Earthquake triggered rock falls and their role in the development of a rock slope: The case of Skolis Mountain, Greece
Graphical abstract
Introduction
A strong feedback exists between geological history, tectonics, lithology and geomorphological evolution of slopes (Jaboyedoff et al., 2011); this is the reason why various types of landslides respond in many ways to tectonic processes. In addition most of the slope features can be considered as the result of long-term geomorphologic evolution under climatic forcing and/or in cases of geologically active areas by the tectonic forcing during earthquakes. Otherwise, topography, lithology and rock mass damage appear to promote or control the rock slope failures (Reid and Iverson, 1992, Zêzere et al., 1999, Agliardi et al., 2013).
Landslides are commonly triggered in the epicentral area of earthquakes or in proximity with active faults (Keefer, 1984a, Keefer, 1984b, Bull et al., 1994, Tibaldi et al., 1995, Burbank and Anderson, 2001, Gallousi and Koukouvelas, 2007). Thus, mapping and analysis of the landslide distribution can be used for hazard assessments (Eisbacher and Clague, 1984, Keefer, 1984a). In addition, landslides in actively deforming areas often provide key data for understanding the delivering of material from hillslopes into valley bottom occupied by rivers, lakes, gulfs or glaciers (Molin et al., 2004, Korup, 2005a, Korup, 2005b, Gallousi and Koukouvelas, 2007).
For the Mediterranean and Greek climatic conditions, the relationships between landslides and extreme rainfall events have been extensively investigated (e.g. Crosta, 1998, Guzzetti et al., 2004, Koukis et al., 2005, Agliardi et al., 2013). However, the role of the earthquakes in landslides is poorly understood in Greece, since a limited number of case studies exist and information from historic data are limited (Koukouvelas et al., 1996, Christaras et al., 1998, Papadopoulos and Plessa, 2000, Ambraseys, 2009). In addition, landslide inventory data are poor for earthquake triggered landslides in terms of their dimensions and it is unlikely that very specific landslide-related parameters, such as material shear strength or phreatic surface levels, are mapped to the detail required for present day landslide susceptibility analysis.
On the 8th of June 2008, a strong earthquake of Mw = 6.4 struck northwestern Peloponnese called hereinafter as the Movri Mountain earthquake (Fig. 1). Northwestern Peloponnese and its surroundings Ionian Islands are located at the most tectonically and seismically active region of Greece (Papazachos and Papazachou, 1997, Ambraseys, 2009, Kokkalas et al., 2013 and references therein). The 2008 event was the largest earthquake to occur in northwestern Peloponnese during the past 30 years (Koukouvelas et al., 2010). The earthquake toppled primarily old buildings in the epicentral area and less reinforced houses and reinforced concrete buildings in the villages and communities nearby its epicenter (unpublished data of our laboratory). The secondary effects that were caused by the Mw = 6.4 earthquake, were landslides, and liquefaction phenomena near the epicentral area (Kokkalas et al., 2008, Koukouvelas, 2008, Pavlides et al., 2008, Koukouvelas et al., 2010, Papadopoulos et al., 2008, Papathanassiou, 2012). Santomerion village suffered by far the most damage due to its proximity to the steep slopes and its high elevation (Fig. 1). As a result, the largest volume of landslide materials, consisting of rock falls and boulders, hit the village. Due to this the Civil Protection Authorities temporarily evacuated the villages in order to avoid casualties and injuries in case of landslide reactivation during strong aftershocks.
The study of Skolis Mountain landslides supports a subdivision of slope failures into two principal types: (a) primarily rock falls and (b) minor rock slides sensu Varnes (1978). Additionally, we estimated rock falls related to seasonal rainfall, and we tried to test if the shadow angle (angle β) of 33°, 27.5° and 24° are applicable in our study area (sensu Evans and Hungr, 1993, Dorren, 2003). The angle β is defined by an imaginary line drawn from the release point down to the maximal runout point of the block. The angles β = 33° and β = 24° correspond with the maximum runout beyond the talus slope and the point where the boulders stop on the slope (Dorren, 2003 and references therein).
