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.

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

Moderate Aftershock Rocks Seddon

Moderate Aftershock Rocks Marlborough Region

Mb 3.7, 5km north-east of Seddon, South Island, New Zealand

The Mb 3.7 Seddon Aftershock, visible just right of the 60 minute timestamp.

A magnitude 3.7 earthquake has struck a short distance north-east of the township of Seddon in the Marlborough Province of the South Island, New Zealand. The earthquake had a focal depth of about 9 kilometres and struck at 11:05am NZST. Felt reports have been received from the local settlements, including Blind River, Seddon, Blenheim, Spring Creek & Renwick. The maximum intensity reported was MM6 in Blenheim; however epicentral shaking intensities of MM5 in both Seddon & Blind River are more in keeping with the expected shaking intensity and are therefore regarded by myself as the maximum felt intensity.

It has been over 15 months now since the Cook Strait sequence began with a moment magnitude 5.5 earthquake in the morning of the 19th July 2013. Since then there have been numerous major earthquakes in the Cook Strait & Seddon regions, including a Mw 6.6 earthquake offshore on the 21st July 2013 & a Mw 6.6 earthquake underneath Lake Grassmere on the 16th August 2013. There were over 4500 recorded quakes in this sequence until the end of September 2013, over 850 of them over a magnitude of 3.0 and thus likely felt. Today's earthquake is one of the smaller aftershocks, but owing to its timing within the aftershock sequence is of considerable merit and interest.

Since the beginning of 2014 there have been 9 earthquakes in the general Cook Strait aftershock zone over a magnitude of 3.5. Of these, four were over a magnitude of 4, the latest a Mb 4.3 quake a considerable distance offshore of Cape Campbell on the 19th June. This earthquake was located within the subducting oceanic Pacific Plate at a depth of 38 kilometres and was widely felt Paraparaumu down to Kaikoura.

The June M4.3 quake was the last earthquake in the general aftershock zone, but within the main quake zone the last over M3.5 was a magnitude 4.1 earthquake about 30 kilometres north-east of Seddon, not too far from the location of the first Mw 5.5 quake in mid-July 2013. This means that today's M3.7 earthquake is the first major Seddon aftershock in 8 months.

Seddon's aftershock sequence has been very consistent with that expected of a highly energetic earthquake swarm. There is a lot of evidence indicating that the Cook Strait earthquakes increased stresses on many Marlborough faults, including the Clarence Fault, Awatere Fault and Wairau Fault. The former may indeed be the culprit which caused the Seddon Sequence.

Whilst this earthquake appears to be just a normal aftershock occurring a relatively long time after it's mainshock, there is the potential for further large earthquakes in the north-east part of the South Island.

This article was written on the 29th October 2014 by J H Gurney.

Nottingham Rocked By Tremor This Evening!

Magnitude 2.6 Earthquake in Hucknall, Nottinghamshire.

Struck at 7:16pm GMT, 28th October 2014.

A magnitude 2.6 earthquake has struck approximately 11 kilometres north of Nottingham. Felt in Nottingham, Carlton, Hucknall and Mansfield. No aftershocks have been observed and few if any are expected. This does not however rule out the potential for a larger or similar size earthquake.

Update 10:04 GMT, 29th October:

This morning BGS released most of its information on yesterday evening's magnitude 2.6 earthquake just north of Hucknall, Nottinghamshire. It struck at 7:16:54pm and had a focal depth of 7.1 kilometres. Many felt reports were given to BGS, most centred in Hucknall, Kirkby in Ashfield, Sutton-in-Ashfield, Mansfield, Ravenshead and north-east Nottingham. The furthest felt report was from south-west Nottingham, some 15 kilometres (10 miles) away.

A map of BGS felt reports to the M2.6 Hucknall Earthquake on the 28th October 2014. All rights to BGS.

BGS also released it's customary historical seismicity map for the earthquake. This shows three significant quakes in the region in the past 250 years, with M4.7 & M4.2 events in 1795 & 1816 respectively, whilst in more recent times a magnitude 4.1 earthquake struck south-east of Nottingham.

