Active normal faults dipping less than 30° have not been documented in the seismic record, but they are commonly observed in geologic structures. One interpretation is that seismogenic low-angle faulting is physically impossible and that the geologic structures do not form at shallow dips. To better examine this problem, teleseismic body waves were inverted to determine source parameters for five large earthquakes in the Woodlark-D'Entrecasteaux region, a rare example of active continental extension associated with Pliocene-Quaternary metamorphic core complexes. The inversions show that at least two and possibly four events are consistent with normal dip slip on shallow-dipping faults. The largest earthquake (Mw = 6.8) is located near the mapped core complexes. The shallow-dipping plane for this event, determined to dip between 10° and 25°, is nearly coplanar to the faults bounding the core complexes and may represent slip on these or other parallel low-angle structures. Although slip on the conjugate high-angle plane cannot be ruled out, observations of Quaternary uplift and the similarity to the adjacent core-complex-bounding structures favor motion on the low-angle plane. Similar earthquake mechanisms have not yet been documented in most other extensional provinces, perhaps because instances of active core-complex exhumation are rare.

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