USGS1972
A program to study the engineering geology of most of the larger Alaska coastal communities and to evaluate their earthquake and other geologic hazards was started promptly after the 1964 Alaska earthquake; this report is a product of that program. Field-study methods were largely reconnaissance, and thus the interpretations in the report are subject to revision as further information becomes available. The report provides broad guidelines for planners and engineers when considering geologic factors during preparation of land-use plans. The use of this information should lead to minimizing future loss of life and property, especially during major earthquakes. Skagway was established in 1897 as a seaport near the head of Taiya Inlet fiord in the northern part of southeastern Alaska. Rugged mountains, steep-walled valleys, fiords, and numerous glaciers and icefields characterize the landscape of the area. Valley floors are narrow and most carry large streams, which end in tidewater deltas. Skagway is situated on the delta and lower valley floor of the Skagway River. Glaciers became vastly enlarged during the Pleistocene Epoch and presumably covered the area at least several times. The last major deglaciation probably occurred about 10,000 years ago. Subsequently, there was minor expansion and then partial retreat of glaciers; land rebound because of glacial melting is still going on today. Bedrock is composed predominantly of plutonic intrusive rocks, chiefly quartz diorite and granodiorite, some metamorphic rocks and a few dikes are present. Most bedrock is of Jurassic and Cretaceous age. An assortment of surficial deposits of Quaternary age form the valley bottoms and locally part of the valley walls. Thick deposits of sand and gravel have accumulated as deltas at the heads of fiords and as alluvium in the main stream valleys; deposits may be as much as S8S feet thick at Skagway. Locally, thin deposits mantle some of the steep bedrock slopes and also form some moderately to gently sloping ground. Manmade fill covers much of the top of the delta and floor of the Skagway valley. The fill is composed chiefly of gravel and sand. Quarried blocks of granodiorite are used as riprap to face river dikes and on fill areas exposed to waves of Taiya Inlet. The geologic structure of the area is imperfectly known. However, it appears that plutonic rocks intruded metamorphic rocks in Jurassic and Cretaceous time. Extensive faulting is strongly indicated by the strikingly linear or curvilinear pattern of fiords and many large and small valleys, but no major faults have been positively identified because of concealment by water or surficial deposits. Inferred faults include those coincident with the lower Skagway valley, Taiya Inlet-Taiya valley, and the Katzehin River delta-Upper Dewey Lake. Principal fault movements probably occurred in middle Tertiary time but some movement might have been in late Tertiary or possibly early Quaternary time. Local faults appear to join the Chilkat River fault, a segment of the important Denali fault system, one of the major tectonic elements of southeastern Alaska. One fault segment of this system shows evidence of movement within the last several hundred years. Southeastern Alaska's other major fault system is the active Fairweather-Queen Charlotte Islands fault system'near the coast of the Pacific Ocean. This fault system passes to within about 100 miles of Skagway. At its northwest end the fault system merges with the Chugach-St. Elias fault. One hundred twenty-two earthquakes, some of them strong, have been felt or possibly felt at Skagway during the years 1898 through 1969. The closest large earthquake (magnitude about 8) causing some damage at Skagway occurred July 10, 1958. Its epicenter was about 100 miles to the southwest. Other earthquakes, as much as 150 miles away, also have caused slight to moderate damage. The closest instrumentally recorded earthquake (magnitude 6) had its epicenter about 30 miles to the west of Skagway. Most earthquakes in southeastern Alaska have occurred southwest, west, or northwest of Skagway, near the coast of the Pacific Ocean. They appear to be related to movement along the Fairweather-Queen Charlotte Islands fault system or the Chugach-St. Elias fault. Most have had their epicenters offshore. Some earthquakes may be related to movement at depth along the Denali fault system. The probability of destructive earthquakes at Skagway is unknown because the tectonics of the region have not been studied in detail. However, on the basis of the seismic record and limited tectonic evidence, we suggest that sometime in the future an earthquake of at least magnitude 6 probably will occur very close to the city, a magnitude 7 earthquake might occur in the general area, and an earthquake of magnitude 8 probably will occur at some distance to the southwest, west, or northwest. Effects from nearby large earthquakes could cause extensive damage at Skagway. Nine principal effects are considered. 1. Surface displacement. Displacement of ground caused by fault movement would affect only structures built athwart the fault. However, a sudden tectonic uplift of land of as much as a few feet might affect a wide area and necessitate extensive dredging and wharf rebuilding. On the other hand, a subsidence of several feet would allow tidewater to reach inland and flood part of the harbor facilities and the business district. 