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5.6.10: Basin and Range (The Klamath Falls Earthquakes of 1993)

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    5993
  • Overview

    Vacations in their native Oregon were a tradition with Ken and Phyllis Campbell. They came at a time when they could avoid the hottest part of the summer at their home in Phoenix, Arizona. Their 1993 excursion had been a grand trip, visiting old high-school friends and taking a cruise ship up the Inside Passage to Alaska. But it was getting late, and Phyllis was anxious to reach their destination, a bed and breakfast in Klamath Falls, a city where she had gone to first grade. Ken was already looking forward to getting back to Phoenix, where he was constructing a workshop to restore classic cars and build toys for his grandchildren. Driving south on U.S. Highway 97 toward Klamath Falls, Phyllis watched the deer along the side of the road.

     

    As they approached Modoc Point, a steep cliff beside the road, it occurred to Phyllis that she wouldn’t see any deer on the left side of the highway because the cliff came right down to the road, and there was no shoulder. Suddenly she saw a blinding flash of light, then another one, and she thought for an instant that it must have been transformers exploding from a power surge.

     

    At that instant, there was a loud crack, and Phyllis heard Ken cry out, “No!” A fourteen-foot boulder smashed down onto their pickup, killing Ken instantly. The windshield collapsed inward, and the truck spun out of control. When the spinning stopped, Phyllis found that she could unhitch her seat belt, but not Ken’s. Nothing worked: she couldn’t get the electric windows to open or the electric locks on the door to work, even though the engine was racing. She tried to turn off the ignition, but the key came off in her hand. She knew that Ken had to be dead, but she did not know how to get out of the truck. Then there was a man at the window, and she was pulled to safety.

     

    The deadly boulder and the breached highway barrier are shown in Figure 6-24.

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    At 8:28 p.m., September 20, 1993, Ken Campbell had become the first fatality caused by an earthquake in Oregon. An eighty-two-year-old woman, Anna Marion Horton of Chiloquin, died of a heart attack because she was frightened by the violent shaking of her house. At the Classico Italian restaurant in downtown Klamath Falls, bricks fell and blocked the sidewalk, and diners left their pasta uneaten and fled the building.

     

    More than a thousand buildings were damaged (Figure 6-25), with a total loss of more than $7.5 million. The Klamath County Courthouse, built in 1924, and the Courthouse Addition suffered damage of more than $3 million. Unreinforced masonry buildings suffered the worst; well-built wood-frame houses that were bolted to their foundations fared relatively well.

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    There had been a warning twelve minutes before: a foreshock of magnitude 3.9. However, this part of Oregon was poorly covered by the existing network of seismographs, and there was no system in place to evaluate the foreshock and issue a warning. Then, more than two hours after the first shock of magnitude 5.9, an even larger earthquake of magnitude 6 struck the region. The depth of the earthquakes was about six miles, much shallower than the Scotts Mills Earthquake. They were located west of Upper Klamath Lake beneath the Mountain Lakes Wilderness, between fifteen and twenty miles west-northwest of Klamath Falls (Figure 6-26). Starting in early December, a new swarm of earthquakes began east of the first group, close to the western shore of the lake, closer to Klamath Falls (Figure 6-26). After the first of the year, the aftershocks slowly began to die away.

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    Unlike the country west of the Cascades, the stark, arid landscape of southeastern Oregon leaves little of its geology to the imagination. Dave Sherrod of the USGS had been mapping the faults of the Klamath Falls region for several years, and early in 1993, before the earthquake, he had met with Klamath Falls officials to discuss the hazard.

     

    clipboard_ec796674f502f3abd3989fddc7f14a154.pngThe basin containing Upper Klamath Lake and Klamath Falls is a graben, down-dropped between faults that dip downward toward and beneath the lake. These are called normal faults, the results when the crust is pulled apart (Figures 3-10a, 6-27). Modoc Point, where Ken Campbell met his death, is part of a fault block. Over hundreds of thousands of years, the countryside east of Highway 97 has been uplifted, and the lowland to the west down-dropped along west-dipping faults so that it now lies beneath the lake. Farther south, other normal faults extend through the main part of Klamath Falls.

     

    West of Upper Klamath Lake are other less prominent normal faults at the west edge of Howard Bay, in the Mountain Lakes Wilderness, and extending beneath Lake of the Woods (Figure 6-26). These faults, which dip east, were activated by the 1993 earthquakes, although there is no evidence that any of them ruptured all the way to the surface.

