Site effects and soil-structure resonance study in the Kobarid basin (NW Slovenia) using microtremors

Abstract

Abstract. The town of Kobarid is located in one of three areas with the highest seismic hazard in Slovenia. It was hit by several [1976][1977] Friuli sequence earthquakes and recently by the 1998 and 2004 Krn Mountains earthquakes which caused damage of intensity up to VII EMS-98 scale. The town is located in a small basin filled with heterogeneous glaciofluvial Quaternary sediments in which site effects due to soft sediments are expected. The existing microzonation which is based on surface geological data only is inadequate, and no borehole or geophysical data are available in the basin that would allow a modelling approach of site effects assessment. The microtremor horizontal-to-vertical spectral ratio (HVSR) method was therefore applied in order to assess the fundamental frequency of the sediments. Investigations were performed on a 100×100 m dense grid and 106 free-field measurements acquired. Clear HVSR peaks were obtained in the majority of the surveyed area. The eastern part of the basin is characterized by two well separated peaks which indicate distinct shallow and deep impedance contrasts. The iso-frequency map of sediments shows a distribution in a broad range of 1.8-22.2 Hz. The observed frequencies can be related to the total thickness of Quaternary sediments (sand, gravel) in the western part of the basin only. They are deposited over bedrock built of Cretaceous flysch. In the eastern part the obtained fundamental frequencies are influenced by the presence of a shallow conglomerate layer inside sandy gravel or lacustrine chalk. The extent of these layers was not known before. Microtremor measurements were also performed inside 19 characteristic buildings of various heights (from two to four stories), and longitudinal and Correspondence to: A. Gosar (andrej.gosar@gov.si) transverse fundamental frequencies determined from amplitude spectra. A potential of soil-structure resonance was assessed by comparing building frequencies with the free-field sediments frequencies derived from the iso-frequency map. For two surveyed buildings a high danger of soil-structure resonance was assessed and for three buildings the danger was of medium level. The building resonant frequency of two-and three-story houses, which prevail in the area, spans the range 4-11 Hz, with an average value of 7.7 Hz. The danger of soil-structure resonance should be therefore sought in this frequency range. Since the majority of Kobarid area is characterized by lower (W part) or higher (E part) frequencies, the danger exist mainly in a relatively narrow transition zone.

Basin Resonance and Seismic Hazard in Jakarta, Indonesia

“…Figure 5b shows amplification of PSA as a function of period for V S30 = 100 m/s, typical of the Jakarta Basin, and values of Z 1.0 ranging from 100 to 1500 m. The amplification curve for Z 1.0 = 100 m exhibits a pronounced peak at period 0.8 s, and this peak broadens and its period increases to 1.5 s and 2.0-2.5 s for Z 1.0 = 500 and 1000 m, respectively, but changes little in amplitude or period range for Z 1.0 = 1500 m. A typical HVSR curve for the Jakarta Basin, on the other hand, exhibits a pronounced peak at around 6 s (some curves such as this one also exhibit a secondary peak at shorter periods, see Cipta et al [13]. While the HVSR curve does not necessarily represent the amplification of seismic waves, its peak period is widely regarded as coinciding with that of S-wave amplification Nakamura [37], it has been used to explain spatial patterns of earthquake damage Gosar [38], and it agrees with the fundamental period of S-wave resonant oscillation calculated form the Jakarta Basin velocity models of both Cipta et al [13] and Saygin et al [39]. Thus, the basin parameters calculated for the Jakarta Basin model of Cipta et al [13], appear to lie outside the range of values used for the development of the deep sediment corrections of CY2014, and the period dependence of CY2014 predictions for PSA using these corrections do not agree with the characteristics of ground motion implied by observed HVSR curves.…”

Application of Environmental Seismic Intensity scale (ESI 2007) to Krn Mountains 1998 <i>M</i><sub>w</sub> = 5.6 earthquake (NW Slovenia) with emphasis on rockfalls

“…Large variations in damage to buildings were observed especially in the Bovec (Gosar, 2007(Gosar, , 2008 and Kobarid (Gosar, 2010) basins; these variations were explained by prominent site and resonance effects between soft sediments and buildings. Apart from substantial damage to nearby settlements, the 1998 earthquake caused considerable changes to the landscape through many rockfalls and some landslides (Vidrih and Ribičič, 1999;Vidrih et al, 2001;Mikoš et al, 2006;Vidrih, 2008).…”

Seismic Vulnerability Maps of Ratu Agung District, Bengkulu City, Indonesia

“…This also presents that the propagated seismic wave could be larger, if it propagated from one medium to other soft mediums compared than the travelled previous wave. Gosar [24] also mentioned that the larger amplification could be caused due to the weak indication of relatively higher impedance contrast between the soft sediment and bedrock. Corresponding to the geological condition in the study area, Ratu Agung District is generally dominated by alluvium terraces composed by sands, silts, clays, and gravels.…”