A detection method for giant earthquake
based on counting a number of seismic
intensity observation stations
中村 洋光
Hiromitsu Nakamura
防災科学技術研究所
National Research Institute for Earth Science andDisaster Prevention, Japan(NIED)Outline
An overview of the earthquake early warning(EEW) during the 2011 Tohoku-Oki earthquake
Problems of the present EEW system
A method for the detection of giant earthquakes

Earthquake Early Warning（緊急地震速報 Kinkyu Jishin Sokuho）

From JMA Web site
The EEW system provides an advance warning of estimated seismic intensities andthe expected arrival time of S-waves. These estimates are based on the prompt
analysis of hypocenter location and earthquake magnitude using data observed byseismographs near the epicenter. The system issues several EEW messages during
the course of an earthquake, improving the accuracy of the warning as the amount ofavailable data increases.

EEW of 2011 Tohoku-Oki earthquake (M9)
Details of issued data
(warning at the 4th message)
No.

Issuance time

Elapsed timesince the E.Q. °Ｎ
detection (s)°ＥArea for which the warning was issued

depth(km)Ｍ12011/03/11-14:46:45.6

5.4

38.2

142.7

10

4.3

2

2011/03/11-14:46:46.7

6.5

38.2

142.7

10

5.9

3

2011/03/11-14:46:47.7

7.5

38.2

142.7

10

6.8

4

2011/03/11-14:46:48.8

8.6

38.2

142.7

10

7.2

5

2011/03/11-14:46:49.8

9.6

38.2

142.7

10

6.3

6

2011/03/11-14:46:50.9

10.7

38.2

142.7

10

6.6

7

2011/03/11-14:46:51.2

11.0

38.2

142.7

10

6.6

8

2011/03/11-14:46:56.1

15.9

38.1

142.9

10

7.2

9

2011/03/11-14:47:02.4

22.2

38.1

142.9

10

7.6

10

2011/03/11-14:47:10.230.0

38.1

142.9

10

7.7

11

2011/03/11-14:47:25.2

45.0

38.1

142.9

10

7.7

12

2011/03/11-14:47:45.3

65.1

38.1

142.9

107.9

13

2011/03/11-14:48:05.2

85.0

38.1

142.9

108.0

14

2011/03/11-14:48:25.2

105.0

38.1

142.9

10

8.1

15

2011/03/11-14:48:37.0

116.8

38.1

142.9

108.1

A total of 15 EEWs were issued during thisearthquake, including forecasts and alerts.
The 4th message was alert.

The EEW (alert) was issued
8.6s after the P-wave arrives
at the closest station to thehypocenter. The S-wave front
had not yet arrived on land.

Warning times from issuing the EEW until the real-time seismic
intensity exceeded 5-lower during the Tohoku-Oki Earthquake.
The contours in the map showwarning times (in seconds)

Warning times before observed seismic intensity
exceeded 5-lower on the JMA scale(equivalent to VII on the MMI scale)

Miyagi
60s

Iwate
Fukushima

90s

30s

IbarakiTochigi

15s
20s

60s

Chiba
Saitama
120s

120s

Tokyo

The EEW played an important rolein providing information in termsof an early warning.

Warning area was underestimated
EEW area

Area where the observed seismic intensity
was greater than or equal to 5-lowergunderestimation

Causes of the underestimation
uWarning information cannot be updated
The EEWs are updated as available data accumulates, but this is when the elapsed
time is less than 60s from the first trigger.
In this giant earthquake, the magnitude continued to increase even after exceeding
the threshold (60s), and therefore the alert information could not be updated.
This problem can be solved by increasing the threshold time.

u Amplitude-magnitude (MJMA) saturationfor events of Mw greater than 8
Fundamental problem concerning the detection of giant earthquakes

MJMA saturation for events of Mw
greater than 8
In the case of giant earthquakes with M
greater than 8, the various types of M aresmall in comparison to the Mw.

