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Applicability of On-Site P-Wave
Earthquake Early Warning to Seismic
Data Observed During the 2011 Off the
Pacific Coast of Tohoku Earthquake,
Japan
Seiji Tsuno*
Seismic Data Analysis Laboratory, Center for Railway Earthquake Engineering Research, Railway Technical Research Institute,
Tokyo, Japan
In this study, the on-site P-wave earthquake early warning (EEW) based on the site-specific
spectral ratio of S-wave to P-wave to efficiently incorporate the site characteristics, which
can potentially issue the earthquake warning by the time of Ts-p, was developed. The
spectral ratio of S-wave to P-wave that are related to the source effects, the path effects,
and the site effects are significantly affected by the site effects contrast to the source
effects and the path effects in practical. At first, the on-site P-wave EEW method which
multiplies a site-specific spectral ratio of S-wave to P-wave prepared in advance by
P-wave observed in the real-time at seismic stations is applied to seismic data for
moderate-sized earthquakes with a magnitude (Mj) of 5.0–6.0, occurred in the eastern
Japan, observed at both the sedimentary basin site and the rock site. As a result, this
method predicted well the observed S-wave in the single indicator of SI within the
logarithmic standard deviation of 0.25 as well as in the frequency of more than 0.5 Hz.
It is, also, confirmed that the site-specific spectral ratio of S-wave to P-wave at a seismic
station was stably retrieved from 20 data samples at least. To investigate the applicability of
this method to earthquake ground motions induced by a large-scaled earthquake, finally,
this method is applied to seismic data during the 2011 off the Pacific coast of Tohoku
earthquake, Japan (Mw 9.0). The prediction of S-wave using a time-window of 10 s after
P-wave arrived, could not reproduce the observation with the underestimation; however,
the prediction of S-wave using a time-window of more than 20 s containing P-wave
propagated from an area generating strong motions in the fault, could reproduce the
observation. Even in the case of the large-scaled earthquake, the on-site P-wave EEW
method based on the site-specific spectral ratio of S-wave to P-wave at a seismic station
availably works by using the gradually increasing time-windows after P-wave arrived in the
single indicator of SI as well as in the frequency content, avoiding the mixture of S-wave
into a part of P-wave.
Keywords: on-site EEW, P-wave, site-specific ratio, prediction of S-wave, real-time, the 2011 off the Pacific coast of
Tohoku earthquake, Tohoku region
Edited by:
Simona Colombelli,
University of Naples Federico II, Italy
Reviewed by:
Masato Motosaka,
Tohoku University, Japan
José Borges,
University of Evora, Portugal
*Correspondence:
Seiji Tsuno
tsuno.seiji.75@rtri.or.jp
Specialty section:
This article was submitted to
Solid Earth Geophysics,
a section of the journal
Frontiers in Earth Science
Received: 16 March 2021
Accepted: 22 October 2021
Published: 09 November 2021
Citation:
Tsuno S (2021) Applicability of On-Site
P-Wave Earthquake Early Warning to
Seismic Data Observed During the
2011 Off the Pacific Coast of Tohoku
Earthquake, Japan.
Front. Earth Sci. 9:681199.
doi: 10.3389/feart.2021.681199
Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 6811991
ORIGINAL RESEARCH
published: 09 November 2021
doi: 10.3389/feart.2021.681199
INTRODUCTION
Earthquake Early Warning (EEW) systems are installed to
many fields in the world based on their own concepts to
provide warning prior to the strength of ground shaking (e.g.,
Allen et al., 2009). The on-site EEW methods, which estimate
the strength of ground shaking at the same location by
generally using a begging part immediately after the arrival
of P-wave, have been developed (e.g., Nakamura, 1988;Allen
and Kanamori, 2003;Odaka et al., 2003;Wu and Kanamori,
2005;Wu et al., 2007). On-site EEW methods, which can be
operated by a single station and/or a seismic network, are
based on the empirical relationships between an amplitude of
P-wave and a strength of ground shaking (Wu and Kanamori,
2005), or based on the estimation of an earthquake magnitude
(Nakamura, 1988;Allen and Kanamori, 2003;Wu et al., 2007)
and an epicentral distance (Odaka et al., 2003)torapidly
predict the strength of ground shaking. Recently, the real-time
ground motion prediction using the observed data at front
stations in the direction of incoming seismic waves have been
developed (e.g., Hoshiba, 2013;Hoshiba and Aoki, 2015;Yang
and Motosaka, 2015). However, especially for the on-site EEW
method operated by a single station, data recorded in the real-
time by a dense seismic network is quite useful, and it is
desirable that site characteristics are efficiently incorporated
to the method to reflect the difference of the strength of
ground shaking in the sites.
