Stress inversions of intermediate-depth focal mechanisms in Vanuatu were used to test two hypotheses: that the slab acts as a stress guide and whether intermediate-focus earthquakes occur on preexisting faults. Although the strike of the Vanuatu slab varies by over 70degrees, the maximum compression sigma(1) is everywhere slab normal and the least compression sigma(3) is everywhere within the plane of the slab. This provides strong confirmation of the stress guide hypothesis. In the depth range 61 - 120 km, the solution space for sigma(3) forms a girdle pattern in the plane of the slab, indicating bi-axial tension. This gradually changes to down-dip tension at greater depth. We find that preexisting faults may become reactivated by intermediate depth earthquakes but only when the slab stresses favor it: Otherwise new faults are formed. This contradicts the idea that the mechanism of intermediate focus earthquakes requires serpentinite dehydration on preexisting faults.
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