Preparing for the Big One

Abstract

Approximately 2.75 million deaths have occurred in 3000 earthquakes in the last 105 years between 1900 and 2004 (Figure 1A). About one-half of these occurred in the seven deadliest events, i.e., a few events dominate historical death count. These events did not necessarily have large magnitudes, but occurred close to heavily populated regions. If these not-so-large earthquakes could cause such destruction, one can only imagine what would happen if an extreme event were to occur. An extreme event can be defined as one of large magnitude occurring in the proximity of a densely populated region. Extreme events are rare because large magnitude events are rare. Shown in Figure 1B is the Gutenberg-Richter relation for all earthquakes that have occurred between 1904 and 2000 (Kanamori and Brodsky 2001). In these 96 years, fewer than one magnitude 8.0 earthquake has occurred on average each year. Traditionally, civil engineers have adopted an observe, learn, and improve approach for earthquake damage mitigation. Unfortunately, with extreme events being rare, the learning process is slow and, as a result, corrective measures are ineffective. In fact, we have not seen the effects of a large magnitude earthquake occurring close to heavily populated urban regions such as Los Angeles, Seattle, Istanbul, Jakarta, Tokyo, Taipei, Kaosiung, Delhi, Mumbai, Calcutta, Beijing, etc. in recent years. The recent magnitude 6.7, January 17, 1994, Northridge earthquake, the magnitude 6.9, January 17, 1995, Kobe earthquake, the magnitude 7.4, August 17, 1999, Kocaeli earthquake, and the magnitude 7.7, September 21, 1999, ChiChi earthquake have provided us with glimpses of what we can expect from a major earthquake. But the data from magnitude 8 earthquakes in urban settings is quite limited. Although the magnitude 8.0, September 19, 1985, Michoacan earthquake killed 10000 people and caused significant damage in Mexico City, it was centered more than 360 km away from Mexico City. Both the magnitude 9.5, May 22, 1960, Chile and the magnitude 9.2, March 28, 1964, Prince William Sound, Alaska earthquakes occurred close to sparsely populated regions. The magnitude 7.8, July 28, 1976, Great Tangshan earthquake, the magnitude 8.3, September 1, 1923, Great Kanto earthquake, and the magnitude 7.7, April 18, 1906, San Francisco earthquake provide the best clue to what could be expected from a large earthquake close to an urban center. The fires following the 1923 and 1906 earthquakes destroyed the cities of Tokyo and San Francisco, respectively, although quite a bit of damage can be attributed to ground shaking as well. Ninety percent of the buildings in the city of Tangshan were flattened in the 1976 earthquake. Unfortunately, recorded data from these earthquakes is minimal. As a result, if we are to prepare for an extreme earthquake striking one of our major metropolitan centers, we cannot rely solely on the traditional approach of learning from observations.

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