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"Tectonic Deformation Re Great Subduction Zone Earthquakes

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_ _ _ _ _ _ _ _ _ _ ____ ._ __ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . 3 TECTONIC DEFORMATION RELATED TO GREAT SUBDUCTION ZONE EARTliQUAKES George Plafker, U.S. Geological Survey Abstract Vertical and horizontal displacements associated with plate convergence at consuming plate margins are the algebraic sum of interseismic, coseismic, and transient deformat- ions through a complete earthquake cycle on a time scale of tens to thousands of years. Elastic and permanent deformations accumulated during the interseismic period are a function of coupling across the megathrust interface between the underthrusting oceanic crust and the upper plate, and of the direction, rate, and duration of relative plate motions. Coseismic deformations result frbm seaward thrusting of the upper plate and depend upon dip of the megathrust, displacement along the megathrust, and the dip and displacements along subsidiary faults that may break through the upper plate. Transient postseismic displacements may occur that result from relatively slow elastic strain i release or creep deformation following an earthquake, | i Coseismic regional vertical displacements typically involve a central broad asymmetric downwarp elongate parallel to the arc with a flanking zone of marked uplift on the sea- | t ward side, and a zone of relatively minor uplift on the landward side. The major zones I of uplift and subsidence may extend from the trench to its associated volcanic are. In the 1960 Chile earthquake (Mw=9.5] deformation occurred for about 1,050 km parallel to the are over an area of 85,000+ km with shoreline vertical displacements to +5.7 m and -2.3 m. In the 1964 Alaska earthquake (Mw=9.2) deformation occurred for 950 km parallel to the arc over 140,000+ km2 with vertical displacements to +11.3 m and -2.3 m; two subsidiary thrust faults cut the surface with up to 7.9 m dip-slip displacement. Both earthquakes generated mejor tsunamis due to sudden upheaval of the sea floor. Horizontal displacments determined by triangulation indicate a minimum of 20 m relative seaward displacement during the 1964 event. Interseismic vertical displacements in the region affected by both uplift and subsidence during the Alaska earthquake are indicated by gradual shoreline submergence of terrestrial vegetation and aboriginal sites along the shores at average rates of 5-7 mm/yr. The submergence suggests a significant component of downwarping in addition to elastic shortening and thickening of the upper plate between coseismic displacements. Tide-gage data and levellings show transient changes and possible reveuals (recovery) of 9-20% of the coseismic vertical displacements in 18 years after the 1964 earthquake. Paleoseismic studics or:dwide indicate recurrence intervals on the order of hundreds to thousands of years for gre Lt subduction zone earthquakes that involve shoreline deformation. In Alaska, paleoseismic studies of a flight of 5 uplifted marine terraces at Middleton Island and of regional pre-1964 submerged shorelines along the mainland and islands of Prince William Sound indicate recurrence intervals for 1964-type earthquakes averaging 800 years with the last previous event about 1400 years ago. References Plafker, George,1972, The Alaskan earthquake ot' 1964 and Chilean earthquake of 1960; implications for are tectonics: Journal of Geophysical Research, v. 77, no. 5, p. 901-925. Plafker, George,1969, Tectonies of the March 27,1964, Alaska Earthquake: U.S. Geological Survey Professional Paper $43-1, 74 p. Plafker, George, and Savage, J.C.,1970, Mechanism of the Chilean earthquakes of May 21-22, 1960: Geological Society of America Bulletin, v. 81, p.1001-1030. Plafker, George, and Rubin, Meyer,1978. Uplift history and earthquake recurrence as deduced from marine terraces on Middleton Island, Alaska, in Proceedings of Conference VI, Methodology for identifying seismic gaps and soon-to-break gaps: U.S. Geological Survey Open-File Report 78-943, p. 687-72, m 8608100165 88061D PDR ADOCK 05000344 p PDR ._ - _- - ._..
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