南极半岛地区地震勒夫波群速度与岩石圈结构

本文依据南极智利贝纳尔多奥伊金斯将军站的来自南桑德韦奇群岛的Ｍｓ＝６．４级地震面波资料，采用最新的适配滤波频时分析技术和网格频散反演方法，计算了南极半岛地区的勒夫波群速度频散，并获得了该区岩石圈结构模型。结果表明，西南极南极半岛地区地壳为成层结构，可分为三层，其分层厚度分别为５ｋｍ、８ｋｍ和１０ｋｍ。地壳厚度为２３ｋｍ，上地幔顶部速度为５．３２ｋｍ／ｓ。在深度５３ｋｍ以下，地幔介质中速度逐步降低为５．１１ｋｍ／ｓ和４．９ｋｍ／ｓ，最低值达４．８ｋｍ／ｓ，可以推断南极地幔亦为成层结构。     

南极沿167°E子午线横贯南极山脉岩石圈速度结构

依据沿大圆弧穿越南极点和斯科特站两地震台的地震瑞利面波波形资料,计算了两台之间的相速度频散,通过反演计算,获得了台间地下２００ｋｍ 岩石圈剪切波速度细结构。结果表明,横贯南极山脉地壳厚度约为４５ｋｍ ,５５～７５ｋｍ 之间存在明显低速带,它预示着这一深度有熔融的岩浆存在。     

Crustal structure of Iraq from receiver functions and surface wave dispersion: implications for understanding the deformation history of the Arabian–Eurasian collision

We report the crustal structure for two locations in Iraq estimated by joint inversion of P -wave receiver functions (RFs) and surface (Rayleigh) wave group velocity dispersion. RFs were computed from teleseismic recordings at two temporary broad-band seismic stations located in Mosul (MSL) in the Zagros Fold Belt and Baghdad (BHD) in the Mesopotamian Foredeep. Group velocity dispersion curves at the sites were derived from continental-scale tomography. The inversion results show that the crustal thicknesses are 39 km at MSL and 43 km at BHD. We observe a strong Ps _Moho at BHD consistent with a sharp Moho discontinuity. However, at MSL we observe a weak Ps _Moho suggesting a transitional Moho where crustal thickening is likely to be occurring in the deep crust. Both sites reveal low velocity surface layers consistent with sedimentary thickness of about 3 km at station MSL and 7 km at BHD and agreeing well with the previous reports. Ignoring the sediments, the crystalline crustal velocities and thicknesses are remarkably similar at both stations. The similarity of crustal structure suggests that the crust of the northeastern proto-Arabian Platform was uniform before subsidence and deposition of the sediments in the Cenozoic. If crystalline crustal structure is uniform across the northern Arabian Platform then crustal thickness variations in the Zagros Fold Belt and Thrust Zone should reveal the history of deformation and crustal shortening in the Arabian–Eurasian collision zone and not reflect pre-existing crustal thickness variations in the Arabian Plate.     

Crust-mantle structure under South Pole and Ross Sea Beach in Antarctica

Ｃｒｕｓｔ－ｍａｎｔｌｅｓｔｒｕｃｔｕｒｅｕｎｄｅｒＳｏｕｔｈＰｏｌｅａｎｄＲｏｓｓＳｅａＢｅａｃｈｉｎＡｎｔａｒｃｔｉｃａＳｈｕＰｅｉｙｉ（束沛镒）ａｎｄＺｈａｎｇＢｏ（张钋）（ＩｎｓｔｉｔｕｔｅｏｆＧｅｏｐｈｙｓｉｃｓ，ＡｃａｄｅｍｉａＳｉｎｉｃ...     

