Characteristics of the Seismic Clusters Bounding the Ramu-Markham Fault Zone, Eastern Papua New Guinea

Abstract

The regionally extensive Ramu-Markham-Fault-Zone (RMFZ) in Papua-New-Guinea (PNG) passes through the seismically-active hinterland of the Bismarck subduction zone in SW-Pacific. The seismicity map for 400km segment of RMFZ shows that higher magnitude earthquakes mainly originate in four spatial clusters (I–IV), asymmetrically disposed from in-land to offshore on either side of RMFZ. The cluster III have produced the 2019 Buloloearthquake (Mw 7.1). The spatial and temporal characters for all four seismic clusters was estimated by: (i) b-value based on maximum-likelihood method; (ii) expected maximum magnitude (Mw) by Gumbel extreme value statistics and surface rupture length; and (iii) the Hurst coefficient (K) and Hurst plot. Hurst plots on sequential seismic moments in the clusters illustrate an alternating positive and negative sloping moment-release pattern over progressive time-period that corresponds to low and high b-values respectively. The regional stress pattern on north and south of RMFZ and for four seismic clusters are analysed by inversion of CMT focal-mechanism data. The result unravels a significant change in regional stress pattern across the RMFZ: (i) a pure-compressive stress regime corresponding to clusters I and II in the ‘PNG Highlands’ that gradually changes to transpressive in the off-shore cluster IV along north of RMFZ, and (ii) the regional stress pattern for earthquakes south of RMFZ including cluster III shows absence of any particular stress orientation and causative faults are randomly oriented. This leads to the presentation that RMFZ is a deep penetrative fault, rather than a crustal ramp fault restricted to 11–18km depth as advocated in literature.