Tuesday, February 16, 2010

Finding faults with LIDAR

My last post on Sumas Mountain and LIDAR indicated that a fault could be observed in the LIDAR image. So here it is. It is my understanding that the initial LIDAR in this area was done to assess river channels on the Nooksack River and someone noticed a straight line on the image.

LIDAR has proven to be a huge help in identifying faults in western Washington. The glaciation of the Puget Sound region as well as the preponderance of thick vegetation has made identifying surface faults very difficult. The identification of surface faults in the Puget Sound region got a big boost when LIDAR began to be used.
A big break came in 1996 when the Kitsap Public Utilities District funded LIDAR mapping of Bainbridge Island in order to get a better handle on the water resources on the island. Greg Berghoff with the District noticed a line cutting across the south portion of the island in the LIDAR image and notified geologists at the University of Washington. This caused a great deal of excitement because the line happens to coincide with the Seattle Fault zone and indicates that the ground surface had been off set substantially since the end of the last glacial period.
Toe Jam Fault on south Bainbridge Island
The trenching experts came in and excavated trenches across the scarp, pieced together disrupted soil horizons and age dated the off sets with carbon. What they found is probable magnitude 7 quakes had taken place along this fault scarp. While large quakes are known to be a risk in Washington State, this discovery recognizes large quakes in close proximity to large population centers are a very real risk. Hence, the Washington State Department of Transportation simulation of what could happen to the Seattle waterfront and the viaduct is very possible as this fault strand is just a few miles from the Seattle waterfront and fits the description of the type of event that would take out the viaduct.
Another fault appears evident across the water to the southwest in the port Orchard area. Both of these faults appear to be associated with the Seattle Fault Zone. It appears that the Puget Sound region has several fault zones. These zones consist of several fault stands instead of a simple single fault line. It may be that only a few of the earthquakes actually cause surface ruptures that will show up on LIDAR imagery. However, the presence of surface ruptures indicates that shallow quakes are possible and allows for estimation of the size of potential quakes associated with these fault zones.

Scarp off setting drumlins, Port Orchard area

As noted a fault scrap was identified in Whatcom County near the Kendal area using LIDAR. This “new” fault caused some interesting dialog between the local media and geologists at Western Washington University. The Western geologist commented on an Environmental Impact Statement (EIS) regarding development of the Bellingham waterfront. One of the issues they raised was concern about the presence of nearby faults including the newly identified fault near Kendal that had not been identified as a potential risk to waterfront development. Much of the waterfront has been constructed on poorly consolidated fill and significant portions are liquefiable. The Bellingham Herald was highly critical of the comments claiming that the information the geologists provided was “nothing new”. The lesson I took from that is that policy makers and those that influence policy may be aware of a potential large subduction zone earthquake, but are unaware of the risks posed by nearby seismic events on much smaller but dangerous faults located near population centers.
In that regard LIDAR has been a huge help and the Seattle media seems to be highly aware of the risk. Perhaps it helps that the Seattle Fault zone is named after the City of Seattle. That said taking a look at Google Earth should show how lateral spreading due to soil liquefatcion can impact an area.

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