Cephalonia Island

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Terrafirma Case Study of Cephalonia, Greece: An Earthquake Preparation Zone

Overview

The area of the central Ionian Sea in Greece, including the islands of Cephalonia and Zakynthos (Fig. 1), is tectonically active. Large earthquakes (M>7) and associated intense co- and post-seismic ground motion have taken place in the past. It is the area of Europe which has the highest seismic energy release, with the most recent large earthquake centred near Lefkas island, to the north of Cephalonia, on a major fault which continues southwards just to the west of Cephalonia. That earthquake, in August 2003, had a magnitude of 6.5 with an aftershock sequence stretching over 80km along the northern section of this fault. A detailed study using Persistent Scatterer Interferometry (PSI) covering the islands, has contributed to a greatly improved understanding of the tectonic behaviour and regime of the broader area, helping to identify a possible pre-earthquake deformation process. This case study focuses on Cephalonia Island, and confirms a dramatic change in tectonic behaviour during 2005.

 Figure 1.  Study area: Cephalonia-Ithaca and Zakynthos islands of Greece

Measuring Deformation using PSI

Technically, the PSI products for Cephalonia were based on 39 descending images from the European Space Agency’s ERS satellite covering the period 1992-2000, and 21 ENVISAT images for the period 2003-2008. More than 860,000 persistent scatterers in descending geometry and 945,000 in ascending geometry were identified, within an areal extent of about 1400 km². Each of these points acts, effectively, like a GPS station (principally for vertical movements) giving an accuracy of better than 1mm/year. Together, they provide a high density network mapping millimetric movements across the island over many years. The ERS PSI products revealed a linear velocity history up to the year 2000 with a data gap until 2003 when ENVISAT was launched. Then, strong deviation from linearity was observed in the ENVISAT PSI product of Cephalonia-Ithaca, signalling a change in the regional tectonic regime of the area from 2005.

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Validation of  PSI Results

PSI products were compared, validated and integrated with geological and tectonic data, together with Differential GPS (DGPS) measurements from local networks established on the islands since 2001. The GPS results showed the same motions and, importantly, changes of motions that were revealed by the PSI study although GPS had a lower density of coverage. This agreement from independent approaches provides the principal validation with geological and seismological data giving further corroboration.

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Interpretation

The ERS and ENVISAT PSI products covering Cephalonia reveal two distinct deformation patterns (Fig. 2a,b), supporting, with much greater detail, DGPS results for the period (2001-2010). The first period (1992 to 2003) generally shows a minor, almost linear subsidence (around 1 mm/yr) which is consistent with the expected neotectonic movements of the island. The second one (2003 to 2010), showing non-linear motion, has been tentatively attributed to dilatancy (a bulging of the Earth’s crust in the area), in which reversal from subsidence to uplift (2-4 mm/yr) occurred mainly along the southern and south-eastern parts of the island, with even greater uplift (>4 mm/yr) taking place on the western part ofCephalonia. Detailed PSI analysis showed that these high velocity, non-linear rates of uplift, started in about mid-2005 with increasing rates in the southern part of the island but decreasing rates in the western part. This pattern may indicate a major regional crustal deformation process in an environment that has previously supported large magnitude offshore earthquakes. The uplifted areas along the southern and western parts were attributed to extensive regional dilatancy taking place offshore and south-west of Cephalonia. This is also supported by the calculated strain field deduced from the DGPS (Fig. 3), where compression is observed along the uplifted areas at the western and the southern parts of the island.

Figure 2. The PSI results from ERS (a) and ENVISAT (b) satellites

Figure 3. The GPS velocity and strain field of Cephalonia – Ithaca for the period 2001-2010

Figure 4 shows a snapshot of the seismicity of the region for the period 2004 to 2006. It includes the aftershock sequence of the August, 2003, magnitude 6.5 earthquake to the north and north-west of Cephalonia which depicts the major offshore fault that continues to the south-west ofCephalonia. The new deformation data, together with a broader analysis of the longer term seismicity pattern, have indicated a seismically critical area south and south west of Cephalonia.

Figure 4. Seismicity of the broader area showing the aftershock sequence (blue dots) of the August 2003, magnitude 6.5 earthquake
against a short sample of the background seismicity around Cephalonia island (centre).

Conclusions

The Terrafirma PSI products based on the ERS and ENVISAT data sets show distinct ground deformation patterns as they changed from a linear, small subsidence regime to a non-linear, significantly uplifting deformation.  The ENVISAT data (2003-2008) confirmed ground motion change first indicated by the DGPS observations in Cephalonia, and more precisely dates the principal onset of change to mid-2005. The PSI and DGPS data, combined with the seismicity pattern analysis, have defined a seismically critical area south and SW of Cephalonia, identifying a possible pre-earthquake deformation process in a region which has the highest seismic energy release, and which has suffered some of the largest earthquakes, in Europe. Recognition of this possible precursor tectonic activity may prove to be a landmark result in the use of geophysical data (PSI plus GPS) to improve the understanding of seismic hazard and the forecasting of large destructive earthquakes.