Deep Continental Crustal Earthquakes and Lithospheric Structure: A Global Synthesis

The distribution of earthquake depths within the continental crust defines the seismogenic thickness (TS), over which at least some part of crustal deformation is accommodated by rapid release of stored elastic strains. Intraplate continental seismicity is often thought to be restricted to the upper crust where TS is within the range of 15 to 20 km. This appears consistent with a lithospheric strength profile involving a weak, ductile lower crust located beneath a stronger, brittle upper crust. With the assumption of a strong uppermost mantle lid, this is often referred to the Jelly Sandwich model of lithosphere rheology. Studies in many places, however, document lower crustal earthquakes beneath continents in apparent disagreement with the model. We explore this and related issues through a survey of where and in what tectonic settings deep intraplate earthquakes are well documented in the continental crust. TS reaches Moho depth in many intraplate regions \--- Sierra Nevada, Colorado Plateau, East African and Baikal Rift Systems, North Island New Zealand, Tien Shan, and the Andean and Alpine forelands. A review of possible deformation mechanisms which could control continental earthquake depth and facilitate seismicity beneath the brittle-ductile transition suggests that the influence of fluids is the only mechanism capable of encouraging earthquake occurrence throughout the continental crust at any tectonic setting. Surface derived fluids can induce pore fluid pressure changes to depths of 25 km and melt-reactions can induce earthquakes at depths throughout continental crust. On a global scale, fluid-enhanced embrittlement is not limited by depth or tectonic environment. We find that deep crustal earthquakes occur where the lithosphere is in a transitional state between primarily stable (e.g., shields) and highly deformed (e.g., U.S. Basin and Range or Southern California). Observations of relative intensity of tectonic deformation and regional percent strain measurements indicate that lower crustal earthquakes exist within continental regions experiencing youthful (generally Neogene to present) tectonism typically with < 15 % strain. We propose a model that describes relative continental deformation and corresponding seismogenic thickness. In stable seismogenic regions, earthquakes are limited to depths where near-surface derived fluids can induce activity and therefore TS < 25 km. As continental lithosphere passes into an intermediate state of deformation, fluids from the near surface and melts formed in situ or derived from the mantle can act to seismically activate the entire continental crust. As deformation continues, percent strains can reach 100 % or more, mantle lithosphere is typically thinned or absent, and the lower crust via heating and/or sufficient weakening becomes aseismic.