Officials remain "quite concerned" about two houses in that area in addition to the one knocked off its foundation. Those 16 homes remained evacuated late Wednesday.
"Being cut off from the road, water and power," residents had to leave, Island County Sheriff Mark Brown said earlier. "It's a pretty massive mudslide."
Another 20 to 25 people were evacuated from the 17 homes higher up the hill.
The slide area remains unstable.
"This afternoon, small bits were sloughing off the bluff," the chief said. "There's no guarantee that will continue to be small amounts."
A geotechnical engineer working for Island County and state Department of Natural Resources geologists took a preliminary look at the area Wednesday and hoped to complete a fuller assessment Thursday.
Area residents were briefed on the status of their homes at a meeting Wednesday night.
There has been no significant rain in recent days, but the area has been prone to slides in the past.
"The west side of the island is prone to slides because of soil conditions and water movement in the ground," Hartin said.
"We have no specific cause as to 'why here, why now, why this big.' "
The slide area extends about 400 to 500 yards across the hillside and down 600 or 700 yards to the water, Hartin said.
Residents who heard the slide about eight miles south of Coupeville described it to KOMO-TV as sounding like thunder.
"It was a mix of rumbling and snapping trees," Hartin said. "We were hearing the same thing when we arrived."
Whidbey Island is about 35 miles long, north to south, and just a mile or two wide in places east to west.
A ferry ride away from the Seattle area, the island offers picturesque farm and water views and has a population of about 60,000, mostly centered around Oak Harbor and the Whidbey Island Naval Air Station.