# What is Seismic Hazard Analysis?

The goal of Seismic Hazard Analysis (SHA) is to state the probability that something of concern will occur given one or more earthquakes. More specifically, SHA states the probability that an Intensity Measure Type (IMT) will exceed some Intensity Measure Level (IML). For example, one might want to know the probability that Peak Ground Acceleration (the IMT) will exceed 0.5 g (the IML).

### Basic Model Components

**Earthquake Rupture Forecast (ERF)**

This gives all possible Earthquake Ruptures and their associated probabilities of occurrence in a region and over some time span.**Intensity Measure Relationship (IMR)**

This gives the conditional probability that an IMT will exceed an IML at a Site, given the occurrence of a specified Earthquake Rupture.

The components and the probabilistic SHA calculation are depicted graphically in the following:

Note that the definition of an IMR is flexible enough to accommodate both empirical Attenuation Relationships, and more complex models based on, for example, 3D waveform modeling from first principles of physics coupled with nonlinear dynamic structural response calculations. Such generality is an underpinning of OpenSHA, the intent being to accommodate both near- and long-term goals.

A wide variety of ERF and IMR models have been, and will continue to be, developed. This diversity reflects differing opinions about how to best model earthquake processes and structural response. Having a variety of viable models is not a conceptual problem in SHA, as the probabilistic formalism includes explicit ways of dealing with differing expert opinion. In fact, proper SHA *requires* that all viable models be accounted for. This presented a practical problem in the past in that we lacked the computational infrastructure necessary to accommodate the wide variety of models under development. In fact, the level of model sophistication was often constrained by the structure of available SHA codes.

The primary goal of OpenSHA has therefore been to develop an object-oriented infrastructure where any arbitrarily complex model may be implemented. In addition, OpenSHA strives to leverage distributed computing resources that reduce maintenance overhead by authors of complex models and components and offloads processor intensive tasks from local applications.

### Modular Framework

Because the different disciplines contributing to SHA often hold different meanings for the same word, also important is establishing a fixed vocabulary. For example, "earthquake" literally means earth shaking, but to some it refers to fault rupture (as in an "earthquake catalog" where no information about ground shaking is contained). Although humans can understand such difference contextually, computers communicating with each other cannot.

A manuscript describing the OpenSHA object-oriented framework, including object definitions and the mathematical details of the SHA calculations, has been published in Seismological Research Letters (see Field et al., 2003).

One can also find definitions in the OpenSHA Glossary.

### Framework Implementation

### Code Verification

### Advanced Information Technologies

- Use of distributed object technologies, which enable the various components to exist anywhere on the internet. This is exemplified in the SHA calculations of Field et al. (2005a); these technologies are discussed more generally in the companion paper of Maechling et al. (2005).
- Use of GRID Computing, where run times are reduced by distributing the computational burdon among any available, idle computers on the network. This is exemplified in the SHA calculations of Field et al. (2005b), and the technology is discussed in more detail in the companion paper of Maechling et al. (2005b).