Markus Eisel

Along strike variations in the electrical structure of the San Andreas Fault at Parkfield, California

Magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the San Andreas Fault at Parkfield, California. A low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km. The base of the wedge is coincident with the shallowest microearthquakes. A change in the electrical and fluid connection of the San Andreas Fault with a low resistivity zone in the Franciscan formation is observed along the Parkfield segment.

ELECTROMAGNETIC STUDY OF THE ACTIVE CONTINENTAL MARGIN IN NORTHERN CHILE

Magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern Chile between 19.5° and 22°S to study the electrical conductivity structures of this active continental margin. The instruments used covered a very broad period range from 10−4 s to approx. 2 × 104 s and thus enabled a resolution of deep as well as shallow structures. In this paper we focus on the interpretation of data from an east-west profile crossing Chile from the Pacific coast to the Western Cordillera at 20.5°S. A decomposition of the impedance tensors using the Groom-Bailey decomposition scheme shows that a two-dimensional interpretation is possible. The resulting regional strike direction is N9°W. Two-dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the Andes. The final model shows a generally high resistivity in the forearc and a very good conductor below the Precordillera. Unlike earlier models from areas further south, a good conductor is not observed below the magmatic arc itself. This correlates with the so-called Pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas.

DC trains and Pc3s: Source effects in mid-latitude geomagnetic transfer functions

Magnetotelluric (MT) data from two sites 150 and 300 km southeast of San Francisco, California (geomagnetic dipole latitude: 43 degrees, L approximately 1.9) show that the usual MT assumption of spatially uniform external magnetic fields is violated to a significant degree in the period range 10–30 s. Inter-station transfer functions exhibit large systematic temporal variations which are consistent with a combination of two distinct sources: electromagnetic noise due to the San Francisco Bay Area Rapid Transit (BART) DC electric railway, and Pc3 geomagnetic pulsations. There is a suggestion in the data that some of the Pc activity may actually be excited by BART.

A magnetotelluric profile across the German Deep Drilling Project (KTB) area: Two‐ and three‐dimensional modeling results

Previous interpretations of magnetotelluric data from the vicinity of the German Deep Drilling Project (KTB) revealed two major structures: a midcrustal layer of increased conductivity and large, regional extent and a highly anisotropic upper crust. Nevertheless, a satisfactory combination of both structures explaining all measurements has not yet been achieved, mostly due to incomplete and qualitatively poor data. Simplified superposition of both structures could not yield an explanation of the observations. On the basis of a carefully processed new data set we apply different modeling approaches to verify the existence of both structures. Models calculated with a two-dimensional modeling program, which allows for general anisotropy, as well as a full three-dimensional code show that an anisotropic upper crust is overlaying a regional east-west striking high-conductivity structure. Nevertheless, this continuous conductive midcrustal layer with a conductance decreasing from north to south must be replaced by a quasi-anisotropic one, at least in the region of the KTB. The final model may still be oversimplified, considering the complexity of the true but unknown geology in this particular area, but it demonstrates which major electrically effective structures could be resolved.