Coseismic
Deformation From the Haiti Earthquake of January 13, 2010, Detected by
ALOS/PALSAR (Ver.021710)
New: Result of ScanSAR
– ScanSAR interferometry was
added.
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Manabu Hashimoto (DPRI,
On
January 13, 2010, a Mw7.0 earthquake hit southern Haiti and caused severe
damages in and around Port au Prince.
This region has not suffered from strong earthquakes since 18th
century, but rather high seismic risk is inferred from GPS observation (Manaker et al., 2008).
This earthquake is considered to have occurred on the Enriquillo fault.
It is essential to reveal coseismic deformation from this earthquake and
estimate fault parameters for the understanding of earthquake generation
process and future seismic hazard evaluation. Therefore we utilize synthetic aperture
radar images.
PALSAR
(Phased Array type L-band SAR) is an L-band synthetic aperture radar onboard
the Japanese satellite named Advanced Land Observation Satellite (hereafter
ALOS). ALOS made an urgent
observation of PALSAR on January 16 from the Path 138 with the off-nadir angle
of 34.3 degree. Performing interferometry with the image acquired on October 11, 2007,
using GAMMA® software, we obtained an interferogram
with CGIAR hole-filled SRTM DEM (Jarvis et al., 2008). Perpendicular baseline is 267m. Precise state vector was available on
Jan. 20, 2010. Figure 1 shows the
result using the precise state vector without flattening. The eastern edge of the obtained image
is about 30km west of the epicenter.
This area corresponds to an about 30km wide peninsula which extends
westward from Port au Prince.
However, we found at least three cycles of fringes (~40cm), which show
increase of range toward the center of fringes, on the south side of the Enriquillo fault.
We also recognized three cycles of fringes on the north side of the
fault, but their gradient is much steeper than the south side.
Figure 1. Coseismic
deformation detected by ALOS/PALSAR. Master and slave images of Path 138
(western path) were acquired on Feb. 28, 2009 and Jan. 16, 2010, respectively,
from an ascending orbit. Images of
Path 136 were acquired on Sept. 12, 2009 and Jan. 28, 2010. The satellite was flying from south to
north emitting microwave eastward.
Incidence angle of microwave on the surface is approximately 39 deg. 1 cycle of color corresponds to 11.8cm
of range (distance between satellite and earth’s surface) changes. The color change in the order of
blue-magenta-yellow-blue means range increase. The red star shows the epicenter by USGS(2010). CMT
solution is given by the Global CMT Project(2010).
On
Jan. 25, 2010, another acquisition of PLASAR image was made from a descending
orbit. Figure 2 is an interferogram of the pair of images acquired on Mar., 9,
2009, and Jan., 25, 2010 (Bperp~798m).
We used provisional orbit information of the image in 2010 and performed
flattening since large orbital error contaminates the interferogram. Concentrated fringes can be
recognized northwest of the epicenter.
We can count 6 fringes with color changing in the order of yellow,
magenta and blue, implying more than 70cm range decrease. Another circular fringe appears just
north of the epicenter. On the
other hand, 3 cycles of fringes with the opposite sense appear on the northern
coast. Broad fringes can be seen on
the south side of the epicenter, which corresponds to that observed in the interferogram in Figure 1. These observation impies
rather complicated rupture process during the main shock than the results of
body wave inversion studies.
Figure
2.
Coseismic deformation derived from the pair of PALSAR images acquired on Mar.
9, 2009, and Jan. 25, 2010, from a descending orbit. The satellite was flying from north to south emitting microwave
westward. See also the legend of
Figure 1.
I
estimated the slip distribution and dip angle of the fault plane by inverting
PALSAR interferograms with the method developed by Fukahata and Wright(2008). Their method can estimate dip angle and
slip distribution simultaneously under the condition of minimum ABIC. The modeled fault plane is assumed to be
100km long and its depth range is 0-50km.
