Measuring Tectonic Velocity
Every active fault has a speed — the rate at which its two sides slide past each other, measured in millimeters per year. This slip rate is the master parameter of earthquake hazard: it determines how fast stress accumulates, how frequently earthquakes must occur, and the total seismic moment budget available for release. Slip rates range from 0.01 mm/yr on slow intraplate faults to 50+ mm/yr on the fastest plate boundary faults.
Geologic Measurement
The classic method identifies a datable feature — a stream channel, glacial moraine, lava flow, or alluvial terrace riser — that has been offset by cumulative fault slip. Measuring the offset and dividing by the feature's age yields the average slip rate over that time span. This method captures thousands to millions of years of fault behavior, averaging over many earthquake cycles, and provides the most reliable long-term rate.
Geodetic Comparison
GPS stations on either side of a fault measure present-day relative motion with sub-millimeter precision. Geodetic rates capture the current strain accumulation but may be biased by the earthquake cycle: elastic strain builds between events, then releases during earthquakes. If the GPS record is short relative to the earthquake cycle, the measured rate may over- or under-estimate the long-term geologic rate.
From Rate to Hazard
Slip rate connects directly to seismic hazard through the moment budget. The total seismic moment rate on a fault equals shear modulus times slip rate times fault area. This moment must be released by earthquakes: either as frequent moderate events or rare great ones. Combined with paleoseismic recurrence data, slip rates form the backbone of probabilistic seismic hazard analysis. This simulation lets you explore how offset measurements, dating, and fault geometry determine the slip rate and its implications.