Seismic tomography reveals a mid-crustal intrusive body, fluid pathways and their relation to the earthquake swarms in West Bohemia/Vogtland

Abstract

The region of West Bohemia/Vogtland in the Czech-German border area is well known for the repeated occurrence of earthquake swarms, CO 2 emanations and mofette fields. We present a local earthquake tomography study undertaken to image the Vp and Vp/Vs structure in the broader area of earthquake swarm activity. In comparison with previous investigations, more details of the near-surface geology, potential fluid pathways and features around and below the swarm focal zone could be revealed. In the uppermost crust, for the first time the Cheb basin and the Bublák/Hartoušov mofette fields were imaged as distinct anomalies of Vp and Vp/Vs. The well-pronounced low-Vp anomaly of the Cheb basin is not continuing into the Eger rift indicating a particular role of the basin within the rift system. A steep channel of increased Vp/Vs is interpreted as the pathway for fluids ascending from the earthquake swarm focal zone up to the Bublák/Hartoušov mofette fields. As a new feature, a mid-crustal body of high Vp and increased Vp/Vs is revealed just below and north of the earthquake swarm focal zone. It may represent a solidified intrusive body which emplaced prior or during the formation of the rift system. We speculate that enhanced fluid flow into the focal zone and triggering of earthquakes could be driven by the presence of the intrusive body if cooling is not fully completed. We consider the assumed intrusive structure as a heterogeneity leading to higher stress particularly at the junction of the rift system with the basin and prominent fault structures. This may additionally contribute to the triggering of earthquakes.

The role of crystallized magma and crustal fluids in intraplate seismic activity in Talala region (Saurashtra), Western India: An insight from local earthquake tomography

“…13), got crystallized and by decompression/differentiation formed the rhyolite and granophyric like rocks as reported in the region (Sethna, 2003). Similar to our observation, the presence of mid-crustal solidified intrusive body (higher Vp and higher Vp/Vs) below the earthquake swarm focal zone has earlier been reported for a non-volcanic regions (e.g., Kato et al, 2010;Mousavi et al, 2015) as well as volcanic region (e.g., Chiarabba and Moretti, 2006;Patanè et al, 2003). We consider the imaged crystallized mafic magma as a pronounced heterogeneity within the crust; and by reducing the crustal strength it provides a locale for the stress accumulation at its contact (McGinnis and Ervin, 1974).…”

“…Further, the seismic swarm-like earthquake activity at shallower depths may be due to the feeding of the crustal fluid, which was released from the mapped crystallized magma (Mousavi et al, 2015). The observed lower b-value (0.67, Rastogi et al, 2013) of the seismic swarm-like earthquake activity in this intraplate setting also indicates the possibility of increase in the pore pressure, possibly due to crustal fluids.…”

Three dimensional velocity structure and accurate earthquake location in Changning–Gongxian area of southeast Sichuan

“…Theoretically, the accuracy of the V S model from arrival time inversion is lower than that of the V P model, due to the lower accuracy of S wave phase picking. Therefore, the velocity ratio V P / V S is generally not determined by division of the inverted P and S velocities; instead, it is acquired from a simultaneous inversion for V P and V P / V S (Thurber, 1993; Thurber and Eberhart‐Phillips, 1999; Mousavi et al, 2015). Nevertheless, if the resolutions of P and S are almost the same and the scale of spatial inhomogeneity of the velocity structure is much greater than the wavelength, division of V p by V S can to some extent describe the true velocity ratio (Nakajima et al, 2001).…”

Locating mofettes using seismic noise records from small dense arrays and matched field processing analysis in the NW Bohemia/Vogtland Region, Czech Republic

“…Mousavi et al . () performed seismic travel‐time tomography of the larger NW Bohemia/Vogtland region. An area of increased P‐ to S‐velocity ratio, pointing to low S‐velocity, is visible south (and partly also north) of the main hypocentral zone, stretching from the hypocentral depth to the surface.…”

“…In spite of these observations, the relation between movements of fluids, CO 2 emissions, and earthquake swarms in the Cheb Basin is not yet fully understood, as the Nový Kostel epicentral zone is situated 10 km north from the main degassing area near Hartoušov. Mousavi et al (2015) performed seismic travel-time tomography of the larger NW Bohemia/Vogtland region. An area of increased P-to S-velocity ratio, pointing to low S-velocity, is visible south (and partly also north) of the main hypocentral zone, stretching from the hypocentral depth to the surface.…”