Decelerating precursory seismicity in Vrancea E.M. Scordilis ⁎ Geophysical Laboratory, Aristotle University, GR-54124, Thessaloniki, Greece Received 1 December 2005; received in revised form 20 March 2006; accepted 16 April 2006 Available online 6 June 2006 Abstract Preshock seismic excitation followed by seismic quiescence has been observed in the seismogenic region of strong shallow mainshocks. The strain released by such preshocks is decelerating with the time to the mainshock and is fitted by a power-law with a power value larger than unit. This model is tested in the present work for the intermediate-depth earthquakes of the Vrancea region, generated in an isolated seismogenic zone proper for such testing. A backward application of this “decelerating preshock strain” model for the case of 4.3.1977 (M = 7.5) earthquake, for which reliable data are available, shows a good fit of the power-law pattern to the seismic activity preceding the main shock. The occurrence rate of recent intermediate-depth shocks in Vrancea indicates that this region is currently in a state of decelerating seismic deformation, which may lead to the generation of a strong intermediate-depth mainshock there at about the beginning of the third decade of the present century. The respective uncertainties are unknown due to lack of previous relative studies. © 2006 Elsevier B.V. All rights reserved. Keywords: Decelerating preshock strain; Intermediate-depth mainshocks; Earthquake prediction; Vrancea region 1. Introduction Considerable research work has been carried out during the last few decades for intermediate-term earthquake prediction on the basis of several preshock seismicity patterns. Two of these patterns have been observed before many strong earthquakes by several researchers and can be considered as the most significant. The first of these patterns lasts several years to a few decades and is characterized by accelerating seismicity, expressed by the generation of moderate magnitude earthquakes that occur before a mainshock in a broad region (critical region), which scales with the fault length of the mainshock (Tocher, 1959; Mogi, 1969; Raleigh et al., 1982; Papadopoulos, 1986; Sykes and Jaumé, 1990; Knopoff et al., 1996; Bowman et al., 1998; Brehm and Braile, 1999; Jaumé and Sykes, 1999; Papazachos and Papazachos, 2000; Tzanis et al., 2000; Robinson, 2000; Papazachos and Papazachos, 2001, among many others). The cumulative Benioff strain, S (sum of square root of seismic energy) varies with time, t, to the mainshock according to the following power- law proposed by Bufe and Varnes (1993): S ðt Þ¼ A þ Bðt c -t Þ m ð1Þ where, t c is the origin time of the oncoming mainshock and A, B and m (< 1) parameters that can be determined by the available data. The second pattern concerns the seismic quiescence (mainly expressed by the decrease of the frequency of Tectonophysics 420 (2006) 509 – 517 www.elsevier.com/locate/tecto ⁎ Tel.: +30 2310 991411; fax: +30 2310 991403. E-mail address: manolis@geo.auth.gr . 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.04.008