1EGU General Assembly 2020 / 4–8 May 2020 online
EGU2020-11999 (Session SM2.5, Thu, 07 May, 14:00–15:45CEST)
https://doi.org/10.5194/egusphere-egu2020-11999

Fault modeling and stress drop estimation
based on millimeter-scale tsunami recordsof an M6 earthquake detected by thedense and wide pressure gauge array
Tatsuya KUBOTA1, Tatsuhiko SAITO1, Wataru SUZUKI1

1. National Research Institute for Earth Science and Disaster Resilience

This work is published in GRL!
https://doi.org/10.1029/2019GL085842

RESEARCH LETTER
10.1029/2019GL085842
Key Points:• Millimeter‐scale tsunamis from an
Mw 6.0 earthquake were capturedby the S‐net, a new nationwide
pressure gauge array off Sanriku,
Japan• Tsunami signals were identiﬁed
from the pressure data adjacent tothe source, which werecontaminated by signals irrelevant
to tsunamis• We inferred the stress drop of theearthquake from the tsunami data
more reliably than could be done
from seismogram analysis
Supporting Information:• Supporting Information S1

Correspondence to:T. Kubota,
kubotatsu@bosai.go.jp[Acknowledgments]

This study used the pressure gauge data recorded by JAMSTEC (KPG1and KPG2) and DART (21346 and 21347). This study was financiallysupported by JSPS KAKENHI 19K14818 and the Sasakawa Scientific
Research Grant by Japan Science Society 2019-2037. The GMTsoftware (Wessel & Smith, 1991) is used for preparing the figures.

Citation:
Kubota, T., Saito, T., & Suzuki, W.
(2020). Millimeter‐scale tsunami
detected by a wide and dense
observation array in the deep ocean:
Fault modeling of an Mw 6.0 interplate
earthquake off Sanriku, NE Japan.
Geophysical Research Letters, 47,e2019GL085842. https://doi.org/10.1029/2019GL085842
Received 16 OCT 2019
Accepted 22 JAN 2020
Accepted article online 24 JAN 2020

Millimeter‐Scale Tsunami Detected by a Wide and DenseObservation Array in the Deep Ocean: Fault Modelingof an Mw 6.0 Interplate Earthquake off Sanriku,NE JapanT. Kubota1, T. Saito1, and W. Suzuki1

1National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan

Abstract A new dense and widely distributed tsunami observation network installed off northeast Japandetected millimeter‐scale tsunamis from an Mw 6.0 shallow interplate earthquake on 20 August 2016.
Based on the fault model deduced from this data set, we obtained a stress drop of 1.5 MPa for this event,similar to those associated with typical interplate earthquakes. The rupture area was unlikely to overlap
with regions where slow earthquakes occur, such as low‐frequency‐tremors and very‐low‐frequency‐earthquakes. The results demonstrated that this new network has dramatically increased the detectabilityof millimeter‐scale tsunamis. Some near‐source stations were contaminated by large pressure offset
signals irrelevant to tsunami, and we must therefore be careful when analyzing these data. Nonetheless,the new array enables estimations of the stress drops of moderate offshore earthquakes and can be used toelucidate the spatial variation of mechanical properties along the plate interface with much higher
resolution than previously possible.

Plain Language Summary Tsunamis are generated when an earthquake occurs beneath theseaﬂoor. Far fewer tsunami observations have been recorded from moderate earthquakes than large to
giant earthquakes because tsunamis created by moderate earthquakes have been too small to beobserved. On 20 August 2016, a moderate earthquake occurred off Sanriku, in northeastern Japan, and atsunami with a height of less than 1 cm was recorded by a new seaﬂoor tsunami observation network.
This network has many tsunami sensors distributed much closer to each other and over a much wider
area than any other previous network in the world. Using these data, this study estimated the source
location and size, and the slip amount of the 2016 earthquake with higher accuracy, which wasimpossible to achieve from past observations because they were too far away from the earthquake and thesignals were too small. Using this source information, we could estimate the stress drop associated withthe earthquake, which is important because the stress drop information deepens our understanding of
how and why earthquakes happen.

M>~7 EQ modeling using offshore pressure gauge (PG) tsunami data

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M >~7 tsunamis have been detected by offshore PGs and used for fault modeling.
Ø However it is challenging to observe M<~6 tsunami– Typical offshore PG networks containMw 6.5 ~100cmMw 7.2 ~101cm
too few stations and remote fromthe source.

Mw 9.0 ~102 cm

Satake et al. (2013BSSA)

Maeda et al. (2011EPS)
Kubota et al. (2019PEPS)

Kubota et al. (2017EPSL)Millimeter-scale tsunami in the 2016 Off-Sanriku EQ (Mw 6.0)

3

ü Recently a new dense and wide observation network (S-net) is constructed.
ü Millimeter-scale tsunami was observed by the S-net during ○ Regular interplate EQs (NIED F-net)● Tremor (Tanaka et al. 2019)an Mw 6.0 EQ off Sanriku on 20 Aug 2016.
● VLFE (Nishikawa et al. 2019)– at northern edge of the 1896 Sanriku EQ.
– near the active regions of the low-frequency tremors andvery-low frequency EQs (VLFEs)(e.g., Tanaka et al. 2019; Nishikawa et al. 2019; Baba et al. 2020)

Ø Purpose– estimate fault modelusing the S-net millimeterscale tsunami data
– examine its relationshipwith other interplate
phenomenaMw 6.0 ~10-1cm

Data processing
ü If only from each single trace,
it is difficult to recognize tsunami
due to noise.
ü When traces are aligned, westwardpropagating tsunami is recognizable.
ü Large step-like signal is observed
at S4N10.

