EARTHQUAKE CLUSTERING IN SHALLOW SUBDUCTION ZONES - KAMCHATKA AND MEXICO

This paper studies regularities in earthquake clustering at shallow de pths (0-60 km) in two Pacific subduction zones, the Mexican and the Ka mchatkan, These zones were formed by subduction under the North Americ an plate of old and fast Pacific plate (Kamchatkan region) and young a nd slow Rivera and Cocos plates (Mexican region). We use events of mag nitude 5.5 and greater (95 events for 1964-1993 from the Kamchatkan re gion and 52 events for 1974-1993 from the Mexican region). We have two active (Kamchatkan and Cocos) and one passive (Rivera) subduction zon es. For the passive Rivera zone, where subduction may be considered as eismic after the great Jalisco event of 1932 (magnitude M(s) 8.2) and its aftershock activity during 1932-1934, we can study only seismic ev ents along the western border of the plate, related to oceanic rises a nd the Rivera fracture zone, Here we have a well-developed clustering of earthquakes (47% of all 17 studied events occurred in clusters), ma inly of swarm type, For the two active subduction zones, the Kamchatka n and the Cocos, we can formulate the following results: (1) most of t he studied events of the Cocos subduction zone (74%) occurred in clust ers, and within the Kamchatkan subduction zone, 45% of the events reco rded were in clusters; (2) the clustering events occurred mainly at a depth interval from 0 to 40 km (98% of all clustering events in Kamcha tka and 61% for the Cocos subduction zone); at the same time, the sing le events were distributed mainly in a depth interval from 30 to 60 km (73% of all single events in Kamchatka and 100% for the Cocos subduct ion zone); (3) comparison of the spatial distribution for the differen t types of clustering id Kamchatka and the Cocos zone shows the absenc e of a band of swarms along the slope of the Middle American trench, w hereas such a band is obvious in the Kamchatkan region; (4) the study of temporal distribution for single events and mainshock-aftershock se quences within the region of large earthquakes shows that there was a change from a long-term sequence of single events to events with after shocks, 1-4 years before a large earthquake; this regularity was obser ved before both large (magnitude greater than seven) earthquakes in Ka mchatka and before the magnitude 8.1 Michoacan earthquake in Mexico.