G3.6 D1727 Short-term SSEs in the Kanto region, central Japan using GNSS data for a quarter century ©Takuya Nishimura. All rights reserved Takuya NISHIMURA Disaster Prevention Research Institute, Kyoto University E-mail: [email protected]

Key points ・ More than 179 short- and imtermidiate- term SSEs (SSEs) with a duration of up to 80 day are detected using 26-years-long GNSS data in the Kanto region, central Japan. ・Two distinctive SSE clusters where relative plate motion is fully released by s-SSEs are found along the . ・ SSEs rarely overlap shallow tremors and regular M>7 earthquakes along the . However, they overlap M5-6 interplate earthquakes considerably.

Detection of small short-term SSEs using GNSS Data Kanto Double Zone and Boso SSEs The Kanto region, central Japan is situated under complex tectonics Start We apply the method of Flowchart of SSE analysis Nishimura et al. (2013) and where the Philippine Sea and the Pacific plates subduct from the Sagami Trough and the Japan Trench, respectively (Fig. 4).Several SSEs were Pre-process of GNSS data Fault model Nishimura (2014) to detect Remove offsets and estimation reported by the previous studies [e.g., Ozawa et al., 2014]. Recently, postseismic transient Rectangular fault on a jump associated with Remove noisy data Nishikawa et al. found tremors along the Japan trench However, plate interface short-term SSEs in GNSS spatiotemporal distribution of SSEs on both plates still remains unclear. Duration estimation time-series and estimate Spatial filtering Stacking method of crustal deformation (Miyaoka and their fault models from NA Yokota, 2012) (Fig. 3) TZ K observed displacements. A N Estimation of daily offsets Iteration 36 N IS Kanto Estimation of offsets T With Step function rectangular fault on the L Tokyo With Ramp function Inubousaki h Comp. of relative plate motion Mt. Fuji c NS, EW, and UD Comp. Odawara n Philippine Sea or the e Boso Peninsula r T

Ito n a Pacific plates is assumed Tokai h g p

u Event Izu a Yes o r Oshima J Fault model for each SSE. The stacking Omaezaki T Izu Peninsula detection a S Boso SSEs g Niijima agam u i Tr (Fig. 2) estimation r ough using AIC u Kouzushima of GNSS time-series based 34 N S No Rectangular fault PAC Shifting a time window h Mikurajima on plate interface ug on the displacement ro Zenisu i T PHS Finish Modified from Nishimura et al.(2013), Nishimura(2014) ka predicted by the fault model an N 100 km B 0.05 [Miyaoka and Yokota, 138 E 140 E 142 E N50ºW Component at 950443 (Sagawa, Kochi Pref.) Fig. 1 Flowchart of 0.04 Fig. 4 Topographic map of the Kanto Region. Fig. 5 Slip distribution of past 6 Boso SSEs (a-f) and their 0.03 SSE analysis 2012] enable us to estimate Modified from Nishimura et al.(2007) recurrence intervals (g). (Ozawa et al., 2019) 0.02 Linear function duration of SSEs by fitting a 0.01 w/wo a step 0.00 ramp function. 0.20 F 93047 N25W Displacement (m) −0.01 Data and Preprocessing E 93041 N25W −0.02 0.15 0 0 0 37˚ 0 6 8 40 −0.03 GNSS station 1 20 Tremor 100 km −0.04 2011 Tohoku-oki D 950226 N25W AIC point 0.10 −0.05 Fig. 2 Example of daily GNSS coordinates parallel to a relative 2008 Ibaraki-oki 80 plate motion and ΔAIC at 950443 station (Nishimura et al., F 80 0.05 60 2013). Downward steps can be recognized, which is possibly 36˚ Kanto E 40 60 - Δ AIC D caused by S-SSEs. Large -ΔAIC means significant step in C 2011 Tohoku-oki AS 0.00 20 40 time-series. 20 C 93022 N80W 0 1923 Kanto B Displacement (m) 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 A −0.05 Year 35˚ Tokai Boso SSE Japan-Izu-Bonin Trench B 93009 N80W

Tokai SSE Sagami Trough −0.10

Category A 33.509N 132.722E D 30.5km L105.4km W 46.1km Strike 227 Dip 8 Rake 104 Slip 0.0144m Mw6.23 RD 2244.0 Izu

35˚ Horizontal displacement (Duration 1 days) 2012/01/02 Duration estimation by stacking of GNSS Timeseries

