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.

Summary: On the Kefalonia Earthquakes 5.11.14 - 13.11.14

On The Kefalonia Earthquakes: 5 to 13 November 2014

Mw 3.9-5.0, Kefalonia, Ionian Islands, Greece, 5-13 November 2014

This is a brief summary of the seismicity that has so far occurred on the island of Kefalonia, Greece, between the 5th November and 13th November 2014. All aftershocks have been reliably provided by NOA up until midnight 13th November; therefore aftershocks to the Mw 4.3 13th November Northern Kefalonia earthquake are not included in this summary. A more in-depth report will be released at a later date.

An interpreted seismogram from the Kalavrita Seismograph (KLV), Achaia Province, Greece. The Mw 4.8 South Gulf of Argostolion Quake can be seen clearly in red.

At 14:22 UTC on the 5^th November 2014 a moment magnitude (M_W) 4.1 earthquake struck the south of the Ionian Island of Kefalonia, Greece. The earthquake was the largest aftershock in eight months to the doublet earthquake of 26^th January & 3^rd February 2014. The earthquake was located at the mouth of the Gulf of Argostolion and had a focal depth of 12 kilometres. According to the National Observatory of Athens (NOA) the earthquake was the result of strike-slip faulting; the moment tensor solution provided had an inferred fault plane striking due east-west, with a dip of 86°.

Moment Tensor Solution for the Mw 4.1 South Gulf of Argostolion Quake

Over the next few hours several minor aftershocks struck along a clear fault plane bearing agreement with that inferred from the moment tensor solution. The largest aftershock struck at 14:54 UTC, a local magnitude (M_L) 2.8 earthquake west of the mainshock’s epicentre, with a focal depth of 21.2 kilometres. In the 24 hours immediately following the mainshock there were 19 aftershocks recorded by NOA over M_L 2.0. At 15:13 UTC on the 6^th November a M_L 2.8 event occurred in about the same location as the previous day’s quake of the same size, with a focal depth of 19.6 kilometres.

At 07:41 UTC on the 7^th November 2014 a moment magnitude 4.8 earthquake struck west of the M_W 4.1 earthquake of the 5^th November. This earthquake struck at a focal depth of 18 kilometres; the moment tensor solution provided by NOA inferred a fault plane striking slightly west-south-west to east-north-east (strike = 263°), with a dip of 76°.

Moment Tensor Solution for the Mw 4.8 South Gulf of Argostolion Quake

This second M_W 4+ earthquake triggered a marked increase in seismic activity across the mouth of the Gulf of Argostolion, with 19 aftershocks recorded by NOA over M_L 2.2 in the 24 hours immediately following this second mainshock. The largest aftershock occurred at 07:53 UTC with a local magnitude of 3.6, at a focal depth of 16.5 kilometres. Four other magnitude 3.0+ earthquakes struck over the next 24 hours. These were: a local magnitude 3.2 at 09:56 UTC, with a focal depth of 19.6 kilometres; a local magnitude 3.3 at 10:54 UTC, with a focal depth of 17.3 kilometres; a local magnitude 3.3 at 17:23 UTC, with a focal depth of 16.5 kilometres; and a local magnitude 3.0 at 10:14 UTC on the 8^th November, with a focal depth of 17.7 kilometres.

At 23:15 UTC on the 8^th November 2014 a moment magnitude 5.0 earthquake struck at about the same location as the M_W 4.8 of the previous day. This earthquake struck at a depth of 14 kilometres; the moment tensor solution provided by NOA inferred a fault plane striking slightly west-south-west to east-north-east (strike = 261°), with a dip of 78°.

Moment Tensor Solution for the Mw 5.0 South Gulf of Argostolion Quake

This third mainshock was the largest earthquake to strike Kefalonia since the 3^rd February M_W 5.9 earthquake. Another marked increase in seismic activity was observed on this strike-slip fault at the mouth of the Gulf of Argostolion, with 32 aftershocks recorded by NOA over M_L 2.0 in the 24 hours immediately following this mainshock. The largest aftershock in this 24 hour period occurred at 01:30 UTC on the 9^th November 2014 with a local magnitude of 3.1, at a focal depth of 19.1 kilometres.

