Combined Ground Deformation Study of Broader Area of Patras Gulf (w. Greece) Using Psi-wap, Dgps and Seismicity Analyses

Long-term ground deformation monitoring using the Persistent Scatterer Interferometry Wide Area Product (PSI-WAP) technique for the period 1992-2003, combined with Differential GPS measurements and seismicity analysis has provided useful information about the tectonic motions of the tectonically complex area of Patras Gulf (Western Greece), and lead to new insights on the geotectonic regime of this region. Descending ERS radar images were used to compile the PSI-WAP product that has been calibrated using the absolute velocity field of available GPS stations in the area. It has been found that the deformation of the southern part of Patras Gulf near the coastline has been characterized by considerable subsidence (>-5mm/yr), where unconsolidated sediments usually prevail, compared to the northern part of the gulf. Significant subsidence has also been identified in areas along the down-throw side of possible faults, as well as areas where extensive ground water pumping has occurred for irrigation. These results correlate well with local GPS and seismicity data. 1. BRIEF GEOΤΕCTONIC SETTING The Eastern Mediterranean is one of the most active tectonic areas in the world [1], resulting from the convergence between the Eurasian and the African continental plates. The majority of the seismic activity is located in the Hellenic territory along the Hellenic Arc, the subduction zone of the African plate beneath the Aegean microplate, with the highest activity observed on the western part of the arc that includes the Central Ionian Islands [1]. Due to the compressional forces applied in the Aegean lithosphere, resulting in a southwestward (relative to Eurasia) movement of the plate, the Aegean domain undergoes crustal extension in a marginal basin environment behind the active subduction system of the Hellenic outer arc. This extensional stress field is concentrated in several zones of graben development, both in front and behind the active volcanic inner arc of the Aegean. In the following, a brief description of the main geotectonic setting is summarized (after [2]). Thrusting of basement and sedimentary rocks is currently active beneath the Hellenic Trench while thin-skinned deformation of sedimentary strata, detached mainly within Triassic salt horizons, occurs outboard of the trench, but does not appear to involve the underlying basement [3]. Extensional faults and linked strike-slip faults of the Central Hellenic Shear Zone cut and disrupt these underlying nappe structures [4], but the converse is not generally observed [5]. These relations also indicate that the extension and strike-slip faulting near the western coast of mainland Greece probably did not begin until Pliocene time [6]. The morphology of the study area reflects the older structures created by thrusting in the Hellenides and also the younger, superposed structures that disrupt them. The broad topographically high areas of western Greece generally correspond to regions of crustal thickening which developed in tandem with thrust faulting and folding. Here, thrust faults and associated folds typically trend NW to NNW, sub parallel to the Hellenic trench. A detailed geological map of the broader area of Patras Gulf is depicted in Fig. 1. It can be seen that alluvial deposits are prevalent in low-lying regions, while limestones of various ages are characterizing the mountainous areas. 2. SEISMICITY ANALYSIS The broader area of Patras Gulf is a very active tectonic area (Fig. 2) that is characterized by intense seismic activity. A large number of earthquakes of magnitude Mw>5 has occurred in the vicinity of the area over the last 30 years. The most notable large-magnitude seismic events are: Figure 1. Detailed Geological Map of the broader area of Patras Gulf, showing also the main faulting features (IGME, Geological Map of Greece, 1983) • 1993: Patras Earthquake (Μw= 5.4) • 1995: Aegion Earthquake (Mw=6.4) • 2002: Vartholomio Earthquake (Mw=5.6) • 2008: Andravida Earthquake (Μw= 6.4) The focal mechanisms of these last three major earthquakes have shown a NE-SW trending zone that crosses the area. Several others events have also occurred adjacent and to the west of Patras Gulf, mainly along the Cephalonia Transform Fault and south of Zakynthos Island [7], [8]. A prominent feature of the seismicity map is the absence of activity in the marine area of gulf when compared to the broader region. This absence of seismicity may be indicative of the lack of major faulting systems in the area, or at least active faults. It has to be noted, though, that the submarine area has not been fully mapped. A major seismic event took place to the east of Patras Gulf in June 1995, the Aegion Earthquake [9]. On June 8, 2008, the Andravida Earthquake, with Mw=6.4 occurred on NW Peloponnese in the vicinity of the Riolos (RLSO) Continuous GPS (CGPS) station (e.g. [10]). Low seismicity was observed in the epicentral area over the past few decades. No historical large events have been reported in this area, where furthermore no evidence of surface faults exists. Strike-slip faulting has been observed in the surrounding region. The source parameters of the Andravida Earthquake were calculated using body-wave modeling. The focal depth was estimated to be 23 km, while the fault plane solution indicates dextral strikeslip faulting, oriented in a NNE-SSW direction. The slip distribution indicates complex rupture with propagation towards the north. The