In the field of railway, Japan, on-site EEW using P-wave
(Nakamura, 1988;Odaka et al., 2003) has been developed to
stop the train quickly during the occurrence of earthquakes. At
the same time, the own dense seismic network has been installed
with the interval distance of 5–40 km and about 100 km along
railway lines and coast lines respectively, especially in the eastern
Japan (Nakamura, 1988;Nakamura, 1996;Miyakoshi et al., 2019).
Recently, after the 2011 off the Pacific coast of Tohoku
earthquake (Mw 9.0) occurred at the plate boundary of the
Pacific plate subducting beneath Tohoku-Japan, seismic
stations were installed with the interval distance of 50 km
inland in the eastern Japan (Yamamoto and Tomori, 2013).
Miyakoshi and Tsuno (2015) illustrated the relationships
between P-wave at the basement and S-wave at the ground
surface with the empirical estimations for the physical
parameters, using seismic data of KiK-net observed in the
Kanto basin, Japan. Tsuno and Miyakoshi (2019) developed
the relationships between P-wave at the ground surface and
S-wave at the ground surface by interpreting the
deconvolution of the transfer function of the P-wave and
the convolution of the transfer function of the S-wave with the
seismic data observed in the Kanto Region. In this study, the
availability of an on-site P-wave EEW based on the site-specific
spectral ratio of S-wave to P-wave at a seismic station, which
directly predicts S-wave from P-wave (e.g. Miyakoshi and Tsuno,
2015;Tsuno and Miyakoshi, 2019;Zhao and Zhao, 2019) without
any estimation of an earthquake magnitude and an epicentral
distance was quantitatively examined by applying to seismic data
in the Tohoku Region for moderate-sized earthquakes with a
magnitude (Mj) of 5.0–6.0, occurred in the eastern Japan. Finally,
the applicability of this method to seismic data for the large-
scaled earthquake of the 2011 off the Pacific coast of Tohoku
earthquake (Mw 9.0), Japan was investigated.
DATA
Seismic data recorded at KS and SS stations respectively
installed in the rock site and in the sedimentary basin site
in the Tohoku Region, Japan by JR East, were used. As for
geophysical information around the seismic stations, AVS30
(Aaverage Velocity of S-wave up to a depth of 30 m) at KS
station located in Ayukawa, Miyagi Pref. is about 470 m/s and
that at SS station located in Shin-Nagamachi, Miyagi Pref. is
about 290 m/s, as shown by J-SHIS (Japan Seismic Hazard
Information Station). Seismic data of moderate-sized
earthquakes with a magnitude (Mj)of5.0–6.0, occurred in
the eastern Japan for a period from November/2007 to August/
2018 and the mainshock of the 2011 off the Pacific coast of
Tohoku earthquake (Mw 9.0) were analyzed in this study. As
for moderate earthquakes, 58 and 95 earthquakes with high
signal-to-noise ratio at KS and SS stations respectively were
selected. Location of epicenters and seismic stations used in
this study are shown in Figure 1. Information of PGAs, PGVs,
and the peak frequency in the seismic data of the moderate-
sized earthquakes with a magnitude (Mj)of5.0–6.0 at KS and
SS stations are shown with those of the 2011 off the Pacific
coast of Tohoku earthquake (Mw 9.0) in Figure 2.Asan
example, waveforms of acceleration for 3 components during
an earthquake (Mj 5.9) occurred on 11th/April 2011 are shown
in Figure 3.
