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The focal mechanisms and depths of small earthquakes as determined from Rayleigh-wave radiation patterns

DEPARTMENT OF EARTH AND SPACE SCIENCES STATE UNIVERSITY OF NEW YORK, STONY BROOK, NEW YORK 11794

Abstract

Many earthquakes of body-wave magnitude less than about 5.5 are located in key positions for defining tectonic processes at plate boundaries and in the interiors of plates. Due to the sparsity of observed body-wave data from these events, their focal mechanisms and depths cannot be well determined from body-wave data alone. Earthquakes with m_b as low as 4.5 radiate surface-wave trains which can be observed at teleseismic distances.

In this study, Rayleigh waves observed at WWSSN stations have been inverted to obtain focal mechanisms and depths of two small earthquakes (m_b = 4.7 and m_b = 5.1) in the Caribbean plate region. An event pair inversion patterned after Weidner and Aki (1973) is contrasted with a single event inversion which makes use of the seismic moment tensor formalism (Mendiguren, 1977; Gilbert, 1970) to linearize the relationship between observed signal and model. The event pair approach requires master events whose focal parameters are well known and which are located near the smaller events. The single event approach, on the other hand, does not require master events, but is limited to amplitude data only. Phase information can be utilized by considering a pair of events located near each other and recovering the difference in phase for the event pair.

For the events studied, the single event analysis yields the same solution as the event pair analysis. In general, amplitude is more sensitive to mechanism than phase. However, noise in amplitude increases more rapidly than noise in phase as the size of the event is reduced.

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