Earthquake Early Warning andRealtime Earthquake Disaster Prevention
Yutaka Nakamura1, 2

Abstract
An EEW, Earthquake Early Warning, is required to trigger realtime
earthquake disaster prevention. However, it is important to avoid too
much trust in EEW for the disaster prevention. This paper describes
the concept of an EEW and gives a brief history which eventually led
to the development of the UrEDAS, the Urgent Earthquake Detection
and Alarm System, the first operational P-wave early warning system,
and its new generation system FREQL, Fast Response Equipment
against Quake Load. A real-world example of disaster prevention by
this system is also described. As a specific example to review the effect for the disaster prevention, the leading time by FREQL is estimated using the strong motion records of the 2009 L’Aquila earthquake. Finally, the role of information in earthquake disaster prevention will be discussed.
1. Introduction
Obviously, the basic of the earthquake disaster prevention is thestrengthening structures and buildings against the earthquake load.
The devastations caused by the 2009 L’Aquila earthquake weremainly based on the lack of the strength of the many facilities andstructures. It is necessary to escape from the situation as meeting witha large earthquake motion to survive from unexpected damage. The
idea to minimize the damage of earthquakes by developing an early
warning system was first published by Cooper in 1868 [1] at San
Francisco. However, Cooper’s idea was never realized and basicallyforgotten. About 100 years later, another but similar concept of earth_______________________________
1 System and Data Research, Tokyo, Japan
2 Tokyo Institute of Technology, Yokohama, Japan

2

Yutaka Nakamura

quake warning was published [2] in Japan. At the same time, many institutions and agencies including JMA, Japan Meteorological Agency,
started to research a possible EEW based on the existing earthquake
observation system. However, there were many problems should be
solved.
JNR, Japanese National Railways, had also started to research possible EEW based on the same concept as JMA, while Nakamura at the
RTRI, Railway Technical Research Institute, of JNR, started to develop a different concept in 1979, using only a single station. He completed the prototype UrEDAS, pronounced “Yuredasu” meaning“shaking begins” in Japanese. This was the first actual P-wave detection and alarm system worldwide and was published in Nakamura [3].
Other organizations, such as JMA, continued their research and development but without implementing operational systems. This madeJNR the only agency to implement an operational and functioning actual warning system in the 20th Century.
2. Concept of the EEW
There are two kinds of earthquake alarms as illustrated in Figure 1.
One is the “On-Site Alarm” which is an alarm based on observations
close to the objects to be warned. The other one is the “Front Alarm”
which is an alarm based on observations near the epicentral area,
which is then used for the warning in possible damage areas awayfrom the epicenter. Because the latter alarm concept requires the useof communication networks, it is also sometimes called “Network
Alarm”. Both alarm types described can make use of two
different triggers, also called“alarms”. One is the trigger/alarm exceeding a preset
level, the so-called “S-wave
Alarm”
or“Triggered
Alarm”; the other one is thetrigger/alarm during the preliminary motion, the so-called
Figure 1. Concept of the EEW“P-wave Alarm”.

Earthquake Early Warning and Realtime Earthquake Disaster Prevention

3. EEW Systems

3

Δ/h=55/70(58/79)km

3.1. UrEDAS and Compact UrEDAS

Epicentral Distance in km

Elemental techniquesfor UrEDAS have been
established for more than
25 years [3] for a description of individual elemental techniques. The
main UrEDAS functions
Figure 2. Example of UrEDAS processing
are the estimation ofmagnitude and location, vulnerability assessment and issuing warnings within a few seconds using initial P-wave motion at a single station. Unlike other automatic seismic observation systems, UrEDAS
does not have to transmit the observed waveform in realtime to a remote processing or centralized system and is therefore comparatively
simple. UrEDAS calculates parameters such as back azimuth, predominant frequency for magnitude evaluation and vertical to horizontal ratio for discrimination between P and S waves, using the amplitude for each sample in realtime as shown in Figure 2. These calculations are processed in realtime without storing waveform data. The
amount of processing of UrEDAS is almost constant regardless ofwhether or not an earthquake is occurring, so the number of computational
processes
No Damage
Possible Damage Area
does not increase
Possible
Damage M
Area
for magnitude
for magnitude M
Damages for Embankmentsin the event of an
Damages for Bridge?
Δ100earthquake. UrEDAS can issue its
al
20G
alarm based on the10
M-Δ diagram in
l
Ga
40
Δ
Figure 3 immediately after the1earthquake detecM 7
3
4
5
6
8
Magnitude M
tion. The M-Δ diaFigure 3. M-Δ diagram
gram is derived
120
Gal

