Johannes Schweitzer

HYPOSAT — An Enhanced Routine to Locate Seismic Events

Abstract — A program package, called HYPOSAT, has been under development that attempts to use the maximum information possible to estimate the hypocenter of a seismic source. The standard input parameters can be used: arrival times of first and later onsets with backazimuths and ray parameters (or apparent velocities). In addition, travel-time differences between different phases observed at the same station can be optionally used. The observed standard deviations are used to weight all input parameters and the inversion is done with a generalized matrix inversion code.¶A starting solution with apriori uncertainties can be calculated as the intersection of all backazimuth observations. If S observations are also available, a preliminary origin time is estimated using Wadatis approach to estimate a source time.¶Global earth models and user-defined horizontally layered local or regional models can be used alone or together to locate seismic events. To gain the best result from all input data, observations of all seismic phases as defined in the IASPEI91 tables can be inverted. Station corrections and corrections for phases with reflection points at the earths surface can be applied by using local velocity structures.

Slowness Corrections — One Way to Improve IDC Products

Abstract — The first step to identify and locate a seismic event is the association of observed onsets with common seismic sources. This is especially important in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT) at the International Data Center (IDC) being developed in Vienna, Austria. Well-defined slowness measurements are very useful for associating seismic phases to presumed seismic events.¶Shortly after installation of the first seismic arrays, systematic discrepancies between measured and theoretically predicted slowness values were observed, and therefore slowness measurements of seismic stations should be calibrated. The observed slownesses measured with small aperture arrays, some of which will be included in the International Monitoring System (IMS) now being implemented for verifying compliance with the CTBT, show large scatter and deviations from theoretically expected values. However, in this study a method is presented, by which mean slowness corrections can be derived, which show relatively stable patterns specific to each array.¶The correction of measured slowness values of these arrays clearly improved the single array location capabilities. Applying slowness corrections with seismic phases observed by ARCES, FINES, GERES, and NORES, and associated to seismic events in the bulletins of the prototype International Data Center (pIDC) in Arlington, VA, also clearly demonstrates the advantages of these corrections. For arrays with large slowness deviations that are due to the influence of a dipping layer, the corrections were modeled with a sine function depending on the measured azimuth. In addition, the measured values can be weighted with the corresponding uncertainties known from the process of deriving the mean corrections.

Comparison of Location Procedures: The Kara Sea Event of 16 August 1997

Various location procedures and velocity models are compared for the ML 3.3 Kara Sea event of 16 August 1997. This event has been the subject of considerable discussion because of its geographical position and the difficulty in obtaining a reliable focal-depth estimate. A comprehensive data set was extracted by (re-)reading the records from all available stations. These readings have then been used in a sequence of location experiments to examine the effect of using different velocity models to describe the travel times of the phases, and also to compare the use of a fully nonlinear scheme (shakeNA; Sambridge and Kennett, 2001) and a linearized location algorithm (HYPOSAT; Schweitzer, 2001, 2002). A standard least-squares misfit criterion has been used for direct comparisons between the two methods. The results confirm both the importance of S-wave information in assessing the depth of regional events, and the need to apply a reliable velocity model to place the strongest constraints on the location of the event. Even with only a limited data set, but an adequate velocity model, it is possible to find the position of the Kara Sea event close to the most probable locations; however, there is then no depth resolution. Reported error ellipses from standard data centers tend to have relatively small error ellipses. With the commonly made assumption that the reading errors and the a posteriori residuals have an unbiased normal distribution, such inversion results may indicate an unreasonably high resolution and accuracy of the solution. The epicenter estimates for the whole data set using the range of different tech- niques agree quite well, with some overlap of the estimated confidence regions. The observed seismic source was most likely an earthquake in the middle or lower crust at about 10-30 km depth.

