The last century has seen unprecedented urban growth in regions of earthquake hazard within the continents. In contrast to plate boundaries, where earthquake hazard is usually confined to narrow zones around the edges of the oceans, active faulting within continental interiors is spread across very wide regions, and with intervals of hundreds, or even thousands of years between large earthquakes in any one area. The long recurrence intervals pose challenges for identification of active faults, and the small database of recent large earthquakes limits our understanding of fault rupture processes in such regions. There is a growing realisation that active faults deep within continental interiors behave in ways that are unexpected given the present understanding of global earthquakes and fault rupture. Tectonic motions in continental interiors are accommodated across diffuse networks of faults, and the occurrence of large earthquakes over historical times suggests that they may be clustered in particular regions at particular times, and may have complex multi-fault sources. These factors have profound implications for the applicability of established scaling relationships to estimate future magnitude, as earthquakes could be significantly larger than expected for a given fault length.
Iran and central Asia offer a natural laboratory for the study of plate interior earthquake occurrence, due to the relatively large number of events, the long historical and archaeological records, and the pristine environment that allows identification and study of earthquake ruptures far into prehistory. In this presentation I describe ongoing efforts to build an understanding of the sources of major earthquakes that have occurred in past centuries and millennia. Particular focus is placed on a number of large earthquakes that occurred within the early to mid 20th century, which offer a bridge between the interpretation of modern and historic seismicity. We bring together elements of seismology, remote sensing, field geology, and quaternary dating to probe the rare, but potentially devastating occurrence of the largest earthquakes across the Asian interior.