Cascadia's Fault - Jerry Thompson [54]
Like Cascadia, there had been no major ruptures of the Alpine fault system in all of recorded history, which in this case amounted to roughly 150 years. There were plenty of signs, however, that land along the mountainfront had been folded and bent and was under extreme stress. Beaches on the Pacific plate had been pushed up into terraces as they had in Cascadia. The Southern Alps mountain chain on New Zealand’s southern islands had been uplifted as a result of compression between the plates, and in places Adams was able to study both vertical and horizontal displacements caused by earthquakes that happened centuries ago.
He noticed several other important things about the timing and the amount of movement along the Alpine system. Some—not all—of the built-up strain had been relieved during big ruptures that happened in the not-too-distant past, but there appeared to be “seismic gaps.” It was pretty obvious that parts of the fault were moving spasmodically in earthquakes. Other segments of the fault, however, showed no evidence of rupture and were either sliding along smoothly or had been stuck together by friction, building up stress for a long time, and they were probably ready to slip again in a big quake. Seismologists call this a “stick–slip” scenario.
Just as he would find years later in Cascadia, Adams learned that the science community was divided about the risk posed by the Alpine fault. Even though it was a “San Andreas–scale” crack in the crust, few seismologists had paid it much attention. “They didn’t see earthquakes,” said Adams. “Their seismic hazard analysis actually ignored it, basically. Whereas the geologists said, ‘This thing has moved recently. You can see the offsets and the other characteristics. And therefore, it has to be an active plate boundary and will generate great earthquakes.’”
The long gaps between rock- and landslides triggered by Alpine earthquakes were eerily similar to the intervals between the deep-sea landslides that Griggs and Kulm had found in the mud off the Oregon coast, so it’s easy to see why Adams was intrigued by their papers when he finally came across them. While Griggs and Kulm weren’t really looking for quakes (they had set out to study the structure and evolution of the deep-sea channel system), serendipity gave them data that would later play a pivotal role in the debate about Cascadia’s fault.
Adams, on the other hand, was definitely searching for seismic fingerprints—earthquake history—which is why he would eventually write to Oregon State University asking permission to examine the mud cores, data logs, and timelines compiled by Griggs and Kulm, Hans Nelson (who would later team up with Chris Goldfinger on a series of follow-up studies), and other members of the original OSU team. Adams wanted a closer look at the patterns. In the meantime, he drew a chilling conclusion about the Alpine fault zone in New Zealand.
While there was ample evidence of seismic activity to the north and south, there had never in recorded history been a major rupture along the central part of the fault zone. Now, with physical evidence from a series of dated landslides, Adams felt confident more of the same would occur. He wrote that large quakes with “a rupture length of 270 kilometres [168 miles], a maximum displacement of 9 metres [30 feet], and magnitudes of approximately 8 are indicated for the central part of the Alpine fault.” New Zealand, like Cascadia, was apparently locked and loaded for a major shockwave.
By the time his Alpine paper was in its final draft and being peer reviewed in the winter of 1979, Adams was already working in North America with a keen interest in the Cascadia Subduction Zone. The Alpine paper was published in a scientific journal called Geology only two months before the eruption of Mount St. Helens, and I suspect its significance—especially the parallels to Cascadia—may have been lost in that spectacular volcanic dust cloud.
Unfazed, Adams continued to work on other evidence that Cascadia was an active threat.