MORPHOLOGY OF THE ALDERWOOD LANDSLIDE; A PROBABLE ORIGIN FOR TSUNAMI IN LYNCH COVE, PUGET SOUND, WASHINGTON

As part of a statewide effort to map unstable landforms in forested watersheds, landslides in the Mason Watershed Administrative Unit (WAU) were evaluated using lidar, orthophotographs, aerial photographs dating from the 1960's to 2000's and reconnaissance field work. During our landslide study, the Alderwood landslide was identified as a major complex in Hood Canal that may have been triggered by seismic shocks. The Sunset Beach fault runs parallel to the Alderwood landslide and intersects with the headscarp. Trench stratigraphy showed one surface displacement event, which is younger than 1.3 thousand years (personal communication, Alan Nelson, USGS).

A tsunami deposit observed by Jovanelly and Moore, 2005, is located to the northeast of the Alderwood landslide. This study indicates that this deposit was correlated to a seismic event approximately 1,100 years ago. The correlation between the tsunami deposit and activity along the Sunset Beach fault make this landslide complex an ideal origin for the tsunami deposit in Lynch Cove. We plan to core sag ponds at the head of the landslide to attempt to confirm this correlation.

Historical aerial photographs document modern movement of the Alderwood landslide as well. Movement was determined by observing head and internal scarp position changes through four flight years at approximately 10 year intervals. Remote sensing, using orthophotos and lidar, was used to map structures and movement within the study time horizon. Water well logs were studied to determine geologic structures and possible failure planes within the landslide and to construct cross sections and evaluate internal structure.