80G
al

EpicenterYutaka Nakamura

4

from past damage experience to JNR facilities and equipment and aids
in decreasing the number of false or needless alarms. This kind of
alarm is referred to as the M-Δ Alarm. UrEDAS can also assist in decisions to safely restart train operations based on the detailed earthquake parameters.
In 1992, UrEDAS started routine operations with full functionality
for the Tokaido Shinkansen line [4]. These UrEDAS issue a P-wave
warning with a processing time of just three seconds after detecting
the P-wave.
After the 1995 Kobe earthquake, it was recognized that an evenshorter warning time would lead to even more effective realtime disaster mitigation. To achieve this, UrEDAS was developed further withthe aim of being able to issue warnings within about one second after
the detection of a damaging earthquake. The resulting system was theCompact UrEDAS which became available in 1997 [5] and commenced routine operations for the Tohoku, Joetsu and Nagano
Shinkansen lines in 1998.
In contrast to UrEDAS which estimates the destructiveness of theearthquake from the earthquake’s parameters, Compact UrEDAS derives this from the earthquake motion directly ‘in realtime’ and thenissues the alarm, if required. To estimate earthquake dangerousness,the power density PD (W/kg) of the earthquake vibration is calculated
from the inner product of the acceleration vector a (cm/s2) and the velocity vector v (cm/s). As this value is a large number, it is normally
expressed in terms of the Destructive Intensity (DI) defined as thelogarithm of absolute value of this inner product (LPD, logarithm of
the power density) as Eq. (1). The concept of DI is illustrated in Figure 4.
E = mv
2/2

DI = log |a·v|= LPD+4.0

= ma
ma
FF =

(1)

The maximum value of DI
during an event, DImax relates to the earthquake damage and is similar to the instrumental intensity scale of

F= ma
m

v

PW = mavFv （（（PW = F∙v = ma∙v
DI = log10 |a∙v|

with cgs unit

PD = PW/m = a∙v
LPD = log10 |a∙v|

with MKS unit

RI = DI + 2.4 = LPD + 6.4
MMI = (11/7)∙RI + 0.5

Figure 4. Earthquake motion indicesEarthquake Early Warning and Realtime Earthquake Disaster Prevention

JMA, IJMA, with the constantdifference of 2.4, and corresponds to MMI, the Modified
Mercalli Intensity. These indices are referred as RI, Realtime Intensity, and MMI, respectively.

5

Acceleration in Gal_ EW

100
50
0
-50
-100

NS

Z

The Change of RI and Acceleration observed at the 2003 Miyagiken-Oki Eq. M7.0

cm/s

RI = DI + 2.4(2)
MMI = (11/7)RI + 0.5= (11/7)DI + 4.27 (3)Figure 5. Acc. waveforms, RI and SI

