Abstract

Abstract. New field work, combined with analysis of high-resolution aerial photographs, digital elevation models, and satellite imagery, has identified an active fault that is traceable for ∼90 km across the Seymchan Basin and is part of the Ulakhan fault system, which is believed to form the Okhotsk–North America plate boundary. Age dating of alluvial fan sediments in a channel system that is disturbed by fault activity suggests the current scarp is a result of a series of large earthquakes (≥Mw 7.5) that have occurred since 11.6±2.7 ka. A possible channel feature offset by 62±4 m associated with these sediments yields a slip rate of 5.3±1.3 mm yr−1, in broad agreement with rates suggested from global plate tectonics. Our results clearly identify the Ulakhan fault as the Okhotsk–North America plate boundary and show that tectonic strain release is strongly concentrated on the boundaries of Okhotsk. In light of our results, the likelihood of recurrence of Mw 7.5 earthquakes is high, suggesting a previously underestimated seismic hazard across the region.

Highlights

  • Our results clearly identify the Ulakhan fault as the Okhotsk–North America plate boundary and show that tectonic strain release is strongly concentrated on the boundaries of Okhotsk

  • Since the earliest days of plate tectonics, the Eurasia (Eur) and North America (NAm) plate boundary zone in northeastern Asia was recognised as a likely location for smaller blocks and micro-plates, even if these could not be precisely identified at the time (Morgan, 1968)

  • In the rest of this paper, we describe the evidence for recent large earthquakes affecting the Seymchan Basin segment of the Ulakhan fault, as well as speculating on longer-term slip rates derived from new age determinations

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Summary

Introduction

Since the earliest days of plate tectonics, the Eurasia (Eur) and North America (NAm) plate boundary zone in northeastern Asia was recognised as a likely location for smaller blocks and micro-plates, even if these could not be precisely identified at the time (Morgan, 1968). The largest recorded earthquake in this region is apparently “intraplate”, and a small number of ∼ M 5.0 earthquakes are all that have been associated with the trace of the Ulakhan fault and the assumed Okh–NAm boundary itself. Hindle et al.: Ulakhan fault surface rupture fluvio-lacustrine deposits and several apparent depocentres with > 1000 m of fill and ongoing sedimentation through the Quaternary (Kuznetsov, 1989), suggests this basin has existed since some time in the mid-Tertiary to early Tertiary (late Paleogene–early Neogene) As such, it forms part of a series of sporadically distributed basins across the northern Okhotsk region, including offshore regions, which initially formed in a generally extensional–transtensional late Paleogene–early Neogene tectonic regime (Worrall et al, 1996; Nokleberg, 2005; Drabkun et al, 1970). In the rest of this paper, we describe the evidence for recent large earthquakes affecting the Seymchan Basin segment of the Ulakhan fault, as well as speculating on longer-term slip rates derived from new age determinations

Field work and remote sensing data
Tectonic geomorphology of the Ulakhan fault across the Buyunda fan
Buyunda fan surface and hydrology
Buyunda fan age determinations and offset rate estimates
Basin-wide fault and scarp features
Elastic displacement modelling and scarp polarity
Seismological and kinematic data
Earthquake size and fault dimensions
Large-scale tectonics
Conclusions
Sample collection
Sample preparation and measurement

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