Some Interesting Central Asian Events on the M, : m,, Diagram

Events from two small regions in Central Asia having nearly equal mb values are found to have a wide range of M, values. One event from Tibet and one from South-eastern Kazakh, USSR have M, : mb ratios typical of explosions. Several other events have borderline ratios. All of the events analysed are classified as shallow focus earthquakes by NOS (i.e. their focal depths are all listed at greater than 20 km in the Preliminary Determination of Epicenter catalogue). The Tibetan events are earth-quakes. The Kazakh event data exhibits a short-period complexity, in addition to an M, : mb ratio, typical of an explosion. However, it cannot be classified as such because its short-period record also contains an apparent depth phase at a time lag corresponding to a focal depth of 22 km. The possibilities of the ' depth phase ' being the result of multiple explosions or of the short-period complexity being the result of unusual source parameters cannot be differentiated with the available data. Discrimination of the event is further complicated by the fact that its Rayleigh wave is swamped by the surface wave of a much larger event from the Tonga Islands Region. The interfering event problem is not too serious here because the short period continental Rayleigh wave from the Kazakh event rides atop the long period oceanic surface wave from the Tonga event in such a way that the effects of each are visually separable