Abstract
Earthquakes are caused by the release of tectonic strain accumulated between events. Recent advances in satellite geodesy mean we can now measure this interseismic strain accumulation with a high degree of accuracy. But it remains unclear how to interpret short-term geodetic observations, measured over decades, when estimating the seismic hazard of faults accumulating strain over centuries. Here, we show that strain accumulation rates calculated from geodetic measurements around a major transform fault are constant for its entire 250-year interseismic period, except in the ~10 years following an earthquake. The shear strain rate history requires a weak fault zone embedded within a strong lower crust with viscosity greater than ~1020 Pa s. The results support the notion that short-term geodetic observations can directly contribute to long-term seismic hazard assessment and suggest that lower-crustal viscosities derived from postseismic studies are not representative of the lower crust at all spatial and temporal scales.
Highlights
Earthquakes are caused by the release of tectonic strain accumulated between events
One of the primary inputs into any probabilistic seismic hazard assessment (PSHA) model is a catalogue of earthquake sources that have occurred in the past[1, 2]
In 2-layer linear Maxwell viscoelastic crustal models of the earthquake deformation cycle, which are commonly used to interpret interseismic deformation, strain rate varies as a function of time between earthquakes[8,9,10], with the shear strain rate in the fault zone decreasing with time since the earthquake
Summary
Earthquakes are caused by the release of tectonic strain accumulated between events. Recent advances in satellite geodesy mean we can measure this interseismic strain accumulation with a high degree of accuracy. The surface strain rate at the fault is proportional to the slip rate and inversely proportional to the locking depth (see Methods section for details).
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