A map of historical and instrumental seismicity around the Mansfield and Nottingham area. The ~M3.6 Mansfield Quake of 1990 can be seen as the moderate-sized red circle just south-east of Mansfield.

BGS says this is the largest earthquake to hit Nottinghamshire since March 1984 (M3.2, East Retford). This is very puzzling as a large earthquake of ~M3.6 was recorded in east Mansfield in February 1990 by the United States Geological Survey (USGS); it is entirely doubtful that USGS overestimated the magnitude of this earthquake by such a large amount so that BGS recorded the 1990 quake as less than M2.6.

If you have any queries or comments about this earthquake or my website, please send correspondences to: sincalveniren@ymail.com

The BGS seismometer station at Charnwood Forest, Leicestshire. The M2.6 Hucknall Earthquake is clearly visible on the 19:00 UTC line - it arrived at Charnwood Forest about 6 seconds after the earthquake began.

7:56PM Two tweets have been found thus far regarding the minor tremor (https://twitter.com/leemarch/status/527184827792232449 & https://twitter.com/steviemac2010/status/527178392022843392).

As of 8:00pm BGS has not put up the earthquake, nor has EMSC noted it.

8:11PM - CEA's magnitude has an uncertainty of ~0.27 magnitude, so magnitude of the earthquake is currently between M3.0 and M3.6. No news agencies picked up quakes, although an editor at Sky has noted the increase in twitter activity (https://twitter.com/SkyNewsEditor/status/527189157228335105)

8:22PM - Both BGS & the Nottingham Post have reported the earthquake being felt in and around Nottingham. BGS is currently working on analysing the earthquake. Very sketchy talk of it being either an earthquake or gas explosion. No word yet from main news agencies in the UK (ITV, BBC or Sky).

8:51PM - BGS has given an official magnitude of 2.6 to this tremor, much smaller than CEA's preliminary magnitude 3.3. This is extremely normal, the French Agency commonly overestimating magnitudes of British earthquakes. Below is a picture of the locations of the earthquake according to CEA & BGS respectively.

Location of earthquake according to BGS (British Geological Survey) & CEA (French Agency). BGS location & magnitude much more reliable/

9:00PM - EMSC quotes BGS data on their website, completing the list of agencies expected to report the Mansfield Earthquake (or Hucknall Earthquake as would appear to be more accurate - see above map!!). BGS say the earthquake had an observed intensity of 3 on the European Macroseismic Scale which seems consistent with reports by people on Twitter over the past two hours.

10:04PM - A look at the archives reveals that there has been relatively few felt earthquakes in the past 25 years. The largest earthquake in the region in that time was a magnitude 3.6 earthquake which hit Mansfield in February 1990. The only other magnitude 2+ earthquake nearby since 2000 was a magnitude 2.5 tremor that struck just south of Cotgrave, near Cropwell Bishop, in February 2013. This earthquake was felt over a reasonable area, with reports coming in via BGS from many local villages and the town of Hucknall, whilst other reports have come from Carlton in East Nottingham & the village of Mapperley west of Nottingham.

Sources:

BGS: http://www.earthquakes.bgs.ac.uk/earthquakes/recent_events/20141028191600.html#page=summary


CEA: http://www-dase.cea.fr/evenement/evenements.php?type=seisme&identifiant=20141028-191657&lang=en

EMSC: http://www.emsc-csem.org/Earthquake/earthquake.php?id=406145

This article was written on the 28th October 2014 by J H Gurney

Monthly Pongaroa Aftershock Report 1: October 2014

Pongaroa – One Month On

A report on aftershock activity to the 23rd September Mb 5.5 Pongaroa Earthquake, New Zealand.