IJMA can only be calculated
after earthquake termination according to its artificial definition without physical background. Contrary to this, DI is defined as the logarithm of Power Density added by a constant. Power Density is aphysical variable related closely to earthquake damage. DI can be calculated in realtime and its value increases immediately after the Pwave arrival. Because DI is sensitive to the P-wave arrival, it can beused to define a P-wave alarm. Figure 5 shows the change of RI as afunction of time with the acceleration waveform and the change of SI,
Spectral Intensity. RI is more sensitive than both, the acceleration andSI, because the value of RI increases drastically at the P-wave detection, about one second prior to the other indices.
The function of the Compact UrEDAS is to issue an early warning
for the area close to the instrument based on the risk of the detectedearthquake motion, which is different from the alarm from the UrEDAS to the area of possible damage based on the estimated locationand magnitude of the detected earthquake. The target area of the
UrEDAS alarm is about 200km around the location of the instrument,
while that of the Compact UrEDAS alarm is only about 20km aroundthe instrument. Thus, to maximize the effectiveness of the warning
system, an ideal strategy is to use the UrEDAS (front alarm or network
alarm) to respond to more distant large earthquakes and use the Compact UrEDAS (on-site alarm) to respond to nearby earthquakes.
3.2. FREQL

Yutaka Nakamura

6

As the next step, the functions of UrEDAS and Compact UrEDAS
were combined which not only reduced the size and weight of the system, but also improved its functionality. This development was completed in 2005 and given the name FREQL. In Japanese, FREQL,
pronounced “furekkuru”, means “wave coming”. FREQL shortened
the processing time required to issue an UrEDAS/P-wave alarm based on the estimated earthquake parameters - from three seconds(for UrEDAS) to one second. The previously minimum time for issuing a P-wave alarm based on the dangerousness of the detected earthquake motion by the Compact UrEDAS of one second was improvedby the FREQL to just 0.2 seconds and further to 0.1 seconds in 2009.
This enables FREQL to issue an alarm before a large earthquake motion even in the case of the epicentral area.
FREQL is not only very valuable in the detection of the initialquake, but is also used during rescue operations after the main shockby detecting, and warning for possible aftershocks. For instance, theTokyo Fire Department Hyper Rescue Team pulled out a small childfrom underneath debris from a land slide during the 2004 NiigatakenChuetsu earthquake. After that they had contacted us for using
FREQL to keep the safety of the rescue staff during the high risk ofaftershocks. Based on their request, a portable version of the
FREQL was developed in 2005,
improving the system’s capabilities further and making it morecompact. The portable FREQL is
now in use by many fire departments nationwide, even accompanying international disaster rescue
teams, such as to the 2006 Pakistan earthquake and the 2008 Sichuan earthquake, see Figure 6.
Rescue teams involved in the aftermath of the 2008 Iwate-MiyagiNairiku earthquake also benefited Figure 6. FREQL used by Japagreatly from the portable FREQL. nese International Rescue Team
Battery Pack

Display Unit

Alert
Unit

Main Body of FREQL(workable without Display Unit)at the Sichuan earthquake

Earthquake Early Warning and Realtime Earthquake Disaster Prevention

7

3.3. Examples of an Integrated System
Tokyo Metro, a subway network
company in the Tokyo metropolitanarea, has set up an earthquake early
warning system consisting of six
FREQL stations and an earthquake
information system consisting of 33
AcCo stations to enable the competent and quick response immediately
after an earthquake. AcCo, Acceleration Collector, is a widely usedportable-size alarm seismometer
which can display seismic intensity
FREQL network
stations:: 66 sites
AcCo networkand horizontal acceleration in real: 33 stations
AcCo
network: 33 sites
time. The locations of the instruments are shown in Figure 7 [6].
Figure 7. EEW and Quick
Response (QR) systems of
After setting up FREQL and AcCo
Tokyo Metro
networks, Tokyo Metro also installeda receiver for the JMA’s EEW following instructions from the transportation authority (JMA is supervised by this authority). The accurate
and quick information from the FREQL-based early warning and the
AcCo-based earthquake information system has helped to minimize
the disruption of train operations
even in the cases of JMA’s EEW
issuing false or delayed alarms.
Additionally, UrEDAS-basedtsunami warning system [7] hasbeen operated by Wakayama prefecture in Japan. This is the only
GPS Clock
UrEDAS
Door Sensor
CPU Communication
example in Japan to keep the Thermo&Hygrometers
with
Lines
Display
A/D
andsafety not only relying on theLong Period Sensor
Modem
Short Period Sensor
JMA information but also monitoring the tsunami independently. Figure 8. Kushimoto UrEDAS for
Figure 8 overviews the Kushi- Tsunami Warning, its location and
Networks of Tokyo Metro

0

1

2

3

4 km

Tokyo Metro operates
the colored subways.