MJMAMJMA

After Utsu (1984)
MJMA is estimated from the maximum displacement which corresponds
to the outputs of seismometers having an eigenperiod of 6s.

GoalWe develop a detection method for giant earthquakes
based on counting a number of seismic intensityobservation stations.
The advantage of this method is that it is not always
necessary to have information regarding theearthquake source.

Distribution of observation stations usedfor analysis
In order to make the distributionof observation stations uniform,
we chose observation stations
established as K-NET in 1996,
with the exception of insularareas.
The spacing between stations isapproximately 20 km.

As of June 1996
（Except insular areas)

Counting results for M6.5 or greater★M9 (Tohoku-Oki earthquake)■M8 (2003 Tokachi-Oki earthquake)▼M7.7 (largest aftershock of M9)◆7.0 <= M >7.4
▲6.5 <= M <= 6.9

The number of observation stations in the M9earthquake was exceptionally large compared toother earthquakes.
For instance, 151 stations with intensity equal toor greater than 5-lower during the M9earthquake, compared to 52 during the 2003
Tokachi-Oki earthquake (M8).
For intensity equal to or greater than 6-lower,
54 stations were observed in the M9 earthquake,compared to 11 in the aftershock that occurredon April 7, 2011.

★M9 (Tohoku-Oki earthquake)■M8 (2003 Tokachi-Oki earthquake)▼M7.7 (largest aftershock of M9)◆7.0 <= M >7.4
▲6.5 <= M <= 6.9

Lower limit of seismic intensity

Real-time counting is possible using real-time
seismic intensity （Kunugi et al., 2008）
Time series of number of stations in 2011 Tohoku-Oki
equal to or greater than 6-upper (12)6-lower (56)
5-upper (100)5-lower (150)
4 (274)3 (454)2 (596)
1 (652)The JMA seismic intensity is
calculated based on one
minute-long waveform data, so
we cannot calculate intensity
in real-time.
However using the real-time
seismic intensity by Kunugi etal. (2008), it is possible tocount the number ofobservation stations in realtime.
An alert can be issued when
the number of stations
exceeds a threshold.

Lapse time (s)Off-line analysis of detection
based on real-time intensity monitoring
The example of the Tohoku-Oki earthquake
Detection pattern:- If the number of observation stations with an intensity
equal to or greater than 5-lower exceeds that (52 stations)of the 2003 Tokachi-Oki earthquake- If the number of stations with an intensity equal to or
greater than 5-lower exceeds 100
Intensity equal toor greater than 5-lower

100 stations

Tokachi-Oki (52)93 s

154 s Lapse time (s)Since the alert is issued
based on the spatial
spreading of strong
motions, the probabilityof overlooking a giant
earthquake is extremelysmallImproving warning information accuracy

EEW

Prediction of strong motion
by determination of
hypocenter location andearthquake magnitude using
P-wave data.

Real-time monitoring ofstrong motion(e.g., Counting method)
Live monitoring of strong
motion is independent of theestimation of EEW.

The use of both types of data will improve theaccuracy of warning information.

Estimating M
Number of observation stations with an intensity equal
to or greater than 5-lower (C5-L)
Shortest observed epicentral distance (Δsd)
Estimated M =5-L

sd

Estimated M

For instance, using thehypocenter location
information from the EEW

Data:
153 earthquakes thatobserved an intensity equal
to or greater than 5-lower

Conclusion
The proposed approach is a simple method that consists ofcounting the number of observation stations. The method iseffective at detecting giant earthquakes that occur inland orrelatively close to the coast.
This method is unique in that it does not use any hypocenter
location information. If the hypocenter location can be obtained,
however, it could be combined with the number of stations toestimate the magnitude of a giant earthquake.
An additional advantage is the facility to perform real-timemonitoring easily in combination with the real-time intensity
calculation method by Kunugi et al. (2008).