# 2.3: Earthquakes in the Crust- Closer to Home

“They laugh and play in the sleepy harbor town
So unaware of the danger that’s around
Livin’ on the fault line
Livin’ on the fault line.
No one can run when it finally comes down
Nobody knows what is stirrin’ underground
Livin’ on the fault line
Livin’ on the fault line.

Doobie Brothers

## 1. Introduction

We have heard the bad news regarding the next great subduction-zone earthquake, but there is one small bit of good news. Based on the edge of the ETS zone, west of which the subduction zone is currently locked, the epicenter is likely to be offshore or close to the coast: more than 50 miles from Portland, one hundred miles from Seattle, and a bit farther from Vancouver (Figure 4-25). This means that due to attenuation, seismic waves will be smaller when they reach the major population centers than when they strike the coast.

Figure 6-1. Quaternary faults of western Washington (heavy dashed lines). Faults with solid lines are major reverse faults, probably not active, with arrows toward the hanging-wall side. Seattle and Tacoma faults are generally east-trending and are close to the cities for which they are named. The fault striking northwest-southeast is the Southern Whidbey Island fault, extending from southeast of Victoria, B.C., southwest of Everett, and into the foothills of the Cascades. The Olympia fault also strikes northwest-southeast.
 A video element has been excluded from this version of the text. You can watch it online here: http://pb.libretexts.org/earry/?p=120 Figure 6-2. Map of Lake Washington, bordering the city of Seattle on the east, locating major landslides and sunken forests (coarse dot pattern). Dashed lines extending across southern half of lake are possible traces of the Seattle Fault. Solid circles are piston core sites that are correlated in Figure 6-5. Modified from Robert Karlin, University of Nevada Reno, and Sally Abella, University of Washington

