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

Mihaela Popa,Mircea Radulian,Nobuo Hurukawa

doi:10.1186/bf03353119

Abstract

AbstractWe relocatedM6 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 (M7.7), 1977 (M7.5) and 1990 (M6.9) major earthquakes were located near the NE edge of the seismic region, while the 1986 (M7.2) 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 nextM7 intermediate-depth earthquake can be expected to occur in Vrancea at a depth interval of 140–160 km sometime early in this century.

Highlights

Isolated intermediate-depth earthquakes occur regularly in the SE corner of the Carpathian arc in central Romania (Fig. 1; see Frohlich, 2006)
An M 7 intermediate-depth earthquake occurs in the Vrancea region of Romania on average every 20–30 years
Combining these and recent smaller earthquakes reported by the Romanian local network, we relocated all of them simultaneously using a Modified Joint Hypocenter Determination (MJHD) method (Hurukawa and Imoto, 1992; Hurukawa, 1995)

Summary

Introduction

Isolated intermediate-depth earthquakes occur regularly in the SE corner of the Carpathian arc in central Romania (Fig. 1; see Frohlich, 2006). The depths of these earthquakes range from 60 to 160 km, and their epicenters are concentrated in an area of 20 × 85 km in the Vrancea region. According to the Romanian earthquake catalog ROMPLUS (Oncescu et al, 1999), five earthquakes of magnitude (M) 7 or greater have occurred every 100-year period over the last 500 years These are intermediate-depth earthquakes, the larger events have caused severe damage in the city of Bucharest, located about 150 km from the epicenter, as well as in other cities in Romania and neighboring countries. In 1977 a moment magnitude (Mw) 7.5 earthquake caused 1,581 deaths

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Conclusion