Fault Sources

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Fault sources are modeled as multi-planar features with earthquake ruptures distributed over the fault plane. The input parameters required to model a fault in a Seismic Hazard Analysis (SHA) are as follows:

• Fault Geometry
• Mechanism (Normal, Reverse, Strike-Slip)
• Magnitude Distribution (required for Probabilistic Seismic Hazard Analysis only)
• Activity Rate (required for Probabilistic Seismic Hazard Analysis only)

Fault Geometry

The fault geometry includes the coordinates in map view, cross-section (depth), width, and the dip.

Mechanism

Each fault should have a specified mechanism: normal, reverse (thrust) or strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as divergent boundaries. Reverse (thrust) faults occur in areas where the crust is being shortened for example at convergent boundary. Strike-slip faults are normally steep structures where the two sides of the fault slip horizontally past each other; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip.

Magnitude Distributions

The magnitude distribution is a probability density function (pdf) that describes the relative number of large magnitude, moderate and small magnitude earthquakes that occur on a seismic source. There are two forms of the magnitude density functions that are considered in PSHA. The functions are a truncated exponential model and the characteristic model. Alternate models are composites of the truncated exponential and the characteristic. One important calculation for the magnitude distributions is the characteristic magnitude.

Crustal faults are normally modeled as both characteristic and truncated exponential; characteristic events are normally assigned a probability of 0.7 and the exponential model a probability of 0.3. The weighting is set as such to balance out the two different models. The truncated exponential distribution predicts a higher ratio of lower magnitudes to higher magnitudes than is observed on a single fault. In contrast, the characteristic model, in its most simple application, predicts fewer earthquakes on a fault as compared to the exponential model.

Characteristic Magnitude

Estimation of the characteristic magnitude is usually computed based on the fault dimensions (length or area). The mean characteristic magnitude is estimated using the source scaling relations for either the fault length or fault area.

Activity Rate

There are two common approaches for estimating activity rates on seismic sources: historical seismicity and geologic information.

Examples

Best Practices Resources

Trainings

Citations:

Revision ID: 3827
Revision Date: 09/30/2022