Moderate Earthquake Strikes North Island, New Zealand

Moderate Ml 5.8 Earthquake Strikes North Island, New Zealand

A local magnitude 5.8 (GNS, New Zealand; moment magnitude 5.4 - GFZ, Germany) earthquake has struck the North Island of New Zealand, about 10 kilometres east of the small village of Pongaroa. The earthquake was the result of a shallow-dip reverse fault.

This earthquake was preceded by several foreshocks, the largest of which was a M3.8 six hours prior to the mainshock. A smattering of aftershocks have followed this moderate-size earthquake, the largest 74 minutes after the mainshock with a magnitude of 4.1.

The earthquake was felt across most of the lower and central North Island, gaining at least 4990 felt reports (likely to exceed 5000 soon). The M4.1 aftershock has gained only 50 felt reports.

It is my expectation that the M4.1 aftershock may only be exceeded by one or two earthquakes hereafter; the maximum is a M4.6, but my preferential forecast is for an aftershock to reach ~M4.3 at maximum. This would agree with Bath's Law for mainshock-aftershock relationship.

This earthquake (or more strictly true, an earthquake of similar magnitude in New Zealand) was anticipated by myself, but only on the basis of a comparative lack of such events in the past 4-6 months. The chances of this Pongaroa earthquake being a foreshock are small, but not impossible; if any further large earthquakes are to follow, it would be in a swarm-like manner, much like the Weber Earthquakes of 1990-1992.

Addendum: Whilst writing, a M4.4 aftershock struck, fulfilling my prediction:

http://geonet.org.nz/quakes/region/newzealand/2015p768685

For more information:
http://geonet.org.nz/quakes/region/newzealand/2015p768477

http://info.geonet.org.nz/display/quake/2015/10/12/Quake+near+Pongaroa

http://geofon.gfz-potsdam.de/data/alerts/2015/gfz2015tzmw/mt.txt

Moment Tensor Solution for the Pongaroa Ml 5.8 Earthquake, as given by the GEOFON Program, GFZ Potsdam, Germany. It shows shallow-dip reverse faulting, corresponding well with the tectonic regime of the eastern North Island.Written by J H Gurney, 11:03 BST (23:03 NZDT), 12th October 2015.

Earthquake Forecast For June 2015 & Beyond

As a new post I have decided to give an attempt at forecasting global seismicity for the rest of this month of June, alongside some forecast for more local regions, which are likely to be less accurate than the global forecast,

As of the 12th June 2015 there have been 36 earthquakes worldwide with a moment magnitude (Mw) of 5.0 or larger according to GEOFON (GFZ Potsdam) in this month. This number includes four events of moment magnitude 6.0 or larger, all of which in fact have been moment magnitude 6.0 earthquakes. These have struck in the following locations:

• Borneo, Malaysia (4th June, depth 14km, Mw 6.0) - killed 18 people

• Offshore Hokkaido, Japan (8th June, depth 58km, Mw 6.0)

• Antofagasta, Northern Chile (10th June, depth 122km, Mw 6.0)

• South of Samoa (12th June, depth 49km, Mw 6.0)

Of the 36 events, only 3 have actually been onshore - the Mw 6.0 Borneo Earthquake on the 4th June, the Mw 6.0 Antofagasta Earthquake on the 10th June & the Mw 5.0 Borneo Earthquake on the 12th June (an aftershock of the 4th June event). This means that there have been no Mw 5.0 or larger events, which would be aftershocks, striking Nepal so far this month. My expectations for Nepal are that there is a 50/50 chance of a Mw 5.0 or larger event striking before the end of the month.

Globally my expectations are the following for the month of June 2015 (as qualifying & recorded by GEOFON):

• Mw 5.0 or larger: 84-96 events

• Mw 6.0 or larger: 8-10 events

• Mw 7.0 or larger: 0-2 events

• Largest magnitude: M6.7-7.6 (most likely at a guess is ~M7.0)

In terms of long-term global expectations I expect there to be at least two more Mw 7.0 or larger earthquakes; the current total of seven is a little below average annual totals of ten events.

There are some regions which have not suffered significant seismic activity thus far this year. Europe is a key region which has only seen a single event over Mw 6.0 (16th April, 29km depth, Mw 6.1: Crete, Greece). In all honesty I do not expect an event of such size to occur this month, but I do expect at least one more to strike somewhere in Europe this year, with likely locations being Greece, Italy & Turkey.

