ruaumoko: Cape Egmont Swarm June 2010

Saturday

Cape Egmont Swarm June 2010

Over the week of 4th – 11th June 2010 a series of 9 earthquakes occurred west of Mt. Taranaki in the area known as the Cape Egmont Fault Zone.
The initial quake appears to be a double quake with the same time reading, but 2.13km apart. The 3.572ML event was reported MM4 (largely observed) at Pungarehu [1], Rahotu [1] and Warea [1].
REF, LAT, LONG, TIME UTC, DEPTH, MAG
3317910, -39.26843, 173.76195, 2010/6/4 15:36:19.5, 15, 3.572
3318090, -39.27583, 173.78476, 2010/6/4 15:36:19.5, 16, 3.627
3320356, -39.27594, 173.80382, 2010/6/9 4:4:28.9, 12, 2.906
3320495, -39.28713, 173.89268, 2010/6/9 10:52:20.9, 19, 1.754
3320667, -39.31802, 173.64392, 2010/6/9 20:12:11.7, 10, 2.724
3320674, -39.29044, 173.80011, 2010/6/9 10:54:16.3, 15, 1.467
3320860, -39.30838, 173.67719, 2010/6/10 6:1:59.4, 8, 3.144
3321217, -39.26763, 173.7896, 2010/6/10 21:24:45.4, 12, 2.768
3321372, -39.27642, 173.81067, 2010/6/11 4:25:55.5, 15, 2.711

A moderate 5.318ML earthquake occurred on Saturday, June 12 2010 at 2:20 am NZST 100km to the south and was reported widely felt throughout the lower North Island and upper South Island. It is unclear if this larger event is related to the Cape Egmont swarm as the faults south of the Cape Egmont Fault Zone in the South Taranaki Bight have yet to be determined west of the Manaia Fault.
3321605, -40.20919, 173.5623, 2010/6/11 14:20:12.6, 166, 5.318
The larger event is shown on the map for reference (zoom out)

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data by geonet. map by google maps. plots by highace using GPS Visualizer

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Earthquake Swarms

Earthquake swarms are events where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey points out that an event may be on the order of days, weeks, or months. They are differentiated from earthquakes succeeded by a series of aftershocks by the observation that no single earthquake in the sequence is obviously the main shock. Earthquake swarms are one of the events typically preceding eruptions of volcanoes

Earthquake Aftershocks

An aftershock is an earthquake that occurs after a previous earthquake (the main shock). An aftershock is in the same region of the main shock but is always of smaller magnitude strength. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are smaller earthquakes formed as the crust around the displaced fault plane adjusts to the effects of the main shock

Aftershocks are dangerous because they are usually unpredictable, can be of a large magnitude, and can collapse buildings that are damaged from the mainshock. Bigger earthquakes have more and larger aftershocks and the sequences can last for years or even longer especially when a large event occurs in a seismically quiet area; see, for example, the New Madrid Seismic Zone, where events still follow Omori's law from the mainshocks of 1811–1812. An aftershock sequence is deemed to have ended when the rate of seismicity drops back to a background level; i.e., no further decay in the number of events with time can be detected.