ON-SITE P-WAVE EARTHQUAKE EARLY
WARNING
Method
Assuming an earthquake ground motion observed in a far field
induced by a double couple point source, the earthquake ground
motions of P-wave and S-wave at the basement in the frequency
domain are expressed, as follows (e.g. Iwata and Irikura, 1986;Aki
and Richard, 2002).
OP
b(ω)RP
θϕ
4πρV3
P
1
rΩ(ω)·exp−ω
2QP
r
V′
P(1)
Ob
S(ω)RS
θϕ
4πρV3
S
1
rΩ(ω)·exp−ω
2QS
r
V′
S(2)
Here, ωis an angular frequency, ρand Vis a density and a
velocity of body waves in and around an earthquake source
region, ris distance from an earthquake source, R
θϕ
is a
radiation coefficient, Ω(ω) is an earthquake source spectrum
and Qis an internal attenuation in the crust. V′is the average
velocity of body waves in the crust. The subscripts of Pand S
represent P-wave and S-wave, respectively. The subscript of b,
also, represents the basement.
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Tsuno On-Site P-wave Earthquake Early Warning
In the logarithmic ratio of Eqs. 1,2, the relationships between
the S-wave at the basement and the P-wave at the basement is
expressed by the following equations.
log OS
b(ω)log OP
b(ω)+a1
(ω)(3)
a1(ω)log V3
P
V3
S
+log RS
θϕ
RP
θϕ
+log e
rω
2−1
QSV′
S+1
QPV′P(4)
a
1
(ω) includes the influence of the source effect and the path
effect, which are V
P
/V
S
around an earthquake source region, the
ratio of the S-wave radiation coefficient to that of the P-wave, and
the internal attenuation of the P-wave and S-wave propagating in
the crust.
The relationships between the P-wave at the basement and
P-wave at the ground surface, and between the S-wave at the
basement and S-wave at the ground surface are expressed using
the transfer functions G
P
(ω) and G
S
(ω) based on the P-wave and
S-wave subsurface structures from the basement to the ground
surface, as follows.
OP
s(ω)OP
b(ω)·GP(ω)(5)
FIGURE 1 | Location of epicenters for moderate-sized earthquakes and seismic stations used in this study. Open circles show the location of epicenters for the
moderate-sized earthquakes with a magnitude (Mj)of5.0–6.0 for a period from November/2007 to August/2018 and, closed triangles show the location of seismic
stations. A closed circle shows location of epicenter for an earthquake (Mj 5.9) occurred on 11th/April 2011. The earthquake source fault plane of the 2011 off the Pacific
coast of Tohoku earthquake (Mw 9.0) occurred on 11th/March 2011 by Suzuki et al. (2011) and the hypocenter by JMA is also shown with a broken rectangle and a
diagonal cross, respectively. Closed rectangles show location of epic enters for the strong motion generation areas (SMGAs of M7-class events) estimated by Asano and
Iwata (2012).
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Tsuno On-Site P-wave Earthquake Early Warning
OS
s(ω)OS
b(ω)·GS(ω)(6)
The subscript of s represents the ground surface.
In the logarithm of Eqs. 5,6,a
2
(ω) and a
3
(ω) which includes
the influence of the site effect related to the P-wave and the
S-wave subsurface structures respectively, are expressed by the
following equations.
log OP
s(ω)log OP
b(ω)+a2(ω)(7)
log OS
s(ω)log OS
b(ω)+a3(ω)(8)
a2(ω)log GP(ω)(9)
a3(ω)log GS(ω)(10)
Finally, the relationships between the P-wave at the surface
and the S-wave at the surface from Eqs. 3,7,8is expressed by the
following equations.
log OS
s(ω)log OP
s(ω)+b(ω)(11)
b(ω)a1(ω)−a2(ω)+a3(ω)(12)
b(ω) includes the influence of the source effect, the path effect,
and the site effect (Tsuno and Miyakoshi, 2019). In practically,
b(ω) is estimated from a spectral ratio of S-wave to P-wave using
seismic data observed at the ground surface at a seismic station.
