THE GROWTH OF NISHINOSHIMA ISLAND AS OBSERVED BY SPACEBORNE SAR
Tadashi Sasagawa1 and Fukashi Maeno2
PASCO CORPORATION, 1-1-2 Higashiyama, Meguro-ku, Tokyo 153-0043, Japan
Email: [email protected]
2
Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
Email:[email protected]
1
KEY WORDS: Nishinoshima Island, Volcanic activity, Spaceborne, SAR, TerraSAR-X
ABSTRACT: An undersea volcanic eruption was spotted on November 20, 2013 in the vicinity of Nishinoshima
Island in the Ogasawara Islands of Japan. This eruption caused the birth of a new land and merging with nearby
Nishinoshima Island and it is still growing. PASCO has been monitoring volcanic activity of the distinct area using
the Synthetic Aperture Radar (SAR) satellite TerraSAR-X since November 22, 2013. Until April 11, 2015, thirty one
(31) TerraSAR-X imageries have been acquired in high resolution SpotLight mode. The new island has been growing
gradually with ongoing eruption and overflow of lava and the current size of the newly formed island measures
several kilometers wide. PASCO is performing time-series monitoring utilizing spaceborne sensors. Volcano
Research Center of Earthquake Research Institute, University of Tokyo is utilizing the TerraSAR-X imageries for the
detailed interpretations of the Nishinoshima Island.
1.
INTRODUCTION
On November 20, 2013, an undersea volcanic eruption was detected in the vicinity of Nishinoshima Island in the
Ogasawara Islands of Japan. This volcanic island is located about 1000 km south of Tokyo. Nishinoshima Island was
site of volcanic eruptions off its coast between 1973 and 1974, which also formed a new island. To prepare for
prolonged volcanic activities, PASCO started monitoring an area around Nishinoshima since November 22, 2013, or
two days after the undersea eruption, by acquiring spaceborne Synthetic Aperture Radar (SAR) TerraSAR-X data.
The Earth Observation (EO) satellites provide up-to-date rapid information about the wide geographical areas of
disaster (Sasagawa, 2014). The weather independence and high resolution of the SAR satellites have made substantial
contribution for the management of natural disasters in the recent years. Independent of weather conditions and
illumination and frequent revisits of the TerraSAR-X (a reliable satellite with of high resolution, multi-polarization
and multi-incidence angle capabilities) we can obtain highly valuable geospatial information for the disaster
monitoring. The eruption of Nishinoshima with lava effusion is still continuing and the company has been doing
time-series monitoring utilizing spaceborne sensors.
2.
MATERIALS AND METHODS
2.1 Utilized Satellite Data
PASCO has been observing a defined area mainly around Nishinoshima Island and until April 11, 2015 (UTC) the
company has acquired 31 imageries at its EO satellite Ground Station (G/S) located in Okinawa. All data acquisitions
were performed by TerraSAR-X and TanDEM-X satellites in the high resolution SpotLight 300MHz mode and
imageries generated were as Ellipsoid Corrected (EEC) and Geocoded Ellipsoid Corrected (GEC) products.
TanDEM-X is TerraSAR-X’s twin satellite. The details of the utilized data are mentioned in Table 1.
The optical data of Pleiades satellites were also utilized for the reference purpose. Pleiades 1A/1B satellites offer
50 cm resolution with an impressive swath of 20 km. Three imageries were acquired on November 30 and December
24 in 2013 and January 29 in 2014.
Table 1. SAR satellite data details
Satellite
Data set
Acquisition period (UTC)
Imaging mode
Resolution
Polarization mode
Incidence angle
Pass direction
Product type
TerraSAR-X/TanDEM-X
31 imageries
November 21, 2013 to April 11, 2015
High Resolution SpotLight 300MHz
1.1 meter (ground range)
HH and VV
37.7 degree
Descending/Ascending
EEC and GEC
The TerraSAR-X imageries are also provided to the Volcano Research Center of Earthquake Research Institute,
University of Tokyo, for the ongoing joint research project with PASCO for monitoring the eruptive activity of
Nishinoshima.
3.
RESULTS AND DISCUSSIONS
3.1 Interpreted Information of the growing Nishinoshima Island
The first data acquisition by TerraSAR-X was performed on November 22, 2013 (JST) after the emerged island
(Figure 1).
The newly formed island was initially about 200 meters long and some 110 meters wide after the eruption on
November 20, 2013. Emerged island is visible within the square in Figure 1.
Figure 1. First data acquisition by TerraSAR-X on November 22, 2013, 05:37 (JST).
Time-series data acquisitions of the Nishinoshima Island starting from December 3, 2013 to April 11, 2015, are
shown in Figure 2.
© 2015 DLR, Distribution Airbus DS / Infoterra GmbH, Sub-Distribution [PASCO]
Figure 2. Time-series data acquisitions.
High resolution (50 cm) optical images of the Pleiades satellites for three different dates, November 30 and December
24 in 2013 and January 29 in 2014, were also utilized for the reference purpose (Figure 3).
© 2015 CNES – Distribution Airbus Defense & Space
Figure 3. High resolution Pleiades imageries.
The newly formed island was initially about 200 meters long and some 110 meters wide, but the observation on
February 18, 2014 found that the island had grown to cover an area of some 549,650 square meters.
Figure 4. Upper is zoomed imagery of TerraSAR-X acquired on 12 August 2014. The illustration on the right side
shows the portions formed and growth after mid-May 2014.
The Figure 4 shows lava flows eastward from mid-July for the image acquisition of 12 August 2014. In addition to the
island is expanding with lava flowing out to the east continues, a new lava flows out also to the north and south of the
main crater group, and covered the lava so far. The size of the island is 1.55 km east-west, 1.2 km north-south and an
area of approximately 1.2 km2. It is observed that the lava dome of about 50 m is formed. Approximately 50 million
m3, eruption rate has fluctuated since mid-May 2014 to eruptive volume but has maintained a level of more than
100,000 m3/day, and it is still active. The outlines of the newly formed Nishinoshima created based on the time-series
imageries of TerraSAR-X and bathymetry chart of the Japan Coast Guard Hydrographic Department is illustrated in
Figure 5 (Maeno, 2014-2015).
Figure 5. Outlines of the newly formed Nishinoshima.
© 2015 DLR, Distribution Airbus DS / Infoterra GmbH, Sub-Distribution [PASCO]
Figure 6. Lava flow as observed on TerraSAR-X imageries of 13 January 2015 and 4 February 2015.
SAR imageries of northwest part of the island clearly showed a boundary portion between the lobes group of newly
formed and old islands (Figure 6). Interpretations indicated the cracks as seen on bread-crust (clefts) that were formed
in the same direction as the direction of growth of the lava lobes. The opening and gradual expanding were observed.
Figure 6 shows the North lava lobes group and the central crater of state. At the central cone with a crater of about
80 m diameter, the shadow part of the crater (recess) is expanding as interpreted from the image of 13 January 2015
(UTC). The image of 4 February 2015 reveals the lava lobe group that is flowing out to the east (Maeno, 2014- 2015).
The rate of area increase of the Nishinoshima is shown in Figure 7. The red line on the graph indicates the temporal
increase of the area.
© 2015 DLR, Distribution Airbus DS / Infoterra GmbH, Sub-Distribution [PASCO]
Figure 7. Temporal variation of the area and rate of area increase.
While observing the central crater since 9 November 2014, to the crater in the horizontal direction, it was noticed that
it was about 40 meters above to the west. The study indicated that lava flows down in the orthogonal direction (north
to south) with respect to the radar transmittance direction (east to west). It was realized about the importance to
consider the microwave signals emitted and received by the SAR sensor (Honda et al. 2015). For the purpose of
precisely recording the development form of volcanic island, and long term observation, the utilization of SAR
satellites is safest and periodical observation is guaranteed with programmed data acquisition.
Although deviated from the entitled paper, the Figure 8 shows the observation
performed by an unmanned aerial vehicle (UAV). The Geospatial Information
Authority of Japan (GSI) conducted data acquisition utilizing UAV on 16
February 2015. The image very clearly shows about the detailed interpreted
information of the emerged island. The interpreted drawings reveal the flow of
the lava overlaid on the UAV image.
