Abstract
In situ micro-displacement monitoring in caves at the seismically active junction of the Southern Alps, Dinarides and Southwestern Pannonian Basin revealed active tectonic micro-deformations. The largest total vertical movement of 0.35 mm (2008–2018) occurred at Kostanjeviška Jama (Southwestern Pannonian Basin–Dinarides). Two abrupt displacements observed at that cave location correspond to pre- and post-seismic episodes in 2014 and 2015. At Pološka Jama (Southern Alps), more gradual and continuous displacement on a monitored bedding plane of 0.13 mm down-slip (2008–2018) was observed. This movement is attributed to slope relaxation as well as tectonic activity on the Julian Alps thrust fault and the seismogenic Ravne Fault, which generated earthquakes at Mw = 5.6 in 1998 and Mw = 5.2 in 2004. At Jama pri Svetih Treh Kraljih in Dinarides, activity on the Ravne Fault was shown as sinistral-horizontal (0.1 mm) and vertical (0.15 mm) movement along an E–W-oriented tectonic structure. This shallowly buried site also experienced cyclic seasonal displacements. At Županova Jama, multiple short-term horizontal dextral and sinistral strike-slip pulses (2016–2018) demonstrated active tectonic micro-deformations within the wider zone of the Dobrepolje and Želimlje faults. At Postojnska Jama, two episodes of vertical movement on a NW–SE-oriented fault are associated with earthquake swarms: the first episode was in 2010 (MLV = 3.7) and the second in 2014 (MLV = 4.3). In-cave flood events do not coincide with periods of micro-displacement activity in the studied caves, and therefore are not the drivers of the micro-displacement.
Highlights
Caves represent natural archives of information beneath the Earth’s surface; they can be more than 100,000 or even more than a million years old and can be treated as “natural boreholes”
Microtectonic displacements detected with TM71 extensometers cannot be related to such small differences in air temperature and humidity
Micro-displacement monitoring using eight TM71 extensometers has been under way since 2004 in five Slovene karst caves at the seismically-active junction of the Southern Alps, the Dinarides and the southwestern Pannonian Basin
Summary
Caves represent natural archives of information beneath the Earth’s surface; they can be more than 100,000 or even more than a million years old and can be treated as “natural boreholes”. To study active tectonics in Slovenian caves by means of micro-displacement monitoring, the first two TM71 extensometers (Košťák et al 2007) were installed in Postojnska Jama in 2004. Another two caves (Kostanjeviška Jama and Pološka Jama) were equipped during 2008, and two more (Jama pri Svetih Treh Kraljih and Županova Jama) in 2011. Monitoring of micro-displacements using TM71 extensometers is taking place at numerous sites (~ 300) in Europe and worldwide and is commonly sited in karst caves in significant faults and fissured zones (Briestenský et al 2007, 2015, 2018; Gosar et al 2009; Šebela et al 2010a; Baroň et al 2016, 2019), along slope deformation structures (Petro et al 2004; Stemberk et al 2017; Baroň et al 2016), on surface outcrops within active tectonic zones (Košťák et al 2007; Stemberk et al 2015, 2019a) and in artificial tunnels (Petro et al 2005, 2011; Stemberk and Košťák 2008)
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