88 – Old Seismic Bulletins to 1920: A Collective Heritage from Early Seismologists

This chapter focuses on collective heritage from early seismologists. Scientists began systematic instrumental observation of earthquakes in the latter part of the 19 th century. Several authors describe the history of the development of an adequate instrumentation for seismology. In the 1880s, scientists in Italy, Japan, and Germany began to record more or less continuously the ground motion with their newly developed seismographs. Because of the limited ability to reproduce the original (analog) paper seismograms, seismologists had to describe their observations in words and numbers. The recording instruments were able to produce seismograms in which one could distinguish between the onsets of all three wave types (i.e., P, S, and surface waves), and a common vocabulary for the description of these records was developed. During the last half of the 19 th century, scientists in many countries began to systematically collect data of macro seismically observed earthquakes and the locations of these events, known only on the basis of such data. In some countries, scientists and/or their governments established special committees or commissions to do this work. The earthquake lists and bulletin data from the early seismic stations not only document the history of seismology but also have intrinsic scientific value. Even today, these old bulletins are needed for event relocation as our technique for earthquake location improves. They are also useful for magnitude analysis to establish a consistent magnitude scale.

Application of the Multichannel Wiener Filter to Regional Event Detection Using NORSAR Seismic-Array Data

Seismic arrays for detection of small earthquakes benefit from reduced noise levels by array processing methods. We present a frequency-dependent multi-channel Wiener fil- tering (MCWF) technique, which employs an adaptive least-squares method to remove coherent noise in seismic array data. The noise records on a number of reference chan- nels are used to predict the noise on a primary channel, which can then be subtracted from the observed data. A sequence of aftershocks caused by a Feb, 21, 2008 main shock in Spitsbergen was recorded in 2008 by the ARCES array in northern Norway. This aftershock sequence was filtered using the multi-channel Wiener filters on both triggered and continuous modes. The Spitsbergen (SPITS) array at a much closer dis- tance to the source region provides reliable reference information on the true number of detectable aftershocks. The triggered procedure detects 631 aftershocks at ARCES in total, where conventional delay-and-sum beamforming combined with a band-pass filter could detect 513 aftershocks with 181 false alarms using a series of constraints, signal-to-noise ratio (SNR), back azimuth and slowness; the multi-channel Wiener fil- tered results found 577 aftershocks with 165 false alarms using the same constraints. A complete automatic multi-channel Wiener procedure is developed for event detec- tion on continuous data. The appropriate SNR threshold for aftershock detection 2.7 is suggested. The MCWF also demonstrates the advantage of reducing false alarms compared to the beamforming method when detecting the same number of aftershocks.

An Empirical Procedure for Rapid Magnitude Estimation in Italy

A composition for cleaning petroleum contaminated surfaces includes 5 to 60 percent by weight of a dioctyl sulfosuccinate salt, 20 to 50 percent by weight of a carrier composition and 10 to 75 percent by weight of a diluent. The preferred carrier composition is a mixture of polyethylene glycol and glycerol. The preferred diluent is water. The preferred concentration of sodium dioctyl sulfosuccinate is about 7% by weight. The preferred dioctyl sulfosuccinate salt is sodium dioctyl sulfosuccinate. A method of cleaning petroleum contaminated surfaces is also provided. The steps include providing a solution containing dioctyl sulfosuccinate salt, a carrier composition, and a diluent as described above and treating the petroleum contaminated surface with the solution in a manner and for a time sufficient to decontaminate the surface. The preferred method of treatment comprises contacting the solution with the petroleum contaminated surface for a time sufficient to create an emulsion and then removing the emulsion. The composition can be used to clean tissue.

BARENTS16: a 1-D velocity model for the western Barents Sea

A minimum 1-D seismic velocity model for routine seismic event location purposes was determined for the area of the western Barents Sea, using a modified version of the VELEST code. The resulting model, BARENTS16, and corresponding station corrections were produced using data from stations at regional distances, the vast majority located in the periphery of the recorded seismic activity, due to the unfavorable land–sea distribution. Recorded seismicity is approached through the listings of a joint bulletin, resulting from the merging of several international and regional bulletins for the region, as well as additional parametric data from temporary deployments. We discuss the challenges posed by this extreme network-seismicity geometry in terms of velocity estimation resolution and result stability. Although the conditions do not facilitate the estimation of meaningful station corrections at the farthermost stations, and even well-resolved corrections do not have a convincing contribution, we show that the process can still converge to a stable velocity average for the crust and upper mantle, in good agreement with a priori information about the regional structure and geology, which reduces adequately errors in event location estimates.