Another region with few events this year is Indonesia. There have been very few large events at shallow depths this year; a Mw 6.9 (27th February, depth 544km) struck the Flores Sea, a Mw 6.2 (3rd March, depth 30km) struck west of Southern Sumatra, a Mw 6.0 (15th March, depth 24km) struck the Minahassa Peninsula, Sulawesi & a Mw 6.2 (17th March, depth 38km) struck the Northern Molucca Sea. Since the Northern Molucca Sea Earthquake there have been no earthquakes over Mw 6.0, the largest in Indonesia since that event was either the Mw 5.9 in Sulawesi (28th March, depth 132km) or the Mw 5.9 in Southern Sumatra (15th May, depth 143km). Therefore this is a likely region to expect a Mw 6.0 or larger event this month.

Again, the Philippines have had relatively few earthquakes; there have only been 11 events of Mw 5.0 or larger, the largest a Mw 5.8 on the 10th January striking offshore of Luzon. This is quite unusual to have so few events in the Philippines, and like my expectations for Europe, I believe there will be a Mw 6.0 or larger event this year, though not necessary this month.

For my two countries of specific interest the past five and a bit months have been relatively interesting. The United Kingdom saw a Mw 3.7 (Ml 4.2) earthquake strike near Ramsgate, Kent, on the 22nd May, the largest earthquake to strike the country since the 2008 Market Rasen Earthquake. I do not expect any further earthquake with a magnitude 4.0 (of any kind), but there may yet be a small number (below six) of M3-3.9 events.

New Zealand has seen three significant earthquakes strike onshore; a Mw 5.6 on the 5th January near Arthur's Pass, South Island; a Mw 6.1 on the 24th April near St Arnaud, South Island; & a Mw 5.6 on the 4th May near Wanaka, South Island. The largest earthquake on the North Island was a Mw 4.6 near Ruatoria on the 14th April.

By a very simple process of deduction (by the simple absence of such events so far this year) I believe there will be at least one Mw 4.8 or larger event which will strike the North Island of New Zealand. Such events may be shallow earthquakes near Whanganui, Wellington, Hawkes Bay or Gisborne, or alternatively moderate depth (70-300km) events beneath the North Island.

My final region of interest is the Pacific West Coast of the United States. I still believe that the next Mw 6.0 or larger event to hit the Los Angeles region is due within the next three years. Currently my focus is on the Newport-Inglewood Fault, which has seen an unusual increase in small magnitude tremors recently. Other parts of the Pacific West Coast at risk of decent earthquake (M5.0 or greater) include the San Francisco Bay Area - in particular the Concord Fault - Cape Mendocino & the vast central area of the San Andreas system, particularly around Soledad & Coalinga.

As a note, these are personal opinions and no official forecasts. They definitely are NOT predictions!

Written by J H Gurney, 14:20 BST, 13th June 2015.

On the Late April 2015 Seddon & St Arnaud Earthquakes, New Zealand

This is just a short update on recent seismic activity, with an unsurprising focus at present on New Zealand.

On Wednesday night at 10:36pm BST a local magnitude 4.4 earthquake struck due east of Cape Campbell on the north-east coast of the South Island of New Zealand. This earthquake is a bit of a surprise – it has been 21 months since the first Mw 5.5 earthquake struck in the Cook Straits & began the Seddon sequence (18^th July 2013 in the UK). This Cape Campbell earthquake proved to be a foreshock to a larger local magnitude 5.1 earthquake, which struck about 7km north-east of Lake Grassmere.

This second Seddon earthquake is probably the more interesting; it struck on what I assumed in my Extended Project was the second fault to rupture with the 21^st July 2013 Mw 6.5 earthquake. This second fault was strike-slip, trending roughly 233° (NE-SW) with a dip of about 70-80°. Though GeoNet have not yet released a moment tensor solution for this earthquake, USGS did give one: Mw 4.65, nodal plane 1 trending 234° (NE-SW), dipping 62°. This matches what I would expect, and its location at the epicentre of the Mw 6.0 earthquake which struck on the 16^th August 2013 (after the second mainshock underneath Lake Grassmere on the same day) seems to infer that this earthquake was indeed on the second fault to rupture during the 2013 sequence.

