Relocation of large intermediate-depth earthquakes in the Vrancea region, Romania, since 1934 and a seismic gap

Abstract

We relocated M 6 or larger intermediate-depth earthquakes in Vrancea, Romania that have occurred since 1934. To determine their absolute hypocenters accurately, we combined them with recent (from January 1996 to November 2003) smaller earthquakes recorded by the Romanian local network and then relocated all of them simultaneously using a modified joint hypocenter determination method. Using the data on recent small earthquakes we first obtained an estimate of the dimensions of the active seismic region of intermediate-depth earthquakes: length, 85 km; along a NE-SW direction; width, 20 km; depth 60-160 km. We then determined that the 1940 (M 7.7), 1977 (M 7.5) and 1990 (M 6.9) major earthquakes were located near the NE edge of the seismic region, while the 1986 (M 7.2) was located inside the seismic region. The focal depths of the 1940, 1977, 1986 and 1990 major earthquakes were calculated to be 124, 98, 135 and 84 km, respectively and their source areas to range from 60 to 140 km in depth. The only unbroken area is at a depth of 140-160 km, which we current consider to be a seismic gap. Based on the regularity of past large earthquakes, we propose that the next M 7 intermediate-depth earthquake can be expected to occur in Vrancea at a depth interval of 140-160 km sometime early in this century.

Fault plane of the 1964 Niigata earthquake, Japan, derived from relocation of the mainshock and aftershocks by using the modified joint hypocenter determination and grid search methods

“…Since the effect of the difference is the same for all the earthquakes, relative locations are still reliable. To remove this artificial bias related to focal depths, Hurukawa et al (2008) introduced a new parameter a 1 , which can shift a group of earthquakes vertically. The larger the a 1 is, the deeper the calculated focal depths are.…”

The stress field of Vrancea region from fault plane solution (FPS)

“…duction rancea seismic zone is located in the South-East of Carpathian region and is characterized ximal seismic activity at intermediate depths, with epicenters distributed within the ellipse in 60×30 km 2 rotated by azimuth θ ≈ 45° according to the major axis (Radu and Polonic Constantinescu and Enescu, 1984) (Fig.1). t relocation of several hypocenters of seismic events has increased the accuracy of the main to 90×20 km 2 (Radulian et al, 2007;Hurukawa et al, 2008). in the high seismic activity of this region, different seismotectonic models were (Constantinescu et al, 1972;Radu, 1974;Constantinescu and Enescu, 1984;Oncescu, escu and Enescu, 1992;Linzer, 1996;Sperner, 1996;Matenco et al, 1997;Enescu and 998;Wortel and Spakman, 2000;Gvirtzman, 2002), but all of them are not in full e with the observational data (Bala, 2000).…”

“…The recent relocation of several hypocenters of seismic events has increased the accuracy of the ellipse domain to 90 × 20 km 2 (Radulian et al, 2007;Hurukawa et al, 2008).…”

Analysis the 1978–2008 crustal and sub-crustal earthquake catalog of Vrancea region

“…The Vrancea seismic zone is characterized by maximal seismic activity at intermediate depths, with epicenters distributed within an ellipsoidal domain 60×30 km 2 , rotated by azimuth θ ≈ 45 • with respect to the major axis (Radu and Polonic, 1982;Constantinescu and Enescu, 1984). The recent relocation of several hypocenters of seismic events has increased the accuracy of the ellipsoidal domain to 90 × 20 km 2 (Radulian et al, 2007;Hurukawa et al, 2008). According to instrumental macroseismic data, the released seismic energy in the Carpathian area originated from the Vrancea seismic area accounts for about 93 %, of which 95 % generated at intermediate depths (Radu and Polonic, 1982;Bala et al, 2003).…”