Simultaneous measurements of compressional wave and shear wave velocities, Poisson's ratio, and Vp/Vs under deep crustal pressure and temperature conditions: Example of silicified pelitic schist from Ryoke Belt, Southwest Japan

The ultrasonic technique for measuring travel times of compressional and shear waves using dual-mode transducers was adapted to a piston cylinder apparatus, allowing simultaneous measurements of travel times of compressional and shear waves of island arc samples under the high pressure and temperature conditions of island arcs. This method enables us to determine elastic properties and their pressure and temperature derivatives simultaneously. Furthermore, Vp/Vs can be directly determined from travel times of compressional and shear waves independently of length change due to compression or thermal expansion of rock samples under deep crustal conditions, providing more accurate Vp/Vs values than those determined from individual measurements of travel times of both elastic wave types using single-mode transducers. Experimental techniques and results are demonstrated using data on silicified pelitic schist from the Ryoke Belt to 0.6 GPa. The simultaneous measurement gives Vp  = 5.60 km/s, ∂ Vp /∂ P  = 0.090 (km/s)/GPa, Vs  = 3.37 km/s, ∂ Vs /∂ P  = 0.05 (km/s)/GPa, σ  = 0.216, and Vp / Vs  = 1.66 at ambient conditions. The temperature derivatives were constrained from fitting using linear functions of temperature, yielding ∂ Vp /∂ T  = −0.518 × 10⁻³ (km/s)/K and ∂ Vs /∂ T  = −0.182 × 10⁻³ (km/s)/K. Performing simultaneous measurements of travel times of compressional and shear waves using dual-mode transducers, it is possible to accurately determine Vp / Vs and Poisson's ratio of crustal minerals and rocks at deep crustal conditions to study the composition of the crustal interior, e.g. rock types and fluids below the hypocentral region of earthquakes or around bright spots.     

腾冲全新世火山区P波和S波速度及其比值

９个发生于观测台网区域上地壳内的地震,用Ｐ波到时定位,从ＴＰ～Ｔs－Ｐ图得出高Ｖ Ｐ／Ｖs值,显示了低速Ｓ波存在的信息。在资料有限情况下,假设震中距不变,反演震源 深度,发震时,ＶＰ和Ｖs,得出ＶＰ＝５．９０ｋｍ／ｓ,Ｖｓ＝３．０４ｋｍ／ｓ,ＶＰ／Ｖｓ＝２．９４。Ｖｐ比区域的同深度平均Ｐ波速度６．０ｋｍ／ｓ低１．７％,Ｖs 比按弹介质的Ｓ波速度＝Ｖｐ／１．７３２低１２．４％。这种低速结果符合富含液性物质     

Modeling of Rayleigh waves dispersion in the Sannio region (Southern Italy) from seismic active refraction data

Short period surface waves, recorded during a seismic refractionsurvey in the Sannio region (Southern Italy), have been modeled to infera shallow velocity model for the area. Based on the decrease of resolutionwith depth, due to the bias on group velocity estimates arising frominterference of the Rayleigh waves with higher modes, we carried out aprocedure of fitting, with synthetic seismograms, of selected filtered traceswith a gaussian filter, having a width at half height equal to 1 Hz and acentral frequency lying in the range [1,4] Hz. We estimated the likelihoodbetween synthetic and observed seismograms by measuring their semblance.In this way we were able to infer a more refined local velocity modelcharacterized by a high Vp and Vs vertical gradient in the sedimentarycover. Two ad hoc resolution studies, based on group velocity andamplitude data respectively, indicate that the local velocity model is a goodvelocity model also for the entire studied area. The increase in the numberof available data when using amplitude information allows us to make amore selective choice in the model parameter space (Vp and Vs of eachlayer) and to solve for the Vp/Vs ratio. The inferred Vp velocity in thehalf-space is equal to 2.8 km/s. This value is in excellent agreement withthat inferred by other authors (3 km/s) by modeling P-wave travel timevs. distance. The best-fit model furnish low Vp/Vs for the sedimentarycover so indicating a high degree of the sediment's compaction in thestudied area. The inferred shallow high-velocity gradient indicates thatthe shallow sedimentary layer in the area could trap and focus the energytraveling into it.     

