[1] We present a new whole mantle P wave tomographic model GAP_P4. We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross correlation. Finite frequency kernels were calculated at the corresponding frequency bands for both long-and short-period data. With respect to an earlier model GAP_P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBSs) deployed in the western Pacific Ocean where station coverage is poor. This new model is different from a model in which the full data set is interpreted with classical ray theory. BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments.

[1] A total of 361 SKS and five local S wave splitting measurements obtained at global and regional seismic network stations in NE China and Mongolia are used to infer the characteristics of mantle fabrics beneath northeast Asia. Fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features. The dominant fast directions at the eastern part, beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone (MTZ), are about 100°from north, which are almost exactly the same as the motion direction of the Eurasian plate relative to the Pacific plate, and are independent of the direction of local geological features. The splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3% anisotropy. The shear wave splitting observations, complemented by the well-established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the Paleozoic era, can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected Pacific slab in the MTZ, or by back-arc asthenospheric flow in the mantle wedge above the slab.

Seismic anisotropy is obtained in the crust in northwestern capital area by shear‐wave splitting analysis, using the SAM technique. The seismic data was recorded at the Capital Area Seismic Network from Jan. 2002 to Dec. 2003. The results at 14 stations in all, every of which has at least 3 records available or more, are statistically discussed in this paper. The statistical results show that the average polarization of fast shearwaves is NE69.9° ±44.5° and the time delay of slow shear‐waves is 4.44±2.93 (ms/km). The average polarization of fast shear‐waves of NE69.9° ±44.5° suggests the direction of maximum horizontal principal compressive stress in this area. The most predominant polarization direction of fast shear‐wave suggests the tectonic implication of horizontal principal compressive stress at the direction NWW or nearly E‐to‐W, which exposes the Zhangjiakou‐Penglai depression fault zones with strike NWW. According to the polarization of fast shear‐wave, this study verifies that the predominant polarizations of fast shear‐wave at stations on active faults are consistent with fault strike. Possibly, both the Nankou‐Sunhe fault and Xiadian fault are two active faults while the Babaoshan fault is possibly a less active fault. The polarizations of fast shear‐wave in the North China Basin show the complexity, consistent with the complicated pattern of regional principal compressive stress controlled locally, induced by many faults crossing in the depression zone within the basin. This study also suggests that the quick change of time delays of slow shear‐waves is possibly related to the temperature change in deep crust.

An isolated swarm of small earthquakes occurred in 1992, near Dongfang on Hainan Island, southern China. The Institute of Geophysics, State Seismological Bureau of China, monitored the swarm with five DCS‐302 digital accelerometers for three months from 1992 June 1. 18 earthquakes, with magnitudes ML ranging from 1.8 to 3.6, were well located by five stations, and shear‐wave splitting varying azimuthally was analysed on 27 seismic records from these events. The mean polarization azimuth of the faster shear wave was WNW. Time delays between the split shear waves at two stations varied with time and space. The time delays at one station fell abruptly after earthquakes of magnitudes 3.1 and 3.6, but did not change significantly at the second station. This behaviour is consistent with the delay‐time changes being caused by changes in the aspect ratio of vertical liquid‐filled (EDA) cracks. Thus, the variation in shear‐wave‐splitting time delay could be due to changes in crustal stress related to nearby small‐magnitude earthquake activity. The connection between earthquake activity and crustal stress variation measured by shear‐wave splitting leaves the door open for possible observations of crustal stress transients related to the onset of an earthquake; however, our data cannot be considered as definite evidence for such precursors.