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 structure of the Baltic Shield region on the basis of DSS and earthquake data

The lithospheric models obtained for the Baltic Shield by using deep seismic soundings are discussed and results from different parts of the shield are compared with models achieved by the investigation of surface waves and of P to S converted waves. The results are found to agree rather well with each other particularly with regard to the first interface (at a depth of 10–15 km) and the whole thickness of the crust (c. 40 km). The macroseismic focal depth determinations of regional earthquakes are correlated with lithospheric structure. The main maximum in the focal depth distributions of Finnish earthquakes occur at a depth of 10–12 km. The geographical distribution of the earthquake epicentres suggests several seismo-active zones in the northern Baltic Shield. One new finding in this field concerns the Lapland zone, which runs in a north-westerly direction.     

Source mechanism of intermediate and deep earthquakes in southern Spain

Focal mechanisms of 10 intermediate-depth earthquakes (30相似文献   

Upper Mantle Domains beneath Central-Southern Italy: Petrological, Geochemical and Geophysical Constraints

—The Italian peninsula shows high complexity of the mantle-crust system and of the Plio-Quaternary magmatism. The lithospheric thickness has remarkable lateral variations from about 110 km to about 30 km. Intermediate and deep-focus earthquakes indicate the presence of a lithospheric slab under the Aeolian-Calabrian area and at the southern end of Campania. Much less extensive intermediate-depth seismicity characterizes the Roman-Tuscany region, where the existence of a relic slab has been hypothesized. The deep seismicity in the southern Tyrrhenian Sea is associated with active calcalkaline to shoshonitic volcanism in the Aeolian arc. Alkaline potassic volcanism occurs in central Italy, and potassic lamproitic magmatism coexists with crustal anatectic and various types of hybrid rocks in the Tuscany area.¶The parallelism between changing magmatism and variation of the structure of the crust-mantle system makes central-southern Italy a key place where petrological and geophysical data can be used to work out an integrated model of the structure and composition of the upper mantle. Beneath Tuscany the upper mantle has been affected by intensive subduction-related metasomatism. This caused the formation of phlogopite-rich veins that cut through residual spinel-harzburgite and dunite. These veins, possibly partially molten, may explain the unusually soft mechanical properties that are detected just below the Moho. In the Roman Province, the upper mantle is formed by a relatively thin lid (the mantle part of the lithosphere) and by metasomatic fertile peridotite, probably connected with the upraise of an asthenospheric mantle wedge above the Apennines subduction zone. Geochemical data indicate that metasomatism, though still related to subduction, had different characteristics and age than in Tuscany. In the eastern sector of the Aeolian arc and in the Neapolitan area, the upper mantle appears to be distinct from the Roman and Tuscany areas and is probably formed by fertile peridotite contaminated by the presently active subduction of the Ionian Sea floor.¶The overall picture is that of a mosaic of various mantle domains that have undergone different evolutionary history in terms of both metasomatism and pre-metasomatic events. The coexistence side by side of these sectors is a key factor that has to be considered by models of the geodynamic evolution of the Central Mediterranean area.     

Source characteristics of the deep Philippine earthquake cluster, 23 and 24 July 2010

We determine the rupture velocity, rupture area, stress drop and duration of four strong deep-focus earthquakes in the Philippines by back-projecting the teleseismic P waves. Four deep-focus earthquakes occurred in a totally consumed Molucca microplate; their focal depths were greater than 550 km and their moment magnitudes were between M _w 6.6 and M _w 7.6. By studying this deep-focus cluster, we are able to estimate the rupture velocity, rupture area and stress drop which would assist in constraining the physical mechanism for earthquakes deeper than 500 km. Since the Molucca microplate is totally consumed, little evidence is left on the surface for us to do research. This deep-focus cluster provides us the opportunity to reveal the properties of this totally consumed microplate by using seismic method for the first time. Four earthquakes in this deep-focus cluster all have multiple rupture subevents. The M _w 7.3 event ruptures in two subevents, the M _w 7.6 and M _w 7.4 events both have three subevents. The M _w 6.6 event has single peak on the amplitude as a function of time; however, its energy releases at two spatially separated areas. Our results show that this deep-focus cluster has a slow rupture velocity which is about 0.27 to 0.43 of the shear wave velocity, long-scaled duration, concentrated energy release area, and high stress drop. These source properties are similar to those of other deep earthquakes occurring in warm slabs and indicate that the totally consumed Molucca microplate possibly is a warm plate.     

