The Makran earthquake of 1983 April 18: A possible analogue to the Puget Sound earthquake of 1965?

Summary On 1983 April 18, a large earthquake ( mb ∼6.5, M 0 = 1.2X10 19 Nm) occurred in the Makran region near the Pakistan‐Iran border. This earthquake is the only event of mb > 6.2 in the entire Makran region since the occurrence of the great Makran earthquake ( M = 8.0–8.3) in 1945. We determined the source parameters of the 1983 event by formally inverting its P ‐ and SH ‐waves recorded by the World‐Wide Standard Seismograph Network (WWSSN) and the Canadian Seismograph Network (CSN) at teleseismic distances. The best‐fitting centroidal solution shows predominantly normal faulting at a depth of about 65 ± 5 km. These results provide an additional constraint on the configuration of the down‐going slab previously inferred from the occurrence of three small‐ to moderate‐sized events located 100–300 km to the west of the event reported here. We also discuss the possibility of drawing an analogy between the occurrence of the 1983 Makran event and that of the Puget Sound earthquake of 1965 (M0–1.4X10 19 Nm, m b ± 6.5), and between the seismotectonic settings of the two regions. If the very large ( M = 8.0–8.3) earthquake off the Makran coast in 1945 is a low‐angle thrust due to slip along the plate interface, its occurrence calls for the possibility of similar seismic risk in the Puget Sound region. However, in contrast to inferences made from geological observations in the field, about a dozen P ‐wave first motions read from historical seismograms and those reported by the International Seismological Summary suggest that the 1945 Makran earthquake has a large component of normal faulting which can be attributed to the relatively old age of the subducted lithosphere.