2. Ground shaking. Because intensity of ground shaking during earthquakes largely depends on type and water content of the geologic material being shaken, the geologic materials are separated into three categories. Those considered susceptible to strongest shaking are grouped into category 1 (containing materials that are saturated, loose, and of medium- to fine-grain sizes); those of intermediate susceptibility in category 2; and those least susceptible to shaking in category 3. 3. Compaction of some medium-grained sediments during strong earthquake shaking could cause local settling of alluvial and deltaic surfaces. Also, some manmade fills near the harbor might undergo marked differential settling. 4. Liquefaction of saturated beds of uniform, fine sand commonly occurs during strong earthquakes. Few such beds, however, are positively identified at Skagway; some may occur within deltaic and alluvial deposits. If present, these beds might liquefy and cause local settling or trigger landslides. 5. Ejection of water-sediment mixtures from earthquake-induced fractures or from point sources, plus some associated ground subsidence, is common during major earthquakes where saturated sand and fine gravel deposits are confined beneath generally impermeable beds. Some alluvial and deltaic deposits at Skagway probably are susceptible to these processes. Locally, ejecta might cover roads and areas between buildings and fill low-lying areas. Associated ground fracturing might damage roadways, foundations of buildings, and other facilities. 6. Subaerial and subaqueous slides occur frequently during earthquakes. Saturated loose sediments on steep slopes are especially susceptible to sliding. During a major earthquake, surficial deposits forming such slopes along the southeast side of the Skagway valley probably would be subject to sliding or earthflowing on an extensive scale. Some sliding might extend onto the valley floor and damage or destroy buildings and part of the railroad. Rockfalls would be numerous and locally very large rockslides might occur. Subaqueous sliding of the Skagway delta is potentially the most damaging of earthquake effects. Sliding may have occurred there during the earthquake of September 16, 1899; any future major earthquake close to the city would cause extensive sliding, possibly triggered in part by liquefaction. If shaking continued for several minutes, successive slides might progressively remove large portions of the delta and allow extensive land spreading and fracturing of Skagway River alluvium as much as several thousand feet landward from the shoreline. 7. Glacier surfaces commonly receive extensive snow avalanches and rockslides during seismic shaking. In the Skagway area, glaciers may be disrupted at their margins, and resulting blocked streams might form lakes in a few places. If these lakes drained suddenly, downstream areas would he flooded. No long-term effects, such as glacier expansion, are expected. 8. Ground- and surface-water levels often are affected during and after strong earthquake shaking. At Skagway, ground-water levels probably would be lowered, but there would be no permanent change in water quality. Earthquake-triggered landslides could dam the Skagway River; the sudden failure of the dams might cause severe flooding. 9. Waves generated by earthquakes include tsunamis, seiche waves, and waves caused by subaerial and submarine sliding and tectonic displacement of land. Damage in the Skagway area would depend on wave height, tidal stage, and warning time. Some waves triggered by subaerial and subaqueous slides have a strong possibility of reaching heights of as much as 60 feet--or possibly even higher. Tsunamis from the open ocean must travel 160 miles of fiords before reaching Skagway, which allows sufficient time for appraisal of expectable wave height and, if necessary, evacuation of the harbor area and other low-lying ground. Geologic hazards other than those hazards associated with earthquakes include nonearthquake-induced subaerial and subaqueous slides, floods, and slow uplift (rebound) of land. Landslides of moderate size are known to have occurred from time to time during heavy rains such as those of September 1967. Subaqueous slides happen intermittently during the normal growth of deltas. Submarine cables on the floor of northern Taiya Inlet presumably were broken by such slides on September 10, 1927. Flooding by the Skagway River has inundated parts of the city many times, usually during heavy rains in the fall. Two floods were reported to have been caused by the sudden draining of glacier-dammed lakes. Dikes protect the city from many smaller floods, but heightening and broadening is needed to give full protection. Slow land uplift at Skagway, because of regional glacioisostatic rebound, averages 0.059 foot per year. On this basis, the shoreline theoretically shifted seaward 500 feet and the harbor shoaled 4.4 feet between 1897 and 1972. It is recommended that future geologic study of the Skagway area include: detailed geologic mapping and collection of data on geologic materials, joints, faults, and slope stability; complete evaluation of earthquake probability and response of materials to shaking; and collection and evaluation of periodic soundings and sediment data from Skagway and Taiya deltas to assist in forecasting the stability of the delta front.