1f-0006

The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020
Paper N° C000661
Registration Code: S-A00565

MICROTREMOR SURVEYS IN LARGE SEDIMENTARY BASINSON THE COAST OF THE SEA OF JAPANK. Asano (1), K. Yoshida (2), K. Miyakoshi (3), M. Ohori (4), T. Iwata (5)(1) Associate Professor, Disaster Prevention Research Institute, Kyoto University, Japan, k-asano@sms.dpri.kyoto-u.ac.jp(2) Senior Researcher, Geo-Research Institute, Japan, yoshida@geor.or.jp(3) Chief Researcher, Geo-Research Institute, Japan, ken@geor.or.jp(4) Associate Professor, Research Institute of Nuclear Engineering, University of Fukui, Japan, ohorim@u-fukui.ac.jp(5) Professor, Disaster Prevention Research Institute, Kyoto University, Japan, iwata@egmdpri01.dpri.kyoto-u.ac.jp

Abstract
Many sedimentary basins or plains were formed during the Tertiary and Quaternary periods on the coast of the Sea ofJapan. These sedimentary basins would be struck by large earthquakes occurring at nearby active fault systems.
Because reliable S-wave velocity structure model is required for quantitatively evaluating seismic hazard, available
information on the S-wave velocity structure model needs to be enriched in such sedimentary basins. As a part ofIntegrated Research Project on Seismic and Tsunami Hazards around the Sea of Japan (2013-2020), we have conductedlarge-scale microtremor array surveys in sedimentary basins on the coast of the Sea of Japan. The surveyed sites include
two sites in the Kaga Plain, two sites in the Ochigata Plain, fifteen sites in the Toyama Plain (including the Niikawa,Tonami, and Imizu Plains), four sites in the western part of the San’in region (Masuda, Hagi, and Nagato cities), fivesites in the Hakodate Plain, and seven sites in the Tsugaru Plain. Each array consists of several circular arrays ofdifferent radii ranging from ten meters to about one thousand meters in order to obtain phase velocities representing
velocity structure from the near-surface sediment layers to the seismic bedrock. The vertical component of microtremor
was analyzed by the spatial auto-correlation method (SPAC) to obtain the phase velocity dispersion curve. Then, the Swave velocity structure was modeled by inverting the obtained phase velocity data. We also additionally measured H/Vspectra by single-station microtremor observations at many sites in the Kaga, Ochigata, Masuda, Hagi, Senzaki(Nagato), and Tsugaru Plains to investigate the spatial variation in the bedrock depth. The depth of the seismic bedrock
is approximately 5–6 km in the Toyama and Tonami plains, which is consistent with existing reflection and refraction
survey results. The Tsugaru Plain also has relatively thick sediments, and those thicknesses are about 3 km in the central
part of the plain. This paper summarizes the results of our microtremor surveys conducted in the last seven years (20132019).
Keywords: microtremor array, SPAC method, H/V spectral ratio, S-wave velocity structure model, sedimentary basin© The 17th World Conference on Earthquake Engineering- 1f-0006 -1f-0006

The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

1. Introduction
Many sedimentary basins were formed during the Neogene and Quaternary periods on the coast of the Sea ofJapan. These sedimentary basins would be struck by large earthquakes occurring at nearby active fault
systems. A reliable S-wave velocity structure model is required for quantitatively predicting strong groundmotions from such earthquakes. For that purpose, nation-wide three-dimensional velocity structure models
for ground motion simulations such as J-SHIS [1, 2] and JIVSM [3] have been developed in Japan bycompiling available geophysical and geological information. Thus, available information needs to beenriched to validate and improve the models. However, S-wave velocity structure is not investigated well for
sedimentary basins on the coast of the Sea of Japan except some well-studied areas such as Niigata [4, 5],
Fukui [6], Tottori [7], and Fukuoka [8] basins. As a part of Integrated Research Project on Seismic andTsunami Hazards around the Sea of Japan (2013–2020), we have conducted large-scale microtremor arraysurveys to obtain information on the S-wave velocity structure down to the seismic bedrock in those
unexplored sedimentary basins.

