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Titolo:
TRAVEL-TIME AND AMPLITUDE ANOMALIES AT THE SEISMIC BROAD-BAND ARRAY GRF
Autore:
WEBER M;
Indirizzi:
SEISMOL ZENT OBSERV SZGRF,KRANKENHAUSSTR 1-3 D-91054 ERLANGEN GERMANY GEOFORSCHUNGSZENTRUM POTSDAM D-14473 POTSDAM GERMANY
Titolo Testata:
Geophysical journal international
fascicolo: 1, volume: 118, anno: 1994,
pagine: 57 - 74
SICI:
0956-540X(1994)118:1<57:TAAAAT>2.0.ZU;2-V
Fonte:
ISI
Lingua:
ENG
Soggetto:
P-WAVES; REFLECTIVITY METHOD; TIME; NORSAR; LITHOSPHERE; SCATTERING; CALIFORNIA; RESIDUALS; CRUST; LASA;
Keywords:
AMPLITUDE RESIDUALS; P-WAVES; SEDIMENTS; SEISMIC ARRAY; SITE EFFECT; TOMOGRAPHY;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Science Citation Index Expanded
Citazioni:
49
Recensione:
Indirizzi per estratti:
Citazione:
M. Weber, "TRAVEL-TIME AND AMPLITUDE ANOMALIES AT THE SEISMIC BROAD-BAND ARRAY GRF", Geophysical journal international, 118(1), 1994, pp. 57-74

Abstract

Traveltime and amplitude residuals of P waves from teleseismic eventsshow 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 stationAl, can be explained by near-surface sediment layers. These shallow structures are known from local and regional geology and are documentedin 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 afterthe 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 ofstations. The main features of the residual patterns correlate well with the main fault systems and inhomogeneities near the array, some ofwhich have been postulated previously. The velocity anomalies causingthe azimuthal- and distance-dependent 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 mightseverely underestimate the influence of local and regional velocity anomalies, since even after the removal of the station average residuallarge 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.

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Documento generato il 01/12/20 alle ore 13:17:32