An attenuation study using earthquakes from the 1997 Umbria-Marche sequence |
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| Authors: | Raúl R Castro Giancarlo Monachesi Luca Trojani Marco Mucciarelli Massimo Frapiccini |
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| Institution: | (1) División Ciencias de la Tierra, CICESE, Apdo. Postal 2732, Ensenada, 22860 Baja California, México;(2) Osservatorio Geofísico Sperimentale di Macerata, Viale Indipendenza 180, Macerata, 62100, Italia;(3) Universita della Basilicata, Potenza, Italia |
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| Abstract: | We studied spatial and temporal characteristics of seismic attenuation inCentral Italy using S- and coda- waves recorded by the MarchesanSeismograph Network from earthquakes located in the epicentral area ofthe 1997 Umbria-Marche sequence. The amplitude decay of the S waveswith distance was defined calculating empirical attenuation functions at 15frequencies between 1 and 25 Hz. We analyzed separately foreshocks andaftershocks and we found the same attenuation functions, suggesting thatthe possible temporal variations could be confined in a small area. Thefrequency dependence of Q
S was approximated by the equation Q
S=18 · f
2.0between 1 and 10 Hz. At higher frequencies (10–25 Hz), the frequencydependence of Q
s weakens, having an average value of Q
S=990. We also estimated Q from coda waves (Q
C) using the single-scattering models of Aki andChouet (1975) and Sato (1977). We found that Q
C=77 · f
0.6, (between 2 and 20Hz) at the western side of the mountain chain, using either foreshocks oraftershocks. This relation is consistent with previous estimates of Q
Creported for the Central Apennines. For a volume sampling the Colfioritobasin, the Apennines and the Marche region we found that Q
C=55 · f
0.8,indicating highattenuation below the mountain belt. To detect small temporal changes ofQ, we calculated spectral ratios of 5 temporal doublets located in theepicentral area and recorded at the closest station. We found temporalchanges of Q that vary from 27% to 56%, depending on the locationof the doublets. This variability suggests that the temporal change ofattenuation may depend on the spatial variation of Q and perhaps on thespatial distribution of tectonic stress in the epicentral area. |
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| Keywords: | coda waves Q S-wave attenuation Umbria-Marche sequence |
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