2. Microtremor Array Observation
Microtremor array observation has been conducted at 35 sites in sedimentary basins on the coast of the Sea
of Japan (Fig.1). Most of those sites were selected referring to the locations of permanent strong motion
stations. The surveyed array sites are composed of two sites in the Kaga Plain, two sites in the Ochigata Plain,fifteen sites in the Toyama Plain, which includes the Niikawa, Tonami, and Imizu Plains, four sites in thewestern part of the San’in region (Masuda, Hagi, and Nagato cities), five sites in the Hakodate Plain, andseven sites in the Tsugaru Plain. Each array observation consisted of several equilateral triangle arrays ofdifferent radii ranging from ten meters to about one thousand meters in order to obtain phase velocities
representing velocity structure from the near-surface sediment layers to the seismic bedrock (Table 1). One
small array observation (r = 10 m) was additionally carried out at MCH (Masuda city hall) because MSD isapart from a strong motion station located at Masuda city hall. We also measured H/V spectra by singlestation microtremor observations at many sites in the Kaga, Ochigata, Masuda, Hagi, Senzaki (Nagato), andTsugaru Plains to investigate the spatial variation in the bedrock depth.

Fig. 1 – Map of microtremor array observation sites

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The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

The observation system consisted of a three-component velocity sensor Lennartz LE-3D/5s having anatural period of 5 s and a data logger Hakusan LS-8800 with 24 bits A/D converter. All recorders in anarray were synchronized by the clock of Global Positioning System. The field works were carried out from
December 2013 to November 2019. The recording duration depends on the size of an array and traffic noisein the surrounding area.
Table 1 – List of microtremor array observation
RegionKaga
Ochigata
ToyamaSan’inHakodate

Tsugaru

Site
KMT
MKH
HKI
NNO
NYZ
UOZ
NMK
TTY
TYB
TYF
OYM
YTO
SIM
DIM
SNM
TNM
OYB
NNT
FKM
MSD
MCH
HAG
NGT
HEK
HAK
MHR
NNE
KMI
OON
HRK
HRS
FJS
IYG
GSW
ING
KRI

Latitude (°N)
36.39902
36.49061
36.89380
37.04567
36.93481
36.82314
36.76432
36.66225
36.72944
36.66900
36.60897
36.58531
36.74025
36.71374
36.78046
36.64767
36.67167
36.58838
36.54680
34.69414
34.67466
34.40722
34.37418
34.39028
41.77032
41.81762
41.86187
41.82532
41.88390
40.61458
40.6012740.65510
40.69692
40.81570
40.88019
40.66417

Longitude (°E)
136.44376
136.49177
136.77949
136.96867
137.50228
137.41153
137.34198
137.31854
137.26300
137.21208
137.28242
137.13992
137.14874
137.03644
137.08285
136.96264
136.89547
136.91946
136.86649
131.83235
131.84351
131.39887
131.18038
131.10141
140.73830
140.75182
140.70154
140.65311
140.64473
140.57196
140.49829
140.50381
140.45653
140.50381
140.39669
140.57972

3© The 17th World Conference on Earthquake Engineering- 1f-0006 -Array radii (m)200, 400, 800, 1600
200, 400, 800, 1600
200, 400, 800, 1600
37, 110, 200, 400
130, 350, 770, 1540
81, 243, 500, 1000
17, 51, 180, 360, 705, 1410
30, 137, 300, 710, 1450
120, 285, 750, 1500
95, 315, 750, 1500
35, 105, 300, 600
140, 300, 666, 1370
30, 143, 300, 675, 1540
20, 60, 235, 470, 750, 1500
78, 234, 500, 1000
125, 280, 676, 1260
26.7, 80, 240, 500, 1000
75, 225, 510, 1020
75, 225, 500, 1000
10, 30, 110, 220
1010, 30, 116, 232
10, 30, 100, 200
10, 30, 140, 280
10, 40, 120, 380, 760
10, 28, 92, 310, 620
10, 58, 174, 444, 888
12, 29, 83, 249, 690, 1380
10, 30, 90, 180, 490, 980
15, 45, 75, 150, 500, 1000
15, 45, 155, 310, 600, 1200
15, 45, 160, 320, 700, 1400
15, 45, 175, 350, 700, 1400
15, 45, 125, 250, 500, 1000
20, 60, 145, 290, 640, 1280
20, 60, 147.5, 295, 590, 1180

Year
2013
2013
2013
2014
2014
2015
2014/2015
2014
2014
2014
2014
2014
2014
2014
2015
2014
2015
2015
2015
2016
2016
2016
2016
2016
2017
2017
2017
2017
2017
2018
2018
2018
2018
2018
2019
2019

