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| A map of the Northern Japan earthquakes according to GEOFON. The information given for each quake is the focal depth, the faulting type (OR = Oblique Reverse, R = Reverse, SS = Strike-Slip) & dip of the fault plane. Important geographical features are named and located clearly (Hokkaido, Honshu & Japan Trench). |
A quick analysis of the recent Northern Japan Earthquake has revealed three important things.
The first is that all of the events offshore of Honshu follows clear patterns; firstly, depth increases with distance westwards from the Japan Trench, concurrent with the subduction model. Whilst this may be fine for the easternmost three events, the Mw 6.7 earthquake on the 16th February is too shallow, as is the Mw 5.4 close to the Honshu coast. Both of these must have occurred within the Eurasian Plate, not in the Pacific Plate or within/near the plate interface.
Secondly, the events offshore of Honshu also follow a pattern in terms of fault plane dip; the three events closest to the Japan Trench have extremely shallow dip angles, consistent with a shallow dipping interface (thrust fault) at the onset of subduction of the Pacific Plate. The Mw 6.7 mainshock (as it is at present - future quakes may change this determination) has a dip angle which would be more expected for its location if it were on the subduction interface, but as it has been determined this is not the case then this is probably a thrust fault caused by compression of the Eurasian Plate in response to the subduction further to the east. The Mw 5.4 Honshu quake is much more difficult to analyse & will be discussed later; by depth it is likely within the asthenosphere above the plate interface & not the overlying Eurasian Plate.
The second revelation from this map is the triggering of other earthquakes by the 16th February moderate subduction event. The Mw 5.4 Hokkaido Earthquake (20th February) is not directly related to the Mw 6.7 Japan Trench event; however it is likely that it was triggered by the moderate M6+ event. Similar instances occurred during the onset of the final foreshock series in March last year preceding the Mw 8.1 Iquique Earthquake. These kind of events were shown by two small M5 events at depth, likely within the overlying South American Plate. It is likely these are stress alleviations at depth to compensate the stress release in the uppermost part of the subduction zone (i.e the shallow earthquakes). Unlike the Chile examples, this earthquake follows the expected subduction characteristics of an event at a more moderate depth (reverse, shallow fault plane angle).
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| The first of the two "moderate depth" Northern Chile earthquakes in March 2014. This event has either a strike-slip or an oblique normal faulting mechanism; by strike of the nodal plane it is more likely to be strike-slip (see NP1). |
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| The second of the two "moderate depth" Northern Chile earthquakes in March 2014. This event has either a slightly oblique normal or very oblique normal faulting mechanism; by strike of the nodal plane it is more likely to be slightly oblique normal (see NP1). |
The third & final revelation adds to a growing picture that has been emerging since I began my records at the beginning of 2014. That is the enigma of moderate depth earthquakes with indeterminable faulting mechanisms. The Honshu Coast Earthquake shows this enigma perfectly; one fault plane suggests strike-slip at an improbably angle of 30 degrees (which would be more consistent with reverse faulting); the other fault plane suggests reverse faulting (or slightly oblique reverse faulting) at an angle of 78 degrees (which would be more consistent with strike-slip faulting). This has puzzled me for months, but I may have a solution - something I have realised in the course of writing this.
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| The "enigma" Honshu, Japan earthquake of the 17th February 2015. This event either had a strike-slip or slightly oblique reverse faulting mechanism; by strike of the nodal plane it is more likely to be strike-slip (see NP1). |
Published this month in Nature, a team from GNS Science in New Zealand (who else would I end up mentioning on the 4th Anniversary of the Port Hills Earthquake? - the earthquake in Christchurch on the 22nd February 2011 at 12:51pm NZDT) have discovered a semi-liquid layer, 5-6 kilometres thick, which lies at the plate interface of subduction zones. Here the two plates can, as such, "slide" past one another. This would cause a strike-slip event (lateral movement) at many different possible depths, with a fault plane angle consistent with the subduction interface angle (5-35 degrees). Therefore, it is tentatively suggested that the Mw 5.4 Honshu earthquake (17th February) may be associated with this layer. Whether the depth remains consistent (quakes at the distance this quake occurred at from the subduction trench are generally around 100-150km deep in New Zealand) with this hypothesis is much more difficult to determine.
What seems to be easy to conclude from this simple map and the associated GEOFON data is that these events are all linked together and likely something quite major. The similarities to the March 2014 Chile swarm are startling, whilst the identification of these events as being directly related to the subduction zone is inescapable.
What remains to be seen is how this develops in the next few days. From the Mw 6.7 Northern Chile earthquake in March 2014 to the Mw 8.1 Iquique earthquake in April 2014, there were 16 days. If things follows suit, a large Japan Trench megathrust earthquake would be anticipated on the 4th March 2015. I am very hesitant to say this large earthquake will even happen - these quakes are, after all, just within the aftershock zone of the March 2011 Mw 9.0 Tohoku Earthquake. Nevertheless, this is unusual aftershock behaviour four years after a mainshock.
Written by J H Gurney, 22:51 UTC, 21/02/2015
