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Title: Rayleigh wave tomography in North-China from ambient seismic noise
Authors: Fang, Lihua
Keywords: Cross CorrelationRayleigh WaveGroup VelocityGenetic AlgorithmNorth China
Issue Date: 29-Mar-2010
Publisher: Università degli studi di Trieste
Abstract: The theory and methodology of ambient noise tomography has been studied and applied to North-China successfully. 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, have been used. The cross correlation technique has been applied to ambient noise data recorded by North-China Seismic Array for each station pairs of the array. Rayleigh wave group velocity dispersion curves are measured at periods between 4 s and 40 s 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 0.25º×0.25º. These maps display higher resolution and extend to shorter periods than previous surface wave tomography maps. Then genetic algorithm is used to invert pure path dispersion curves. The 3-D shear wave velocity structure from 0 to 50 km depth is readily constructed. To the authors' knowledge, the resolution presented here is, so far, the highest one in China mainland. The original results of this thesis are: 1, The SNR of Green Function is proportional to the square root of observation time and can be enhanced by using the symmetric component. The inhomogeneous distribution of seismic noise gives rise to the asymmetry of Green Function. Using more than one year's data, one can get more symmetric and higher SNR Green Function. 2, The characteristics of ambient seismic noise are different for different period bands. In 4-10 s, a coherent phase with large amplitude near zero lag time is observed. In 10-20 s, the sources of ambient seismic noise have 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 range, Rayleigh wave Green Functions are almost symmetrical and show less seasonal variation in both signal strength and directivity, which indicates that the distribution of noise is - ii - almost homogeneous. In 4-20s range, the amplitudes of positive and negative components of Green Functions are obviously asymmetrical, but the arrival times are almost identical, indicating that the distribution of noise has much influence on the amplitude of Green Function, but less influence on arrival time. 3, Tomographic 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. 4, The 3-D S-wave crustal velocity model in North China shows a distinct low velocity belt with NW trend at 10 km of 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 border line. There is a well-defined low velocity zone in middle-to-lower crust (15-25 km) in the Beijing-Tianjin-Tangshan region, which may be caused by intrusion of hot mantle materials. 5, We analyzed the seismogenic structure near Tangshan,Luanxian and Ninghe earthquake region. 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 the 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 its viscosity. Some minerals are dehydrated. The water moves up and is trapped in the middle crust. The existence of liquid affects the structure and composition of the fault zone, further changes the stress state, weakens the seismotectonic region and triggers the earthquakes.
Description: 2008/2009
NBN: urn:nbn:it:units-8976
Appears in Collections:Scienze della terra

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