In this paper we investigate the distribution of rock falls and isolated rock falls over the period from 1945 until 2008 (63 years) across the Skolis Mountain and how these were modified during the Movri Mountain earthquake. Note that our study area is characterized primarily as forest and semi natural and thus human activity is rather limited (Fig. 1). Thus, the Skolis Mountain is a key example of earthquake triggered mass movements overlapping in time onto identical type of mass movements caused by climatic forcing. Phreatic surface is too deep in the area to have any impact on mass movements and thus this paper addresses: (a) The role of the inherited array of faults and joints to the localization of the slope failure on meteorologically or earthquake triggered landslides (Fig. 2). (b) The impact of the earthquakes in triggering meteorologically controlled rock falls. (c) An assessment of rock fall hazard due to earthquake activity by defining the boulder release zone and run-out zone of boulders.
Section snippets
Earthquake landslides and dispersal of boulders on slopes
According to Keefer, 1984a, Keefer, 2002, moderate to strong earthquakes have long been recognized as a major cause of landslides. Earthquake triggered landslides can be either located on active faults or in the epicentral area (Keefer, 1984a, Keefer, 1984b). Furthermore, earthquakes with magnitudes greater than 4.0 can trigger landslides on susceptible slopes, and earthquakes with magnitudes greater than 6.0 can generate widespread landsliding (Jibson and Keefer, 1993). Seismically induced
Materials and methods
Immediately after the Movri Mountain earthquake eyewitnesses provided us the information that the western bluff of the Skolis Mountain was entirely covered by a dust cloud. Testimonies of ten persons, being in the Santomerion village during the earthquake, describe how bunching blocks were crushing on the fences and demolished two houses. Also it is important to say that the bunching boulders started from the south of Santomerion and progressively covered all the way through the mountain's
Structural analysis
The Skolis Mountain is a fault related open asymmetric anticline 9 km long by 1.5 km wide (Fig. 2b cross sections A and B). The mountain consists of late Cretaceous–Eocene carbonates and late Eocene–Oligocene flysch in the surrounding area (Fig. 1, Fig. 2). The anticline is developed above an east facing thrust fault, compartmentalized into two splays, that is called hereinafter as the Skolis Thrust (Fig. 2b cross sections A and B). The thrust, currently referred as inactive and is characterized
Landslides at Skolis Mountain
The Skolis Mountain is a long morphological feature with N–S orientation with maximum elevation of about 965 m. It is characterized by steep slopes, especially at its western part. The height of steep slopes across the Skolis Mountain exceeds 100 m for the case of Santomerion, whereas in the case of Portai, the height of the limestone bluffs is in the order of 50–80 m, while on the northernmost end of the mountain slope does not exceed 10 m. The majority of unstable sites are located in the Skolis
Aspects of risk management
The local tectonic grain in the study area, which is mainly represented by Skolis Thrust Fault and an anticline, has formed and shaped the characteristic steep slopes of Skolis Mountain. The fold and joint related deformation has caused dense fracturing in the carbonate rocks which in combination with the erosion process and the last period seismicity, led to slope instability phenomena, which are manifested as rock falls. Thus, the Skolis Mountain located in an area prone to strong earthquakes
Discussion
The mapping of rock falls along the western slope of the Skolis Mountain indicated that these rock slope instabilities are controlled by long term climatic and tectonic forcing. Climatic forcing over the last 63 years has been outweighed by the earthquake activity. This raises the problem of rock fall risk during earthquakes in combination with the population growth development and construction engineering in natural slopes prone to strong earthquakes. In our case the earthquakes happened in the
Conclusions
In this study we map and analyze the geological evolution of primarily rock fall sites for sixty three years before the 2008 Movri Mountain earthquake (Mw 6.4) in NW Peloponnese. Our work represents an inventory of landslides which is composed of individual data and results, derived from field observations, structural mapping and orthorectified air-photo analysis across the Skolis Mountain. The results are used in order to understand the role of earthquakes in triggering and/or inflation of
Acknowledgments
The first author expresses his thanks to P. Xypolias and S. Kokkalas for their help during the fieldwork just after the earthquake and during the 1996 structural mapping of the study area. This work was supported by Grant E078 to V. Zygouri from the Research Committee of the University of Patras (Programme K. Karatheodori). The authors are grateful to the referees for their constructive and positive comments and the managing senior Co-Editor C. Carranza-Torres for insightful comments and
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