Geology topics

Richard W. Lemke

Publications and source records attributed to Richard W. Lemke.

12 recordsLinked to original sources

USGS1975

Reconnaissance engineering geology of the Ketchikan area, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

The Alaska earthquake of March 27, 1964, dramatically emphasized the need for engineering geologic studies of urban areas in seismically active regions. A reconnaissance study of the Ketchikan area in southeastern Alaska is part of a program to evaluate earthquake and other geologic hazards in most of the larger Alaska coastal communities. These evaluations in the Ketchikan area should provide broad guidelines useful in city and land-use planning. Ketchikan, which had a population of approximately 7,000 in 1970, is built on the southwestern end of Revillagigedo Island along the northeastern coastline of Tongass Narrows. Altitudes reach 1,000 feet (305 m) within half a mile (0.8 km) of the coast and near-vertical cliffs characterize the terrain in places. The climate is predominantly marine. Average precipitation is approximately 152 inches (386 cm). The Ketchikan area was covered by glacier ice at least once and probably several times during the Pleistocene Epoch. The present topography, characterized by elongate lakes, U-shaped valleys, fiords, inlets, and passages, clearly reflects the effects of glaciation. The presence of emergent marine deposits, at least 300 feet (91 m) above sea level, shows that the land has been uplifted relative to sea level since the last deglaciation of the region. Bedrock is exposed or is near the surface throughout most of the mapped area. The bedrock consists chiefly of metamorphic rocks. In a few places these rocks have been intruded by igneous rocks. Exposed metamorphic rocks are mostly thinly foliated schists and phyllites, metamorphosed to greenschist facies. Foliation generally strikes northwest with moderate to steep dips to the northeast. Most of the rock is fairly competent and near-vertical cuts tend to be stable. The more indurated metamorphic rock can be used for riprap, but more durable blocks generally can be obtained from the igneous rock. The surficial deposits have been divided into the following map units on the basis of their time of deposition, mode of origin, and grain size: (1) undifferentiated drift (Qd), (2) elevated marine deposits (Qm), (3) stream alluvium (Qa), (4) fan-delta deposits (Qf), and (5) modern beach deposits (Qb). Manmade fill (f) also is mapped as a separate unit. Muskeg, colluvium, and offshore deposits are not included as map units but are discussed in the report under the heading "Surficial deposits (not shown on map)." The undifferentiated drift deposits consist mostly of till or other diamictons, generally less than 25 feet (7.6 m) thick. Exposed elevated marine deposits (Qm) generally consist of sand and gravel less than 5 feet (1.5 m) thick. Stream alluvium (Qa) is chiefly sand, gravel, cobbles, and boulders probably everywhere less than 15 feet (4.6 m) thick. Fan-delta deposits (Qf) consist mostly of loose sand, gravel, and boulders as much as 50 feet (15 m) thick. Modern beach deposits (Qb) are mostly loose sand and gravel generally less than 10 feet (3 m) thick. Two basically different types of manmade fill are present: (1) large fills along the waterfront, commonly 5 to 15 feet (1.5-4.6 m) thick, consisting of silt, sand, gravel, rock, and diverse other materials, and (2) fills, generally less than 10 feet (3 m) thick and consisting of sand, gravel, or crushed rock, placed inland from the waterfront and used as pads for buildings and parking areas. Fairly thick deposits of muskeg may be present in the southeastern part of the mapped area but have not been examined in the field. Colluvial deposits, locally 5 to 8 feet (1.5-2.4 m) thick, consist mostly of decomposing bedrock fragments. Offshore deposits are poorly known; near-shore loose sand and gravel rest on a sloping bedrock surface. Southeastern Alaska lies within the circum-Pacific seismic belt that rims the northern Pacific Basin and has been tectonically active since at least early Paleozoic time. Large-scale faulting has been common. The two most prominent fault systems in southeastern Alaska and surrounding regions are (1) the Denali fault system, and (2) the Fairweather-Queen Charlotte Islands fault system. Of the two, the Fairweather-Queen Charlotte Islands fault system is the more active and of most significance in relation