Let’s just have one more look at magnitude before moving on to other topics. Some people have noticed that the magnitudes being given for deep earthquakes under the North Island by GeoNet Rapid (Beta) are much lower than the official Local Magnitudes being given on www.geonet.org.nz. This is related to GeoNet Rapid (Beta) moving to magnitudes based on estimates of Moment Magnitude, as discussed in my last blog. It highlights why understanding earthquake magnitude can be complicated – particularly in New Zealand where we have deep earthquakes. The magnitude estimate used by GeoNet Rapid (Beta) is removing the bias in the Local Magnitude caused by the way the earthquake waves lose or do not lose energy as they travel through the complicated earth structure beneath the North Island. To understand this let’s look in a little detail at what lies below our feet (assuming you live in the North Island as I do).
Under the North Island of New Zealand the Pacific and Australian tectonic plates are colliding, and the Pacific plate is being pushed down (subducted) under the Australian plate (for more details see article in Te Ara). It is a slow collision compared to a car crash at only around 5 cm a year, but reasonably fast in geological terms. This can be seen in the image of earthquakes under the North Island of New Zealand (see diagram) – shallower earthquakes (orange) near the east coast give way to deeper earthquakes (green, blues to purple) as we travel west outlining the Pacific plate getting deeper beneath the Australian plate. By the Taranaki area the earthquakes are hundreds of kilometres deep and by Auckland you have moved out of the region where there is a subducting Pacific slab at depth. Above the Pacific plate under much of the central North Island, the material has been disturbed by this collision and subduction process forming a region of volcanic and geothermal activity.
When an earthquake happens deep under the North Island the earthquake waves travel up and along the colder rock of the Pacific plate without losing much shaking energy, but the waves travelling up through the hotter volcanic zone lose most of their shaking energy. This explains why these earthquakes are often strongly felt on the East Coast of the island but are sometimes not even felt directly above where they occur! Putting all this together we see why measuring the magnitude of a deep North Island earthquake is difficult. Our instruments record high levels of shaking along the East Coast of the North Island and even felt levels of shaking along the same coast in the South Island, but low levels of shaking directly above the earthquake and to the west (depending on the location).
When the New Zealand Local Magnitude scale was devised in the 1970s, these complications were not taken into account fully and so deep earthquakes under the North Island are assigned higher magnitudes than newer techniques like Moment Magnitude give. This explains why GeoNet Rapid (Beta) which uses a magnitude estimate based on Moment Magnitude gives values lower than Local Magnitude for deeper North Island earthquakes by around 0.5 units or more. Similar effects happen for deep earthquakes in the Fiordland region of the South Island.