Strike is 262deg from the north.
The center of the fault plane is close to the epicenter and the fault
plane is dipping northward. I
searched dip angle within the range of 40 – 90 deg with step of 2 deg. I discarded data in the northern region
>19N and from the island.
Figure 3
is the estimated slip distribution.
Optimal dip angle is estimated to be 52 deg, which is much smaller than
70 deg of CMT. There are three
asperities, one of which is located near the
hypocenter and has maximum of about 2.5m at the depth of 15km. Left lateral strike slip with slight
thrust motion is recognized.
Another asperity is about 13km west at a depth of 5km and has about 3m
slip with significant thrust component.
Last asperity is located further west at the depth of about 12km. Figures 4 and 5 show synthesized interferograms, which roughly explain the characteristics
in the observed ones in Figures 1 and 2.
Geodetic moment is estimated to be 1.03×1020Nm (Mw7.28) with
a rigidity of 33GPa.
I will try to search
more favorable model by changing strike and/or location with data acquired
during the next couple of weeks.
Figure 3. Estimated slip
distribution. Slip is projected
onto the vertical plane oriented in the direction of N262E seen from the north
side.
Figure 4. Synthesized interferogram for the ascending orbit. The rectangle is the surface projection
of the modeled fault plane with the contours of slip distribution. The star indicates the epicenter by USGS(2010).
Figure 5. Synthesized interferogram for the descending orbit. See also the legend of Figure 2.
New: Analysis of ScanSAR – ScanSAR interferometry
On February 11, 2010, an acquisition of WB1
mode (ScanSAR) was made. Observation with WB1 mode was made four times
before the mainshock for this path 448 and frame 3250. We have made an interferometric
analysis of this image with the image acquired on September 26, 2009. Using the program provided by Prof. David
Sandwell, SIO, UCSD, we divided original Level1.0 image
into 5 images according to off-nadir angle/PRF. After this division, we performed usual interferometric analysis with Gamma. Perpendicular baselines for 5 swaths are estimated
to be 259.8, 236.6, 216.7, 200.5, and 190.1m from 1 to 5, respectively, which are
the smallest among the analyses we have made. Figure 6 is flattened interferogram.
Original interferogram
contains large orbital fringes. Coherence
is very high due to small Bperp, and continuity is good
between swaths, except the westernmost swath5. We observe coseismic fringes similar to those
in Figure 2, but they are much clearer. Broad fringe north of Port au Prince might
be coseismic one, judging from its shape. On the both ends of this interferogram, short-wavelength regular disturbances appear.
This type of fringes sometimes appears
in ScanSAR-ScanSAR interferograms.
So far we don’t know the reason but
suspect the problem of coregistraion of images or matching
of bursts.
PALSAR level1.0 data were provided by the Japan Aerospace
Exploration Agency via the Geographical Survey Institute under the project of
the Evaluation of Utilization of Land Observation Satellite for Disaster
Mitigation, Earthquake Working Group.
The ownership of PALSAR products belong to JAXA and Ministry of Economy,
Trade and Industry.
We thank Professor David Sandwell,
Scripps Institute of Oceanography,
Fukahata, Y., and T.J. Wright, 2008, A
non-linear geodetic data inversion using ABIC for slip distribution on a fault
with an unknown dip angle, Geophys. J. Int., 173, 353-364,
doi:10.1111/j.1365-246X.2007.03713.x
Global CMT Project,
2010, http://neic.usgs.gov/neis/bulletin/neic_rja6_hrv.html
Jarvis, A., et al.,
2008, Hole-filled SRTM data V4, International Centre for Tropical Agriculture
(CIAT),
http://www.srtm.csi.cgiar.org.
Manaker, D.M., et al.,
2008, Interseismic plate coupling and strain
partitioning in the northeastern
USGS, 2010, http://earthquake.usgs.gov/earthquakes/eqinthenews/2010/us2010rja6/.