Gonzalo Payo

Moment Magnitudes for Early (1923–1961) Instrumental Iberian Earthquakes

A collection of 18 Iberian earthquakes, felt with epicentral intensity I 0 ≥ VI (MSK) in the Iberian Peninsula during the period 1923–1961, just before the introduction of the WWSSN stations in 1962, have been investigated through digital reconstruction of old seismograms recorded in four Spanish seismological observatories: Alicante, Almeria, Ebro, and Toledo. Spectral analysis of the seismic waves recorded at the different stations has been performed and the seismic moment of the early instrumentally recorded earthquakes considered has been calculated according to the model of Brune (1970) and applying corrections by instrumental response, anelastic attenuation, free surface, and radiation pattern. A seismic moment-magnitude relation has been obtained, and the result has been compared with other regressions obtained for different seismo-active regions in Spain and in the world. The moment magnitudes of the earthquakes considered have been calculated according to the scale of Kanamori (1977) and Hanks and Kanamori (1979). Finally, a magnitude correction for the conversion of surface wave magnitude to moment magnitude has been proposed: M = 0.96 M (A/T) – 0.36. This conversion should be considered valid for the 4.0–6.0 surface-wave magnitude interval, which spans well the characteristic range for the moderate seismic activity in the Iberian Peninsula.

Magnitude Formulae and Intensity–Magnitude Relations for Early Instrumental Earthquakes in the Iberian Region

The magnitudes of early instrumentally recorded earthquakes in the Iberian region (1912–1962) have been studied through processing of digitized seismograms of Wiechert seismograph and analysis of macroseismic information. A magnitude system based on instrumental registrations and macroseismic observations has been proposed. It consists of two compatible magnitude formulae depending on the total duration of seismic oscillations and on the maximum ground amplitude/period ratio of surface waves and includes correspondent intensity–magnitude relationships.

Preliminary shear Q structure in the Mediterranean Sea from Rayleigh wave observations

Abstract Fundamental-mode Rayleigh wave attenuation coefficients have been obtained for the Mediterranean Sea using the two-station method applied to seismograph stations located in the coastal regions of Spain and Israel. The average attenuation coefficients vary between 7.36 10 −4 km −1 , at a 16 s period, to 0.76 10 −4 km −1 at a 70 s period. The associated Rayleigh wave anelastic factors range from 42 to 157. The above results indicate a relatively highly attenuating region. Inversion methods applied to the determined Rayleigh wave attenuation coefficients indicate that a strong and well-developped shear Q structure seems to be located under the Mediterranean Sea.