# Earthquake Hazard Assessment

A seismic hazard assessment is essentially an effort to predict the potential ground shaking intensity from future earthquakes.

While the first seismic hazard assessments were deterministic, the common approach now is *Probabilistic Seismic Hazard Analysis* (PSHA), in which the expected ground motions at a given site are evaluated for different probability levels. PSHA requires at its core four steps as exemplified in the figure below: 1) Definition of the *earthquake sources*; 2) Definition of a *recurrence model* (expressed by frequency of occurrence vs. magnitude) for each source; 3) Defining *ground motion attenuation* as function of distance and frequency, and 4) Computation of ground shaking intensity versus probability of exceedance (the *hazard* curve).

*The fundamental steps of a PSHA leading to the hazard curve.*

*Examples of response spectra for a suite of return periods (reciprocal of probability of exceedance).*

In case that individual hazard curves are computed for a suite of spectral ground motion periods but for the very same probability of exceedance a so-called *equal* *hazard spectrum* is generated. This spectrum is an artificially created *response spectrum *which represents the expected ground motion intensity levels versus the spectral period (representing the fundamental period of various type of structures. Response spectra, in combination with ground shaking intensity maps, generally establish the basis for earthquake resistant design.

*Hazard maps for Norway from NORSAR and NGI (1998), which are today used in EC8 (NS-EN 1998-1:2014) and distributed through Standard Norway (www.standard.no).*

A PSHA computation is based on a mathematical model that attempts to resemble the empirical knowledge. The mathematical approximations established through seismotectonic interpretations comprise significant *uncertainties*, which are contained within the probabilistic computation models. *Epistemic* uncertainties relate to our incomplete knowledge, while *aleatory* uncertainties relate to the apparent randomness of the earthquake processes.

Nowadays, most countries have building regulations that consider earthquake shaking as a load that any structure should withstand, and with stricter regulations (i.e., lower annual probabilities accepted) for sensitive structures (hospitals, power stations etc.). In Norway, the Eurocode 8 provision applies through NS-EN 1998-1:2014. The strictest safety requirements of all apply to *nuclear installations*, which are designed to withstand ground shaking intensities with very low *exceedance probabilities*. Special criteria therefore apply to the seismic hazard methodology for such installations.