At 02:41 NZST on the 23^rd September 2014 a magnitude 5.5 earthquake struck about 20 kilometres west of Pongaroa, waking up most of the lower North Island of New Zealand. The earthquake gained 3489 felt reports on GeoNet, with highest intensities at Herbertville (east coast) & Terrace End (Palmerston North) at MMI7. The earthquake struck at a depth of 25 kilometres and was followed 16 seconds later by a magnitude 5.2 earthquake slightly further north at a shallower depth of 11 kilometres. This second earthquake was not identified until 61 hours after the earthquake occurred.

The two main shocks on the 23^rd September were in fact the largest aftershocks to occur after the moment magnitude 6.2 Eketahuna earthquake on the 20^th January 2014. That earthquake occurred about 11 kilometres south-west of September’s aftershock, showing the length of the apparent ruptured fault line. Aftershock activity to the January earthquake was relatively quick to decay over time, with 6 aftershocks over magnitude 4 striking within a 48-period, but only two further aftershocks over magnitude 4 striking before the September quake. All of these initial aftershocks were relatively well contained to a south-west to north-east striking fault line, about ten kilometres in length.

Aftershocks to the Pongaroa earthquake are seemingly following a very similar trend. In the first month since the Mb 5.5 earthquake there have been 138 earthquakes over magnitude 1 in the immediate aftershock region. A more detailed table of them can be seen below:

Total Aftershocks: 138

Aftershocks >M3: 4

Aftershocks M2-2.9: 37

Aftershocks M1-1.9: 97

Felt earthquakes: 5 (M2.6 to M3.7)

Most of the earthquakes to have occurred after the Pongaroa Mb 5.5 earthquake have been below M2.5; in fact there have been only 7 aftershocks over M2.5. This seems to correlate quite well with a reinvigoration in seismic activity caused by a large aftershock, as seen during the 2010-2012 Christchurch sequence when large aftershocks in June & December 2011 caused brief increases in seismicity for a matter of days to weeks before the aftershock activity returned to previous large-aftershock activity rates. In the case of Pongaroa, the following has been observed:

00 hours to 24 hours after Mb 5.5: 19 aftershocks

24 hours to 72 hours after Mb 5.5: 10 aftershocks

3 days to 7 days after Mb 5.5: 17 aftershocks

7 days to 14 days after Mb 5.5: 35 aftershocks

14 days to 21 days after Mb 5.5: 23 aftershocks

21 days to 30 days after Mb 5.5: 32 aftershocks

To show this is a more regular pattern, daily intervals have been taken since the mainshock and given as a bar chart:

A bar chart showing the frequency of earthquakes in individual 24-hour periods after the Mb 5.5 Pongaroa quake on the 23rd September 2014: each 24-hour period begins and terminates at 02:41:38 NZST.

As can be clearly seen from this bar chart, there is much variation still in the aftershock sequence to the Mw 6.2 January Earthquake, and only a brief peak is seen after the Mb 5.5 September Earthquake. The largest aftershocks occurred on Days 15 & 16 (M3.6 & M3.0), and both of these aftershocks in fact occurred near original ruptured fault or fault segment south of the September Earthquake. This shows that the Mb 5.5 has had little overall effect on the aftershocks sequence as a whole, and normal minor aftershock activity as would be expected at this time of the January sequence is what is being observed.

This in-depth article was written on the 24^th October 2014 by J H Gurney.

Post Scriptum: Another update will be made in a month’s time on this sequence, and a second monthly report set shall begin in mid-November on the Puysegur Trench aftershock sequence, for which data is a little scarcer. If any other significant earthquakes occur in New Zealand they shall also be noted, and a New Zealand specific page will also be created in the next few days to weeks when time allows – JHG.

Dunedin Rocked By Moderate Quake

Moderate Quake Rocks Otago Province

Mb 4.1, 30km west of Dunedin, South Island, New Zealand.

The Magnitude 4.1 Lee Stream Earthquake as seen on the New Zealand National Seismograph Network

A magnitude 4.1 earthquake has struck a short distance away from the South Island's third city, Dunedin, gaining over 1200 felt reports and a maximum intensity within the city of MM6. The quake's effects were relatively localised, mainly due to the sparsity of major settlements, so that the quake was felt at Oamaru to the north, Balclutha to the south and Kingston to the west. A supposed felt report from Arthur's Point, Queenstown, is not to be believed due to the lack of reports from Queenstown and it's associated suburbs.

