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.

Large earthquake strikes offshore of El Salvador

Large earthquake strikes offshore of El Salvador

Magnitude 7.2, offshore south of San Miguel, El Salvador

A large subduction earthquake struck about 100 kilometres off the coast of El Salvador at 03:51 UTC, causing damage in many several communities near to the Pacific coast. Reports indicate minor damage in San Miguel, including a major hospital and some walls with heights in excess of 4 metres.

The earthquake was caused by the subduction of the Cocos Plate below the Caribbean Plate. Earthquakes occur commonly along this convergent plate boundary, including the April Mw 6.6 quake in Nicaragua, and the March Mw 6.1 quake. Quakes are rarely deadly, and the last magnitude 7.0+ quake in this region was the magnitude 7.6 quake in Costa Rica in 2012.

Nicaragua's seismic institute INETER has recorded nine aftershocks to the earthquake over Ml 3.0, the largest a magnitude 4.6 at 13:27 UTC. This is about 1.5 magnitude levels below the expected magnitude for the largest aftershock, according to the Gutenberg-Richter Law. There also appear to have been a relatively low number of aftershocks over Ml 3.0; for example, the Van Earthquake of October 2011 had 34 aftershocks over magnitude 4.0 in the first ten hours of its aftershock sequence, which is about 11 times as many as this quake has had in the same time period.

List of Aftershocks (INETER)

Ml 4.0 - 04:01 UTC

Ml 3.8 - 04:06 UTC

Ml 3.6 - 04:47 UTC

Ml 3.4 - 04:49 UTC

Ml 3.6 - 05:04 UTC

Ml 4.2 - 06:09 UTC

Ml 3.7 - 06:11 UTC

Ml 3.1 - 07:18 UTC

Ml 4.6 - 13:27 UTC

This article was written on the 14th October 2014 by J H Gurney.

Magnitude 2.0 Aftershock Strikes Jersey

Aftershock strikes near Jersey

Magnitude 2.0, near Jersey, Channel Islands

A minor earthquake struck south-west of Jersey in the Channel Islands, with reports sent to the British Geological Survey of people on the island of Jersey feeling the tremor. It is another aftershock of the magnitude 4.3 earthquake which struck on the 11th July this year.

The magnitude 2.0 earthquake struck at 9:49pm, and had a focal depth of 7 kilometres. Previous seismicity in this area since the magnitude 4.3 earthquake on the 11th July have struck at depths of between 6 and 12 kilometres along a north-south striking line. Although the immediate conclusion is this line may well represent the features of a fault, it has yet to be confirmed that this is the case. In fact very little information has been forthcoming regarding the July earthquake from BGS, something which is rather bizarre for the largest earthquake to strike the Channel Islands since 1932.

The Channel Islands have a long recorded history of felt earthquakes on or near the islands. The earliest known earthquake to have been felt on the Channel Islands occurred in 1091 and this tremor brought down the copper cock on the spire of Coutances Cathedral in France; it quite likely also brought down stones from the church at Angers. Since this quake there have been numerous large quakes in the Channel Islands or adjacent parts of France, including numerous shakes in April 1773, a large earthquake offshore of Caen in 1878, another large earthquake offshore of Caen in 1889, two earthquakes in 1926 and 1927 east of Jersey of large magnitudes, and the 1932 earthquake.

Further earthquake in the Channel Islands over the next two or three decades are to be expected. Due to the lack of recent earthquake of significance, it is unknown what sort of magnitudes to expect, but in general magnitude of 5.5 at maximum are not to be dismissed for the Channel Islands themselves, and magnitude 6 earthquakes in the Cotentin or southern Brittany near Vannes are distinct possibilities.

This article was written on the 4th October 2014 by J H Gurney.