OpenQuake’s Seismic Hazard Engine Component
The OpenQuake hazard component (OQ-H) leverages from OpenSHA – an open source, Java-based platform for conducting Seismic Hazard Analysis (SHA).
OQ-H computes hazard following two different approaches: one termed Classical Probabilistic Seismic Hazard Analysis (PSHA) and a second one based on the stochastic simulation of earthquake activity and of the shaking produced by each event.
OQ-H takes as an input a seismic source model (i.e. a set of sources described in terms of their geometry and capability to generate earthquakes) and a set of ground-motion prediction equations (GMPEs – empirical equations that given – at least – a magnitude and a rupture-site distance compute the value of a parameter representative of ground shaking intensity). Seismic sources are described using a standardized description that currently includes four different typologies:
- Area source: a geographical region with uniform seismicity parameters;
- Point source: a single geographical location of concentrated seismicity;
- Simple fault source: rectangular 3D surface allowing simulation of finite earthquake ruptures;
- Complex fault source: arbitrarily shaped 3D surface allowing simulation of finite earthquake ruptures.
Epistemic uncertainties related to the seismic source and GMPE models (i.e. uncertainties due to limited data or knowledge) are described by means of logic trees. A logic tree allows to consider multiple plausible source/GMPE models and also specific parameter uncertainties.
Further extensions of OQ-H will include:
- The development of a disaggregation calculator i.e. a calculator computing contributions to a given value of hazard in one site for discrete combinations of basic parameters considered in the hazard calculation (e.g. magnitude, rupture-site distance);
- The extension of the standardized description of seismic sources with new source types and additional description parameters;
- A toolbox for the preparation of PSHA input models. The toolbox will contain – for example – applications for plotting inputs and outputs, applications for processing basic information in order to derive information commonly used for the definition of PSHA input models (e.g. parameters characterizing a frequency-magnitude distribution) and, tools for studying the sensitivity of results to the structure and weights characterizing the logic trees.
The main results provided by OQ-H are:
- Hazard curves (curves providing probabilities of exceedance in a given time span for given values of a ground motion parameter);
- Hazard maps (maps describing the geographic distribution of values of a ground motion parameter with a fixed probability of exceedence in a given time span);
- Uniform hazard spectra (spectral acceleration/velocity/displacement values associated with different ground motion periods for a given probability of exceedence in a specified time span);
- Stochastic Event Set (a set of earthquake ruptures in a given time span obtained through random sampling of a seismic source model);
- Ground Motion Fields Set (each ground motion field describes the ground motion shaking values for a set of sites computed considering an earthquake rupture and a GMPE);
- Magnitude-Distance-epsilon and geographic disaggregation.