Miyakoshi and Tsuno (2015) investigated the relationships
between P-wave at the basement and S-wave at the ground
surface, by the theoretical technique, the empirical formulas,
and observation data. As a result, Miyakoshi and Tsuno
(2015) concluded that the spectral ratio of S-wave at the
ground surface to P-wave at the basement was significantly
affected by the site effects contrast to the source effects and
the path effects.
The method of on-site P-wave EEW directly predicts S-wave
by multiplying a site-specific spectral ratio of S-wave to P-wave
prepared in advance by P-wave observed in the real-time at a
seismic station, as expressed by the Eq. 11 in the frequency
domain with a logarithm (Tsuno and Miyakoshi, 2019;Zhao and
Zhao, 2019). Therefore, this method using P-wave can
significantly reduce the time of Ts-p to issue the warning than
FIGURE 2 | Information of PGAs, PGVs, and the peak frequency in the seismic data used in this study. Nin the figures indicates the number of earthquakes used.
Crosses and a circle in the figure of PGAs and PGVs at KS and SS stations show data of the moderate-si zed earthquakes with a magnitude (Mj)of5.0–6.0 and data of the
2011 off the Pacific coast of Tohoku earthquake (Mw 9.0), respectively. Histograms of the dominant frequency (Hz) for data of the moderate-sized earthquakes observed
at KS and SS stations are shown with that of the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) by a circle in the figure. (A) KS station (B) SS station.
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Tsuno On-Site P-wave Earthquake Early Warning
the previous method mainly using S-wave. Specifically, S-wave
was predicted in the frequency domain, by using P-wave as shown
in Figure 3 with an underline for UD component.
Site-specific Ratio of S-Wave to P-Wave
Observed
A site-specific spectral ratio of S-wave to P-wave for Fourier
spectrum and pseudo-velocity response spectrum (a damping
coefficient of 20%), and a site-specific ratio of S-wave to P-wave
for SI (Spectral Intensity; Housner, 1965) using seismic data of
moderate-sized earthquakes at KS and SS stations as shown in
Figure 1 were estimated. SI is calculated by averaging velocity
response with a damping coefficient of 20% from a period of
0.1–2.5 s. At first, onsets of P-wave and S-wave for all the data
were visually read as shown in Figure 3. Fourier spectrum and
pseudo-velocity response spectrum of S-wave in horizontal
components and P-wave in a vertical component without a
FIGURE 3 | Waveforms of acceleration for 3 components during an earthquake (Mj 5.9) occu rredon 11th/April 2011 observed at KS and SS stations. Broken lines
show an onset of P-wave for UD component and onsets of S-wave for NS and EW components. Under lines show a time-window of 10.24 s for each component, to
estimate a spectral ratio of S-wave to P-wave. (A) KS station (B) SS station.
FIGURE 4 | Estimated site-specific spectral ratios of S-wave to P-wave for a time-window of 10.24 s at KS and SS stations. Nin the figures indicates the number of
earthquakes used. (A) KS station (B) SS station.
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Tsuno On-Site P-wave Earthquake Early Warning
smoothing process were calculated using time-windows of 10.24 s
after the onsets. The time of Ts-p in the seismic data used was
sufficiently secured to avoid mixing S-wave into a part of P-wave,
which means the time of Ts-p is longer than 10.24 s in this study.
Spectral ratios of S-wave to P-wave were calculated by dividing
the Fourier spectrum and pseudo-velocity response spectrum of
S-wave which is the arithmetic mean for the NS component and
the EW component, by those of P-wave for the UD component.
Finally, the spectral ratios of S-wave to P-wave and SI ratio of
S-wave to P-wave were averaged by those for all the seismic data
at each seismic station.
