Large Earthquakes in Low-Strain Regions of Central Asia, Mongolia, and Iran

Abstract:

Although often characterized by an apparent low level of seismicity, areas of low-strain accumulation are subjected to considerable seismic and associated natural hazards. However, we are often unaware of these hazards, because in contrast to active plate boundaries, recurrence intervals of ground-rupturing earthquakes are typically on the order of thousands of years. Geodetic or seismicity records may thus not yet reflect full seismic cycles. Low-strain regions may occur in stable continental interiors or tectonically active intracontinental mountain belts, which often comprise areas of spatially disparate reverse and strike-slip faulting, up to thousands of kilometers away from plate boundaries. These regions may be shortened at rates on the order of centimeters per year, with no clear spatiotemporal pattern of seismicity, and single fault-slip rates may attain less than one to a few millimeters per year.

The Kyrgyz and Kazakh Tien Shan, the Mongolian Hangay or the Iranian Alborz mountains are prime examples that share these characteristics. These regions are located along the northern rim of the ongoing Indian-Eurasian or Arabian-Eurasian collision, but are hundreds of kilometers away from the corresponding plate boundaries. Historical earthquakes M > 7 are known from the Alborz mountains and several events with M > 8 are known from the northern Tien Shan and Mongolia – all with poorly constrained recurrence rates.

We review our ongoing tectonic studies of these areas, which differ in faulting mechanisms, the preservation potential of primary and secondary rupture evidence, and the level of population density and value concentrations. We use paleoseismic trenching, geomorphic and terrestrial LiDAR data analysis, and geochronology to gain insights into the complex deformation processes that govern these areas to better understand how present-day deformation is accommodated in areally extensive deformation zones in continental interiors.