邢台震源区波速比异常与地震的关系

通过对忻州─泰安人工地震测深剖面Ｐ波、Ｓ波的联合解释，得到沿剖面不同地质单元隆起区与裂陷区、震源区与非震区的速度和波速比结构．鲁西隆起和太行、山西隆起为较均匀的成层构造，地壳厚度分别为３２ｋｍ和４０－４３ｋｍ，波速比为１．７４．中段裂陷区构造变化较大，地壳厚度约３０－３３ｋｍ，波速比为１．７５－１．７７．邢台地震区上地壳下部和中地壳出现高波速比１．７７的异常，与裂陷区东的１．７３形成明显的差异．由此推测，地震的发生不仅与震源区的构造有关，更主要是与震源区岩石的性质有关。     

天山造山带地壳结构与构造──乌鲁木齐─库尔勒地震转换波测深剖面

沿乌鲁木齐－库尔勒３５０余公里长的测线上完成了穿越天山的地震转换波野外观测，得出天山造山带的深部构造剖面图和壳内Ｐ，Ｓ波速度结构模型．天山的结晶地壳可分为４层，各层的速度分别是：Ｖｐ值为５．９，６．３，６．６，６．９ｋｍ／ｓ；Ｖｓ值为３．２，３．５，３．７，３．９ｋｍ／ｓ。上地幔顶部的波速：Ｖｐ值为８．１０ｋｍ／ｓ。Ｖｓ值为４．６ｋｍ／ｓ。天山地区的地壳厚度为４２－５６ｋｍ，中天山最厚，达到５０－５６ｋｍ。地壳中部存在低速透镜体。探测结果表明，天山大地构造可分为北天山、中天山和南天山３个区，其地壳深部构造特征各不相同。用颤动构造理论来解释天山造山带的形成机制和演化较板块构造理论更为合理。     

天山造山带地壳结构与构造──乌鲁木齐─库尔勒地震转换波测深剖面

沿乌鲁木齐－库尔勒３５０余公里长的测线上完成了穿越天山的地震转换波野外观测，得出天山造山带的深部构造剖面图和壳内Ｐ，Ｓ波速度结构模型．天山的结晶地壳可分为４层，各层的速度分别是：Ｖｐ值为５．９，６．３，６．６，６．９ｋｍ／ｓ；Ｖｓ值为３．２，３．５，３．７，３．９ｋｍ／ｓ。上地幔顶部的波速：Ｖｐ值为８．１０ｋｍ／ｓ。Ｖｓ值为４．６ｋｍ／ｓ。天山地区的地壳厚度为４２－５６ｋｍ，中天山最厚，达到５０－５６ｋｍ。地壳中部存在低速透镜体。探测结果表明，天山大地构造可分为北天山、中天山和南天山３个区，其地壳深部构造特征各不相同。用颤动构造理论来解释天山造山带的形成机制和演化较板块构造理论更为合理。     

Seismic imaging of the middle and upper crust by double-difference tomography: the Haicheng earthquake (Ms 7.3) in Liaoning Province

The Haicheng earthquake (Ms 7.3) occurred in Liaoning Province (39°N–43°N, 120°E–126°E ), China on February 4, 1975. The mortality rate was only 0.02% owing to the first timely and accurate prediction, although the area affected by the earthquake was 9200 km² and covered cities with a population density of 1000 p/km². In this study, the doubledifference (DD) tomography method was used to obtain high-resolution three-dimensional (3D) P- and S-wave velocity (Vp and Vs) structures and Vp/Vs as well as the earthquake locations. Tomography results suggest that velocity structure at shallow depth coincides well with topography and sediment thickness. The earthquake locations form a northwest-striking zone associated with the Jinzhou(JZ) Fault and a northeast-striking zone associated with the Haichenghe-Dayanghe (HD) Fault, and suggest that the JZ Fault consists of three faults and the Ms 7.3 Haicheng earthquake originated at the intersection of the JZ and the Faults. Lowvelocity zones (LVZs) with low Vp/Vs are observed at 15–20 km depth beneath the Haicheng (HC) region. We interpret the LVZs in the middle crust as regions of fluids, suggesting rock dehydration at high temperatures. The LVZs and low Vp/Vs in the upper crust are attributed to groundwater-filled cracks and pores. We believe that large crustal earthquakes in this area are caused by the combination of faulting and fluid movement in the middle crust.     

