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 23rd 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 23rd September were in
fact the largest aftershocks to occur after the moment magnitude 6.2 Eketahuna earthquake
on the 20th 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 24th 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.
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