Earthquake Rate Models

An Earthquake Rate Model gives the long-term rate of all possible damaging earthquakes (magnitude, rupture surface, and average rake) throughout the entire region. By "damaging" we mean above some magnitude threshold (typically M>5) and by "all possible" we mean at some level of discretization that is sufficient for hazard and loss estimation. Although a tectonic region may evolve (with old faults healing and new faults being created), the concept of a long-term model is legitimate in that there will be some absolute rate of ruptures over any given time span. By "long" we simply mean long enough to capture the statistics of events relevant to the forecast duration, and short enough that the system does not significantly change. An Earthquake Rate Model generally has two components:

1. Fault-Based Earthquake Rate Model:
   This gives the rate of events on larger, well characterized faults. These models are generally constructed by: a) making assumptions on the spatial extent of rupture surfaces (e.g., segmentation) and using magnitude-area relationships to define magnitudes; b) assuming a magnitude-frequency distribution for the fault and using magnitude-area relationships to define possible rupture surfaces; or c) a combination of these two. The long-term rate of each earthquake rupture is constrained by the known fault slip rate (moment balancing).
2. Background-Seismicity Model:
   This accounts for events that occur elsewhere (off the larger, well characterized faults). These models generally provide Gutenberg-Richter magnitude-frequency distribution at each point on a spatial grid (with typical grid spacing of 5 to 10 km). Technically speaking, this model constitutes a hypocenter forecast in that finite rupture surfaces are not provided (something SHA calculations need to account for). Combing these two sub-components gives a complete Earthquake Rate Model, which obviously should be moment balance with respect to the deformation model and consistent with observed seismicity rates. Listed below are the versions that are available, under development, or envisioned.

Earthquake Rate Model 1

Status: Available

This model is that used in UCERF 1, which is a slight modification of that used in the 2002 USGS National Seismic Hazard Maps for California (the modification being an added rupture model ("model 3") for the southern San Andreas Fault).

In short, Type A Faults were segmented and ruptures were sometimes allowed to include more than one segment, and Type B Faults were assumed to have 67% moment release in full-fault (characteristic) ruptures and 33% moment as Gutenberg-Richter events (with a lower magnitude of 6.5 and the upper magnitude defined by the full-fault rupture). These faults and slip rates were based on Deformation Model 1 (which is Fault Section Database 1) Background seismicity was based on smoothed historical seismicity (Historical California Earthquake Catalog 1).

For details, please see the UCERF 1 Report and/or the 2002 USGS National Seismic Hazard Map documentation.

Earthquake Rate Model 2

Status: Available

Full documentation is available in the UCERF2 Report and related appendices.

Earthquake Rate Model 3

Status: In development

This is a more experimental model that attempts to relax segmentation and allow fault-to-fault jumps. The idea is to divide all faults into ~5km sections, define all possible combination of ruptures (including fault-to-fault jumps), and constraint the rate of each by satisfying fault slip rates, a regional Gutenberg-Richter constraint, historical seismicity, paleoseismic data, and whatever other information exists. The problem will be posed as a formal inversion that can be resolved whenever new constraints are available. The solution space will define the range of viable models.

Please see the UCERF 3 Project Plan for more information.

Future Earthquake Rate Models

Status: Envisioned

Incorporate earthquake rates derived from physics-based earthquake simulators?