Multiple geophysical investigations to characterize massive slope failure deposits: application to the Balta rockslide, Carpathians

Abstract

Summary Origins of ancient rockslides in seismic regions can be controversial and must not necessarily be seismic. Certain slope morphologies hint at a possible co-seismic development, though further analyses are required to better comprehend their failure history, such as modelling the slope in its pre-failure state and failure development in static and dynamic conditions. To this effect, a geophysical characterisation of the landslide body is crucial to estimate the possible failure history of the slope. The Balta rockslide analysed in this paper is located in the seismic region of Vrancea-Buzau, Romanian Carpathian Mountains, and presents a deep detachment scarp as well as a massive body of landslide deposits. We applied several geophysical techniques on the landslide body, as well as on the mountain crest above the detachment scarp, in order to characterise the fractured rock material as well as the dimension of failure. Electrical resistivity measurements revealed a possible trend of increasing fragmentation of rockslide material towards the valley bottom, accompanied by increasing soil moisture. Several seismic refraction surveys were performed on the deposits and analysed in form of P-wave refraction tomographies as well as surface waves, allowing to quantify elastic parameters of rock. In addition, a seismic array was installed close to the detachment scarp to analyse the surface wave dispersion properties from seismic ambient noise; the latter was analysed together with a co-located active surface wave analysis survey. Single-station ambient noise measurements completed all over the slope and deposits were used to further reveal impedance contrasts of the fragmented material over in-situ rock, representing an important parameter to estimate the depth of the shearing horizon at several locations of the study area. The combined methods allowed the detection of a profound contrast of 70-90 m, supposedly associated with the maximum landslide material thickness. The entirety of geophysical results was used as basis to build up a geomodel of the rockslide, allowing to estimate the geometry and volume of the failed mass, i.e., approximately 28.5-33.5 million m3.

Evaluating landslide response in seismic and rainfall regime: A case study from the SE Carpathians, Romania

“…The parameters and their values used in the static and dynamic analysis are mentioned in Table 1. Mreyen et al, (2021). ³Based on Hoek and Brown (1997) and field observation.…”

“…In view of the similar litho-tectonic conditions and spatial proximity, the shear-wave velocity (Vs) values in the present study are based on Mreyen et al (2021). For the loose overburden (soil) and rockmass, the Vs are taken as ~400 m/sec and ~900 m/sec, respectively.…”

“…In order to achieve the aforementioned objectives, a massive (~9.1 Mm²) landslide in the Vrancea Seismic Zone, SE Carpathians is chosen as a case study area. The region has been subjected to frequent earthquakes and relatively wet climatic conditions that induce frequent landslides and related socio-economic losses (Micu et al 2013;Micu, 2019;Mreyen et al 2021).…”

“…1 It was inferred from the empirical equation ofKahraman (2001) using the Vs and Vp data ofMreyen et al (2021). ²These values were inferred from the empirical equations of McDowell (1990) using the P & S wave velocity of…”

Analysis of the Influence of Structural Geology on the Massive Seismic Slope Failure Potential Supported by Numerical Modelling

“…On the basis of the actual morphology of the Balta rockslide and analyses of the failed rock volume (presented in Mreyen et al [63]), a possible slope surface prior to failure was created. The volume balance of pre and post-failure state of the slope (considering a bulking factor of 15%, i.e., volume expansion during collapse; cf.…”