A Century of Aftershocks Related to Large Intracontinental Earthquakes: A Seismic Behavior for Hazard Assessment.

Abstract:

The sequence of 4 large earthquakes of magnitude 8 and more, which occurred in Mongolia and its border during the XX century, is unique in the world. Starting in 1905 July 7 by the Tsetserleg earthquake of Mw≈8, it was followed by the great Bolnay earthquake the 1905 July 23 Mw≈8.4, the FuYun earthquake the 1931 August 10 Mw≈8, and the Bogd (Gobi-Altay) earthquake the 1957 December 4 Mw≈8.1. The seismicity recorded since 1964 by the RCAG (Research Center of Astronomy and Geophysics, Mongolia) is concentrated, still up to now, along the large surfaces ruptures induced by theses four events with a cumulative length of more than 1000 km. We do not observe such high seismicity along the other active faults in the region that produced similar large events several centuries ago. The recurrence period estimated of these large earthquakes is more than 3000 years related to the low deformation rate along the faults (estimated between 1 to 3 mm/year). Thus, an important part of the observed seismicity is related to “late aftershocks”.

Despite the seismic network started to cover Mongolia only by 1964, it covers a period allowing us to reconstruct one century of aftershocks decay by combining these 4 events of “similar” magnitude. The completeness magnitude of the catalog is considered over various periods. Yet, the area of the 1905 events has a more complex aftershocks behavior probably due to the proximity of the Hovsgol extension basin at the north and the close location of the two 1905 events (the rupture of the Bolnay event touches the ones of the Tsetserleg). We show the very long duration of aftershocks related to such great intracontinental earthquakes. Therefore, the hazard assessment in such context should include the aftershocks as a characteristic of the area.