Waiting for the Big One
from the book, "Petrified Lightning"
Waiting for the Big One
The term terra firma ("solid earth") is comforting but a blatant misnomer because the earth's crust is truly dynamic and almost constantly in motion. This is evidenced by the occurrence of more than a million earthquakes per year throughout the world. An earthquake can be felt somewhere in the world every few minutes, but happily for most creatures and things only 1 in 50 causes damage.
The Richter scale, which measures the ground motion as recorded on seismographs, charts the magnitude of every quake. It was originally designated as a logarithmic scale to classify the amount of energy released in an earthquake. Calculated in megatons (one megaton is equivalent to a million tons of tnt), it is an open—ended scale, from one through nine—plus, that compares the amount of energy released to an equal number of hydrogen bombs. The 1906 San Francisco earthquake, estimated at 8.3, produced the same energy as several thousand atomic explosions.
California is indeed earthquake country; the state experiences around 10,000 quakes per year. Most are imperceptible to humans, but Californians do feel about 500 annually. Fewer than one per year causes any damage or death, partly because of rigid building codes throughout the state. Most California quakes are generated by the famous San Andreas Fault that runs through southern California for a distance of 650 miles and cuts through the earth's crust about 20 to 30 miles deep.
The San Andreas is the hinge line between the American Plate on the east and the Pacific Plate on the west. It seems to be acting out the Old Testament prophecy of Zechariah that states, "The Mount of Olives shall cleave in the midst toward the east and west, and half the mountain shall move toward the north and half to the south." The plates slide horizontally past each other at the rate of two to three inches per year. This seems to describe a strike—slip motion along a fault as the Pacific Plate moves to the north and the American Plate to the south. This movement along the San Andreas tends to stretch and squeeze the adjacent land which, to relieve the stresses placed on it, will fracture and move. The pressure is transferred to other sections of the earth's crust, and they will also fracture. It appears to be an almost endless process.
The southern extension of the San Andreas underlies the Gulf of California; the northern extension enters the San Francisco Bay and terminates against the Juan de Fuca Plate near Cape Mendocino in northern California. The American northwest is relatively free from the activity on the San Andreas Fault. The Juan de Fuca Plate is a more menacing culprit.
At 5:34 a.m. on March 26, 1993, Oregon was shaken by a 5.3 earthquake that rattled much of the state and caused considerable damage, although fortunately no deaths. This was a rude awakening for Oregonians, who think of earthquakes as a California anomaly. In California, a Richter scale reading of 5.3 is not even considered a moderate quake, although it can cause damage to old, unreinforced structures. This is what occurred in Oregon on March 26. Oregonians may be surprised to realize that they undergo about a thousand earthquakes per year, most of such low energy levels that they go unnoticed.
This situation is most likely going to change. A team of scientists from Washington State has found disturbing evidence that the Pacific Northwest has experienced several powerful earthquakes in the recent geologic past and may be headed for another catastrophic shock in the not too distant future. Their concern is even more intense because northwestern cities such as Vancouver, Seattle, and Portland are not prepared for high—magnitude quakes. Should a trembler of 81 on the Richter scale strike these areas, the destruction could exceed the damage done in the San Francisco disaster of 1906.
If, or when, a giant quake rattles the Pacific Northwest, the rock slippage will originate 10 or more miles beneath the surface from a tectonic structure called the Cascadia subduction zone. The zone runs offshore from Vancouver Island to Cape Mendocino and marks the place where a piece of ocean floor, the small Juan de Fuca Plate, is slowly crashing into the edge of the North American Plate. As they undergo collision, the lighter and more buoyant North American Plate runs over the Juan de Fuca, pushing it down into the earth's interior. Because of friction with the North American Plate, some of the Juan de Fuca undergoes melting. The molten material finds its way to the surface through cracks and fissures and produces the series of active volcanoes such as Mount St. Helens.
Earthquakes originating in subductive zones are fairly common. They are often of great magnitude, such as the 1960 quake in Chile that reached 8.9 on the Richter scale. Even more unforgettable was the Alaskan quake of 1964, which reached a Richter high of 9.1. It released 12,000 times more energy than was released by the atom bomb dropped on Hiroshima. Such massive jolts occur typically when the subducting plate fails to slide smoothly under the overriding plate. When two plates lock together, as they do from time to time, they will build up strain for hundreds of years. The pressure responsible for this stress continues to thrust and shove on the plate. When the strain builds to a point where the locked zone can no longer resist the stress, the plates suddenly slip past each other, generating a massive earth shudder.
Concern is growing about the Cascadia subduction zone. Sensitive seismic instruments have not detected any large tremors originating from the contact of the North American and Juan de Fuca Plates. This could indicate that the plates are tending to stick and are therefore preparing to unleash a killer quake. Historic records are of little use, since they go back only to the early 1800s and since that time no large earthquakes have struck the northwestern part of the United States.
For earlier evidence the scientists have investigated the geologic record. In 1987 they reported finding strong evidence that sections of the Washington coast subsided at least six times in the last several thousand years. The scientists have traced the evidence up and down the coast and, by using radiometric carbon—14 dating, have established the timing of these events. These instances of sudden subsidence or uplift have been traced as far down the coast as northern California.
Although land can rise and subside for reasons other than earthquakes, the evidence here leaves little doubt as to the quake origins. Mud that had collected in estuaries was found in the sediments deposited directly on top of dry land soil layers. This suggests that the lowland areas abruptly dropped below the high—tide levels and were quickly covered by a deposit of marine mud. If the lowland area had subsided slowly, there would be a gradual transitional zone between the terrestrial soil beds and the overlying marine mud instead of the sharp distinction between the two.
In these same sedimentary deposits were valid signs of tidal waves (tsunami) that coastal earthquakes can generate. Some of the sediments that subsided show a sheet of coarse sand packed between soil and mud layers. The sandwiched sand layers reflect a series of enormous quake—generated tsunami that crashed over the subsided sections of coastline. The tidal waves deposited the sand, which was immediately covered by fine—grained marine mud. Research geologists examined the sedimentary deposits still visible on the Chilean coast after the 1960 quake. Comparing them with the northwestern sediments yielded similar examples of subsided coastline and tsunami deposits of mud.
Some of the most dramatic evidence of prehistoric quakes in the Pacific Northwest is found in the numerous trunks of red cedar trees that have remained standing centuries after they perished. Tidal muds buried the lower trunks, suggesting that the ground level dropped below the high—tide mark, and saltwater flooded their root systems. Tree ring dating established that the red cedar trees died somewhere between 1684 and 1687, all at the same time! This implies that a section of the Washington coast over 90 kilometers (56 miles) long dropped suddenly below sea level about three centuries ago. Tree ring evidence coordinates quite accurately with the data provided by radiometric carbon—14 dating.
Based on the combined evidence, the conclusion is that most of the coastal areas along the subductive zone suddenly subsided or uplifted during the late 1600s. Unfortunately, the Cascadia quakes do not hit with predictable frequency. Although the events average about 600 years apart, the span between quakes has been as short as three centuries. Moreover, these coastal areas have been subjected to this type of tectonic plate disaster for at least the last 10,000 years.
Accumulated evidence seems to indicate that the Cascadia subductive zone is storing up energy for a future blockbuster quake. Data collected over the last 60 years have shown that the coastline bordering the subductive zone has undergone significant warping during that time. Apparently this zone is at least partially locked and is building up an incredible amount of strain as it bends the edge of the North American Plate. When the plate has had all the stress bending it can take, it will snap back wildly. When this happens, Portland or Seattle may become the first city on the moon!