Questions:
1) How many trees are visible on the island from the highway?
2) What was the estimated height of the tsunami when it hit the shore?
3) From where did the records of this phenomenon originate?
4) How many hills comprise the island?
Information:
The Cascadia Earthquake was a megathrust earthquake that occurred in the 18th century. It's estimated to have had a moment magnitude scale (Mw) reading of 8.7 - 9.2 M. This occured due to seismic activity at the The Juan de Fuca Plate, which is a small portion of the larger Cascadia Subduction Zone. The zone itself is an off-shore tectonic area that subducts underneath the North American Plate. It spans from Vancouver Island, Canada, to Northern California. The tsunami that occured in 1700 was most recent fault rupture, spanning 1000 km long, and nearly 20 m meters in width. The effects of the tsunami were seen in the Pacific Northwest, and even across the Pacific Ocean in Japan. Native Americans, such as the Chinook tribe, witnessed the event and recorded it in their mytholgy.
Here in Siletz Bay, one can see the end result of the tsunami over 300 years later. The peninsula, which temporarily becomes an island during high tide events, is historical evidence of how powerful water can be as an erosional force. It is the remnants of a basalt deposit, which is a common material the comprises oceanic plates. Weaker geologic materials were readily swept away, resulting in an embankment. Over time, soil deposits on the island and weathering of parent material has allowed an environment for plants to grow. It's a very photogenic area, especially at sunset.
Sources and further reading:
Great Tsunami of 1700
The Cascadia Subduction Zone
The Story of Thunderbird and the Orphan Tsunami
Erosion from Ancient Tsunami
More information about the area:
Visit Lincoln City
Siletz Bay Activities