The October 2008 Nový Kostel earthquake swarm and its gas geochemical precursor

Abstract

A gas geochemical precursor anomaly was identified prior to the October 2008 Nový Kostel (Czech Republic) earthquake swarm with a peak magnitude ML of 3.8. This anomaly was observed as a deviation of CO2 concentrations from the long‐term annual CO2 concentration trend in the gas extracted from the scree at the Nový Kostel and Oldřišská gas monitoring stations, which are directly above the Plesná valley‐Počátky and Mariánské Lázně fault systems. Both sites are located within the major focal zone of the NW Bohemian swarm earthquake region at the northern edge of the Cheb Basin. A decrease in CO2 concentration started at Nový Kostel in September 2008, 17 days before the swarm, opposite to the usually increasing annual trend in the autumn period, and ended with a nearly coseismic drop immediately prior to the onset of the first swarm. The CO2 concentrations at Oldřišská, deviating from the annual trend, did not further increase after August 2008. The calculated horizontal strain field, based on the data of two permanent Global Navigation Satellite Systems stations, proved there was horizontal compression in this period. The increasing compression along the Plesná valley‐Počátky and Mariánské Lázně fault systems during the stress build‐up reduced the fault permeability prior to this earthquake swarm as indicated by the decrease in CO2 concentration. The 17‐day duration of the earthquake precursor at Nový Kostel and about 65 days at Oldřišská lie within the range of the precursor times that are hypothesized worldwide for an ML = 3.8 earthquake. The nature of earthquake precursors and their origin are discussed, for example, as an indication of changed fault permeability by stress build‐up in the case of the Nový Kostel swarm earthquake precursor or as fault opening in other cases.

Short-Term Seismic Precursor Anomalies of Hydrogen Concentration in Luojishan Hot Spring Bubbling Gas, Eastern Tibetan Plateau

“…The high uncertainty and low predictability of earthquakes during forecasting make earthquakes one of the worst natural disasters, which may lead to the instant loss of lives and properties (Gupta, 2001;Wen et al, 2008). The anomalously high concentration of helium, hydrogen, carbon dioxide and radon in hot spring bubbling gas have been widely studied in seismically active faults in recent years, especially in Japan, China, Italy and United States, in hunt of changes that might be premonitory of for earthquake prediction (King, 1986;Sugisaki and Sugiura, 1986;Nagamine, 1994;Cicerone et al, 2009;Babuška et al, 2016;Weinlich et al, 2016;Fischer et al, 2017;Huang et al, 2017). Temporal abnormal variations of gas (He, H 2 , CO 2 , Rn, CH 4 ) concentration have been observed with durations of a few hours to several days before and after a lot of large earthquakes at some gas monitoring stations at the epicentral distances of up to several hundreds of kilometers, which are usually related to the chemical and physical changes occurring in the active faults as enhanced/reduced water-rock interaction, crustal stress/strain, permeability structure, etc.…”

“…Temporal abnormal variations of gas (He, H 2 , CO 2 , Rn, CH 4 ) concentration have been observed with durations of a few hours to several days before and after a lot of large earthquakes at some gas monitoring stations at the epicentral distances of up to several hundreds of kilometers, which are usually related to the chemical and physical changes occurring in the active faults as enhanced/reduced water-rock interaction, crustal stress/strain, permeability structure, etc. before and after earthquakes (Sugisaki et al, 1983(Sugisaki et al, , 1996Cicerone et al, 2009;Umeda et al, 2013;Weinlich et al, 2016). The large change of gas geochemical anomalies as regards shape, duration, delay/ anticipation time and parameters-association with respect to foreshocks, main-shock and aftershocks are considered to be highly site specific (King, 1986;Sugisaki et al, 1996).…”