A 950216 N80W

Islands

Suruga Trough (Miyaoka and Yokota, 2012) −0.15

Cleaned time-series SSE 34˚ 34˚

137˚ 138˚ 139˚ 140˚ 141˚ 142˚ 143˚

Stacking time-series

33˚ ’ SNR 1.48

2 mm −0.20 Cal. Obs. ~ Source time function 20 km Fig. 6 Used GNSS stations and assumed slab geometry. 94 95 96 97 98 99 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 35˚ Vertical displacement ２(SNR 3.86) The geometry is based on the model complied by Dr. F. Fig. 7 Time-series of daily GNSS displacements for 26 ② 34˚ Hirose. Time-series of A-E stations are shown in Fig.8. ・Removal a linear ② ’ SNR 2.69 ・Stacking in order years. Six Boso SSEs (black arrows) have been identified 33˚ of high SNR. (SNR 5.15) trend 4 mm 15 by previous studies. A linear trend is removed. See Fig. 6 20 km Obs. Cal. 131・˚ Normalized132˚ 133˚ 134˚ by135˚ ・Stop stacking at for station location. noise level the highest SNR of ’ SNR 2.16 ・Reverse polarity stacking timeseries 40 (SNR 6.06) The used daily coordinates at 294 stations from April 25, 1994 to of data if predicted signal is negative. February 21, 2020 are estimated using GIPSY 6.4 software. Offsets ’ SNR 2.69 Fitting of 72 (SNR 6.65) Ramp function related M≥6 earthquakes and maintenance are removed. Although co- 1000 11 days and post- seismic displacement of the 2011 Tohoku-oki earthquake is 200 (SNR 4.63) ’ SNR 2.04 removed by fitting Heaviside, logarithmic, and exponential functions, we excluded a period from December 11, 2010 to September 11, 2011 for ’ SNR 2.19 Estimate duration by fitting the SSE detection because of large scattered data. The transient Normalized amplitude a rump function displacement related the 2000 Miyake-Kouzu volcanic event is also Fig. 3 An example of an analysis procedure for stacking of GNSS Timeseries. Stacked timeseries are used to es- removed by using the model prediction (Nishimura et al., 2001). N80W, timate a source time function of small SSEs. N25W, and N40W components are used to detect steps for SSEs along the Japan Trench, Sagami Trough, and Suruga Trough, respectively.

SSEs detected on the PHS SSEs detected on the PAC

37˚ 37˚ 1.2 37˚ 37˚

00 2011 0 (A) 0 100 80 60 1 60 (B) 80 2 −40 1.1 (A) 40 (B) −2 1.5

1.4

1.0 0.5 1.3

0.9 2008 1.2

0.8 1.1

36˚ 36˚ 36˚ 1.0 8 0.7 36˚ 2011AS2011AS 80 0 0 −4 0.9

0.6 60 −40 20 −60 0.8 −20 0.5 0.7

0.6 1923 0.4 5 0. 1.5 0.5

0.3 Total slip(m) 1 0.4 35˚ 35˚ 35˚ 35˚ 1 Total slip(m) 0.2 0.3 TokaiTokai 0.5 0.5 0.1 0.2 0.1 0.0 0.5 0.0 TonankaiTonankai 50 km 50 km 50 km 50 km 34˚ 34˚ 34˚ 34˚ 137˚ 138˚ 139˚ 140˚ 141˚ 142˚ 137˚ 138˚ 139˚ 140˚ 141˚ 142˚ 139˚ 140˚ 141˚ 142˚ 143˚ 139˚ 140˚ 141˚ 142˚ 143˚ Fig. 8 Fault models for detected S-SSEs on the . (a) Rectangular fault Fig. 9 Fault models for detected S-SSEs on the . (a) Rectangular fault models models (blue rectangles) and their slip vectors (yellow vectors). (b) Cumulative slip of SSEs. (blue rectangles) and their slip vectors (yellow vectors). (b) Cumulative slip of SSEs. Focal Focal mechanism is plotted for Mw>4.6 interplate earthquakes (240º ≤ Strike ≤ 300º, 0º ≤ Dip ≤ mechanism is plotted for Mw>4.6 interplate earthquakes (160º ≤ Strike ≤ 220º, 0º ≤ Dip ≤ 30º, 30º, and 45º ≤ Rake ≤ 135º) during 1997-2019. Green regions represent source regions of the and 45º ≤ Rake ≤ 135º) during 1997-2019. Green regions represent source regions of the past past megathrust earthquakes. megathrust earthquakes. Findings and Implications Findings and Implications ・Two distinctive SSE patches (Regions A and B) ・Many SSEs occur in two depth ranges, i.e., 10-20 km and 40-60 km. ・Total slip on these patches is equal to the relative plate motion. ・Little overlapping of S-SSEs and tremor regions in the shallow(10-20 km) depth range. ・Medium-size interplate earthquakes occur at a downdip edge ・Both SSEs and fast earthquakes coexist in the deep depth range (40-60 km). It is partly attributed of SSEs patches. to detected SSEs include co- and post-seismic slip events of M~6 earthquakes. ・Small cumulative slip in the region of past megathrust ・No significant change of SSE slip behavior at the 2011 Tohoku-oki earthquake (Fig. 10). ・Small cumulative slip in the region of past megathrust earthquakes.

