This earthquake happened in the Australia plate, west of the deep sea trench. <\/p>\n
The deep sea trench owes its existence to the plate boundary fault system there, a convergent plate boundary where plates move towards each other. The plate boundary here is formed by the subduction<\/a> of the Australia plate beneath the North Fiji Basin.<\/p>\n The largest earthquakes that happen on Earth happen on these subduction zone faults.<\/p>\n At first I thought that this was an interface earthquake along the megathrust subduction zone fault. These are called interface events because they happen along the fault interface between the two plates. They are also called interplate earthquakes.<\/p>\n However, the location showed this to be west of the subduction zone. Also, as the earthquake mechanisms (e.g., focal mechanism or moment tensor) were calculated and posted online, it was clear that this was not a megathrust earthquake. <\/p>\n Here is an illustration that shows a cross section of a subduction zone. I show hypothetical locations for different types of earthquakes. I include earthquake mechanisms (as they would be viewed from map view) for these different types of earthquakes.<\/p>\n Here is a legend for these different mechanisms. We can see what the mechanisms look like from map view (from looking down onto Earth from outer space or from flying in an airplane) and what they look like from the side.<\/p>\n The mechanism for the M 7.7 Loyalty Isles earthquake is an extensional (normal) type of an earthquake that happened in the slab of the Australia plate.<\/p>\n Typically, the extension in these slab events is perpendicular to the plate boundary fault because that is the direction that the plate is pulling down (slab pull) due to gravity or that is the orientation of bending of the plate that causes this extension.<\/p>\n There are also records of tsunami and seismic waves on water level sensors in this region. A tsunami was observed on the Ouinne (New Caledonia) tide gage.<\/p>\n Here are the tide gage data from https:\/\/webcritech.jrc.ec.europa.eu\/SeaLevelsDb\/Home<\/a>. <\/p>\n This M 7.7 earthquake is having lots of aftershocks and the largest was a M 7.1.<\/p>\n https:\/\/earthquake.usgs.gov\/earthquakes\/eventpage\/us6000kdce\/executive<\/a><\/p>\n Not only did the M 7.7 generate a tsunami but so did the M 7.1 earthquake. The plot below shows both earthquake times relative to the tsunami produced by those events. <\/p>\n These tsunami were recorded on other gages as well but this was the best record I saw.<\/p>\n My initial tweet called this normal faulting event a tensional earthquake. Dr. Harold Tobin provided an excellent overview of the difference between tension and extension.<\/p>\n In the Earth, all 3 principal stresses are (nearly) always compressional or inward-directed. A tensional stress state is a rare condition indeed. If the biggest principal stress is vertical, then high-angle (but not vertical!) extensional normal faults are the result. pic.twitter.com\/cvYar2f010<\/a><\/p>\n — Harold Tobin (@Harold_Tobin) May 20, 2023<\/a><\/p><\/blockquote>\n
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