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.
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| 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 5th November 2014 a moment magnitude (MW)
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 26th January & 3rd 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°.
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| 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 (ML) 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 ML 2.0. At 15:13 UTC on the 6th
November a ML 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 7th November 2014 a moment magnitude 4.8 earthquake
struck west of the MW 4.1 earthquake of the 5th 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°.
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| Moment Tensor Solution for the Mw 4.8 South Gulf of Argostolion Quake |
This
second MW 4+ earthquake triggered a marked increase in seismic
activity across the mouth of the Gulf of Argostolion, with 19 aftershocks
recorded by NOA over ML 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 8th November, with a focal depth
of 17.7 kilometres.
At
23:15 UTC on the 8th November 2014 a moment magnitude 5.0 earthquake
struck at about the same location as the MW 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°.
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| 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 3rd
February MW 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 ML 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 9th November 2014 with a
local magnitude of 3.1, at a focal depth of 19.1 kilometres.
Since
the MW 5.0 earthquake of the 8th November there have been
no further mainshocks within this apparent swarm. In total since the 5th
November MW 4.1 mainshock there have been 154 aftershocks recorded
by NOA; of these 49 have been smaller than ML 2.0, 96 have been of ML
2.0-2.9, 7 have been of ML 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 MW 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 12th 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.
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| Moment Tensor Solution for the Mw 3.9 Northern Paliki Peninsula Quake |
At
09:37 UTC on the 13th 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.
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| 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 5th November MW 4.1 earthquake. This
does not run parallel to the other faults identified (26th January
Fault, 3rd February Fault & potential 13th 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 MW
4.0+ earthquakes in and around Kefalonia. This is due to the increased seismic
activity after an 8 month hiatus between the 5th March MW
4.8 earthquake and the 5th November MW 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
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