Evidence for anomalous earthquake size distributions in regions of minimal strain

Abstract

Some regions have no earthquakes above magnitude ∼5.5, and thus apparently contain only small active faults, with length less than the thickness of the brittle upper crust. Britain and SSW Turkey are examples, with strain rates ∼3×10−20 and ∼3×10−17 s−1. Their seismicity has Gutenberg‐Richter parameter b ∼1.5, larger than ∼1 typical elsewhere, sufficient for equal strain to be associated with each order‐of‐magnitude of fault length present. These regions deform by increasing lengths and number densities of small faults. When strain reaches ∼0.01, these faults will be so closely‐spaced that they may interact, forming large faults that cut the brittle layer and take up most later deformation. Deformation involving only small faults will last <1 Myr in regions with strain rate ∼10−15 s−1, and is unlikely to be observed.