Where We Are Today (9/13/98)

(on the eastern end of the 1905 Bulnay fault rupture)

Progress Report:

Hi again from the eastern Bulnay fault rupture. Today dawned clear and cold (really cold, judging from the frozen water droplets on the tent flys), and warmed into one of the more pleasant days we've had in Mongolia, with blue skies and no wind. We split into two teams today to work on the portion of the fault near our camp. Suzanne Hecker, Bill Lund, and Heidi Stenner, along with Gantulga, Dashaa, and Sandag (one of our drivers) worked to expose a trench across a mole track about 7 km west of our camp, while David Schwartz, Dan Ponti, A. Bayasgalan, Enkhbaatar, and Ganbold (our other driver) reconnoitered about 25 km of the fault east of camp. Tomorrow we will move westward along the Bulnay fault. Bu'st Nuur (lake) or bust!

Geology:

Between us we've worked along major strike-slip faults such as the San Andreas, the North Anatolia, the Motagua, and the Bogd, but we've never seen as spectacular fault geomorphology as we've observed today along the Bulnay. A major goal of our work is to define how often great earthquakes on this fault occurred. To do this we need locations that preserve evidence of past events. Today we found one such site, about 4 km east of our camp, where a series of stream terraces have been progressively offset along the fault. At this location we were able to measure 10.8 meters of left-lateral offset from the 1905 event and 1.2 meters of vertical separation. From our preliminary observations, it appears that this flight of terraces has been affected by three earthquakes during the past 11-12 thousand years.

Along this stretch of the fault, the small mole tracks associated with the 1905 earthquake have, over time, grown into large hills that deform the surface of alluvial fans. In numerous places the fault steps left as much as a kilometer, producing depressions that commonly are filled with lakes. In these stepover zones, tension gashes have formed, some several meters deep and up to 100 meters long. Several generations of these tensional features can be observed from repeated earthquakes.

A major problem we have with all of this is that we'd like to look at every centimeter of the fault, but only have time to spend a few hours at what looks to be the most promising places. We'll have to come back!

Miscellaneous:

Our work along the fault has generated a lot of interest from the local residents, who ride up from their gers on horseback to observe what we're doing. All have been keenly interested, friendly, and helpful, including loaning nails to our trenching crew to help in the logging. One fellow moved what must have been 100 pound boulders to assist our Jeep in fording a stream. We continue to be impressed with the extraordinary skill of our drivers, who can traverse marshy terrain that we fear to even walk on. But even the best of these can occasionally meet his match, as can be seen here. The moral here is never to travel in Mongolia without a winch!