GLOBAL CREEPER DISTRIBUTION AND ITS RELATION TO EARTHQUAKE-SOURCE GEOMETRY AND TECTONIC ORIGIN

The M(s):m(b) ratio, conventionally used for the discrimination of ear thquakes and underground nuclear explosions, has been studied for a pu re earthquake catalogue in an attempt to classify earthquakes into two categories, one related to extremely low-frequency events of creep an d the other to relatively high-frequency explosions. M(s) and m(b) mea sured at 20 s and 1 s respectively are considered as estimates of sour ce spectra at two separate points, thus allowing one to compare large samples of earthquakes from the spectral point of view. Creeper, or th e deviation of (M(s), m(b))-points from the orthogonal regression of M (s) on m(b), is by definition a parameter independent of the size of s eismic events. It has been estimated from NEIC magnitudes and is studi ed in relation to parameters of central moment tensor solutions of the Harvard group. For this purpose, the parameters of central moment ten sors have been factorized to make classes of equivalence of events wit h the same dip and rake and arbitrary strike and size. The resulting t riangle representation of source mechanisms has been used to correct t he global distribution of creeper for the type of source mechanism. Di p-slip events have consistently lower creeper than strike-slip ones. H owever, the global distribution of creeper, either corrected or uncorr ected for source mechanism, has a clear tectonic pattern: negative val ues in most of the subduction zones and positive ones in mid-ocean rid ges. The dependence of creeper on focal depth is negligible according to empirical data for events shallower than 40 km. Limitation of the s ample by considering only events in this depth range does not diminish considerably the amount of data and does not change the tectonic patt ern of the creepex.