Michael Becken

Correlation between deep fluids, tremor and creep along the central San Andreas fault

The seismicity pattern along the San Andreas fault near Parkfield and Cholame, California, varies distinctly over a length of only fifty kilometres. Within the brittle crust, the presence of frictionally weak minerals, fault-weakening high fluid pressures and chemical weakening are considered possible causes of an anomalously weak fault northwest of Parkfield. Non-volcanic tremor from lower-crustal and upper-mantle depths is most pronounced about thirty kilometres southeast of Parkfield and is thought to be associated with high pore-fluid pressures at depth. Here we present geophysical evidence of fluids migrating into the creeping section of the San Andreas fault that seem to originate in the region of the uppermost mantle that also stimulates tremor, and evidence that along-strike variations in tremor activity and amplitude are related to strength variations in the lower crust and upper mantle. Interconnected fluids can explain a deep zone of anomalously low electrical resistivity that has been imaged by magnetotelluric data southwest of the Parkfield–Cholame segment. Near Cholame, where fluids seem to be trapped below a high-resistivity cap, tremor concentrates adjacent to the inferred fluids within a mechanically strong zone of high resistivity. By contrast, subvertical zones of low resistivity breach the entire crust near the drill hole of the San Andreas Fault Observatory at Depth, northwest of Parkfield, and imply pathways for deep fluids into the eastern fault block, coincident with a mechanically weak crust and the lower tremor amplitudes in the lower crust. Fluid influx to the fault system is consistent with hypotheses of fault-weakening high fluid pressures in the brittle crust.

Anatomy of the Dead Sea Transform from lithospheric to microscopic scale

Fault zones are the locations where motion of tectonic plates, often associated with earthquakes, is accommodated. Despite a rapid increase in the understanding of faults in the last decades, our knowledge of their geometry, petrophysical properties, and controlling processes remains incomplete. The central questions addressed here in our study of the Dead Sea Transform (DST) in the Middle East are as follows: (1) What are the structure and kinematics of a large fault zone? (2) What controls its structure and kinematics? (3) How does the DST compare to other plate boundary fault zones? The DST has accommodated a total of 105 km of left-lateral transform motion between the African and Arabian plates since early Miocene (similar to 20 Ma). The DST segment between the Dead Sea and the Red Sea, called the Arava/Araba Fault (AF), is studied here using a multidisciplinary and multiscale approach from the mu m to the plate tectonic scale. We observe that under the DST a narrow, subvertical zone cuts through crust and lithosphere. First, from west to east the crustal thickness increases smoothly from 26 to 39 km, and a subhorizontal lower crustal reflector is detected east of the AF. Second, several faults exist in the upper crust in a 40 km wide zone centered on the AF, but none have kilometer-size zones of decreased seismic velocities or zones of high electrical conductivities in the upper crust expected for large damage zones. Third, the AF is the main branch of the DST system, even though it has accommodated only a part (up to 60 km) of the overall 105 km of sinistral plate motion. Fourth, the AF acts as a barrier to fluids to a depth of 4 km, and the lithology changes abruptly across it. Fifth, in the top few hundred meters of the AF a locally transpressional regime is observed in a 100-300 m wide zone of deformed and displaced material, bordered by subparallel faults forming a positive flower structure. Other segments of the AF have a transtensional character with small pull-aparts along them. The damage zones of the individual faults are only 5-20 m wide at this depth range. Sixth, two areas on the AF show mesoscale to microscale faulting and veining in limestone sequences with faulting depths between 2 and 5 km. Seventh, fluids in the AF are carried downward into the fault zone. Only a minor fraction of fluids is derived from ascending hydrothermal fluids. However, we found that on the kilometer scale the AF does not act as an important fluid conduit. Most of these findings are corroborated using thermomechanical modeling where shear deformation in the upper crust is localized in one or two major faults; at larger depth, shear deformation occurs in a 20-40 km wide zone with a mechanically weak decoupling zone extending subvertically through the entire lithosphere.

