Rayleigh Wave Tomography In North-China From Ambient Seismic Noise

North China (NC) has the oldest craton of China mainland. NC had experienced significant tectonic rejuvenation in the late Mesozoic and Cenozoic, which is evidenced by the widespread lithospheric extension and thinning. NC is not only the political, economical and cultural center of China, but also a region with strong seismic activity. Many strong earthquakes have occurred in this region and cause great damages to our country and people. There are large numbers of faults in this region. The tectonic and geologic setting is extremely complex. So, many researchers studied the velocity structure of crust and upper mantle and the cause of destructive earthquakes using multiple geophysical methods, and obtained many significant results.Extensive geophysical investigations have been conducted in NC. However, these previous results are mainly concentrated on P wave velocity structure. Many surface wave tomography studies have also been conducted, but most of them are at China mainland scale and the lateral resolution is in the order of 100-200km. Yet few studies of surface waves in NC have been undertaken to date, which is due to the relatively low level of seismicity and the consequent logistic limitation suffered by earthquake-based surface wave tomography. Previous studies almost exclusively used teleseismic earthquakes. It is difficult to obtain reliable short-period (<10s) dispersion measurements from distant earthquakes due to intrinsic attenuation and scattering along the ray paths, and it is these short-period waves that are most useful for constraining the structure of the crust and uppermost mantle. Moreover, the long paths also result in broad lateral sensitivity kernels which limit resolution to hundreds of kilometers. For these reasons, high-resolution surface wave tomography results are scarce.In order to study the formation and evolution of NC, to obtain crust and upper mantle structure and to verify various proposed mechanisms for the interpretation of the lithospheric process occurred in North China caton, a seismic experiment (North China Seismic Array, NCSA) has been carried out since the winter of 2006. 250 portable stations were deployed in North China, of which 190 are broadband stations, 10 are very broadband stations and 50 are short period stations. In the thesis, we studied the theory and methodology of ambient noise tomography and applied this method to NC sucessfully. Continuous vertical-component seismograms, spanning the period from January 1, 2007 to February 28, 2008 recorded by 190 broadband stations and 10 very broadband stations, are used in this study. We apply the cross correlation technique to the ambient noise data for each station pairs recorded by NCSA. Rayleigh wave group velocity dispersion curves are measured at periods between 4s and 40s by multiple filter technique. We obtain 5630 high quality dispersion curves. Surface wave tomography is conducted to generate group velocity maps with a grid spacing of 0.25°x0.25°. These maps display higher resolution and span to shorter periods than previous surface wave tomography maps. Then genetic algorithum was used to invert pure path dispersion curves. The 3-D shear wave velocity structure from 0 to 50km depth was readily constructed. To the authors' knowledge, the resolution presented here is, so far, the highest one in China mainland.Follows are the original points of this thesis:1, Studied the theory about extracting Green Function's from ambient seismic noise, dispersion curve measurement and inversion, and surface wave tomography. Developed ambient noise tomography processing codes and dispersion curve measurement program. We can obtain very good dispersion measurement with multiple filter technique. For Green Functions with lower signal-to-noise ratio (SNR), we can use phase matched filter to get continuous and smooth dipsersion curves.2, We can obtain more reliable dispersion measurment by introducing maximum cutoff period. The maximum period extracted from ambient seismic noise is related to the aperture of the seismic array. The larger the aperture, the wider the spectrum band is. The SNR of Green Function is propational to the square root of observation time and can be enhanced by using symmtric component. The inhomogeneous distribution of seismic noise gives rise to the asymmetry of Green Function. Using more than one year's data, we can get more symmetric and higher SNR Green Function.3, We analized the characteristic of ambient seismic noise and found the characteristics are different for different period band. Between 4-10s period band, there is a coherent phase with large amplitude near zero lag time. Because of absorbing and attenuation of seismic waves, the Green Function's SNR is relative lower. In 10-20s period band, the sources of ambient seismic