Shallow and deep crustal conductivity studies in the Miramichi earthquake zone, New Brunswick

Abstract

The occurrence in early 1982 of four earthquakes in north-central New Brunswick with magnitudes ranging from 5.0 to 5.7 mb prompted detailed electromagnetic surveys of the epicentral region. Scalar audiomagnetotelluric (AMT) measurements, spaced at 100 m intervals or less, along a 7.5 km east–west profile located two conductors but did not find anomalies that could be associated with the proposed fault plane defined by the earthquake hypocentres.The 150 AMT measurements, combined with broad-band tensor soundings at 11 sites in a confined region (6 km × 7.5 km), provided an opportunity to study the distorting effects of near-surface anomalies and also to determine the regional conductivity structure. The apparent resistivity and phase curves from all tensor stations, calculated in a common coordinate system, were remarkably similar (except for static shift of the apparent resistivity curves) to those derived from the rotationally invariant Berdichevsky determinant averages. These averages appear to be very effective for deriving a first-order estimate of the conductivity structure in areas for which near-surface anomalies are a problem.A geometric mean of the AMT measurements was used to estimate the correct level for the high-frequency asymptotes of the tensor apparent resistivity curves. A one-dimensional inversion of the tensor magnetotelluric (MT) data, with the apparent resistivity shifted to the AMT average, yields a four-layer electrical conductivity model for the crust, with depths from the surface of 2.4, 19, and 32 km and with resistivities of 10 000, >100 000, 10 000, and 300 Ω∙m. The crustal resistivities in the Miramichi region are considerably larger than those in other regions in eastern North America but are typical for the Precambrian Shield. As well, there is an indication of lower crustal and (or) upper mantle electrical anisotropy. The geomagnetic transfer function data suggest the presence of a north-northwest-trending structure 7–12 km east of the Miramichi survey area. At present there is little other geophysical or geological evidence for this conductive anomaly.