Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

RAPID SOURCE PARAMETER ESTIMATIONOF GREAT EARTHQUAKES FOR TSUNAMIWARNING
Akio KATSUMATA1, Shigeki AOKI2, Yasuhiro YOSHIDA3, Hiroshi UENO4
and Takashi YOKOTA5
1

Head of The Second Research Laboratory, Seismology and Volcanology Research Department,Meteorological Research Institute, Japan Meteorological Agency,
Tsukuba, Japan, akatsuma@mri-jma.go.jp
2Senior Researcher, Seismology and Volcanology Research Department, MRI, JMA,Tsukuba, Japan, shigaoki@mri-jma.go.jp
3Senior Researcher, Seismology and Volcanology Research Department, MRI, JMA,Tsukuba, Japan, yyoshida@mri-jma.go.jp
4Senior Researcher, Seismology and Volcanology Research Department, MRI, JMA,Tsukuba, Japan, hueno@mri-jma.go.jp
5Director of Seismology and Volcanology Research Department, MRI, JMA,Tsukuba, Japan, yokotat@met.kishou.go.jp
ABSTRACT: One of major problems in the tsunami warning for the 2011 off the Pacificcoast of Tohoku Earthquake (Mw 9.0) was a lack of awareness of underestimation of theearthquake magnitude at the time soon after the occurrence. Displacement magnitude,which is usually used for the first tsunami warning a few minutes after the earthquake
occurrence, could not evaluate such large magnitude. Seismic moment could not be
determined from the regional seismological network data due to over range of broadband
sensor outputs. To overcome these difficulties in earthquake magnitude estimation,
several methods are being developed to estimate proper magnitude roughly and tounderstand possible magnitude underestimation soon after such large earthquakes.
Key Words: rapid magnitude determination, great earthquakes, tsunami warning, area ofstrong shaking, strong motion duration

INTRODUCTION
The tsunami height of the first tsunami warning by Japan Meteorological Agency (JMA) is estimatedbased on earthquake magnitude. The first tsunami warning should be issued within a few minutes after
the detection of the earthquake occurrence. Displacement magnitude (Katsumata, 2004) is used for thefirst tsunami warning. Seismic moment tensor is also estimated after the first tsunami warning toestimate size of the earthquake more accurately (Usui et al., 2010). The tsunami height estimation is
updated from data including the seismic moment tensor and sea level observations. At the time of theoccurrence of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0), the magnitude was not
estimated properly due to too short cutoff period (six seconds) of the filter for the displacement592

magnitude compared with the rupture duration (about three minutes [Yoshida et al., 2011a]).
Seismic moment could not be estimated from the regional seismological network data due toover-range of broadband sensor (STS-1 and STS-2) outputs, and it took longer time to estimate it fromglobal data. These caused a lack of awareness of underestimation of the earthquake magnitude at thetime soon after the occurrence.
To overcome these difficulties in earthquake magnitude estimation, we are developing several
methods to estimate proper magnitude roughly and to understand possible magnitude underestimation
soon after such large earthquakes. A large earthquake causes strong shaking in a wide area, longstrong motion duration, and large seismic amplitude in long period range as well as in short period
range. These observations could be used for the magnitude estimation.

MAGNITUDE DETERMINATION FROM SPAN OF STRONG MOTION AREA
Large earthquakes cause strong motion in a wide area. Span of strong-motion area is related toearthquake magnitude. Seismic intensity distribution in Japan can be known in a few minutes afterearthquake occurrence owing to a dense on-line network of seismic intensity meter in Japan. It ispossible to estimate earthquake magnitude roughly from the area of strong shaking.
Figure 1 shows the distributions of seismic intensity of the 2011 off the Pacific coast of Tohoku
Earthquake (Mw 9.0) and the 2003 Off-Tokachi Earthquake (Mw 8.0). The span of 5-lower or thegreater of the JMA seismic intensity scale of the 2011 off the Pacific coast of Tohoku Earthquake
reached about 700 km. The span of the 2003 Off-Tokachi Earthquake was about 300 km, which wasmuch less than that of the 2011 off the Pacific coast of Tohoku Earthquake.

Fig. 1. Distributions of seismic intensity of the 2011 off the Pacific coast of Tohoku Earthquake andthe 2003 Off-Tokachi Earthquake. The contours in maps denote slip distributions estimated byYoshida (2005) and Yoshida et al. (2011a).

