Karl Koch

Regional Ground-Motion Scaling in Central Europe

Regressions of 2700 horizontal-component broadband seismograms from 213 seismic events recorded by the German Regional Seismic Network (67 earthquakes and 146 large mining explosions and rockbursts) are carried out to study the scaling relationships of high-frequency S -wave motion for central Europe. At a set of sampling frequencies, regressions were performed on the logarithms of the peak amplitudes of narrow bandpass-filtered seismograms, as well as on the logarithms of the Fourier components of the velocity spectra. At a fixed frequency f , these values are written as \[\mathrm{AMP}(f,{\ }r)=\mathrm{EXC}(f,{\ }r_{\mathrm{ref}})+\mathrm{SITE}(f)+D(r,{\ }r_{\mathrm{ref}},{\ }f).\] EXC( f, r ref ) is the excitation at an arbitrary reference hypocentral distance, r ref , SITE( f ) is a site term, and D ( r, r ref , f ) describes the crustal attenuation in the region. The crustal propagation term, empirically estimated in the (0.5–16.0 Hz) frequency band and (40–600 km) distance range, is modeled using a complex geometrical spreading function and a frequency-dependent crustal Q . We suggest \[Q(f)=Q_{0}{\cdot}\left(\frac{f}{f_{\mathrm{ref}}}\right)^{{\eta}},{\ }\begin{array}{ccc}Q_{0}&=&400\\{\eta}&=&0.42\\f_{\mathrm{ref}}&=&1.0{\ }\mathrm{Hz}\end{array}\] and a log-log quadrilinear geometrical spreading. A factor exp(-πκ 0 f ) is used to fit the high-frequency roll-off of the inverted excitation terms. Since we deal with two different kinds of sources (explosions-rockbursts and earthquakes), we use \[{\kappa}_{0}=\begin{array}{l}0.08{\ }\mathrm{sec}{\ }(\mathrm{for\ explosions});\\0.05{\ }\mathrm{sec}{\ }(\mathrm{for\ earthquakes}).\end{array}\] The same Brune spectral model, characterized by a stress drop Δσ = 30 bars, is used to fit both earthquakes and explosive excitation terms. A regression on the effective duration of the ground motion following the S -wave onset is also carried out. In central Europe, duration is observed to be almost frequency-independent. This property might be explained in terms of a self-similar distribution of crustal scatterers.

Identification of Earthquakes and Explosions Using Amplitude Ratios: The Vogtland Area Revisited

Abstract — Identification of seismic events is a major scientific issue in the framework of verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Of special interest in this context is the identification of the numerous low-yield mining or blasting events, especially those occurring in the same area as earthquakes, such as the Vogtland area in the border region of Germany and the Czech Republic. Seismic events in this area were investigated by WÜSTER (1993, 1995), who achieved complete discrimination using measures of spectral decay and spectral variance at the GERESS array and a quadratic discrimination function.¶A subset of these events, for which ground-truth information is available, has been analyzed in this study using multivariate statistical analysis. Various parameters based on measurements from seismic waveforms of the broadband stations of the German Regional Seismic Network (GRSN) and short-period elements of the GERESS array are tested for statistical significance in a linear regression analysis, in particular spectral amplitude ratios for the Lg phase and Pg/Sg amplitude ratios. The subset includes a total of 35 explosions and 24 earthquakes. The results of our study argue that identification based on spectral Lg and high-frequency Pg/Sg ratios is promising. However, discrimination success is strongly varying from station to station; thus, weighting according to station success rates could improve the overall identification capability.

Discrimination between Earthquakes and Chemical Explosions by Multivariate Statistical Analysis: A Case Study for Switzerland

A data set of 43 confirmed earthquakes and 42 explosions (1.3 ≤ M L ≤ 3.8) is analyzed by multivariate statistical analysis. The explosion events include quarry blasts and explosions detonated for seismic experiments as well as an explosion of an ammunition storage site. The events of this training data set are mainly located in the Alpine mountains and Central Switzerland, with the seismic sources having excellent distance ( R < 300 km) and azimuthal coverage for the recording stations. Multivariate statistical analysis is used to derive general discriminant functions based on regional waveform data for the training set. The discriminant functions are then tested for consistency on the events of the training data set, with the result that 97% of these events are correctly identified by the multivariate discriminant functions. Of the total training data set, 17% is classified as ambiguous cases, based on marginal discriminant functions. The method is subsequently applied to an extended data set including nighttime events from 1992-1996, which are hence regarded as earthquakes, and a set of presumed explosions from the years 1984-1997. For Central Switzerland, the events of the extended data set can clearly be identified with the discriminant functions derived from the training data set. The identification rate is lower for other regions in Switzerland, which can be related to tectonic units. For these regions, considered to have a laterally homogeneous crust, a linear correction term in the discriminant function providing for station corrections can be found that significantly decreases the misidentification rate. Online material: table of earthquake and explosion events Manuscript received 11 May 2001.

Determination of Mislocation Vectors to Evaluate bias at GSETT-3 primary stations

Using the database provided by the Reviewed Event Bulletins (REBs) for the first 2.5 years of the Group of Scientific Experts Technical Test-3 (GSETT-3) experiment, we compiled mislocation vectors for both arrays and selected three-component stations of the primary network from the published slowness and azimuth information gained through f-k- and polarization analysis. Imposing constraints such as a minimum signal-to-noise ratio (SNR) and number of defining phases, we aim at eliminating location bias as the hypocentral parameters are taken from the REBs. Results from 14 arrays with apertures from about 1 km to more than 20 km are presented as well as from 18 three-component stations, which indicate that the mislocation vectors in many cases can improve location accuracy considerably. If these mislocation vectors are compiled to provide coverage of a sufficient portion of the slowness domain these empirical corrections can easily be applied prior to location processing. In the context of the Comprehensive Nuclear Test-Ban Treaty (CTBT), these mislocation patterns could be essential for providing accurate event location of suspicious low-magnitude events, as these location parameters will be used to pinpoint the area where to conduct an on-site inspection.