南极点和罗斯海滨壳幔结构

本文根据美国极地台（南极点）和新西兰斯科特台罗斯海滨的ＷＷＳＳＮ台所接收的长周期深源远震图Ｐ波波形的理论地震图拟合，探讨南极洲壳幔构造特征。结果表明，南极点下方地壳厚约４５ｋｍ，双层结构；罗斯海滨地层厚约２４ｋｍ，３５０～４５０ｋｍ深度有明显低速带存在，它们反映了南极横贯山脉两侧不同的构造特征     

Joint inversion of receiver function and surface wave dispersion observations

We implement a method to invert jointly teleseismic P wave receiver functions and surface wave group and phase velocities for a mutually consistent estimate of earth structure. Receiver functions are primarily sensitive to shear wave velocity contrasts and vertical traveltimes, and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages. Their combination may bridge resolution gaps associated with each individual data set. We formulate a linearized shear velocity inversion that is solved using a damped leastsquares scheme that incorporates a priori smoothness constraints for velocities in adjacent layers. The data sets are equalized for the number of data points and physical units in the inversion process. The combination of information produces a relatively simple model with a minimal number of sharp velocity contrasts. We illustrate the approach using noisefree and realistic noise simulations and conclude with an inversion of observations from the Saudi Arabian Shield. Inversion results for station SODA, located in the Arabian Shield, include a crust with a sharp gradient near the surface (shear velocity changing from 1.8 to 3.5 km s¹ in 3 km) underlain by a 5kmthick layer with a shear velocity of 3.5 km s¹ and a 27kmthick layer with a shear velocity of 3.8 km s¹, and an upper mantle with an average shear velocity of 4.7 km s¹. The crustmantle transition has a significant gradient, with velocity values varying from 3.8 to 4.7 km s¹ between 35 and 40 km depth. Our results are compatible with independent inversions for crustal structure using refraction data.     

Upper crustal shear velocity models from higher mode Rayleigh wave dispersion in Scotland

Summary. Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8–4.8 Hz, generated from a local earthquake of magnitude 3.7 M _L in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s⁻¹ in the upper 2 km, with a slow increase to around 3.8 km s⁻¹. P -wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments.     

Waveform inversion of mantle Love waves:the Born seismogram approach

Summary. Normal mode theory, extended to the slightly laterally heterogeneous earth by the first-order Born approximation, is applied to the waveform inversion of mantle Love wave (200–500 s) for the Earth's lateral heterogeneity at l = 2 and a spherically symmétric anelasticity ( Q _μ) structure. The data are from the Global Digital Seismograph Network (GDSN). The l =2 pattern is very similar to the results of other studies that used either different méthods, such as phase velocity measurements and multiplet location measurements, or a different data set, such as mantle Rayleigh waves from different instruments. The results are carefully analysed for variance reduction and are most naturally explained by heterogeneity in the upper 420 km. Because of the poor resolution of the data set for the deep interior, however, a fairly large heterogeneity in the transition zones, of the order of up to 3.5 per cent in shear wave velocity, is allowed. It is noteworthy that Love waves of this period range cannot constrain the structure below 420 km and thus any model presented by similar studies below this depth are likely to be constrained by Rayleigh waves (spheroidal modes) only.
The calculated modal Q values for the obtained Q _μ model fall within the error bars of the observations. The result demonstrates the discrepancy of Rayleigh wave Q and Love wave Q and indicates that care must be taken when both Rayleigh and Love wave data, including amplitude information, are inverted simultaneously.
Anomalous amplitude inversions of G2 and G3, for example, are observed for some source-receiver pairs. This is due to multipathing effects. One example near the epicentral region, which is modelled by the obtained l = 2 heterogeneity, is shown.     

The crustal structure beneath the northwest fjords, Iceland, from receiver functions and surface waves

Five broad-band seismic stations were operated in the northwest fjords area of Iceland from 1996 to 1998 as part of the Iceland Hotspot project. The structures of the upper 35  km or so beneath these stations were determined by the modelling and joint inversion of receiver functions and regional surface wave phase velocities. More than 40 teleseismic events and a few regional events containing high-quality surface wave trains were used. Although the middle period passband of the seismograms is corrupted by oceanic microseismic noise, which hinders the interpretation of structural details, the inversions reveal the overall features. Many profiles obtained exhibit large velocity gradients in the upper 5  km or so, smaller zero gradients below this, and, at ~23  km depth, a zone 2–4  km thick with higher velocity gradients. The two shallower intervals are fairly consistent with the 'upper' and 'lower' crust, defined by Flovenz (1980 ). The deep zone of enhanced velocity gradient seems to correspond to the sharp reflector first reported by Bjarnason et al . (1993 ) and identified by them as the 'Moho'. However, this type of structure is not ubiquitous beneath the northwest fjords area. The distinctiveness of the three intervals is variable, and in some cases a structure with velocity gradient increasing smoothly with depth is observed. We term these two end-members structures of the first and second types respectively. Structures of the second type correlate with older areas. Substantial variation in fundamental structure is to be expected in Iceland because of the great geological heterogeneity there.     

