Earthquake Early Warning in Japan

Hiromitsu Nakamura
National Research Institute for Earth Science andDisaster Prevention, Japan(NIED)Outline
Overview of the earthquake early warning (EEW)
system in Japan
Transmission and utilization of the EEW
Problems of the present EEW system

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

after JMA Web site
The EEW system provides advance warning of estimated seismic intensities andexpected arrival time of S-waves. These estimates are based on prompt analysis ofhypocenter location and earthquake magnitude using data observed by seismographs
near the epicenter. The system issues several EEW messages during the course of
one earthquake, improving the accuracy of the warning as the amount of available
data increases.

Seismic stations for EEW

NIED：Hi-net（800 stations）• High-sensitivity seismograph• Borehole with depth of 100 m
or deeper

JMA：network for tsunami warning●NIED▲JMAand earthquake information services（200 stations）• Strong-motion seismographReal-Time Earthquake Information System (REIS)by the not-yet-arrived method applied to Hi-net data
voice alarm
focalmechanism

estimatedintensity

countdown toS-wave arrivalreceiverpointepicenter

seismograms
and computedP arrival

S-wave front

stationamplitudes

REIS can be used to determine earthquake parameters andthen transmit these results to JMA in order to issue an EEW.

Earthquake Early Warning System in Japan

The practical service of EEW in Japan has been in
operation since Oct. 2007.

Problems of the present EEW system(1) Sometimes EEW is issued after S-wave arrival
Warning times become negative within an area about 30km from theepicenter. When a large earthquake occurs, the closer to the hypocenter
the greater the likelihood of damage.

(2) Underestimation of seismic intensity during a
massive earthquake
In the 2011 Tohoku-Oki earthquake (M9), the EEW was issued to thearea close to the hypocenter earlier than the S-wave arrival. But the EEWcannot be issued to areas further away from the hypocenter, where theobserved seismic intensity is greater than 5-lower.

(3) False alarm
Earthquakes sometimes occurred simultaneously over the entire fault
region, such that the EEW system became confused, and didn’t always
determine the hypocenter location and earthquake magnitude correctly.

Time delay of first EEW reports andnegative warning area -problem(1)Warning time before S-wave arrivals
2008 Iwate Miyagi inland earthquake

Histogram of the time delay in issuing
the first report after P-wave arrives atthe closest station

Negative warning
area

70

Data: 197 evens with
Is greater than 4 since2007

60event

50403020
1005101520

Time delay of first report (s)peak at 3 to 5 seconds that roughly corresponds to an areawithin 30 km of the epicenter where warning time is negative.

Importance of Near-field Strong Ground Motion
Data for EEW

Near-field strong ground motion data are very effective in reducing
the size of the negative warning area
2008 Iwate-Miyagi inland
earthquake

Off -line analysis result
warning time of the timing of over intensity 5lower observed at IWTH25 where the epicentral
distance is about 3km
Comparison of near-field strong motion dataobserved at KiK-net IWTH25 (epicentral
distance 3km) and timing of EEWFirst EEW report
P-wave08:43:54.2
08:43:46.7

Warning time offirst EEW report (JMA)

ACC. EW

The negative warning ACC. NS
area is within a few
km from the epicenter ACC. UD
mic intensity

Negative warning
area

Real –timeSeismic intensity

Over intensity 4.5
08:43:48.0
Time(s)The EEW of the 2011 Tohoku-Oki earthquake-problem(2)-Warning area

Area where the observed seismic intensity
is greater than or equal to 5-lowerg

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

underestimation

This underestimation can probably be attributed to theamplitude-magnitude (MJMA) saturation for events of Mw
greater than 8 and the large extent of the fault rupture, anextent of about 40,000 square kilometers.

What should we do?
EEW

Real-time monitoring ofstrong motion

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

Live monitoring of strong
motion is independent of theestimation of EEW.

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

Warning simulation of the 2011 Tohoku-Oki earthquake
EEW

Off-line analysis of on-site warning
using K-NET & KiK-net data

Actually issued

The on-site warning for eacharea is issued when theobserved real-time seismic
intensity exceeds a specificthreshold.
In this simulation, thewarning threshold is seismic
intensity 4 in real-time.

The EEW can be issued early to the area near the hypocenter.
The on-site warning can be issued to the area where the EEW can’t besufficiently responded.

Conclusions
The NIED has developed the real-time earthquake
information system (REIS) which is able to determine
hypocenter locations and earthquake magnitude within afew seconds.
The JMA has been issuing EEW, which contain the resultsof REIS, to the general public since October, 2007.
The EEW is transmitted to many kinds of devices and used
for personal safety and automatic control.
It is very important to observe strong motion in real-timeusing a dense network in order to improve the EEW system.