The June 2000 M_w 7.9 earthquakes south of Sumatra: Deformation in the India–Australia Plate

Abstract

[1] Two large (M w 7.9) earthquakes occurred on 4 and 18 June 2000, south of Sumatra, beneath the Indian Ocean. Both earthquakes were predominantly left-lateral strike-slip on vertical N-S trending faults that we interpret to be reactivated fracture zones. The 4 June Enggano earthquake occurred at the edge of the rupture area of the 1833 subduction earthquake. The first strike-slip subevent within the subducting plate triggered a thrust subevent on the plate interface, which comprised at least 35% of the total moment and ruptured SE away from the 1833 earthquake. The 18 June earthquake in the Wharton Basin is one of the largest shallow strike-slip faulting earthquakes ever recorded. A small second subevent with reverse slip is required to fit the body waves. The orientation of both subevents in our preferred model is consistent with the current stress field in the region. Both the June 2000 earthquakes are consistent with recent models of distributed deformation in the India-Australia composite plate. The occurrence of the Enggano earthquake implies that the stress field within the Indian plate continues to a depth of 50 km in the subducting slab. The purely strike-slip source model of the Wharton Basin earthquake obtained by Robinson et al. [2001] matches the P waves very poorly and fits the S waves no better than our preferred model. The strike-slip subevents of both earthquakes had few aftershocks and higher stress drops than the subduction thrust subevent of the Enggano earthquake. This difference is consistent with previous observations of oceanic and subduction earthquakes.

Re-evaluation of the surface ruptures of the November 1951 earthquake series in eastern Taiwan, and its neotectonic implications

“…of the earthquake's moment was produced by strike-slip rupture on a fault in the downgoing oceanic slab. This was followed by failure of a patch of the overlying subduction interface (Abercrombie et al, 2003).…”

Seismic evidence of a two-layer lithospheric deformation in the Indian Ocean

“…To the west of the NER in the CIB, deformation is taking place along E-W trending high-angle thrust faults and associated folds [1][2][3] due to the N-S compression resulting from the continental collision of India with Eurasia. To the east of the NER in the WB, the direction of the maximum stress is NW-SE, and the deformation is accommodated along N5°E-trending re-activated fracture zones with left-lateral strike-slip movements 2,5,10,11 . This was recently confirmed by the occurrence of the Mw ¼ 8.6 earthquake on April 11, 2012 in the WB, the largest strike-slip earthquake ever observed on the Earth.…”

“…Based on waveform analyses, Robinson et al 17 suggested that the 2000 Mw ¼ 7.9 Cocos earthquake did not only rupture a re-activated N5°E fracture zone but possibly also an orthogonal E-W trending ridge parallel fault. Some active thrust faults are also reported with an orientation perpendicular to the NW-SE compressive axis 10 . The 10 January foreshock (Mw ¼ 7.2) and 11 April aftershock (Mw ¼ 8.2) seem to have ruptured the re-activated fracture F6 (Fig.…”

The 2 March 2016 Wharton Basin Mw 7.8 earthquake: High stress drop north‐south strike‐slip rupture in the diffuse oceanic deformation zone between the Indian and Australian Plates

“…Hill et al [2015] estimate an average stress drop of 17 MPa for this event. The Wharton Basin extends southeast of these events and has experienced other large, predominantly strike-slip earthquakes such as the 18 June 2000 M w 7.9 earthquake (Figure 1) west of the Investigator Fracture Zone [Robinson et al, 2001;Abercrombie et al, 2003]. This event included secondary faulting on a thrust fault, but the stress drop for the primary strike-slip rupture was estimated as 5-10 MPa by Abercrombie et al Supporting Information:…”

“…The Australian plate east of the Ninety East Ridge has numerous north-south striking fracture zones associated with leftlateral offsets of the east-west trending Wharton fossil spreading center, which was active between 45 and 85 Ma [e.g., Deplus et al, 1998;Matthews et al, 2011]. Some of these structures appear to influence earthquake faulting geometry or are directly reactivated by present day deformation within the intraplate oceanic basins and in the subducting slab [e.g., Bull and Scrutton, 1990;Deplus et al, 1998;Abercrombie et al, 2003;Delescluse et al, 2008;Rajendran et al, 2011;Geersen et al, 2015;Aderhold and Abercrombie, 2016].…”