Using the INQUA Scale for the Assessment of Intensity: Case Study of 14/08/2003 Lefkada Earthquake, Greece
by
George Papathanassiou
Department of Geology, Aristotle University of Thessaloniki, Greece
Coauthors: Pavlides Spyros

The Lefkada (Ionian Islands, Greece) earthquake of 2003 (Mw=6.2, Ms=6.4) occurred few kilometers offshore the northwestern part of the island. According to the National Observatory of Athens, Institute of Geodynamics (NOAGI), its focus was located at 38.790 N, 20.560 E at depth h = 12 km. The maximum intensity has been evaluated as VIII (EMS), Papadopoulos et al. (2003), at Lefkada municipality. Few hours after the event, field surveys have been conducted in order to report the ground failures triggered by the earthquake.

The aim of this research is to compare the intensity parameters of the 2003 earthquake based on the EMS scale, with those assessed using the INQUA intensity scale. The intensity values, provided by the INQUA scale, are based solely on the ground effects.

This event was selected because of: a) the shock caused ground failure at several sites while the structural damages were few, b) the historical seismicity of the area indicates that at least two events during the 20th century (1914 and 1948) induced similar phenomena on the island, and c) these two past events occurred before the application of the first Greek seismic code (1959) to the building's construction.

In particular, the 2003 earthquake caused considerable effects on the northern part of the island where the reported rockfalls-landslides were widespread while liquefaction occurrences were observed at the municipality of Lefkada. On the southern part, the environmental effects are classified as marginal to modest; the rockfalls were rare and occurred along slopes where equilibrium is unstable. Building damages were reported mainly at the town of Lefkada (northern part) while many port facilities have been damaged in the whole island. Intensity values in the range between V to VIII have been assessed by Papadopoulos et al. (2003) for the 14/08/2003 event based on the EMS scale.

The historical records of the island provide well-documented information about ground failure triggered by two past earthquakes (27/11/1914 and 30/06/1948). According to the available seismic catalogues (e.g., Papazachos et al. 2000) these events had approximately the same source characteristics as the shock of 2003. Moreover, the reported environmental effects were distributed at the same sites of those triggered by the last earthquake. Galanopoulos (1950) reported that rockfalls and sand crater, probably due to densification, triggered by the 1914 and 1948 earthquakes at the road of Tsoukalades-Ag. Nikitas and the Pefkoulia beach respectively (similar phenomena were observed during the field observations, few days after the event of August 14th, 2003). The induced structural damages were significant, since many houses collapsed while others were severely damaged. For the seismic events of 1914 and 1948 the intensity values were in the range between VIII to IX based on Rossi-Forel scale, Critikos (1916), and between VII and X based on Mercalli-Graden scale, Galanopoulos (1950), respectively. These values had been assessed for the northern part of the island. The maximum epicentral intensities of these past earthquakes were evaluated as IX (MM scale) in both cases according to Papazachos et al., 2000.

The first Greek seismic code was issued in 1959, after the devastating Kefalonia's earthquake of 1953, and it was revised in 1984. A new revision took place in 1992 and updated in 2000 and 2003. According to the 1992 Greek Seismic code, the PGA coefficient for the Lefkada's seismic zone was established to a=0.36g with a spectral magnification factor bo = 2, 5. Obviously, the implementation of this code contributed to the amelioration of the building strength. Therefore, the low-rise buildings of Lefkada were not heavily damaged by the event of 2003.

The intensity values of the 2003 earthquake (INQUA scale), that were assessed at 18 sites on Lefkada's island based on field observations, were compared with those provided by Papadopoulos et al. 2003 and with the evaluated intensity parameters of the past events. This comparison's conclusions are:

a) the major differences between the evaluated intensity parameters for the 2003 event, based on INQUA and EMS scale, are concentrated mainly at 4 sites, i.e. at the villages of Nydri (VII- INQUA, V EMS scale), Lygia (VII INQUA, V+ EMS), Kalamitsi (VII INQUA, V EMS) and Ag. Nikitas (VII+ INQUA, VI+ EMS),

b) at the same sites are also observed significant differences among the intensity values of the past events with the values of the last one, based on EMS scale,

c) the intensity values based on INQUA scale are more closed to the assessed values of the past events than the evaluated intensities based on EMS scale.

Date received: July 4, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-22.