Earthquake swarms in non‐volcanic regions: What fluids have to say

Abstract

The detailed processes generating earthquake swarms are complex and not fully understood. Most earthquake swarms occur in volcanic regions and mid‐ocean rifts. Here, we report new 3 He/ 4 He data of free gases monitored at CO 2 ‐rich degassing locations close to the Nový Kostel focal zone (NKFZ) located in the western Eger rift. The NKFZ is known for the recurrence of earthquake swarms at which the focal zone ranges between 6 and 12 km depth. At degassing locations neighboring to the NKFZ a progressive increase of mantle‐derived helium has been observed during the last 15 years – actually the highest 3 He/ 4 He ratios (>6 Ra) in Central Europe. The 3 He/ 4 He anomalies indicate hidden magmatic activity. We assume that the latest strong earthquake swarm in October 2008 was initiated by a hidden magma intrusion process from the upper mantle into the lower crust that has been indicated by a three month lasting increase of the 3 He/ 4 He ratios in spring 2006 at all degassing locations near the NKFZ.