Research report for the ERI visit from April 15 to June 17, 2018
Dr. Lucile BRUHAT (Visitor), Dr. Junichi FUKUDA (Host)
July 31, 2018

I visited ERI from April 15 to June 17, 2018. The original plan was to apply the methodology
I developed during my PhD to the Japan Trench megathrust, in particular the region affectedby the 2011 Tōhoku earthquake. Following recent study that Dr. Fukuda had done about slow
slip events in the Bōsō Peninsula area, he suggested that I analyze interseismic rates in the sameregions. This is the project I focused on during my stay at ERI. I also took advantage of my visit
to give a seminar at ERI on Friday, May 11, to participate to the weekly Blue Earth Seismology
Seminar organized by Prof. Kawakatsu, to present my research at the JpGU annual meeting, andto visit JAMSTEC at the Yokohama Institute for Earth Sciences.

Research Project: Temporal evolution of fault coupling associated with theoccurrence of slow slip events in central Japan
Background
Although interseismic coupling has often been considered to be stationary in time, there is increasing evidence that fault locking can vary both spatially and temporally during the interseismicperiod. The detection of transient slip behavior in the proximity of locked regions, such as slow
slip events or decadal-scale uncoupling events, suggests in fact that the notion of a characteristic
interseismic coupling distribution might not be appropriate.
This study focuses on interseismic deformation rates in the southeastern part of the Kantōarea in Japan. This region lies at junction of two subduction zones, leading to a particularlycomplicated tectonic setting. On the Eastern side, the Pacific plate subducts under the Okhotsk
Plate at the Japan Trench, while the Sagami Trough to the south evidences the subduction of thePhilippine Sea Plate under the Okhotsk Plate. The Philippine Sea Plate interface has hosted M8
megathrust earthquakes in the vicinity of Tokyo metropolitan area, such as the 1923 Great Kantō
earthquake. Studies of interseismic deformation rates (Sagiya, 2004; Nishimura et al., 2007) haveshown that these megathrust events are consistent with the presence of a strongly locked asperityon the western extent of the Philippine Sea Plate interface at depths above 15-20km. Meanwhile,offshore the Bōsō Peninsula, i.e. on the eastern side of the interface, recurrent slow slip events
have been detected in 1996, 2002, 2007, 2011, 2013-2014, and 2018. These events of magnitude
6.4–6.7 last between 14 to 43 days and are associated to transient slip located at the downdip
transition between the locked region and the deep freely creeping zone (e.g., Fukuda, 2018; Hiroseet al., 2012, 2014).
Data analysis
In this study, we analyze deformation rates from GEONET GPS measurements at stations in thesouthern Kantō region (see Figure 1).
1All Stations
36.5

3006
0584
3008
30103011 3013 0224 3012
3014
3015
3016 3018
3020
3019
3023
0228
0225
3024
3026
3025
3027
3028
3030
3033
08043032
3037
3034
3036
0756
3067
3045
0226
0759
303909593044 3041
3007

Latitude

36

35.5

3009
3021

3022

0227
3047

35

34.5

139

139.5

140

140.5

141

Longitude

Figure 1: GPS stations used in this study
Time series are corrected for offsets due to antenna changes, for the 2000 volcano-seismologicalevent in Izu Islands, and for co- and post-seismic deformation due to local earthquakes (e.g., theseries of magnitude 6-6.9 which occurred on May 8th, 2008, off the coast of Ibaraki Prefecture).
Time series are also filtered from seasonal variations and from common-mode errors following aprocedure developed by Mavrommatis et al. (2014). Figure 2 presents an example of corrected
time series.
We estimate horizontal and vertical interseismic rates at four different inter-SSE periods from1994 to 2011. We notice changes in the linear trend, between inter-SSE periods for both horizontal
and vertical components, which may be associated to changes in fault coupling.
Preliminary results and perspectives
Deformation rates are then inverted to produce coupling maps at the considered inter-SSE periods.
We finally investigate possible relationships between the characteristics of the slow slip events andthe inferred changes in interseismic fault coupling.
This work will be presented at the 19th General Assembly of WEGENER in Grenoble (France)in September 2018, and at the 2018 AGU Fall meeting in Washington, D.C. (USA).

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Figure 2: Displacement at the station 3015. Corrected from seasonal variations, antenna changes
and earthquake deformation. During considered time period, three slow slip events (SSEs) occurred: May 11th - 24th, 1996; October 2nd - November 13th, 2002; and August 12th - 25th,
2007. SSEs are indicated by thin solid colored lines in the time series.

Acknowledgement
I would like to thank Dr. Fukuda for hosting me during these two months at ERI. I would also
thank Prof. Kawakatsu for welcoming me in his weekly seminar. Finally, thank you to all thestudents and researchers I have interacted with, and all the staff at the ERI International Office
for making my visit so enjoyable.

References
Fukuda, J. (2018), Variability of the Space-Time Evolution of Slow Slip Events Off the Boso
Peninsula, Central Japan, From 1996 to 2014, Journal of Geophysical Research: Solid Earth,123 (1), 732–760, doi:10.1002/2017JB014709.
Hirose, H., H. Kimura, B. Enescu, and S. Aoi (2012), Recurrent slow slip event likely hastened3

by the 2011 Tohoku earthquake, Proceedings of the National Academy of Sciences, 109 (38),15,157–15,161, doi:10.1073/pnas.1202709109.
Hirose, H., T. Matsuzawa, T. Kimura, and H. Kimura (2014), The Boso slow slip events in 2007
and 2011 as a driving process for the accompanying earthquake swarm, Geophysical Research
Letters, 41 (8), 2778–2785, doi:10.1002/2014GL059791.
Mavrommatis, A. P., P. Segall, and K. M. Johnson (2014), A decadal-scale deformation transient
prior to the 2011 M w 9.0 Tohoku-oki earthquake, Geophysical Research Letters, 41 (13), 4486–4494, doi:10.1002/2014GL060139.
Nishimura, T., T. Sagiya, and R. S. Stein (2007), Crustal block kinematics and seismic potential of
the northernmost Philippine Sea plate and Izu microplate, central Japan, inferred from GPS andleveling data, Journal of Geophysical Research, 112 (B5), B05,414, doi:10.1029/2005JB004102.
Sagiya, T. (2004), A decade of GEONET: 1994–2003 —The continuous GPS observation in
Japan and its impact on earthquake studies—, Earth, Planets and Space, 56 (8), xxix–xli, doi:
10.1186/BF03353077.

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