Scientific Background and Field Plan
In September 1998, we investigated the surface ruptures associated with the 1905 Bulnay earthquakes. The team was led by David Schwartz (USGS, Menlo Park, CA), with Suzanne Hecker, Daniel Ponti, (both at USGS, Menlo Park), A. Bayasgalan (Cambridge University and Mongolian Centre for Informatics and Remote Sensing), William Lund (Utah Geological Survey), and Heidi Stenner (Arizona State University). The 1905 Bulnay earthquake was actually a sequence of large events. The first occurred on July 9, 1905 and ruptured 130 km (80 miles) of the left-lateral Tsesterleg fault (see Figure 2 for location). Based on this rupture length, we estimate the magnitude of that earthquake to be 7.4. This was followed fourteen days later by a 350 km-long rupture (216 miles) on the left-lateral Bulnay fault, and rupture of the shorter Teregtiyn thrust. The estimated magnitude of 8.1 for the July 23 event makes this one of the largest earthquakes in shallow continental crust to have occurred anywhere in the world this century. Displacements on the Bulnay fault were reported to be as large as 11 m (36 ft). Figure 3 shows how a portion of this surface rupture looked in 1990. The rupture is still extremely well preserved considering that the earthquake occurred nearly 90 years before the photo was taken.
Figure 3. Photograph of ground rupture produced by the 1905 Bulnay earthquake, taken near Tsavdan Nuur, in northwestern Mongolia (from Baljinnyam et al, 1993). The fault rupture is characterized by tension gashes and linear mounds referred to as mole tracks (it should be obvious why this term was developed!). This surface rupture pattern is characteristic of strike-slip faulting.
During our field reconnaissance we hand excavated small trenches with the goal of exposing the evidence of the penultimate earthquake on each of the 1905 rupture components, and sampling it for radiocarbon dates. This allows us to determine whether the complexity of the 1905 rupture is a repeat of previous earthquakes. It also lets us compare the timing of paleoearthquakes with those on the 1957 Gobi-Altay rupture as part of the analysis of regional earthquake clustering. We also hope to identify alluvial fans that have been offset repeatedly by the fault, measure the amount of offset, sample the fan surfaces for cosmogenic isotope dating, and calculate slip rates for these faults. Finally, we measured additional offsets from 1905 to develop a better idea of how earthquake slip is distributed on large faults. All of this will improve our understanding of how and why large earthquakes occur far from the active boundaries of large plates.
The 1998 field party arrived in Ulaan Bataar, the capital of Mongolia, on September 8, local time. The following day, the scientific team, along with a small Mongolian support crew, embarked across-country (literally, as roads are few and poorly maintained) via 4-wheel drive vehicles, to the western end of the 1905 Bulnay fault ruptures. From that time until September 24, we conducted our studies while moving progressively eastward along the fault, dry-camping in the Mongolian mountain steppe along the way. During the field expedition, we sent updates of our observations and progress in near real-time by uploading web pages from the Bulnay fault directly to this site. These reports summarize our geologic findings and approaches, along with our experiences and observations from Mongolia (along with a few photographs, too).
