Low Qp Research Articles

Seismic reflections from the crystalline crust below the Continental Deep Drilling Site KTB: Modeling and inference on reflector properties

The three‐dimensional (3‐D) reflection‐seismic data set ISO‐89 3D was recorded near the deep borehole KTB in southeastern Germany. Reflections from the SE1 reflector and from the top of the Erbendorf body (EB) in the upper crystalline crust can be identified in 5–10% of the single‐shot sections. The reflectors have been first identified in previous studies of stacked and migrated seismic data. In this paper the velocity and density variations of these two structures are estimated in a new way using true amplitude single‐shot (vibroseis) data. The method uses the direct wave Pg as a reference phase and models the amplitude ratios of the SE1 and EB reflections to Pg. Modeling in this paper uses a combination of ray theory and the reflectivity method, and the SE1 and the top of the EB are assumed to be obliquely oriented 1‐D structures. Pg modeling shows that a depth‐dependent velocity function within the uppermost crystalline basement explains the amplitudes and travel times of this phase with sufficient accuracy. The largest observed amplitude ratios SE1/Pg and EB/Pg are explained by laminated models with strong velocity contrasts and with reflection coefficients of magnitude 0.1–0.2 (SE1) and 0.05–0.15 (EB). The total thickness of the reflecting zones is less than ∼300 m. Pg amplitude modeling requires low Qp factors (<100) to a depth of ∼1 km, whereas at larger depths, values of several hundred are necessary to keep the SE1 and EB velocity contrasts in realistic ranges. Both reflectors can be interpreted as cataclastic zones. For the SE1 this interpretation agrees with the view that it is a steeply dipping thrust fault which continues the tectonic Franconian Lineament into the upper crust. We assume that the EB is the fractured top of a high‐velocity zone at depths below ∼10 km, known from earlier wide‐angle measurements. Both reflectors have large weakly reflecting or nonreflecting parts. The SE1 is nonreflecting at the intersection with the KTB borehole.

The Bolivian Altiplano conductivity anomaly

A long‐period magnetotelluric study was carried out in the central Andes between latitudes 19.5°S and 21°S along two almost parallel profiles of 220 and 380 km length, respectively. The investigation area extends from the Pacific coast to the southern Altiplano Plateau in the back arc of the South American subduction zone. The main geoelectrical structure resolved is a broad and probably deep‐reaching highly conductive zone in the middle and deeper crust beneath the high plateau. Although the data show deviations from two‐dimensionality, a two‐dimensional approach is justified for large parts of the profiles. Sensitivity studies were carried out in order to constrain the depth extent. Another electrically conductive structure was resolved in the middle crust of the Chilean forearc, thought to be connected with the Precordillera fault system. The Andean Continental Research Program (ANCORP) seismic reflection profile, carried out along the same line at 21°S, revealed highly reflective zones below the Altiplano, in good correlation with the upper boundary of the Altiplano conductor. This highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp/vs ratio and, although not well resolved, with low Qp seismic quality factors. Taking into account the enhanced heat flow and a derived temperature model, the most probable explanation lies in the assumption of granitic partial melts. The good conductor below the volcanic arc which was found in regions farther south at 22°S gradually vanishes toward the north; this is consistent with the results of seismic tomography concerning Qp values and a gap of recent volcanism.

Attenuation tomography in the western central Andes: A detailed insight into the structure of a magmatic arc