Damage was estimated at more than $28,000,000, with$4,500,000 to the State Capitol alone. (The ultimate cost of retrofitting the Capitol was later estimated at more than 67,000,000!) Surprisingly, there were no deaths. Injuries were limited to those from falling glass and bricks and to some of the employees of a large Walmart store overcome by fumes from bottles and cans of garden chemicals that had crashed to the floor. Unreinforced masonry buildings suffered a disproportionate share of the damage. The timing of the earthquake was fortunate: early in the morning during the week of spring vacation, preventing deaths at the unreinforced Molalla High School building. The fortunate timing gave the earthquake its name: the Spring Break Quake. Losses would have been much higher if the earthquake had struck one of the larger communities of the Willamette Valley rather than a rural area in the foothills of the Cascades. Former senator Ron Cease of Portland, a member of the legislature at the time, may have said it best: not being able to walk beneath the rotunda on their way to work had an educational effect on Oregon’s legislators in terms of earthquake hazards! As shown in Figure 6-16 and 6-17, there are other faults in the Willamette Valley. The Corvallis Fault was mapped by Chris Goldfinger on the northwest side of the city of Corvallis in low hills slated for urban development. Despite considerable efforts, none of these faults can be shown to displace Holocene deposits (younger than ten thousand years). Accordingly, we cannot state that these faults are active. The faults can be marked on the maps of areas being considered for urban development, and developers, local government, and potential buyers can make up their own minds about the potential for fault rupture. If you were considering purchase of a new house in the Willamette Valley, would you want to be told by local government that your house would be built on or close to a major fault, even though it could not be said that the fault was active or not? ## 7. Southwest British Columbia Northern Vancouver Island just doesn’t seem like Earthquake Country. The highway north of Victoria runs past small towns along the east coast of the island; it is lined with firs, with breathtaking views of the Georgia Strait, the Gulf Islands, and on a clear day, the snow peaks of the Coast Mountains. The road passes through Courtenay to Campbell River, past fishing villages and logging camps. One branch of the road crosses the Forbidden Plateau and Strathcona Provincial Park on its way to a lonely, storm-swept fjord below Gold River, on the Pacific Ocean side of the island. This thinly populated region was the location of the largest crustal earthquakes in the short recorded history of the Cascadia region, an event of M 7.0 on December 6, 1918 and a larger earthquake of M 7.3 on June 23, 1946 (located on Figure 6-19). Figure 6-19. Earthquakes northeast of Nootka fault zone and central Vancouver Island. The Nootka fault zone separates the Juan de Fuca and Explorer plates. The beach-ball plots show that all earthquakes were due to left-lateral strike slip; larger earthquakes have larger beach balls. The 1918 and 1946 earthquakes were in North American continental crust. The 1946 earthquake was the largest recorded crustal earthquake in the Cascadia region. QCF: Queen Charlotte fault. The 1946 earthquake produced extensive chimney damage in Campbell River, Courtenay, and Comox, and there were many landslides in the mountains and liquefaction and slumping of coastal sediment. Despite extensive areas of intensity VIII from Campbell River to Courtenay, only one person was killed when his boat at Deep Bay was swamped by a wave, possibly generated by slumping of sediment into the water. The 1918 earthquake struck along the primitive west coast of Vancouver Island, damaging the lighthouse at Estevan Point, south of Nootka Sound. The area of highest intensity was thinly populated, with widely scattered fishing villages accessible only by boat, and damage was slight. The focus of the earthquake was about ten miles deep, and intensities up to VI were recorded. It was felt as far away as Seattle and the town of Kelowna in the Okanagan Valley east of the Cascades. The seismograms of both earthquakes, as recorded at distant stations, show that the motion was consistent with left-lateral strike slip on a crustal fault (or faults) striking northeasterly. This is the same strike as the Nootka Fault, a major left-lateral strike-slip transform fault on the deep ocean floor west of the continental slope, a fault that forms the boundary between the Juan de Fuca Plate and the Explorer Plate (Figure 6-19). However, the earthquakes are not located directly on the landward projection of the Nootka Fault but are offset about forty miles to the east. The more heavily populated regions of Vancouver and Victoria experience quite a few small earthquakes, indicating that the region is a northern continuation of the seismically active crust beneath Puget Sound. This poses a dilemma for seismologists such as Garry Rogers of the Pacific Geoscience Centre in Sidney, B.C., concerned about estimates of seismic hazards in these areas. Should Rogers and his colleagues consider that earthquakes as large as the 1946 event, M 7.3, are possible in Vancouver or Victoria, or anywhere else in the shallow continental crust of southwestern British Columbia? Or should they conclude that the large crustal earthquakes in central Vancouver Island are part of a zone that has an unusually high seismic hazard because of its proximity to the offshore Nootka Fault, thereby reducing the perception of hazard to Vancouver and Victoria? The answers to those questions are not yet at hand. ## 8. Eastern Washington and Northeastern Oregon John McBride and his partner, Jack Ingram, were in trouble with the law. Contemporaries referred to them as “border ruffians . . . scoundrels who for pure cussedness could not be excelled anywhere on the border,” probably a compliment in the Washington Territory in 1872. Things had started out well; they had set up the first trading post in Wenatchee. But they were caught selling liquor to the Indians, and this got them arrested in Yakima. They bribed the prosecutor and were set free, but John McBride was then rearrested by federal marshals in Walla Walla, and he posted bond. He and Ingram had sold the