r - The Hong Kong Polytechnic University

Problem with Seismic Hazard Analysis
Actually, the 2011 Tohoku earthquake is not the sole example that defies the results of our current adopted
seismic hazard analysis. In fact, most of the notable earthquakes occurred in the last 30 years had repeatedly
illustrated that our seismic hazard analysis is inadequate. For example, the Chelungpu fault before the 1999
Mw 7.6 Chi-Chi Earthquake is largely inactive. So, it came as a surprise in Taiwan when it killed 2415 people
on September 21, 1999. The largest historical earthquake induced by the Awaji fault is M=6 in 1916, yet it
produced the 1995 7.2 Kobe Earthquake that killed 5,094. Kanamori (1995) reported that the Awaji fault,
which generated the 1995 Kobe earthquake, is not included in the 1994 review of potential seismic hazard the
Kinki district by the “Committee of Earthquake Observation and Research” in the Kansai Area. Therefore,
1995 kobe Earthquake also came as a surprise. The more recent 2008 Wenchuan earthquake occurred on the
Longmenshan fault, at which the largest historical earthquake is the 1657 April 21 Wenchuan Earthquake (M =
6.5) (Chau, 2009). The estimated seismic hazard level by the China Earthquake Administration at Yingxiu and
Beichuan (the worst hit towns) before the 2008 Wenchuan earthquake is 0.1g which is even lower than 0.15g of
Hong Kong. The 2010 Mw 6.9 Yushu Earthquake happened on a segment of the Ganzi-Yushu fault with a
largest historical earthquake of 6.5 (Chau, 2010b; Zhou et al., 1997). I think it is obvious that we have major
problem with our current terminology in estimating seismic hazard. Probably, it is time to revolutionize the
idea of Cornell’s (1968) method.
UNCERTAINTY IN ESTIMATING THE FOCAL MECHANISM
The focal mechanism of the huge 2011 Tohoku earthquake has been studied by various researchers. Figure 2
compiled six different versions of the focal mechanism. These versions were proposed by: (a) USGS (United
States Geological Survey); (b) NIED (National Institute for Earth Science and Disaster Prevention); (c) Caltech
(California Institute of Technology); (d) Nagoya University; (e) Tsukuba University; and (f) GFZ (German
Research Center for Geosciences). The activated fault plane is normally interpreted from the distribution of
the aftershocks. Therefore, the fault plane is rather consistent amount these different models. However, the
slip distributions as well as the maximum slips are substantially different. The maximum slips are 18m, 25m,
23m, 10m, 30m, and 30m for the models by USGS, NIED, Caltech, Nagoya University, Tsukuba University,
and GFZ respectively. Clearly, they are completely different. Except for the NIED model, all these back
analyses were based on the far field velocity data observed at seismic stations thousands of kilometres from the
epicentre. The locations and numbers of seismic stations used in back analyses vary from one model to the
other. The sensitivity of slip distribution is clearly not high from far field data. Unfortunately, seismologists
and geophysicists did not seek for a better back analysis technique. To be fair, most of their results are
questionable since all of them fit real data well and it also means that none of them are really reliable.
(a)
(c)
(e)
(b)
(d)
(f)
Figure 2 Focal mechanisms in terms of slip distribution given by (a) USGS, (b) NIED, (c) Caltech, (d) Nagoya
University, (e) Tsukuba University, (f) GFZ.
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