Lithospheric structure of NW Iran from P and S receiver functions

We computed P and S receiver functions to investigate the lithospheric structure beneath the northwest Iran and compute the Vp/Vs ratio within the crust of this seismologically active area. Our results enabled us to map the lateral variations of the Moho as well as those of the lithosphere–asthenosphere boundary (LAB) beneath this region. We selected data from teleseismic events (M_b?>?5.5, epicentral distance between 30° and 95° for P receiver functions and M_b?>?5.7, epicentral distance between 60° and 85° for S receiver functions) recorded from 1995 to 2008 at 8 three-component short-period stations of Tabriz Telemetry Seismic Network. Our results obtained from P receiver functions indicate clear conversions at the Moho boundary. The Moho depth was firstly estimated from the delay time of the Moho converted phase relative to the direct P wave. Then we used the H-Vp/Vs stacking algorithm of Zhu and Kanamori to estimate the crustal thickness and Vp/Vs ratio underneath the stations with clear Moho multiples. We found an average Moho depth of 48 km, which varies between 38.5 and 53 km. The Moho boundary showed a significant deepening towards east and north. This may reveal a crustal thickening towards northeast possibly due to the collision between the Central Iran and South Caspian plates. The obtained average Vp/Vs ratio was estimated to be 1.76, which varies between 1.73 and 1.82. The crustal structure was also determined by modeling of P receiver functions. We obtained a three-layered model for the crust beneath this area. The thickness of the layers is estimated to be 6–11, 18–35, and 38–53 km, respectively. The average of the shear wave velocity was calculated to be 3.4 km/s in the crust and reaches 4.3 km/s below the Moho discontinuity. The crustal thickness values obtained from P receiver functions are in good agreement with those derived by S receiver functions. In addition, clear conversions with negative polarity were observed at ~8.7 s in S receiver functions, which could be related to the conversion at the LAB. This may show a relatively thin continental lithosphere of about 85 km implying that the lithosphere was influenced by various geodynamical reworking processes in the past.     

基于三相孔隙弹性理论的火山岩气层岩石物理响应特征分析

Unlike previous theories with velocity and/or elastic modulus averaging, we use a three-phase porous rock physics model developed by Santos for analyzing the seismic response of two immiscible fluids in saturated porous media. Considering reservoir reference pressure and coupling drag of two fluids in pores, the effects of frequency, porosity, and gas saturation on the phase velocities of the P-and S-waves are discussed in detail under field conditions. The effects of porosity and gas saturation on Vp/Vs are also provided. The data for our numerical experiments are from a sample of deep volcanic rock from Daqing. The numerical results show that the frequency dispersion effect can be ignored for deep volcanic rocks with low porosity and low permeability. It is concluded that for deep volcanic rocks the effect of gas content in pores on Vp/Vs is negligible but the effect of porosity is significant when there is a certain amount of water contained in the pores. The accurate estimate of lithology and porosity in this case is relatively more important.     

Merging active and passive data sets in traveltime tomography: The case study of Campi Flegrei caldera (Southern Italy)

We propose a strategy for merging both active and passive data sets in linearized tomographic inversion. We illustrate this in the reconstruction of 3D images of a complex volcanic structure, the Campi Flegrei caldera, located in the vicinity of the city of Naples, southern Italy. The caldera is occasionally the site of significant unrests characterized by large ground uplifts and seismicity. The P and S velocity models of the caldera structure are obtained by a tomographic inversion based on travel times recorded during two distinct experiments. The first data set is composed of 606 earthquakes recorded in 1984 and the second set is composed of recordings for 1528 shots produced during the SERAPIS experiment in 2001. The tomographic inversion is performed using an improved method based on an accurate finite‐difference traveltime computation and a simultaneous inversion of both velocity models and earthquake locations. In order to determine the adequate inversion parameters and relative data weighting factors, we perform massive synthetic simulations allowing one to merge the two types of data optimally. The proper merging provides high resolution velocity models, which allow one to reliably retrieve velocity anomalies over a large part of the tomography area. The obtained images confirm the presence of a high P velocity ring in the southern part of the bay of Pozzuoli and extends its trace inland as compared to previous results. This annular anomaly represents the buried trace of the rim of the Campi Flegrei caldera. Its shape at 1.5 km depth is in good agreement with the location of hydrothermalized lava inferred by gravimetric data modelling. The Vp/Vs model confirms the presence of two characteristic features. At about 1 km depth a very high Vp/Vs anomaly is observed below the town of Pozzuoli and is interpreted as due to the presence of rocks that contain fluids in the liquid phase. A low Vp/Vs body extending at about 3–4 km depth below a large part of the caldera is interpreted as the top of formations that are enriched in gas under supercritical conditions.     

