Age and Slip Distribution of Past Earthquakes Along the Bogd Fault (Mongolia)
Abstract
We carried out morphotectonic and paleoseismologic studies along the left lateral strike-slip Bogd fault, the principal structure involved in the Gobi-Altay earthquake of December 4, 1957 (published magnitudes range from 7.8-8.3). The Bogd fault is 260 km long and can be subdivided into 5 main geometric segments based on the variation in strike direction. From west to east these segments trend: N095°E along West Ih Bogd (WIB), N105°E along North Ih Bogd (NIB), N095°E along East Ih Bogd (EIB), N075°E along West Baga Bogd (WBB), N115°E along North Baga Bogd (NBB), respectively. Our goals are to refine and complete slip rate estimates on the Bogd fault and to determine the ages and the slip distribution of pre-1957 surface ruptures.Morphological analysis of offset streams, terrace risers, and ridges at five sites (two at Ulaan Bulag on the WIB segment, one at Noyan Uul on the WIB segment, one at Bitut on the NIB segment, and one at Zadgay Sayhr on the EIB segment) distributed across the three western segments show that the slip along each segment is characteristic (the different offsets measured at a point are multiple of the 1957 slip at this point). The horizontal components per event are between 5-5.5 m at Ulaan Bulag, 4.4-5.2 m at Noyan Uul, 2.8-3.4 m at Bitut, and 4-5 at Zadgay Sayhr. A paleoseismic study on the western segment at Ulaan Bulag suggests a mean earthquake recurrence interval of ~ 3500- 3700 years for the late Pleistocene-Holocene. There have been four events during the past 14±1.65 ka and three events since 10.6±0.71 ka (11.1±0.71 ka IRSL date), OSL dates bracket the age of the penultimate event between 3.4±0.34 ka and 5.44±0.34 ka (5.05±0.3 ka IRSL date). At Bitut (NIB) alluvial surfaces that have preliminary age dates between 3.71±0.46 ka (OSL) and 4.5±0.8 ka (in situ 10Be) are only offset by the 1957 event. This is consistent with the interval between the 1957 and penultimate events at Ulaan Bulag. We suggest that both the WIB and NIB segments ruptured together during the penultimate earthquake along this section of the Bogd fault.Our still-in-progress slip rate study indicates that the horizontal slip rate along the Bogd fault decreases progressively from ~1.3 mm/yr to ~0.8 mm/yr between Ulaan Bulag (WIB segment) and Bitut (NIB segment). This decrease reflects the progressive change of kinematics from pure left-lateral strike-slip faulting to left-lateral-reverse faulting consistently with the change of strike from WIB segment to NIB segment.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T21B1959R
- Keywords:
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- 0468 Natural hazards;
- 1105 Quaternary geochronology;
- 7221 Paleoseismology (8036);
- 8010 Fractures and faults;
- 8036 Paleoseismology (7221)