trading post and were living in a cabin west of the Columbia River near the Wenatchee River while awaiting trial. In the early morning hours of December 15, 1872, they were awakened by a loud noise, as if the stove had fallen over. As they were pulling on their clothes, they were thrown to the floor, and they realized that they were experiencing an earthquake. They made their way to the Wenatchee trading post, six miles away, where they found the new owners in a state of confusion, with sacks of flour thrown about and damage to the roof and upper logs of the cabin and to the kitchen. Great masses of earth came down from the hills, and the gulches were filled with debris. A group of Spokane Indians crowded around the white settlers, crying out that the world was coming to an end. North along the Columbia River, a fifteen-year-old Indian boy, Peter Wapato, told of a landslide at Ribbon Cliff near Winesap (present-day Entiat) that dammed the Columbia River for several hours. This landslide was also reported by the Indians to a settler, Elizabeth Ann Coonc, camped downstream. Decades later, geologist I. C. Russell of the USGS would describe this landslide at a place that became known as Earthquake Point. The Indians called it Coxit (Broken-off) Point. Chilliwack and Lake Osoyoos, B.C., and Snoqualmie Pass and Kittitas Valley, Washington, reported intensities of VII. Port Townsend, Seattle, Olympia, Vancouver, and Walla Walla, Washington, and Victoria, B.C., experienced intensities of VI. A century later, the 1872 earthquake was the subject of great speculation because of plans for nuclear power plants by the Washington Public Power Supply System and Seattle City Light. The epicenter was variously located in the north Cascades, in the western foothills of the north Cascades, even in British Columbia, with magnitude estimates as high as 7.4. Bill Bakun of the USGS and his colleagues used the distribution of felt reports to locate the epicenter near Entiat and to estimate the magnitude as MI 6.8 (see Chapter 3 and Figure 2-6a), which made it the largest historical crustal earthquake in the Pacific Northwest except for Vancouver Island. No source fault has been found. The eastern edge of the north Cascades near the Columbia River continues to be a source of small earthquakes, including an earthquake of M 5-5.4 on August 5, 1951, near Chelan. If there is something special about the Entiat region that should cause it to be more seismogenic than other areas, it is not known what it is. On June 25, 2001, Spokane was rattled by a very shallow magnitude 3.7 earthquake that was followed by several aftershocks lasting into August. The distribution of the aftershocks suggested that they originated on a fault called the Hangman or Latah Creek fault, although no surface rupture related to these earthquakes was found. Such earthquakes are referred to as anearthquake swarm, in which there is a series of small earthquakes rather than a main shock. Another earthquake swarm was recorded in 1987 in the Columbia Plateau near Othello, Washington, with more than two hundred events over a period of about a year. Like the 1872 earthquake, these could not be assigned to a specific fault. The largest earthquake to strike northeastern Oregon shook the Milton-Freewater area shortly before midnight on July 16, 1936 (Figure 6-20). This earthquake has been given a magnitude as high as 6.1 and maximum intensity of VIII, although a recent study assigned it an intensity magnitude of MI 5.1 to 5.5 and maximum intensities of only VI. It was preceded by two foreshocks and followed by many aftershocks. Damage was reported in Milton-Freewater, Umapine, and Stateline, Oregon, and it was strongly felt in Walla Walla, Washington. Chimneys were damaged, houses were moved off their foundations, and liquefaction and landsliding were reported. A video element has been excluded from this version of the text. You can watch it online here: http://pb.libretexts.org/earry/?p=120 Figure 6-20. Faults in northeastern Oregon in the vicinity of the 1936 Milton-Freewater Earthquake. Shaded areas represent sediment-filled lowlands, including the fault-bounded Grande Ronde Graben and Baker Valley. Solid circles represent earthquakes, with the largest circle the Milton-Freewater Earthquake. This earthquake struck close to the Wallula Fault, but there is no evidence that this fault ruptured the surface during the earthquake. This area is classified as having relatively low seismic hazard by the USGS. Faults and earthquakes from Gary Mann, USGS  A video element has been excluded from this version of the text. You can watch it online here: http://pb.libretexts.org/earry/?p=120 Figure 6-21. (Top) Tectonic map of Yakima Fold Belt, including the Hanford Reservation. Anticlines (arrows facing away from solid lines) are upfolded ridges of Columbia River Basalt that may be underlain by blind faults that could be the sources of earthquakes. (Bottom) Regional map showing location of Yakima Fold Belt and the Olympic-Wallowa Lineament and the distribution of the Columbia River Basalt. Olympic-Wallowa Lineament may have been formed by regional strike-slip faulting that could generate earthquakes. The southeastern part, in Oregon, includes the faults shown in Figure 6-15. As in the previous examples, no source fault was immediately found. But in this case, a possible culprit has been identified: the Olympia-Wallowa Lineament, otherwise known as the OWL (Figure 6-21). This subtle structural alignment can be traced from the Olympic Peninsula across the Cascades and Hanford Reservation to the Wallowa Mountains in northeastern Oregon. Geologists have had difficulty in mapping the OWL on the ground, even though a straight-line feature can be observed from space (Fig. 6-22). However, geology students from Whitman College at Walla Walla found evidence that a branch of this structure may cut deposits only a few thousand years old. The Wallula Fault Zone cutting the Columbia River Basalts near Milton-Freewater could be part of the OWL (Figure 6-21), and one branch, the Umapine Fault, may have evidence of Holocene activity. The southeast end of the OWL is the linear northeast range front of the high Wallowa Mountains, Oregon’s version of the Swiss Alps, although glacial moraines 140,000 years old do not appear to be cut by a range-front fault. Other faults mark the boundaries of basins within the Blue