Vrancea slab earthquakes triggered by static stress transfer

Abstract

Abstract. The purpose of this paper is to study the interaction of the Vrancea seismic activity (Romania) in space as result of Coulomb, static stress transfer during M = 7+ events. In this area, three large events occurred in 1977, 1986 and 1990 at mid-lower, lithospheric depths and with similar focal mechanisms. Assuming elastic rheology for the deforming rocks it is suggested that frictional sliding on pre-existing fault produced the 1986 M = 7.1 event (depth 131 km), that was possibly triggered by the 1977 M = 7.4 event (depth 94 km). We calculated a static stress transfer of 0.52-0.78 bar to the hypocentre of the 1986 event. On the contrary, the occurrence of the 1990 event is uncertain: it is located inside the relaxed (shadow) zone of the combined 1977 and 1986 static stress field considering an azimuth for maximum compression of N307 • E. It follows that, the 1990 earthquake most likely represents an unbroken patch (asperity) of the 1977 rupture plane that failed due to loading. However, if a different compression azimuth is assumed (N323 • E) then the 1990 event was also possibly triggered by static stress transfer of the 1977 and 1986 events (combined). Our modeling is a first-order approximation of the kind of earthquake interaction we might expect at intermediate lithospheric depths (80-90 to 130-140 km). It is also suggested that static stress transfer may explain the clustering of Vrancea earthquakes in space by the rupturing of two (possibly three) NW-dipping major zones of weakness (faults) which accommodate the extension (vertical elongation) of the slab.

Earthquake mechanism and characterization of seismogenic zones in south-eastern part of Romania

“…The angular dia- grams for the largest Vrancea earthquakes (M w ≥ 4.7) recorded between 1977 and 2013 based on the CMT fault-plane solutions (http://www.globalcmt.org/) are plotted in the Figure 7. There are 20 events partly considered in the papers of Ganas et al [2010] and Radulian [2014]. They show the same NE-SW tendency of fault plane orientation as for the entire data set of subcrustal earthquakes in Figure 6.…”

Stochastic Simulation of Ground Motions Generated by Vrancea Intermediate-Depth Seismic Source

“…The magnitude of the seismic fault according to Oth et al [16] was 1.2×1.8 km and the stress drop was 75 bars [18]. From the point of view of the rupture mechanism, according to Ganas et al [18], the earthquake was produced at a 219 o strike angle, the dip being 81 o , with a rake of 107 o .   .…”

“…Starting from the assumption that in the simulations made by Boore [11], Pavel and Văcăreanu [25], Oth [16], Sokolov [20] the stress drop parameter was chosen higher than could be found in the specialized literature (for example, for the earthquakes of 1977, 1986 -Ganas et al, [18], Oncescu and Bonjer [29]), the stress drop parameter was changed from 75 bars to 300 bars, and 4 sets of 400 simulations for the Source 1, H/V amplification profile were made. An average arithmetic of peak simulation type acceleration ranging from 17.97 cm/s 2 to 41.92 cm/s 2 were obtained (Table I).…”

Assessment of the ground motion levels for the Vrancea (Romania) November 1940 earthquake

“…The corresponding stress drop values are taken from the work of Gusev et al (2002) and from the paper of Ganas et al (2010). According to the paper of Gusev et al (2002), Enescu et al (1979) assigned a stress drop value of 13.3 MPa for the Vrancea 1940 earthquake, which represents a value much larger than the ones computed for the Vrancea seismic events of 1977, 1986 and 1990. The simulations are performed with the software SMSIM (Boore 2005).…”