SOURCE AREA ESTIMATION FROM SEISMIC INTENSITY DISTRIBUTION
The place of observed strong motion would be close to the seismic source. Source area can be
estimated from the area of strong-motion. At each station, distance from the fault is estimated from theseismic intensity with the formula by Si and Midorikawa (1999). To estimate the source area, grid
points are set on the plate boundary. At each grid points, number of stations where the distance
between the station and the grid point is larger than the estimated fault distance is counted. If the grid593

point is on the source area, the number of inconsistent stations (where the distance between the stationand the grid point is larger than the estimated fault distance) should not exceed some number. Figure 2
shows the estimated source area of the 2011 off the Pacific coast of Tohoku Earthquake and the 2003
Off-Tokachi Earthquake from the number of the inconsistent stations. Whereas it is possible toestimate the close edge of a fault with this method, it is difficult to estimate the far edge of an offshore
fault properly. A different color map is used for the area off the trench axis in Figure 2. When theseismic fault lies along the island arc, like the 2011 off the Pacific coast of Tohoku Earthquake, it isable to estimate length of the fault.

Fig. 2. Estimated source area of the 2011 off the Pacific coast of Tohoku Earthquake and the 2003
Off-Tokachi Earthquake from seismic intensity distribution [Yokota and Kaida, 2011].

DURATION OF STRONG MOTION
The duration of the strong motion becomes also longer for larger earthquakes. Durations of strong
motion were investigated for large earthquakes in and around Japan.
Figure 3 shows distribution of strong-motion durations for the 2003 Off-Tokachi Earthquake. Thecolors denote strong-motion duration at stations. Data from K-NET [Kinoshita, 1998] and KiK-net[Aoi et al. 2000] is used for this analysis. The durations in the north of the epicenter are shorter thanthose in the other directions in the figure. The duration difference is considered due to directivity
effect. Figure 4 shows azimuthal distribution of the duration, which shapes a sinusoidal curve. It ispossible to estimate fault length and rupture direction from the distribution (Izutani and Hiraswa,1987). The arrow in Figure 3 indicates estimated fault length and rupture direction. When the faulting
is simple unilateral, this method is useful for rapid estimation of the fault parameters.
Figure 5 shows relationship between moment magnitude and strong-motion duration ofearthquakes which occurred in and around Japan. Good correlation is seen between strong-motion
duration and earthquake magnitude. The duration of the earthquake in March, 2011 exceeded eighty
seconds, which was the longest among those of the large earthquakes. It is difficult to estimate theaccurate moment magnitude on the basis of the duration due to the large scatter. We can, however,judge possible magnitude underestimation by the duration.

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Fig. 3. Distribution of strong-motion duration of the 2003 Off-Tokachi Earthquake [Aoki et al., 2011].
The star indicates the epicenter of the earthquake, and the arrow indicates estimated fault lengthand rupture direction by the method of Izutani and Hirasawa (1987).

Fig. 4. Azimuthal distribution of strong-motion duration of the 2003 Off-Tokachi Earthquake [Aoki etal., 2011]. Blue curve indicates the theoretical distribution of the estimated fault.

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Fig. 5. Relationship between moment magnitude (after the Global CMT Project) and strong-motion
duration Dobs of earthquakes which occurred in and around Japan [Aoki, et al., 2011]. The redcircle denotes that of the 2011 off the Pacific coast of Tohoku Earthquake.

MAGNITUDE ESTIMATION FROM P-WAVE
Some magnitude determination methods from P-wave have been proposed. Yoshida (1995) proposeda magnitude determination method from P-wave displacement amplitude, Mp. Figure 6 shows thestation magnitude Mp of the 2011 off the Pacific coast of Tohoku Earthquake. The horizontal axis ofFigure 6 denotes time of magnitude determination from the origin time of the earthquake whichcorresponds to S-wave arrival time at the station. The magnitude grows before about four minutes.
This reflects the extension of the rupture area.
Tsuboi et al. (1995) developed a method of estimating moment magnitude, Mwp, from integrated
P-wave. Figure 7 shows the magnitude Mwp of the 2011 off the Pacific coast of Tohoku Earthquake.
Ogawara et al. (2004) showed a magnitude determination method, Mwliss, based on squared
amplitude of broadband seismic wave. Figure 8 shows the magnitude Mwliss of the 2011 off thePacific coast of Tohoku Earthquake.
These magnitudes show similar variation along the time from the origin time, and reach about ninein five minutes. It is possible to determine the magnitude of this earthquake properly in five minuteswith these methods. Dispersion of Mwp is smaller compared with other P-wave magnitudes.

