THE 1989 (MS 6.3) UNGAVA, QUEBEC, EARTHQUAKE - A COMPLEX INTRAPLATE EVENT

On 25 December 1989 the largest earthquake (Ms 6.3) in northern Quebec in at least 60 yr occurred in the Ungava Peninsula. This earthquake w as unique in that it marked the first time surface faulting could be c onfirmed for a historical eastern North American earthquake. Field obs ervations indicated a combination of thrust and strike-slip motion. Wa veforms modeled in this study show that the earthquake consisted of tw o subevents-a thrust subevent on a NE-SW striking plane followed by a larger strike-slip subevent on a NNE-SSW striking plane. The surface w aves give an equivalent double-couple moment of 1.3 x 10(25) dyne cm. The sum of the subevent moments is greater than the simple source mome nt by almost a factor of 2. Both subevents occurred at a depth of 3 km . They were separated by 0.9 sec in time with the second subevent occu rring 5.2 km southwest of the first. The source parameters determined by modeling and those inferred from the ground observations generally agree, but there are some differences with respect to the subevent off set. These may be reconciled if the strike-slip motion was distributed among the several small faults at the southwestern end of the rupture or if the complete surface rupture was not found. The stress orientat ion (NW compression) inferred from the fault plane solution is consist ent with the focal mechanisms of many other recent northeastern Canadi an events. The Ungava earthquake and many intraplate events from sever al continents share a number of characteristics, including shallow hyp ocenters, source complexity, and occurrence on faults that showed no e vidence for previous ruptures.