This morning, at 4:36am BST a large earthquake struck underneath the Kaikoura Ranges in the north-east of New Zealand’s South Island, about 35km south-east of St Arnaud. This earthquake had a local magnitude of 6.2 (GeoNet) - moment magnitude of 6.0 (GEOFON) & 6.1 (USGS) and had a focal depth of 52km (GeoNet). The earthquake was a long one due to its location, depth and magnitude – most people who felt it in New Zealand reported on Twitter that shaking lasted at least 20 seconds. The earthquake was felt across the whole country – over 6500 felt reports were received by GeoNet, from Auckland in the north, to Dunedin in the south.

As soon as I saw it (I was unfortunately awake when the earthquake struck!) I could discern several things. First of all, there was unlikely to be any structural damage. If the earthquake had struck Christchurch or Wellington city centres (as the Port Hills quake did in February 2011) then there would have been severe damage and likely fatalities; however the quake struck in the sparsely populated Kaikoura Ranges between Kaikoura and St Arnaud, whilst the depth meant there was unlikely to be any damage regardless to modern structures in New Zealand (and probably only very minor damage to older buildings).

Secondly, the type and depth of earthquake indicated that there would be few aftershocks and those which did occur would not follow Båth’s Law. Generally the largest aftershock is expected to be one magnitude unit below that of the mainshock – in this case it would be about magnitude 5.0. However, moderate depth earthquakes rarely follow such a rule – generally the largest aftershock is 1.5-2 magnitudes below that of the mainshock. So far there have been three major aftershocks – a M3.7 (32km depth, 4:46am BST), a M3.8 (36km depth, 4:53am BST) & a M4.2 (96km depth, 3:35pm BST – not revised). I immediately wrote this tweets on my UKEQ account:

“M6.3 strikes upper South Island, NZ, ~70km depth. Large aftershock (M5+) are unlikely #eqnz”

Unsurprisingly, when the first detailed news article was put out by GeoNet (http://info.geonet.org.nz/display/quake/2015/04/24/Quake+hits+inland+Marlborough), they agreed with my assessment entirely. So far my assessment for aftershocks has proven correct and will very likely remain accurate.

The only remaining thing which I immediately asked was if there was a relationship between the Kaikoura Ranges earthquake & the Seddon earthquakes the day before. I concluded that though it was very unlikely the M4.4 & M5.1 earthquakes directly triggered the M6.2 Kaikoura Ranges earthquake, it was plausible that the Seddon earthquakes as a whole could have been a catalyst or contributor to this morning’s shake. This may well be investigated by seismologists in the next few months to years (perhaps a journal article will appear). Once again, the news article above corroborated with my own beliefs. It is therefore very evident that my experience with New Zealand seismicity – particularly South Island seismicity – has meant that my conclusions and assumptions are well founded from observations over the past 50 or so months.

Recent Japan Earthquakes A Reason For Concern

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).

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).

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.

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

Earthquake Swarm Strikes Near Yedisu Fault, Eastern Turkey

A minor earthquake swarm has afflicted a part of Eastern Turkey which has probably the highest risk of a major earthquake along the North Anatolian Fault other than the Marmara Sea.

At 9:58pm UTC last night a Ml 3.6 earthquake struck in Tunceli Province, about 35 kilometres (22 miles) due west of the town of Yedisu. In the next two hours four further earthquakes, the largest a Ml 4.1 event at 10:52pm UTC, struck in the same location. All of these events are located perilouslyclose to the as-yet unruptured 65-kilometres long Yedisu Fault.

In 1939 a massive earthquake (Mw 7.9) struck west of the city of Erzincan. Ever since, twelve large earthquake (>Mw 6.5) have unzipped the North Anatolian Fault from east of Erzincan to Izmit on the Marmara Sea. The last major earthquakes on the fault was the Mw 7.4 earthquake near Izmit in August 1999 & the Mw 7.2 near Duzce in November 1999.

Prior to the Izmit earthquakes, Stein et al. had published a paper in the Geophysics Journal (Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering, 1996) which had noted how each earthquake along the North Anatolian Fault had increased stress on the adjacent segments which subsequently ruptured. They therefore successfully predicted the Izmit earthquakes three years before they struck.

