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Subduction-zone mega-thrust #earthquakes, the most powerful earthquakes, can produce #tsunamis through a variety of structures that are missed by simple models. (THREAD)
The basic model shows rock above the convergent plate boundary being compressed and bent as it stores elastic energy. Once friction is overcome, the overriding plate slides abruptly up the fault causing the leading edge to heave seawater upward generating a #tsunami.
In 2011, the M9 #Japan #earthquake ruptured a fault area 500kms x 200kms over an interval of nearly 3 min. Surveys of sea-floor bathymetry before and after the earthquake document up to 50 m of fault displacement at near the Japan Trench.
During the rupture process, the overriding plate slid up the west-dipping fault plane that uplifted the sea floor by 10m producing the local #tsunami that rushed onshore within 20 min of the earthquake & the distant tsunami that spread over the Pacific Ocean Basin.
BUT, deformation of the overriding plate, as well as fault displacement, can produce a tsunami.
The 2010 M8.8 Chile earthquake tore a rupture zone 600 km x 130 km wide. Although the rupture began at approximately the same distance from the trench, there was little, to no displacement at the trench as there was during the Japan earthquake.
Fault displacement ranged from 5m at the hypocenter beneath the coast to 10m midway to the trench, then decreased to little or no displacement at the trench. Westward displacement of the continental shelf above the plate boundary caused broad uplift of the seafloor.
Additionally, the change in displacement along the plate boundary caused internal deformation of the overriding plate. These effects combined to uplift the ocean floor by 2- 3 m. This sudden uplift generated the deadly local #tsunami along the coast & at nearby islands.
The 1964 M9.2 Great #Alaska Earthquake ruptured a major segment of the eastern Aleutian subduction zone 800 km long lasting over 4 min. The continental shelf and slope of the overriding North American Plate was uplifted over 9 m while inboard areas subsided as much as 2 m.
This sudden displacement of the ocean floor generated the #tsunami that claimed lives from Alaska to northern California. Less than 2 m of regional uplift occurred at the Trench. So what caused the 9m uplift of the continental shelf and generated the tsunami?
During this earthquake, displacement started on the plate-boundary megathrust fault producing 2 to 4 m of regional seafloor uplift & subsidence. BUT, most of the fault offset ramped up to the seafloor on “splay” faults like the Patton Bay Fault that was displaced 9 m.
Because displacement on steeply-dipping splay faults causes larger uplift of the ocean floor, a bigger #tsunami is produced and it starts closer to shore providing less time to evacuate.
In steep-sided fiords and inlets, ground shaking caused #landslides, some entirely below sea level, resulting in surges of seawater up to 50 m.
The Alaska 1964, Chile 2010, and Japan 2011, as well as the catastrophic 2004 Sumatra subduction zone megathrust earthquakes have delivered powerful lessons that rapid evacuation of tsunami inundation zones is a life-saving emergency response.
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