Transitional continental–oceanic structure beneath the Norwegian Sea from inversion of surface wave group velocity data

We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50  s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18  km in the Norwegian Sea area and 9  km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9  km  s⁻¹ for most paths. An average lithospheric thickness of 60  km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30  km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.     

Crustal and uppermost mantle structure in southern Africa revealed from ambient noise and teleseismic tomography

Rayleigh wave phase velocity maps in southern Africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the Southern Africa Seismic Experiment (SASE) deployed between 1997 and 1999. These phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two-plane-wave method by Li & Burke. In the period range of overlap (25–40 s), the ambient noise and two-plane-wave methods yield similar phase velocity maps. Dispersion curves from 6 to 143 s period were used to estimate the 3-D shear wave structure of the crust and uppermost mantle on an 1°× 1° grid beneath southern Africa to a depth of about 100 km. Average shear wave velocity in the crust is found to vary from 3.6 km s^–1 at 0–10 km depths to 3.86 km s^–1 from 20 to 40 km, and velocity anomalies in these layers correlate with known tectonic features. Shear wave velocity in the lower crust is on average low in the Kaapvaal and Zimbabwe cratons and higher in the surrounding Proterozoic terranes, such as the Limpopo and the Namaqua-Natal belts, which suggests that the lower crust underlying the Archean cratons is probably less mafic than beneath the Proterozoic terranes. Crustal thickness estimates agree well with a previous receiver function study of Nair et al. . Archean crust is relatively thin and light and underlain by a fast uppermost mantle, whereas the Proterozoic crust is thick and dense with a slower underlying mantle. These observations are consistent with the southern African Archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust, if the foundering process became less vigorous in arc environments during the Proterozoic.     

Detailed S-wave structure in the Dublin Basin and its northern margin

Summary. The seismic structure has been measured to a depth of about 3 km along a 30 km seismic profile in east central Ireland. This profile is unusual in that it is the S -wave velocity—depth structure that has been measured to a degree of precision more normally associated with P -wave results. One reason for this is that the sources used were quarry blasts which generated strong S -waves and short-period surface waves but rather weak P -waves.
The results show a layer of Carboniferous limestone with shear velocity 2.65 km⁻¹ s overlying a layer with a velocity of 3.06 km s⁻¹. This second layer was interpreted as Lower Palaeozoic strata (Silurian/Ordovician) since this velocity was evident in an inlier seen at the surface at the northern end of the line. A third refraction horizon, shear velocity 3.45 km s⁻¹ and displaying a basinal structure, was also recognized. This may be Cambrian or Precambrian basement.     

Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis – II. Crustal and upper-mantle structure

We determine the 3-D shear wave speed variations in the crust and upper mantle in the southeastern borderland of the Tibetan Plateau, SW China, with data from 25 temporary broad-band stations and one permanent station. Interstation Rayleigh wave (phase velocity) dispersion curves were obtained at periods from 10 to 50 s from empirical Green's function (EGF) derived from (ambient noise) interferometry and from 20 to 150 s from traditional two-station (TS) analysis. Here, we use these measurements to construct phase velocity maps (from 10 to 150 s, using the average interstation dispersion from the EGF and TS methods between 20 and 50 s) and estimate from them (with the Neighbourhood Algorithm) the 3-D wave speed variations and their uncertainty. The crust structure, parametrized in three layers, can be well resolved with a horizontal resolution about of 100 km or less. Because of the possible effect of mechanically weak layers on regional deformation, of particular interest is the existence and geometry of low (shear) velocity layers (LVLs). In some regions prominent LVLs occur in the middle crust, in others they may appear in the lower crust. In some cases the lateral transition of shear wave speed coincides with major fault zones. The spatial variation in strength and depth of crustal LVLs suggests that the 3-D geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur. Consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity.     