Crustal structure variation along 30°N in the eastern Tibetan Plateau and its tectonic implications

We determined crustal structure along the latitude 30°N through the eastern Tibetan Plateau using a teleseismic receiver function analysis. The data came mostly from seismic stations deployed in eastern Tibet and western Sichuan region from 2004 to 2006. Crustal thickness and Vp/Vs ratio at each station were estimated by the H–k stacking method. On the profile, the mean crustal thickness and Vp/Vs ratio were found to be 62.3 km and 1.74 in the Lhasa block, 71.2 km and 1.79 near the Bangong–Nujiang suture, 66.3 km and 1.80 in the Qiangtang block, 59.8 km and 1.81 in the Songpan–Garze block, and 42.9 km and 1.76 in the Yangtze block, respectively. The estimated crustal thicknesses are consistent with predictions based on the topography and the Airy isostasy, except near the Bangong–Nujiang suture and in the Qiangtang block where the crust is 5–10 km thicker than predicted, indicating that the crust may be denser, possibly due to mafic underplating. We also inverted receiver functions for crustal velocity structure along the profile, which reveals a low S-wave velocity zone in the lower crust beneath the eastern Tibetan Plateau, although the extent of the low-velocity zone varies considerably. The low-velocity zone, together with previous results, suggests limited partial melting and localized crustal flow in the lower crust of the eastern Tibetan Plateau.     

A New Passive Tomography of the Aigion Area (Gulf of Corinth, Greece) from the 2002 Data Set

We present the results of a tomographic study performed in the framework of the 3F-Corinth project. The aim of this work is to better understand the rifting process by imaging the crustal structure of the western Gulf of Corinth. Forty-nine stations were deployed for a period of six months, allowing us to monitor the microseismicity. Delayed P and S first-arrival times have been simultaneously inverted for both hypocenter locations and 3-D velocity distributions. We use an improved linearized tomography method based on an accurate finite-difference travel-time computation to invert the data set. The obtained Vp and Vs models confirm the presence of a two-layer vertical structure characterized by a sharp velocity gradient lying at 5–7 km depth, which may be interpreted as a lithological contrast. The shallower part of the crust (down to 5 km depth) is controlled by the N-S extension and lacks seismicity. The deeper part (7–13 km depth) matches the seismogenic zone and is characterized by faster and more heterogeneous anomalies. In this zone, the background seismicity reveals a low-angle active surface dipping about 20° toward the north and striking WNW-ESE. The position of this active structure is consistent with both high Vp/Vs and low Vp.Vs anomalies identified at 8–12 km depth and suggesting a highly fracturated and fluid-saturated zone. Both the geometry of the active structure beneath the gulf and the presence of fluids at 8–12 km depth are in accordance with a low-angle detachment model for the western part of the Gulf of Corinth. S. Gautier and D. Latorre formerly at Géosciences Azur     

Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust,Shimanto Belt,southwest Japan

To investigate the physical property anisotropies of foliated fault rocks in subduction zones, the hanging wall phyllites and footwall cataclasites exhumed along the Nobeoka Thrust, a fossilized out‐of‐sequence‐thrust in the Shimanto Belt, Japan, was focused. Discrete physical property (electric resistivity, P‐ and S‐wave velocities, and porosity) measurements were conducted employing geologic coordinates (depth‐parallel direction, strike direction, and maximum dip direction of foliation), using the core samples obtained from the Nobeoka Thrust Drilling Project and compared the data to borehole geophysical logs. A higher sample P‐wave velocity (Vp), lower S‐wave velocity (Vs), higher Vp/Vs, and lower sample porosity and resistivity compared to the logs, are inferred to have been caused by the larger sampling scale of the logs and lower fluid saturation of the borehole. The phyllites and cataclasites exhibited substantial vertical and horizontal anisotropy of Vp (0.4–17.3 % and 2.7–13.8 %, respectively), Vs (0.5–56 % and 7.7–43 %, respectively), and resistivity (0.9–119 % and 2.0–65.9 %, respectively). The physical property anisotropies are primarily affected by the dip angles of foliation. The fault rocks that have gentler dip angles exhibit a higher Vp in the strike and maximum dip direction and a lower Vp in the depth‐parallel direction. In contrast, the fault rocks that have steeply dipping structures show a higher Vp in the strike and depth‐parallel directions with a lower velocity in the maximum dip direction. Resistivity anisotropy show a trend opposite to that of the Vp in relation to the dip angles. Our results show lower Vp anisotropy than those obtained in previous studies, which measured wave speeds perpendicular or parallel to foliation under confining pressure. This study highlights the significance of dip angles on vertical properties in geophysical surveys across foliated fault rocks.     