Since the M_W 5.0 earthquake of the 8^th November there have been no further mainshocks within this apparent swarm. In total since the 5^th November M_W 4.1 mainshock there have been 154 aftershocks recorded by NOA; of these 49 have been smaller than M_L 2.0, 96 have been of M_L 2.0-2.9, 7 have been of M_L 3.0-3.9 and 2 larger earthquakes (as noted above). Most of these aftershocks have occurred at focal depths of 14-22 kilometres, with only 4 earthquakes having depths less than 14 kilometres – this number does not include the initial M_W 4.1 earthquake, which is noted as having a revised focal depth of 16.6 kilometres when the moment tensor solution data is ignored.

Although the Gulf of Argostolion sequence appears to have eased up, two moderate earthquakes have subsequently struck the north of Kefalonia. At 06:31 UTC on the 12^th November a moment magnitude 3.9 earthquake struck onshore of the Paliki Peninsula, about 12 kilometres north of Argostolion. This earthquake struck at a depth of 6 kilometres; the moment tensor solution provided by NOA inferred a fault plane striking slightly north-north-east to south-south west (strike = 7°), with a dip of 35°. Unlike the southern earthquakes, this earthquake was the result of thrust (compressional) faulting. Very few aftershock were observed to this earthquake.

Moment Tensor Solution for the Mw 3.9 Northern Paliki Peninsula Quake

At 09:37 UTC on the 13^th November a moment magnitude 4.3 earthquake struck offshore, west of the Erisos Municipality in northern Kefalonia. This earthquake struck at a depth of 6 kilometres; the moment tensor solution provided by NOA inferred a fault plane striking north-north-east to south-south-west (strike = 15°), with a dip of 78°. This earthquake was the result of strike-slip faulting similar to the southern earthquakes.

Moment Tensor Solution for the Mw 4.3 Northern Kefalonia Quake

However this particular earthquake appears to correlate well with the mainshocks of January & February; the inferred fault plane matches those mainshocks, and the alignment infers that this aftershock occurred on a parallel to sub-parallel strike-slip fault. Subsequent reading of published work has shown that this earthquake, as well as those of January & February, occurred on the Kefalonia Transform Fault Zone (KTFZ). The form of the KTFZ is still debated, with it either being a homogenous single strike-slip fault running from the Ambracian Gulf in the north to a point south-west of Kefalonia in the south, or a series of smaller (~10 kilometres long) parallel and sub-parallel strike-slip faults. This year’s Kefalonia earthquakes may provide valuable evidence to resolve this question and bring a better understanding of the seismic hazard in the Central Ionian Islands.

This poses an intriguing question regarding the strike-slip fault which began to rupture with the 5^th November M_W 4.1 earthquake. This does not run parallel to the other faults identified (26^th January Fault, 3^rd February Fault & potential 13^th November Fault) and therefore is arguably not a part of the KTFZ. Nevertheless it must be related in one form or another due to its proximity to the KTFZ and is probably the result of the same tectonic processes that have formed the KTFZ.

Over the next few weeks the author proposes that there may well be further M_W 4.0+ earthquakes in and around Kefalonia. This is due to the increased seismic activity after an 8 month hiatus between the 5^th March M_W 4.8 earthquake and the 5^th November M_W 4.1 earthquake being so obviously broken. The lack of significant strain energy release in the period previous to November’s activity infers that a trigger was required for further moderate to large earthquakes to occur. As with any earthquake sequence there is the potential for a larger earthquake to occur; Kefalonia has borne witness to two magnitude 7 events since 1950, the first in 1953 and the last occurring in 1983 (Papadimitriou, E.E. (2002), Bulletin of the Seismological Society of America, Vol. 92, No. 8, Table 1, pp. 3296). With a debatable recurrence interval of 30 years, the next magnitude 7 is apparently due.

References:

NOA Revised Locations Catalogue: http://bbnet.gein.noa.gr/HL/seismicity/catalogues/manual-alerts

NOA Moment Tensors Catalogue: http://bbnet.gein.noa.gr/HL/seismicity/moment-tensors

NOA Valsamata (VLS) Seismograph: http://bbnet.gein.noa.gr/plots/VLS.html

NOA Kalavrita (KLV) Seismograph: http://bbnet.gein.noa.gr/plots/KLV.html

NOA Stations Heliplot History: http://bbnet.gein.noa.gr/HL/real-time-plotting/old-dates

Kefalonia Rocked By Two Aftershocks

Two Strong Aftershocks Rock Kefalonia

Mw 4.1, 9km south of Argostoli, Kefalonia, Greece, 14:22 UTC, 05/11/14; Mw 4.8, 10km south-south-west of Argostoli, Kefalonia, Greece, 07:41 UTC, 07/11/14.