Mountains, including Grande Ronde Valley containing the city of La Grande, and Baker Valley containing Baker City (Figure 6-20). The Baker Valley Fault at the base of the Elkhorn Mountains has evidence of Late Quaternary (although not Holocene) displacement. The West Grande Ronde and East Grande Ronde faults also have evidence of Late Quaternary movement. Both faults are expressed in tectonic topography. Farther southeast, other faults coincide with a zone of high seismicity near the Snake River in both Oregon and Idaho. ## 9. The Pasco Basin: Nuclear Wastes and Earthquakes The military aircraft droned over the bleak December landscape of eastern Washington, and its lone passenger took note of what he saw through the window. As he gazed down at the sagebrush-covered Hanford Reach, with the broad ribbon of the Columbia River curving away in the distance, Lt. Col. Franklin Matthias knew that he had the site he wanted: raw desert, virtually unpopulated, but with a dependable water source, the Columbia River, close at hand (Figure 6-22). The nearest large city, Spokane, was nearly one hundred and twenty miles away. Matthias would report back to his superior, General Leslie Groves, military overseer for the top-secret Manhattan Project, that Hanford was suitable for a large super-secret government operation related to the war effort. The year was 1942. Fig. 6-22. Oblique digital image of Yakima fold belt, view north. Image courtesy of William Bowen, California State University Northridge and California Geographical Survey. Columbia River doubles back on itself at Wallula Gap and flows west toward Portland. Linear features are Yakima folds. Farther west are three Cascade volcanoes and in upper left corner, the waters of Puget Sound. Compare with top map of Figure 6-21, which identifies major Yakima folds. Soon after, in 1943, the few Indians and farmers who had been scratching out a living in the Hanford Reach were hustled out, and the government took over for a crash project to manufacture plutonium for an atomic bomb, the first of which would be dropped two years later on Nagasaki, Japan, bringing an end to World War II. Then came the Cold War, and Hanford continued to expand, still in secrecy, bringing jobs and prosperity to the Pasco Basin and the Tri-Cities of Richland, Pasco, and Kennewick. In addition to manufacturing plutonium, atomic reactors produced energy for the Bonneville power grid, and nuclear wastes began to be stored on the Hanford Reach. In the 1980s, the site was proposed as a national nuclear waste dump, the Basalt Waste Isolation Project. By this time, though, serious reservations had been expressed about nuclear waste disposal in general and the Hanford site in particular. The Hanford N Reactor and the plutonium manufacturing facilities were shut down, and later, the proposed waste disposal site was shifted to Yucca Mountain in Nevada. But still the legacy of nuclear wastes already stored at Hanford hangs over the Tri-Cities, and so it is useful now to look at the geologic setting and consider Hanford’s hazard from earthquakes. Clearly, geology and earthquakes were not considered at all in Col. Matthias’s report to General Groves. Now, however, a nuclear reactor is considered to be a critical facility, meaning that it is necessary to conduct exhaustive site studies to determine its long-term stability to hazards, even those that might be very unlikely, including earthquakes. Are the reactors and the plutonium manufacturing plants able to withstand earthquake shaking? Would highly toxic radioactive waste stored in subterranean tunnels leak out following a major earthquake? To answer these questions, we look for evidence for past earthquakes in the geology around the site, especially in the long ridges of basalt known as the Yakima Folds (Figure 6-23). Figure 6-23. Geologic cross section across Yakima Fold Belt west of Hanford Reservation. Folds in basalt are interpreted as being forced up by compressional faults in rigid crust beneath the basalt; these faults may be earthquake sources. South is to the left. Between Wenatchee and Hanford, the Columbia River turns southeast through a sagebrush-covered black-rock wasteland, away from the ocean, to cut a succession of gorges through basalt ridges on its way to the last canyon, Wallula Gap, where it turns sharply back on itself and heads west to Portland (Figures 6-21, 6-22). These basalt ridges, Frenchman Hills, Saddle Mountain, and Rattlesnake Mountain, are anticlinal folds in the Columbia River Basalt, crumpled like a heavy carpet after a sofa has been pushed over it (Figure 6-23). The Columbia has eroded through these anticlines as they formed. The anticlines are best seen in the canyon of the Yakima River between the towns of Ellensburg and Yakima—not from Interstate 82, which soars high over the gorge, but on lonely State Highway 821, which twists along the banks of the Yakima as the river lazes across broad synclines and churns through anticlinal cliffs of basalt. Project managers working at the Hanford Nuclear Reservation tended to downplay the role earthquakes may have had in forming these anticlinal ridges, perhaps from wishful thinking, perhaps because they did not want to answer questions they had not been asked. One theory was that the anticlines formed millions of years ago, during or soon after the eruption of basalt, and were no longer active or an earthquake risk. In fairness to the geologists and managers at Hanford, anticlines were not considered as harbingers of earthquakes until 1983, when an earthquake of M 6.7 trashed the downtown section of Coalinga, California, a small town on the west side of the San Joaquin Valley. There was no active fault at the surface at Coalinga, but the forces accompanying the earthquake were shown to add to the folding of an anticline at the surface. The implication of active folding is that the fold is underlain by a blind reverse fault orblind thrust, one that does not reach the surface, but tends to force one block over another: faulting at depth, but only bending at the surface (Figure 3-10b). The 1994 Northridge California, Earthquake was caused by rupture on a blind thrust. I once saw a Volkswagen bus that had been in a highway accident. There had been a carpet on the floor, as if its owner had been camping inside the bus. During the wreck, the flooring was buckled and broken, but the carpet