R. A. Sloan

Multisegment rupture in the 11 July 1889 Chilik earthquake (Mw 8.0–8.3), Kazakh Tien Shan, interpreted from remote sensing, field survey, and paleoseismic trenching

The 11 July 1889 Chilik earthquake (M-w 8.0-8.3) forms part of a remarkable sequence of large earthquakes in the late nineteenth and early twentieth centuries in the northern Tien Shan. Despite its importance, the source of the 1889 earthquake remains unknown, though the macroseismic epicenter is sited in the Chilik valley, similar to 100 km southeast of Almaty, Kazakhstan (similar to 2 million population). Several short fault segments that have been inferred to have ruptured in 1889 are too short on their own to account for the estimated magnitude. In this paper we perform detailed surveying and trenching of the similar to 30 km long Saty fault, one of the previously inferred sources, and find that it was formed in a single earthquake within the last 700 years, involving surface slip of up to 10 m. The scarp-forming event, likely to be the 1889 earthquake, was the only surface-rupturing event for at least 5000 years and potentially for much longer. From satellite imagery we extend the mapped length of fresh scarps within the 1889 epicentral zone to a total of similar to 175 km, which we also suggest as candidate ruptures from the 1889 earthquake. The 175 km of rupture involves conjugate oblique left-lateral and right-lateral slip on three separate faults, with step overs of several kilometers between them. All three faults were essentially invisible in the Holocene geomorphology prior to the last slip. The recurrence interval between large earthquakes on any of these faults, and presumably on other faults of the Tien Shan, may be longer than the timescale over which the landscape is reset, providing a challenge for delineating sources of future hazard.

The structure and late Quaternary slip rate of the Rafsanjan strike-slip fault, SE Iran

The Rafsanjan right-lateral strike-slip fault in SE Iran has a clear expression in the geomorphology, is sited close to several large population centers, and yet its role in the regional tectonics, its rate of activity, and its potential to generate destructive earthquakes are unknown. We use high-satellite imagery and field investigation to identify the active strands of the fault system and show that the overall north-south right-lateral shearing across the region is spatially separated into almost pure strike slip on the NW-SE–trending Rafsanjan fault and an orthogonal component of shortening on parallel thrust faults in the lowlands. Possible remnants of ruptures, involving right-lateral slip of ∼3 m, from an earthquake of ∼Mw >7 are identified along the eastern part of the Rafsanjan fault. We speculate that these ruptures result from the destructive 1923 Laleh event. An alluvial fan displaced right laterally by 48 ± 4 m and tentatively dated at ∼120 ka yields an average slip-rate estimate of ∼0.4 mm/yr. Our slip-rate estimate is consistent with known estimates of late Quaternary slip rate on other faults within eastern Iran and with global positioning system (GPS) measurements of present-day deformation in this part of the country. Our results therefore suggest that the slip rates of faults in eastern Iran do not vary substantially through the late Quaternary: a result that is important for the interpretation of geodetic and late Quaternary measurements of slip rate in regions of distributed strike-slip faulting.

The Egiin Davaa prehistoric rupture, central Mongolia: a large magnitude normal faulting earthquake on a reactivated fault with little cumulative slip located in a slowly deforming intraplate setting

Abstract The prehistoric Egiin Davaa earthquake rupture is well-preserved in late Quaternary deposits within the Hangay Mountains of central Mongolia. The rupture is expressed by a semi-continuous 80 km-long topographic scarp. Geomorphological reconstructions reveal a relatively constant scarp height of 4–4.5 m and a NW-directed slip vector. Previous researchers have suggested that the scarps exceptional geomorphological preservation indicates that it may correspond to an earthquake that occurred in the region c. 500 years ago. However, we constrain the last rupture to have been at least 4 ka ago from morphological dating and <7.4 ka ago based on radiocarbon dating from one of two palaeoseismic trenches. Our study shows that discrete earthquake ruptures, along with details such as the locations of partially infilled fissures, can be preserved for periods well in excess of 1000 years in the interior of Asia, providing an archive of fault movements that can be directly read from the Earths surface over a timescale appropriate for the study of slowly deforming continental interiors. The Egiin Davaa rupture involved c. 8 m of slip which, along with the observations that it is largely unsegmented along its length and that the ratio of cumulative slip (c. 250 m) to fault length (c. 80 km) is small, suggests relatively recent reactivation of a pre-existing geological structure. Supplementary material: All scarp profiles are available at http://www.geolsoc.org.uk/SUP18871