 

The Mw 4.8 Argostoli Earthquake recorded on the Valsamata Seismograph, NOA NET. Two sizable aftershocks are visible (both in red on the right of the screenshot): Ml 3.6, 07:56 UTC & Ml 3.3, 10:54 UTC.

In the past two days two moderate aftershocks have struck the island of Kefalonia, Western Greece. The two earthquakes have occurred on a fault located near Kefalonia Airport in the south of the island.

Graphic created by J H Gurney showing the assumed strike-slip fault as suggested by the moment tensors of the 5th November Mw 4.1 & 7th November Mw 4.8 events - this graphic was created before the 7th November event was given a moment tensor solution by NOA.

An Mw 4.1 (Ml 4.2) earthquake struck on the 5th November at a depth of 12 kilometres. The moment tensor solution by NOA indicates a strike-slip faulting mechanism on an east-west striking fault, bisecting the Gulf of Argostoli at its mouth with the Ionian Sea.

The moment tensor solution information from NOA for the 5th November Mw 4.1 event. The beachball plot (top left) and map (bottom left) show a clear strike-slip earthquake at the mouth of the Gulf of Argostoli. Data on the right infers a nearly horizontal (dip = 86) fault plane with a east-west strike (strike = 270).  The rake (rake = -20) infers a minor normal faulting element to the quake. The earthquake is given a focal depth of 12 kilometres.

Earlier this morning an Mw 4.8 (Ml 4.7) earthquake struck at a depth of 18 kilometres. No moment tensor solution has yet been given by NOA, but judging by the many small earthquakes since the Mw 4.1 on the 5th November this earthquake will also be a strike-slip earthquake on the same fault.

The moment tensor solution information from NOA for the 7th November Mw 4.8 event. The beachball plot (top left) and map (bottom left) show a strike-slip earthquake with a minor reverse element (rake = 15) at the mouth of the Gulf of Argostoli. Data on the right infers a nearly steep (dip = 69) fault plane with a roughly east-west strike (strike = 263). The earthquake is a given a focal depth of 18 kilometres.

In January & February 2014 two magnitude 6 earthquakes struck the island, spawning numerous aftershocks and severely damaging the harbours at both Argostoli & Lixouri. Incredibly, despite the proximity to these major towns and the very shallow depth of the quakes, nobody was killed and between the two quakes only 23 people were injured.

Since the 3^rd February Mw 5.9 earthquake there has been a rapidly declining number of moderate aftershocks. These two earthquakes are of particular interest due to their location south of Argostoli, on the fringe of the immediate aftershock zone, and the fact these have occurred a full nine months after the mainshock. More shocks in the region of magnitude 4 are possible in the next few days and weeks.

This article was written on the 7th November 2014 by J H Gurney.

Hobbiton Hit By Doublet Earthquake

Real-Life Hobbiton Shaken By Two Moderate Quakes

Mb 3.6, 25km north-east of Cambridge, New Zealand, 02:47 UTC, 05/11/14; Mb 3.8, 25km south of Te Aroha, New Zealand, 14:43 UTC, 06/11/14

The M3.8 earthquake today, seen on the Tahuroa Road seismograph just above the 4-hour timestamp. Other quakes: at 10 hours M2.7; two quakes after M3.8 are estimated at M2.9 & M1.5 respectively (not put up by GeoNet); at 2 hours M2.3.

Two moderate earthquake have struck near the town of Matamata, Northland Region, New Zealand, home to the farm in which the set for Hobbiton was built. Yesterday a local magnitude 3.6 earthquake struck at a depth of 7 kilometres; felt reports were received from Onemana (Coromandel Peninsula) in the north, Whakatane in the east, Putaruru in the south & Hamilton in the west. The worst felt intensities were received from the nearby towns of Matamata, Waharoa and Cambridge, with moderate felt intensities reported (MMI5). The earthquake gained 269 felt reports, a reasonable figure for a moderate quake in the Northland Region.

