Paleoseismologic investigation of the fault rupture of the 14 April 1928 Chirpan earthquake (M 6.8), southern Bulgaria

Seventy‐five years after the destructive Chirpan earthquake of 14 April 1928, we conducted a paleoseismologic study of the causative fault combining a review of contemporary literature, geomorphology, geophysical prospecting, and trenching. We reidentified the fault scarp in the field, and mapped it over a distance of 12.5 km. Geophysical profiles and boreholes demonstrate that Chirpan scarp is the surface expression of a normal fault that was active throughout the Pleistocene and Holocene. In 2002, we excavated a paleoseismologic trench to study the faulting history. A narrow fault zone separates Plio‐Pleistocene alluvial sand in the footwall from Holocene alluvial and colluvial silt in the hanging wall. The 1928 earthquake is recorded by 0.45 m vertical offset of the topsoil, in accordance with contemporary descriptions. We identified three colluvial wedge‐like units in the hanging wall sediments next to the fault, evidencing at least three surface‐rupturing paleoearthquakes since the Atlantic. Their timing could only be loosely constrained using pollen. The penultimate event had an offset of 0.40–0.45 m and occurred after circa 2600 calibrated years before present (cal years B.P.). Event 3 displaced a Subboreal semiarid calcic soil 0.55–0.70 m between circa 5750 and 2600 cal years B.P. The fourth event had a minimal offset of 0.50–0.70 m and occurred between circa 8900 cal years B.P. and 4900 B.C., when the region was first settled. We obtain a Holocene fault slip rate of 0.22 ± 0.12 mm/yr and an average recurrence interval of 2350 ± 643 years for earthquakes comparable to or larger than the 1928 event.