Determination of focal mechanism from SV/P amplitude ratios at small distances

Abstract A computational scheme for determining focal mechanisms of small earthquakes recorded at short distances by high-gain, short-period seismographs has been developed and tested. The steps in the procedure are explained. The most serious difficulty is the inherent nonlinearity of the problem and the attendant question of determining when a convergent solution is a well-constrained representation of the real focal mechanism.

Seismic Event Identification of Earthquakes and Explosions in Germany Using Spectral Lg Ratios

Abstract — At the German NDC initial work on seismic event identification has focused on the application of spectral amplitude ratios for Lg in order to discriminate naturally occurring seismic events from other events associated with mining and quarry activities. Only about 10% of all seismic events occurring in Germany and adjacent areas are due to natural seismicity and are mostly constrained to the Alpine regions and areas along the Rhinegraben, Rhenish massif, Swabian Jura, and the Bohemian massif (Vogtland region). Using data from the broadband GRSN network, spectral amplitude ratios are calculated from maximum trace amplitudes in the 1–2 Hz and 6–8 Hz frequency bands, which are within the passbands of the deployed STS-2 instruments and the recorded 20 Hz data streams. These amplitude ratios then must be corrected with an appropriate attenuation model in order to remove propagation paths effects. For event identification, a scoring scheme is applied across the GRSN network, based on station-dependent scoring thresholds. In a case study aimed at testing the identification scheme, events are investigated from a quarry in southern Germany that provided ground-truth information for six events in 1997 to demonstrate the suitability of this identification approach. Except for one event with a rather strong earthquake signature, i.e., a low spectral Lg ratio, these events could be screened out from the earthquake population by their large Lg ratios. In a second step, aimed at applying the identification scheme, all events in Germany and neighboring areas that occurred in 1995 were processed, with approximately 800 out of more than 1200 events showing explosion-type Lg ratios, while only 10% remain in the earthquake population. However, specific mining areas appear to consistently produce earthquake-type spectral ratios indicative of particular blasting practices.

A Quantitative Study of the Site Effects Observed at the GERESS Array

Site effects were studied for the individual sites of the 25 elements of the German Experimental Seismic System (GERESS) array based on the traditional spectral ratio (TSR) technique and horizontal-to-vertical ratios (HVRs) for earthquakes and microtremors, commonly referred to as Nakamura9s technique. An understanding of these site effects is required for various applications, in particular for the discrimination of seismic events. The spectral ratio technique identifies three stations (A0, A3, B3) as showing prominent peaks in the frequency band from 1 to 17 Hz up to a factor of 30, with A0 and B3 having the largest site effect at a frequency of 7 Hz. These stations are situated in the immediate vicinity of the array center. A number of other stations, such as A1, B1, B4, B5, C1, C3–C5, D2, D5, and D7–D9, display a broad frequency range preferably above 10 Hz, where the spectral ratios indicate up to 10 times higher spectral amplitudes compared to the reference site, C2. The remaining stations, A2, B2, C6, C7, D1, D3, D4, and D6, show no noticeable site effects. Further study considers the appropriateness for the selection of C2 as reference site for the TSR technique and the comparison of TSR and HVR techniques for the three-component elements of the array. Although typically site response refers to the site amplification of horizontal ground motions, the assessed site response of vertical ground motions seems to yield comparable results. The spectral variance, a parameter based on the smoothness of stacked array spectra from the best 17 array elements of the GERESS array, has been used successfully to distinguish between local earthquakes and mining explosions. It is shown that the spectral variance estimate is considerably improved using all 25 array elements and applying a site response correction.

The use of teleseismic P-wave complexity for seismic event screening – results determined from GRF and GERESS array data

Seismic data recorded at the broad-band teleseismic GRF array and theshort-period regional GERESS array, which is a designated IMS primarystation, are analyzed to determine the effectiveness of teleseismic P-wave complexity for the purpose of seismic event screening within theframework of Comprehensive Nuclear-Test-Ban Treaty verification. For theGRF array, seismic waveform data from nearly 200 nuclear explosions havebeen recorded since its installation in the late 1970s, which were studiedalong with several thousand earthquakes from the last few years.Additionally, we investigated teleseismic P wave complexity for a similarnumber of earthquakes recorded at GERESS. However, owing to itsoperation starting in 1991, only a limited number of nuclear explosionseismograms are available for study.For nuclear explosions, complexity does not exceed levels of 0.3 except fora number of events from the Nevada Test Site recorded only at the GRFarray and located at a large distance where PcP may interfere with the initialP wavelet. Since all events with complexity at GRF larger than 0.3 areexclusively located on Pahute Mesa within the Nevada Test Site,near-source geology or topography must play a dominant role for theseincreased complexity values, while PcP may not contribute significantly tothe high-frequency energy measured by the complexity parameter.Although many earthquakes show complexities below this level, for morethan 25% of the earthquakes investigated the complexities determined arelarger than 0.7, thus showing distinctly larger values than nuclearexplosions. Therefore, this percentage may be screened as earthquakes fromall seismic events detected. As currently only about half of the eventsdetected by the global IMS network are screened out based on focal depthand the mb:Ms criterion, teleseismic P-wavecomplexity may contribute significantly to the task of seismic eventscreening.