Over the last month, Seddon & Wellington have been rattled by a swarm of earthquakes that started with a 5.7 at 9:06am on July 19th. This swarm reached a head with a magnitude 6.5 shake on Sunday 21st July at 5.09pm. This quake was centred 25km east of Seddon and had a focal depth of 13km. An aftershock sequence followed with over 2500 earthquakes over magnitude 2.0.
Just as the previous sequence was starting to quieten down, a magnitude 6.6 earthquake struck at 2.31pm on August 16th, near Lake Grassmere. This quake heavily damaged Seddon, and startled workers and residents of Wellington. This quake also has its own aftershock sequence, of which over 2000 quakes over magnitude 2 have been recorded.
Points to note
I have been asked quite a few questions about earthquakes in the last few weeks, as one of a few staff members with a geological background. These are a few of the more common answers to questions I have been asked:
1. Earthquakes are made up of two parts – P-wave, which comes first and is a compressional, push-pull wave, and S-wave, which comes second and is the side-to-side shaking that is normally what causes damage. Very often you won’t feel the P-wave – your pets might, though, which is what gave rise to the folktale that animals ‘sense’ earthquakes.
2. There’s no real difference between an earthquake and an aftershock – they’re just words we use to qualitatively differentiate between a big event and any little ones that happen around it.
3. A swarm is where a localised area is struck by many earthquakes over a short period of time. They are usually around the same size and are different from aftershocks in that they are not preceded by a large event.
Where can I learn more?
I must confess that I am not a seismologist, but I have studied seismology and do have a degree in geology. Even so, I get a lot of my information from GeoNet. It is the most reliable source of information (I would take what they say in news articles with a few grains of salt).
These GeoNet pages are very very good:
1. GeoNet News: Damaging quake hits the north-east of the South Island : This page gives you nice brief, easy to understand scientific descriptions of what has happened, and it compares the 6.5 and 6.6 quakes.
2. GeoNet: M6.6 Lake Grassmere earthquake : This page shows more info on the 6.6 and also the latest information that GNS have on the quake.
3. GeoNet: Cook Strait aftershocks and forecast probabilities : This page gives you the lastest aftershock forecast probabilities (These are NOT predictions) and the information on the 10 largest quakes so far.