Shallow dips of normal faults during rapid extension: Earthquakes in the Woodlark‐D'Entrecasteaux rift system, Papua New Guinea

Abstract

Earthquakes in the Woodlark‐D'Entrecasteaux rift system reveal changes in fault geometry along strike, as extension progresses from continental to oceanic modes. In one segment, evidence is found for earthquakes on shallow‐dipping normal faults. To examine these trends, locations of 147 earthquakes are redetermined, and focal mechanisms for 21 events are derived from waveform inversion. Where seafloor spreading occurs, east of 152°E, most earthquakes illuminate a NE‐SW strike‐slip fault north of the oceanic rift system along the Woodlark Rise. Elsewhere, earthquakes occur along a well‐defined rift zone, and focal mechanisms reveal normal faulting with north‐south T axes and depths less than 10 km. Most normal‐faulting earthquakes show high‐angle fault planes (dips 40°–50°), but several are consistent with slip on moderately low‐angle planes dipping 15°–35°. These low‐angle nodal planes are only found in a region between 150.5°E and 152.5°E, where the transition occurs from seafloor spreading to continental rifting. A seismic reflection profile near 151°E reveals a major normal fault that dips to the north at 25°–35° just landward of the oceanic rift tip. That fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied. Hence normal faults dipping ∼30° are likely to be seismically active in at least one part of this rift system. In this region just landward from the rift tip the highest extension rates are expected (∼40 mm/yr) and strain appears localized on a single system of normal faults. Because shallow dips for normal faults are seen near the rift tip, but not elsewhere along strike, high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips. Perhaps high shear traction at the base of the upper crust, localized by nearby spreading, sufficiently alters the local stress regime.