accepted paper, 15WCEE

Comparison of the P-wave Earthquake Alarm by
Multi-Station and Single Station Detection System

Tsutomu Sato, Y. Nakamura
System and Data Research Co., Ltd., JapanSUMMARY:
The earthquake motion of the 2011 off the Pacific coast of Tohoku Earthquake grew gradually larger andcontinued unusually long with high amplitude more than JMA intensity 4 in wide area. The single-station
detection system FREQL succeeded to issue the P-wave alarm and estimated the earthquake parameters,
although it took 15 seconds after detection. Proposed technique estimates the origin time and location from theP-wave detection time of at least 5 sites. In case of the 2011 off the Pacific coast of Tohoku Earthquake. The
location estimated 3.67 seconds after the first P-wave detection at MYG011 Oshika station. The magnitude grewup, P-wave alarm can be issued around 6 seconds after P-wave detection. Although P-wave alarm withsingle-station observation could be issued 14 seconds after P-wave detection, multi-station observation method
can be expected to shorten the alarm time for 8 seconds.
Keywords: Single-Station Detection System, Multi-Station Observation System, FREQL, P-wave Alarm,Earthquake Parameter Estimation

1. INTRODUCTION
The earthquake motion of the 2011 off the Pacific coast of Tohoku Earthquake (hereafter "the 3.11earthquake") grew gradually larger and continued unusually long with high amplitude more than JMA
intensity Ijma 4 (corresponding to MMI 7) in wide area. Although final seismic intensity Ijma reached
6+ (corresponding to MMI 10-11), there was relative little damage caused by the earthquake motion.
In such situation, some early warning system had worked to issue alarm before large motion, but manyof them could not perform up to expectation. Especially a single-station-detection early warning
system developed by JMA, Japan Meteorological Agency, could not recognize the 3.11 earthquake
danger and could not issue the warning. And also the alarm system for Shinkansen, bullet train, is thealarm system based on the JMA system replace after the 2004 Niigata-ken Chuetsu earthquake, and it
issued the alarm lately. Fortunately, no high speed train run into a damaged area, but the train wasunder a risky condition. At the time of the 2004 Niigata-ken Chuetsu earthquake, Compact UrEDAS,
the other early warning system for JMA system, worked as expected and kept safety for the passengersand crews, totally 154 persons, however the system for Shinkansen train had been replaced. A
single-detection early warning system FREQL is a successor of UrEDAS and Compact UrEDAS. Here,
FREQL is an earthquake early warning system essentially differs from that of JMA. FREQL is uniquesystem characterized by its functions to distinguish P wave from ground motion, to determine theearthquake parameters independently at installed site. Also FREQL can issue P wave alarm based on
the dangerousness of the detected earthquake motion with realtime data processing. FREQL has been
operated as practical system for a disaster prevention system of railway companies or otherorganization. Please see the detail of FREQL in the paper (Nakamura, 2007). One FREQL station
nearby base of Oshika Peninsula succeeded to detect the 3.11 earthquake, to issue P wave alarm and todetermine the earthquake parameters reasonably without magnitude. But it needed 15 seconds to issue
P wave alarm.
So this paper compares the early P wave alarm between ordinal FREQL single estimation and theaccepted paper, 15WCEE
other multi-detection method using FREQL technique, and then considers a possibility of earlier
earthquake warning.

2. ESTIMATION OF EARTHQUAKE PARAMETERS USING MULTI STATIONINFORMATION
2.1. Estimation of Origin Time and Location of a Source
Origin time and location of a source (Latitude, Longitude and Depth) are estimated from times of P
wave detection of more than five stations using least squares method, assuming a P wave propagation
velocity 6 km/sec. Consideration below uses strong motion records of K-NET and KiK-net andassumes that the station has an instrument corresponds to FREQL and send a time of P wave detection
in real time, and monitors a detection, an alarm, growth of magnitude and so on. That is, initialestimation is done with P wave detection information from first five stations, and the estimation is
updated as often as receiving the information from the other station and the result is compared with theresult before. If the scatter of the results are in predefined range (5 km in this paper), the result will befixed. If a density of strong motion observatories is less than 20 km, it will be expected that thelocation is fixed within 4 seconds after first P wave detection. In case of 2004 Niigata-ken Chuetsu
earthquake and 2010 Taiwan Kaohsiung earthquake, the source locations are determined in 1.04
second and 1.69 seconds after P wave detection, respectively. In contrast, the JMA method with multi
observation required 5.4 seconds in average and another 5.4 seconds for issuing the first information
for the 3.11 earthquake.
2.2. Estimation of Magnitude
Magnitude is estimated by a formula below (Takayama et al, 1981), based on the amplitude of P wave
and its frequency by Gutenberg-Richter.
M=1.59(logVp＋logR)+1.53(2.1)

