Cascadia's Fault - Jerry Thompson [55]
In 1982 Reilinger and Adams took the Washington highway data from the earlier study and extended it southward by adding new measurements from five more resurveyed east–west highways crossing through the mountains in Oregon. They showed that survey markers located near the coast had been lifted up a noticeable amount in the less than eighty years since the last set of surveys. Tide gauge data along the coast showed pretty much the same thing; the beaches had been lifted as well. Put it all together and you got a picture of a mountain range about 370 miles long and 37 miles wide (600 km by 60 km) being hoisted up along its western edge and tilted, en masse, toward the east.
In this new mountain-tilting paper Adams and Reilinger drew attention to another worrisome study published only six months earlier by Jim Savage and a team at the USGS that showed land in the Puget Sound lowlands around Seattle apparently being compressed: squished together in a northeasterly direction. That was the same direction the Juan de Fuca plate was supposed to be moving. If the ocean floor was actively sliding underneath the continent and if the two plates were locked together by friction, this kind of compression, or “crustal shortening,” near Seattle was exactly what you’d expect to find. It was also exactly contrary to what Robert Crosson and Ando and Balazs had said earlier.
When Ando looked at the vertical shift of outer coast survey markers and the eastward tilting of the Coast Range, he and Balazs reasoned that the long-term, apparently quake-free uplift meant the two tectonic plates were not locked. That’s why Cascadia was seismically quiet. So how could one explain Savage’s new compression data? How does the ground squeeze together if the plates are not locked?
A series of measurements of the distances between geodetic survey monuments on opposite sides of Puget Sound, spaced six to eighteen miles (10–30 km) apart across the sound, revealed a surprising and somewhat baffling trend. Between 1972 and 1979, Savage and his colleagues used a Geodolite, a powerful and precise distance-measuring instrument that fired a laser beam from a survey marker on one side of Puget Sound across to a similar marker on the other side. There, a bank of highly polished mirrors bounced the laser back to the Geodolite, which measured how long the beam took to make the round trip.
If it took less time in 1979 than it did in 1972 the two markers had to be closer together, and that’s exactly what they found. They figured the accuracy of the Geodolite was within 0.2 inches (5 mm) and that the amount of squeezing of the valley floor was statistically significant. Savage and his coauthors (Mike Lisowski and Bill Prescott) recognized that their new data were “not easily reconciled” with Ando’s aseismic subduction concept, but they published them anyway. They concluded that the laser measurements were evidence of strain building up, probably caused by the subduction of the Juan de Fuca plate underneath North America.
The significant point was that the two plates had to be locked together for this kind of strain to accumulate. Savage and his team were saying as politely as they could that Ando and Balazs must be wrong. “The implication is clearly that the Washington and Vancouver Island coasts are subject to great, shallow, thrust earthquakes,” they wrote in June 1981.
A year later, when John Adams was drafting his conclusions for the paper on Coast Range tilting based on highway survey data, he spoke to Savage on the phone and heard his idea about why the