Ground movements associated with M8.1 earthquake in
Solomon Islands on April 1, 2007, detected by
ALOS/PALSAR
Yousuke Miyagi
(1)
, Taku Ozawa
(2)
, Yuichi Nishimura
(3)
, and Masanobu Shimada
(1)
(1)
Japan Aerospace Exploration Agency / Earth Observation Research Center, Tsukuba Space Center, 2-1-1 Sengen,
Tsukuba, Ibaraki 305-8505, Japan, E-mail: miyagi.yousuke@jaxa.jp, shimada.masanobu@jaxa.jp
(2)
National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, Ibaraki 305-0006,
Japan, E-mail: taku@bosai.go.jp
(3)
Institute of Seismology and Volcanology, Hokkaido University, N10W8 Kita-ku, Sapporo, Hokkaido 060-0810, Japan
Abstract
We describe the large M8.1 earthquake occurred in
Solomon Islands on April 1, 2007, as an example of
disaster monitoring and observation in remote locations
by ALOS/PALSAR. Comparing PALSAR images
observed before and after the earthquake, we could find
some changes associated with the earthquake. We went
to the Solomon Islands in the end of July and
investigated the damaged area. Ground deformations
were detected over a wide area using a DInSAR
technique. We presume that such deformations represent
co-seismic deformations caused by faulting, and try to
model these observed deformations using a fault model.
Results from this modeling can account for observed
data well and are in good agreement with results from
modeling using a teleseismic body wave.
Figure 1. Study area (Solomon Islands) with
plate boundary defined by PB2002 [Bird,
2003]. Red star represents the epicenter.
Keywords: PALSAR, DInSAR, ground movement,
earthquake, Solomon Islands
A remote-sensing technique has the advantage of being
able to observe and monitor a disaster that has occurred
in a remote location like the Solomon Islands that is
difficult to access and receives few geophysical
observations. The Phased Array type L-band Synthetic
Aperture Rader (PALSAR) installed on ALOS is
especially well-suited for these purposes in contrast to an
optical sensor that often suffers from cloud coverage in
the tropical region. PALSAR provides much data
because it can observe the target day and night and even
under cloudy conditions. Furthermore, PALSAR data can
be applied in Differential Interferometric SAR (DInSAR)
techniques to detect precise ground deformation. Such
precise geodetic data is helpful for inferring a fault
model. If it is possible, field investigation is helpful for
verifying the remotely sensed data. We went to the
Solomon Islands in late July 2007 and investigated the
area damaged by the earthquake and tsunami.
1. INTRODUCTION
The Solomon Islands is a nation composed of about 10
main islands and a thousand small islands located in the
southwest Pacific Ocean. Two major plates, the Pacific
and Australian plates, and two minor plates, the Solomon
Sea and Woodlark plates, produce complicated tectonics
in the southwest off the Solomon Islands [Tregoning et
al., 1998] (Fig. 1). On April 1, 2007 (UTC), a M8.1
interplate earthquake occurred in the subduction zone
between the Pacific Plate and the Australian Plate
(S8.48°, E156.98°) [see the USGS web site for time of
occurrence, type, magnitude, and location of the
earthquake:
tsunami and caused considerable damage in the area. The
Japan Aerospace Exploration Agency (JAXA) performed
emergency observation using the Advanced Land
Observing Satellite (ALOS) and tried to acquire
information on the afflicted area as soon as possible.
In this paper, we compare the PALSAR images
observed before and after the earthquake in Solomon
Islands to detect changes associated with the earthquake
and compare the data from remote sensing with that from
field investigation. We then present the observed ground
deformation using the DInSAR technique and the result
of fault modeling with DInSAR data.
2. SATELLITE AND FIELD DATA
Figures 2a and 2b are amplitude images of the southern
part of Ranongga Island acquired before and after the
earthquake. Comparing these images, we find an
apparent increase of coastal land area in Fig. 2b in spite
of the rising tide level. We presume that some uplift
associated with the earthquake occurred in these areas,
although it is difficult to assess quantitatively using just
these data. Figure 3 is a picture of the area enclosed by
the red circle in Fig. 2b on July 28, 2007. In the field
investigation of this area, we could see evidence of a
major uplift of three or more meters. Figures 4a and 4b
are amplitude images of the northern part of Simbo
Island acquired before and after the earthquake. In
contrast to Ranongga Island, Simbo Island exhibits
decreasing land area for the period. This is not clear
evidence of subsidence because it may be caused by tidal
changes. However, at least, there was no uplift as on the
neighboring Ranongga Island.