Anomalous propagation of surface waves in the Barents Sea as inferred from NORSAR recordings

Summary. NORSAR recordings of Rayleigh waves generated by presumed nuclear explosions on central and southern Novaya Zemlya and in northwestern Siberia have been studied. Using a frequency time analysing technique and correcting for presumed known dispersion effects across the Baltic Shield, dispersion curves for two different paths across the southern part of the Barents Sea were obtained. The curves are very unusual in that they give extremely low velocities even for periods up to 20 s. For the path to the middle part of the island, the inversion of the data gives a model with sediments and consolidated sediments down to 25 km, followed by a 15-km thick basaltic layer and an upper mantle with a P velocity as low as 7.9 km/s. For the path to the southern part of Novaya Zemlya the data inversion gives a somewhat different model with sediments and consolidated sediments down to 8 km, followed by a 17-km thick zone with velocities close to granitic and a 15-km thick layer with basaltic velocities. Again the upper-mantle P velocity is only 7.9 km/s. Other indications of lateral inhomogeneities in the Barents Sea are obtained by utilizing the array's capability to determine the angle of approach of seismic waves. It is demonstrated that reflections both from inhomogeneities in the Barents Sea and the continental margin off Norway can be detected. For waves from the southern end of the island, a reflection from a strong discontinuity close to the direct path to the middle part of the island is found, whereas signals from this area include a reflected wave possibly coming from the edge of the Svalbard platform.     

Surface-wave propagation across the Mexican Volcanic Belt and the origin of the long-period seismic-wave amplification in the Valley of Mexico

A network of nine broad-band seismographs was operated from March to May 1994 to study the propagation of seismic waves across the Mexican Volcanic Belt (MVB) in the region of the Valley of Mexico. Analysis of the data from the network reveals an amplification of seismic waves in a wide period band al the stations situated in the southern part of the MVB.
The group velocities of the fundamental mode of the Rayleigh wave in the period range 2–13 s are found to be lower in the southern part of the MVB than in its northern part and in the region south of the MVB. The inversion of dispersion curves shows that the difference in group velocities is due to the presence of a superficial low-velocity layer (with an average S -wave velocity of 1.7 km s^-1 and an average thickness of 2 km) beneath the southern part of the MVB. This low-velocity zone is associated with the region of active volcanism.
Numerical simulations show that this superficial low-velocity layer causes a regional amplification of 8–10 s period signals, which is of the same order as the amplification measured from the data. This layer also increases the signal duration significantly because of the dispersion of the surface waves. These results confirm the hypothesis of Singh et al. (1995), who suggested that the regional amplification observed in the Valley of Mexico is due to the anomalously low shear-wave velocity of the shallow volcanic rocks in the southern MVB     

The correlation of microtremors: empirical limits and relations between results in frequency and time domains

The use of the correlation of microtremor records is on its way to develop into a common tool to estimate local shear wave velocity structure. For this reason, the establishment of the conditions for the correct use of this method and its limitations when applied to real data is becoming increasingly important. In addition to the use of frequency domain spatial correlation technique [the Spatial Autocorrelation (SPAC) method], the use of time domain correlation to obtain the Green's function of the medium is rapidly gaining presence. We explore the use of microtremor correlation techniques in the time domain to determine local velocity structure and compare with previous results obtained with the same data using SPAC. Our data come from three experiments carried out in Parkway and Wainuiomata valleys in New Zealand, using broad-band portable stations. Interstation distances range from 5 m to 2.1 km, and our results are useful in the frequency band from 0.1 to 7 Hz. Frequency domain correlation requires an isotropic microtremor field, a condition that need not be satisfied in the time domain. Two station correlations provide useful results due to the temporal stationarity and isotropy, in average, of the microtremor wavefield. This manifests itself in the symmetry of the temporal correlation functions with respect to zero time. Our results show that the local velocity structure and the interstation distance are the key factors conditioning the frequency range where surface wave dispersion can be correctly measured either in frequency or time domains. We confirm that, when the interstation distance becomes much larger than the dominant wavelengths, only the correlation in time domain is useful. All our results indicate that the signal obtained in the correlation of vertical component microtremors is due to the fundamental mode of Rayleigh waves, which appears as the most stable propagation mode, without any indication of body waves.     

