Traveltime and amplitude anomalies at the seismic broad-band may GRF

Abstract

S U M M A R Y Traveltime and amplitude residuals of P waves from teleseismic events show a positive correlation at the stations of the German seismic broad-band array GRF. Fast arrivals consistently have reduced amplitudes in the four frequency ranges considered (short-period bandpass, WWSSN-SP filter, broad-band velocity and WWSSN-LP filter).With the exception of the WWSSN-LP filter, 40-60 per cent of the average of the relative traveltime and almost 100 per cent of the average of the amplitude residuals at individual stations, with respect to the reference station A l , can be explained by near-surface sediment layers. These shallow structures are known from local and regional geology and are documented in borehole, local refraction and polarization studies. This aximuthal-and distance-independent term (station average) reaches values of up to -0.55 s and -0.15 units in log amplitude (magnitude).Even after the removal of these station averages, which also contain a trend of increasing crustal and uppermost mantle velocity from north to south, large azimuthal-and distance-dependent relative traveltime and magnitude residuals can be observed across the array. These residuals vary between -0.8 and +1.1 s and -0.54 and +0.72 magnitude units. Negative (fast) traveltime residuals are again related to negative magnitude residuals (small amplitudes). The azimuthal-and distance-dependent patterns of these residuals are identical within three distinct clusters of stations. The main features of the residual patterns correlate well with the main fault systems and inhomogeneities near the array, some of which have been postulated previously. The velocity anomalies causing the azimuthal-and distancedependent residuals are located in crust and uppermost mantle under the array and its vicinity.The fact that local effects like sedimentary covers are responsible for 40 per cent or more of the station average residuals demonstrates again that site effects should be removed before tomographic methods are employed. The simple stations corrections usually applied in global tomography might severely underestimate the influence of local and regional velocity anomalies, since even after the removal of the station average residual large azimuthal-and distance-dependent traveltime and amplitude anomalies remain between stations less than 100 km apart. These anomalies due to shallow inhomogeneities might then erroneously be mapped into features at greater depth.

Broadband frequency-dependent amplification of seismic waves across Bucharest, Romania

“…Largescale structures spanning several wavelengths may cause laterally systematic amplitude variations. An example is the amplitude pattern at the Gräfenberg Array, Germany, which is due to a dipping sedimentary wedge (Weber 1994). …”

Small 660‐km seismic discontinuity beneath Tibet implies resting ground for detached lithosphere

“…To this end, we use traveltime residuals, the difference between observed and predicted traveltimes, from earthquakes at teleseismic distances (30°to 90°). At teleseismic distances, effects of wave propagation in the deep earth are simple and well understood; and the approach used here is known to be effective for studying the TZ [Weber, 1994;Brudzinski et al, 1997]. We determine traveltime residuals from up to 35 teleseismic earthquakes at 50 stations whose data are analyzed to constrain V P under Tibet (see Table S1).…”

“…1 We use only earthquakes whose back azimuths fall between 110°and 150°, the same general direction as earthquake sources used for seismic profiles, so any azimuthal dependence of station corrections are taken into account [Weber, 1994;Brudzinski et al, 1997].…”

High-resolution teleseismic tomography of upper-mantle structure using ana priorithree-dimensional crustal model

“…Resolution in teleseismic tomography of upper‐mantle structure depends critically on our understanding of the 3‐D crustal velocity structure beneath a receiver array. Because of the low frequencies and near‐vertical angle of incidence of incoming waves, teleseismic traveltime tomography resolves the crustal‐scale structure poorly, though complex crustal structures influence teleseismic traveltimes strongly (Baer 1980; Guyoton 1991; Weber 1994; Waldhauser 1996). When propagating through regions with a rough topography of the crust–mantle boundary and with strong intracrustal velocity inhomogeneities, teleseismic wave fronts undergo distortions that lead to significant traveltime and amplitude anomalies observed at the Earth's surface.…”