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Fig. 6. Mp of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) [Yoshida et al., 2011b].
The red circle denotes station magnitude calculated from seismic wave between P and S arrivals
at each station. The blue small dot denotes that calculated from seismic wave contaminated withS-wave. The horizontal axis shows the time when the magnitude is estimated at each station
measured from the origin time.

Fig. 7. Mwp [Tsuboi et al., 1995] of the 2011 off the Pacific coast of Tohoku Earthquake [Yoshida etal., 2011b]. The symbols are the same as those in Figure 6.

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Fig. 8. Mwliss [Ogawara et al., 2004] of the 2011 off the Pacific coast of Tohoku Earthquake[Yoshida et al., 2011b]. The symbols are the same as those in Figure 6.
MAGNITUDE DETERMINATION FROM LONG-PERIOD SEISMIC WAVE
It takes a long time to complete a rupture of a large earthquake. Excitation of long-period seismic
wave is one of features of large earthquakes. The cutoff period (6 s) for the displacement magnitudeused for the first tsunami warning was too short for the 2011 off the Pacific coast of Tohoku
Earthquake. Usage of long period components of seismic wave would help to estimate earthquake
magnitude properly.
Figure 9 shows seismic waves processed with filters of different frequency responses. The figure
shows seismic waves of the 2011 off the Pacific coast of Tohoku Earthquake (the left) andthe 2003 Off-Tokachi Earthquake (the right). The upper of the figure shows seismic waves with thesame response for the displacement magnitude. The lower shows those processed with a filter of
200-1000 second pass-band. While amplitudes of short-period seismic wave are not so different
between the two earthquakes, those of long-period seismic wave differ very much. This is one method
to distinguish the difference of these magnitudes.
Magnitude determination methods from various frequency ranges are developed. Peakdisplacements of seismic waves of 1, 2, 5, 10, 20, 50 and 100 second cutoff periods are used todetermine magnitude. Phase type is not cared in this method, and peaks are possibly those of S-waves.
Figure 10 shows estimated durations to determine magnitude only from P-wave and from S-wave. It isnecessary to wait for the completion of fault rupture to get enough length of data. At a station close tosource area, S-P time would be shorter than the rupture duration. It is considered that magnitude
determination from S-wave peak is more rapid than that only from P-wave peaks when local seismic
data are available. Figure 11 shows growth of the magnitude with time from the origin time on thehorizontal axis. The averaged magnitudes calculated from closest ten stations are shown in the figure.
The magnitudes from long-period seismic wave reach the final values within three minutes for theearthquake.

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Fig. 9. Filtered Seismic waves of the 2011 off the Pacific coast of Tohoku Earthquake (the left) andthe 2003 Off-Tokachi Earthquake (the right) [Yoshida et al., 2011b]. The upper shows seismic
waves for the displacement magnitude used for the tsunami warning, and the lower shows those
processed with a filter of a 200-1000 second pass-pand.

Fig. 10. Assumed time to estimate earthquake magnitude from P-wave (the green broken curve) andS-wave (the red solid curve) [Katsumata et al., 2011]. The curves indicate relationships between
epicentral distance and sum of travel time and assumed rupture duration. Sixty seconds isassumed as the rupture duration here.

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Fig. 11. Growth of magnitude from seismic waves of various cutoff-periods [Katsumata et al., 2011].
The horizontal axis shows time from the origin time of the earthquake. The numerals indicate
cutoff periods of the filters.

CONCLUSIONS
Several magnitude determination methods are being developed to estimate earthquake magnitude soonafter occurrence of a large earthquake for tsunami warning. Magnitude estimation from span ofstrong-motion area, strong-motion duration, P-wave, and S-wave amplitudes were examined.
Combination of these methods is expected to help us to issue a proper tsunami warning for the next
great earthquake.

ACKNOWLEDGMENTS
Data from NIED K-NET, KiK-net, F-net, and the JMA seismic network were used in this study.
Hypocenter parameters of the unified seismic catalog of Japan and Global CMT Project are referred to.
The unified seismic catalog of Japan is based on data from the National Research Institute for Earth
Science and Disaster Prevention, Hokkaido University, Hirosaki University, Tohoku University,
University of Tokyo, Nagoya University, Kyoto University, Kochi University, Kyushu University,
Kagoshima University, the National Institute of Advanced Industrial Science and Technology, Tokyo
metropolitan government, Shizuoka prefectural government, Kanagawa prefectural government, theCity of Yokohama, the Japan Marine Science and Technology Center, and the Japan MeteorologicalAgency. We thank Y. Kaida for many valuable discussions about source area estimation from seismic
intensity distribution.
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