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We compare recent magnetotelluric investigations of four large fault systems: (i) the actively deforming, ocean-continent interplate San Andreas Fault (SAF), (ii) the actively deforming, continent-continent interplate Dead Sea Transform (DST), (iii) the currently inactive, trench-linked intraplate West Fault (WF) in northern Chile, and (iv) the Waterberg Fault/Omaruru Lineament (WF/OL) in Namibia, a fossilized intraplate shear zone formed during early Proterozoic continental collision. These fault zones show both similarities and marked differences in their electrical subsurface structure. The central segment of the SAF is characterized by a zone of high conductivity extending to several kilometres depth and attributed to fluids within a highly fractured damage zone. The WF exhibits a less pronounced but similar fault zone conductor (FZC) which can be explained by meteoric waters entering the fault zone. The DST appears different as it shows a distinct lack of a FZC and seems to act primarily as an impermeable barrier to cross-fault fluid transport. Differences in the electrical structure of these faults within the upper crust may be linked to the degree of deformation localization within the fault zone. At the DST, with no observable fault zone conductor, strain may have been localized for a considerable time span along a narrow, meter-scale damage zone with a sustained strength difference between the shear plane and the surrounding host rock. In the case of the SAF, a positive correlation of conductance and fault activity is observed, with more active fault segments associated with wider, deeper, and more conductive fault zone anomalies. Fault zone conductors, however, do not uniquely identify specific architectural or hydrological units of a fault. A more comprehensive whole-fault picture for the brittle crust can be developed in combination with seismicity and structural information. Giving a window into lower-crustal shear zones, the fossil WF/OL in Namibia is imaged as a sub vertical, 14 km deep, 10 km wide zone of high and anisotropic conductivity. The present level of exhumation suggests that the WF/OL penetrated the entire crust as a relatively narrow shear zone. Contrary to the fluid driven conductivity anomalies of active faults, the anomaly here is attributed to graphitic enrichment along former shear planes. Once created, graphite is stable over very long time spans and thus fault/shear zones may remain conductive long after activity ceases.

To address one of the central questions of plate tectonics-How do large transform systems work and what are their typical features?-seismic investigations across the Dead Sea Transform (DST), the boundary between the African and Arabian plates in the Middle East, were conducted for the first time. A major component of these investigations was a combined reflection/ refraction survey across the territories of Palestine, Israel and Jordan. The main results of this study are: (1) The seismic basement is offset by 3-5 km under the DST, (2) The DST cuts through the entire crust, broadening in the lower crust, (3) Strong lower crustal reflectors are imaged only on one side of the DST, (4) The seismic velocity sections show a steady increase in the depth of the crust-mantle transition (Moho) from 26 km at the Mediterranean to 39 km under the Jordan highlands, with only a small but visible, asymmetric topography of the Moho under the DST. These observations can be linked to the left-lateral movement of 105 km of the two plates in the last 17 Myr, accompanied by strong deformation within a narrow zone cutting through the entire crust. Comparing the DST and the San Andreas Fault (SAF) system, a strong asymmetry in subhorizontal lower crustal reflectors and a deep reaching deformation zone both occur around the DST and the SAF. The fact that such lower crustal reflectors and deep deformation zones are observed in such different transform systems suggests that these structures are possibly fundamental features of large transform plate boundaries

We give details of a planned new magnetotelluric (MT) experiment at Parkfield and present results of a similarly designed investigation of the Dead Sea Transform fault. MT studies have always played an important role in the investigation of the San Andreas Fault (SAF) but due to short profile lengths, they have generally focused on the geometry and nature of the upper crustal fault. Continental transform faults separate plates of lithosphere but it is still debatable if these faults cut rigid blocks as narrow shear zones or if the lower crust decouples and deforms continuously over a wide area. A geophysical transect across the Dead Sea Transform (DST) in Jordan and Israel reveals a narrow, approximately 3-5 km wide, sub-vertical zone of high electrical conductivity penetrating the lithosphere to a depth of at least 30 km. This image strongly suggests that the DST is spatially confined even in the ductile lower crust. However, the situation may be different at the SAF. Results from the brittle crust show some similarities in these major transform fault systems, but also marked differences. Field work in Parkfield will start in spring 2005 with a high resolution MT survey following the 50 km reflection / refraction seismic profile of Hole & Ryberg. Additional MT sites will be deployed along and off the main 50 km profile for areal coverage. Acquiring seismic and magnetotelluric data along a coincident profile gives the opportunity to utilize the combined imaging power of the two methods.

Upper crustal fault zones, either fossil or active, are often connected with electrical conductivity anomalies. These anomalies depend on properties such as the porosity/permeability of the fault zone material, the fluid content or the state of healing/cementation of the fault-fracture mesh; properties that moreover control the ability of a fault to accumulate strain. Structural heterogeneities caused by the faulting process are therefore believed to either increase or decrease the electrical conductivity in the fault's vicinity. We show results of two combined magnetotelluric and structural studies of large scale strike-slip dominated fault zones. The trench-linked West Fault (WF) in Northern Chile shows a pronounced anomaly of high conductivity confined to the central region of the fault. The zone of high conductivity is approximately 400 m wide and 1.5 km deep. Structural mapping reveals that this conductivity enhancement is closely related to a mesh of faults and fractures ('damage zone') that most likely provides a pathway for fluids. In contrast to this, the Dead Sea Transform Fault (DST) in Jordan shows no obvious evidence of such a fault zone conductor as the DST is expressed as the boundary between two different domains of conductivity on either side of the fault. Correspondingly, a marked macroscopic fault-fracture mesh in the fault core region is not developed. Comparison of the results from the WF with published data from the San Andreas Fault suggests generally a positive correlation of fault activity with geometric extent and conductivity of the fault zone conductor. However, the Dead Sea Transform Fault apparently does not comply with this scheme although it is active. It is possible that intense localisation of deformation caused the formation of a very narrow fault gouge, which cannot be resolved with the MT experiments. This result could suggest that the existence or non-existence of high conductivity in the central parts of large scale strike-slip fault zones is an indicator for the degree of strain localisation during faulting.

We investigated a segment of a 170 km long strike-slip fault zone belonging to the trench parallel Precordilleran Fault System in the Central Andes of northern Chile using structural geology, magnetotelluric (MT) and geochemical methods. A maximum fracture density of 3.3 m-1 at the main fault trace decreases to background within a zone of ~1 km width. Individual fault segments in this damage zone are steeply inclined with varying dip angle and direction. A 400 m wide coloured fluid alteration zone marks the fault core. The MT experiment with a dense site spacing of 100 m across the fault reveals a zone of enhanced conductivity about 300 m wide that coincides with the fault core. This shows that the 'conductive' width of the fault zone is 1/3 of the 'structural' width, which depends on deformation history. The ratio of fault width to fault length ranges from 2 x 10-3 to 6 x 10-3. The conducting zone dips steeply to the east, in agreement with most shear planes in the fault core. Geochemical analysis of the alteration zone rocks indicates a meteoric origin of fault zone fluids. We assume that water entering the ruptured zone of the fault core is most likely responsible for the conductivity enhancement. A second MT profile 4 km further south, with broader site spacing, again shows a good conductor located on the eastern side of the surface fault trace. The conducting zone reaches a maximum depth of 1500 m in both profiles. This suggests that permeability of the fault zone at greater depth is reduced due to healing and cementation of the fracture system. Two other conductive features that have no surface expression are imaged along the dense MT profile. These structures could indicate further fault branches of a wider fault zone.

Im Sommer 1997 wurden erstmalig im Rahmen des DFG-Schwerpunktes MERAPI magnetotellurische Messungen am Hochrisikovulkan Merapi durchgeführt. Erste Kenntnisse der elektrischen Leitfähigkeit im lokalen wie im regionalen Umfeld des Vulkans bilden die Grundlage, um ein elektrisches Abbild des Merapi zu erstellen. Ziel ist die Erkundung von signifikanten Leitfähigkeitskontrasten, die mit der Entstehung oder mit der rezenten Aktivität des Vulkans zusammenhängen. Ursachen für hohe Leitfähigkeiten können . Schwächezonen sein, die den Aufstieg des Magmas während der Entstehung des Vulkans begünstigen und noch heute beeinflussen, . hydrothermale Fluide, die in Konvektionszellen, angetrieben vom heißen Magma, zirkulieren, . hydrothermale Ablagerungen sowie . das heiße Magma. Um einen ersten Überblick zu gewinnen, wurden auf zwei Profilen Messungen zur Bestimmung der elektrischen Leitfähigkeit im Untergrund durchgeführt: (1) auf einem Profil von 165 km Länge zur Untersuchung der regionalen Leitfähigkeitsstruktur, (2) auf einem Profil von 9 km Länge zur Untersuchung der elektrischen Leitfähigkeit direkt am Vulkan.

Magmadynamische Prozesse im Inneren eines Vulkans können an der Oberfläche als seismische Signale registriert werden. Die bekannteste Signalform ist der vulkanische Tremor, dessen harmonische Anteile im Spektrum auf Instabilitäten des Magmaflusses hinweisen. Um diese Instabilitäten zu erklären, wird von Chouet (1) und Schick (2) ein Zweiphasenflußmodell vorgeschlagen. Das Vorhandensein einer freien Gasphase erhöht die Kompressibilität des Magmas, was die Entstehung von extrem starken Druckgradienten ermöglicht. Der Zweiphasenfluß in einem porösen Medium aber erzeugt zeitvariable elektrische Strömungspotentiale (3), die von den Druckgradienten abhängen. Unsere Idee war daher, an gleichzeitig registrierten seismischen und magnetotellurischen Zeitreihen, vom Tremor erzeugte Änderungen des elektrikschen Feldes festzustellen

Galvanic distortion of the regional MT respones was inferred from the analyses of the BC87 dataset presented at the MT-DIW1 in 1992. Since perturbation currents also produce an anomalous magnetic field, the GDS response functions were suspected to be distorted in the long period range. In order to reveal the correct regional strike direction, hypothetical event analysis was applied to the data from all sites. With this technique, common regional information, such as strike and impedance phase, can be recovered from distorted response functions. Although the BC 87 GDS data are noisy, and the distortion is more of an inductive than of a purely galvanic nature, the resulting regional strike direction of 60° agrees well with the azimuth suggested from previous MT decomposition methods. Thin sheet modelling studies of the geometry of the deep crustal conductor (L-shaped or oblique) at 1000 s imply that along the BC87 profile induction arrows are generally produced by current concentrations around a resistive region associated with the Nelson Batholith. Comparison of the model induction arrows with those determined from the BC87 data indicates that the model featuring the oblique, NE striking conductor (SABC) may be favoured.