was still continuous over the flooring, although it had a large hump in it over the break in the flooring. I thought about that VW bus as I studied the Northridge Earthquake—the bump in the carpet was the anticline, giving a silent clue to the unseen fault beneath. The same analogy could be made for the basalt ridges in the Pasco Basin. Two college teachers, Bob Bentley of Central Washington University in Ellensburg and Newell Campbell of Yakima Valley College, trudged into Yakama Indian territory to examine Toppenish Ridge, a narrow anticline south of the city of Yakima (left center, Figure 6-21, top). They found normal faults on the crest of the anticline and reverse faults on its north flank where the anticline had been thrust northward toward the plowed fields of the Yakima Valley. These structures are not the same age as the Columbia River Basalt; they are much younger, possibly still active. Similar evidence later showed that the east end of the Saddle Mountain Anticline, east of the Columbia and north of Hanford, is also active. As shown in Figure 6-23, the prominent anticlines overlie and provide evidence for blind reverse faults beneath, faults that themselves could produce large earthquakes at the nuclear reservation. The Olympia-Wallowa Lineament (OWL) traverses southeast across the Hanford Reach and across the Yakima folds. Although it is visible on satellite images and on computer-generated digital topographic maps (Figure 6-22), its earthquake significance is unclear. In summary, as Hanford’s nuclear operations change into environmental cleanup mode, and the Tri-Cities await their fate, an earthquake assessment seems long overdue. The Hanford installation is not the only critical facility in the Pasco Basin; there are also the Wanapum, Priest Rapids, and McNary dams on the Columbia River. Failure of one of these dams could cause a repeat of the catastrophic floods of the Pleistocene, although on a greatly reduced scale. Critical facilities will be considered in a later chapter. ## 10. Basin and Range: the Klamath Falls Earthquakes of 1993 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. Figure 6-24. Boulder at Modoc Point, alongside U.S. Highway 97, that breached a roadside barrier and took the life of Ken Campbell during the September 20, 1993, Klamath Falls Earthquake. Boulder has been pushed back behind barrier. Figure 6-25. Downtown Klamath Falls, Oregon, after the earthquakes of September 20, 1993. Automobile parked in front of Swan’s Bakery was crushed by falling bricks from an unreinforced parapet. 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 than7.5 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. 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. Figure 6-26. Earthquakes and aftershocks of the Klamath Falls earthquake sequence, September-December, 1993. Size of circles proportional to magnitude with the largest M 6.0. Open circles show earthquakes from September 20 to the time of an aftershock of M 5.1 on December 4. Solid circles show aftershocks from December 4 to 16. Second sequence is closer to Klamath Falls but is still west of Upper Klamath Lake. Note the absence of earthquakes in the city of Klamath Falls itself. Thin solid lines are faults; note that faults east of lake did not have earthquakes in 1993. From USGS 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.  A video element has been excluded from this version of the text. You can watch it online here: http://pb.libretexts.org/earry/?p=120 Figure 6-27. Structure of the Basin and Range Province of southeastern Oregon. Because the crust is extending east-west, normal faults form. The basin formed between two opposing normal faults is called a graben. Upper Klamath Lake (Figure 6-20) occupies a graben. The basin containing Upper Klamath Lake and Klamath Falls is a graben, downdropped between faults that dip downward toward and beneath the lake. These are called normal faults, and they result 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 downdropped 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). Figure 6-28. Steens Mountain in southeastern Oregon. An active fault with evidence for Holocene displacement is found at the base of Steens Mountain, separating it from the Alvord Desert in the foreground. Steens Mountain has been uplifted along the normal fault at its base accompanied by earthquakes over several million years. Photo by Robert Yeats 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 activein 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). Figure 6-29. Map showing locations of recent earthquake swarms in southern Oregon and northern California, including the Warner Valley, Oregon, earthquakes of 1968. From USGS. 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.) Figure 6-30. Computer-generated topographic map of Bend, Oregon, region showing young faults (linear features marked F). Image is illuminated by a light source from northeast that is 15 degrees above the horizon; accordingly, fault scarps that face northeast are brightly lit, whereas fault scarps facing southwest (such as the lineation marked F? north of Pilot Butte) are in shadow. These faults cut volcanic materials and sediments as young as late Pleistocene, but are not known to cut Holocene deposits. The prominence of these faults may be due to the greater consolidation of the deposits cut by them rather than their Holocene age. The fault scarp in Bend may be seen on Bend city streets. Pilot Butte and Awbrey Butte are volcanoes. Image created by Rose Wallick, Oregon State University 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 cenury 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. Figure 6-31. A Computer-generated topographic map of the Basin and Range Province. The linear pattern is formed by block-fault mountain ranges bound by normal faults and separated by valleys and grabens. From USGS. ## 11. Pacific Coast and Offshore The Northwest coastline is struck on occasion by winter storms of great ferocity, among the most violent in the world. The ocean waves that crash against the rocky headlands and from time to time across Highway 101 are agents of geologic change. They grind down rocky platforms and tide pools and eat into the base of the sea cliffs, occasionally causing beachfront homes and condos built on top of the cliffs to topple into the sea. The boundary between the rocky platform