Tuesday, June 26, 2007
Earthquakes on Whidbey Island
Although W.I. is a young island, composed of mostly glacial Till (consequently dampening a lot of shaking by underlying Earth crust), it nevertheless has experienced some violent quakes. These are shown in accompanying photos, as evidence of what has happened in the past.
An attempt has been made to determine the frequency of quakes, from the time-spacing (vertically-separated layers) of obvious disruptions in the cliffs along the beach. The beach is almost the only place where earth movements can be assessed, since most of the island is covered by vegetation, rubble from glaciation, and water in ponds, creeks, and lakes.
The western beaches and cliffs are the ones which are cleaned by yearly storms, so that these should be relied upon as most revealing. These cliffs are sheared by slumping caused by storms and quakes, so that each year fresh views are presented. The evidence seen last year is later removed, so that a motion picture in one’s mind is made of a part of the layers as they retreat from one photo to the next. This amounts to as much as one foot yearly, and the conclusions reached in previous years can be tested for those scenes found NOW.
The accompanying photos, either at the top of the text or on ones following and preceding are from the Ledgewood portion of W.I., where the first location of a massive fault and its attending quakes can be found, along with the resulting house collapse. The casual observer would proclaim that this is simply slumping, and indeed slumping is common near the beaches on the south and west sides of W.I. But the next inquiry should be: what caused the slumping, particularly in an area where it is not so common?
In this case, a slab of a garage up the hill has been checked, to find that the slab moved to the right relative to the house slab, as shown by bent nails connecting a 2x6 between them. This is a Left lateral fault- meaning that when one stands on either side of the fault, the opposing side is displaced to the left. The house owner might have thought that he was the Victim of Bad Fortune to have the most slumping in the neighborhood, but he happened to have built his house on the fault line- where slumping is pronounced. You can see that the houseowner on the cliffs eventually capitulated! Cliffs and houses on the east side of Puget Sound islands are much less affected by winter storms, which carry away the fault and slumping debris. This feature requires the analyst to take information primarily from the west side (even though faulting ocurs on all sides of the islands). A fault found near Mariner's Cove on the east near Silver Lake and Creek results in little visual evidence and much less slumping.
The second photo looks in the direction offshore, along the NW-SE faultline, and finds that a saddle occurs where Seattle Pacific College resides (between Admiralty Head and Camp Casey). This is an excellent example which shows that a saddle on a topographic map, or a visual sighting, can strongly indicate a fault line.
The Bird got in the Act!
As Luck would have it, as I was aligning my camera with the NW-SE orientation, a lusty Raven felt the Earth’s impulse, and acted on his own to signal to me (via squawking and pointing) that we were getting a signal from the Earth. Since I am under Edgar Allen Poe’s influence, I immediately undertook to explore, what was meant by the Lost Lenore, and that which was most visual just beneath my Camera Door! All of this, you understand, while receiving a cell phone call from the notorious Black (as the Raven) Bart, a.k.a. Glenn Wasson. I had forthwith to dispatch the Bart, giving the ebony bird precedence; the Raven was most accommodating.
The 2nd separate cliff photo above, in a preceding Blog, shows the actual evidence of violent quaking, as seen in large blocks of angular rock (actually Pre-stone from the same general area, within a kilometer), which have fallen into crevices opened up in the earth during an earthquake. These angular blocks were later entombed in the earth, with the crevices closing- for viewing in cliffs which are eroding now. I have seen these episodes in at least four different elevations above the sea, indicating that the cyclicity of earthquakes for Whidbey island is on the order of 40,000 years/4 incidents = 10k years per violent quake (those paroxysms which opened up the ground surface and allowed nearby blocks to fall into the crevices).