Fault zone fluids and seismicity in compressional and extensional environmen inferred from electrical conductivity: the New Zealand Southern Alps and U. S. Great Basin

Abstract

Seismicity in both compressional and extensional settings is a function of local and regional stresses, rheological contrasts, and the distribution of fluids. The influence of these factors can be illustrated through their effects on electrical geophysical structure, since this structure reflects fluid composition, porosity, interconnection and pathways. In the compressional, amagmatic New Zealand South Island, magnetotelluric (MT) data imply a concave-upward ("U"-shaped), middle to lower crustal conductive zone beneath the west-central portion of the island due to fluids generated from prograde metamorphism within a thickening crust. Change of the conductor to near-vertical orientation at middle-upper crustal depths is interpreted to occur as fluids cross the brittle-ductile transition during uplift, and approach the surface through induced hydrofractures. The central South Island is relatively weak in seismicity compared to its more subduction-related northern and southern ends, and the production of deep crustal fluids through metamorphism may promote slip before high stresses are built up. The deep crustal conductivity is highly anisotropic, with the greater conductivity along strike, consistent with fault zone models of long-range interconnection versus degree of deformation. The central Great Basin province of the western U.S. by contrast is extensional at present although it has experienced diverse tectonic events throughout the Paleozoic. MT profiling throughout the province reveals a quasi one-dimensional conductor spanning the lower half of the crust which is interpreted to reflect high temperature fluids and perhaps melting caused ultimately by exsolution from crystallizing underplated basalts. The brittle, upper half of the crust is generally resistive, but also characterized by numerous steep, narrow conductors extending from near-surface to the middle crust where they contact the deep crustal conductive layer. These are suggested to represent fluidised/altered fault zones, with at least some fluids contributed from the deeper magmatic exsolution. The best-known faults imaged geophysically before this have been the listric normal faults bounding graben sediments as imaged by reflection seismology. However, the major damaging earthquakes of the Great Basin appear to nucleate near mid-crustal depths on near-vertical fault planes, which we suggest are being imaged with the MT transect data, and where triggering fluids from the ductile lower crust are available. In both compressional and extensional examples, the fluidised fault zones are hypothesized to act to concentrate slip, with major earthquakes resulting in asperities along the fault surface.

Resistivity structure around the focal area of the 2004 Rumoi-Nanbu earthquake (M 6.1), northern Hokkaido, Japan

“…For example, the observed low resistivity images along the faults could be explained as the existence of an abundant quantity of fluid in cracks or fault breccias (e.g., Unsworth et al, 1997;Hoffmann-Rothe et al, 2004). Recent magnetotelluric (MT) investigations have also revealed low resistivity zones distributed in the mid to lower crust beneath the intraplate earthquake zones (e.g., Mitsuhata et al, 2001;Ogawa et al, 2001;Ogawa and Honkura, 2004;Wannamaker et al, 2004;Uyeshima et al, 2005;Yoshimura et al, 2008). Those low resistivity zones are interpreted as fluid reservoirs from which the fluid is supplied, triggering the intraplate earthquake in the upper layer (e.g., Iio et al, 2002).…”

Electrical conductivity of a locked fault: investigation of the Ganos segment of the North Anatolian Fault using three-dimensional magnetotellurics

“…This is due to the reduction of the effective normal stress or rheological alteration at the boundary by fluid migration from conductive zones through resistive blocks (Wannamaker et al 2004). Local earthquakes are distributed just north of the GF; some outliers can be found south of our study area.…”

“…Another way of examining the dimensionality of MT data is to perform magnetotelluric phase tensor analyses developed by Caldwell et al (2004). Phase tensor analyses have the advantage of providing distortion-free dimensionality information.…”

“…Graphical representations of phase tensors are given by ellipses; the major axis represents the polarization direction, and β values (i.e., skew angles) provide essential dimensionality information (Caldwell et al 2004). Extreme colors (dark red and dark blue) in these ellipses represent higher and lower β values as indicator of threedimensionality.…”

Magnetotelluric, Basin Structure and Hydrodynamics; South West of Western Australia

“…Several studies indicate a separation between TE and TM mode in both apparent resistivity and phase at low frequencies in proximity to the coast line (Pous et al, 2002;Lezaeta & Haak, 2003). Large scale faults also impact magnetotelluric data and inversion, and similarly many MT studies have investigated the influence of these structures (Unsworth & Bedrosian, 2004;Bedrosian et al, 2002;Wannamaker et al, 2004;Karaş et al, 2017). In most of these studies, MT data was collected across the fault zone.…”