Vp和Vp/Vs联合反演:在日本东北地区深部结构成像(英文)

本文提出了一种新的反演方法:通过采用纵、横波走时数据对(从相同的震源产生的P和S波被同一台站记录)来联合反演纵波速度(Vp)和纵、横波速度比(Vp/Vs),然后单独反演横波速度Vs,在反演过程中同时对地震参数进行定位。该方法不需要假设P和S波的射线路径一致,它是沿着P和S波射线路径计算相对慢度扰动值。该方法直接把Vp/Vs作为一个模型参数,由此能获得比采用从独立反演获得的Vp和Vs计算出Vp/Vs的方法更精确的速度比值。该新方法被应用到反演日本东北地区的壳幔速度及波速比结构的研究中,获得了较好的效果。反演结果表明,在日本东北地区,太平洋俯冲板块为一高Vp,高Vs和低Vp/Vs异常区,而在活火山下方的浅部地幔楔以及背弧深部地区为低Vp,低VS和高Vp/VS异常。虽然这些特征在前人的研究中已经报道过,但与前人的研究结果相比,本次研究所获得的Vp/Vs的空间分布具有较小的分散性,同时,它的分布特征能较好的与地震波速度结构相吻合。     

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.     

Strength and deformation behavior of the Shimanto accretionary complex across the Nobeoka thrust

A rapid reduction in sediment porosity from 60 to 70 % at seafloor to less than 10 % at several kilometers depth can play an important role in deformation and seismicity in the shallow portion of subduction zones. We conducted deformation experiments on rocks from an ancient accretionary complex, the Shimanto Belt, across the Nobeoka Thrust to understand the deformation behaviors of rocks along plate boundary faults at seismogenic depth. Our experimental results for phyllites in the hanging wall and shale‐tuff mélanges in the footwall of the Nobeoka Thrust indicate that the Shimanto Belt rocks fail brittlely accompanied by a stress drop at effective pressures < 80 MPa, whereas they exhibit strain hardening at higher effective pressures. The transition from brittle to ductile behavior in the shale–tuff mélanges lies on the same trend in effective stress–porosity space as that for clay‐rich and tuffaceous sediments subducting into the modern Nankai subduction zone. Both the absolute yield strength and the effective pressure at the brittle–ductile transition for the phyllosilicate‐rich materials are much lower than for sandstones. These results suggest that as the clay‐rich or tuffaceous sediments subduct and their porosities are reduced, their deformation behavior gradually transitions from ductile to brittle and their yield strength increases. Our results also suggest that samples of the ancient Shimanto accretionary prism can serve as an analog for underthrust rocks at seismogenic depth in the modern Nankai Trough.     

Internal structures and high-velocity frictional properties of a bedding-parallel carbonate fault at Xiaojiaqiao outcrop activated by the 2008 Wenchuan earthquake

This paper reports internal structures of a bedding-parallel fault in Permian limestone at Xiaojiaqiao outcrop that was moved by about 0.5 m during the 2008 M_W7.9 Wenchuan earthquake. The fault is located about 3 km to the south from the middle part of Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system that was moved during the earthquake. The outcrop is also located at Anxian transfer zone between the northern and central segments of Yingxiu-Beichuan fault where fault system is complex. Thus the fault is an example of subsidiary faults activated by Wenchuan earthquake. The fault has a strike of 243° or N63°E and a dip of 38°NW and is nearly optimally oriented for thrust motion, in contrast to high-angle coseismic faults at most places. Surface outcrop and two shallow drilling studies reveal that the fault zone is several centimeters wide at most and that the coseismic slip zone during Wenchuan earthquake is about 1 mm thick. Fault zone contains foliated cataclasite, fault breccia, black gouge and yellowish gouge. Many clasts of foliated cataclasite and black gouge contained in fault breccia indicate multiple slip events along this fault. But fossils on both sides of fault do not indicate clear age difference and overall displacement along this fault should not be large. We also report results from high-velocity friction experiments conducted on yellowish gouge from the fault zone using a rotary shear low to high-velocity frictional testing apparatus. Dry experiments at normal stresses of 0.4 to 1.8 MPa and at slip rates of 0.08 to 1.35 m/s reveal dramatic slip weakening from the peak friction coefficient of around 0.6 to very low steady-state friction coefficient of 0.1-0.2. Slip weakening parameters of this carbonate fault zone are similar to those of clayey fault gouge from Yingxiu-Beichuan fault at Hongkou outcrop and from Pingxi fault zone. Our experimental result will provide a condition for triggering movement of subsidiary faults or off-fault damage during a large earthquake.     