Then earlier today a preliminary magnitude 3.8 earthquake struck in pretty much the same location, at a focal depth of 7 kilometres. The quake was felt as far north as Coromandel, as far east as Te Puke, as far south as Taupo and as far west as Raglan. The worst felt intensities were once again in the nearby towns of Matamata, Waharoa & Cambridge, but also in Tirau (further south), Wilton (north-west of Waharoa) & the city of Tauranga on the Bay of Plenty coast, all with moderate felt intensities reported (MMI5). The earthquake has gained over 660 felt reports (as of 19:52 UTC), indicating a greater magnitude than the 5^th November earthquake.

Earthquakes in this part of New Zealand are comparatively rare. The last earthquake to strike this particular area occurred in August 2011, a magnitude 3.4 event. Other events in the Northland Region is recent years include a swarm on the Coromandel Peninsula in June 2013 (including magnitude 3.0 and magnitude 3.4 events) and a doublet earthquake in March 2013 north-east of Auckland (magnitude 3.1 & magnitude 3.9). The latter mentioned Auckland earthquake is the most felt quake in GeoNet’s history, with nearly 14,000 felt reports received and a maximum intensity of MMI6.

The current Matamata sequence (so far six recorded earthquakes by GeoNet, with two others between M1.5 and M2.9 unrecorded) is unlikely to continue for too much longer; like other swarm activity in the Northland region it very often has one or two moderate earthquakes and then disappears again very quickly. Nevertheless there is the potential for further earthquakes of similar size in the next few days in the Matamata region.

This article was written on the 6^th November 2014 by J H Gurney.

Second Moderate Earthquake In 24 Hours Rattles North-West Nevada

North-West Nevada Swarm Still Energetic

Mw 4.3, 68km ESE of Lakeview, Oregon, 08:34 UTC, 06/11/2014

A moment magnitude 4.3 (body-wave magnitude 4.5) earthquake has struck the Sheldon Contiguous Study Area in north-west Nevada. This is the second-largest earthquake to strike in a nearly four-month long swarm that has rattled this rural part of the American West. The earthquake had a focal depth of 8 kilometres and was centred 68 kilometres east-south-east of Lakeview, Oregon. The quake was the result of a normal faulting mechanism, with a major strike-slip element; the strike of the fault plane was roughly north-east to south-west.

After yesterday’s Mw 4.6 earthquake a large number of moderate high-magnitude 3 earthquakes have afflicted the region, including an Mw 3.9 event at 15:18 UTC on the 5^th November, with four more earthquakes of similar size striking today (including a preliminary local magnitude 4.2 at 15:58 UTC). This is a much more energetic phase within the sequence than at any point in the past and may well indicate that we are in the peak of activity within the swarm. A similar situation was seen during the Hawthorne Sequence of 2011 where eight earthquake over magnitude 3.5 struck within a 36-hour period (15^th to 16^th April 2011), including a magnitude 4.6 event.

Only the largest events have been felt people, mainly due to the extremely rural nature of this region and the timing of the largest quakes (from the late hours of night until the early hours of the morning). If this swarm had been occurring in a less rural region then it is likely that many people would have felt even some of the smaller shocks (M2.0-3.0), mainly because of the extremely shallow depths of these earthquakes. Examples of this are present throughout the world, but were highlighted recently by the Christchurch earthquakes, where small quakes of around magnitude 2.7 have been felt very well in the southern and eastern suburbs.

My personal belief is that this swarm will perhaps continue at this sort of intensity for another 12-36 hours before beginning to fade away. This does not mean there will not be further magnitude 3.5+ events weeks or even months from now, but the frequency with which these earthquakes strike should decrease over time. Unlike the earthquakes in central and northern Oklahoma, these earthquakes are tectonic in nature; therefore there is a limit as to how much energy can be released as the stress being released cannot be quickly accumulated again.

It is plausible, though less likely than the scenario described above, that the swarm will continue at similar intensities for longer than the 36-hour window I predict. This would be of great interest as seismic observations in this part of Nevada are scarce and may help answer some of the questions regarding the causes of the seismicity in this part of the United States. In this scenario the swarm would continue at similar intensities as at present, punctuated by several magnitude 4 events, before losing energy very quickly with the frequency of earthquakes fading fast.

A far less likely scenario is that this swarm is a precursor for a much larger event, perhaps in the order of magnitude 5-6. This is considered very unlikely as there is no evidence currently that the fault or faults rupturing in this swarm are large enough to generate such an earthquake. Nevertheless similar occurrences have happened in the recent past, with a large swarm preceding the Mw 8.1 Iquique Earthquake in northern Chile. This is extremely unlikely to occur as the Nevada quakes are not located near a subduction zone and therefore the chance of this swarm preceding a larger earthquake is extremely remote.

