ALBERT

Description: High-resolution seismic tomography at local and regional scales requires large and consistent sets of arrival-time data. Algorithms combining accurate picking with an automated quality classification can be used for repicking waveforms and compiling large arrival-time data sets suitable for tomographic inversion. S-wave velocities represent a key parameter for petrological interpretation, improved hypocenter determination, as well as for seismic hazard models. In our approach, we combine three commonly used phase detection and picking methods in a robust S-wave picking procedure. Information from the different techniques provides an in situ estimate of timing uncertainty and of the reliability of the automatic phase identification. Automatic picks are compared against manually picked reference picks of selected earthquakes in the Alpine region. The average accuracy of automatic picks and their classification is comparable with the reference picks, although a higher number of picks is downgraded to lower quality classes by the automatic picker. In the production-mode, we apply the picker to a data set of 552 earthquakes in the Alps recorded at epicentral distances 〈 or =150 km. The resulting data set includes about 2500 S phases with an upper error bound of 0.27 sec.

Description: A simple theoretical analysis shows that both local magnitude M (sub L) and seismic moment M (sub 0) or equivalently moment magnitude M (sub w) are, in principle, measures of basic properties of the earthquake source: M (sub L) is proportional to the maximum of the moment-rate function, whereas M (sub 0) is proportional to its integral. Thus, in theory, this implies that M (sub L) varies as (2/3) log M (sub 0) and M (sub L) = M (sub w) over the entire range for which M (sub L) can be determined. In practice, observed differences between M (sub L) and M (sub w) are telling us something either about the physics of the earthquake source or about inadequacies in our wave-propagation model and in our ways of measuring M (sub L) . If influences of propagation and instrument response were properly corrected for and if effects of radiation pattern and rupture directivity were averaged out, then systematic deviations of M (sub L) relative to M (sub w) could be interpreted in terms of changes in stress drop or rupture velocity. However, model calculations show that, because of the way attenuation along the path is usually corrected for, we have to expect that, in most cases, M (sub L) for small events (M (sub w) 〈2) is systematically underestimated by as much as a whole unit. Moreover, for small events with few recordings, single-station scatter due to radiation pattern and directivity can be responsible for random errors that are also on the order of a whole unit. Thus systematic and random errors in the determination of M (sub L) for small earthquakes are likely to be much greater than the variability of M (sub L) with respect to M (sub w) , which could be expected from variations in source properties. The extrapolation of constant offset corrections between regional M (sub L) scales and M (sub w) to smaller events, for which independent determinations of M (sub 0) are usually lacking, is not advisable: in most cases the large random errors and systematic underestimation of M (sub L) can contribute a significant bias to magnitude recurrence relations.

Description: Using 3D Green's functions we determine full and constrained moment tensor solutions of icequakes near the base of Gornergletscher, Switzerland. The seismic events were recorded in the summer of 2004 using a high-density seismometer array. The seismic velocity model used in the generation of Green's functions is based on radio-echo soundings to approximate the basal topography, which beneath the study site exhibits a strong inclination. As the basal conditions are not well known, we try moment tensor inversions with seismic velocity profiles consisting of two and three media. The former case consists of homogeneous ice resting on bedrock, whereas the latter case includes a thin basal layer with slow seismic velocities representing eroded material or highly fractured ice. Effects of errors in Green's functions are estimated by sensitivity studies in which we invert 1D and 3D synthetics using Green's functions of wrong velocity models. The results show that calculations of source types and fault plane orientations of tensile cracks are rather robust with respect to errors in Green's functions. However, the quality of the waveform fits depends on strike and dip of the synthetic source. When inverting seismograms, Green's functions of the seismic model that includes the basal slow velocity layer are found to give the most realistic source types as well as the best waveform fits. The fault mechanisms derived from constrained moment tensor inversions are near-horizontal tensile cracks, which suggest a complex time-dependent basal stress field.

Description: An earthquake catalog containing a uniform size estimate is important for long-term seismic hazard assessment in regions of low-to-moderate seismicity. During the update of the Earthquake Catalog of Switzerland (ECOS), we performed regression analyses to convert all earthquake size information in ECOS to physically meaningful moment magnitude M (sub W) . For 34 events in and near Switzerland, we determined seismic moment (thus M (sub W) ) by regional waveform inversion. Independent M (sub W) estimates for the same events do not exist; however, M (sub W) from European-Mediterranean events, obtained in the same way, agree with M (sub W) from Harvard CMT solutions. All other size estimates, M (sub L) , M (sub D) , m (sub b) , M (sub S) , and intensities, are calibrated relative to these 34 events. Teleseismic M (sub S) and m (sub b) from international data centers are directly regressed against M (sub W) . Most observations in ECOS consist of local magnitudes (M (sub L) , M (sub D) ) and intensities. For local magnitudes, we first calibrated the Swiss Seismological Service's M (sub L) . Then we calibrated magnitudes from observatories in neighboring countries (France, Germany, Italy) using only events in the border region (e.g., France-Switzerland). Modern instrumental records exist only since the mid-1970s. We calibrated the macroseismic dataset, which represents by far the largest period in the catalog, by determining surface wave magnitude M (sub S) for stronger twentieth century Swiss earthquakes from analog seismograms. These M (sub S) , which were converted to M (sub W) , connect intensities and M (sub W) . After calibration, all 20,300 events in ECOS have a unified M (sub W) , including a class-type uncertainty estimate based on the original magnitude scale. ECOS covers the period 250-2001, from 44 degrees N to 51 degrees N and 4 degrees E to 13 degrees E. The largest event in ECOS is the 1356 M (sub W) 6.9 Basle earthquake.

