Seismic hazard analysis in a historical context: experience at caltrans and elsewhere

Abstract I entered my profession (1972–2005) in California in seismic hazard determination, which started in earnest after the destructive 1971 San Fernando earthquake. I have been fortunate to survive my work of more than 3 decades with the satisfaction of public service as well as passing through the oppositions to making and applying the state seismic hazard map of California twice. Determining the largest earthquakes on seismogenic faults based on its spatial dimension, with no regard to recurrence or probability, and estimating the anticipated ground motion hazard at a site from those largest earthquakes are the two components of seismic hazard determination. The earthquake is called a maximum credible earthquake (MCE) in magnitude. For earthquake-resistant design, the highest ground motion at a site from one or more of the MCEs is used. The ground motion is estimated by using empirical attenuation curves or calculated by using assumed earthquake source representation and seismic waves propagation, such as that applied in Neo-Deterministic Seismic Hazard Analysis (NDSHA). The deterministic approach has been easily understood and well accepted with confidence by engineers in California for decades for almost all applications. It has never been opposed as too extreme, because it is not so, and is reasonable for the intended purposes. Years later, that method has been called the deterministic method by those who promoted the use of likelihood or other probabilistic measure such as in weather forecasting. Failure in weather forecasting is well known but the consequences are not devastating. Failure in earthquakes could be deadly and therefore not acceptable. Likelihood of earthquake will trivialize earthquake hazards. More importantly, buildings based on such likelihood cannot be easily modified if the likelihood becomes known to be higher. Likelihood must not be used for earthquake policy concerning public safety. The hazards of recent destructive earthquakes exceeded those presented in probabilistic hazard maps with deadly consequences but would not exceed deterministic estimates (e.g., International Seismic Safety Organization, 2012). Despite this fact, most people in this field today seem to know only the probabilistic approach as the only way to define seismic hazards. For safety and peace of mind, the profession must know and use deterministic hazards that will not be exceeded by future events. My experience has shown why and how the probabilistic approach has become so dominant despite its continuing failures. Writing this unconventional article forced me to recall my experience about how this had been started in California and was exported out all over the world. I am still in the never-ending discovery stage of what really had happened. It deserves a serious investigation. The deterministic approach would be the best option for seismic hazard definition for public safety policy because it is transparent, easy to understand, economic, and realistic for engineering; and is conservative for public safety as the hazard would not be exceeded. It has a proven track record in California. It can incorporate new information, advanced earthquake source representations, and seismic wave propagation for any applications. With its simple but sound framework and minimum analysis, it has never been proven wrong or inappropriate for the intended purposes.