De-tided
BPF 100-1800s

45Note: abrupt pressure increase at S4N10
ü Rotation of 5.7o was observed based on co-located accelerometerü PG sensor is sensitive to its rotation (Chadwick et al. 2006)(Takagi et al. 2019)~20 hPa
increase

DetidedLPF 100s
BPF 100-1800s

Pressure (psi)Ø The step-like signals are likely due to the sensor rotation
associated with the seafloor seismic motion.
Ø Careful analysis is required to use the near-source
S-net PG data.

Chadwick et al. (2006)

Angle of rotation (deg)θ = 5.72°
Takagi et al. (2019)

Analysis (1): Estimation of fault horizontal location
ü Horizontal location of the fault was estimated via thegrid-search analysis- Scaling law of Blaser et al. (2010) is used for assuming the rectangularfault dimension (L and W).
- GCMT seismic moment is used to determine slip amount D.
- Depth is fixed to coincide with the plate boundary model of Iwasaki
et al. (2015).
- Tsunami is calculated by solving linear-dispersive equation.

GCMT𝑀!
𝐷=(𝜇 = 40GPa)
𝜇𝐿𝑊

6Analysis (1): Estimation of fault horizontal location
ü Optimum fault was estimated at ~10 km west of the GCMT centroid.
Ø When shifting the location by ~ 5km, the arrival time cannot be explained.
→ horizontal resolution
is ± ~5 km.

GCMT

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Analysis (2): Fault dimension modelingü Optimum model had L=17 km, W=5 km, and
D=40.5 cm (Mw 6.0, μ=40GPa)
ü Static stress drop of Δσ ~ 1.5 MPa
Ø Waveforms were reproduced well except for S4N10Δ𝜎 =8𝜇 𝑀!
3𝜋 𝐿𝑊 "/$8Analysis (2): Fault dimension modelingØ Fault dimensions of L > 20 km or W > 7 km cannot explain the observation.
→ Uncertainty of fault dimension is L ≤ 20 km and W ≤ 7 km

910Stress drop uncertainty
We evaluate plausible range of fault
parameters by changing L or W
ü Models with relatively high variancereduction (VR) had ranges of15 ≤ L ≤ 20km, W ≤ 7 km, and0.7 ≤ Δσ ≤ 5 MPa
Ø Δσ seems not so small as expected in”tsunami earthquake” like the 1896
Sanriku earthquake, characterized byextremely small Δσ (<< 1 MPa)Δ𝜎 =8𝜇 𝑀!
3𝜋 𝐿𝑊 "/$𝑉𝑅 = 1 −∑% 𝑥%&'( − 𝑥%)*+$∑% 𝑥%)*+$×100 %
*Mo was fixed to that of the optimum model.

11Discussion (1): spatial heterogeneity of stress drop
ü Δσ inside the rupture area of the 1992 Nicaragua tsunami EQ wassignificantly smaller than outside (Bilek et al. 2016)– However:no systematic spatial difference of Δσ was seen offSanriku (e.g., Yamashita et al. 2004).
Ø To discuss in more detail, we needto:– compare Δσ deduced from theS-net seismometers.
– examine more examples ofnearby moderate EQs.

~0.8MPa
Out study area

~1.5MPa~5.5MPa

Bilek et al. (2016)↑Yamashita et al. (2004 M thesis, Tohoku Univ.) →1

Discussion (2): relationship with other interplate phenomena
ü The 2016 fault is located at northern edge of the1896 Sanriku tsunami EQ (Satake et al. 2015)
ü The 2016 event is isolated from thelow-frequency tremors and VLFEs(Tanaka et al. 2019; Nishikawa et al. 2019)Ø These may reflect spatial difference infrictional properties along plate interface?
(e.g., Nishikawa et al. 2019)– further investigation of regular EQs
will be important to discuss
the heterogeneous frictional property.

12○ Regular interplate EQs (NIED F-net)● Tremor (Tanaka et al. 2019)● VLFE (Nishikawa et al. 2019)

Summary
We investigated the S-net millimeter-scale tsunami records during the 2016 OffSanriku EQ, recorded by the S-net, new seafloor pressure gauge network.

ü The fault was located ~10 km to the west of the GCMT centroid and was unlikely to overlap
with regions where slow earthquakes phenomena occur such as the tremors and VLFEs.
ü Stress drop seemed not so small as expected in tsunami EQ like the 1896 Sanriku EQ, which
may reflect the spatial heterogeneity of frictional property along the plate interface.

Ø Take-home massage!
– S-net array dramatically increases the detectability of a millimeterscale tsunami and the constraints on earthquake source parametersof moderate EQs off eastern Japan.
– More tsunami examples due to minor-to-moderate EQs by this S-net
dense and wide array will reveal the spatial variation of the stressdrops or heterogeneity of mechanical properties along the plateinterface, with much higher resolution than previously possible.

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