and the sea cliff is called the shoreline angle (Figure 6-32), and it is formed at sea level. Figure 6-32. View south from Cape Foulweather along the central Oregon coast. The Inn at Otter Crest is in foreground, and Otter Rock village and Devil’s Punchbowl are behind. Within the surf are outcrops of basalt that have been planed off by wave erosion to a flat platform. The angle between the eroded platform and the beach cliff is called theshoreline angle and is at sea level. The Inn at Otter Crest and Otter Rock are built on an older marine erosion platform that may be eighty thousand years old (note horizontal layer in sea cliff). It, too, has a shoreline angle marking sea level in late Pleistocene time. Photo by Alan Niem, Oregon State University Highway 101 and many of the resort cities and fishing villages of the coast rest on older, higher sand-covered marine platforms that were eroded during the late Pleistocene. A marine platform 125,000 years old marks a time when sea level was as much as twenty feet higher than it is today. At places like Cape Arago, Oregon, several of these platforms of different ages lie at different elevations, like giant stair steps, the oldest more than two hundred thousand years old. The shoreline angles of these old marine platforms indicate the position of ancient Pleistocene sea levels. Careful surveying by Harvey Kelsey of Humboldt State University in Arcata, California, and his colleagues and students shows that these shoreline angles are not horizontal, like the modern one is, but they rise and fall, and in some places are cut by faults (Figures 6-33). Because the shoreline angles reflect ancient sea levels, meaning that they were once horizontal, their deformation allowed Kelsey to measure tectonic crustal deformation along the Pacific coast. Figure 6-33. (Top) Map of Cape Arago-Coos Bay region, southwest Oregon, showing marine terrace platforms and active faults. (Bottom) Cross section along the coast from Cape Arago to Coos Bay showing tilting and faulting of Whiskey Run marine terrace and platform, eighty thousand years old. One cross section shows the vertical scale exaggerated ten times the horizontal scale, the other shows the cross section without vertical exaggeration. From G. McInelly and H. Kelsey, Humboldt State University The seismicity of the coastal regions north of California is relatively low, and there is no direct evidence that the formerly horizontal shoreline angles were deformed by earthquakes. Deformed marine terraces have been described by Lisa McNeill of Oregon State University (now at Southampton University in England) and Pat McCrory of the USGS. McNeill found that some of the downwarps along the coast, such as South Slough near Coos Bay, Oregon, and the mouth of the Queets River in Washington, correspond to active folds offshore, and these structural lows contain peat deposits that were downdropped suddenly by great earthquakes. Even Willapa Bay, the site of Atwater’s discovery of buried marshes in Niawiakum Estuary, is the location of an active syncline offshore. Deformation along the Olympic coast mapped by McNeill and McCrory may be correlated to the north-south shortening of one-fourth inch per year recorded by GPS in the Puget Sound region. In summary, the low seismicity may mean that deformation of these shoreline angles and downdropping of the structural depressions may be secondary crustal responses to past great earthquakes on the Cascadia Subduction Zone. Alternatively, they may be related to earthquakes in the crust that were not associated with movement on the subduction zone. Offshore, on the continental shelf and slope, active deformation is more intense. The continental shelf itself, very broad off Washington, narrow off southern Oregon and northern California, was eroded to a flat surface during times of Pleistocene glacial advance, when great expanses of ice had taken up water that otherwise would have returned to the sea. During these times of ice advance, sea level was almost four hundred feet lower than it is today, and the continental shelf was dry land. Chris Goldfinger of Oregon State University wondered if the coastline at the time of maximum ice advance twenty-one thousand years ago, when sea level was four hundred feet lower, shows the same evidence of erosion as the modern coast does. To answer this question, he and I and our colleagues surveyed the edges of Nehalem Bank, Heceta Bank, and Coquille Bank on the Oregon continental shelf, using side-scan sonar and Delta, a two-person submersible. What we discovered was truly remarkable: another Oregon coast, drowned beneath the sea at the edge of the shelf, complete with rocky headlands, estuaries, and barrier-island sand bars (Figure 6-34). Delta cruised along this Pleistocene beach, now covered by soft mud, and we observed holes at the base of the cliff rather like the holes made by organisms at the base of modern sea cliffs. The rise of sea level approximately fourteen thousand years ago had been so rapid, more than an inch per year, that these shoreline features were preserved, like the wreck of the Titanic, rather than being destroyed by wave erosion. Figure 6-34. The other Oregon coast. Sidescan sonar imagery outlines a shoreline angle developed at the maximum Pleistocene ice advance twenty-one thousand years ago on the west side of Heceta Bank west of Florence, now covered with four hundred feet of sea water. Visible are rocky cliffs (like the present coastline north of Otter Rock) and the mouth of a Pleistocene river. Some of the rocks at the base of the cliffs have been bored in shallow water by marine organisms. The dark gray region in the lower left half of the picture is the former marine erosion platform, now covered with Holocene mud. The Heceta Bank shoreline angle is warped and deformed, evidence of deformation of the Oregon continental shelf. But unlike the present shoreline angle, which is at sea level and is horizontal, these shoreline angles rise and fall, like the shoreline angles of the raised Pleistocene beaches along the coast. The continental shelf had been warped and tilted, possibly during earthquakes. One of our most memorable discoveries was during our survey of Stonewall Bank southwest of Newport, Oregon, an area known to local commercial fishers as “the rock pile.” Our side-scan sonar imagery showed that Stonewall Bank is a rocky ridge split