crust-mantle transition, but an analysis of the phase and group velocity inversions results has indicated that the total thickness of the crust should be between 30 and 39 km. The crustal and upper mantle model, obtained with phase velocity inversion, can be used as a preliminary model for the Brazilian coast.     

Subcrustal structure of the black sea basin from seismological data

The P-wave travel time data from the earthquakes offshore and onshore around the Black Sea are used for the tomographic reconstruction of the three-dimensional (3D) velocity distribution in the lithosphere of the region. The preliminary refinement of the foci parameters (the coordinates and origin time) has reduced the random errors in the travel-time data. The earthquake data were supplemented by the previous deep seismic sounding (DSS) data on the profiles in Crimea and offshore off the Black Sea. The dataset included more than 4000 travel times overall. In order to eliminate the crustal effect, the travel times were reduced to a surface at a depth of 35 km corresponding to the mean Moho depth in the region. The improved crustal model was used for removing the contribution of the crust from the initial data. The new tomography method, which was recently developed by one of the authors and which relies on the assumption of smoothness of the lateral velocity variations, was applied for reconstructing the velocity structure of the upper mantle beneath the Black Sea up to a depth of 95 km. The lateral velocity variation maps at different depths and the vertical velocity distributions along the meridional and sublatitudinal cross sections across the Black Sea were constructed. High velocities were revealed in the subcrustal lithosphere, and the structural difference below two subbasins—the West Black Sea (WBS) and the East Black Sea (EBS) ones—was established. It shows that the high-velocity body below the WBS is located deeper than below the EBS and is distinguished by higher velocities. Based on these results, it is concluded that the lithosphere beneath the Black Sea has a continental origin.     

汶川MS8.0级地震余震分布及周边区域P波三维速度结构研究

利用川滇地区长期积累的地震走时观测资料和汶川地震余震观测资料对汶川地震震源区及周边区域地壳和上地幔P波三维速度结构进行了研究.结果表明,浅部P波速度分布与地表地质之间具有很好的对应关系.龙门山断裂带在20 km以上深度表现为高速异常带,彭灌杂岩体和宝兴杂岩体为局部高速异常区.龙门山断裂带中上地壳的局部高速异常体对汶川地震的余震分布具有明显的控制作用.在余震带南端,余震全部发生在与宝兴杂岩体对应的高速异常体的东北侧;在余震带的中段,与彭灌杂岩体对应的高速异常体在一定程度上控制了余震的分布;在余震带的东北端,宁强-勉县一带的高速异常体可能阻止了余震进一步向东北扩展.龙门山断裂带中上地壳的P波高速异常表明介质具有相对较高的强度,在青藏高原物质向东挤出过程中起到了较强的阻挡作用,有利于深部能量积累.在30 km深度之下,扬子地块具有明显的高速特征,其前缘随深度增加向青藏高原方向扩展,在下地壳和上地幔顶部已达到龙门山断裂带以西.     

The D″ region

Two very different types of models are currently being proposed for D″, the lowest region of the earth's mantle: (a) those in which the P and S velocities vary smoothly down to the core-mantle boundary, without any extreme change in gradient; (b) those in which the velocity gradients decrease fairly abruptly at a height of 100 km or so above the core-mantle boundary, and maintain a value close to the critical gradient down to the boundary.Type (a) is represented by model UTD124A′ of Dziewonski and Gilbert (1972) and model B1 of Jordan and Anderson (1974). Both models are in good agreement with most travel time and free oscillation data. Their validity rests on the supposition, supported in part by theoretical studies, that data which suggest the presence of a low velocity zone in D″ result from distortion of seismic waves by the core-mantle boundary.On the other hand, slowness and amplitude data from short period P waves indicate a fairly rapid decrease in velocity gradient at a depth corresponding to an epicentral distance of about 92°, and it is very unlikely that these data can be interpreted as interface phenomena. The measured P and S times at distances beyond about 96° also indicate reduced velocities in D″. The suggestion that the measured velocities are in error as a result of interface effects is weakened by the fact that the results are apparently not wavelength-dependent.Type (b) is represented by model B2 of Jordan (1972), Bolt's (1972) model, and a new model designated as ANU2. All models have high density gradients indicative of inhomogeneity in the region. Model B2 fits the oscillation data reasonably well, but has an unjustifiably low S velocity at the core-mantle boundary. In Bolt's model the P and S velocities at the top of D″ are based on the models of Herrin et al. (1968) and Jeffreys (1939), whereas in ANU2 the values are taken from Hales and Herrin (1972) and Hales and Roberts (1970b). The velocities at the core-mantle boundary in Bolt's model and ANU2 are based on observations of “diffracted” P and S. Both of these models were designed to produce flattening of the P curve at about 92°. Both may require some modification in order to be compatible with free oscillation data.     