Description: We have determined seismic source mechanisms for shallow and intermediate-depth icequake clusters recorded on the glacier Gornergletscher, Switzerland, during the summers of 2004 and 2006. The selected seismic events are part of a large data set of over 80,000 seismic events acquired with a dense seismic network deployed in order to study the yearly rapid drainage of Gornersee lake, a nearby ice-marginal lake. Using simple frequency and distance scaling and Green"s functions for a homogeneous half-space, we calculated moment tensor solutions for icequakes with M (sub w) -1.5 using a full-waveform inversion method usually applied to moderate seismic events (M (sub w) 〉4) recorded at local to regional distances ( nearly equal 50-700 km). Inversions from typical shallow events are shown to represent tensile crack openings. This explains well the dominating Rayleigh waves and compressive first motions observed at all recording seismograms. As these characteristics can be observed in most icequake signals, we believe that the vast majority of icequakes recorded in the 2 yr is due to tensile faulting, most likely caused by surface crevasse openings. We also identified a shallow cluster with somewhat atypical waveforms in that they show less dominant Rayleigh waves and quadrantal radiation patterns of first motions. Their moment tensors are dominated by a large double-couple component, which is strong evidence for shear faulting. Although less than a dozen such icequakes have been identified, this is a substantial result as it shows that shear faulting in glacier ice is generally possible even in the absence of extreme flow changes such as during glacier surges. A third source of icequakes was located at 100 m depth. These sources can be represented by tensile crack openings. Because of the high-hydrostatic pressure within the ice at these depths, these events are most likely related to the presence of water lenses that reduce the effective stress to allow for tensile faulting.

Description: We estimate moment magnitudes M (sub w) for earthquakes in Switzerland recorded between 1998 and 2009 using three different spectral methods. The M (sub w) estimation in Switzerland is extended to lower magnitudes (local magnitude M (sub L) 0.1), and scaling relations between M (sub L) and M (sub w) are investigated. Above M (sub L) 4, the obtained M (sub w) estimates are consistent with the previously obtained scaling relation of M (sub w) =M (sub L) -0.3 at the Swiss Seismological Service (SED). Below M (sub L) 4, all three methods indicate that a 1:1-type relationship is inappropriate. Therefore, we propose a new piecewise empirical scaling relation for earthquakes in Switzerland. The scaling is linear below M (sub L) 2 and above M (sub L) 4. To obtain a smooth transition between the two linear scales we fit a quadratic relation in between (2〈 or =M (sub L) 〈 or =4). This scaling relation is also consistent with M (sub w) estimates from moment-tensor (MT) solutions based on broadband waveform fitting of local earthquakes with M (sub L) 〉3.0. We have tested all three methods carefully to ensure that the observed break in scale at around M 3 cannot be attributed to bias in the M (sub w) determination. However, we cannot determine with certainty from the dataset at hand whether the break in scaling is due to bias in the routine determination of M (sub L) or to physical properties of the source.

Description: Theoretical considerations and empirical regressions show that, in the magnitude range between 3 and 5, local magnitude, M (sub L) , and moment magnitude, M (sub w) , scale 1:1. Previous studies suggest that for smaller magnitudes this 1:1 scaling breaks down. However, the scatter between M (sub L) and M (sub w) at small magnitudes is usually large and the resulting scaling relations are therefore uncertain. In an attempt to reduce these uncertainties, we first analyze the M (sub L) versus M (sub w) relation based on 195 events, induced by the stimulation of a geothermal reservoir below the city of Basel, Switzerland. Values of M (sub L) range from 0.7 to 3.4. From these data we derive a scaling of M (sub L) approximately 1.5M (sub w) over the given magnitude range. We then compare peak Wood-Anderson amplitudes to the low-frequency plateau of the displacement spectra for six sequences of similar earthquakes in Switzerland in the range of 0.5〈 or =M (sub L) 〈 or =4.1. Because effects due to the radiation pattern and to the propagation path between source and receiver are nearly identical at a particular station for all events in a given sequence, the scatter in the data is substantially reduced. Again we obtain a scaling equivalent to M (sub L) approximately 1.5M (sub w) . Based on simulations using synthetic source time functions for different magnitudes and Q values estimated from spectral ratios between downhole and surface recordings, we conclude that the observed scaling can be explained by attenuation and scattering along the path. Other effects that could explain the observed magnitude scaling, such as a possible systematic increase of stress drop or rupture velocity with moment magnitude, are masked by attenuation along the path.