ALBERT

Description: We present observations of cross coupled spheroidal modes in the Earth's free oscillation spectrum recorded by the vertical component G-ring laser (Geodetical Station Wettzell) of the 2011 M w 9.0 Tohoku-Oki earthquake. In an attempt to determine which are the mechanisms responsible for spheroidal energy in a vertical axes rotational spectra, we first rule out instrumental effects as well as the effect of local heterogeneity. Secondly, we carry out simulations of an ideal rotational sensor taking into account the effects of the Earth's rotation, its hydrostatic ellipticity and structural heterogeneity, which results in a good fit to the data. Simulations considering each effect separately are performed in order to evaluate the sensitivity of rotational motions to various phenomena compared to traditional translation measurements.

Description: Microzonation, the estimation of (shear) wave velocity profiles of the upper few 100 m in dense 2D profiles, is one of the key methods for understanding the variation in seismic damage caused by ground-shaking events and thus for mitigating the risk of damage in the future. In this article, we present a novel method for estimating the Love-wave phase velocity dispersion using ambient noise recordings. We use the vertical component of rotational motions inherently present in ambient noise and the established relation to simultaneous recordings of transverse acceleration, in which the phase velocity of a plane SH (or Love)-type wave acts as a proportionality factor. We demonstrate that the developed inversion technique shows comparable results to more classical, array-based methods. Furthermore, we demonstrate that if portable weak-motion rotational motion sensors are available and the installation of a seismic network or array is not possible, a single point, multicomponent approach for estimating the dominant direction of the incident wavefield and the local velocity structure will be feasible with similar performance compared to more classical techniques.

Description: Dynamic tilts (rotational motion around horizontal axes) change the projection of local gravity onto the horizontal components of seismometers. This causes sensitivity of these components to tilt, especially at low frequencies. We analyse the consequences of this effect onto moment tensor inversion for very long period (vlp) events in the near field of active volcanoes on the basis of synthetic examples using the station distribution of a real deployed seismic network and the topography of Mt. Merapi volcano (Java, Indonesia). The examples show that for periods in the vlp range of 10–30 s tilt can have a strong effect on the moment tensor inversion, although its effect on the horizontal seismograms is significant only for few stations. We show that tilts can be accurately computed using the spectral element method and include them in the Green's functions. The (simulated) tilts might be largely influenced by strain–tilt coupling (stc). However, due to the frequency dependence of the tilt contribution to the horizontal seismograms, only the largest tilt signals affect the source inversion in the vlp frequency range. As these are less sensitive to stc than the weaker signals, the effect of stc can likely be neglected in this application. In the converse argument, this is not necessarily true for longer periods, where the horizontal seismograms are dominated by the tilt signal and rotational sensors would be necessary to account for it. As these are not yet commercially available, this study underlines the necessity for the development of such instruments.

Description: SUMMARY Electromagnetic induction (EMI)-based metal detectors are the most widely used sensing techniques in landmine clearance operations; however, they are negatively influenced by frequency dependence of magnetic susceptibility. A total of 466 rock and soil samples collected from across the tropics are investigated in this study. The data show that frequency-dependent susceptibility depends on the parent material as well as on the degree of weathering. Ultramafic and mafic rocks and their derivatives have higher susceptibility and absolute frequency dependence than material originating from intermediate, felsic and sedimentary rocks. Within each parent material group, absolute frequency dependence increases steadily with increasing alteration from unweathered rock to topsoil. This effect is likely due to either the residual enrichment of weathering resistant ferrimagnetic minerals including superparamagnetic (SP) grains, the comminution of larger ferrimagnetic minerals or the neoformation of SP minerals during soil formation. Relative frequency dependence is generally lower than 15 per cent for the investigated samples with a few exceptions. It increases with alteration for igneous rocks but remains at the initially high level for sediments. This finding indicates that the relative concentration of SP minerals changes with respect to the total magnetic fraction for igneous rocks but remains constant for sediments. Soils derived from ultramafic, mafic and intermediate rocks show low relative frequency dependence, and their magnetic susceptibility is mainly the result of multidomain lithogenic minerals. In contrast, soils derived from felsic rocks and sediments show the highest values, and their susceptibility is due to SP minerals that are either formed during pedogenesis or residually enriched. The average and extreme values of the absolute frequency dependence within each subgroup, based on parent material and alteration grade, are used to design a prognosis system for assessing the impact of the subsurface on EMI sensors for landmine detection. In general, intermediate, felsic and sedimentary rocks have no influence on the detectors and only a weak influence in the most extreme cases. Soils derived from these rocks typically have no influence; however, they can have a very severe influence in a few cases. In contrast, ultramafic and mafic rocks typically have a moderate influence and a very severe influence in extreme cases, with the associated soils resulting in a severe influence in general. The deduced prognosis system can be used by demining organizations to help them predict metal detector performance in tropical regions on the basis of geologic and/or soil maps, which do not supply information on electromagnetic properties. In this way, such a system may eventually help with the planning of demining campaigns and selection of appropriate sensors.