by a broad former river channel, the seaward extension of the Yaquina River when sea level was lower than it is today (Figure 6-35). Surprisingly, the river channel now slopes about twenty-five feet eastward toward Newport. Since water originally must have run downhill toward the west, we concluded that the river channel was tilted back toward its source during the last twelve thousand years. We had discovered the eastern flank of a broad anticline beneath Stonewall Bank, an anticline formed by a blind reverse fault like the fault that ruptured during the Northridge Earthquake and the faults that may underlie the folded basalt ridges of the Pasco Basin (Figure 6-23). Figure 6-35. Sidescan sonar image of a river channel crossing Stonewall Bank, southwest of Newport, now covered with two hundred feet of sea water. The channel, marking the seaward continuation of the Yaquina River, is now tilted eastward, evidence of deformation of the Oregon continental shelf in the last twelve thousand years. Image courtesy of Chris Goldfinger, College of Oceanic and Atmospheric Sciences, Oregon State University; see also Yeats et al. (1998) The three sources of northern California earthquakes—the subduction zone, Gorda Plate, and the crust—are so interconnected that it is difficult to isolate faults and earthquakes that are limited to North American continental crust. Where the Cascadia Subduction Zone turns to the southeast near the Mendocino Fracture Zone, it is not a single fault but a zone, fifty to sixty miles wide (Figure 5-2), of thrust faults and warped marine terraces in addition to the buried fault that ruptured in the 1992 Cape Mendocino Earthquake. Although many crustal faults in this region may have some Holocene displacement, two zones are the most active: the Mad River Fault Zone between Trinidad and Arcata, which includes the Mad River and McKinleyville faults, and the Little Salmon Fault south of Eureka (Figure 4-22). These structures account for about a third of an inch of shortening per year, which is about 20-25 percent of the convergence rate between the Gorda and North America plates. Backhoe trench excavations by Gary Carver of Humboldt State University across these fault zones (Figure 6-36) provide paleoseismologic evidence that the last two earthquakes on the McKinleyville Fault and Mad River Fault produced displacement of at least eight feet for each event, evidence that these earthquakes were greater than M 7. Trench excavations across the Little Salmon River Fault reveal evidence for three earthquakes in the last seventeen hundred years, each with displacements of eight to ten feet. The last earthquake struck about three hundred years ago. The late Holocene slip rate on the Little Salmon River Fault alone is one-fifth inch (three to seven millimeters) per year. Figure 6-36. Log of side of backhoe trench across a scarp of the Mad River Thrust Fault at McKinleyville, California, showing how bedrock has been thrust over sediments that are radiocarbon dated at about ten thousand years. Laboratory errors are ± 60-80 years. The bedrock is overlain by a wave-cut platform which is itself overlain by terrace deposits (Qt). These were folded, indicating that the Mad River Fault is a blind thrust at this locality. The terrace deposits are overlain by debris from the rising fault scarp (C1 through C6); each unit may have been deposited during an earthquake. Ca marks the active slope wash and debris. Modified from a sketch by Gary Carver, Humboldt State University At Clam Beach, near the McKinleyville Fault, Carver found an uplifted beach cliff and tide-pool platform carved by waves from an ancient sea. The beach sand resting on this platform contains a driftwood log that is one thousand to twelve hundred years old, based on radiocarbon dating. Another beach sand deposit overlies the driftwood log. This sand was colonized by beach grass and a coastal forest. A dead tree in this forest, still rooted in a soil on top of the beach deposit, is no more than three hundred years old. This tree and its soil are overlain by still another beach deposit, perhaps recording subsidence related to movement on the McKinleyville Fault at the time of the A.D. 1700 Cascadia Subduction Zone Earthquake. Between these two fault zones are Arcata Bay and Humboldt Bay, where subsided marshes have been found (Figure 4-12). At first, it was thought that the marsh subsidence was related to rebound from a subduction-zone earthquake, like marshes in Oregon and Washington and in marshes downdropped during the 1964 Alaska Earthquake (Figure 4-15). But this area is so close to the subduction zone that the coast would have been uplifted during an earthquake, just as islands close to the Alaska subduction zone were uplifted in 1964 (Figure 4-15). In addition, the coastline was uplifted in the 1992 Cape Mendocino Earthquake on the subduction zone. The bay was downwarped due to crustal deformation, especially slip on the Little Salmon Fault. Because the age of the drowned marsh is three hundred years, like the age of the youngest subsided marshes in Oregon and Washington, the crustal deformation probably occurred at the same time as the most recent subduction-zone earthquake. Uplifted marine terraces cut by storm waves provide additional evidence of crustal deformation. If there were no crustal deformation, the older, uplifted marine terraces would be completely level, like the present marine platform is. But the older terraces are tilted and warped, as is evident by viewing the coast north from Patricks Point State Park. This provides evidence that the Earth’s crust in this region is on the move, up and down, through folding and faulting, producing earthquakes in the process. An earthquake of M 6.4 on June 6, 1932, near Arcata produced intensity as high as VIII, resulting in one death and considerable damage in Eureka. On December 21, 1954, an earthquake of M 6.5-6.6 struck twelve miles east of Arcata in the vicinity of the McKinleyville Fault Zone, causing one death and$3.1 million in damage. And on August 17, 1991, a M 6.2 earthquake struck at seven miles depth beneath the community of Honeydew on the Mattole River. The official estimate of damage in this relatively unpopulated region was fifty thousand dollars, but this estimate is probably low. Intensities of VII and VIII were encountered, as they were in the two earlier crustal earthquakes.

