Geoengineering assessment of liquefaction hazard of the town of Larissa, Central Greece Un essaie pour estimer le risqué de liquéfaction a la ville de Larissa, Grèce Centrale G. Papathanassiou & B. Christaras Aristotle University of Thessaloniki, Department of Geology M. Nanas, Th. Pappas, K. Seggis, P. Staikos, V. Mermigas Georesearch G.P. Larissa, Greece ABSTRACT The goal of this paper is the assessment of the liquefaction hazard using data from borings with SPT. The town of Larissa is situated upon sediments, deposited by the river of Pinios which traverses the city, that are considered as potentially liquefiable regarding their depositional process. In addition, the region of Thessaly, where Larissa is located, is characterized as an earthquake prone area where the design acceleration on seismic bedrock is assigned as 0.24g according to the Greek seismic code. Moreover, liquefactioninduced ground deformations, triggered by the event of 1941, were reported at the town of Larissa according to historical seismicity. Therefore, a detailed geotechnical investigation is necessary in order to evaluate the liquefaction potential of the soil elements. Initially, geotechnical profiles of borings with SPT were collected and the factors of safety against liquefaction of the soil layers were estimated. Afterwards, the liquefaction potential index, LPI, of each boring and the possibility of liquefactioninduced surface disruption per site were computed in order to compile a liquefaction hazard map. The results of this study indicate the existence of liquefiable soil layers below the surface. However, the probability of liquefactioninduced surface disruption is very small at the urban area of Larissa for an earthquake magnitude and a PGA value of 7 and 0.24g, respectively. Only in two sites, at the northern part of the city, the percent of probability approach the value of 50% that can be defined as the boundary between the occurrence and nonoccurrence of liquefaction induced ground disruption. Keywords: Liquefaction, probability, Larissa, susceptibility, Thessaly, Greece 1 INTRODUCTION The town of Larissa located at central Greece, at the region of Thessaly. This area is characterized as medium to high seismicity due to the fact that strong earthquakes occurred at the 20 th century. Particularly, severe ground deformations such as surface faulting, rockfalls, liquefaction, ground cracks etc., triggered by the events of 1941 (M 6.3), 1954 (M 6.7), 1955 (M 6.6), 1957 (M 6.2) and 1980 (M 6.4), were widespread to the whole area. According to the Greek Seismic Code (EAK, 2000) Larissa located to the second seismic hazard zone where the design acceleration on seismic bedrock is assigned as 0.24g while the maximum earthquake magnitude for the same recurrence interval is M = 7 (Papazachos & Papazachou, 1997). The last decade, the town of Larissa was rapidly developed and the urban area was extended on new areas that were inhabitant in the past. Some of this areas included sites close to the river of Pinios, where the surface geology consisted by quaternary sediments. The initial scope of this paper was the evaluation of the liquefaction susceptibility of the sediments at the town of Larissa regarding the surface geology. In order to achieve this, the geological units of the urban area of Larissa were correlated to the recommendations of CDMG (1999). Furthermore, the liquefaction susceptibility of the subsoil layers was investigated based on the modified susceptibility criteria of Seed et al. (2003) while the liquefaction potential was computed according to the recommendation of Youd et al. (2001). For the assessment of the liquefaction susceptibility and the potential of the subsoil of the urban area of Larissa, data provided by borings with SPT were used. The earthquake parameters that were employed for the evaluation of liquefaction potential and the hazard at the town of Larissa were earthquake magnitude equal to 7 and PGA equal to 0.24g.