Earthquakes in which two blocks of crust grind sideways past each other rarely move enough water to make a serious tsunami. That is why the magnitude 7.6 shock off Sand Point in Alaska’s Aleutian chain on 19 October 2020 left researchers scratching their heads. The fault there shifted mostly horizontally, by close to 2 metres, yet the sea still produced a wave that, while small, was bigger than that style of rupture should allow.
A study led by geophysicist Ronni Grapenthin of the University of Alaska Fairbanks, published in the journal Seismica, now offers an answer. Modelling suggests the main quake loaded extra stress onto parts of the Alaska-Aleutian megathrust, the vast boundary where the Pacific plate dives beneath the North American plate. About 100 kilometres south of the epicentre, a small patch of that boundary then began to move, but only up to eight minutes after the initial jolt and at an unusually gentle pace.
According to the team, this delayed motion pushed the seafloor up by around 40 centimetres, enough to set the wave going. NOAA buoys nearby logged a rise of about 5 centimetres, and roughly 25 centimetres reached Hawaii. Land-based seismometers missed the slip because it released relatively little energy. Grapenthin put it vividly: after the instruments “go berserk” during the main event, a slower movement follows that “went unrecorded”. A pair of strong aftershocks that struck shortly afterwards could be part of the same delayed process, the team suggests.
The finding matters beyond Alaska. If such quiet, after-the-fact slips can feed tsunamis elsewhere without registering on land, warning systems may be underestimating the hazard from some large earthquakes.
