A series of earthquakes, all of them of magnitude 7 or more, struck the Yakut region of Alaska. The main quake had a magnitude of 8 and struck on September 10, 1899. The whole coastal area of Alaska north and south of Yakutat Bay was shaken for a distance of six hundred miles.
During the month of September 1899 the region of Yakutat Bay was shaken by a series of very severe earthquakes, all of them of magnitude 7 or more. Fortunately, the number of people living in this region was very small and no one was killed. All of these events profoundly changed the topography of the area, raising the elevation of the land in places by as much as forty-seven feet, the greatest amount of displacement by an earthquake ever known in historic times.
The most powerful earthquake of the series was the magnitude 8 quake that struck on the tenth of September. This earthquake was experienced as a strong shaking from Fairbanks all the way to Sitka, a distance of more than six hundred miles. The geologists who visited the area five years later found dead barnacles and other shellfish everywhere. They saw several uplifted beaches and areas of subsidence as deep as six feet. The tenth of September earthquake lasted ninety minutes and was followed by many aftershocks.
The whole southern coast of Alaska, including the Aleutian chain of islands that stretch westward as far as the International Date Line, is a 1,500-mile-long chain of volcanic activity constituting the volcanic capital of America. There are more active volcanoes or earthquakes here than in all the other United States combined, an average of one event occurs every year. The Aleutian Islands, many of them standing high above sea level, together with the mountains of the main part of Alaska, are all outcomes of millions of years of volcanic eruptions and earthquakes.
The Pacific Tectonic Plate is constantly pressing against and sliding beneath the North American Plate. The contact area between these two massive plates is curved and this condition gave rise to subsidiary movements around faults, or cracks in the seabed, that force the main plates to slide past each other rather than collide. Often the sliding past type of action can be more destructive than a head-on collision. Two of these subsidiary faults, the Queen Charlotte and the Transition, especially the Transition, are the ones most involved in the earthquake of the tenth of September 1899. Their actions in causing earthquakes are not yet fully understood but it is clear from what happened that they are very important.
Yakutat Bay is a deep indentation in an otherwise unbroken concave stretch of coastline between Cross Sound and Controller Bay. Eastward of this coastline are the St. Elias and Fairweather ranges—St. Elias with heights of 18,000 and 19,500 feet respectively in Mount St. Elias and Mount Logan. These mountains do not rise directly from the sea, but are faced by a low foreland, or coastal plain, made up of glacial debris. The northwest side of Yakutat Bay is still occupied by the ice plateau of the Malaspina Glacier.
On its west side, the bay is bordered by a foreland of glacial gravels which are still being deposited by streams issuing from the Malaspina and other glaciers. The changes that the earthquake effected in the glaciers included a rapid retreat of Muir Glacier, 150 miles to the southeast, and a general advance of several other glaciers near Yakutat Bay. Muir Glacier, which hundreds of travelers had visited annually up to 1899, became inaccessible to tourist vessels in that year and remained so until 1907. By 1903 the glacier had retreated by as much as three miles and, by 1907, almost eight miles. Before examining further the details of the earthquake of September 10, 1899, a short account of Alaska’s earthquake history will help to set the stage.
Early reports of earthquakes in Alaska were fragmentary. The first event in this incomplete record occurred in July 1788 when a tsunami inundated the islands of Sanak and Unga and a part of the Alaska Peninsula. Overall, the record of earthquakes identifies two areas as having experienced most of the state’s seismic activity—one area is the Aleutian Island Chain and the other is the coastal and inland region north and south of Yakutat Bay. From 1899 to 1969, eight earthquakes of magnitude 8 or more on the Richter Scale occurred in Alaska. Four of these caused extensive property damage and topographic changes; the other four were centered in areas with no nearby towns, and, except for being recorded by seismographs, went relatively unnoticed.
The Alaskan earthquake that is outstanding in the memory of most is the Anchorage quake of 1964. It had a magnitude of more than 9. In October of 1900 a magnitude 7.9 earthquake was felt from Yakutat Bay to Kodiak, and probably even farther westward. On Kodiak Island chimneys were downed, and a man was thrown from his bed. The shock was probably centered in southeastern Alaska. Property damage was very moderate for such a great shock because of the low population densities in the affected areas.
Andreanof Island sustained an earthquake of magnitude 8.8 in 1957 which caused very severe damage on Adak and Unimak Islands. This earthquake initiated a tsunami and its forty-foot wall of water smashed the coastline of Unimak Island. On Adak Island two bridges were destroyed and considerable damage was done to roads and docks. The tsunami caused millions of dollars of damage in Japan and Hawaii, both parts of the world that have suffered damage from Alaskan tsunamis from time to time. During the period 1899 to 1969, eight great earthquakes of magnitudes 7–7.9 occurred in Alaska. Thirteen earthquakes occurred in or near populated regions and caused minor to severe damage.
On July 22, 1937, a magnitude 7.3 earthquake occurred in central Alaska, about twenty-five miles southeast of Fairbanks. It was felt over most of Alaska’s interior, over an area of 300,000 square miles. About ten years later, on October 15, 1947, a magnitude 7.3 shock occurred in the same region. It was preceded by a swarm of shocks, some very minute, others violent. On April 7, 1958, a magnitude 7.3 earthquake hit central Alaska. Within a 40–50 miles radius of Huslia, cracks in lake and river ice, and many ground cracks and mudflows, were observed. The strongest shock since those of September 1899 at Yakutat hit southeastern Alaska on July 9, 1958. It was rated magnitude 7.9 on the Richter Scale.
