Reconnaissance Engineering Geology of the Wrangell Area, Alaska, with Emphasis on Evaluation of Earthquake and Other Geologic Hazards
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UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Reconnaissance Engineering Geology of the Wrangell area, Alaska, with Emphasis on Evaluation of Earthquake and Other Geologic Hazards By Richard W. Lemke Open-file report 1974 This report is preliminary and has not been edited or reviewed for conformity with U.S. Geological Survey standards or nomenclature CONTENTS Page SUMMARY --------------- ------------_----_ _______ i Purpose of study---------------------------------------- 1 Geography------------------------------------------------ 1 Glaciation and associated land-level changes ---------- \ Bedrock-------------------------------------------------- 2 Surficial deposits--------------------------------------- 2 Structure------------------------------------------------ 2 Seismicity of southeastern Alaska------------------------ 2 Seismicity of the Wrangell area-------------------------- 3 Earthquake probability of the Wrangell area-------------- 3 Inferred effects from future earthquakes- ------------- 3 Inferred future effects from geologic hazards other than those caused by earthquakes---------------------------- 5 Recommendations for additional studies- ---------------- 5 INTRODUCTION------- ----------------------------- --- 6 Purpose and scope of study------------------------------- 6 Methods of study and acknowledgments ------------------ 5 GEOGRAPHY--------------------------------------------- 8 Location and extent of area------------------------------ 8 Topographic setting-------------------------------------- 8 Bathymetry and tides------------------------------------- 11 Climate and vegetation---------------------------------- 11 Economy and population----------------------------------- 12 Transportation and other facilities---------------------- 12 GLACIATION AND ASSOCIATED LAND- AND SEA-LEVEL CHANGES------- 15 DESCRIPTIVE GEOLOGY----------------------------------- ---- 16 Bedrock-------------------------------------------------- 16 Surficial deposits (shown on map)- - -- 18 Elevated fine-grained marine deposits (Qem)--------- 19 Elevated beach deposits (Qeb)----------------------- 23 Muskeg (Qn)- ------------------------------- ---- 27 Late shore deposits (Qs)- ------------------------- 30 Delta deposits of the intertidal zone (Qd)---------- 31 Manmade fill (mf)- -------------------------------- 34 1 Surficial deposits (not shown on map)-------------------- 36 Diamicton------------------------------------------- 35 Colluvial deposits---------------------------------- 37 Offshore deposits----------------------------------- 37 STRUCTURE---------------------------------------------------- 41 Summary of regional structure---------------------------- 41 Denali fault system and associated faults----------- 42 Fairweather-Queen Charlotte Islands fault system---- 45 Clarence Strait lineament ------------------------ 45 Coast Range lineament------------------------------- 46 Local structure------------------------------------------ 46 Page EARTHQUAKE PROBABILITY--------------------------------- 47 Seismicity----------------------------------------------- 47 Relation of earthquakes to known or inferred faults and recency of fault movement--------- 4----------------- 55 Assessment of earthquake probability in the Wrangell area--------------------------------------------------- 58 INFERRED EFFECTS FRCM FUTURE EARTHQUAKES------ - -- 67 Surface displacement on faults and other tectonic land-level changes--------------------*---------------- 68 Introduction ---------------------4---------------- 68 Inferred effects in the Wrangell ar0a- ----------- 55 Ground shaking--------------------------*---------------- 69 Introduction---------------------------------------- 69 Inferred effects in the Wrangell ar^a ------------ 73 Geologic units in category 1 (strongest expectable shaking)-------------------------- 73 Manmade fill, mf (part ofiunit)----------- 73 Delta deposits of the inte^rtidal zone, Qd- 74 Late shore deposits, QS- ---------------- 74 Muskeg deposits, Qn --------------------- 74 Geologic units in category 2 (intermediate expectable shaking)----------^- ------------- 75 Manmade fill, mf (part of unit)----------- 75 Elevated beach deposits, C(eb- ----------- 75 Elevated fine-grained marine deposits, Qem- ------------------------ 75 Geologic units in category 3 (least expectable shaking)------------------------------------- 75 Bedrock, bex and be- -------------------- 75 Compaction- -------------------------------------------- 76 Introduction---------------------------------------- 76 Compaction in the Wrangell area--------------------- 76 Liquefaction in cohesionless materials------------------- 77 