Cascadia's Fault - Jerry Thompson [99]
Judging by the reported height of the waves in Japan, the orphan tsunami was too big to have come from Alaska, the Aleutians, or the Kamchatka Peninsula. The way the plate boundaries line up, those subduction zones are almost parallel to Japan rather than perpendicular to it. So movements along those faults generate waves with primary energy vectors that usually miss Japan. The 1964 event in Alaska, for example, sent large and destructive waves to Vancouver Island and California, but the side-angle waves that hit Japan were less than a foot (0.3 m) above the tide.
To make the much larger waves (some as high as sixteen feet, or 5 m) that ran up the beaches of Japan in 1700, a quake in Alaska or Kamchatka would have had to be magnitude 9 or larger. But there were no written records or geologic evidence for a megathrust event of that size in either region in January of that year. Therefore, by process of elimination, Satake and his colleagues concluded that Cascadia was “the most likely source” of the orphan tsunami.
To estimate the magnitude of the parent earthquake, Satake compared the damage reports from the orphan tsunami with those from the 1960 Chile wave, which killed 140 people in Japan, and found the wave heights to be similar. To create a tsunami as big as the one that came ashore in 1700, Satake and his team figured the Cascadia earthquake must have been at least magnitude 9 or larger. From orphan tsunami to parent quake, these geologic sleuths had traced the last big failure of Cascadia’s fault. The next step was to figure out the exact time it happened.
They knew from other research how fast a tsunami could move. “The deep-ocean speed is approximately the same as a jetliner,” Satake told me, “so it would have taken about nine or ten hours from the West Coast to Japan.” Next they took the confirmed arrival times of waves hitting the beach in Japan and calculated backward to figure out when the earthquake must have triggered the tsunami.
“The earliest documented tsunami arrival time was around midnight on 27 January, Japan time,” wrote Satake and his colleagues in Nature. “Because tsunami travel time from Cascadia to Japan is about 10 hours, the earthquake origin time is estimated at around 5:00 on 27 January GMT or 21:00 on 26 January local time in Cascadia. This time is consistent with Native American legends that an earthquake occurred on a winter night.”
And so the conceptual or philosophical barrier was finally breached. It would no longer be possible to argue that Cascadia was probably harmless. The doubters could not point to an absence of great earthquakes in “all of recorded history” and make their case for aseismic subduction. Thanks to a handful of local officials in Japan, the written history of North America’s west coast had been extended back more than three centuries—far enough to reveal a new and ominous picture of this hidden plate boundary.
The evidence that Cascadia’s subduction zone had generated earthquakes big enough to send damaging waves all the way to Japan seemed quite convincing. All of Brian Atwater’s tenacious digging in the mud, together with Kenji Satake’s eureka moment and deductive reasoning, had paid off. “We were really very excited to find this evidence,” Satake said with a modest smile.
“It used to be that we would say that Cascadia made an earthquake and tsunami, or a series of earthquakes and tsunamis, about three hundred years ago,” Atwater began to tell coastal audiences at tsunamisafety workshops. “Now, we