to the Ketchikan area. Ketchikan lies within the northwest trend of the Gravina-Nutzotin belt of fault thrusting. The trends of at least some of the linear fiords near the mapped area are controlled by faults. However, it is not known whether a major fault extends up Tongass Narrows offshore from Ketchikan. Between 1899 and 1970, five earthquakes having magnitudes of 8 or greater occurred in or near southeastern Alaska or in adjacent offshore areas; three have occurred having magnitudes of between 7 and 8, at least eight with magnitudes of between 6 and 7, 15 with magnitudes of between 5 and 6, and about 140 have been recorded with magnitudes of less than 5 or of unassigned magnitudes. All of the earthquakes with magnitudes greater than 8, and a large proportion of the others, appear to be related to the Fair-weather-Queen Charlotte Islands fault system or to the connecting Chugach-St. Elias fault to the northwest. Within a 50-mile (80-km) radius of Ketchikan, epicenters of three earthquakes with magnitudes of 5 or less have been recorded. Within a radius of 100 miles (160 km), 10 epicenters have been recorded, two with magnitudes between 6 and 7 and eight with magnitudes of 5 or less. Although no instrumentally recorded earthquakes had epicenters in the mapped area, at least 32 earthquakes that had epicenters elsewhere were felt or possibly felt in Ketchikan. Most of these earthquakes probably had epicenters along the Queen Charlotte Islands fault. Ketchikan is tentatively assigned by me to seismic zone 2. This is a zone in which magnitudes of the largest expectable earthquake would range from 4.5 to 6.0 and where moderate damage could be expected. Large earthquakes of magnitude 8 or greater, however, can be expected to occur from time to time along the Queen Charlotte Islands fault. Ground motion from these earthquakes, although attenuated with distance, may still be sufficiently strong at Ketchikan to cause substantial damage. Possible future earthquake effects include: (1) land-level changes caused by local faulting or by large-scale regional deformation, (2) ground shaking, (3) compaction, (4) liquefaction, (5) subaerial and submarine sliding, (6) water-sediment ejection and ground fracturing, (7) reaction of sensitive and quick clays, and (8) effects of tsunamis, seiches, and other abnormal water waves. Although land-level changes due to local faulting are unlikely, large-scale regional deformation may cause uplift or subsidence in Ketchikan. Adverse effects would be confined mainly to the waterfront area. This area also would be most heavily damaged if Ketchikan were strongly shaken by an earthquake. Nonengineered, loose, manmade fills and fan-delta deposits in this area probably would be subject to the strongest shaking. These deposits probably also are most subject to compaction, liquefaction, sliding, and water-sediment ejection. Earthquake effects expectably would be considerably fewer and less severe for the part of Ketchikan upslope from the harbor area because bedrock is at or near the surface in large parts of the area. No sensitive clays have been identified but, if present, they probably are confined to the till and other diamicton deposits in the northeastern part of the mapped area. Tsunami waves are not expected to have a local generation source. Those arriving from a distant source, although potentially highly destructive, probably would be greatly attenuated before arriving at Ketchikan. Seiche waves may develop on lakes near the mapped area and possibly cause failure of earth-fill dams. Destructive waves generated by earthquake-induced local submarine sliding appear to be unlikely in the Ketchikan area. Geologic hazards in the area that are not caused by earthquakes are believed to be relatively minor. They include: (1) landsliding and subaqueous sliding, and (2) flooding. Only minor landsliding has occurred in the mapped area, but the potential for sliding may increase as the city expands and heavily timbered areas are cleared, with attendant accelerated erosion and mass wasting. The greatest potential for subaqueous sliding is along the shoreline, where fairly thick fan-delta deposits rest on a sloping bedrock surface. Periodic flooding has occurred on some creeks in the mapped area and can be expected to occur from time to time in the future. In order that more accurate evaluations of geologic hazards can be made in the future, several recommendations are made for additional studies.