(a)
(b)
Figure 2. PALSAR amplitude images observed before
(a) and after (b) the earthquake in the southern part of
Ranongga Island (red rectangle in Fig. 1).
Figure 3. Huge uplift in Lale (Red circle in
Fig. 2b). Author stands beside a 3 m scale in
the picture.
In Tapurai, a small village located on the northern part of
Simbo Island (yellow circle in Fig. 4b), almost all
buildings were destroyed by a large tsunami. In contrast,
there was almost no tsunami damage in Lale, a village
located on the southern Ranongga Island (red circle in
Fig. 2b) only 10km north of Tapurai. This suggests that
the uplift occurred before the tsunami in Lale, and there
was no uplift in Tapurai. There may be a boundary of
uplift and subsidence between Simbo and Ranongga
islands.
(a)
(b)
Figure 4. PALSAR amplitude images observed before (a)
and after (b) the earthquake in the northern part of
Simbo Island (green rectangle in Fig. 1).
3. DIFFERENTIAL INSAR AND FAULT MODEL
Although we can see that something occurred or the
pattern of some phenomena by comparing amplitude
images, we can’t understand what it was or how great it
was. However, ground deformation derived from
DInSAR processing is quite precise geodetic information
and can be used for quantitative discussion. Figure 5
presents interferograms in the study area, composed of
three consecutive paths (Paths: 343 to 345). The color
fringe in Fig. 5 represents the ground deformation in a
slant range component, which can be interpreted as being
induced by a reverse dip-slip in the supposed seismic
fault.
Figure 5. Interferograms generated by DInSAR
technique showing ground deformation associated
with the earthquake.
We next tried to model the observed geodetic
information by fault modeling. In this modeling, we first
set locations and a strike direction of the fault area so as
to include the plate boundary defined by PB2002 [Bird,
2003]. We assume the size of the fault area 360km long
and 150km wide, divide it into sub-faults of 24×10, and
estimate the slip vector for each sub-fault. A dip angle of
the fault is determined so as to minimize the ABIC. To
avoid the problem of not knowing the absolute
displacements and the relationship of displacements
among islands, we set biased displacements for each
island. An orbit tuning is not performed in the DInSAR
processing, so there is a possibility that a non-crustal
deformation component remains in each interferogram.
Because the remnant of the orbital component can be
considered as a uniformly inclined plane due to a precise
determination of the orbit information of ALOS satellite,
we define non-crustal deformation component as a plane
and estimate them for each orbital path.
Figure 8. Slip distribution on the supposed fault plane.
Figure 6 depicts the calculated interferogram using best
fit parameters, and Fig. 7 illustrates the residuals
between observed and calculated interferograms for each
path. The model well explains the observed data and the
pattern of slip distribution (Fig. 8), which has a two-eyed
large slip area around the hypocenter and northwest of it,
is similar to that deduced from the teleseismic body wave
data [Yamanaka, 2007].
4. CONCLUSIONS
ALOS/PALSAR could acquire favorable data for
understanding phenomena associated with the large
earthquake in the Solomon Islands. Comparison of
amplitude images before and after the earthquake
revealed that something had occurred and this was
confirmed by the field investigation. Satellite and the
field investigation data revealed a major uplift in
Ranongga Island and small uplift or subsidence in Simbo
Island and this agreed with the difference in tsunami
damage on the two islands. Significant ground
deformation was detected over the wide area using the
DInSAR technique. From these geodetic data, we
inferred a fault model and slip distribution for the
earthquake. Although it is a preliminary result, the model
well explains the observed deformation, and exhibits
good agreement with that inferred from the teleseismic
data, in which they had a two-eyed large slip area.
Figure 6. Calculated interferogram using
best-fit parameters. Color fringe represents
displacement in slant range component.
5. ACKNOWLEDGEMENT
Figure 7. Residuals between observed and
calculated interferograms. The small residual
indicates good agreement between both
interferograms.
Copyrights of ALOS/PALSAR data used in this study are
all reserved by JAXA and METI.
REFERENCES
[1] Tregoning, P., et al., Estimation of current plate
motions in Papua New Guinea from Global Positioning
System observations, J. Geophys. Res., 103,
pp.12,181-12,203, 1998.
[2] Bird, P., An updated digital model of plate boundaries,
G. Geophys. Geo., Vol. 4, No. 3, 1027,
doi:10.1029/2001GC000252, 2003.
[3] Yamanaka, Y., NGY seismology note on the web: