Günter Bock

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.

Long‐period S to P converted waves and the onset of partial melting beneath Oahu, Hawaii

Strong S-P converted waves are observed at the long-period Global Digital Seismograph Network (GDSN) station Honolulu (HON), Oahu, Hawaii, preceding mantle S by 11±1 s. The lead times and polarities of precursors relative to S are consistent with a seismic velocity decrease at a depth between 70 and 80 km probably marking the asthenosphere-lithosphere boundary (ALB) beneath HON. Comparison of the data with synthetic seismograms calculated for models containing a first order seismic discontinuity at 75 km depth suggests that the S wave velocity decreases by at least 15% across the ALB. The sharpness of the ALB cannot be resolved with long-period data; the same results are obtained if the velocity decrease extends over a depth interval up to 40 km wide. Assuming aperidodite composition for the uppermost mantle, a lower limit of 8% for the volume percentage of partial melt and a minimum temperature of about 1450°C in the asthenosphere is consistent with the observations.

Rupture history of the Great Bolivian Earthquake: Slab interaction with the 660‐km discontinuity?

Teleseismic body waves of the great Bolivian earthquake of June 9, 1994 are analyzed to determine the focal parameters and rupture history. Broadband seismograms reveal a complex rupture process: A small initial event (Mw 7.2) was followed about 10 s later by a large moment release pulse of about 40 s duration. Focal mechanisms determined for the mainshock indicate normal faulting with one very shallow NE dipping plane. Azimuthal variation in body-wave displacement pulse widths suggest northward rupture. From master event and body-wave inversion, the main moment release is located 25–50 km NE of the initiation point at about 650-km depth with only small depth variations between the initial and main event. This suggests that rupture was N directed on the near-horizontal plane. Because the slab along other parts of the Andean arc at about 600-km depth dips steeply, a sub-horizontal plane may imply shearing perpendicular to slab dip. Downdip compression on a sub-horizontal plane would then imply that the slab does not penetrate the 660-km seismic discontinuity, but rather, is being sheared out to the NE. This interpretation is not unique as other scenarios are also possible. Such an event immediately above the 660-km discontinuity suggests massive deformation above the discontinuity with no smooth slab penetration into the lower mantle.

Investigation of mantle discontinuities from a single deep earthquake

Converted phases have been identified in the P-wave coda of teleseismic seismograms from a single large (Mw 6.8), deep (360 km) earthquake south of Honshu, Japan. Standard array techniques (slantstacking), applied to broadband data from the global broadband network, the German Regional Seismic Network (GRSN) and the Grafenberg (GRF) array, and synthetic seismogram modeling, reveal that these unusual phases are S to P conversions from under the seismic source. Timing of the converted phase indicates that subduction has had little, if any, effect on the depth of the ‘660 km’ discontinuity. This study shows that upper mantle seismic structure can be constrained from single events using the presently available broadband seismic data and applying array techniques to network data.

Upper Mantle Structure Beneath the Eifel from Receiver Functions

The average Moho depth in the Eifel is approximately 30 km, thinning to ca. 28 km under the Eifel volcanic fields. Receiver function (RF) images suggest the existence of a low velocity zone at about 60 to 90 km depth underneath the West Eifel. This observation is supported by P- and S-wave tomographic results and absorption (Ritter this volume). Indications for a zone of increased velocity near 200 km depth, again agree with S-wave and absorption tomographic results. This anomaly, surprisingly not visible in P-wave tomography, could be due to an area of S-wave anisotropy that compensates for elevated plume temperatures. All three RF anomalies — at the Moho, at 60 to 90 km and near 200 km depth — have a lateral extent of about 100 km. The aperture of the Eifel network limits the resolution of tomographic methods to the upper 400 km. The RF method does not suffer from this limitation and can resolve deeper structures. The 410 km discontinuity under the Eifel is depressed by 15 to 25 km. Lowering of the 410 km discontinuity could be explained by a maximum temperature increase of +200 to +300 °C. The second surprising feature in the 3-D RF image of the Eifel Plume is the occurrence of two additional, currently unexplained conversions between 410 and 550 km depth. They could represent remnants of previous subduction or anomalies due to delayed phase changes. The lateral extent of the two additional conversions and the depression of the 410 km discontinuity is about 200 km. The 660 km discontinuity, in contrast to the 410 km discontinuity, does not show any depth deviation from its expected value, a scenario also encountered in the western US. Based on these observations we present the following scenario for the Eifel plume. The Eifel plume is a plume with temperature excess relative to the surrounding mantle of about +200 to +300 °C. The plume is imaged in the upper mantle and might be fed by regions imaged as low velocity anomalies in the lower mantle under Central Europe. Seismological methods provide only a blurred present day snap-shot. Thus we can not exclude the possibility that ascent of plume material, possibly coming even from the lower mantle, is intermittent and we see only the present day effects and configuration of the plume.