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The excitation and attenuation of seismic crustal phases in Iran

DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES SAINT LOUIS UNIVERSITY, ST. LOUIS, MISSOURI 63103

Abstract

The most prominent crustal phases of Iranian earthquakes recorded by stations in Iran are the first P (Pn or refracted P), Pg, Sn, and Lg. Pg, Sn, and Lg appear as trains of waves whose attenuation behaves as that of dispersed surface waves. For 1-sec period Pg, Sn and Lg, the coefficient of anelastic attenuation has an average value of 0.0045 km^–1, similar to that for California but much greater than that for eastern North America. For 3-sec period Lg waves, the average value is 0.003 km^–1. The excitation of 1-sec Pg and Lg waves is approximately equal. The amplitude of their vertical components for an m_b = 5.0 earthquake at 10-km epicentral distance is 270 µm. The 10-km amplitude of vertical-component, 3-sec period Lg waves is 800 µm for an m_b = 5.0 earthquake. For all epicentral distances the resultant horizontal, 1-sec period Lg motion is twice that of the vertical component. The 10-km value of the resultant horizontal Sn motion for an m_b = 5.0 earthquake is 240 µm, slightly less than half that of Lg. The amplitude of the first P motion is, in general, one or more orders of magnitude smaller than that of the other crustal phases to distances of 500 km. However, at distances of 1000 to 1500 km the amplitude of first P is comparable to that of the other phases. Formulas are given to compute body-wave magnitude of Iranian earthquakes from the amplitudes of 1-sec period, vertical component Pg and Lg waves as well as amplitudes of the first arrival.

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