In their paper Stein et al. also noted that there was one major seismic gap still remaining - the 65-km long Yedisu Fault, bounded by previous events which occurred in 1949 (Mw 7.1) & 1992 (Mw 6.5). These events, alongside all the other major earthquakes since 1939, had increased stress on the Yedisu Fault by a massive 10.1 bars - this is the equivalent to 10.1 atmospheres. Other stress increase caused by the 1949 & 1992 events had led to the 1971 Bingol earthquake (M 6.3) & 2003 Bingol earthquake (Mw 6.4) to the south-east of the Yedisu Fault.

It is therefore possible that these earthquakes which occurred last night may well be precursors to the expected >M7 Yedisu earthquake. If this were to strike, much like the 1939 & 1992 Erzincan earthquakes they would be expected to cause serious damage to the city of Erzincan and likely kill hundreds if not thousands of people.

Post Scriptum: The Moment Tensor Solution provided by the Kandilli Observatory seems to indicate these earthquakes did not occur on the Yedisu Fault but likely on a nearby strike-slip fault with an alignment more akin to the faults which ruptured in 1971 with the M 6.3 Bingol earthquake.

Ml 3.6: http://www.emsc-csem.org/Earthquake/earthquake.php?id=426568

Ml 3.1: http://www.emsc-csem.org/Earthquake/earthquake.php?id=426571

Mw 4.1: http://www.emsc-csem.org/Earthquake/earthquake.php?id=426578

Ml 3.0: http://www.emsc-csem.org/Earthquake/earthquake.php?id=426581

Ml 2.2: http://www.emsc-csem.org/Earthquake/earthquake.php?id=426604

Mislocated Earthquakes From Auckland Islands Earthquake

Minor Earthquake Strikes Auckland Islands: Mislocated Earthquakes on South Island - GeoNet

Mw 5.6, ~415 kilometres south of Invercargill, New Zealand, 13:04 UTC, 01/12/2014

The Mw 5.6 Auckland Islands Earthquake picked up on the New Zealand National Seismograph Network (bottom right of picture). The individual P-waves and S-waves can be seen here on the seismograms of stations such as Wellington or Denniston North.

At 13:04:23 UTC a moment magnitude 5.6 earthquake struck north-west of the Auckland Islands, about 400 kilometres south-south-east of New Zealand, at a depth of 32 kilometres. This earthquake had an oblique reverse faulting mechanism, something consistent with seismic events in this part of the world. The quake was caused by the relatively weak subduction of the Australian Plate beneath the Pacific Plate at a feature known as the Puysegur Trench. The earthquake was picked up by GeoNet, the national natural hazards programme run by the New Zealand government, and mislocated as two small local events on the South Island of New Zealand. In this article the processes behind these mislocations are explained and compared to similar instances in GeoNet’s monitoring.

At 13:05:21 UTC the GeoNet automatic detection system for seismic events computed a local magnitude 4.1 earthquake 10 kilometres north-east of Alexandra, South Island. This earthquake would be a very rare event, the McKenzie Country of western Otago Province generally proving aseismic, even with numerous faults being present in the region. The earthquake was given a likely depth of 15 kilometres, something compatible with previous earthquakes in the area.

About one minute later (13:06:26 UTC) a local magnitude 3.9 earthquake was computed by the automatic detection system about 25km north-east of Murchison, South Island. This earthquake would be a relatively unusual event, not owing to its location, where both strike-slip events, associated with the Marlborough Fault System, and reverse subduction events, associated with the Kaikoura Ranges (the only onshore imprint of the Hikurangi Trench subduction zone) occur. However this earthquake was given a focal depth of 142 kilometres, something more commonly associated with subduction events in the western sections of the Cook Straits or the Taranaki Region of the North Island.

The Auckland Islands Earthquake as seen at the nearest seismograph on the New Zealand National Seismograph Network to the focus - The Paps, on Stewart Island, south of Invercargill. This shows a very clear spike at the bottom left; note the S-waves cannot easily be picked out from the P-waves.

These two automatic detection earthquakes are what have come to be known as “ghost quakes”. When a distant earthquake is detected by seismograms in New Zealand, GeoNet’s automatic detection system attempts to compute a local event as the cause of the arriving waves. Unfortunately the arriving seismic waves are often more distinct than those arriving from a local event. Therefore two separate waves are detected by GeoNet’s seismograph stations – the initial P-waves, followed by the S-waves. The difference between these two sets of waves means that the further away from the true focus of the earthquake a second “S-wave” earthquake is located – the S-wave is far easier determined at these more distant seismograph stations than at stations closer to the source (an example is seen below).