Introduction---------------------------------------- 77 Inferred effects in the Wrangell area ------------ 78 Earthquake-induced slides and slumps- ------------------ 79 Introduction ------------------------------------- 79 Inferred effects in the Wrangell are^- ------------ 79 Water-sediment ejection and associated subsidence and ground fracturing---------------------------------- 81 Introduction- - ------------------ - - -- - - ------- 81 Inferred effects in the Wrangell area--------------- 81 Reaction of sensitive and quick clays-------------------- 82 Introduction---------------------------------------- 82 Inferred effects in the Wrangell area- ----------- g2 Effects of tsunamis, seiches, and other abnormal water waves-------------------------------------------------- 82 Introduction- ------------------------------------- 82 Inferred effects in the Wrangell area--------------- 83 11 Page INFERRED EFFECTS FRCM FUTURE EARTHQUAKES Continued Ground water and streamflow------------------------------ 84 Introduction--------------------------------------- 84 Inferred effects in the Wrangell area--------------- 85 Glaciers and related features---------------------------- 85 Introduction---------------------------------------- 85 Inferred effects in the Wrangell area- ----------- 85 INFERRED FUTURE EFFECTS FRCM GEOLOGIC HAZARDS OTHER THAN THOSE CAUSED BY EARTHQUAKES--------------------- 86 Landsliding and subaqueous sliding- ------------------- 86 Flooding- -------- 87 Sediment accumulation---- - -------- -------------------- 87 RECOMMENDATIONS FOR ADDITIONAL STUDIES-- 87 GLOSSARY---------- ----- - -- - --- 90 REFERENCES CITED - ----- --- ---- 94 111 ILLUSTRATIONS Page Figure 1. Map of southeastern Alaska and adjacent Canada showing pertinent geographic features ----- - 9 2. Location map of Wrangell and surrounding region, Alaska-- ------------------ ----- ----------------- IQ 3. Reconnaissance geologic map of th£ Wrangell area, Alaska-------------------------------- In pocket 4. Map of Alaska showing major elements of the Denali and Fa irweather -Queen Charlotte! Islands fault systems----------------------------------- ---- 43 5. Map of southeastern Alaska and adjacent Canada showing major faults and selected other linea ments interpreted to be probable or possible faults, shear zones, or joints------------------- 44 6. Map of the Wrangell area and vicinity showing faults or other lineaments----------------------- 48 7. Map showing locations of epicenters and approxi mate magnitudes of earthquakes in southeastern Alaska and adjacent areas, 1899^1972 and July 1, 1973 * 50 8. Seismic probability map for most of Alaska as modified from U.S. Army Corps of Engineers, Alaska District---------------------- --------- 59 9. Seismic zone map of Alaska +- -.-- - 60 10. Seismic zone map of western Canada----------------- 61 11. Strain- release map of seismic energy 1898-1960, inclusive, in southeastern Alaska and part of adjacent Canada---------------- 4 ----------------- 62 12. One-hundred-year probability map showing peak earthquake accelerations for southeastern Alaska and part of adjacent Canada---------------------- 64 IV TABLES Page Table 1. Physical properties of a sample of elevated fine grained marine deposits (Qem) from the Wrangell area, Alaska-------------------------------------- 21 2. Cenozoic marine megafossils in elevated fine-grained marine deposits (Qem) from the Wrangell area, Alaska ------ --- - -- 24 3. Partial list of earthquakes felt and possibly felt at Wrangell, Alaska, 1847-1972, and July 1, 1973-- 52 4. Approximate relations between earthquake magnitude, energy, ground acceleration, acceleration in relation to gravity, and intensity---------------- 65 RECONNAISSANCE ENGINEERING GEOLOGY OF THE WRANGELL AREA, ALASKA, WITH EMPHASIS ON EVALUATION OF EARTHQUAKE AND OTHER GEOLOGIC HAZARDS By Richard W. Lemke SUMMARY Purpose of study The Alaska earthquake of March 27, 1964, pointed up the need for engi neering geologic studies of urban areas in seismically active regions. A reconnaissance study of the Wrangell area constitutes part of an overall program to evaluate earthquake and other geologic hazards in most of the larger Alaska coastal communities. The resulting tentative evaluations should be useful in city and regional land-use planning so that new facilities can be built, as nearly as possible, in the best geologic environment. Geography The mapped area described in the report, which comprises about 2 square miles (5 km2 ) of land in and immediately adjacent to the city of Wrangell, is about 140 miles (225 km) southeast of Juneau and 90 miles (145 km) northwest of Ketchikan. Wrangell, in 1970, had a population of about 2,000 people. Except for two hills, which rise to heights of approximately 400 and 300 feet (122 and 91 m), all of the mapped area lies below 150 feet (46 m) and is characterized by gentle