Initial 30 seconds of the 2011 off the Pacific coast of Tohoku Earthquake (M w 9.0)—amplitude and τ c for magnitude estimation for Earthquake Early Warning—

Early magnitude and potential damage zone estimates for the great Mw 9 Tohoku‐Oki earthquake

“…This study extends the analysis of Hoshiba and Iwakiri [2011], by overcoming the limitation owing to the use of the closest four accelerometric stations along the coast (at a maximum epicentral distance of 164 km). For such stations the S‐waves are expected to arrive within the selected 30 s time window and their inclusion within the PTW may introduce a significant bias in magnitude estimations because of a different scaling between magnitude and early warning parameters for P‐ and S‐waves [ Zollo et al , 2006].…”

“…For such stations the S‐waves are expected to arrive within the selected 30 s time window and their inclusion within the PTW may introduce a significant bias in magnitude estimations because of a different scaling between magnitude and early warning parameters for P‐ and S‐waves [ Zollo et al , 2006]. Furthermore, the limited distance range analyzed by Hoshiba and Iwakiri [2011] is expected to provide a partial image of the ongoing source process, whose full investigation requires broader time and distance/azimuth ranges. We came to the conclusion that both P d and τ c estimate an earthquake magnitude M > 8 when using large PTWs (>25 s), although uncertainties in magnitude estimation from P d (∼0.3) are significantly smaller than the uncertainties from τ c (∼1.0).…”

“…Here we want to investigate the reliability of existing EW methodologies and empirical regression relations for the Tohoku‐Oki earthquake. For the same earthquake, Hoshiba and Iwakiri [2011] analyzed the initial peak ground motion and the τ c parameter in the first 30 s after the first arrival at four accelerometric stations along the coast. Here we generalize this approach by expanding the analysis to larger PTW (up to 60 s) and epicentral distances (up to 530 km), showing that the early measurements of P‐peak displacement and τ c at the whole Japanese accelerometric network provided relevant insights on the ongoing earthquake rupture process and reliable estimations of the potential damage area.…”

A Fast Magnitude Estimation for the 2011 Mw 9.0 Great Tohoku Earthquake

“…Therefore, using the first few seconds of the waveform from an initial nucleation rupture for estimating the eventual magnitude, which is the concept behind the earthquake early warning (EEW) system, may be difficult for extremely large earthquakes. In the case of the 2011 Tohoku earthquake, the EEW estimated magnitude from the amplitude and frequency content (Hoshiba and Iwakiri, 2011) is smaller than the reported magnitude of M w 9.0. Our results indicate that the effective shaking ( Es p ) method of Wu and Teng (2004) provides an excellent magnitude estimation for the 2011 Tohoku earthquake, without a saturation problem.…”

Outline of the 2011 off the Pacific coast of Tohoku Earthquake (M w 9.0) —Earthquake Early Warning and observed seismic intensity—

“…The first EEW "forecast", the first of 15 announcements, was issued 5.4 s later. The magnitude at the time was estimated to be 4.3, because the waveform started with small amplitude, which was comparable to noise level for displacement (Hoshiba and Iwakiri, 2011). The small amplitude does not indicate that the initial rupture of the M w 9.0 event is large, and does not suggest a large magnitude event.…”