Significant Earthquake Strikes Northern Molucca Sea

On The Northern Molucca Sea Sequence, November 2014 – A Doublet Earthquake & Separate Triggered Mainshock

Mw 6.7, ~100km west of Halmahera, Indonesia, 14;33 UTC, 26/11/14.

At 14:33 UTC on the 26^th November 2014 a moment magnitude 6.7 earthquake struck the Northern Molucca Sea, some 100 kilometres west of the Indonesian island of Halmahera. This follows two previous earthquakes in the area in the past 11 days: a moment magnitude 7.0 earthquake struck on the 15^th November, whilst a moment magnitude 6.5 earthquake struck on the 21^st November. All three events were the result of reverse faulting, at depths of 35-40 kilometres, on fault planes with unusually steep dip angles (35-50°).

The initial mainshock struck at 02:31 UTC on the 15^th November 2014. The earthquake had a focal depth of 39 kilometres and a fault plane with a strike = 35° and a dip = 41°. This is not indicative of a normal subduction zone & therefore must belong to some other sort of tectonic feature with reverse faulting characteristics.

Initial analysis of the Mw 7.0 earthquake showed an aftershock sequence which did not correlate with expected aftershock activity – the largest aftershock was an Mw 5.6 event 95 minutes after the mainshock (focal depth = 54km, strike = 124°, dip = 36°); a second aftershock struck on the 18^th November with an Mw 5.6 (focal depth = 27km, strike = 232°, dip = 41°). The ΔMw = 1.4 was slightly too high to match the Båth’s Law, which predicts that ΔMw =1.1-1.2. Therefore an aftershock in the region of Mw 6.0 was anticipated.

On the 21^st November a significant Mw 6.5 event struck about 63 kilometres north-east of the 15^th November event. The earthquake had a focal depth of 32 kilometres and a fault plane with a strike = 341° and a dip = 38°. This fault plane does not match the initial mainshock, and thus I tentatively call this a triggered event on a separate fault, and therefore not an aftershock. This is debatable, and with such a small ΔMw it is tempting to call these two mainshock a doublet earthquake. There were two potential aftershocks to this quake recorded by GEOFON – an Mb 4.6 on the 24^th November & an Mb 4.7 on the 25^th November. Neither were manually revised, thus their depths at 62km and 46km respectively.

On the 26^th November a significant Mw 6.7 event struck at 14:33 UTC, about 32 kilometres west of the 15^th November event. The earthquake had a focal depth of 38 kilometres and a fault plane a strike = 30° and a dip = 37°. The error with location may indicate this earthquake occurred on the same fault as the 15^th November event; the strike and dip of the 26^th November event are strikingly similar. This earthquake is too large to be called an aftershock, and thus is a mainshock – this means that there is a doublet earthquake which is comprised of the 15^th November Mw 7.0 & the 26^th November Mw 6.7 events.

Aftershocks to the Mw 6.7 earthquake are following an expected pattern for a second event in a doublet earthquake – few in number, but occurring a relative short time afterwards and of reasonable magnitude. The largest aftershock was an Mw 5.6 event 16 minutes after the mainshock (focal depth = 36km, strike = 31° and dip = 46°), which matches Båth’s Law perfectly – this is in contradiction to the largest aftershocks to the 15^th November Mw 7.0 event.

This first aftershock to the 26^th November earthquake infers that the Northern Molucca Sequence has reached its natural end and no further mainshocks are to be expected. Further aftershocks may occur, with a maximum moment magnitude of 5.6.

Written at 16:00 UTC on the 26^th November 2014 by J H Gurney. All data sourced from GEOFON.