High‐quality data from 1498 local earthquakes recorded by the PISCO ’94 (Proyecto de Investigatión Sismológica de la Cordillera Occidental, 1994) and ANCORP ’96 (Andean Continental Research Project, 1996) temporary seismological networks allowed the detailed determination of the three‐dimensional (3‐D) attenuation structure (Qp−1) beneath the recent magmatic arc in the western central Andes (20° to 24°S). Assuming a frequency‐independent Qp−1 in a frequency band between 1 and 30 Hz, whole path attenuation (t*) was estimated from the amplitude spectra of the P waves using spectral ratios and a spectral inversion technique. The damped least squares inversion (tomography) of the data reveals a complex attenuation structure. Crust and mantle of the forearc and subducting slab are generally characterized by low attenuation (Qp > 1000). Crust and mantle beneath the magmatic arc show elevated attenuation. The strongest anomaly of extremely low Qp is found in the crust between 22° and 23°S beneath the recent volcanic arc (Qp < 100). N‐S variations can be observed: The western flank of the crustal attenuation anomaly follows the curved course of the volcanic front. North of 21°S the attenuation is less developed. In the northern part of the study area the low‐Qp zone penetrates in the forearc mantle down to the subducting slab. In the south a deeper zone of high attenuation is resolved between 23° and 24°S directly above the subducting slab. Low Qp in the mantle correlates with earthquake clusters. The strong crustal attenuation is confined to the distribution of young ignimbrites and silicic volcanism and is interpreted as a thermally weakened zone with partial melts. The attenuation pattern in the upper mantle might reflect the variable extent of the asthenosphere and maps variations of subduction‐related hydration processes in the mantle wedge from slab‐derived fluids.

Effect of dithiothreitol (DTT) and diethylditiocarbamate (DDC) on watered and water stressed bean leaves

The effect of infiltration of 3 and 5 mM DTT and 10 mM DDC in watered and water stressed bean leaves, upon low or high intensity light has been assessed, in order to compare the relative importance of non-photochemical energy dissipation and active oxygen scavenging on photosynthesis. qN, in high light, was reduced by 3 and 5 mM DTT in stressed plants, and only by 5 mM in watered. The low qP value in stressed leaves was not further reduced by DTT, instead, Fs increased. Restrictions on the capacity for qN formation, induced by DTT, however, did not result in a lower capacity to recover Fm' after 20 min darkness in neither watered or stressed leaves upon high light, or watered plants at low light. On the contrary, stressed plants under low light, recovered Fm' up to a slightly higher value compared to the other treatments and control. DDC, only affected qN in low light stressed plants, slightly increasing it, as well as Fs, also decreasing qP. In Watered plants, DDC negatively affected the Fm' recovery, in high and low light. Despite the big differences in qN and qP, among water regimes, both reached similar photosynthetic rates in low light. The increased qP after DTT infiltration, as well as the higher Fm' recovery in stressed plants under low light, suggest that qN might reduce, under such conditions, activation of assimilatory enzymes. Stressed leaves were more resistant to qN and SOD inhibitors compared to watered. Supported by FONDECYT 1990077

Seismological evidence for magmatic and hydrothermal structure in Long Valley Caldera from local earthquake attenuation and velocity tomography

We compare the results of six different tomographic inversions for the three‐dimensional seismic attenuation and velocity structure in Long Valley caldera, California, in order to constrain interpretations on the location and character of magmatic and geothermal structure in the caldera crust. P and S wave amplitudes and arrival times obtained in previous studies from seismograms of local earthquakes recorded on vertical‐ and three‐component seismographs were inverted for QSV−1, QSH−1, ΔQS−1, ΔQP−1, and ΔVP/VS in the upper 9 km of the caldera crust. Detailed comparisons of the results of the inversions suggest significant locations of high temperatures and perhaps magma and hydrothermal zones in the caldera. A region of low Qs with locally variable QP and VP/VS is imaged in the 7–9 km depth range beneath the resurgent dome. Overlying and adjacent to this anomaly are significant low‐QP and low‐VP/VS anomalies as well as smaller zones of low QS and high VP/VS. Together, these suggest zones of high temperature/magma below and adjacent to zones of supercritical hydrothermal fluids. At 4–5 km depth along the southern rim of the caldera just east of Mammoth Mountain a significant anomaly with relatively low‐QP and relatively low‐VP/VS is imaged and is interpreted as a supercritical hydrothermal zone. Poorly resolved relatively low QP and relatively high VP/VS are seen at 4–5 km depth beneath the Inyo craters, suggesting possible high temperatures and perhaps magma. Because of lack of data in the individual inversions the resultant images have spatially variable error and resolution. However, by comparing multiple images of different seismic parameters we have improved our ability to determine the significance of and constrain interpretation of seismic structure anomalies.