Abstract
Over the last decade electromagnetic (EM) measurements have provided new constraints on the upper-crustal structure of the major fault zones in the world, both when they act as conduit and as a barrier, due to strong sensitivity of resistivity to fluids circulation and mineralization. On the track of a high impact magnetotelluric (MT) study performed across the San Andreas Fault, high resolution EM data were collected in the Colfiorito epicentral area along profiles crossing some main fault lineaments. Being the study focussed both on shallow that on intermediate resistivity distribution in the brittle upper-crust, a MT profile was integrated by several electrical resistivity tomographies (ERT). The latter were successful in locating faults even where the structures are buried by a wide covering of Quaternary deposits and in the recognition of different electrical signatures of the faults. MT resistivity model crossing Mt. Prefoglio normal fault clearly imaged the typical thrust structures of the area and a high conductive zone spatially related to the fault. Seismicity seems to be located outside such conductive area, whose behaviour suggests a fluidised and altered zone incapable of supporting significant stress internally.
Highlights
The presence of an electrically conducting fluid, such as saline pore water, strongly influences the overall resistivity of the crustal rocks, favouring the application of prospecting methods based on electrical and electromagnetic physical properties, in detailed imaging of fault zones.In particular, the MT method uses the fluctuation of natural electromagnetic (EM) fields toIn traditional MT survey usually large separations between MT soundings were adopted to detect fault area anomalies, mis-estimating both the true extent of the fault area and the correct resistivity contrast with the host rocks
This paper presents and discusses the results obtained from the combined application of the two methodologies
The Colfiorito area is located in the inner part of the Umbria-Marche Apennines, an eastward verging fold and thrust belt, representing the Shallow to intermediate resistivity features of the Colfiorito Fault System inferred by DC and MT survey of the CoFS
Summary
The presence of an electrically conducting fluid, such as saline pore water, strongly influences the overall resistivity of the crustal rocks, favouring the application of prospecting methods based on electrical and electromagnetic physical properties, in detailed imaging of fault zones. In order to control the shallow part of the subsurface, magnetotelluric measurements can be integrated with DC survey This approach both constrains the MT model in the first hundred metres of depth and estimates eventual static shift effects in the high frequency MT data. The study area is characterized by compressional structures (Upper Miocene in age), displaced by a system of later normal faults, generated during the subsequent (Late Pliocene to present) extensional phase. These faults are still active, and responsible for the moderate to high (5
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