Procedure of reduction of travel-time curves ofP andS waves at the focal surface

A method for the calculation of seismic waves velocities at focal depth is here proposed. A stratified earth model with spherical symmetry and the analytical relationship between the epicentral distance and the travel times of seismic waves are used.This method, applied to the southern Tyrrhenian region and to the Japanese islands, allows to reduce the observed travel times to the focal depth independently of a particular velocity model.     

3-D Crustal Velocity Structure in Northwestern Greece

— The three-dimensional crustal velocity structure in the area of the northwestern Greek mainland was determined by P-wave travel time inversion, applying a two-step tomography procedure. The data set consists of the travel-time residuals of 584 well located earthquakes. In order to improve the initial (reference) velocity model, before the inversion of travel times, the minimum 1-D model was determined. Several tests were conducted to estimate model stability and hypocenter uncertainties. The velocity distribution in the shallow layers (4 and 7 km) is strongly affected by the crustal thickness variation and the complex tectonics. A first, well-defined velocity discontinuity appears at a depth of 3–6 km, along the Hellenides Mountain chain. A second low velocity anomaly is detected at a depth of 9–12 km and may be connected with the Alpidic orogenesis. Another interesting feature appears beneath the Amvrakikos Gulf (horstgraben structure), where relatively low velocities (<6.0 km^-1) appear to a depth of 20 km. Finally, a well-pronounced velocity boundary is found at a depth of 16 km. In general, low velocities are predominant along the Dinarides-Hellenides Mountain chain, rather typical for the upper crust.Acknowledgement. The authors thank the referees for their useful comments. Moreover, we would like to thank the General Secretariat for Research and Technology of Greece, for the partial support of this study.     

UK 1-D regional velocity models by analysis of variance of P-wave travel times from local earthquakes

A method is presented for deriving 1-D velocity depth models from earthquake bulletin data. The models can be used as initial models for more advanced modelling techniques such as tomographic inversion. The method is useful when there is little or no refraction and long-range reflection survey data. The bulletin travel times are subjected to an analysis of variance, where they are separated into source, distance, and receiving station terms. The distance terms describe the variation of travel time with distance, and the associated trend lines allow 1-D velocity models for the crustal layers to be determined. The velocity models provide an average crustal model for the region derived from local data. This does not include superficial layers which are necessarily poorly determined. Earthquake bulletin P-wave data from propagation paths across three different regions of the UK are employed to illustrate the use of the technique.     

Crustal structure and local seismicity in Colombia

Using P-wave travel time data from local seismicity, the crustal structure ofthe central and southern part of Colombia was determined. A very stableand narrow range of possible velocity models for the region was obtainedusing travel time inversion. This range of models was tested with earthquakelocations to select the best velocity model. The 1D velocity modelproposed has five layers over a halfspace, with interfaces at depths of 4,25, 32, 40 and 100 km and P-wave velocities of 4.8, 6.6, 7.0, 8.0, 8.1and 8.2 km/sec, respectively. According to this model the Moho lies at32 km depth on average. For P-waves, the station corrections range from–0.62 to 0.44 sec and for S-wave they range from –1.17 to 0.62 sec.These low variations in station residuals indicate small lateral velocitychanges and therefore the velocity model found should be well suited forearthquake locations and future starting model for 3D tomography studies.Using this new velocity model, the local earthquakes were relocated. Theshallow seismicity, < 30 km, clearly shows the borders betweentectonic plates and also the main fault systems in the region. The deepseismicity, > 80 km, shows two subduction zones in the country: theCauca subduction zone with a strike of N120^°E, dip of 35^°and thickness of 35 km, and the Bucaramanga subduction zone which has,for the northern part, a strike of N103^°E, dip of 27^° andthickness undetermined and, for the southern part, a strike ofN115^°E, dip of 40^° and thickness of 20 km. Based ondifferences of thickness of brittle crust in the subducted slab and spatialdistribution of the seismicity, the Cauca and Bucaramanga subduction zonesseem to represent independent processes. The Cauca subduction seems tobe connected to the process of the Nazca plate being subducted under theNorth Andes Block. In the Bucaramanga subduction zone, the transitionbetween southern and northern parts and changes in geometry of the slabseem to be gradual and there is no evidence of a tear in the slab, howeverthe local seismicity does not allow us to determine which plate or plates arebeing subducted. The Bucaramanga nest appears to be included into thesubducted slab.