noise has a very clear seasonal variability. The azimuthal distributions of noise share a great similarity with the map of average ocean wave height map obtained by TOPEX-Poseidon. In 20-50s period band, Rayleigh wave Greeen Functions are near symmetrical and have less seasonal variation in both signal strength and directivity, which indicate the distribution of noise is almost homogeneous. In 4-20s period band, the amplitudes of positive and negtive components of Green Functions are obviously asymmetrical, but the arrival times are near identical, which indicate the distribution of noise has much influnce on the amplitude of Green Function, but less influnce on arrival time.4, Tomographic maps, with a grid spacing of 0.25°×0.25°, are computed between 4s and 40s period band. The maps at short periods reveal an evident lateral heterogeneity in the curst of North-China, quite well in agreement with known geological and tectonic features. The North China Basin is imaged as a broad low velocity area, while the Taihangshan and Yanshan uplifts and Ordos block are imaged as high velocity zones, and the Quaternary intermountain basins show up as small low-velocity anomalies. We computed the velocity gradients of the group velocity maps at 9s, 10s, 11s and 12s, using 0.25°×0.25°cells, and compare them with the space distribution of all the large historic earthquakes (M≥6.0) occurred in this region since 780 BC. We grouped the velocity gradients into three intervals: <0.05 km/s, 0.05-0.15 km/s, >0.15 km/s, we find that most of the large earthquakes (about two-thirds) occurred in regions with moderate velocity gradients.5, We constructed the 3-D shear wave crustal velocity model in North China by inverting the pure path Rayleigh wave dispersion curves at 432 nodes using genetic algorithum. The inversion results revals the shear wave velocity structure in North China crust, the thickness of sedimentary cover and the lateral variation of Moho interface very well. Our results show that the thickness of sedimentary cover is less than 2km in Taihangshan and Yanshan uplifts, about 5km in Yanqing-Huailai basin and about 3km in Datong basin. The thickness of sedimentary cover in North China basin is more than 6km and has lateral variation in different tectonic units. The thickness of sedimentary cover is about 8km in Cangxian uplift and about 11km in Jizhong and Huanghua depressions. The depth of Moho interface is thicker in westert part than those in the eastern part. The crustal thickness increases from 28km in Bohai Bay to 44km in Zhangjiakou. Near Taihangshan fault belt, the depth of Moho interface increases from 36km in the east to 40km in the west. In Yanshan uplift, the crustal thickness increases from 32km in Bohai Bay to 44km in the west.6, S-wave velocity maps at different depths show that Taihangshan fault is a boundary of high and low velocity anomaly in 0-12km dpeth. Below 13km, the boundary of high and low veloity anomaly is visible only at some parts of Taihangshan fault.We infer that Taihangshan fault only extends to Moho interface in some regions. From 0 to 8km, Yanshan uplift is mapped as broad high velocity anomaly, while its southern margin is mapped as high and low velocity anomalies alternatively, which may be caused by the NNE-NE trend faults. There is a distinct low velocity belt with NW trend at 10km depth near Zhangjiakou-Bohai seismic zone. This low velocity belt and the southern margin of Yanshan high velocity anomaly draw the outline of Zhangjiakou-Bohai seismic zone and its northern boarderline. There is a well-defined low velocity zone in middle to lower crust (15-25km) in the Beijing-Tianjin-Tangshan region, which may be caused by intrusion of mantle materials.7, We analized the S-wave velocity structure near Tangshan, Luanxian and Ninghe earthquake region. We find the focal depth of Ninghe earthquake is located in the trasition zones of high and low velocity bodies. The focal depths of Tangshan and Luanxian earthquake are located in high velocity bodies. There is a low velocity zone beneath the focuses of Tangshan and Luanxian earthquake. Moho interface is locally uplifted in Ninghe and Luanxian. The S-wave velocity is lower in the uppermost mantle of these three earthquake regions. We infer that these three earthquakes are mainly caused by vertical deformation of upper mantle and material exchange between crust and upper mantle. The magma intrudes crust along faults near the boundary of crust and upper mantle, which leads to the low velocity anomaly in the uppermost mantle. The magma intrusion heats up the lower crustal material and drops the viscosity. Some minerals are dehydrated. The water move up and store in the middle crust. The exsitence of liquid affects the structure and composition of fault zone, further changes the stress state, weaken the seismotectonic region and triggers the earthquakes.