6th Colloquium on historical earthquakes & paleoseismology studies

First assessment of recent tectonics and paleoearthquakes along the Irtysh fault (eastern Kazakhstan)

24 Oct 2018, 13:45

35m

Han-sur-Lesse

Poster Preference Session 2 Posters Poster Session 1

Speaker

Mr Stéphane BAIZE (Institut de Radioprotection et Sûreté Nucléaire)

Description

The Asian plate interiors are known to have host strong earthquakes with magnitudes up to M≈8 in recent history, especially around the border area between Mongolia, Kazakhstan, China and Russia (e.g., M7.3 Chuya earthquake, 2003). Their recurrence times seem to be long, because of the relative low slip rates (less than 1 mm/y) of the faults which caused them. In this study, we focus on a large inherited fault zone (namely the Irtysh Fault Zone) with no historical earthquakes larger than M6, to test whether this structure could have generated surface-rupturing events during the last thousands of years, like some other historically silent similar faults of cratonic southern Kazakhstan did in pre-historic times. To do so, we use tectonic-morphological analyses of satellite images and trenching across several fault portions to detect potential paleoearthquakes. The Irtysh Fault Zone (IFZ) is a 250+ km long basement set of faults that marks a major tectonic block boundary between different units with Paleozoic magmatic rocks and thick deposits of Late Paleozoic age. The formation of the IFZ dates back into the Late Paleozoic times during collision between Siberia-Kazakh cratons and it was repeatedly reactivated in later times (e.g., Oligo-Miocene). The Quaternary activity of the Irtysh Fault Zone has never been addressed in modern studies and, in this study, we document for the first time that the IFZ is an active fault and is a potential source of large earthquakes in easternmost Kazakhstan. Our analyses actually revealed, at different spots along the IFZ, the occurrence of a set of lineaments offsetting lithologies or deflecting streams and other landscape features. Trenching sites across those lineaments eventually confirmed that they were active fault strands and in some trenches, 14C-dated Holocene-Late Pleistocene deposits are clearly faulted. Those recent deposits include organic soils, loess layers and colluvium directly overlying the Paleozoic rocks. Our findings lead to the conclusions that the IFZ is clearly active along several strands of its trace in bedrock. During the Holocene, it hosted earthquakes with surface rupture individual displacements of up to 1.5 meters, suggesting events with a magnitude around M≈7. However, our investigations do not allow to confident estimation of slip rates: we propose for one fault section parallel to the main fault a preliminary estimation of ~0.22 mm/y vertical displacement rate, based on a single trench. Based on regional morphoclimatic arguments and landscape features offsets, we also propose a first assessment of the lateral slip rate along the major fault between 0.3 and 0.5 mm/y. Additional investigations should be undertaken to refine surface fault mapping, the paleoearthquake calendar, the slip rate, and potential along-strike evolution, in order to constrain future seismic hazard analyses with fault models in this region of Eastern Kazakhstan.