|Ramon Arrowsmith ; Arizona State University |
Collaborative Research: Magnitude of Slip, Slip Rate, and Slip Distribution along the Cenozoic Altyn Tagh Fault System
University of California, Los Angeles: Collaborative Research
Our Chinese colleagues from the Institute of Geomechanics of the Chinese Academy of Sciences in Beijing have collaborated with us scientifically and provided logistical support.
Activities and findings:
1) Fault trace mapping. The major effort in the last year (3/99-3/00) was an 8 week field excursion along the central Altyn Tagh Fault (ATF) in western China. Arrowsmith along with ASU graduat student Zack Washburn and Chinese colleagues, mapped a 150 km reach of the central ATF east of our 1998 mapping area. This work included mapping on airphoto and satellite image basemaps the Quaternary geology along the fault zone. Within the mapping area, sites were chosen for more detailed work, which included topographic mapping using a total station, and sampling for numerical dating (14C, thermoluminesence). 2) Earthquake geology investigations. We excavated 7 trenches including two sets of large ones (~50 cubic meters) with the help of local laborers to determine the timing of the most recent and penultimate earthquakes along the Altyn Tagh Fault. We also measured more than 50 offset landforms in order to estimate the slip distribution in the last earthquakes. 3) Geo-rectification of Declassified Intelligence Satellite Photographs. These photographs are quite valuable as cheap alternatives to SPOT or other stereo satellite imagery. However, they are geometrically a bit difficult. We have spent a considerable effort at their geo-rectification and have nearly completed rectification of the first set. These images are quite useful for documenting the landforms and fault zone geometry in this region, and once they have been rectified, we can compile our geologic and geomorphic mapping in absolute coordinates and produce digital final maps and derivative products. Other data analysis and production has included preparation of basemaps of digital topography and seismicity for the region using GMT and GTOPO30. 4) Workshop and meeting participation has included consultation with colleagues and presentation at the Fall AGU meeting and the Altyn Tagh Fault working group meeting at UCLA in late February, 2000.
Activities and findings:
1) Preliminary results are that the fault zone records significant evidence for major prehistoric earthquakes. The tectonic landforms that we have documented are world class examples of strike-slip related deformation. Here is text that comes from our 1999 AGU abstract that summarizes our findings: The sinistral Altyn Tagh fault (ATF) is one of the key structures in Asian collisional tectonics. To answer questions about the earthquake geology of the ATF, we conducted fault trace mapping and paleoseismic investigations between Wu Zun Xiao Er and Suerkuli Valley. The results of this work document the fault trace structure, offset distribution, min. rupture length, and also provide numerical age constraints for the last 2 earthquakes. We also estimate relative geomorphic ages of surface rupture and min. magnitude of recent events. We identified 3 geometric segments along the 164-km-long reach that was mapped. From east to west they are 1) the > 87-km-long Suerkuli Valley segment, 2) the 35-km Snowy Mountain segment (38.61N, 90.6E to 38.56N, 60.45E), and 3) the > 42-km Wu Zun Xiao Er segment. The Suerkuli segment ruptures Holocene alluvium and is characterized by a 065 - 072 striking, 30-1000 m wide, linear fault zone. The fault zone is dominantly strike-slip, but has active normal faults on its periphery. In contrast to the linear Suerkuli segment, the Snowy Mtn. segment has a highly variable strike (065 - 110) and ruptures through mountainous topography. The 20-150 m wide, active fault zone consists of 1-5 km long, discontinuous strike-slip faults. The active strike-slip trace has a larger, S side down, dip-slip component than that of Suerkuli. Geomorphic features, such as strongly developed wineglass valleys and triangular facets, also support a significant normal component of slip along this segment. From east to west, the strike of the Wu Zun Xiao Er segment progressively bends from 090 to 050 and then resumes its characteristic 070 strike as it leaves the fractured bedrock of the Snowy Mtn. range and enters Holocene alluvium. In bedrock, the active trace is expressed as 20-100 m long, right stepping, en echelon strike-slip faults. Dilation between en echelon faults is common, but slip vector measurements show that this segment has a lower component of dip slip than the other two segments. Air photo interpretation combined with field mapping show the Suerkuli segment has continuous surface rupture for > 130km with min. offset values of 2.0 to 11.2 m. The double-stranded mole track has a fresh morphology that is continuous over most of the segment length and implies that the last event probably occurred in the past few hundred years. Seven paleoseismic excavations reveal at least 3 events in Holocene units and confirm that peripheral normal faults are synchronous with the main strike-slip rupture. The surface rupture of the Snowy Mtn. segment appears to be older than that of the Suerkuli segment because the moletrack is geomorphically less distinctive. Minimum offsets of 3.1 to 5.7 m are notably lower than those seen on the Suerkuli segment. Combined field work from the previous year show the Wu Zun Xiao Er segment has a surface rupture length > 79 km. Offsets for this event range from 2.7 to 5.1 m and the moletrack appears to be geomorhically older than Suerkuli, but younger than Snowy Mtn. An excavation exposed lower N-dipping fractured silts that have been faulted twice while the overlying gravel and S-dipping silts are only broken by fractures that cut to within a few cm of the surface. The 1215 to 1295 cal AD 14C date on pieces of buried shrubs above the lower silts and below the gravels provides a min. age for the penultimate event and a max age for the last earthquake. In summary, 2 separate events probably ruptured 129 km of the ATF in the field area within the last 700 years.
Graduate and undergraduate student training in large scale logistics, image processing, and graduate student training in active tectonics and paleoseismology and numerical dating.
Education and Outreach:
Arrowsmith presented a brown bag seminar to the ASU Geology Department that was attended by over 30 graduate students, faculty, and people from the community.
These pages summarize the main aspects of our research and present some of the raw map and image data.
Other Specific Products:
Contributions within Discipline:
Our findings help to focus inquiry on the Altyn Tagh fault and whether it is like other continental transform faults such as the San Andreas Fault. It is not very productive in instrumental seismicity, yet it has a high slip rate. Our investigations indicate that the last major earthquake along the central Altyn Tagh Fault was no more than about 700 years ago and that it had surface offset of 2-6 m. This has implications for related studies of the current crustal deformation in the region.
Training and testing for earthquake hazard studies
Web pages may be useful for training in studies of major strike-slip fault systems.
Special Requirements for Annual Project Report: