Talk slides on deformation of Usu Volcano given at the IPGP-ERI workshop at IPG Paris.

1. Posteruptive thermoelastic deflation
of intruded magma in Usu Volcano,
Japan, 1992–2017
Xiaowen Wang1,2 and Yosuke Aoki1
1 Earthquake Research Institute, The University of Tokyo
2 Now at Southwest Jiaotong University, Chengdu, China
J. Geophys. Res. Solid Earth, 124, 335-357
doi:10.1029/2018JB016729 (2019)
2 October 2019
Workshop IPGP-ERI
Paris, France

2. Inter-eruptive volcano deflation
Nabro, Eritrea
(Hamlyn et al.,
Prog. Earth Planet. Sci., 2018)
Asama, Japan
(Aoki et al., Geol. Soc. Lond. Spec. Publ., 2013)
Kutcharo, Japan
(Fujiwara et al.,
Earth Planet.
Space, 2017;
Yamasaki et al.,
JVGR, 2018)

3. Why studying volcano deflation?
Potential mechanisms of volcano deflation
 Viscoelastic relaxation (Hamlyn et al., 2018; Yamasaki et al.,
2018)
 Contraction of magma reservoir (e.g. Hamlyn et al., 2018)
 Cooling of emplaced lava (Wittmann et al., JGR Solid Earth,
2017)
Temporal evolution of volcano deflation could carry various
information such as rheology of intruded magma and host rock.

4. Toya caldera
 Usu volcano is located at the rim of
Toya caldera which ejected >100 km3
of magma ~114,000 years ago.
 Eruption of Usu volcano in historical
time:
1663: VEI=5
1769: VEI=4
1822: VEI=4
1853: VEI=4
1910: VEI=2
1944: VEI=2
1977: VEI=3
2000: VEI=2

5. Volcanic activity of Usu Volcano
1910
Activity time
Eruptive
Interval (yr)
Location Eruption type
Upheaval
height (m)
July ̶ Nov. 1910 57 North flank Phreatic 170
Dec. 1943 ̶ Sep.
1945
33 East flank Phreatomagmatic 280
Aug. 1977 ̶ Mar.
1982
32 Summit Phreatomagmatic 180
Mar. ̶ Aug. 2000 18 West flank Phreatomagmatic 80

6. A phreatomagmatic eruption in 2000
Showa Shinzan that emerged during
the 1943-1945 eruption
280 m

7. Subsidence of the 1943 vent
 Persistent subsidence observed by leveling survey.
 Subsidence of 54 mm/yr (1965-1975) and 32 mm/yr (1975-1990)
 Current deformation? Spatial variation?
Yokoyama & Seino
(EPS, 2000)

8. SAR data processing
JERS-1ALOS-1ALOS-2
Ascending Descending
 A total of 111 scenes from JERS-1
(1992-1998), ALOS (2006-2011),
and ALOS-2 (2014-2017)．
 Time-series analysis from all
possible interferograms (a total of
239 pairs).

9. LOS changes The 2000 eruption (Nishiyama)
• Two subsidences
• 38 mm/yr of LOS extension (mainly
subsidence) between 2006 and 2011.
• Negligible LOS changes between 2014
and 2018．
 The 1977-1982 eruption (summit)
• LOS extension rate declines from 66
mm/yr (1992-1998) to 45 mm/yr (2006-
2011) and 43 mm/yr (2014-2017).
 The 1943-1945 eruption（Showa
Shinzan）:
• Stationary LOS extension rate of ~20
mm/yr
DescendingAscending
2000
1977
1943

10. Decomposing (quasi-)EW and vertical velocities
• EW contraction and
subsidence.
• The subsidence rate
is higher than the
contraction rate.
1977
19432000
1977
1943
NC KC
ALOS-1 (2006-2011) ALOS-2 (2014-2017)

11. Modeling by thermal contraction
V
d
Sea level
Intruded
magma body
Surface
Temperature
Time elapse
High Low
V: source volume ;
d: depth of the source;
T: magma temperature (1200 K);
a: thermal expansivity ( 2×10-5);
k: thermal diffusivity;
v: poisson ratio (0.25);
u(x, t) = f (x, t, V, d, T, a, k, v)
 Assumed an intrusion of a
spherical body (Furuya,
2004, 2005).
Black: fixed
Blue: model parameters

12. Optimum parameters
 The depth of the intruded magma is shallower than 400 m bsl．
 The apparent thermal expansivity is an order higher than the
lab-derived value except for the 1943-1945 case.
Longitude
(°)
Latitude
(°)
Depth
( m b.s.l)
Volume
(×106 m3)
Thermal
diffusivity
(×10-5 m2/s)
Misfit Data source
2000 site
140.8034 42.5541 213±19 6.67±0.21 8.21±1.01 2.78 ALOS-1 (NC)
140.8118 42.5563 100±13 2.05±0.13 8.06±1.20 2.02 ALOS-1 (KC)
1977 site 140.8353 42.5416 396±29 132.18±5.21 10.05±1.09 5.06 JERS+ALOS-1+ALOS-2
1943 site 140.8662 42.5426 92±12 49.51±2.12 1.65±0.22 1.03 JERS+ALOS-1+ALOS-2

13. Observation vs calculation
2000 vent
1977 vent 1943 vent
ALOS-
1
JERSALOS-1ALOS-
2
JERSALOS-
1
ALOS-
2

14. The vent of the 1943-1945 eruption
KC
NC
Vertical velocity map
P1943
P1977
P2000

15. KC
NC
Vertical velocity map
P1943
P1977
P2000
The vent of the 1977-1982 eruption

16. KC
NC
Vertical velocity map
P1943
P1977
P2000
The vent of the 2000 eruption

17. Why is the apparent thermal diffusivity high?
Hydrothermal convection effectively release heat
from magma right after the intrusion?
 Lake Toya is right next to the volcano, providing groundwater.
 Frequent phreatomagmatic eruptions
Question:
Why is the apparent thermal diffusivity in the 1943 vent normal?
Possible collaboration with IPGP:
Reconstructing hydrothermal circulation beneath Usu volcano by
numerical simulation.

18. Summary
 We measured ground deformation of Usu volcano by SAR images.
 Deformation is concentrated around lava domes that emerged during
previous eruptions.
 The observed deformation is explained by thermal contraction of the
intruded lava dome.
 The inferred apparent thermal diffusivity is larger than the lab-derived
value especially right after the intrusion.
 Hydrothermal circulation effectively cools the intruded magma?

19. Earth Planets and Space Special Issue
L-band Synthetic Aperture Radar:
Current and future applications to
Earth Sciences
https://earth-planets-space.springeropen.com/lbsar
Submission due: 31 December 2019
Guest Editors：
Yosuke Aoki (Univ. Tokyo), Masato Furuya (Hokkaido Univ.),
Francesco de Zan (DLR), Marie-Pierre Doin (ISTerre),
Michael Eineder (DLR), Masato Ohki (JAXA), Mark Simons (Caltech),
Tim Wright (Univ. Leeds)