Implication of Secondary Geodynamic Phenomena on Co-seismic Interferometric Coherence
Issaak Parcharidis(1) , Michael Foumelis(2)
, and Efthimios Lekkas(2)
Harokopio University of Athens,
El. Venizelou 70, Kallithea,
176 71, Athens,
(2) National and Kapodistrian University of Athens, Panepistimioupolis, Ilissia, 157 84, Athens, Greece
The tectonic regime of the wider area of Lefkada island (Western Greece) is determined by the subduction of the African plate beneath the Aegean microplate. The Ionian Islands represents a transitional zone between the northern ending of the active subsidence and the continental collision, between the Apoulian platform and the Aegean microplate.
On August 14, 2003 at 08:15 local time (05:15 GMT) a magnitude 6.4R earthquake strokes the island of Lefkada. The epicenter was located off shore, about 30 km WNW of the town of Lefkada. The main shock was followed by three major after-shocks.
A significant number of earthquake-related phenomena such as ground fissures, landslides, rockfalls, soil liquefaction and coastline changes were observed. Moderate damages related to these phenomena were recorded mainly at the western and northern parts of the island. The primary and secondary road networks as well as a number of ports of the island were heavily affected by the earthquake-triggered phenomena. Local ground conditions played an important role to the distribution of the damages.
Displacement resolution recorded in differential interferograms (minimum phase difference recorded in each pixel) varies as the successful measurement of phase difference is largely determined by image coherence. Loss of coherence, a major limitation of DInSAR technique, occurs when the physical and/or geometrical nature of the ground changes (vegetation, water, ploughed field etc.), and as a consequence the stability of the phase signal is lost. Thus Interferometric coherence can be seen as a measure of the quality of the interferometric analysis.
However, coherence between two passes could be degraded and the measurement of phase difference becomes disrupted by too high gradient and chaotic deformation, caused by landslides, creeping, rockfalls etc. Such phenomena lead to incoherence if phase variation across a pixel exceeds one cycle.
The authors in a previous work have shown that the locations of secondary phenomena are in good agreement with the spatial distribution of ground displacement detected by means of Differential SAR Interferometry (DInSAR).
The present paper is focused on the degree of degradation of interferometric coherence caused by earthquake-triggered geodynamic phenomena. The study was based on coherence image produced by applying the two passes interferometric technique using two ENVISAT ASAR scenes acquired on 23/03/03 and 12/09/03 forming a co-seismic interferometric pair (Bp=53 meters). Although a fringe pattern was clearly recognized as the majority of the triggered phenomena occurred in that specific area, considerable noise is been inherited in the interferogram due to the lack of coherence. A correlation analysis between coherence values and the location of secondary phenomena recorded through field observation was performed. Additionally, temporal decorrelation caused by the ground cover changes (large parts of the island are covered by vegetation) is also taken in consideration.