Site and Basin Effects on Seismic Hazard in Indonesia:Sulawesi and Jakarta Case Studies
Date
2019
Authors
Cipta, Athanasius
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Abstract
Earthquakes are among the most costly, devastating and deadly
natural hazards. The
extent of the seismic hazard is often influenced by factors like
the source location and site
characteristics, while the susceptibility of assets is influenced
by the population density,
building design, infrastructure and urban planning. A
comprehensive knowledge of the
nature of source and local geology enables the establishment of
an effective urban planning
that takes into account the potential seismic hazard, which in
turn may reduce the degree
of vulnerability.
The first probabilistic seismic hazard assessment (PSHA)
incorporating the effects of local site characteristic for the
island of Sulawesi in Indonesia has been conducted.
Most of the island, with the exception of South Sulawesi, is
undergoing rapid deformation.
This leads to high hazard in most regions (such that PGA > 0.4g
at 500 year return period
including site effects) and extremely high hazard (like PGA > 0.8
g at 500 year return
period) along fast-slipping crustal fault. On the other hand, a
distant site relative to fault
might suffer higher ground motion if that site is composed of
soft soil. This research has
proven that incorporating near-surface physical properties, in
this case is represented by
VS30, surface geology contribute significantly to ground motions,
consequently, responsible
for potential building damage.
The PSHA study that took place in Sulawesi took us move further,
investigate
the effect of deep structure on seismic waves. Jakarta was chosen
for its location sitting
on less known deep sediment basin and economic and political
importances. A dense
portable-seismic-broadband network, comprising 96 stations, has
been operated within
four months covering the Jakarta. The seismic network sampled
broadband seismic-noise
mostly originating from ocean waves and anthropogenic activity.
We used Horizontal-toVertical Spectral Ratio (HVSR) measurements
of the ambient seismic noise to estimate the
fundamental-mode Rayleigh wave ellipticity curves, which were
used to infer the seismic
velocity structure of the Jakarta Basin. By mapping and modeling
the spatial variation of
low-frequency (0.124{0.249 Hz) HVSR peaks, this study reveals
variations in the depth to
the Miocene basement. To map these velocity profiles of unknown
complexity, we employ
a Transdimensional-Bayesian framework for the inversion of HVSR
curves for 1D profiles
of velocity and density beneath each station. The inverted
velocity profiles show a sudden
change of basement depth from 400 to 1350 m along N-S profile
through the center of the
city, with an otherwise gentle increase in basin depth from south
to north.
Seismic wave modelings are conducted afterward and shows that for
very deep
basin of Jakarta, available ground motion prediction equation
(GMPE) is less sufficient
in capturing the effect of basin geometry on seismic waves.
Earrthquake scenario modeling using SPECFEM2D is performed to
comprehend the effect of deep basin on ground
motions. This modeling reveals that the city may experience high
peak ground velocity
(PGV) during large megathrust earthquake. The complexity of the
basin is responsible
for magnifying ground motions observed in the basin.
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Keywords
earthquake hazard, microtremor, HVSR inversion, basin resonance, Jakarta, Sulawesi
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