Estimated site-specific spectral ratios, response spectral
ratios, and SI ratios of S-wave to P-wave for a time-
window of 10.24 s at KS and SS stations are shown in
Figures 4–6with a plus and a minus of one standard
deviation, respectively. The spectral ratio of S-wave to
P-wave at the ground surface was significantly affected by
the subsurface velocity structure and therefore, the spectral
ratio of S-wave to P-wave at SS station in the sedimentary
basin site had a large amplification in a wide frequency range
of 0.3–10 Hz. On the other hand, the spectral ratio of S-wave
to P-wave at KS station in the rock site had a large
amplification in a frequency range of 3–10 Hz, especially it
had the peak at the high frequency of around 10 Hz. The
negative value for the spectral ratio of S-wave to P-wave in
the frequency indicates that site amplification of P-wave is
larger than that of S-wave as shown in the Eq. 12.Site-
specificresponsespectralratioshaveamoresmoothed
tendency than site-specificspectralratiosinthe
frequency/period domain without the negative value. Site-
specificSIratiosofS-wavetoP-waveare6and4.5atKSand
SS stations, respectively.
A stability of site-specific spectral ratio of S-wave to P-wave,
using seismic data with the different number of datasets was
examined. RMS (Root Mean Square) between the spectral
ratio of S-wave to P-wave using seismic data to the full
datasets, for KS and SS stations is shown in Figure 7.The
number of full datasets is 58 and 95 for KS and SS stations,
respectively. In general, as the number of datasets increases,
the spectral ratio becomes stable. In this study, RMS becomes
stable enough by 40 and 20 data samples for KS and SS
stations, respectively.
Prediction of S-Wave From P-Wave in the
Real-Time
S-wave was predicted by multiplying the site-specificspectral
ratioofS-wavetoP-wavepreparedinadvancebyP-wave
observed in the real-time at each seismic station, using the
Eq. 11. As an example, Fourier spectra of S-wave for the
earthquake of Mj 5.9, occurred on 11th/April 2011 at KS and
SS stations whose accelerations are shown in Figure 3,predicted
FIGURE 5 | Estimated site-specific response spectral ratios of S-wave to P-wave for a tim e-window of 10.24 s at KS and SS stations. Nin the figures indicates the
number of earthquakes used. (A) KS station (B) SS station.
FIGURE 6 | Estimated site-specific SI ratios of S-wave to P-wave for a
time-window of 10.24 s at KS and SS stations.
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Tsuno On-Site P-wave Earthquake Early Warning
by this method were shown in Figure 8. Predicted Fourier
spectrum of S-wave was in good agreement to the observed with
thedominantfrequencyofabout10HzfortheearthquakeatKS
station. On the other hand, predicted Fourier spectrum of
S-wave was in good agreement to the observed with the
dominant frequency of 1–2HzfortheearthquakeatSS
station. The on-site P-wave EEW method could well explain
Fourier spectrum of the S-wave in the frequency of more than
0.5 Hz at both KS and SS stations in which the site conditions
are different. However, this method could not well explain
Fourier spectrum of the S-wave in the frequency of less than
0.5 Hz. Miyakoshi and Tsuno (2015) pointed out that the
spectral ratio of S-wave to P-wave at the basement is
relatively affected by both the source effects and the path
effects in the low frequency and in the high frequency,
respectively. Even at the ground surface, therefore, the
difference between the observation and the prediction in
thelowfrequencyarecausedbytheinfluence of the
radiation coefficient in a
1
(ω)oftheEq. 4. Pseudo-velocity
response spectra (a damping coefficient of 20%) of S-wave for
the earthquake of Mj 5.9, occurred on 11th/April 2011 at KS
and SS stations whose accelerations are shown in Figure 3,
predicted by this method were shown in Figure 9,withthe
average ±the standard deviation. The predicted response
spectra of S-wave were in good agreement to those observed
in periods of 0.1–10 s at both KS and SS stations, indicating
the observations mostly within the average ±one standard
deviation of the predictions. Predicted SIs of S-wave for all
the earthquakes at KS and SS stations against those observed
are shown in Figure 10. The predictions of SI of S-waves were
in good agreements with the observations within the
logarithmic standard deviation of 0.25.
