Spatiotemporal behavior of active faults in diffuse continental deformation zones

Abstract:

The present study is a compilation of neotectonic data offering an unprecedented view into spatial and temporal patterns of seismic strain release in diffusely deforming crust, at timescales of 1 kyr to 1 Myr. We compiled paleoseismologic and Quaternary geology data from 159 published studies, for some 167 faults across the Great basin region (Western US), an archetype for diffuse deformation of the continental crust. Our database is accessible online through an interactive GIS interface and an Excel file.

Results from the database indicate a mean vertical displacement for earthquakes on Basin and Range normal faults of 2 m. The distribution of earthquake recurrence intervals is far more scattered, with a mode value of 1-3 kyr and a mean value of ~11 kyr. A significant fraction of recurrence intervals (24%) are >10 kyr, and about half that (12%) are >20 kyr. Displacement per event and recurrence interval seems not clearly correlated. Finally, in 2/3 of the measurements, the difference in vertical displacement with a previous event on the same fault is < ±1 m.

We then investigate the spatial and temporal variations of displacement rates over varying time windows during the Quaternary. Over the last 15 ka, the deformation has been concentrated along the edges of the province (Wasatch front, central Nevada seismic belt), the central part of the Great Basin being almost inactive. However, the deformation is evenly distributed in the overall region when considering a 150 ka time window, with homogeneous finite displacement rates of ~0.2-0.3 mm/yr on the faults. Spatial paleoearthquakes kinematics is not random: we can distinguish "local clusters" (episode of events on a single fault) from "regional clusters" (episode of events, distributed on several faults, each with a single event). We thus propose a general model where any given fault alternates between: (1) fast transient displacement rate episodes (1-2 mm/yr), lasting ~20-50 kyr, associated to “local clusters”; (2) slow transient displacement rate episode (0.05-0.1 mm/yr), lasting up to 400 kyr, associated to “regional clusters”. More speculatively, we comment on possible “local clusters” migrations and connections between clustering and climatic forcing.