GEOMETRICAL SPREADING AND ATTENUATION OF LG WAVES - A COMPARISON BETWEEN WESTERN ANATOLIA (TURKEY) AND SOUTHERN SPAIN

We compared the attenuation and geometrical spreading coefficient of Lg waves for crustal paths in western Anatolia and southern Spain, at source-receiver distances between 80 and 400 km. We used the coda normalization method which is based on the elimination of the source and site effects from observed spectra of Lg waves. Q for Lg waves in the crust was determined as a function of frequency in the range 1.5-10.0 Hz for western Anatolia and in the range 1.5-12 Hz for southern Spain. We found that Q(Lg) shows a strong frequency dependence in both regions. In western Anatolia, the frequency dependence of Q(Lg) is Q(Lg)(f)= 82.f(1.0) and the geometrical spreading coefficient ranges from 0.6 to 0.9. We found Q(Lg) = 83.f(0.88) and a geometrical spreading coefficient close to 0.5 in southern Spain. Comparing our results with code-Q values obtained in both regions, Q(Lg) and Q(Lg) are similar. Q(Lg) is close to Q(i) in all frequency bands. We interpret the geometrical spreading coefficient higher than 0.5 for western Anatolia as due to a strong layered and heterogeneous crustal structure

Source Parameters of Microearthquakes At Phlegraean Fields (southern Italy) Volcanic Area

The seismic activity that occurred at the Pblegraean Fields (Southern Italy) volcanic area during a pronounced episode of ground uplift has been analysed. One hundred and eighty-one three component seismograms from a digital network operating in the period January—May 1984 were processed to obtain seismic moments, source radii and stress dropsfor32microearthquakes(0.7<ML< 3.2).Aninversemethodbasedonaleastsquaresfittingofdisplacement spectra with a theoretical model has been applied to infer the spectral parameters of P-radial and SH records. This method has the advantage of giving statistical errors on the spectral estimates. The computed seismic moments range between 0.5 x 1018 and 0.13 x 1021 dyne-cm. Stress drops are approximately constant, and do not clearly depend on seismic moments, which is in contrast with other reported data from microearthquake sequences. Small values of source radii (few tens of metres) are found.Published25-423.1. Fisica dei terremotiJCR Journalreserve