SUBCRUSTAL MICROEARTHQUAKE SEISMICITY AND FAULT PLANE SOLUTIONS BENEATH THE HELLENIC ARC

Subcrustal seismicity recorded in the southern Aegean sea during a 7-week microearthquake study was low compared with shallow seismicity. Most intermediate depth seismicity occurred beneath the western and eastern ends of the Hellenic arc. This distribution confirms that a slab of lithosphere is being subducted at a very shallow (<15-degrees) angle for 200 km beneath the western end (Peloponnese) but more steeply beneath the eastern end (Dodecanese). We could locate only one intermediate depth event beneath the central part of the arc, where teleseimically located intermediate depth earthquakes also are infrequent. T axes for most of the 22 focal mechanisms of subcrustal earthquakes are Toughly parallel to the local dip direction of the seismic zone. Between depths of 40 and 80 km, the mechanisms are more confused than at greater depth, perhaps because some of these earthquakes did not occur within the downgoing slab. Earthquakes deeper than 80 km, and within the subducted slab, have nearly horizontal P axes that trend NNE-SSW in the eastern part and NNW-SSE in the western pan of the arc. These deeper mechanisms show horizontal P axes along strike, perhaps in response to the contortion of the slab or to the westward motion of Turkey, as well as lengthening downdip, probably in response to gravity acting on excess mass in the slab. Thus the short slab, both downdip and along strike, subducting beneath the Aegean is subjected to a more complex set of forces than the long slabs of the Pacific.