The U.S. team of geologists who visited Yakutat Bay in 1905 came upon clear evidence of recent uplift in barnacles attached to ledges high above the reach of the present tide. They conducted detailed observations along the affected shoreline until practically every foot of its 150 miles had been examined, and evidences documented of uplift, depression, faulting, avalanches, earthquake waves, and changes in the locations of glaciers. Local native fishermen and prospectors provided detailed accounts of what had happened when the earthquake struck.
For example, eight men were in the fiord portion of the Yakutat Bay inlet on the tenth of September. They had camped on the east side of the moraine-covered margin of Hubbard Glacier. They were prospectors, washing the gravels they had collected in hope of finding gold. Alaska and Yukon Territory in Canada had been the center of gold rushes a few years earlier and the findings from that time were still inspiring new seekers of gold. These men lost everything they possessed when the earthquake struck and they almost lost their lives too. For the team of geologists they provided good descriptions, as much as they could remember in the context of a terrifying experience.
As nearly as can be made out from the prospectors’ descriptions, their camps were on the moraines and alluvial fans. Three men were on one side of a glacial stream and the others were on the opposite side. As is common in major earthquakes or volcanic eruptions, there are minor tremors that precede the main event. The men decided to rig up a simple device that would indicate the strengths of these tremors. In 1899 very little was known about the forces that cause earthquakes and there were no instruments suitable for measuring their strengths.
All the men could do was improvise. They hung up two hunting knives in such a way that their points were touching. Any earth tremor would cause a jingle between the knives and the strength of the sound would give them some idea of the power of the tremor. These men were well acquainted with earthquakes from their experiences in Alaska and they hoped to be able to detect danger moments among the tremors and so get away to a safe place before the main quake struck. They counted fifty-two tremors in all before the big quake struck but it was difficult to identify increasing strengths in them. Their plan was a clever one; unfortunately, it failed to give them the kind of advance warning they needed.
One man was about six miles from the spot where the shore was lifted up forty-seven feet and two miles away from another spot where the shore had been uplifted seven feet. It was impossible for anyone to stand unaided as everything around was moving. Even the ground cover of alder bushes kept shaking and bending over as if they were under the influence of a strong wind. All of the enormous uplifts of shore areas began to take place early, around nine, and this was followed by a succession of further shocks that ended with the biggest shock of the day around two in the afternoon. At that time the men were sitting in their tent. As they tried to get out one of them was physically thrown over the camp stove and across the length of the tent. The other men took hold of the tent pole and held on to it for the duration of the quake. The ground below was moving like waves in the sea for the minutes it took for the earthquake to subside.
Immediately, a pause occurred in the total amount of movement all around, the men then ran outside, leaving everything. They never recovered any of their possessions. As they watched they saw the Hubbard Glacier, one that measured five miles in diameter, slide out half a mile into Yakutat Bay. At the same time a lake of two acres in size and thirty feet in depth that had stood above the men broke away from its site and crashed down on the place where their camp had stood. The men were already away from the area by this time.
They had started running away from their camp as soon as there was a slight pause in the violent ground shaking. They felt that the safest place was the lowest elevation and so they had made for the shore. The collapse of the lake brought water, rocks, and debris down the mountainside but, within a short time, a tidal wave, triggered by the uplift of land, brought a wall of water twenty feet high on to the high ground, sweeping the debris from the lake back up on to the moraines. The men heard the sound of this ocean wave but by that time they had found a place of safety and they stayed there. Notching the fiord walls at various levels was a series of sea cliffs, which the waves had cut in the headlands and which, with their associated rock benches, were hoisted above sea level during the faulting.
The benches are broadest and the cliffs highest where the weaker rocks outcrop on exposed points and they are narrowest and the cliffs lowest where the more resistant strata occur. These elevated benches are not remnants of glacial marginal channels, as is proved by the barnacles and other sea forms still attached to their ledges. All in all, they form one of the most striking, obvious, and spectacular of the physiographic evidences of uplift. The amount of land created from the sea during these changes of level far exceeded the amount submerged by the sea in places where there were depressions.
In summary, these earthquakes were most severe on two dates, September 3 and 10, especially on the tenth, when there were more than fifty small shocks and two violent ones, the second of which was extremely violent and probably caused the greater part of the changes observed in and around Yakutat Bay. There were many additional shocks. The view of the geologists who visited the area within five years of the events concluded that they had no record of any other region on earth that experienced such shaking as had occurred here.
The great earthquake in the afternoon of the tenth of September was concentrated in Yakutat Bay. The shock of it, though locally sharp and of world-shaking caliber, seems to have been more restricted and was observed at fewer localities in Alaska than the earlier earthquake on the same day. The volume affected by the earth movement must be great in order to shake such a wide area. During all these seismic disturbances there was no recorded loss of life and little damage to property, simply because of the sparseness of population in the shaken area and the fact that few buildings were there. Most of the buildings were low, one-story rustic cabins built loosely of heavy logs or boards. Past experience has shown that homes of this kind are well able to withstand earthquakes as they are difficult to tear apart by shaking.