Initial 30 seconds of the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0)—amplitude and τ c for magnitude estimation for Earthquake Early Warning—
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 7 April 2011
Accepted: 10 June 2011
Published: 27 September 2011
We analyzed the waveforms of the mainshock (Mw 9.0) and three foreshocks of the 2011 off the Pacific coast of Tohoku Earthquake during the initial 30 s after P-wave onset to determine the maximum amplitudes of acceleration, velocity, and displacement, and τc (the period parameter of the waveform). The amplitudes for the Mw 9.0 event were quite small for the first several seconds, as small as those of the Mw 6 foreshocks, and the τc value was also as small as those of the foreshocks. For the first 30 s, the amplitude of the Mw 9.0 event was larger than that of the Mw 7.3 foreshock whereas τc was smaller. These results suggest that it is difficult to determine the eventual magnitude for very large earthquakes from the initial several seconds, that an updating procedure is important for Earthquake Early Warning using ongoing waveforms, and that τc might not be reliable for magnitude estimation at least for the main shock.
Key wordsEarthquake Early Warning magnitude τ c the 2011 off the Pacific coast of Tohoku Earthquake
Earthquake Early Warning (EEW) systems have been researched and developed in Japan, Mexico, the United States, Taiwan, Italy, Turkey, and other countries (e.g., Hoshiba et al., 2008; Alcik et al., 2009; Allen et al., 2009; Espinosa Aranda et al., 2009; Hsiao et al., 2009; Kamigaichi et al., 2009; Nakamura et al., 2009; Zollo et al., 2009). One of the important aspects of EEW is the rapid and reliable estimation of magnitude using the early portion of ongoing waveforms. Maximum amplitudes have commonly been used for magnitude estimates, and these are adopted in some EEW systems (e.g., Yamamoto et al., 2008; Kamigaichi et al., 2009), in which the estimate of magnitude is updated repeatedly using ongoing waveforms.
New algorithms, such as τc and , have been proposed for making estimates of the eventual magnitude by using the frequency contents of the very early portion of the waveforms; these algorithms have been investigated by many researchers (e.g., Nakamura, 1988; Wu and Kanamori, 2005; Wu et al., 2007; Allen and Kanamori, 2003; Yamada and Mori, 2009; Brown et al., 2009; Zollo et al., 2010). Most of these researches have concluded that the frequency contents are more sensitive to the magnitude than the ground-motion amplitude during the initial several seconds of the P wave. They have also claimed that the sensitivity is appropriate for EEW and that it is possible to determine the eventual size of the events from the initial several seconds. However, Rydelek and Horiuchi (2006) and Rydelek et al. (2007) using the data from accelerometers networks and high-sensitivity velocity-meter networks concluded that the size of larger earthquakes are difficult to estimate from only the early part of the records. Yamada and Ide (2008) came to a similar conclusion after considering a complex-source model. The validity of the new algorithms such as τc and for estimates of magnitude is not yet fully accepted.
The 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0) occurred on March 11, 2011, following foreshock activity. We have used the waveform data of the mainshock and three foreshocks to estimate the maximum acceleration, maximum velocity, maximum displacement and τc from the initial tN seconds of the waveform from the onset of the P wave using tN values from 3 to 30 s. We then consider whether the amplitudes or τc could discriminate the main-shock from the smaller foreshocks during the initial portion of their waveforms.
Some previous studies of τc used waveform data from many earthquake events and many recording stations, with differences in the distance between source and recording stations: many of them were from less than 100 km, and some were from larger than 100 km for larger events. The many different source-site geometries and site amplification factors may have affected the results of those studies. Variations due to differences in path factor were small in our analysis because we used only a few events that were close together, and a few observation sites that were also close together.
4. Results and Discussion
When the analysis was performed using a different high-pass filter of 0.166 Hz, and when the second filtering pass was omitted when obtaining ud(t), the results were similar.
Our results suggest that it would be difficult to estimate the eventual magnitude of the Mw 9.0 earthquake from the initial several seconds, even if the parameters based on frequency contents, such as τc and , were used, and indicates that an updating procedure is necessary, using ongoing waveforms, for EEW purposes. It would also be difficult to recognize that the Mw 9.0 event would be larger than the Mw 7.3 event using only τc based on the initial 30 s of the record, a time during which the ground-motion amplitude is clearly larger than that of the Mw 7.3 event. This is contrary to the claim that the frequency contents are more sensitive to earthquake magnitude than ground-motion amplitude.
As shown in Fig. 5, the frequency contents of the Mw 9.0 earthquake may be quite different from that expected for earthquakes of this size. The source process of the Mw 9.0 earthquake might be extraordinary for an Mw 9 class earthquake, and this analysis is only one case of an Mw 9 class earthquake. It is impossible to conclude from this case that τc and are ineffective for magnitude estimation for all larger events. But even so, our analysis shows that the extraordinary is not necessarily improbable. We believe that the EEW system should be robust even for extraordinary cases, especially for larger events.
The authors thank the anonymous reviewer who encouraged them to analyze and τp(t) in addition to τc, and Dr. M. Böse and Professor K. Yomogida (editor), who gave useful comments for improving the manuscript. The waveform data are from K-NET and KiK-net of NIED. The hypocenter locations of the events are based on the unified hypocenter catalog of the Japan Meteorological Agency (JMA). Moment magnitudes are taken from the JMA CMT catalog and the F-net catalog of NIED. We thank all of these entities for their efforts in maintaining these observations and providing the data, despite the exigencies resulting from the disaster. The figures were made using Generic Mapping Tools (Wessel and Smith, 1995).
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