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Abstract:
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This work aims to study the variation in subduction
zone geometry along and across the arc and the fault pattern within the subducting
plate. Depth of penetration as well as the dip of the Benioff zone varies considerably
along the arc which corresponds to the curvature of the fold- thrust belt which varies
from concave to convex in different sectors of the arc. The entire arc is divided into 27
segments and depth sections thus prepared are utilized to investigate the average dip
of the Benioff zone in the different parts of the entire arc, penetration depth of the
subducting lithosphere, the subduction zone geometry underlying the trench, the arctrench
gap, etc.The study also
describes how different seismogenic sources are identified in the
region, estimation of moment release rate and deformation pattern. The region is
divided into broad seismogenic belts. Based on these previous studies and seismicity
Pattern, we identified several broad distinct seismogenic belts/sources. These are l)
the Outer arc region consisting of Andaman-Nicobar islands 2) the back-arc Andaman Sea 3)The Sumatran fault zone(SFZ)4)Java onshore region termed as Jave Fault Zone(JFZ)5)Sumatran fore arc silver plate consisting of Mentawai fault(MFZ)6) The offshore java fore arc region 7)The Sunda Strait region.As the Seismicity is variable,it is difficult to demarcate individual seismogenic sources.Hence, we employed a moving window method having a window length of 3—4° and with 50%
overlapping starting from one end to the other. We succeeded in defining 4 sources
each in the Andaman fore arc and Back arc region, 9 such sources (moving windows)
in the Sumatran Fault zone (SFZ), 9 sources in the offshore SFZ region and 7 sources
in the offshore Java region. Because of the low seismicity along JFZ, it is separated
into three seismogenic sources namely West Java, Central Java and East Java. The
Sunda strait is considered as a single seismogenic source.The deformation rates for
each of the seismogenic zones have been computed. A detailed error analysis of
velocity tensors using Monte—Carlo simulation method has been carried out in order
to obtain uncertainties. The eigen values and the respective eigen vectors of the
velocity tensor are computed to analyze the actual deformation pattem for different
zones. The results obtained have been discussed in the light of regional tectonics, and
their implications in terms of geodynamics have been enumerated.ln the light of recent major earthquakes (26th December 2004 and 28th March
2005 events) and the ongoing seismic activity, we have recalculated the variation in
the crustal deformation rates prior and after these earthquakes in Andaman—Sumatra
region including the data up to 2005 and the significant results has been presented.ln this
chapter, the down going lithosphere along the subduction zone is modeled using the
free air gravity data by taking into consideration the thickness of the crustal layer, the
thickness of the subducting slab, sediment thickness, presence of volcanism, the
proximity of the continental crust etc. Here a systematic and detailed gravity
interpretation constrained by seismicity and seismic data in the Andaman arc and the
Andaman Sea region in order to delineate the crustal structure and density heterogeneities a Io nagnd across the arc and its correlation with the seismogenic behaviour is presented. |