The earthquake struck at 6:44pm NZST, and had a focal depth of 4 kilometres. It was located near the small community of Lee Stream, and only 10 kilometres north-west of the town of Outram.

The Dunedin region is one of the most aseismic parts of New Zealand, with seismic events quite rare. A quick look into the archives finds that since 1980, there have only been 6 earthquakes over magnitude 4 recorded by GeoNet, three of them offshore (M4.2 in 1982, M4.3 in 1989 & M4.1 in 1991). 

The other three quakes (including today's) are located west of Dunedin, the northernmost near Middlemarch (M4.1, 1986). The other two are in closer proximity and therefore it is likely they occurred on the same fault, giving a very rough recurrence interval for a quake of the same size as today's. These two quakes occurred near Lee Stream. In 1982 an M4.0 struck at a depth of 12km, only a short distance east of today's epicentre. Today's quake struck at a depth of 4km, and with the increase in seismograph stations across New Zealand since the early 1980s the location and depth are almost certainly much more accurate than the 1982 event.

The largest quake to strike the region in the past century was an offshore M5.0 event in April 1974. The earthquake struck at 7:50pm local time, about 10km south of central Dunedin, at a depth of about 12km. The earthquake caused chimney damage, masonry to crack and tiles to come loose in Dunedin and its immediate suburbs. The fact that there have been five offshore earthquakes over M4 between 1941 and 1991, all south of Dunedin, infers a fault system there which is possibly, but not necessarily, quite constrained in size.

In recent years there have been seven earthquakes over M3 within 30 kilometres of Dunedin. Three such earthquakes occurred south-west of the city (M3-3.5), two south of the city offshore (M3.2-M4.1), one to the north of the city (M3.1) and today's earthquake. There have been many smaller earthquake, particularly to the south-west of the city on the Akatore Fault (see below).

Possibly the scariest thing about this is how little the public knows of the threat to the eastern Otago region. Only in recent years has there been any sort of effort to analyse seismic hazards in this region, with three major faults discovered: the Akatore Fault, which runs parallel to the coast from the Taieri River mouth to about 30km south-west of Dunedin, the Titri Fault which runs for 60 kilometres parallel to the Akatore Fault, and the lesser known Green Island fault, which is probably responsible for the offshore quakes mentioned above. The Titri Fault or a related fault is most probably responsible for today's earthquake.

The warning here is rather disconcerting and worrisome - the Greendale & Port Hills Faults were unknown before the Darfield & Lyttelton Earthquakes in 2010-11, and the damage they caused to Christchurch and the wider Canterbury Plains was massive. Seismicity in the region was known, with at least three quakes known from the 19th century underneath the city itself, and a M5.6-5.8 quake known to have occurred underneath Lake Ellesmere in 1870. None of these previous quakes were in living memory, nor on the faults which ultimately ruptured.

Perhaps the same is true for Dunedin and the Otago region. Dunedin is primarily built atop a Miocene basalt bedrock, very similar to that which Lyttelton and Akaroa are built in on Bank Peninsula, and the type of bedrock which was undoubtedly home to the deadly Port Hills Fault. Maybe it should be seen that, much like in Christchurch, the people of Dunedin should be aware of and prepare for the threat of a damaging earthquake in the future.

Search Parameters for Dunedin Data

See: https://docs.google.com/file/d/0BwHSzEDw4wXsVEdCRWhoZ0Y5VUk/

General Information on Dunedin Seismic Risk

See: http://www.orc.govt.nz/Documents/Publications/Natural%20Hazards/Hazards%20on%20the%20Taieri%20Plains/Taieri%20Report%20-%20Seismic%20Hazards.pdf

This in-depth article was written on the 16th October 2014 by J H Gurney.