Attenuation of multiple ScS waves beneath the Japanese Arc

36 pairs of multiply-reflected ScS waves from deep earthquakes around Japan are analyzed to investigate the anelastic properties of the mantle on the continental and oceanic sides of the dipping slab. The average Q-value for shear waves passing through the mantle on the oceanic side is found to be 226 in the frequency range 10–40 mHz. This Q-value is in good agreement with the Q models SL8 (Anderson and Hart, 1978) and QBS (Sailor and Dziewonski, 1978) which have been derived from free oscillation data. Assuming that the Q-value for the mantle deeper than 400 km on the continental side of the Japanese Arc is the same as that for the model SL8, we obtain a value of Q = 53 in the upper mantle above the dipping slab beneath the Sea of Japan. Higher Q-values are obtained for the mantle behind the northern Izu-Bonin arc.     

Influence of fluids and magma on earthquakes: seismological evidence

In this paper, we present seismological evidence for the influence of fluids and magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the Japan Islands. The relationship between seismic tomography and large crustal earthquakes (M=5.7-8.0) in Japan during a period of 116 years from 1885 to 2000 is investigated and it is found that most of the large crustal earthquakes occurred in or around the areas of low seismic velocity. The low-velocity zones represent weak sections of the seismogenic crust. The crustal weakening is closely related to the subduction process in this region. Along the volcanic front and in back-arc areas, the crustal weakening is caused by active volcanoes and arc magma resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab. In the forearc region of southwest Japan, fluids are suggested in the 1995 Kobe earthquake source zone, which have contributed to the rupture nucleation. The fluids originate from the dehydration of the subducting Philippine Sea slab. The recent 2001 Geiyo earthquake (M=6.8) occurred at 50 km depth within the subducting Philippine Sea slab, and it was also related to the slab dehydration process. A detailed 3D velocity structure is determined for the northeast Japan forearc region using data from 598 earthquakes that occurred under the Pacific Ocean with hypocenters well located with SP depth phases. The results show that strong lateral heterogeneities exist along the slab boundary, which represent asperities and results of slab dehydration and affect the degree and extent of the interplate seismic coupling. These results indicate that large earthquakes do not strike anywhere, but only anomalous areas which can be detected with geophysical methods. The generation of a large earthquake is not a pure mechanical process, but is closely related to physical and chemical properties of materials in the crust and upper mantle, such as magma, fluids, etc.     

Seismicity of the Prince William Sound Region for over Thirty Years Following the 1964 Great Alaskan Earthquake

—In this paper we present results of body wave-form modeling of 19 earthquakes (generally m _b ≥5.7) occurring from 1964 to 1983 in the vicinity and down-dip of the large asperity within the Prince William Sound region that ruptured in 1964. These data are supplemented with source parameters from studies of more recent (post-1980) events. Our results suggest that moderate earthquakes which occurred in the region between 1964 and 1984 were predominantly located in the vicinity of the Prince William Sound asperity and could be assigned to two groups. The first group consists of events occurring above the plate interface within Prince William Sound along reverse faults or low angle thrusts. The second group occurs at 35 to 60 km depth in the region north of Prince William Sound, and represents normal to normal-oblique faulting within the subducted Pacific crust or upper mantle. These earthquakes occur below the northern edge of the 1964 asperity in a region where the subducting plate undergoes a rapid change in strike and dip. A third group of events occurs in Cook Inlet well down-dip of the 1964 asperity and below the plate interface. These events exhibit a variety of mechanisms and many at depths of 50 to 70 km may be associated with complexities in the shape of the downgoing slab. Most of the Cook Inlet events occurred after 1984, whereas a few events of similar magnitude have occurred in the vicinity of the Prince William Sound asperity since 1984.     