GSI has carried out recent UAV acquisitions as well. Such information derived
from the UAVs can lead to the potential of data fusion from multiple sensors
and collection platforms for improved knowledge about monitoring volcanoes.
[UAV Image credit: Geospatial Information Authority of Japan, Ministry of
Land, Infrastructure, Transport and Tourism (MLIT)]
Figure 8. Observation by UAV.
The latest TerraSAR-X image was acquired on April 12, 2015, JST (Figure 9). The location of Nishinoshima on the
right hand side of the map of Japan is also shown in the below image.
© 2015 DLR, Distribution Airbus DS / Infoterra GmbH, Sub-Distribution [PASCO]
Figure 9. Nishinoshima as observed on April 12, 2015.
It is interesting to observe, while comparing the satellite imageries, how the emerged island looked in the past months
and how it has drastically changed in the recent months and still becoming bigger. The SAR characteristics of the
imageries distinctly show that the new island continues to grow and signs of building lava plateau.
PASCO is monitoring the island by keeping the same acquisition parameters of TerraSAR-X and performing data
collections regularly. It seems that no numerous data sets acquisitions from spaceborne sensors have been conducted
so far for the Nishinoshima. It is important to note that these observations were obtained utilizing satellite imagery
and no confirmation has been made on the site of Nishinoshima by PASCO.
4. CONCLUSIONS
The Nishinoshima is still growing due to continuing volcanic eruptions and comparatively steady lava effusion. The
growth is significantly judged from the collected SAR satellite images. Monitoring of volcanoes is necessary for
doing predictions and appropriate action plans. The continuous lava accumulation may cause collapse of the slopes
of the island and cause a tsunami. PASCO is committed for the purpose of continuing to offer this kind of satellite
image monitoring.
The greatest advantage of the SAR system is their weather and daylight independent data collection and the measured
reflected signal ensures the acquisition of an area of interest. PASCO is performing the spaceborne SAR data
acquisitions regularly. The EO satellite G/S of the company is located in Okinawa. The satellite data reception cone is
wide-ranging, approximately 2,300 km, and useful for highly speedy data delivery immediately after performing the
direct tasking the satellite and data downlink as near real-time at the G/S. The products are generated quickly and
information is provided and shared among the experts on the same day. The acquired time-series TerraSAR-X
imageries of the Nishinoshima Island are regularly published on PASCO’s website. Earthquake Research Institute,
The University of Tokyo is also publishing the detailed results pertaining to the eruptive activity of Nishinoshima.
ACKNOWLEDGMENTS
The author would like to express his appreciation and sincere gratitude to Prof. Setsuya Nakada, Asst. Prof. Takayuki
Kaneko, Volcano research Center of the University of Tokyo, for their great contribution for the research of the
volcanic island Nishinoshima and for providing valuable latest findings for this paper.
RREFERENCES
Honda, T., Udono T., Shimomura H., Nozaki T., Nakada S., Kaneko T., Maeno F. 2015. Usefulness of long-term
monitoring of volcanic eruptions by synthetic aperture radar, Japan Geoscience Union Meeting 2015. Also from
http://www.eri.u-tokyo.ac.jp/?page_id=183&id=4805#maeno0213
Maeno, F. 2014-2015. 21st November, 2013 Volcanic activity in Bonin Islands, On the eruptive activity of
Nishinoshima, from (English) http://www.eri.u-tokyo.ac.jp/en/?page_id=183&id=1026,
(Japanese) http://www.eri.u-tokyo.ac.jp/VRC/nishinoshima/
Monitoring of the volcanic activity of Nishinoshima. 2014, from (Japanese) http://www.pasco.co.jp/disaster_info/141118/
Sasagawa, T. 2014. Approaches for Mitigating Flood Disasters Utilizing Remote Sensing Information, ACRS
Proceeding 2014: http://www.a-a-r-s.org/acrs/index.php/acrs/acrs-overview/proceedings-1?view=publication&task=show&id=1508