Graham, K.M.; Gerstenberger, M.C.; Rhoades, D.A.; Christophersen, A. 2024 Time-dependent seismic hazard assessment: current practises and future directions. Lower Hutt, N.Z.: GNS Science. GNS Science report 2024/03. 56 p.; doi: 10.21420/H4YG-KN82
Abstract
The ability to successfully forecast the future behavior of a system is a strong indication that the system is well understood. Certainly, many details of the earthquake forecasting system remain obscure, but several hypotheses related to earthquake occurrence and seismic hazard have been offered. Here, we review the concept and recent progress on the study of time-dependent seismic hazard and risk assessment. Probabilistic seismic hazard assessment (PSHA) provides the conceptual framework to quantify the rate (or likelihood) of exceeding various ground-motion levels for a spatial grid within a given time span given all possible earthquakes. For decades, PSHA has been utilized by governments and industry for making official national hazard maps, developing building code requirements, determining earthquake insurance rates and even for deciding safety criteria for large infrastructure such as nuclear power plants and hydroelectric dams. PSHA in its classical formulation adopts the removal of foreshocks and aftershocks from earthquake catalogues to separate dependent and independent seismic activity. In so doing, it neglects some events that can generate damaging ground motions, resulting in underestimation of the seismic hazard. Hence, there is a need for time-dependent PSHA. Time-dependent applications can be classified into two groups: long-term models based on renewal statistics and short- and medium-term models based on clustering statistics. The latter honors the fact that earthquake occurrence is not stationary by including spatiotemporal clustering (e.g. aftershock sequences and precursory seismicity). Combining seismic hazard with different spatiotemporal scales is a practical way to achieve a reduction in the exposure (e.g. through the medium- to long-term planning of land use) or the vulnerability (e.g. through short-term structural reinforcement or assessment). Thus, in terms of reducing seismic disaster risk and answering the increasing needs of social sustainability (toward earthquake resilience), integrating a fully time-dependent PSHA is warranted. However, the current state of the art is under-reported in the literature. It is important to note that achieving this integration is not just about our implementation; it also necessitates a substantial upskilling on the part of end-users. This report underscores the critical need for incorporating time-dependent seismic hazard analysis and risk assessment into current PSHA and risk assessment methodologies, an area currently not sufficiently addressed.