Temporal evolution of SSE moment release PAC SSEs controlled by subducting sea-mounts and overriding plates 2×1020 4×1020 Japan Trench 37˚ SSE on PAC plate Sagami Trough Region A × 20 × 20 Many shallow SSEs on the PAC locate in a region 1.5 10 Region B 3 10 where the overriding plate is the North American Suruga Trough 36˚ plate, not the Philippine Sea plate (Fig. 11). This may 1×1020 2×1020 reflect on a difference of interplate coupling controlled by geology of the overriding plate [Uchida et al., 0.5×1020 1×1020 35˚ 2009]. It is also suggested that the SSE cluster at (Nm) Trench Moment for Japan ~35.5ºN corresponds to a subducted sea-mount (Nm) Troughs Moment for Sagami and Surga 0 0 induced from a bathymetry and gravity anomaly. 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 34˚ Calendar year 139˚ 140˚ 141˚ 142˚ 143˚ Fig. 10 Moment evolution of SSEs in the three analyzed Fig. 11 Distribution of the PAC SSEs and bathymetry. SSE region. A gray period is excluded in the analysis because of clusters at ~35.5ºN corresponds to a subducted sea-mounts postseismic deformation of the 2011 Tohoku-oki earthquake. chain corresponds. Reference Acknowledgments Miyaoka, K. and T. Yokota, Development of stacking method for the detection of crustal deformation -Application to the early detection of slow slip phenomena on the plate boundary in the Tokai region using strain data, J. Seismol. Soc. Jpn. (Zisin), 65, 205-218, 2012. Nishikawa, T., Matsuzawa, T., Ohta, K., Uchida, N., Nishimura, T., and Ide, S. The slow earthquake spectrum in the Japan Trench illuminated by the S-net seafloor observatories. Science, 365(6455), 808-813, 2019. We thank the Geospatial Information Authority of Japan and the Nishimura, T., Ozawa, S., Murakami, M., Sagiya, T., Tada, T., Kaidzu, M., and Ukawa, M. Crustal deformation caused by magma migration in the northern Izu Islands, Japan. Geophysical Research Letters, 28(19), 3745-3748, 2001. Nishimura, T., T. Sagiya, and R. S. Stein, Crustal block kinematics and seismic potential of the northernmost Philippine Sea plate and Izu Microplate, central Japan, inferred from GPS and leveling data, J. Geophys. Res., 112, B05414, doi:10.1029/2005JB004102, 2007. Japan Coast Guard for providing GNSS RINEX data. The Japan Nishimura, T., T. Matsuzawa, and K. Obara, Detection of short-term slow slip events along the , southwest Japan using GNSS data, J. Geophys. Res. Solid Earth, 118, 3112-3125, doi:10.1002/jgrb.50222, 2013. Nishimura, T., Short-term slow slip events along the , southwestern Japan, observed by continuous GNSS, Progress in Earth and Planetary Science, 1:22, doi:10.1186/s40645-014-0022-5, 2014. Meteorological Agency provided the unified catalogue of earthquakes. Ozawa, S., Shortening of recurrence interval of Boso slow slip events in Japan. Geophysical Research Letters, 41(8), 2762-2768, 2014. Uchida, N., Nakajima, J., Hasegawa, A., and Matsuzawa, T. What controls interplate coupling?: Evidence for abrupt change in coupling across a border between two overlying plates in the NE Japan subduction zone. Earth and Planetary Science Letters, 283(1), 111-121, 2009.