Transformation of VLF anomaly maps into apparent resistivity and phase.

We investigate a transformation of magnetic transfer functions into the tangential‐electric mode part of the impedance tensor in the scope of the plane‐wave electromagnetic tensor–VLF method. The transformation, which is applicable to any 2D data representing the response of arbitrary 3D geoelectric structures, overcomes the difficulties of quantitative interpretation of magnetic transfer functions, which predominantly provide a measure of the lateral changes of the electrical conductivity in the earth. We require densely sampled magnetic transfer functions of one frequency as input data. These may be decomposed into their normal and anomalous parts (deviation from the response of a layered earth) for a unit external plane‐wave source field using the Hilbert transform relationship between the magnetic field components. Faradays law then directly provides the anomalous toroidal electric field. Unfortunately, there is no chance to estimate the normal electric field from magnetic data, since the magnetic fi...

Magnetotelluric Studies at the San Andreas Fault Zone: Implications for the Role of Fluids

Fluids residing in interconnected porosity networks have a significant weakening effect on the rheology of rocks and can strongly influence deformation along fault zones. The magnetotelluric (MT) technique is sensitive to interconnected fluid networks and can image these zones on crustal and upper mantle scales. MT images have revealed several prominent electrical conductivity anomalies at the San Andreas Fault which have been attributed to the presence of saline fluids within such networks and which have been associated with tectonic processes. These models suggest that ongoing fluid release in the upper mantle and lower crust is closely related to the mechanical state of the crust. Where fluids are drained into the brittle crust, and where these fluids are kept at high pressures, fault creep is supported. Fluid fluxes from deeper levels, in combination with meteoric and crustal metamorphic fluid inflow, and in response to fault creep, leads to high-conductivity zones developing as fault zone conductors in the brittle portion of crust. In turn, the absence of crustal fluid pathways may be characteristic for mechanically locked segments of the fault. Here, MT models suggest that fluids are trapped at depth and kept at high pressures. We speculate that fluids may infiltrate neighboring rocks and in their wake induce non-volcanic tremor.

Strategies for land-based controlled-source electromagnetic surveying in high-noise regions

Marine controlled-source electromagnetics (CSEM) has become an established exploration tool in the hydrocarbon industry over the last decade; however, this is not the case for land-based CSEM. A possible reason is the perception that electromagnetic field energy traveling through air masks target responses much more than for marine surveys in which the “airwave” energy is attenuated by the conductive water. Various strategies for mitigating air-wave effects have been proposed recently (Loseth et al., 2010; Chen and Alumbaugh, 2011). In this contribution, we focus on other problems limiting the use of land-based CSEM. These include logistical and technical challenges related to high-power current transmission, and the high levels of cultural electromagnetic noise typical of target areas located in populated regions.

Equivalent images derived from very-low-frequency (VLF) profile data

We describe the implementation of a new fast imaging technique for filtering very-low-frequency (VLF) data measured on profiles into corresponding equivalent current systems in the earth. Single-frequency VLF data using magnetic measurements alone are often used to delineate lateral changes in electrical conductivity, e.g., fracture zones in crystalline terrains or changes in lithology in the sedimentary cover. Here, an attempt is made to add depth information to the conductivity distribution by realizing that the single-frequency VLF profile data contain information about (1) the background medium through their decay away from the conductors, (2) the position, and (3) the depth of the dominating conductors through the relative contribution of in-phase and quadrature components to the VLF anomaly in addition to the rate of change of the anomaly close to the conductors. Synthetic data from a model containing a shallow and a deeper conductor are filtered to show that the estimated current distributions coincide well with the horizontal position of the conductors, but even they provide some smeared images of the depth distribution of the conductors. A comparison with models obtained from regularized inversion of the same data shows good correspondence. The VLF field data from an area with clay lenses overlying wet sand and crystalline basement are filtered into current distributions that grossly mimic the electrical conductivity distribution of the clay lenses as obtained from radiomagnetotelluric measurements along the same profile.

Electrical resistivity image of the Jingsutu Graben at the NE margin of the Ejina Basin (NW China) and implications for the basin development

[1]xa0Magnetotelluric and transient electromagnetic data reveal the Graben geometry of a NNE striking depression within crystalline basement at the NE margin of the Ejina Basin. We interpret this structure as a Late Mesozoic pull-apart basin, which is delineated by NNE-striking normal faults to the east and west. Satellite imagery, SRTM elevation data and previously published geophysical data of our work group suggest that the Graben is part of normal-fault system that continues at least 140 km farther to the south, thus forming the eastern margin of the Ejina Basin. NNE striking normal faults are atypical in a regional context, where NW striking and ENE striking fault systems prevail. Thus, this feature marks a distinct difference of the Ejina Basin when compared to other basin structures in the China-Mongolia border region.

An Analysis of ZTEM Data Over the Mt Milligan Porphyry Copper Deposit, British Columbia

ZTEM data acquired across the Mt Milligan Cu-Au porphyry system are compared with overlapping VTEM data. Conductivity-depth sections derived from both data sets show broad agreement, but indicate better spatial resolution for the VTEM data. Neither data set appears to show a significant response from the Cu-Au mineralization. Products derived from the ZTEM data, including apparent conductivity, phase and Karous-Hjelt filtered grids appear to map geologic structure.

Correction to “Anatomy of the Dead Sea Transform from lithospheric to microscopic scale”

Weber, M., Abu‐Ayyash, K., Abueladas, A., Agnon, A., Alasonati‐Tašárová, Z., Al‐Zubi, H., Babeyko, A., Bartov, Y., Bauer, K., Becken, M., Bedrosian, P. A., Ben‐Avraham, Z., Bock, G., Bohnhoff, M., Bribach, J., Dulski, P., Ebbing, J., El‐Kelani, R., Förster, A., Förster, H.‐J., Frieslander, U., Garfunkel, Z., Goetze, H. J., Haak, V., Haberland, C., Hassouneh, M., Helwig, S., Hofstetter, A., Hoffmann‐Rothe, A., Jäckel, K. H., Janssen, C., Jaser, D., Kesten, D., Khatib, M., Kind, R., Koch, O., Koulakov, I., Laske, G., Maercklin, N., Masarweh, R., Masri, A., Matar, A., Mechie, J., Meqbel, N., Plessen, B., Möller, P., Mohsen, A., Oberhänsli, R., Oreshin, S., Petrunin, A., Qabbani, I., Rabba, I., Ritter, O., Romer, R. L., Rümpker, G., Rybakov, M., Ryberg, T., Saul, J., Scherbaum, F., Schmidt, S., Schulze, A., Sobolev, S. V., Stiller, M., Stromeyer, D., Tarawneh, K., Trela, C., Weckmann, U., Wetzel, U., Wylegalla, K. (2010): Correction to Anatomy of the Dead Sea Transform from lithospheric to microscopic scale. ‐ Reviews of Geophysics, 48, RG1003

Utilizing Impressed Current Cathodic Protection as the Source for Electromagnetic Exploration

Central Europe is criss-crossed by pipelines to transport water, gas and oil. Metal pipelines are routinely protected against electrochemical corrosion with a coating supplemented with an impressed current cathodic protection (ICCP) system. For pipeline integrity tests, the rectified injection current is temporarily switched on and off. The switching scheme effectively generates time-varying electrical currents and induces secondary electric and magnetic fields in the subsurface, which decay spatially and temporally as a function of subsurface electrical resistivity. Here, we describe our first attempts to measure and to analyze the induced electromagnetic fields generated by switched cathodic protection currents in order to determine the subsurface electrical resistivity structure in the upper (few) kilometers depth range. This approach is closely related to controlled source electromagnetics. It may provide a cheap complement to existing electromagnetic geophysical sounding techniques, which is applicable in noisy environments without facing the logistical challenge of the installation a strong current source in the field. The methodology can aid in geophysical subsurface reconnaissance addressed in the exploration and monitoring of resources, reservoirs and geological storages.