M 7.0 - 114 km NNW of Popondetta, Papua New Guinea

Tectonic Summary

The July 17, 2020, M 7.0 Papua New Guinea earthquake occurred as a result of oblique normal faulting at an intermediate depth, approximately 80 km beneath eastern Papua New Guinea, near the northern edge of the Australia plate. Focal mechanism solutions indicate that rupture occurred either on a steeply dipping fault striking to the northwest, or a moderately dipping fault striking to the northeast. At the location of the earthquake, the Australia plate moves to the north relative to the Pacific plate at a velocity of about 73 mm/yr. Earthquakes in this geographical region are generally associated with the large-scale convergence of these two major plates and with the complex interactions of several associated microplates, most notably the South Bismarck plate, the Solomon Sea microplate, and the Woodlark plate. The depth of the July 17th earthquake implies that it occurred in subducted or foundered lithosphere.

Although there is still not a complete geophysical consensus on the location and configuration of subducted plates in the region, the location of the earthquake is consistent with it occurring in a subducted fragment of the Australia plate (Solomon Sea microplate) that is bent about an approximately east-west axis and subducting both to the north and to the south or southwest. In northern Papua New Guinea, approximately 200 km to the north of the July 17th event, clear north-directed subduction of Australia beneath Pacific (and the South Bismarck microplate) occurs, with the associated Wadati-Benioff zone active to depths of more than 600 km beneath the Bismarck Sea to the northeast of New Guinea. The location of the July 17th earthquake places it in the gently south-southwest-dipping segment of the Solomon Sea microplate. Slip on a fault aligned with either nodal plane of the focal mechanism solution is consistent with this intraplate setting.

Papua New Guinea experiences a high rate of seismic activity, with 64 earthquakes occurring of M 6 and larger within 250 km of this event in the past 50 years, 8 of which were M7 or larger. Most of the previous seismicity has occurred to the north of this event. Recently, on May 7th, 2019, a M7.1 earthquake occurred 175 km to the northwest of this July 17th event at a depth of 146 km and destroyed at least 130 homes. A M7.4 earthquake on February 8th, 1987, 200km north of this event caused landslides that resulted in 3 fatalities.

Earthquakes like the July 17th event, with focal depths between 70 and 300 km are commonly termed “intermediate-depth” earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interface between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar-magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distances from their epicenters.