Coupling Crustal Seismicity to Crustal Permeability – Power-law Spatial Correlation for EGS-induced and Hydrothermal Seismicity

Abstract

Direct association of induced seismicity with crustal fluid flow properties has long been assumed, particularly for active crustal fault systems, but for a variety of reasons it has been difficult to describe the relationship in terms of subsurface properties. We present microseismicity evidence that, at least in the absence of active faulting, this relationship can be understood in terms of the empirics of power-law scaling porosity-related crustal properties in a critically-strained brittle crust. In summary, the observed power-law scaling two-point spatial-correlation property of microearthquake locations – e.g., induced-seismicity “clouds” – can be directly related to fluctuations in crustal permeability controlled by power-law scaling spatial fluctuations in crustal porosity. Evidence for such a relation comes from seismicity observed at two geothermal developments: (1) fluid injection at 6.5 km depth at a Finnish EGS site, and (2) natural seismicity at 3 km depth in an Indonesian geothermal field. Microearthquake locations from both sites show power-law scaling two-point correlation distributions in event separation distance r, Γeq(r) ~ 1/r^n, n ~ ½. The observed microseismicity spatial correlation systematics follow from a trio of empirical properties expressed in terms of spatial frequency k, and crustal porosity φ and permeability K: • Crustal porosity fluctuation power scales inversely as a power-law in k, Pφ(k) ~ 1/k; • Crustal permeability is closely associated with crustal porosity, K ~ exp(αφ), where mean value of the exponent has value ~ 3-4; • In stationary random systems spatial correlation distributions Γ(r) relate to spectral fluctuation power distributions via Fourier transformation, P(k) ~ ∫exp(ikr)Γ(r)dr (Wiener-Khinchin theorem). Numerical simulations of permeability K for a range of porosity spatial correlations Pφ(k) ~ 1/k^m, 0 less than m