Change Laws of Intensity of Moderate-Small Earthquakes (2.0≤Ms≤5.5)

The change laws of intensity of nearly 200 moderate-small earthquakes with Ms=2.0~5.5 occurring on the Chinese mainland since 1954 have been analyzed and are compared with that of moderately strong earthquakes(Ms≥5.0).The results show that there is quite obvious regionality in the statistical laws by both moderate-small earthquakes and moderately strong earthquakes:attenuation of intensity in Eastern China is slower than that in Western China and the ratio of the major axis to the minor axis of iso-seismal lines in Eastern China is smaller than that in Western China.The results also show that for a fixed magnitude the attenuation of intensity of moderate-small earthquakes is faster when intensity is higher than V and slower when intensity is lower than V-in comparison with that of moderately strong earthquakes.Furthermore,there also have been some differences in the aspects as intensity-magnitude relation between statistical laws of moderate-small earthquakes and moderately strong earthquakes.These di     

Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and 40Ar/39Ar dating

Seven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the ⁴⁰Ar/³⁹Ar and Rb–Sr methods. Despite extensive efforts, phengite dated by the ⁴⁰Ar/³⁹Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene–phengite, epidote–phengite and whole‐rock–phengite Rb–Sr isochrons for the same samples yielded ages of 78 ± 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 °C at c. 78 ± 2 Ma, and that the ⁴⁰Ar/³⁹Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites. Our new results lead us to conclude that high‐pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE‐dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10–15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high‐pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.     

Two-dimensional modelling of subduction zone anisotropy with application to southwestern Japan

We present a series of 2-D numerical models of viscous flow in the mantle wedge induced by a subducting lithospheric plate. We use a kinematically defined slab geometry approximating the subduction of the Philippine Sea plate beneath Eurasia. Through finite element modelling we explore the effects of different rheological and thermal constraints (e.g. a low-viscosity region in the wedge corner, power law versus Newtonian rheology, the inclusion of thermal buoyancy forces and a temperature-dependent viscosity law) on the velocity and finite strain field in the mantle wedge. From the numerical flow models we construct models of anisotropy in the wedge by calculating the evolution of the finite strain ellipse and combining its geometry with appropriate elastic constants for effective transversely isotropic mantle material. We then predict shear wave splitting for stations located above the model domain using expressions derived from anisotropic perturbation theory, and compare the predictions to ∼500 previously published shear wave splitting measurements from seventeen stations of the broad-band F-net array located in southwestern Japan. Although the use of different model parameters can have a substantial effect on the character of the finite strain field, the effect on the average predicted splitting parameters is small. However, the variations with backazimuth and ray parameter of individual splitting intensity measurements at a given station for different models are often different, and rigorous analysis of details in the splitting patterns allows us to discriminate among different rheological models for flow in the mantle wedge. The splitting observed in southwestern Japan agrees well with the predictions of trench-perpendicular flow in the mantle wedge along with B-type olivine fabric dominating in a region from the wedge corner to about 125 km from the trench.     

Geometry and brittle deformation of the subducting Nazca Plate, Central Chile and Argentina

We use data from the Chile Argentina Geophysical Experiment (CHARGE) broad-band seismic deployment to refine past observations of the geometry and deformation within the subducting slab in the South American subduction zone between 30°S and 36°S. This region contains a zone of flat slab subduction where the subducting Nazca Plate flattens at a depth of ∼100 km and extends ∼300 km eastward before continuing its descent into the mantle. We use a grid-search multiple-event earthquake relocation technique to relocate 1098 events within the subducting slab and generate contours of the Wadati-Benioff zone. These contours reflect slab geometries from previous studies of intermediate-depth seismicity in this region with some small but important deviations. Our hypocentres indicate that the shallowest portion of the flat slab is associated with the inferred location of the subducting Juan Fernández Ridge at 31°S and that the slab deepens both to the south and the north of this region. We have also determined first motion focal mechanisms for ∼180 of the slab earthquakes. The subhorizontal T -axis solutions for these events are almost entirely consistent with a slab pull interpretation, especially when compared to our newly inferred slab geometry. Deviations of T -axes from the direction of slab dip may be explained with a gap within the subducting slab below 150 km in the vicinity of the transition from flat to normal subducting geometry around 33°S.     

Numerical comparison of different convergent plate contacts: subduction channel and subduction fault

At convergent plate boundaries, the properties of the actual plate contact are important for the overall dynamics. Convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain. We investigate two fundamental physical states of the subduction contact: one based on a fault and the other based on a subduction channel. Using a finite element method, we determine the specific signatures of both states of the subduction contact. We pay particular attention to the overriding plate. In a tectonic setting of converging plates, where the subducting plate is freely moving, the subduction channel reduces compression relative to the fault model. In a land-locked basin setting, where the relative motion between the far field of the plates is zero, the subduction channel model produces tensile stress regime in the overriding plate, even though the amount of slab roll-back is small. The fault model shows a stronger development of slab roll-back and a compressive stress regime in the upper plate. Based on a consistent comparison of fault and channel numerical models, we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension. We conclude that, the type of plate contact plays a decisive role in controlling the backarc state of stress. To obtain backarc extension, roll-back is required as an underling geodynamic process, but it is not always a sufficient condition.     

洋壳深俯冲超高压变质作用研究及其地质意义

洋壳深俯冲经历超高压变质作用的岩石比较稀少,是目前超高压变质作用研究的重要前沿领域,有关这方面的研究对于建立新的冷俯冲带变质作用类型、探讨地幔水流体的成因、建立更广泛的超高压变质岩石的抬升机制具有重要意义。另外,洋壳深俯冲高压变质带往往与低压高温变质带组成双变质带,对双变质带的深入研究对于完整地重塑俯冲碰撞造山带的形成演化过程具有重要意义。简要地介绍了新疆西天山洋壳深俯冲超高压变质带的研究现状和存在的问题。     

Attenuation studies based on local earthquake Coda waves in the southern Indian peninsular shield

The Dharwar craton, Cuddapah basin and the Godavari graben characterise three diverse geological and tectonic settings in the peninsular shield of India. Owing to their contiguous proximity, they offer a unique opportunity to document differences, if any, in their seismic wave attenuation characteristics that might have a bearing on the seismogenic nature of the crust in a craton, basin and a rift-like graben structure. An attempt is made here to bring out these differences using constraints from coda-Q. We considered local earthquakes with epicentral distances ranging from␣14 to 150 km recorded at the digital broadband stations at Dharwar (DHD), Cuddapah (CUD) and Kothagudem (KGD) regions to derive the frequency-dependent coda-Q relations. Using the single scattering method, we obtained the frequency-dependent Q _C relationship (Q _C = Q ₀ f ⁿ )for each of the three geological units separately: DHD: Q _C = (730.62 ± 0.09)f ^{(0.54 ± 0.01)}; CUD: Q _C = (535.06 ± 0.13)f ^{(0.59 ± 0.01)} and KGD: Q _C = (150.56 ± 0.08)f ^{(0.91 ± 0.01)}. The Q _C values obtained for all the three sub-regions show moderate to strong frequency dependence and essentially reflect the level of crustal heterogeneities to varying degrees.