1f-0006

The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

3. Data Processing
The vertical component of the observed microtremors was analyzed to obtain the spatial autocorrelation(SPAC) coefficients [9] after dividing the records into small segments (20.48s, 40.96s, or 81.92 s). The phasevelocity dispersion curve was estimated by the extended SPAC method [10, 11] using the obtained SPAC
coefficients. Then, we estimated the one-dimensional layered S-wave velocity structure model at each site byassuming that the phase velocity could be modeled as the fundamental mode of the Rayleigh wave. The Swave velocity of each layer was fixed at the value given in the layered model of J-SHIS V2 [2] except forsome additional layers, which was necessary for reproducing the observed dispersion curves. The thicknessof each sedimentary layer and the S-wave velocity of the additional layers were estimated by using thegenetic algorithms [12] or the Markov chain Monte Carlo method [13].

4. Results and Discussions
4.1 Kaga and Ochigata Plains
The observed phase velocity (blue circles) and the estimated S-wave velocity structure model (red line) at
two sites in the southern part of the Kaga Plain and two sites in the Ochigata Plain, central Japan, are shown

Fig. 2 – Phase velocity dispersion curves, S-wave velocity models, and H/V peak frequencies (coloredsquares on the map) in the Kaga and Ochigata Plains

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The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

in Fig.2 together with the model of J-SHIS V2 (green line). The depth in the list within the figure refers to
the top depth of each layer (The same applies hereinafter). The detail of the survey in this area has been
published in our previous paper [14]. The S-wave velocity model for NNO was revised by another fieldobservations conducted after that study. The estimated S-wave profiles at KMT and MKH and the spatial
variation in the peak frequency of H/V spectral ratio indicate that the thickness of the Quaternary depositschanges at around the Tedori River. The peak frequency of H/V spectra decreases gradually along the linefrom Nanao (NNO) to Hakui (HKI) in the Ochigata Plain, indicating gradual change in the bedrock depth. It
is also consistent with the Bouguer anomaly in this area [15].

4.2 Toyama Plain
The observed phase velocity (blue circles) and the estimated S-wave velocity structure model (red line) at
fifteen sites in the Toyama Plain, central Japan, are shown in Fig.3. The present J-SHIS V2 model (greenline) reproduces well at several sites particularly in the lower frequency range (< 1 Hz). The observed phasevelocity in the higher frequency range (> 1 Hz) is relatively low at alluvium sites in the Izumi Plain (SNMand SIM) and NMK compared to other surveyed sites mainly covered by fan and terrace deposits. The
bedrock depth is deep (5–6 km) at most of the surveyed sites in Toyama, Izumi, and Tonami Plains. The
deep seismic reflection profile across the Tonami Plain and the Kureha Hills by Ishiyama et al. (2017) showsthat the Neogene sedimentary basin deposits fill up to approximately 5 km depth beneath the plain [16], andthat result is quite consistent with our microtremor surveys at OYB and TNM.

Fig. 3 – Phase velocity dispersion curves and S-wave velocity models in the Toyama Plain

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The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

Fig. 3 – Continued

4.3 Western San’in region
The observed phase velocity (blue circles) and the estimated S-wave velocity structure model (red line) atfive sites in the western part of the San’in region, western Japan, are shown in Fig.4. The estimated phasevelocity is limited in the frequency range at NGT and HEK due to extremely low power spectra of theobserved microtremors. The S-wave velocity of the topmost layer is 0.12–0.20 km/s in the Masuda plain(MSD and MCH) and the Hagi plain (HAG), indicating existence of Holocene delta deposits. The estimated6© The 17th World Conference on Earthquake Engineering- 1f-0006 -1f-0006

The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

Fig. 4 – Phase velocity dispersion curves, S-wave velocity models, and H/V peak frequencies (coloredsquares on the map) in Masuda, Hagi, and Nagato cities, western San’in region

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The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

S-wave velocity model at each site has thicker sediments than the present J-SHIS V2 model (green line). The
spatial distribution of the peak frequency of H/V spectra is relatively complicated in the Senzaki (Fukagawa)plain (NGT) compared to the Hagi and Masuda plains.