The distribution of earthquakes with depth and stress in subducting slabs

We explain the global variation of Benioff zone seismicity with depth and the orientation of stress axes of deep and intermediate earthquakes using numerical models of subducting slabs. Models that match the seismicity and stress require a barrier to flow at the 670 km seismic discontinuity. The barrier may be a viscosity increase of at least an order of magnitude or a chemical discontinuity. Instantaneous flow is subparallel to the slabs for models with a viscosity increase but contorted for models with a chemical barrier. Log N (number of earthquakes) decreases linearly to 250–300 km depth and increases thereafter. Stress magnitude in our models shows the same pattern, in accord with experiments showing N proportional to e^kσ, with k a constant and σ stress magnitude. The models predict downdip compression in the slabs at depths below 300–400 km, as observed for earthquake stress axes.     

Velocity properties of the lithosphere in the ocean-continent transition zone in the Kamchatka region from seismic tomography data

Arrival times of P and S waves from local earthquakes in the Kamchatka area of the Kurile-Kamchatka Island Arc are used for calculating a spatial model of the elastic wave velocity distribution to a depth of 200 km. The lithosphere is shown to be strongly stratified in its velocity properties and laterally heterogeneous within the mantle wedge and seismic focal zone. A lower velocity layer (an asthenospheric wedge) is identified at depths of 70–130 km beneath the Eastern Kamchatka volcanic belt. The morphology of the Moho interface and the velocity properties of the crust are studied. The main tectonic structures of the region are shown to be closely interrelated with deep velocity heterogeneities. Regular patterns in the statistics of the earthquakes are analyzed in relation to variations in the elastic wave velocities in the focal layer. A mechanism of lithospheric block displacements along weakened zones of the lower crust and upper mantle is proposed.     

Formation time of the big mantle wedge beneath eastern China and a new lithospheric thinning mechanism of the North China craton—Geodynamic effects of deep recycled carbon

High-resolution P wave tomography shows that the subducting Pacific slab is stagnant in the mantle transition zone and forms a big mantle wedge beneath eastern China. The Mg isotopic investigation of large numbers of mantle-derived volcanic rocks from eastern China has revealed that carbonates carried by the subducted slab have been recycled into the upper mantle and formed carbonated peridotite overlying the mantle transition zone, which becomes the sources of various basalts. These basalts display light Mg isotopic compositions(δ26 Mg = –0.60‰ to –0.30‰) and relatively low87 Sr/86 Sr ratios(0.70314–0.70564) with ages ranging from 106 Ma to Quaternary, suggesting that their mantle source had been hybridized by recycled magnesite with minor dolomite and their initial melting occurred at 300-360 km in depth. Therefore, the carbonate metasomatism of their mantle source should have occurred at the depth larger than 360 km, which means that the subducted slab should be stagnant in the mantle transition zone forming the big mantle wedge before 106 Ma. This timing supports the rollback model of subducting slab to form the big mantle wedge. Based on high P-T experiment results, when carbonated silicate melts produced by partial melting of carbonated peridotite was raising and reached the bottom(180–120 km in depth) of cratonic lithosphere in North China, the carbonated silicate melts should have 25–18 wt% CO2 contents, with lower Si O2 and Al2 O3 contents, and higher Ca O/Al2 O3 values, similar to those of nephelinites and basanites, and have higher εNdvalues(2 to 6). The carbonatited silicate melts migrated upward and metasomatized the overlying lithospheric mantle, resulting in carbonated peridotite in the bottom of continental lithosphere beneath eastern China. As the craton lithospheric geotherm intersects the solidus of carbonated peridotite at 130 km in depth, the carbonated peridotite in the bottom of cratonic lithosphere should be partially melted, thus its physical characters are similar to the asthenosphere and it could be easily replaced by convective mantle. The newly formed carbonated silicate melts will migrate upward and metasomatize the overlying lithospheric mantle. Similarly, such metasomatism and partial melting processes repeat, and as a result the cratonic lithosphere in North China would be thinning and the carbonated silicate partial melts will be transformed to high-Si O2 alkali basalts with lower εNdvalues(to-2). As the lithospheric thinning goes on,initial melting depth of carbonated peridotite must decrease from 130 km to close 70 km, because the craton geotherm changed to approach oceanic lithosphere geotherm along with lithospheric thinning of the North China craton. Consequently, the interaction between carbonated silicate melt and cratonic lithosphere is a possible mechanism for lithosphere thinning of the North China craton during the late Cretaceous and Cenozoic. Based on the age statistics of low δ26 Mg basalts in eastern China, the lithospheric thinning processes caused by carbonated metasomatism and partial melting in eastern China are limited in a timespan from 106 to25 Ma, but increased quickly after 25 Ma. Therefore, there are two peak times for the lithospheric thinning of the North China craton: the first peak in 135-115 Ma simultaneously with the cratonic destruction, and the second peak caused by interaction between carbonated silicate melt and lithosphere mainly after 25 Ma. The later decreased the lithospheric thickness to about70 km in the eastern part of North China craton.     

