Please use this identifier to cite or link to this item: http://hdl.handle.net/10077/3623
Title: Rayleigh wave tomography in North-China from ambient seismic noise
Authors: Fang, Lihua
Supervisore/Tutore: Panza, Giuliano
Zhifeng, Ding
Cosupervisore: Romanelli, Fabio
Vaccari, Franco
Jianping, Wu
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.
Ciclo di dottorato: XXI Ciclo
metadata.dc.subject.classification: GEOFISICA DELLA LITOSFERA E GEODINAMICA
Description: 
2008/2009
Keywords: Cross Correlation
Rayleigh Wave
Group Velocity
Genetic Algorithm
North China
Language: en
Type: Doctoral Thesis
Settore scientifico-disciplinare: GEO/10 GEOFISICA DELLA TERRA SOLIDA
NBN: urn:nbn:it:units-8976
Appears in Collections:Scienze della terra

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