It was indicated that the on-site P-wave EEW method, based
on the site-specific spectral ratio of S-wave to P-wave can predict
the observed S-wave in the single indicator of SI as well as in the
frequency/period content for the moderate-sized earthquakes.
APPLICATION OF THE ON-SITE P-WAVE
EEW METHOD TO THE 2011 OFF THE
PACIFIC COAST OF TOHOKU
EARTHQUAKE
The applicability of the on-site P-wave EEW method to seismic
data observed at KS and SS stations, during the 2011 off the
Pacific coast of Tohoku earthquake (Mw 9.0), Japan was
investigated. Waveforms of acceleration for 3 components
during the 2011 off the Pacific coast of Tohoku earthquake
(Mw 9.0) occurred on 11th/March 2011 observed at KS and
SS stations are shown Figure 11.
In the real-time, S-waves were predicted by multiplying the
site-specific spectral ratio of S-wave to P-wave prepared in
advance (See Figure 4) by the different time-windows of 10,
20, and 25 s after P-wave arrived. Predicted Fourier spectra of
FIGURE 7 | RMS between the spectral ratio of S-wave to P-wave using
seismic data to the full datasets, for KS and SS stations. The number of full
datasets is 58 and 95 for KS and SS stations, respectively.
FIGURE 8 | Predicted Fourier spectra of S-wave for the earthquake of Mj 5.9 on 11th/April 2011 at KS and SS stations whose accelerations are shown in Figure 3.
Observed Fourier spectra of P-wave for the earthquake are also shown. (A) KS station (B) SS station.
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Tsuno On-Site P-wave Earthquake Early Warning
S-wave for time-windows of 10, 20, and 25 s at KS and SS stations
were shown in Figures 12,13, respectively. As a result, the S-wave
predicted for the 2011 off the Pacific coast of Tohoku earthquake,
using the time-window of 10 s in P-wave after P-wave arrived,
could not reproduce the S-wave observed with the
underestimation at both KS and SS stations. On the other
hand, the S-wave predicted using the time-windows of 20 and
25 s in P-wave could reproduce the S-wave observed at both KS
and SS stations. It was point out that an area generating strong
motions in the earthquake source fault differed from an area
which the fault rupture started in the large-scaled earthquake
event. To directly predict S-wave from P-wave observed in the
real-time, therefore, a time-window containing P-wave induced
by an area generating strong motions in the fault is necessary to
be analyzed. As for strong motion generation areas (SMGAs) for
the 2011 off the Pacific coast of Tohoku earthquake as shown in
Figure 1,(Asano and Iwata, 2012) reported that two strong
motion generation areas (SMGA1 and SMGA2) are identified in
the Miyagi-oki region west of the hypocenter and another two
strong motion generation areas (SMGA3 and SMGA4) are in the
Fukushima-oki region southwest of the hypocenter. Also, they
indicated that the strong ground motions in the frequency range
0.1–10 Hz along the Pacific coast are mainly controlled by these
SMGAs of M7-class events existing in the deeper portion of the
source fault plane. In their results, the strong ground motions
observed in Miyagi Pref. where the KS and SS stations are located,
are significantly affected by SMGA1 (Length: 36 km, width:
36 km, rise time: 6.9 s, and rupture velocity: 4 km/s) and
FIGURE 9 | Predicted pseudo-velocity response spectra (a damping coefficient of 20%) of S-wave for the earthquake of Mj 5.9 on 11th/April 2011 at KS and SS
stations whose accelerations are shown in Figure 3, with the average ±the standard deviation. Observed pseudo-velocity response spectra of P-wave for the
earthquake are also shown. (A) KS station (B) SS station.