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.     

Seismic evidence for a metastable olivine wedge in the subducting Pacific slab under Japan Sea

We apply a forward-modeling approach to high-quality arrival time data from 23 deep earthquakes greater than 400 km depth to investigate the detailed structure of the subducting Pacific slab beneath the Japan Sea. Our results show that a finger-like anomaly exists within the subducting Pacific slab below 400 km depth, which has a P-wave velocity 5% lower than the surrounding slab velocity (or 3% lower than that of the normal mantle), suggesting the existence of a metastable olivine wedge (MOW) in the slab. The MOW top and bottom depths are 400 and 560 km, respectively. The MOW is estimated to be about 50 km wide at 400 km depth and close to the slab upper boundary. At 560 km depth the MOW is located at about 25 km below the slab upper boundary. Most of the deep earthquakes are located in the MOW. Our results favor transformational faulting as the mechanism for deep earthquakes.     

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:

上海地区地壳精细结构的综合地球物理探测研究

通过在上海地区开展深、浅地震反射、地震宽角反射/折射、高分辨地震折射和大地电磁测深等联合剖面探测, 获得了该地区近地表至Moho面的精细速度结构、电性结构和深浅构造关系.结果表明, 该地区地壳可划分为上、中、下三个组成部分.其中,上地壳厚为12～14 km,波速为5.7～5.9 km/s;中地壳厚度约为10 km,波速为5.9～6.2 km/s;下地壳厚为10～11 km, 波速为6.2～6.3 km/s,Moho面深度约为32 km.剖面浅部地质构造复杂,共解释出12条特征明显的断裂.其中,除3条断裂错断结晶基底（G界面）并向下延伸至上地壳底界面外,其他断裂均在深度3～5 km以上终止或收敛于G界面之上.此外,仅在剖面西侧基底下部约13～15 km埋深处发现一厚度在2 km左右的壳内高导层.所以,在综合各方面资料后分析认为,在剖面经过地区不存在发生大地震的深部构造条件,近地表所存在的活动断层是未来产生对该区有影响地震的震源区.     

