570 Bulletin of the Seismological Society of America, Vol. 92, No. 2, pp. 570–580, March 2002 Accelerating Seismic Crustal Deformation in the Southern Aegean Area by C. B. Papazachos, G. F. Karakaisis, A. S. Savvaidis, and B. C. Papazachos Abstract A region of intense accelerating seismic crustal deformation has been identified in the southwestern part of the Hellenic arc (broader area of Cythera island). The identification is performed using a detailed parametric grid search of the broader southern Aegean area for accelerating energy release behavior. The identified region has similar properties with past preshock (critical) regions, which have been identi- fied for strong mainshocks in the Aegean area. Based on such observations, which suggest that this region is at a critical state that can lead to a critical point, that is, to the generation of a mainshock, an estimation is made of the possible epicenter coordinates, magnitude, and origin time of this oncoming large (M 7.0) earthquake. The estimation procedure is validated on the basis of retrospective analysis of strong events in the Aegean area, as well as by appropriate application on synthetic random catalogs. These results, the existence of similar observations of accelerating seismic deformation in eastern part of southern Aegean and independent information on the time distribution of large earthquakes (M 6.8) for the whole southern Aegean indicate that the generation of strong earthquakes in this area in the next few years must be considered as very probable. Introduction The southern Aegean (34° N–38.5° N, 19° E–30° E) is seismically one of the most active parts of western Eurasia (Fig. 1). The most dominant feature of the area is the Hel- lenic trench, where subduction of the eastern Mediterranean lithosphere (front part of the African lithospheric plate) takes place under the Aegean microplate. To the north of the trench, the sedimentary part of the Hellenic arc (Peloponnesus– Cythera–Crete–Rhodes) represents the accretionary prism. Moving farther north we can identify the other typical ele- ments of a subduction system, namely the southern Aegean trough (sea of Crete) and the volcanic arc (Sousaki, Meth- ana, Milos, Santorini, and Nisyros). Shallow- and inter- mediate-depth earthquakes with magnitudes up to about 8.0 have occurred in this area (e.g., Papazachos, 1990). The main geotectonic features of the southern Aegean are shown in Figure 1, whereas the inset in Figure 1 shows the broader area with the solid-line box delineating the investigated area. During the time period 1948–1957 four shallow main- shocks with magnitudes 7.1 (9.2.1948, 35.5° N–27.2° E), 7.0 (17.12.1952, 34.4° N–24.5° E), 7.5 (9.7.1956, 36.6° N–26.0° E) and 7.2 (25.4.1957, 36.5° N–28.8° E) occurred in this area (magnitudes reported hereafter are moment magnitudes or equivalent moment magnitudes, following Papazachos et al. [1999]). Before all these mainshocks, accelerating inter- mediate magnitude seismicity (typically in the range M 4.5– 6.0) was observed (B. C. Papazachos and C. B. Papazachos, 2000; C. B. Papazachos and B. C. Papazachos, 2000). A similar accelerating seismicity release pattern that has started several years ago is currently observed in southern Aegean and is the main factor that motivated the present study. An additional factor is the lack of generation of strong earth- quakes in the entire southern Aegean in the past several years (Papazachos and Papazachou, 1997). Observations on accelerating intermediate magnitude seismicity have been reported during the last 40 yr by vari- ous researchers (e.g., Tocher, 1959; Mogi, 1969; Sykes and Jaume, 1990; Knopoff et al., 1996). However, during the last decade the related research efforts have been intensified, as it has been realized that the process of generation of in- termediate magnitude shocks (preshocks) can be considered as a critical phenomenon and the largest earthquake (main- shock) as a critical point (e.g., Sornette and Sornette, 1990; Sornette and Sammis, 1995; Huang et al., 1998; Jaume and Sykes, 1999). By the term intermediate magnitude we usu- ally indicate seismicity around 2–3 magnitude units less than the mainshock magnitude (Jaume and Sykes, 1999). A con- sequence of this concept is that the time variation of mea- sures of preshock–mainshock seismic crustal deformation (seismic energy, seismic moment, Benioff strain) follows a power law. Based on this law, Bufe and Varnes (1993) pro- posed the so-called time-to-failure method, which has been used in several attempts to predict earthquakes (Varnes, 1989; Sornette and Sammis, 1995; Bufe et al., 1994). Bow- man et al. (1998) proposed an algorithm to quantify the ac- celerating seismic energy release (specifically Benioff strain) by minimizing a curvature parameter, C, which is defined as