Here, Vp is P wave amplitude of vertical motion in mkine (= 0.001 cm/sec) and R is epicentral
distance in km. Although equation 2.1 supposes that the value of Vp is observed on a hard ground,
K-NET station is not always installed on a hard ground. So the earthquake motion amplification factor
A for each station is estimated as Table 2.1 based on the estimate formula for seismic intensity (Saitaet al, 2005), and equation 2.1 is modified as follows.
M=1.59(logVp+logR)+1.53-1.59logA

Table 2.1

The amplification characteristics at station of K-NET and KiK-net

Site
MYG011
MYG008MYG002
MYGH03
MYGH12
MYG010
IWTH25
IWTH26
IWT010
AKT023
IWTH24
MYGH02
FKSH03(2.2)

OshikaKitakami
UtatsuKarakuwa(kik)Shizugawa(kik)Ishinomaki
Ichinoseki-W(kik)Ichinoseki-E(kik)Ichinoseki
Tsubakidai
Kanegasaki(kik)Naruko(kik)Takasato(kik)Sum ofdata
9111413131210119810612

Averaege of RI
amplification
1.504
0.749
1.203
0.375
0.350
1.683
0.854
1.203
0.805
0.500
1.133
0.136
1.128

Dispersion of2logA
0.885
0.602
0.570
0.589
0.629
0.864
0.740
0.858
0.888
0.706
0.771
0.712
0.319

Constant termof Fig.(2)
0.33
0.93
0.57
1.23
1.25
0.19
0.85
0.57
0.89
1.13
0.63
1.42
0.63

accepted paper, 15WCEE

Tsubakidai Ｍ fixed
Ichinoseki-E Ｍ fixed Naruko Ｍ fixed
Ichinoseki-W Ｍ fixed9MedutingaM

8Tsubakidai
Kanegasaki Ichinoseki-EM7.2 by JMA

7Swave arrival.
Ｓ波到着時刻高郷を除くTakasato) Ichinoseki-W(exclude

IchinosekiTakasato
at Takasato,

6

S wave arrival is later.
Ichinoseki Ｍ fixed
Takago Ｍ almost fixed

Naruko5Kanegasaki Ｍ fixed

43-2-1Figure 2.10123Time from earthquake detection (sec)karakuwa Ｍ fixed Kitakami Ｍ fixed
Shizugawa Ｍ fixed
Utatsu Ｍ fixed
Shizugawa
Kitakami
Alarm level Ｍ7.7
karakuwa

8Utatsu OshikaMedutinga
M-5
Figure 2.20awiaamgawukuz
akithair
SKatt
Kaatmamrarmarlalaaleveav
eaav
wwawPPP

3ikuasm to tan Uihst
Iat marm lar aaleav
e av
w
aw P
P

Ishinomaki

akihs
Otamralaev
awP

5

Takasato
TakasatoＭ

SＳ波到着時刻
wave arrival.
Takasato
高郷
高郷を除くTakago)(exclude765

The growth of magnitude of the 2008 Iwate and Miyagi Earthquake

M9.0 by JMA

944Ishinomaki Ｍ fixedOshika Ｍ fixed

510152025
Time from earthquake detection (sec)30

35

The growth of magnitude of the 20112011 off the Pacific coast of Tohoku Earthquake

Although epicentral distance is calculated based on a rupture start point estimated in first few seconds,
epicentral distance will be changed by the progress of fault rupture. And also main shock will arrive
after the initial motion. So equation 2.2 can be adopted for a time window R/4Cr after P-wave
detection, a rupture time for 1/4 of the epicentral distance from the start point of the rupture. In thistime, the scatter of the estimated magnitude is less than +/- 0.2 during the applying time.
After fixing the epicentre, magnitude is estimated from equation 2.2 and then the P wave alarm isissued based on the M-∆ relation if a station seems to be danger. Because this method can grasp thelocation of the earthquake relatively accurately, it is possible to determine an area needed alarm withtremendous precision. Finally averaged magnitude will be the magnitude of whole system. Thispaper concerns the change history of the estimated magnitude for the 3.11 earthquake and the 2008
Iwate and Miyagi earthquake.

accepted paper, 15WCEE

Scale of real-time seismic intensity RI : 5
PI alarm at Ishinomaki
Ishinomaki RI=0.0

Corresponding to Hypocentral distance

PI alarm at Shizugawa

PI alarm at Karakuwa
Shizugawa RI=0.0
Karakuwa RI=0.0
Utatsu RI=0.0

PI alarm at Utatsu

Kitakami RI=0.0

PI alarm at Kitakami.
dexifecruosfoemitehTnoitcetedevaw
Pfoemitht
6noitcetedevaw
Pfoemitts
135(The fastest alarm by FREQL)

P-alarm at Kitakami, Karakuwa
and Shizugawa using
Multi-FREQL information

4045
Standard time in Japan:

PI alarm at Oshika
Oshika RI=0.050
2011/3/11 14:46:**

55

Figure 2.3 P wave alarm time of first 6 sites of the 20112011 off the Pacific coast of Tohoku Earthquake

