Global Creepex Distribution and Its Relation to Earthquake-Source Geometry and Tectonic Origin

Abstract

SUMMARY The Ms:mb ratio, conventionally used for the discrimination of earthquakes and underground nuclear explosions, has been studied for a pure earthquake catalogue in an attempt to classify earthquakes into two categories, one related to extremely lowfrequency events of creep and the other to relatively high-frequency explosions. Ms and mb measured at 20 s and 1 s respectively are considered as estimates of source spectra at two separate points, thus allowing one to compare large samples of earthquakes from the spectral point of view. Creepex, or the deviation of (Ms, mfc)-points from the orthogonal regression of Ms on mb, is by definition a parameter independent of the size of seismic events. It has been estimated from NEIC magnitudes and is studied in relation to parameters of central moment tensor solutions of the Harvard group. For this purpose, the parameters of central moment tensors have been factorized to make classes of equivalence of events with the same dip and rake and arbitrary strike and size. the resulting triangle representation of source mechanisms has been used to correct the global distribution of creepex for the type of source mechanism. Dip-slip events have consistently lower creepex than strike-slip ones. However, the global distribution of creepex, either corrected or uncorrected for source mechanism, has a clear tectonic pattern: negative values in most of the subduction zones and positive ones in mid-ocean ridges. the dependence of creepex on focal depth is negligible according to empirical data for events shallower than 40 km. Limitation of the sample by considering only events in this depth range does not diminish considerably the amount of data and does not change the tectonic pattern of the creepex.