Flat-Slab Thermal Structure and Evolution Beneath Central Mexico

Recent seismic and magnetotelluric experiments, aimed at better characterizing the shape and state of the subducting slab and continental crust beneath Central Mexico, exposed significant differences with conclusions of previous studies. A new slab geometry is revealed in which the subducting Cocos slab is perfectly flat between 120 to 290?km from the trench, after which it plunges into the asthenosphere at a dip angle of ~65°, in sharp contrast with the previously proposed ~20° dip angle. Seismic tomography studies show negative P-wave velocity anomalies (?2 to ?4%) in the mantle wedge beneath the Mexican Volcanic Belt, and positive anomalies (+2 to +3%) for the subducted Cocos slab. Magnetotelluric experiments exposed a very low-resistivity area (1?C10? ??m) located within the continental crust just below the Mexican Volcanic Arc. Finally, several spots of non-volcanic tremors (NVTs) have been recorded inside the continental crust above the flat-slab segment. While all these experiments provide a better picture of the subduction system beneath Central Mexico, several key processes need further investigation. In this study, we take advantage of these new observations to better constrain the thermal structure beneath Central Mexico. Two different thermal models are computed for a mantle potential temperature (T _p) of 1,350 and 1,450°C, respectively. The new thermal structures are then converted into P-wave velocity anomalies and compared with the observed V _p anomalies. We found that a T _p of 1,450°C produced larger V _p anomalies that do not fit the observations. However, using a T _p of only 1,350°C, our predicted V _p anomalies are positive (+2 to +3%) for the cold slab and negative (?2 to ?4%) in the mantle wedge. These V _p estimates are consistent with the observed seismic tomography from P-wave arrivals, and therefore we conclude that a T _p of 1,350°C is a better estimate for the mantle potential temperature beneath Central Mexico. The new thermal model, in conjunction with phase diagrams for sediments, hydrated basalt and lithospheric mantle, have been used to estimate the amount and location of fluids released from the subducting Cocos slab. Several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat-slab. We found a good correlation between the pattern of these dehydration pulses and the location of NVTs, suggesting that slab dehydration is responsible for triggering the tremors. We suggest that NVT bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening, a process that has been going on since 15?Ma ago when the Cocos slab entered into a flat-slab regime. Such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15?Ma, allowing the slab to easily roll back. The continuous low-resistivity region recorded beneath the volcanic front in Central Mexico might represent the evidence of slab dehydration and crust weakening over time.     

Q and its effect on the observation of upper mantle travel-time branches

There is broad agreement among various seismological studies that the upper mantle has two regions where high positive velocity gradients or transition zones exist. The presence of these zones implies that two major triplications should exist in the travel-time curve for distances less than 30°. Approximately 200 earthquakes from the New Guinea, New Britain, and Solomon Island regions recorded at the Warramunga Array were analyzed using adaptive processing methods in an attempt to identify the positions of the later arrival branches. From measurements made along the first 20 sec of the arrivals, a retrogade travel-time branch associated with the “650-km” discontinuity was clearly identified as extending from 21° to 26°, and some evidence was found near 16° for the lower portion of the triplication associated with the “400-km” discontinuity. A careful search revealed however that the upper portions of the replicated travel-time branches were missing. There were no observed values ofdt/dΔ in the 12–13 sec/deg range for Δ greater than 20°. In this study it was found that if anelastic effects (Q) were not taken into consideration or ifQ were kept constant, the models derived from observed travel-time data all predicted large amplitude arrivals where non existed. The difficulty with the first triplication was resolved by the introduction of a lowQ region at depths of 85–315 km. This region may be associated with “the low-velocity region” but it is not necessary to decrease the P velocity to explain the observations.The difficulty with the second triplication was resolved by the introduction of a layer at a depth of 575–657 km which has no velocity gradient and a value ofQ significantly less than that for the material just below the “650-km” discontinuity. This layer may well represent the return path for an upper mantle convection cell.     