3. RESULT AND DISCUSSION
In case of the 2008 Iwate and Miyagi earthquake, information from five stations is observed 3.46
seconds after first detection at IWTH25 Ichinoseki-Nishi station, and the location of the epicenter isdetermined latitude 39.008˚N, longitude 140.875˚E and depth of -4.6 km. Then after 0.55 seconds,
information from MYGH02 Naruko station was added and the location of the epicenter moved tolatitude 39.027˚N, longitude 140.867˚E and depth of -2.1 km. Because the difference was within 5 km,
the estimation was fixed. The depth was minus value but less than 5 km so the depth fixed as 0 km.
Also, the epicenter by JMA was latitude 39.028˚N, longitude 140.880˚E and depth of 8 km.
In case of the earthquake, information from five stations is observed 2.54 seconds after first detection
at MYG001 Oshika station, and the location of the epicenter is determined latitude 36.376˚N,longitude 146.094˚E and depth of 381.6 km. Then after 1.13 seconds, information from MYG010
Ishinomaki station was added and the location of the epicenter moved to latitude 38.023˚N, longitude
143.331˚E and depth of 10.8 km. The difference of estimation was quite large. After 0.89 seconds,
information from MYG030 Towa station was additionally arrived and the location of the epicenter
moved to latitude 38.003˚N, longitude 143.356˚E and depth of 81.1 km. Because the difference of thelocation is within 5 km but the depth was quite differ for each other but the reliability of the depth at
this station was low, the fixed estimation became the second one with 4.56 seconds. Also, theepicenter by JMA was latitude of 38.103˚N, longitude 142.860˚E and depth of 24 km.
Figures 2.1 and 2.2 show the progress of the magnitude based on equation 2.2 with the information of

accepted paper, 15WCEE
first six stations for each earthquake. Information of FKSH03 Takasato station was added as an
example of little far station as a reference for both figures.
In case of the 2008 earthquake, applicable time is only 1-2 seconds for the first six stations and themagnitude grew drastically and reached and was fixed 7.2 around one second after detection. In caseof FKSH03 Takasato station about 170 km away, the magnitude grew relatively slow and stopped
within about three seconds. Finally the magnitude was fixed about 7.
In case of the 3.11 earthquake, applicable time was about 13-15 seconds during the growth of themagnitude and the magnitude was fixed. In case of FKSH03 Takasato station about 300 km away, themagnitude grew yet slower and stopped after the applicable time. The magnitude reached or wasestimated to reach further finally more than 8 for each station. Magnitude 7.7 seems to be the alarmlevel for each station based on the M-∆ relation and the alarm could be issued mainly about six
seconds and 15 seconds at least after detection. Thus it shows that it is possible to issue the alarm six
seconds at the earliest after detection. This is almost same time for the earliest P-wave alarm of simple
detection method for over all sites. For example Karakuwa station detected P wave at 14:46:40 andissued 14:46:46. Because in case of the single-station detection system seen in Figure 2.3, the alarm
seemed to be issued at 14:46:54, this multi-detection method can be expected to gain time 8 seconds.
Figure 2.3 shows the alarm time of the multi-detection method on the time-line of real time intensity
RI (Nakamura, 2003) with RI = 1.5 as P wave alarm level.

4. CONCLUSIONSThis paper compares between the early P wave alarm using multi-detection method with FREQL P
wave detection and that of single-station detection. As a result, a tendency is seemed that the slower
magnitude grew, the farther the station locates. Although P wave alarm is relative earlier for thedistant earthquake, it is confirmed that some station can issue earlier alarm with the multi-detection
method and more exact magnitude and location of the epicenter can be determined promptly. Because
the exact earthquake parameters are fundamentals for proper countermeasure soon after the event, wewould like to construct a system to grasp the source information in a certain level of accuracy fordisaster prevention under isolation of the information.

AKCNOWLEDGEMENT
In this paper, the waveform data was mainly provided by K-KET and KIK-net of NIED, National ResearchInstitute for Earth Science and Disaster Prevention. The authors would like to sincerely express our highest
appreciation and gratitude to people and the organizations.

REFERENCES

Eds, P.Gasparini et., al. (2007). FREQL and AcCo for a Quick Responce to Earthquakes, Earthquake Early
Warning Systems, 307-324, Springer-Verlag.
Takayama, A., Emoto, Y,. Abe, H., and Nakamura, Y. (1981). A study of earthquake early warning system,
JSCE Magazine, Vol.66, No.5, 51-56.
Saita, J. and Nakamura, Y. (2005). The development of a formula for seismic intensity estimation added thecharacteristics on strong motion observation, Proceeding of the 4th Annual Meeting of Japan Associationfor Earthquake Engineering, 308-309.
Nakamura, Y. (2003). Examination of a Rational Strong Motion Index - Relationship between the DI value andthe other strong motion indices -, Abstract Volume of the 27th JSCE Earthquake Engineering Symposium,
54.
Sato, T. A. and Nakamura, Y. (2005). The seismic source estimation using the variation of seismic damage
intensity DI value at 2004 Niigata-Chuetsu earthquake, Proceeding of the 4th Annual Meeting of Japan
Association for Earthquake Engineering, 298-299.