The seismograph at Top House (upper South Island, New Zealand) shows the Auckland Islands Earthquake (left of picture, near 4-hour line); the waveform recorded shows clear P-waves followed by weaker S-waves. The other clear spike (bottom centre, near 2-hour line) is a local magnitude 4.5 aftershock to the Mw 6.7 Hikurangi Earthquake of the 17th November 2014.

In the case today this involved a distant moment magnitude 5.6 earthquake located north-west of the Auckland Islands within the Puysegur Trench system, about 415 kilometres south-south-west of Invercargill. This meant, as the seismic waves were detected further and further to the north-north-east from the focus the waveform recorded became more and more deformed and elongated.

At The Paps Seismograph (north-west of Oban, Stewart Island) the waveform recorded was stretched, but the P-wave and S-wave was not easily distinguishable. This is indicative of a sizable earthquake but at a reasonable distance from the seismograph station – in this case about 400 kilometres away.

At Top House Seismograph (north-east of St Arnaud, South Island) the waveform recorded was incredibly stretched, the P-wave and S-wave easily distinguishable from one another. This sort of waveform is representative of the seismograph station being a great distance from the epicentre – in this case about 1070 kilometres away.

The easily distinguishable S-wave at Top House & nearby seismograph stations in the north part of the South Island led to the automatic detection system to compute an earthquake in this region. Due to the highly deformed waveform it was also computed that the earthquake must have been quite deep. The inability to distinguish the P-waves and S-waves in the lower South Island meant that a shallower earthquake was computed in this region.

Generally speaking this sort of issue is not associated with Puysegur Trench earthquakes but rather Kermadec Trench earthquakes. Numerous examples have been noted in the past by myself, including a moment magnitude 5.6 earthquake which struck the Kermadec Arc on the 21^st September 2013. More distant events of significant magnitudes (such as the moment magnitude 6.9 event on the 1^st November 2014) are also often given “ghost quakes” despite this origin being over 1200 kilometres to the north of New Zealand. The bias here is due to the tectonic setting of New Zealand and the simple geography of the south and north of New Zealand.

The two "ghost" quakes computed by GeoNet, as seen on the quake list on the GeoNet website at about 13:26 UTC. These earthquakes will be subsequently deleted come morning on the 2nd December NZDT.

The Puysegur Trench is an incipient subduction zone which acts as a transfer zone on the Australian-Pacific Plate Boundary. Unlike the Kermadec-Tonga Trench system to the north, the Puysegur Trench does not appear to cause earthquake with focal depths of more than 120 kilometres – these events are generally restricted to the Fiordland Region of New Zealand’s South Island. The Puysegur Trench connects the 600 kilometres long Alpine Fault in the north to the 800 kilometres long Macquarie Fault Zone. Large events have occurred along this part of the greater plate boundary, but they are infrequent. This reflects general seismic activity levels.

The Kermadec Trench takes the majority of the subduction element of the collision of the Pacific Plate and Australian Plate to the north of New Zealand. Here earthquake are incredibly common, perhaps not helped by the parallel Havre back-arc basin which accounts for the extension within the overlying Australian Plate (as it bends and flexes due to the stresses exerted by the subducting Pacific Plate). Earthquakes here can reach depths in excess of 500 kilometres, whereas further to the north nearer Tonga and Fiji earthquake focal depths reach the greatest found globally at around 700 kilometres (only events under the Sea of Okhotsk, part of the Kuril-Kamchatka Trench, can match these depths).

Thus I have recorded such an example of a rare Puysegur Trench earthquake, distant from New Zealand, recorded by GeoNet, and how the issues with the automatic detection system are very similar in their nature to those faced by the system after distant Kermadec Trench earthquakes.

Sources

13:04 Auckland Islands Earthquake: http://geofon.gfz-potsdam.de/data/alerts/2014/gfz2014xnfx/mt.txt

13:05 Alexandra “earthquake”: geonet.org.nz/quakes/region/newzealand/2014p904176

13:06 Murchison “earthquake”: geonet.org.nz/quakes/region/newzealand/2014p904178

Link to previous article on 21^st September 2013 Mw 5.6 Kermadec Earthquake: http://sincalquakes.wordpress.com/2013/09/22/interpreting-a-significant-kermadec-quake/

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.