     

    Fortunately for Klamath Falls, the faults on the west side of the graben ruptured rather than the faults on the east side, which extend directly through the city. If the east side faults had ruptured with earthquakes of comparable magnitudes, the damage to Klamath Falls, with its unreinforced masonry buildings, would have been disastrous, resulting in many deaths.

     

    Eastward from the Cascades from Bend and Klamath Falls to the Owyhee River country stretch the block-fault mountains and the dry-lake grabens that make up the Oregon Basin and Range: Green Ridge and Walker Rim, Summer Lake and Winter Ridge, Lake Abert and Abert Rim, and finally, higher than all the rest, and with evidence of Pleistocene glaciers, Steens Mountain, followed by the Alvord Desert (Figure 6-28).

     

     

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    Mark Hemphill-Haley, then with Woodward-Clyde Consultants, found a fault at the foot of the Steens, snaking along the west edge of the Alvord Desert Graben. The Steens Mountain Fault shows geological evidence of a Holocene earthquake within the last ten thousand years, based on trench excavations. Hemphill-Haley could then conclude on the basis of geologic evidence alone that the fault at the foot of the Steens is active in the legal sense of the word, which means that special precautions should be taken to guard any major structures against seismic shaking. Fortunately, there are only a few ranches and herds of livestock, and they would probably survive a magnitude 7 quake without much problem.

     

    Hemphill-Haley had the answer to why Steens Mountain is there in the first place. It has been gradually raised up from the desert floor along its range-front fault, accompanied by literally thousands of earthquakes over a period of millions of years, each earthquake lifting the mountain up just a few feet. The cumulative effect of all these individual uplifts is the massive, rugged fault-block mountain we see today, snow-capped much of the year, towering over the playa flats of the Alvord Desert to the east (Figure 6-28).

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    West of Steens Mountain, a swarm of earthquakes struck the small town of Adel, in Warner Valley, in 1968, with the largest of magnitude 5.1 (Figure 6-29). Silvio Pezzopane and Ray Weldon of the University of Oregon found other active faults in the desert west of Abert Rim, and they applied the new science of paleoseismology to find evidence of prehistoric earthquakes in backhoe trenches across fault scarps. Faults that are active on the basis of offset Holocene deposits were found in Paulina Marsh, at the west edge of Summer Lake near Winter Rim, and along the west boundary of Abert Rim. Normal faults in eastern Oregon are seen on computer-generated topographic images, including faults in and near Bend, Oregon (Figure 6-30). (The Bend fault scarps may be active, but faulting involves sediments in part derived from Cascade volcanoes to the west, and they might be due to resistance to erosion of volcanic-derived sediments rather than Holocene faulting.)

     

     

    The Oregon Basin and Range is the northern continuation of the Basin and Range of Nevada (Figure 6-31), including the Central Nevada Seismic Zone, which was rocked repeatedly by a series of eight earthquakes, starting in 1903 and ending in 1954, the largest of magnitude 7.5. Fault scarps that formed during several of these earthquakes are magnificently preserved in the desert climate (Figure 3-7) and can be seen by driving a back road south of Winnemucca, Nevada, through Pleasant Valley at the western foot of the Sonoma and Tobin ranges, over the Sou Hills, down Dixie Valley east of the Stillwater Range, to U.S. Highway 50, itself broken by a surface rupture accompanying an earthquake of magnitude 7.2 on December 16, 1954. Like the Steens country, the Central Nevada Seismic Zone is thinly populated, and although the earthquakes were felt over large areas, the losses were small.

     

     

     

    Despite the intense seismic activity in this century, long-term slip rates on faults in the Central Nevada Seismic Zone are extremely slow, comparable to slip rates on faults in the Oregon Basin and Range. Paleoseismology shows that prior to the twentieth century, earthquakes occurred many thousands of years ago. We refer to the Nevada earthquakes of the twentieth century as an earthquake cluster, characterized by intense activity over a short period of time separated by thousands of years of quiet. The Oregon Basin and Range is similar to the Central Nevada Seismic Zone, but its seismic silence shows that it is in a quiet period. We know that this quiet period will end someday, but we do not know when—tomorrow or thousands of years from now. Sadly, forecasts made in terms of many thousands of years do not answer the societal questions about timing (next year or fifty years from now?) that are of interest to you and me and those around us.

     

     

     

     

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