Kusihimoto

Seismometers

the block diagram of the system

Yutaka Nakamura

8

Processing 警報処理時間
Time for P-wave
in sec.
）（ 秒Alarm

10
moto UrEDAS observatory inSDR products
5
stalled in March 2000. In SepUrEDAS 3 sec.
UrEDAS
3
tember 2004, this system deTest observation2tected two tsunami earth1
Compact UrEDAS 1 sec.
quakes at offshore Kushimoto
0.5
and reported it three seconds
Min.
FREQL Minimum 0.2 sec.
after the detection. Estimated
0.1
0.2
sec.
magnitudes were M6.0 and
0.1
M7.4, and the official magni1980
1985
1990
1995
2000
2005
2010
Year
tudes were M6.9 and M7.4 by
Figure 9. Processing time of the
JMA.
EEW systems
Figure 9 shows the timeline for the processing time required by the various warning systems
to issue a P-wave alarm. It is obvious that FREQL exhibits the shortest alarm processing time for all shown P-wave detection alarm systems. In addition, it is important to note that FREQL simultaneously
also calculates and displays RI.
cf. Equipment developed by JMA

Average 5.4 sec.

JR Tokaido Shinkansen
Tsunami warning system Minimum 2.0 sec.
for Wakayama Prefecture

Tohoku Shinkansen
Tokyo Metro

Tokyo Metro, Odakyu
Fire Departments of Tokyo, Osaka and so on

4. Example of a Successful Earthquake Early Warning Event
0 1 2 3 4 5 6 7 8 9 1 0 11 1 2 1 3The derailment
P-wave Arrival
P-wave Alarmof the Shinkansen
S-wave Arrival
train Toki #325
Principal Motion
during the 2004
Main Derailments
Niigata-ken ChuSuccessive Vehicle ②:
etsu
earthquake
Derailment
Example ofLocation
impressively
Compact
UrEDAS
demonstrated thenormal vehicle
Warningderailment vehicle byeffectiveness andflange climbing
benefits of thelarge relative displacement
railway deformation
early warning sysFront ⑩
Rear ①
tem for Shinkanderailed train
sen train operaFigure 10. The Shinkansen derailment during
tions. The fact thatthe 2004 Niigata-Ken Chuetsu earthquake
there were no fatalities or injuries to the passengers and crew of the train, totally 155
x100m

5 seconds

Time

You can see derailment situationand contact situation between bodyand railroad,

Final Derailment
Final Situation
Situation(a)先頭車Tunnel Exit(b)最後尾車

Earthquake Early Warning and Realtime Earthquake Disaster Prevention

9

persons, is clearly a result of the availability of the early warning system. Figure 10 shows a summary and timeline of the events (see thedetail in [8]).
A subsequent simulation of the alarm behaviour of a FREQL system using the recorded waveform showed that the train would havereceived the P-wave alarm another 0.8 seconds earlier, if a FREQL
had been installed, because the FREQL would have issued the P-wave
alarm already 0.2 seconds after the P-wave detection.
There are several more examples of successful warnings by UrEDAS or Compact UrEDAS in 1995, 2003, 2005 and as detailed in [9].
After 2006, UrEDAS and Compact UrEDAS for Shinkansen lines
have been replaced by the JMA type system. The reason of replacement is still not clear because the operator explained that the replacement aimed to make the alarm faster but the new system had not been
earlier than UrEDAS or Compact UrEDAS even now.
5. Leading Time by FREQL Simulated with Strong Motion Records
In this section, a simulated EEW of FREQL during an earthquake
is described and compared to the actual performance of JMA’s EEW,
based on the observed data.
JMA’s system for EEW had been developed since around 2000 andofficially operated in November 2007 [10].This system is so called anintermittent processing system, so the system determines the necessary parameters by fitting aP-wave Alarm ( RI=1.5) by FREQL
45
Triggered Alarm (5HzPGA=10Gal) by AcCofunction with stored waveBeginning of Principal Motion
40nOccurrence Tim e of Maxim um Motion
form data. According to
otio
35
M
m
FREQL Quicker Zone
JMA, the duration of the
imu
30
x
a
M
stored data is required at
25
Damaged Area
least two seconds and aver20
15
aged process time is 5.4
Alarm by JMA
10seconds. The parameters of
JMA
Alarm
m
r
a
l
5
A
L
each observation site are
RE Q
0F
gathered in JMA’s central0
10
20
30
40
50
60
70
80
90 100 110 120
Epi-central Distance in km
ized system, and then procFigure 11. EEW timing for the 2008
essed again. After this procIwate-Miyagi-Nairiku earthquake
aximum Motion