Seismic tomography of Central São Miguel, Azores

We determine the three-dimensional distribution of P- and S-wave velocities for Central São Miguel Island (Azores, Portugal) by tomographic inversion of local earthquake arrival times. We use P- and S-phases from 289 earthquakes recorded by a network of 20 seismometers. The model shows good resolution in the shallowest 5–6 km, as illustrated by different resolution tests. There are several velocity anomalies, interpreted as pyroclastic deposits, intrusive bodies, geothermal fields, and the effects of tectonics. A low Vp zone marks Furnas caldera, probably evidencing volcaniclastic sediments with development of intense geothermal activity. Another low Vp zone extends in correspondence of the highly fractured area between Fogo and the north coast. Conversely, strong positive anomalies are found south of Fogo and northwest of Furnas. They are interpreted in terms of high-density deposits and remnants of a plutonic intrusion. These interpretations are supported by the distribution of Vp/Vs, and are consistent with previous geological, geochemical, and geophysical data.     

On the Vp/Vs–Mg# correlation in mantle peridotites: Implications for the identification of thermal and compositional anomalies in the upper mantle

We use thermodynamically self-consistent and hybrid methods to analyze the correlation of important physical parameters (e.g. bulk density, elastic moduli) with bulk Mg# and modal composition in mantle peridotites at upper mantle conditions. Temperature (anharmonic and anelastic), pressure and compositional derivatives for all these parameters are evaluated. The results show that the widely used correlations between Vp/Vs and Mg# in peridotites are strictly valid only for garnet-bearing assemblages at temperatures < 900 °C. The correlation breaks down when: i) spinel is the stable Al-rich phase in the assemblage and ii) when anelastic attenuation of seismic velocities becomes important (T ? 900 °C). This implies that the range of applicability of published Vp/Vs–Mg# correlations for the upper mantle is limited to a depth interval between the spinel–garnet phase transition and the 900 °C isotherm. We use numerical simulations to show that this depth interval is virtually nonexistent in lithospheres thinner than ～ 140 km and can comprise up to ～ 50% of the lithospheric mantle in thick (> 220 km) lithospheric domains. In addition, we show that for most of the upper mantle the expected Δ(Vp/Vs) values associated with compositional variations are smaller than the resolution limit of current seismological methods. All these considerations suggest that the Vp/Vs ratio is not a reliable measure of compositional variations and that for large parts of the upper mantle compositional anomalies cannot be separated from thermal anomalies on the basis of seismological studies only. We further confirm that the only reliable indicator of compositional anomalies in a peridotitic mantle is the ratio of density to shear wave velocities (ρ/Vs). Our results demonstrate that geophysical–petrological models (forward or inverse) that model these two fields (i.e. density and Vs) self-consistently within a robust thermodynamic framework are necessary for characterizing the small-scale thermal and compositional structure of the lithosphere and sublithospheric upper mantle.     

Structure of crust and upper mantle beneath the Ordos Block and the Yinshan Mountains revealed by receiver function analysis

A temporary seismological network of broadband three-component stations has been deployed N–S to investigate the crust and upper mantle structure across the Ordos Block and the Yinshan Mountains. P wave receiver functions reveal the Moho depth to be about 41 km beneath the central Ordos Block and down to 45 km beneath the northern Ordos Block, a slight uplifting to 42–43 km beneath the Hetao Graben, increasing to 47–48 km beneath the Yinshan Mountains and then decreasing to 44 km beneath the northern Yinshan Mountains along the profile. In the Ordos Block, the crustal Vp/Vs ratio (about 1.80) south to the Hetao Graben differs from that (about 1.75) beneath the center Ordos Block. The crustal Vp/Vs ratio is significantly lower (about 1.65–1.70) beneath the Yinshan Mountains. The P wave receiver function migration imaging suggests relatively flat discontinuities at 410 and 660 km, indicating the lack of a strong thermal anomaly beneath this profile at these depths, and a low S wave velocity anomaly in the upper mantle beneath the Hetao Graben. We suggest that the low S wave velocity anomaly may be attributable to heat and that the thermal softening advances the evolution of the Hetao Graben, while the lower-crustal ductile flows transfer from the Hetao Graben to the northern Ordos Block, resulting in crustal thickening.