Abstract
During the two mainshocks of September 26, 1997 inthe Umbria-Marche border a strong-motion accelerographrecorded peak ground accelerations as large as 0.6 g,approximately, in the town of Nocera Umbra, atdistances of 10 to 15 km from the epicentres. Thisvalue is significantly larger than expected on thebasis of the usual regressions with magnitude anddistance. A broad-band amplification up to a factor of10 was consistently estimated in previous papers,using both weak and strong motion data recorded at theaccelerograph site during local moderate earthquakes.To study the cause of this amplification we deployedsix seismologic stations across the tectonic contactbetween the Ceno-Mesozoic limestone and the Mesozoicmarly sandstone where the accelerograph is installed.Seismograms of 21 shallow aftershocks in the magnituderange from 2.2 to 4.0 and a subcrustal Mw = 5.3event are analysed. Regardless of epicentre location,waveforms show a large complexity in an approximately200 m wide band adjacent to the tectonic contact. Thisis interpreted as the effect of trapped waves in thehighly fractured, lower velocity materials within thefault zone.
Similar content being viewed by others
References
Amato, A., Azzara, R., Chiarabba, C., Cimini, G.B., Cocco, C., Di Bona, M., Margheriti, L., Mazza, S., Mele, F., Selvaggi, G., Basili, A., Boschi, E., Courboulex, F., Deschamps, A., Gaffet, S., Bittarelli, G., Chiaraluce, L., Piccinini, D. and Ripepe, M., 1998, The Colfiorito, Umbria-Marche earthquake sequence in central Italy (Sept.-Nov., 1997): a first look to mainshocks and aftershocks, Geophys. Res. Lett. 25(15), 2861–2864.
Bonilla, L.F., Steidl, J.H., Lindley, G.T., Tumarkin, A.G. and Archuleta, R.J., 1997, Site amplification in the San Fernando Valley, California: variability of site-effect estimation using the S-wave, coda and H/V methods, Bull. Seism. Soc. Am. 87, 710–730.
Caserta, A., Bellucci, F., Cultrera, G., Donati, S., Marra, F., Mele, G., Palombo, B. and Rovelli, A., 2000, Site effects in the area of Nocera Umbra (central Italy) during the 1997 Umbria-Marche seismic sequence, Journal of Seismology, this volume.
Cello, G., Mazzoli, S., Tondi, E. and Turco, E., 1997, Active tectonics in the Central Apennines and possible implication for seismic hazard analysis in peninsular Italy, Tectonophysics 272, 43–60.
Cormier, V.F. and Spudich, P., 1984, Amplification of ground motion and waveform complexity in fault zones: examples from the San Andreas and Calaveras Faults, Geophys. J. R. astr. Soc. 79, 135–152.
Cranswick, E., 1988, The information content of high-frequency seismograms and the near-surface geologic structure of 'hard rock' recording sites, Pageoph 128, 333–363.
Ekstrom, G., Morelli, A., Boschi, E. and Dziewonski, A.M., 1998, Moment tensor analysis of the central Italy earthquake sequence of September-October 1997, Geophys. Res. Lett. 25(11), 1971–1974.
Field, E.H. and Jacob, K., 1995, A comparison and test of various site response estimation techniques, including three that are non reference-site dependent, Bull. Seism. Soc. Am. 85, 1127–1143.
GNDT, 1999, Nocera Umbra: elementi di microzonazione sismica, GNDT - SSN Report, http://daphne.irrs.mi.cnr.it/seism/Ital/homeum.html.
Lachet, C. and Bard, P.-Y., 1994, Numerical and theoretical investigations on the possibilities and limitations of the Nakamura's technique, J. Phys. Earth 42, 377–397.
Lermo, J.F. and Chavéz Garcia, F.J., 1993, Site effect evaluation using spectral ratios with only one station, Bull. Seism. Soc. Am. 83, 1574–1594.
Li, Y.-G. and Leary, P.C., 1990, Fault zone trapped seismic waves, Bull. Seism. Soc. Am. 80, 1245–1271.
Li, Y.-G., Leary, P., Aki, K. and Malin, P., 1990, Seismic trapped modes in the Oroville and San Andreas Fault zones, Science 249, 763–766.
Li, Y.-G., Vidale, J.E., Aki, K., Marone, C.J., and Lee, W.H.K., 1994, Fine structure of the Landers fault zone: segmentation and the rupture process, Science 265, 367–370.
Li, Y.-G., Vidale, J.E., Aki, K., Xu, F. and Burdette, T., 1998, Evidence of shallow fault zone strengthening after the 1992 M7.5 Landers, California, earthquake, Science 279, 217–219.
Nakamura, Y., 1989, A method for dynamic characteristics estimation of subsurface using microtremors on the ground surface, Quarterly Rept. RTRI, Jpn. 30, 25–33.
Ohmachi, T., Nakamura, Y. and Toshinawa, T., 1991, Ground Motion Characteristics in the San Francisco Bay Area Detected by Microtremor Measurements, Proceedings of the Second International Conference on Recent Advances in Geotechnical Engineering and Soil Dynamics, March 11- 15, St. Louis, Missouri, pp. 1643–1648.
Pino, N.A., Mazza, S., and Boschi, E., 1999, Rupture directivity of the major shocks in the 1997 Umbria Marche (Central Italy) sequence from regional broad band waveforms, Geophys. Res. Lett. 26, 2101–2104.
Rovelli, A., Bonamassa, O., Cocco, M., Di Bona, M. and Mazza, S., 1988, Scaling laws and spectral parameters of the ground motion in active extensional areas in Italy, Bull. Seism. Soc. Am. 78, 530–560.
Sabetta, F. and Pugliese, A., 1987, Attenuation of peak horizontal acceleration and velocity from Italian strong-motion records, Bull. Seism. Soc. Am. 77, 1491–1513.
Tucker, B.E. and King, J.L., 1984, Dependence of sediment-filled valley response on input amplitude and valley properties, Bull. Seism. Soc. Am. 74, 153–165.
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Marra, F., Azzara, R., Bellucci, F. et al. Large amplification of ground motion at rock sites within a fault zone in Nocera Umbra (central Italy). Journal of Seismology 4, 543–554 (2000). https://doi.org/10.1023/A:1026559901378
Issue Date:
DOI: https://doi.org/10.1023/A:1026559901378