Analysis of the Granada (Spain) earthquake of 24 June, 1984 (M=5) with emphasis on seismic hazard in the Granada Basin

Abstract

The earthquake of 24 June, 1984, with epicenter in the Granada Basin (36.86 degrees N, 3.76 degrees W

H = 12 km

m(b) = 4.9), was the first event to produce accelerograms in Spain. It was recorded at three sites, Beznar, Alhama, and Santa Fe, within about 40 km of the epicenter. It was also recorded by three NARS network stations (NE11, NE13 and NE14), which were in operation in Spain at the time. The data from Alhama and NE14, however, are not usable because of instrumental problems, The fact that the epicenter of the 1984 event was relatively close to the highly destructive earthquake of 25 December 1884 (M = 6.7-7.1) makes the source study of the 1984 event very interesting and useful. By synthetic modelling of the regional seismograms at NE11, NE13 and Toledo, the seismic moment is estimated as 3.0 x 10(23) dyne-cm (M(w) = 5.0). Source spectra estimated from accelerograms at Beznar and Santa Fe suggest that Holocene sediments (200 m in thickness) at Santa Fe may cause broadband amplification of about 10. From the spectra obtained from the Beznar accelerogram, the corner frequency is estimated as 1 Hz, which gives a stress drop of about 100 bar. We have synthesized expected ground motions at Santa Fe and Beznar from a postulated M(w) = 7.0 earthquake (similar to the 1884 event) using recordings of the 1984 event as empirical Green's functions. The simulations suggest that the horizontal accelerations during the postulated event may be 8-10 times greater than those observed during the 1984 event, exceeding 1/4 g at Santa Fe. Because of their close proximity and similar subsoil conditions, estimations at Santa Fe may also be valid for the lower part of the city of Granada. We finally compare the expected response spectra for the postulated event with the proposed design spectra.