ei sm ol og y A re - evaluation of the great Aleutian and Chilean earthquakes of 1906 August 17

S U M M A R Y We investigate two great earthquakes that occurred in the Aleutian Islands and Chile, within 30 min of each other, on 1906 August 17, based on a collection of seismograms compiled shortly after the events by scientists at Strasbourg. The method of Preliminary Determination of Focal Mechanisms (PDFM) is applied to 14 mantle waves from seven stations, in order to resolve the moment tensors of the two shocks. It is complemented by examination of body wave polarities at Japanese stations to lift the remaining indeterminacy in focal mechanism. The Chilean earthquake, occurring second, is a regular subduction event, whose moment (2.8 × 1028 dyn cm) is revised downwards from previous estimates (except Kanamori’s), suggesting that its rupture did not involve more than ∼200 km of fault. The Aleutian earthquake, occurring first, has a larger moment (3.8 × 1028 dyn cm), but features mantle wave radiation patterns and body wave polarities incompatible with both underthrusting at the Aleutian subduction zone and tensional buckling at the outer rise. Rather, we suggest that it is an intraplate, somewhat deeper (∼50 km) earthquake: this is supported by tentative relocation of available arrival times north of it. The origin of the earthquake may be related to the presence of the Bowers ridge north of the Amchitka pass in the epicentral area. Finally, hydrodynamic simulations using our source mechanisms support the observation that the Chilean event was the source of the reported transpacific tsunami; the report of a 3.5-m wave at Maui constitutes a misassociation, as its timing is shown to be non-causal for both events.