FIGURE 10 | Predicted SIs of S-wave for all the earthquakes at KS and SS stations against those observed. Nand σin the figures indicate the number of
earthquakes and one logarithmic standard deviation, respectively. (A) KS station (B) SS station.
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Tsuno On-Site P-wave Earthquake Early Warning
SMGA2 (Length: 36 km, width: 36 km, rise time: 6.9 s, and
rupture velocity: 4 km/s). Specially, the first and the second
wave packets of strong ground motions observed at the KS
and SS stations as shown in Figure 11 are generated by
SMGA1 and SMGA2, respectively (Asano and Iwata, 2012).
To accurately predict the amplitude of S-wave by P-wave
observed in the real-time during the 2011 off the Pacific coast
of Tohoku earthquake, it is necessary that strong ground motions
generated by SMGA1 is contained in the observed P-wave. In this
study, the time-window of 20 s in a UD component after P-wave
arrived at KS and SS stations includes strong ground motions
generated by SMGA1 at least. Therefore, the S-wave predicted
using the time-windows of 20 and 25 s in P-wave could reproduce
the S-wave observed at both KS and SS stations.
FIGURE 11 | Waveforms of acceleration for 3 components during the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) occurred on 11th/March 2011
observed at KS and SS stations. Broken lines show onsets of P-wave for UD component and S-wave for NS and EW components. Under lines show time-windows of
10, 20, and 25 s for each component. The first and secon dwave packet are generat ed by different the strong motion generation areas (SMGAs) estimated by Asano and
Iwata (2012).(A) KS station (B) SS station.
FIGURE 12 | Predicted Fourier spectra of S-wave for time-windows of 10, 20, and 25 s during the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) at KS
station. (A) Time-window of 10 s (B) Time-window of 20 s (C) Time-window of 25 s.
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Tsuno On-Site P-wave Earthquake Early Warning
On the other hand, the discrepancy between the S-wave
observed and the S-wave predicted even using the appropriate
time-windows in P-wave at KS and SS stations is larger in the low
frequency than in the high frequency caused by the source effects,
as shown in Figures 12B,13C. It might be caused by that the
prediction of S-wave for this large-scaled earthquake (Mw 9.0)
which significantly affected the ground motions in the low
frequency, was performed by the site-specific spectral ratio of
S-wave to P-wave using seismic data of moderate-sized
earthquakes with a magnitude (Mj) of 5.0–6.0.
DISCUSSION
In the real-time, SI of S-wave by multiplying the site-specific
spectral ratio of SI prepared in advance (See Figure 6)bySIof
P-wave observed were predicted. The predicted SI of S-wave and
the observed SI of S-wave with the time interval of one second
until the time that S-wave arrived at each KS and SS stations
during the 2011 off the Pacific coast of Tohoku earthquake is
shown in Figure 14. The predicted SI of S-wave is gradually
increasing until S-wave for the first wave packet arrives at each KS
and SS stations. The values of the predicted SI immediately before
S-wave for the first wave packet arrives reproduce well the
maximum values of S-wave observed at both KS and SS
stations. On the other hand, the lead time from the last
prediction to the arrival of S-wave is not sufficient at KS
station, because SMGA1 generating the strong ground motions
for the first wave packet which is in the Miyagi-oki region west of
the hypocenter, is close to KS station. At SS station which is
located to more inland than KS station, however, the lead time of
10 s could be earned by this on-site P-wave EEW method.
Even in the case of the large-scaled earthquake, the on-site
P-wave EEW method availably works by using the gradually
increasing time-windows after P-wave arrived in the single
indicator of SI as well as in the frequency content. Note that
in principle, this method can be applied to P-wave observed in the
real-time until S-wave arrives. In case of mixing S-wave into a
part of P-wave, the prediction overestimated the observation, as
the predicted Fourier spectra of S-wave for the time-windows of
25 s after P-wave arrived at KS station shown in Figure 12C.
