At Umnak Island in the Unalaska seismic gap, Witter and his colleagues discovered drift logs stranded up to 75 feet (23 meters) above sea level. There were sand deposits in the island's marshes from nine past tsunamis in the last 2,200 years. The team also found evidence of six past tsunamis on nearby Sedanka Island in the last 1,600 years. The tsunami sediments imply repeated earthquakes in the Unalaska seismic gap every 280 to 325 years, Witter said. (By modeling potential seismic wave sources, geologists can determine whether a tsunami was likely triggered by a landslide or an earthquake.) A separate USGS study reported six tsunamis in the past 1,770 years on Unalaska Island, which is also in the Unalaska seismic gap.
The drift logs, combined with evidence for several prior tsunamis, suggest that the Unalaska seismic gap isn't a gap at all, Witter told Live Science's Our Amazing Planet. Instead, this part of the fault broke in 1957 during the magnitude-8.6 Andreanof Islands earthquake, he said. (Thanks to an early warning system, the tsunami from this quake did not cause any deaths, though it did destroy buildings and boats along beaches in Alaska and Hawaii.)
Farther east, however, the Shumagin seismic gap seems to be one of the calmest zones along this hazardous fault. In a separate study, Simeonof Island within the gap shows no evidence of uplift (or land-level change) from earthquakes in the past 3,400 years, nor signs of damaging tsunamis, Witter and his colleagues reported April 9 in the journal Geophysical Research Letters.
Instead of rupturing in large earthquakes, the Shumagin gap looks to be creeping, a type of movement in which each side of a fault slides past the other without storing up powerful earthquake energy. The central segment of the San Andreas Fault also creeps, as do portions of Indonesia's subduction zone near the Batu Islands, Briggs said.
While evidence for earthquakes in the Shumagin seismic gap could be buried offshore, the lack of evidence for changes in height or for tsunamis limits the possibility that mega-earthquakes occurred in the gap, the researchers said.
"We found no evidence of great earthquakes in the Shumagin seismic gap in the past 3,400 years," Witter said. If such events had occurred, "the earthquake and its tsunami would have left clues along the shoreline."
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Article originally appeared on Live Science's Our Amazing Planet.