Alaska↗

USGS1969

The Alaska earthquake, March 27, 1964: Effects on communities

This is the second in a series of six reports that the U.S. Geological Survey published on the results of a comprehensive geologic study that began, as a reconnaissance survey, within 24 hours after the March 27, 1964, Magnitude 9.2 Great Alaska Earthquake and extended, as detailed investigations, through several field seasons. The 1964 Great Alaska earthquake was the largest earthquake in the U.S. since 1700. Professional Paper 542, in 7 parts, describes the effects of the earthquake on Alaskan communities.

Alaska↗

USGS1967

Effects of the earthquake of March 27, 1964, at Seward, Alaska

Seward, in south-central Alaska, was one of the towns most devastated by the Alaska earthquake of March 27, 1964. The greater part of Seward is built on an alluvial fan-delta near the head of Resurrection Bay on the southeast coast of the Kenai Peninsula. It is one of the few ports in south-central Alaska that is ice free all year, and the town’s economy is almost entirely dependent upon its port facilities. The Alaska earthquake of March 27, 1964, magnitude approximately 8.3–8.4, began at 6:36 p.m. Its epicenter was in the northern part of the Prince William Sound area; focal depth was 20–50 km. Strong ground motion at Seward lasted 3–4 minutes. During the shaking, a strip of land 50–400 feet wide along the Seward waterfront, together with docks and other harbor facilities, slid into Resurrection Bay as a result of large-scale submarine landsliding. Fractures ruptured the ground for'severa1 hundred feet back from the landslide scarps. Additional ground was fractured in the Forest Acres subdivision and on the alluvial floor of the Resurrection River valley; fountaining and sand boils accompanied the ground fracturing. Slide-generated wares, possibly seiche waves, and seismic sea waves crashed onto shore; ware runup was as much as 30 feet above mean lower low water and caused tremendous damage; fire from burning oil tanks added to the destruction. Damage from strong ground motion itself was comparatively minor. Tectonic subsidence of about 3.6 feet resulted in low areas being inundated at high tide. Thirteen people were killed and five were injured as a result of the earthquake. Eighty-six houses were totally destroyed and 260 were heavily damaged. The harbor facilities were almost completely destroyed, and the entire economic base of the town was wiped out. The total cost to replace the destroyed public and private facilities was estimated at $22 million. Seward lies on the axis of the Chugach Mountains geosyncline. The main structural trend in the mapped area, where the rocks consist almost entirely of graywacke and phyllite, is from near north to N. 20° E. Beds and cleavage of the rocks commonly dip 70° W. or NW. to near vertical. Locally, the rocks are complexly folded or contorted. So major faults were found in the mapped area, but small faults, shear zones, and joints are common. Surficial deposits of the area hare been divided for mapping into the following units: drift deposits, alluvial fan deposits, valley alluvium, intertidal deposits, landslide deposits, and artificial fill. Most of these units intergrade and were deposited more or less contemporaneously. The drift deposits consist chiefly of till that forms moraines along the lower flanks of the Resurrection River valley and up tributary valleys. The till is predominantly silt and sand and lesser amounts of clay-size particles, gravel, cobbles, and boulders. Glacial outwash and stratified ice-contact deposits constitute the remainder of the drift deposits. Fans and fan-deltas have been deposited at the valley mouths of tributary streams. Some, including the one upon which Seward built, project into Resurrection Bay, and deltaic-type deposits form their distal edges. The larger fans—composed chiefly of loosely compacted and poorly sorted silt, sand, and gravel—form broad aprons having low gradients. The fan deposits range in thickness from about 100 feet to possibly several hundred feet and, at least in some places, lie on a platform of compact drift. Smaller fans at the mouths of several canyons have steep gradients and considerable local relief. Valley alluvium, deposited chiefly by the Resurrection River, consists mostly of coarse sand and fine to medium gravel. In the axial part of the valley it is probably more than 100 feet thick. Near the head of Resurrection Bay, the alluvium is underlain by at least 75 feet of marine deltaic sediments, which are in turn underlain by 600 or more feet of drift in the deepest part of the bedrock valley. Beach, deltaic, and estuarine sediments, deposited on intertidal flats at the head of the bay and along far1 margins that extend into the bay, arc mapped as intertidal deposits. They consist mostly of silt, sand, and fine gravel, and lesser amounts of clay-size particles. The earthquake reactivated old slides and trigged new ones in the mountains. Rock and snow avalanches, debris flows, and creep of talus deposits characterized slide activity on the steeper slops. The Seward waterfront had been extended before the earthquake by adding artificial fill consisting of loose sand and gravel; part of the lagoon area had been filled with refuse. After