focal mechanism of the main shock showed a right lateral strike-slip motion. The spatial distribution of the aftershocks covers an area of about 40km in length and oriented in a NNE-SSW direction, in agreement with the constrained focal mechanism of the main shock. The length of the main rupture was approximately 25km, while the aftershock area extends to more than 40km. The seismicity at the two edges of the activated fault can be attributed to stress transfer caused by the main shock, as revealed by Coulomb Stress Analysis [11] in the area. Note that the causative fault was striking NE–SW and dipping 85°NW. It was of right-lateral slip. Thus, this fault acts as a transition between the extensional area of Patras Gulf–Corinth rift and the compressional area of the Ionian Sea. There is no historical evidence that the 2008 fault segment was ruptured by strong earthquake in at least the last 300 years. Figure 2. Seismic activity in the broader area of Patras Gulf (Mw>3.6) for the period 1980-2012. Low seismicity was observed in the epicentral area for the years 1964-2007, setting it as a likely candidate for a forthcoming event [11]. Strike-slip faulting has been observed in the surrounding region, while the type of motion in the study area remained under discussion. More specifically, the two moderate Chalandritsa events (Mw=4.5) occurred NE of the focal area four months before the Andravida Earthquake. In addition, reverse faulting dominates along the Hellenic Arc to the west, while to the NE in the Gulf of Corinth the main active faults are normal with an approximately E-W orientation. After the occurrence of the Andravida Earthquake, an important number of aftershocks were located using data from the Hellenic Unified Seismological Network (HUSN). The best located events for the period 8-26 June 2008 were 490 [12]. Following, cross-correlation and nearest-neighbor linkage [13] were applied, resulting in several multiple clusters. Finally, relocation was performed [14] with both catalogue and crosscorrelation differential travel-time data. The spatial distribution of the relocated aftershocks covers an area of about 40km in length with a NNE-SSW direction, in agreement with the constrained focal mechanism of the mainshock. Fault plane solutions of the largest aftershocks indicate a type of faulting similar to the one of the major event. The length of the main rupture was approximately 25km, while, as previously mentioned, the aftershock area extends to more than 40km. The seismicity at the two edges of the activated fault can be attributed to stress transfer caused by the mainshock, as revealed by Coulomb Stress Analysis [11]. Pick Ground Velocity (PGV) distribution [15] was obtained using the determined slip model. High PGV values between 17 and 20 cm/sec were observed at the northern edge, where many structural damages were reported, but in an area that was not ruptured. The observed damage can be attributed to stress transfer. 3. THE GPS MEASUREMENTS A GPS network (Fig. 3) was established in the broader area of Patras Gulf in August 1994 to study the crustal deformation, as well as the tectonic settings and motions at both local and regional scales. The Patras Network consists of ten stations equally distributed on both sides of the gulf (5 stations on the northern side and 5 stations on the southern side). The network was fully remeasured on three periods that is in August 1994, in October 1996 and in January 2006. The analysis presented in the following is based on [16]. The Bernese software v.5.0 [17] was used for the post-processing of all the GPS observations. On a regional scale, horizontal and vertical motion of the broader area was estimated with respect to ITRF2000 using data also from the CGPS station RLSO. A ground deformation having direction of ENE to NE with a rate of 10.6 22.2 mm/yr was observed for the northern part of Patras Gulf (Fig. 3a). The southern part exhibited an ESE to SE displacement with rate of 6.311.6 mm/yr. Concerning the vertical component, uplift was generally observed. The rate was about 3 mm/yr in the northern part of the gulf, while in the southern part the rate was higher ranging from 5.9-12.1 mm/yr (Fig. 3b). A distinct differential movement between the northern and the southern parts of the gulf is apparent from these images. An opening up of the gulf is evident from the horizontal vector, while the vertical component indicates uplift of the southern part compared to the northern part. The horizontal deformation vectors revealed an extension of the Patras Gulf: The southern part seems to be extending towards the SSW direction with a rate of 8-13 mm/yr, while in the north, there is a clear differentiation between the westernmost part (station No 35) and the eastern part. The main N-S faulting zone that crosses the area seems to affect these movements. The absence of seismic activity in the marine area of the gulf shows that the extension that takes place in the area is of an aseismic/plastic character on a soft soil environment, without active faulting zones. These results are consistent with geological [18], tectonic [19], seismological [9] and other GPS studies [20] in the area. Previous studies determined that the extension rate increases from 5 mm/yr at the eastern Corinth Gulf to 15 mm/yr on the west, near Aegion (e.g.[20]). A mean extension rate in the Patras Gulf of 8-10 mm/yr was determined based on the tectonic evolution of the area [4], while 20-30 mm/yr of dextral strike-slip occurs along the Cephalonia Transform Fault (e.g [8], [9], [20]).