Transcript Crust and upper mantle structure of Tien Shan

Crust and upper mantle structure of Tien Shan Orogen and its surroundings by
ambient noise tomography and earthquake tomography
Yong
a
Zheng ,
Yingjie
a,b
Yang ,
Weisen
b
Shen ,
and Michael H.
b
Ritzwoller
a. State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics,
Chinese Academy of Sciences, Wuhan, 430077, China.
b. Department of Physics, University of Colorado, Boulder, CO 80303, USA
Abstract
We calculate the ambient noise Green’s Functions
between the station pairs between Chinese stations
and other PASSCAL ans IRIS stations distributed in
Tienshan and its surroundings, and obtain the phase
velocity maps for the Tien Shan orogen in the periods
range from 6~40s. Since ambient noise is not sensitive
to the deeper structure, in order to obtain the clues of
the deformation process, earthquake data are
important for compensating the information of the
lithosphere and athenosphere. In this work we
collected teleseismic data recorded by the CEArray
stations, and applied two plane wave tomography
method to pick out the dispersion curves in the periods
between 30s and 80s. Based on careful data quality
control, we merge the results from ambient noise
tomography and two plane wave tomography to
construct integrated dispersion maps at the periods
range from 6s to 100s. The preliminary result shows
that high velocity anomalies can be observed under the
central Tien Shan orogen at the periods between 35s
and 66s, while there is a low velocity anomaly in
Western Tienshan, which may imply that dynamic
processes are different between the central and the
western part of Tien Shan orogen..
Data and Methods
In this work, ambient noise tomography method
Bensen et al. (2007) and Two Plane Wave Tomography
method (TPWT) (Forsyth & Li, 2004; Yang & Forsyth)
are applied to pick out the phase maps both for short
periods and long periods. The ambient noise data
processing procedure is outlined below. First, raw
seismograms are filtered in broad period band from 10
to 50 s after the mean and trend are removed. The
filtered seismograms are whiten in frequency domain
and normalized using running average in time domain
to suppress earthquake signals, then calculate the
cross correlation functions by stacking data for two
years. At last, the method of frequency-time analysis
(FTAN) and phase-match filtering is applied to obtain
phase velocities. As for the TPWT work, we choose
earthquakes range epicentral distance of 30~120
degree recorded by CEArray and other stations, and
pickout the data based on the arrival time front. The
stations distibution, earthquake distribution and data
are shown in figure 4, 5 and 6.
Figure 4. Cross-correlation functions stacked with two years data. (a) cross-correlation
functions between PASSCAl and CEArray stations. The data are filtered with bandpass filter
0.018~0.08Hz. (b) Cross-correlation functions between CEAray stations. The data are filtered
with filter of 0.015~0.1Hz.
Figure 5. Teleseismic seismograms section recorded .
By CEArray and the other stations. The magnitude of
the event is M6.4. The data filtered with bandpass
filter of 0.01~0.025Hz.
Figure 6. Number of paths used in ANT
tomography and TPWT tomography. Red line
shows the number of ANT paths, Green and
blue lines show the data used for the east and
west part of Tien Shan region
Preliminary Results (Phase velocity maps)
Introduction
The Tien Shan orogen situates in central Asia about 2000km
away from the collision boundary between Indian plate and
Eurasian plate, is one of the highest, youngest, and most
active mountain belts on the earth. Although many studies
have been made on the dynamic process of the Tien Shan
orogen, its tectonic rejuvenation and uplift mechanism are
still being debated. Some researchers owed the Tien Shan
mountain building to the shortening in the crust
accompanied by a similar style in the lithosphere, and the
shortening may be caused by the collision of the Indian plate
with the Eurasian plate; other people suggested that the
Tien Shan orogenic belt was caused by the underthrusting
of the Tarim Basin and Kazakh Shield beneath the Tien
Shan. Surface wave velocity maps and 3D seismic velocity
structure may help us to decipher this puzzle. In recent
years there are hundreds of broadband seismic stations
have been deployed in this region, Which form the basis of
this work
Figure3.
Event
distribution
of
earthquakes. The red dots show the
distribution of the teleseimic events.
All of the earthquakes are in the
depicentral distance range of 30~120
degress, with magnitudes larger or
equal to 5.5. The black lines show the
paths between the events and the
stations. The grey triangle shows the
average location of the studied region.
Figure 2 . Stations distribution for ANT
and TPWT. In ambient noise work,
CEA array has been used to calculate
the cross-correlation functions with the
continuous data recorded between
2007.8~2009.7. the blue stations are
the IRIS stations. For TPWT study, we
collected data from CEA array, XW, XP,
KZ and KR stations, which are shown
in the upper plot.
Stations and events
Stations used in this work mainly composed by Chinese CEA Array data and the
other broadband seismic stations, including 62 Xinjiang stations and 54 other
stations for ANT work, 62 Xinjiang stations and 127 other stations for TPWT work.
For the TPWT work, the operation period for Xinjiang network is 2007.8~2009.7,
for XW is 1997.8~2001.12, for XP network is 2005.7~2007.12, for KR network is
2008.1~2012.10, and for KZ station is 1997.8~2012.10.
Figure1. Tectonic units and topography of Tienshan Orogen and its
surroundings
Figure7. Phase velocity maps in Tien Shan and surroundings obtained by
ambient noise tomography. The maps show periods of 8s, 16s, 25s, 30s,
and 40s. Since ambient noise data are mainly sensitive to shorter periods.
The reference velocities are: 2.92km/s, 3.18km/s, 3.28km/s, 3.42km/s,
3.54km/s, and 3.74km/s respectively.
Figure8.
Phase velocity maps in Tien Shan orogen and
surroundings inversed by TPWT method. The maps show periods of
40s, 50s, 66s, 83s, and 100s. The reference velocities are: 3.74km/s,
3.85km/s, 3.96km/s, 4.05km/s, and 4.11km/s, respectively. The
subplot in the lower right corner shows the misfit at period of 50s.
Conclusion
1. Ambient noise tomography of the Tien Shan and surroundings is performed;
2. Two plane wave tomography with finite frequency effects is performed in Tien Shan orogen.
3. strong variations can be observed between the Eastern and Western part of Tien Shan orogen;
4.Between 30~66s, low velocity anomalies can be observed under western part of Tien Shan Orogen.
5.Velocities in Kazakh Shield and Tarim Basin are relatively faster than those of Tien Shan orogen.