Landslide of the Week – Alderwood Landslide
April 27, 2009
Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.
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 (Jovanelly and Moore, 2005; Moore, 1991) . 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 (Bucknam, et al, 1992). Wood samples from a core of the sag pond, to shattered till below, dated at 900AD (limiting age).
One of the most interesting things about this landslide is its similarity to landslides that moved during the 1964 Good Friday earthquake in Alaska. Looking at cross sections from Turnagain Heights and Government Hill landslides show similar lateral spread and earthflow processes (Barnhardt and Kayen, 2000; Hansen, 1965). This has some interesting implications, are Washington’s Puget Sound shorelines vulnerable to lateral spreads during large Cascadia Earthquakes (like the January 26, 1700 earthquake), or is this a fluke or a confluence of numerous factors, such as a fault along the shoreline?
Barnhardt, W.A.; Kayen, R.E., 2000, Radar Structure of Earthquake-Induced Coastal Landslides in Anchorage, Alaska, Environmental Geosciences, Volume 7, Number 1, 2000 38-45.
Bucknam, R.C.; Hemphill-Haley, E.; Leopold, E.B., 1992, Abrupt uplift within the past 1700 years at southern Puget Sound, Washington: Science, v. 257, p.1611-1614.
Hansen, W. R., 1965 Effects of the Earthquake of March 27, 1964, at Anchorage, Alaska, USGS Professional Paper 542-A.
Jovanelly, T.J.; Moore, A.L., 2005, Tsunami origin for an 1,100 year old enigmatic sand sheet in Lynch Cove, Puget Sound, Washington, U.S.A. [abstract]: Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 65.
Moore, A.L., 1991, Evidence for a tsunami in Puget Sound 1,100 years ago [abstract]: Eos (American Geophysical Union Transactions, v. 72, no. 44, Supplement, p. 315.