A critique of probabilistic versus deterministic seismic hazard analysis with special reference to the New Madrid seismic zone

Scientific understanding of earthquakes in the New Madrid seismic zone has advanced greatly in recent years, but these advances have resulted in neither better assessment of seismic hazard and risk nor better mitigation policy. The main reasons for this are (1) misunderstanding about the National Seismic Hazard Maps and (2) confusion about seismic hazard and risk. Seismic hazard and seismic risk are two fundamentally different concepts, even though they have often been used interchangeably. Both are used differently in policy decision making, but seismic risk is the deciding factor, not seismic hazard. Even though the input parameters are scientifically sound, we contend that the National Seismic Hazard Maps produced for the New Madrid region are flawed because they were produced from probabilistic seismic hazard analysis (PSHA). PSHA is scientifically flawed: As a complex computer model, it could not pass a simple sensitivity test with a single input earthquake, and the annual probability of exceedance (i.e., exceedance probability in one year and a dimensionless quantity) has been erroneously interpreted and used as the annual frequency or rate of exceedance (i.e., the number of event exceedances per year and a dimensional quantity). Thus, the seismic hazard and resulting seismic risk estimates from PSHA can be viewed as artifacts, and the mitigation policies developed, the NEHRP (National Earthquake Hazards Reduction Program) provisions and resulting building codes in particular, are problematic. Scenario seismic hazard analysis is a more appropriate approach for seismic hazard assessment, seismic risk assessment, as well as policy development in the New Madrid region.