Yutaka Nakamura

10

Time in seconds after Earthquake
Occurrence

50
ess, a warning is isThe 2009 L'Aquila Earthquake
RI_max45
2009/4/6, Mw6.3, h9.5km
sued if necessary.
RI>2
40
Maximum RI = 5.68 (MMI = 9.43)
The 2008 Iwateat
AQV(epicentral
distance
4.4km)
35
S-waveMiyagi-Nairiku
30
RI>3
RI>1
earthquake caused
25
RI>0
severe damage in20
the epicentral area,
P-wave15 RI>4
yet JMA’s EEW
10Onsite FREQL Alarm: RI>1.5
system was unable
First FREQL Alarm: RI>1.5
5
to issue an alarm0
before the start of
0
10
20
30
40
50
60
70
80
90 100
Epicentral
Distanceinkm
shaking in thedamagedarea Figure 12. EEW timing for the 2009 L’Aquila Eq.
within approximately 25km of the epicentre. Figure 11 shows thetime of the actual alarms issued by JMA’s EEW and compares themwith alarms which an on-site FREQL system would have been able toissue, based on a simulation using the recorded waveforms. The depthof this earthquake was approximately 8km below the surface. The
alarm from JMA’s EEW was issued 3.5 seconds after the event detection to the primary customer and one second later to the public, whichis equivalent to about 10 seconds after the earthquake occurrence. Although the timing of the alarm in this case was faster than the averaged processing time of 5.4 seconds (see Figure 9), the warning wasstill only delivered after the beginning of the strong motion. In contrast to this, FREQL is expected to issue an alarm 0.2 seconds after
the P-wave detection in the damaged area, i.e. the time margin before
the beginning of the strong motion is several seconds, even at the epicentre itself.
Until today, there has been no example where JMA’s EEW wasable to effectively be used for disaster prevention by means of early
warning.
Figure 12 shows a result of simulation for the 2009 L’Aquila
earthquake using strong motion records [11]. According to this figure,
FREQL was expected to get a few seconds as a leading time at even
epicentral area. It is very short time, but during the leading time people could be evacuate into some sturdy desk before falling down the
2 (3.6)4 (6.8)4 (6.8)RI>4

