The 29 July 2025 M8.8 Kamchatka earthquake occurred on a megathrust fault- these are the largest faults on Earth and are capable of hosting the largest earthquakes on Earth.  The shallow parts of megathrust faults are underwater – so they also pose a significant tsunami risk.  The 2011 Japan M9 and 2004 Sumatra-Anadaman M9.1 are recent, devastating megathrust earthquake examples.  Megathrusts exist at subduction zones, where a denser oceanic tectonic plate sinks below a less dense overriding tectonic plate.  At the location of the 29 July 2025 earthquake the Pacific plate is sinking below the North American plate.  Megathrust faults allow earthquake rupture along great lengths and given its size the M8.8 is likely to have ruptured over a length of 300-400 km.  The depth is reported by USGS to be 20 km, which is quite shallow for megathrust earthquakes.  The shallow rupture of the fault means the earthquake has more chance of causing the seabed to move which can cause a tsunami.  An earthquake of this size in this area is not a surprise – in 1952 there was a M9.0 earthquake close to this latest rupture.  This is a subduction zone well known for being able to produce large earthquakes.