This article was written on the 6^th November 2014 by J H Gurney.

Moderate Earthquake Rattles North-West Nevada

Moderate Earthquake Rattles North-West Nevada

Mw 4.6, 63km ESE of Lakeview, Oregon, 07:23 UTC, 05/11/14.

A moment magnitude 4.6 (body-wave magnitude 4.6) earthquake has struck the Sheldon Contiguous Study Area in north-west Nevada. This is the just the latest in a large swarm of minor earthquakes that has been rumbling on since mid-July. The earthquake had a focal depth of 8 kilometres and was centred about 63 kilometres east-south-east of Lakeview, Oregon. The quake was the result of normal faulting, with a minor strike-slip element; the strike of the fault is roughly north-north-east to south-south-west.

In the past few days the earthquake swarm has surged in intensity after a short period of inactivity, with two earthquakes of Mw 3.9 (30^th October) & Mw 3.8 (4^th November) the main quakes in this new period of activity. The largest earthquake previous to this appears to have been an Mw 4.0 event on the 1^st October. All three of these events have been the result of oblique normal faulting, indicating that the quakes are the result of tensional forces within the tectonic setting of this region.

The greater part of Nevada is a small section of what is generally known as the Basin & Range Province, stretching from north-western Mexico all the way up to south-east Oregon. Boundaries of the region in the western United States are the Sierra Nevada Mountains to the west and the Wasatch Fault to the east. A magnitude 6.0 earthquake which struck the town of Wells, Nevada in 2008 was the largest earthquake to occur in this region since 2000.

A swarm similar to the Sheldon Area Sequence occurred in 2011. The Hawthorne Swarm of March to May 2011 included several events over magnitude 4, the largest a magnitude 4.6 on the 16^th April 2011. Most of these events occurred at depth of 10-15 kilometres, although later on in the swarm events occurred at shallower depth of between 2-5 kilometres.

The Sheldon Area Sequence has now been going on for nearly four months and has included many moderate earthquakes of the high-magnitude 3 to low-magnitude 4 range. Today’s earthquake may well indicate the height of seismic activity in this sequence, but there have been quite a few aftershocks so far to today’s earthquake (including a moment magnitude 3.5 quake three hours after the Mw 4.6*). This swarm is certainly something of interest to keep an eye on for the weeks to come.

*This aftershock fits the Gutenberg-Richter Law of aftershock magnitudes relating to their mainshock almost perfectly, being just over 1 magnitude below the mainshock’s.

This article was written on the 5^th November 2014 by J H Gurney.

Second Quake In A Week Rocks Seddon

Second Aftershock Rocks Marlborough Region

Mb 3.7, 10km south-east of Seddon, South Island, New Zealand

The Mb 3.7 Seddon Earthquake, visible just right of 80 minutes before timestamp

A magnitude 3.7 earthquake has struck just south-east of the township of Seddon in the Marlborough Province, South Island, New Zealand. The earthquake had a focal depth of 19 kilometres and struck at 5:15 NZST. Felt reports have been received from Renwick in the west, Ward in the east and Mount Cook, Wellington in the north. The maximum felt intensity was MM4 (light shaking), most probably because of the hour and the focal depth being reasonably deep.

This is the second moderate Seddon aftershock in the past week, with a magnitude 3.7 striking on the 30th October to the north-east of Seddon. Whereas last Thursday's quake had its epicentre on the Awatere River, today's quake occurred underneath Lake Grassmere, where last August's Mw 6.6 mainshock occurred. This is an expected aftershock on what is possibly a northern extension of the Clarence Fault near Cook Strait; looking at Wednesday's aftershock it is plausible that may have occurred on a northern extension of the Awatere Fault, therefore meaning these two moderate aftershock have occurred on two of the main fault lines of the Marlborough Fault System.

It is not unusual for such sizable aftershocks to occur over a year after a sequence's mainshock, although it is relatively unusual for them to be temporally quite close. I still do not believe there is any cause for alarm, especially as both aftershocks have occurred within the general aftershock zone from last July's & August's quakes. Nevertheless the residents of Marlborough must be vigilant as last summer's quakes are likely to have increased the rupturing of more faults in the region in magnitude 5 or higher events.

This article was written on the 3rd October 2014 by J H Gurney