The Mid-Atlantic Ridge: structure at 45° N

Summary. A structural model of the Mid-Atlantic Ridge at 45° N is proposed on the basis of travel-time data, amplitudes and synthetic seismograms. The crustal structure seems to be similar to that in the FAMOUS area (Fowler). At the ridge axis there is an absorptive zone in the upper mantle, the depth below the seabed to the top of this zone being about 6 km. Away from the ridge axis there is a positive velocity gradient of about 0.04 to 0.05 km/(skm) in the top 5 to 8 km of the upper mantle. Shear waves propagate across the ridge axis, suggesting that there is no sizeable crustal magma chamber. The shear-wave velocity of the uppermost mantle is 4.35 km/s.     

Simultaneous inversion of seismic data

Summary. The resolving power of different data sets, consisting of surface-wave dispersion measurements and S travel times, are compared for a continental structure. The shear velocity in the low-velocity zone can be resolved in some detail with higher-mode phase-velocity data. Sufficient resolution for small density contrasts (0.03 g cm⁻³) until depths of ∼ 300 km can be reached if higher-mode group velocities are available as well, even at a precision as low as 0.10 km/s. At greater depths the density is not resolved, and here travel-time data are superior to higher modes in resolving the shear velocity.     

Lithospheric structure of the continent–continent collision zone: eastern Turkey

We infer the lithospheric structure in eastern Turkey using teleseismic and regional events recorded by 29 broad-band stations from the Eastern Turkey Seismic Experiment (ETSE). We combine the surface wave group velocities (Rayleigh and Love) with telesesimic receiver functions to jointly invert for the S -wave velocity structure, Moho depth and mantle-lid (lithospheric mantle) thickness. We also estimated the transverse anisotropy due to Love and Rayleigh velocity discrepancies. We found anomalously low shear wave velocities underneath the Anatolian Plateau. Average crustal thickness is 36 km in the Arabian Plate, 44 km in Anatolian Block and 48 km in the Anatolian Plateau. We observe very low shear wave velocities at the crustal portion (30–38 km) of the northeastern part of the Anatolian Plateau. The lithospheric mantle thickness is either not thick enough to resolve it or it is completely removed underneath the Anatolian Plateau. The shear velocities and anisotropy down to 100 km depth suggest that the average lithosphere–asthenosphere boundary in the Arabian Plate is about 90 and 70 km in Anatolian block. Adding the surface waves to the receiver functions is necessary to constrain the trade-off between velocity and the thickness. We find slower velocities than with the receiver function data alone. The study reveals three different lithospheric structures in eastern Turkey: the Anatolian plateau (east of Karliova Triple Junction), the Anatolian block and the northernmost portion of the Arabian plate. The boundary of lithospheric structure differences coincides with the major tectonic boundaries.     

Resolution potential of surface wave phase velocity measurements at small arrays

The deployment of temporary arrays of broadband seismological stations over dedicated targets is common practice. Measurement of surface wave phase velocity across a small array and its depth-inversion gives us information about the structure below the array which is complementary to the information obtained from body-wave analysis. The question is however: what do we actually measure when the array is much smaller than the wave length, and how does the measured phase velocity relates to the real structure below the array? We quantify this relationship by performing a series of numerical simulations of surface wave propagation in 3-D structures and by measuring the apparent phase velocity across the array on the synthetics. A principal conclusion is that heterogeneities located outside the array can map in a complex way onto the phase velocities measured by the array. In order to minimize this effect, it is necessary to have a large number of events and to average measurements from events well-distributed in backazimuth. A second observation is that the period of the wave has a remarkably small influence on the lateral resolution of the measurement, which is dominantly controlled by the size of the array. We analyse if the artefacts created by heterogeneities can be mistaken for azimuthal variations caused by anisotropy. We also show that if the amplitude of the surface waves can be measured precisely enough, phase velocities can be corrected and the artefacts which occur due to reflections and diffractions in 3-D structures greatly reduced.