the earth- quake, fill, consisting of silt and sand dredged from the head of the bay, was pumped onto part of the lagoon area and also on land at the northwest corner of the bay. Response to the disaster was immediate and decisive. City, State, and Federal agencies, as well as other organizations and individuals, gave unstintingly of their time and facilities. Within a few days, there was temporary restoration of water, sewerage, and electrical facilities. The U.S. Army Corps of Engineers was authorized to select sites and construct a new dock for the Alaska Railroad, a new small-boat basin, and related facilities. The firm of Shannon and Wilson, Inc., under contract to the Corps of Engineers, investigated subsurface soils extensively to determine the factors responsible for the sliding along the Seward waterfront and to assist in site selection for reconstruction of the destroyed harbor facilities. Borings also made along the Seward waterfront and at the head of the bay, and laboratory tests were conducted on pertinent samples. These studies were augmented by geophysical studies both on land and in the bay. In addition, the Corps of Engineers made shallow borings on the intertidal flats at the head of the bay and performed pile-driving and load tests. Borings also were drilled and test pits were dug in the subdivision of Forest Acres. Sliding along the Seward waterfront markedly deepened the water along the former shoreline. Post-earthquake slopes of the bay floor immediately offshore also are steeper in places than before the earthquake. The strong ground motion of the earthquake triggered the landsliding, but several factors may have contributed to the magnitude and characteristics of the slides. These factors are: (1) the long duration of strong ground motion, (2) the grain size and texture of the material involved in the sliding, (3) the probability that the finer grained materials liquefied and flowed seaward, and (4) the added load of manmade facilities built on the edge of the shore, Secondary effects of the slides themselves—sudden drawdown of water, followed by the weight of returning waves—also may have contributed to the destruction. Submarine sliding at the northwest corner of the bay occurred in fine-grained deltaic deposits whose frontal slopes probably were in metastable equilibrium under static conditions. Uplift pressures from aquifers under hydrostatic head, combined with the probable liquefaction characteristics of the sediments when vibrated by strong ground motion, probably caused the material to slide and flow seaward as a heavy slurry. Under static conditions, no major shoreline or submarine landsliding is expected in the Seward area; in the event of another severe earthquake, however, additional sliding is likely along the Seward waterfront and also in the deltaic deposits at the northwest corner of the bay. Fractured ground in back of the present shoreline along the Seward waterfront is an area of incipient landslides that would be unstable under strong shaking. For this reason the Scientific and Engineering Task Force placed the area in a high-risk classification and recommended no repair, rehabilitation, or new construction in this area involving use of Federal funds; it was further recommended that the area should be reserved for park or other uses that do not involve large congregations of people. The deltaic deposits at the head of the bay probably also would be susceptible to sliding during another large earthquake. This sliding would result in further landward retreat of the present shoreline toward the new railroad dock. Specifications for the new dock, whose seaward end is now approximately 1,100 feet from the back scarp of the subaqueous landslide, require design pro- visions to withstand seismic shock up to certain limits. Earthquake-induced fracturing of the ground in the subdivision of Forest Acres was confined to the lower part of a broad alluvial fan. There, sewer and water lines were ruptured and the foundations of some homes were heavily damaged. Landsliding, such as occurred along the shoreline of the bay, was not a contributing cause of the fracturing. Two hypotheses are offered to explain the fracturing: 1. Seismic energy was transformed into visible surface waves of such amplitude that the strength of surface layer was exceeded and rupturing occurred; tensional and compressional stresses alternately opened and closed the fractures and forced out water and mud. 2. Compaction by vibration of the fine-grained deposits of the fan caused ground settlement and fracturing; ground water under temporary hydrostatic head was forced to the surface as fountains and carried the finer material with it. Water waves that crashed onto shore, while shaking was still continuing, were generated chiefly by onshore and offshore landsliding. Waves that overran the shores about 25 minutes after shaking stopped and that continued to arrive for the next several hours are believed to be seismic sea waves (tsunamis) that originated in an uplifted area in the Gulf of Alaska. During the time of seismic sea-wave activity and perhaps preceding it, seiche wares also may have been generated within Resurrection Bay and complicated the wave effects along the shoreline.

Alaska↗

USGS1952