Fig. 6-37. Active fault scarp on continental shelf west of Newport, Oregon, photographed from submersible DELTA. The scarp is too deep to be affected by active wave action. Two light dots are 20 cm apart. Photo by Gary Huftile, then of OSU.

It is clear that for their size, the crustal earthquakes were more damaging than Gorda Plate earthquakes. They struck at shallow depth close to population centers, whereas most of the Gorda Plate earthquakes were offshore, some so far offshore that onshore damage was minimal.

Curiously, under a new California insurance plan discussed in Chapter 10, the Eureka region will be charged earthquake insurance rates that are among California’s lowest, despite accounting for a quarter of the state’s seismicity!

This chapter closes with an image from the submersible DELTA of a fault scarp at a depth greater than 750 feet, too deep for active wave action (Figure 6-37). This fault could have been formed during a crustal earthquake, or it could have been the result of a secondary fault related to a subduction-zone earthquake. The answer to this question is not yet at hand.

## 12. Summary

In estimating the seismic hazard from crustal earthquakes, we study three lines of evidence: geology, seismology, and geodetic evidence using GPS. In the Puget Sound region, we have all three: Holocene active faults and folds, high instrumental seismicity, and GPS evidence of shortening. In northern California, we also have geological and seismological evidence of earthquake hazard, including damaging historical earthquakes that have caused fatalities. The two Oregon earthquakes come close: the Scotts Mills earthquake probably took place on the Mt. Angel fault, and the Klamath Falls earthquakes were the result of motion on normal faults bounding the Klamath Falls graben.

In other places, the evidence is less complete. The largest crustal earthquakes in the Pacific Northwest on Vancouver Island and near Entiat in northern Washington took place in areas with little or no geological evidence of young faulting. The active Portland Hills Fault is in an area of moderate seismicity, but many of the earthquakes around Portland cannot be correlated to that fault. The Milton-Freewater Earthquake was not assigned to a specific fault, but it may be part of an active fault system following the Olympic-Wallowa Lineament (OWL).

Some areas have geological evidence for young faulting, but have not experienced large earthquakes. These areas include the Oregon Basin and Range east and north of Klamath Falls and the folded basalt ridges of the Pasco Basin in Washington. The faults around La Grande and Baker City, Oregon, show geological evidence of activity, but they have not been the source of large earthquakes. The southeastern end of the OWL has moderate seismicity, but as yet, this area has not been damaged by a large earthquake.

What about the rest of the Northwest? The Oregon Coast Range and the Klamath-Siskiyou regions of Oregon have no clear evidence of active faulting and also have very few earthquakes. Similarly, the Coast Mountains of British Columbia, the Columbia Plateau of Washington, and much of the Blue Mountains of Oregon have low seismicity and little evidence of active faulting. At present, these areas are placed in a lower-risk category, but the next earthquake could prove this assessment wrong.

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