The Andreanof Islands earthquake of May 7, 1986 (Mw = 8.0) was followed in the first 1.5 months by a series of shallow upper-plate earthquakes, the five largest of which range in magnitude between Mw = 5.3 and Mw = 5.6 and are consistent with right-lateral motion on arc-parallel transform faults [Ekström and Engdahl, 1989]. The February 4, 1965 Rat Islands earthquake (Mw = 8.7) was followed on July 4, 1966 by a shallow mb = 6.2 upper-plate strike-slip event, which can be interpreted as a right-lateral earthquake in a relative position and time very similar to the Andreanof Islands earthquakes, here associated with slip on the eastern, strongest asperity of the 1965 main event. Finally, the Biak (Indonesia) earthquake of Feb. 17, 1996 (Mw = 8.2) was followed by two upper-plate events, one left-lateral strike-slip (Mw = 6.5, Feb. 17) and the other extensional (Mw= 6.4, Feb. 18) relative to an arc-parallel back-arc transform feature suggested by the locations of mainly left-lateral upper-plate events from 1979 (probably triggered by a Sept. 12, 1979 earthquake nearby).
Subduction in the Aleutians is such that it principally produces coseismic
increases in extensional and left-lateral shear stress on back-arc transforms.
For a net increase in right-lateral coulomb shear stress, consistent with
the strike-slip events in the Andreanof and Rat Islands, the increase in
extensional stress multiplied by f must be greater than the increase
in left-lateral shear stress. Positions in the back arc where this
occurs depend strongly on distance perpendicular from the trench and the
choice of f. For a 2D model with homogeneous slip along strike during
the main event (on an interplate interface of downdip width L) at
an obliquity of 30 from the trench normal, increases in right lateral Coulomb
shear stress in the back arc are achieved for distances > 1.2L from
the trench with values of f = 0.4. For smaller obliquity
as found for actual slip during the main events in the Andreanof and Rat
Islands, any value of f = 0.2 produces an increase in right-lateral
Coulomb shear stress for distances > L. A 2D model also predicts
Coulomb stress increases both extensional and left-lateral consistent with
the back arc activity in Indonesia. However, if there is a significant
slip heterogeneity during these events, as known to be the case for the
Aleutians, then stress changes may be better estimated from a simple 3D
model [Dmowska et al., 1996] of a localized asperity sustaining
strong stress drop in the center of an interface rupture zone with free
slip around it. Such modeling for oblique subduction reveals that
Coulomb stress changes favor back arc strike slip events only to one side
of the asperity and extensional primarily to the other. The modeling
is consistent with locations of the strike-slip events in the Aleutians
relative to the inferred asperities. For Biak, it suggests that if
there is a similar slip asperity on the interplate interface, then it must
be located along strike between the 17 Feb. left-lateral strike-slip and
18 Feb. extensional back-arc events.