1 (2.1)0 (0.5)3 (5.2)5 (8.4)RI>5

3 (5.2)4 (6.8)Figures in brackets correspond to MMI

Earthquake Early Warning and Realtime Earthquake Disaster Prevention

11

floor or the ceiling. Of course it is needless to say that the most important measure is to build the structures enough earthquake-proof. Even
after the reinforcement against the earthquake, because there is stillrisk on the falling objects, the repeated emergency drills are essential
to acquire the image of the damage situation after the earthquake forthe reasonable countermeasures. Anyway there is no system to issue
the earthquake alarm quicker than on-site FREQL, so it is advisable totake proper countermeasures without exaggerating the effect of EEW.
6. What Information is required for Earthquake Disaster Mitigation?
From the operational side, there are three pieces of information
relevant to realtime earthquake assessment:(1) An early warning before the onset of the strong shaking for thearea of certain damage,(2) Information before the shaking starts about the area that will
not be damaged, and(3) A precise and detailed estimate of the location and extent of thearea where serious damage can be expected.
The most important issue for public agencies is to have a reliable
assessment as listed in Item (3) above, so that rescue activities in thedamaged areas can be initiated immediately. For the public agencies it
is impossible to deliver information as under Item (1) due to the longprocessing and communication times. Only on-site early warnings candeliver that information. In general, the early warning can only be
used to trigger individual or locally based social group help or action.
Governmental assistance will mainly focus on the rescue work in theseriously damaged areas evaluated by information as under Item (3).
Information as under Item (2) has no relevance to damage, and is unnecessary information for disaster prevention. For most events, unfortunately, JMA’s EEW seems usually only capable to deliver information as under Item (2).
It is important to receive rapid and accurate earthquake informationfrom organizations such as JMA in Japan, but it is also important not
to restrict the delivery of the information as under Item (3) from otherorganizations, such as local universities or institutions.

12

Yutaka Nakamura

For the most appropriate and quick response after the earthquake,the “actual observed” seismic motion as intensity, not an estimatedvalue, is the most important information because the degree of damage can be estimated from it.
It became very obvious that an M7 class earthquake causes catastrophic damage especially in the area close to the epicentre. Although
M7 class earthquakes are quite common in Japan, it is a potentially fatal flaw in the warning system that the JMA’s EEW cannot issue atimely and accurate estimate of the large motion in the catastrophically damaged area.
For M8 class events (which occur statistically once every several
tens of years in Japan), the JMA’s EEW seems to be capable to issue awarning with a useful time margin for the strong motion. Yet such awarning is only useful, if it is readily available for the damaged area
more than 50 km from the epicentre. This, however, might not be thecase, because in such a serious situation it is quite possible that the information is lost or delayed by problems in the communication system.
On-site alarm equipment such as the FREQL, is capable to issue
the alarm in as short a time as 0.1 seconds after the P-wave and canthus issue the warning before the onset of the large earthquake motion,even for locations close to the epicentre. If then a local FM radio station would broadcast the FREQL information, there would be a time
margin for preventative action within the damaged area.
7. Concluding Remarks
Realtime earthquake warning was initially aimed at disaster prevention for the train system in Japan, but is by now used in manyother environments as well. To be of wide-ranging benefit, the time
margin of the warning must be very short on a time scale of seconds toenable quick action by the general public, such as to seek shelter (likeunder a sturdy desk) or to evacuate immediately to a pre-determinedsafety zone.
It is clear that the primary preventative action by the people at thetime of the earthquake is to keep away from places where they could

Earthquake Early Warning and Realtime Earthquake Disaster Prevention

13

be hit by falling or loose objects. To be able to do this obviously requires a timely and immediate warning to be issued.
A warning cannot prevent the possible collapse of structures, but it
can most definitely assist people evacuating from collapsing facilities.
National organizations should be able to specify the area of expectedcatastrophic damage accurately and initiate rescue operations without
delay based on information from responsible and capable organizations in realtime.
Acknowledgment
The author wish to thank Tokyo Metro and the Tokyo Fire Department
for their cooperation in the implementation the systems: FREQL-based early
warning system and AcCo-based earthquake information system. The simulation described in this article partially used the waveform data from K-NETand KiK-net of NIED, National Institute of Earth Science for Disaster Prevention, and from ItacaNet [11]. I would like to express my sincere thanks to
Mr. J. Saita and Mr. T. Sato, the senior research engineers of SDR, Systemand Data Research, and all individuals that contributed to this paper.

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[2] M. Hakuno, H. Takahashi, Earthquake warning system 10 seconds before the strong motion (inJapanese), Shizen, September 1972.
[3] Y. Nakamura, On the urgent earthquake detection and alarm system (UrEDAS), Proceedings of
9th World Conference on Earthquake Engineering (WCEE), Vol. VIII, 1988, pp.673-678.
[4] Y. Nakamura, Real-time information systems for hazards mitigation, Proceedings of 11th WCEE,
1996.
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