4.4 Hakodate Plain
The observed phase velocity (blue circles) and the estimated S-wave velocity structure model (red line) atfive sites in the Hakodate Plain, southern Hokkaido, are shown in Fig.5. MHR is located on the middle
terrace, and the other stations are located on the alluvium. The estimated velocity model explains theobserved phase velocities better than the J-SHIS V2 model (green line). The thickness of Quaternary and
Neogene sediments at MHR is relatively thin, which reflects its higher phase velocity in the low frequency
range (<3 Hz) compared to the other sites. The layers with S-wave velocity ranging from 1.1 to 1.7 km/s
dominates in the sedimentary layers, that is, the Hakodate Plain is mostly filled by Miocene sediments. The
S-wave velocity of the topmost layer at NNE is very small (0.07 km/s), which is might be due to peat in thisarea. The active reverse fault zone along the western margin of the Hakodate Plain [17] makes thickness of
Quaternary sediments deepen in the western part of the plain (KMI and OON) compared to the eastern part
(NNE).

Fig. 5 – Phase velocity dispersion curves and S-wave velocity models in the Hakodate Plain

4.5 Tsugaru Plain
The results at seven sites in the Tsugaru Plain, northeastern Japan, are shown in Fig.6. The estimated model(red line) explains the observed phase velocities (blue circles) much better than J-SHIS V2 model (green

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The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020line). The bedrock depth is estimated to be about 1.5–1.6 km in the northern part of the Tsugaru Plain (ING
and GSW), and it is more than 3 km in the central part of the plain (IYG and FJS). The top depth of VS 0.6
km/s is shallow in the central part of the plain (FJS and HRS). The layers with VS 1.1–1.7 km/s are relatively
thick among sedimentary layers, and they are more than 1 km at most sites. This fact indicates Miocene

Fig. 6 – Phase velocity dispersion curves and S-wave velocity models in the Tsugaru Plain

9© The 17th World Conference on Earthquake Engineering- 1f-0006 -1f-0006

The 17th World Conference on Earthquake Engineering

17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

sedimentary layers are thick in the Tsugaru Plain. The S-wave velocity of the topmost layer is small (0.08–
0.13 km/s) at sites on alluvium deposits (Jusanko Formation) along the Iwaki River (ING, GSW, IYG and
FJS) and relatively large (0.20–0.23 km/s) at the other sites (HRS, HRK, and KRI). The results of H/Vspectra by single-station microtremor observations will be used to interpolate the structure model over the
Tsugaru Plain.

5. Conclusions
We have carried out large-scale microtremor array surveys at 35 sites in the sedimentary basins on the coastof Sea of Japan to investigate S-wave velocity structure model down to the bedrock. We also additionallymeasured H/V spectra by single-station microtremor observations at many sites in the Kaga, Ochigata,
Masuda, Hagi, Senzaki (Nagato), and Tsugaru Plains to investigate the spatial variation in the bedrock depth.
The estimated depth of the seismic bedrock is approximately 5–6 km in the Toyama and Tonami plains incentral Japan, which is consistent with existing reflection and refraction surveys. The Tsugaru Plain in
northeast Japan also has relatively thick sediments, and those thicknesses are more than 3 km in the central
part of the plain. The sedimentary layers having S-wave velocity ranging from 1.1 to 1.7 km/s, which might
be correspond to Neogene sedimentary rocks, are relatively thick compared to Quaternary sediments (Vs < 1km/s) in the sedimentary basins studied during this project. These survey results would be quite useful forrevising the nation-wide and regional three-dimensional velocity structure models in near future.

6. Acknowledgements
This work has been conducted as a part of Integrated Research Project on Seismic and Tsunami Hazards
around the Sea of Japan (FY2013-2020, PI: Prof. M. Shinohara, Earthquake Research Institute, University ofTokyo) funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.
The field observation and data analysis were carried out in collaboration with Dr. Suqun Ling. Researchers
from Geo-Research Institute and Geo-Analysis Institute Co., Ltd. and students from Kyoto University andKanazawa University participated in the field work. The maps were created based on the Digital Maps (Basic
Geospatial Information) of the Geospatial Information Authority of Japan [18] and JTOPO30v2 of the
Marine Information Research Center, Japan Hydrographic Association. The Generic Mapping Tools version
6 [19] were used to draw figures.

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17th World Conference on Earthquake Engineering, 17WCEE
Sendai, Japan - September 13th to 18th 2020

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