Studies of coda using array and three-component processing

The application of standard array processing techniques to the study of coda presents difficulties due to the design criteria of these techniques. Typically the techniques are designed to analyze isolated, short arrivals with definite phase velocity and azimuth and have been useful in the frequency range around 1 Hz. Coda is long in time and may contain waves of different types, phase velocities and azimuths. Nonetheless, it has proved possible to use or adapt array methods to answer two questions: what types of waves are present in coda and where are they scattered? Most work has been carried out on teleseismicP coda; work on local coda has lagged due to lack of suitable data and the difficulties of dealing with high frequencies. The time domain methods of beamforming and Vespagram analysis have shown that there is coherent energy with a high phase velocity comparable toP orPP in teleseismicP coda. These methods can detect this “coherent” coda because it has a fairly definite phase velocity and the same, or close to, azimuth as firstP orPP. This component must consist ofP waves and is either scattered near the source, or reflected in the mantle path as apdpP or precursorPP reflection. The Fourier transform method of the frequency-wavenumber spectrum has been adapted by integrating around circles of constant phase velocity (constant total wavenumber) to produce the wavenumber spectrum, which shows power as a function of wavenumber, or phase velocity. For teleseismicP coda, wavenumber spectra demonstrate that there is a “diffuse” coda of shear,Lg or surface waves scattered from teleseismicP near the receiver. Wavenumber spectra also suggest that the coherent coda is produced by near-source scattering in the crust, not mantle reflection, since it is absent or weak for deep-focus events. Crustal earthquakes have a very strong coherent component of teleseismic coda, suggesting scattering from shear to teleseismicP near the source. Three-component analysis of single-station data has shown the presence of off-azimuth arrivals and may lead to the identification of waves scattered from a single scatterer.     

Imaging 3-D crustal P-wave velocity structure of western Yunnan with bulletin data

Western Yunnan is a region with intensive tectonic activity and serious earthquake risk. It is of significant importance to study three dimensional crustal structure of this region to understand the tectonic setting and disaster mechanism. Densification and digitalization of seismic networks in this region provides an opportunity to study the velocity structure with bulletin data. In this study, we collect P-wave data of 10 403 regional earthquakes recorded by 79 seismic stations from January 2008 to December 2010. In addition to first arrivals data (Pg with epicentral distance less than 200 km and Pn), the Pg (or P) data with epicentral distance more than 200 km are also considered as later direct arrivals in the tomographic inversion. We also compare the quantity and the quality of the seismic data before 2010 and after 2010. The test results show that adding the follow-up Pg phase can effectively improve the inversion ability of crustal imaging, and quantity and the data quality are significantly improved since 2010. The tomographic results show that: (1) The Honghe fault zone, which is the major fault systems in this region, may cut through the entire crust, and the velocity contrasts between two sides at lower crust beneath the Honghe fault are estimated at higher than 10%, while the velocity difference below Nujiang fault zone extends only in the upper crust; (2) Most of the earthquakes in the region occurred at the interface of high-velocity media and low-velocity media, i.e., the areas with high velocity gradient, which has been validated in other areas.     

应用远震有限频率层析成像反演首都圈上地幔速度结构

本文选用首都圈数字地震台网2003年9月~2005年12月记录的300多个远震事件的波形资料,采用分频带多道互相关方法得到三个不同频段的P波相对走时数据共18499个,计算了每个频段的走时灵敏度核,应用有限频率层析成像反演得到首都圈地区的上地幔三维P波速度结构模型.利用检测板估计了反演结果的分辨率,并与射线层析成像方法的结果进行了比较,说明了反演结果的可靠性.研究结果表明,各构造单元具有明显不同的速度结构特征,其差异可到150 km深:燕山隆起区表现高速;太行山隆起区整体以低速为主并存在小范围高速块体;华北盆地、渤海湾下浅层上地幔中存在大范围的强低速异常,其顶面在50~70 km,可视为软流圈顶面的埋深,这一结果说明华北盆地、渤海湾下岩石圈明显减薄;张家口—蓬莱断裂带是上地幔浅部速度结构的变异带,也是岩石圈减薄的边界带,区内大部分强震都发生在该构造带上,由此看来该带上强震的发生不仅与地壳结构的不均匀性有关,还可能有较深的构造背景.