Geodynamics and slip rates of the Ulakhan fault and northwestern Okhotsk plate, Northeast Russia
Final Report Abstract
The project involved sampling old river sediments from along a plate boundary (Okhotsk-NorthAmerica) which is located in eastern Siberia. We visited a region called the Seymchan basin, which is very interesting because two large rivers, the Kolyma and the Buyunda, cross the plate boundary here. We discovered a fault scarp, a clear indication of an earthquake, or several earthquakes which have offset the ground and produce a clear step in the landscape, which nobody had recognized as belonging to a plate bounding fault before. The sediments from this location were then analyzed using a special method (optically stimulated luminescence of quartz) to date the moment they were deposited and buried and no longer exposed to light. We also believe this coincides with the moment of a large earthquake that helped to form the scarp. When we looked closer and using other data, like aerial photographs and the newest satellite topography, we interpreted one feature next to the scarp to show the offset due to movement on the plate boundary, and we interpreted the age of this offset to be the same as the age of deposition of the sediments we sampled. By measuring the offset and dividing by the age of the sediments, we can give an estimate of the slip rate of the fault over the last 10,000 years (5-6 mm/yr). Up until now, nobody was absolutely sure where the plate boundary between Okhotsk and North America ran because it moves so slowly that there have been no earthquakes on it related to big movements between plates in all the time humans have been studying the earth. But this look into the deeper past tells us that it is very likely that the fault we visited is a part of the plate boundary today, and will be active again some time in the future, probably with big earthquakes (M 7.5). We also found indications for the total length of the fault that regularly slips in the region we visited. To do this, we compared the theoretical vertical motion around a fault with a vertical orientation and a "strike slip" sense of motion, to the uplift patterns we could identify on parts of the fault from satellite topography. This kind of work has only recently become possible with the newest data available from a special satellite called TANDEM. We also looked at the gorge of the Kolyma river which has cut deep into the landscape, next to the plate boundary fault. Here, we were very surprised how fast this "incision" appears to have happened. In fact, it seems as if the Kolyma has incised deep into bedrock before "filling in" again, and then incising and removing the infill once more, all in the last 30,000 years. We can see from this that enormous volumes of material move down the river, probably in discrete pulses, but we do not fully understand the reasons why. Nevertheless, the data themselves could prove very useful in better understanding how rivers respond to a range of factors like tectonics, climate change and so forth.
Publications
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The Ulakhan fault surface rupture and the seismicity of the Okhotsk-North America plate boundary, Solid Earth Discuss.
Hindle, D., Sedov, B., Lindauer, S., and Mackey, K.