CONCLUSIONS
In this study, at first, the on-site P-wave EEW method which
multiplies a site-specific spectral ratio of S-wave to P-wave
prepared in advance by P-wave observed in the real-time at
seismic stations is applied to seismic data for moderate-sized
FIGURE 13 | Predicted Fourier spectra of S-wave for time-windows of 10, 20, and 25 s during the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0) at SS
station. (A) Time-window of 10 s (B) Time-window of 20 s (C) Time-window of 25 s.
FIGURE 14 | Predicted SI of S-wave and observed SI of S-wave with the
time interval of one second during the 2011 off the Pacific coast of Tohoku
earthquake (Mw 9.0) at KS and SS stations. Predicted SI of S-wave by using
P-wave in the real-time, is shown until the time that S-wave arrived at
each KS and SS station.
Frontiers in Earth Science | www.frontiersin.org November 2021 | Volume 9 | Article 68119910
Tsuno On-Site P-wave Earthquake Early Warning
earthquakes with a magnitude (Mj) of 5.0–6.0, occurred in the
eastern Japan, observed at both the sedimentary basin site and the
rock site. As a result, this method predicted well the observed
S-wave in the single indicator of SI within the logarithmic
standard deviation of 0.25 as well as in the frequency of more
than 0.5 Hz. It is, also, confirmed that the site-specific spectral
ratio of S-wave to P-wave at a seismic station was stably retrieved
from 20 data samples at least. To investigate the applicability of
this method to earthquake ground motions induced by a large-
scaled earthquake, finally, this method is applied to seismic data
during the 2011 off the Pacific coast of Tohoku earthquake, Japan
(Mw 9.0). The prediction of S-wave using a time-window of 10 s
after P-wave arrived, could not reproduce the observation with
the underestimation; however, the prediction of S-wave using a
time-window of more than 20 s containing P-wave propagated
from an area generating strong motions in the fault, could
reproduce the observation. Even in the case of the large-scaled
earthquake, the on-site P-wave EEW method based on the site-
specific spectral ratio of S-wave to P-wave at a seismic station
availably works by using the gradually increasing time-windows
after P-wave arrived in the single indicator of SI as well as in the
frequency content, avoiding the mixture of S-wave into a part of
P-wave.
Practically, this on-site P-wave EEW will be installed with the
regional EEW (Odaka et al., 2003) in the field of railway, Japan, to
further improve the safety from earthquakes. The threshold levels
for the single indicators to issue earthquake early warning is
empirically set on around less than 10 cm/s in SI and/or less than
100 cm/s
2
in PGA and therefore, the nonlinearity of the soft soil at
the surface layers is not necessary to be considered from a
practical point of view. As the future work, however, the
influence of nonlinearity of the soft soil at the surface layers
should be investigated to accurately predict earthquake ground
motions in the case of strong ground motions. Also, the effects of
2-D and 3-D irregularity of the sedimentary basin should be
investigated as well as the nonlinearity of the soft soil at the
surface layers.
DATA AVAILABILITY STATEMENT
The data analyzed in this study is subject to the following licenses/
restrictions: Original data is provided by JR-East. ST has the
results processed in this study. Requests to access these datasets
should be directed to Seiji Tsuno, tsuno.seiji.75@rtri.or.jp.
AUTHOR CONTRIBUTIONS
ST analyzed the seismic data. ST drafted the manuscript.
ACKNOWLEDGMENTS
Seismic data recorded at two seismic stations maintained by
JR-East were used. The geophysical information of a site
condition in J-SHIS by NIED (National Research Institute for
Earth Science and Disaster Resilience) at seismic stations were
used. Location of hypocenters for the moderate-sized earthquakes
with a magnitude (Mj) of 5.0–6.0 and the 2011 off the Pacific
coast of Tohoku earthquake estimated by JMA (Japan
Meteorological Agency) were used. We also thank two
reviewers for their comments and suggestions, which
significantly contributed to improving the quality of this paper.
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Conflict of Interest: The author declares that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
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