Magnitude determination using duration of high frequency energy radiation for the 2011 Off the Pacific Coast of Tohoku Earthquake

Transcription

1 Magnitude determination using duration of high frequency energy radiation for the 2011 Off the Pacific Coast of Tohoku Earthquake Tatsuhiko Hara International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute 1

2 Summary The source time of the 2011 Tohoku earthquake is around 170 s. The rupture that generated strong high frequency signals propagated in the southwest direction. The magnitude estimate based on the formula of Hara (2007a) is Compared to the 2004 Sumatra earthquake, the duration of high frequency energy radiation is shorter (about 43 %), the maximum amplitude (with the distance correction) is larger (about 170 %). 2

3 International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute The IISEE is providing training courses in fields of seismology, tsunami, and earthquake engineering for participants from developing countries. Among them, the Global Seismological Observation course is provided to contribute to promotion of CTBT (T5-P14). 3

4 Magnitude using HFER and displacement amplitude We developed a new method to determine earthquake magnitudes using durations of high frequency energy radiation (HFER), t (s), and the maximum displacement amplitudes, A (m), measured from first arriving P-waves (Hara, 2007a, EPS, 59, ): M where = 0.79 log A log logt (km) is a epicentral distance. The above formula is applicable to M9 events (Hara, 2007a) and to tsunami earthquakes (Hara, 2007b, EPS, 59, ). Hara (2007a) showed that HFER durations well correlate with source times. 4

5 Measurement of HFER duration Observed broadband records 2-4 Hz band-pass filtered time series Smoothed time series Event: 2011 Tohoku, Station: MBWA End of HFER: 25 % of the peak We use one sixth of the difference between P wave arrival time and the time when the amplitude of band-pass filtered time series becomes the largest as the moving window for smoothing. 5

6 Measurement of HFER duration of the 2011 Tohoku earthquake We retrieved broadband waveforms recorded at GSN stations in teleseismic distance range from IRIS DMC. We applied the HFER measurement procedure of Hara (2007a) to these data. 6

7 HFER duration of the 2011 Tohoku earthquake The mean of the measured HFER durations is s. The azimuthal dependence suggests that the rupture that generated strong high frequency signals propagated in the southwest direction. This suggests two distinct rupture propagations; the other is the rupture in the east direction that caused huge tsunamis (e.g., Fujii and Satake, 7

8 Magnitude determination The magnitude estimate from the formula of Hara (2007a) is In terms of HFER duration and maximum amplitudes, the 2011 Tohoku earthquake is not tsunami-earthquake like. 8

9 High Frequency Peak Time Hara (2008, EPS, 60, ) investigated distribution of time difference between P wave arrival time and the time when the amplitude of band-pass filtered time series becomes the largest, which we call the high frequency peak time. We measure high frequency peak times following these steps: 1. Baseline correction; 2. Application of 2-4 Hz high band-pass filter; 3. Square each data points; 4. Detect P-wave arrival by STA/LTA with manual correction; 5. Find the peak of the squared time series within 400 s from the arrival of P-wave. 9

10 Temporal distribution of Normalized High Frequency Peak Times Normalized High = Frequency Peak Time High Frequency Peak time 2 (Centroid timeshift) 100 (%) 2011 Tohoku earthquake Events whose centroid time shifts > 40s (modified from Hara [2008]) 10

11 Conclusion The HFER duration and magnitude of the 2011 Tohoku earthquake are s and 8.96, respectively. The directivity of HFER durations suggest that the rupture that generated strong high frequency signals propagated in the southwest direction. In terms of HFER duration and amplitude, the 2011 Tohoku earthquake is not tsunami-earthquake like. The distribution of normalized high frequency peak times of this event is consistent with that of a set of large earthquakes obtained by Hara (2008). The above distribution implies that HFER is weak in the vicinity of rupture starting areas. 11