Abstract
Abstract. As concerns landslide prevention and mitigation policies at the urban scale, the ability of Geographical Information Systems (GIS) to combine multi-layered information with high precision enables technicians and researchers to devote efforts in managing multiple hazards, such as seismically induced instability in urbanized areas. As a matter of fact, many villages in the Italian Apennines, placed near high-energy seismic sources, are characterized by active sliding that are seasonally remobilized by rainfall. GIS tools can be useful whether accurate Digital Elevation Models (DEM) are available and detailed mechanical and hydraulic characterization of superficial deposits over significant portion of the urban territory is undertaken. Moreover, the classic methods for estimating the seismic-induced permanent displacements within natural slopes are drawn from the generalization of Newmark's method. Such method can be applied to planar sliding mechanism that can be considered still valid wherever shallow landslides are generated by an earthquake. The failure mechanism depends on the mechanical properties of the superficial deposits. In this paper, the town of Castelfranci (Campania, southern Italy) has been studied. This small town, hosting two thousand inhabitants, suffers from the seasonal reactivation of landslides in clayey soil deposits due to rainfall. Furthermore, the site is seismically classified by means of the peak ground acceleration (PGA) equal to 0.246 g with respect to a 475 yr return period. Several studies on the evolution of slopes have been undertaken at Castelfranci and maps have been drawn at the urban scale not taking into any account the seismic hazard. This paper shows possible seismically induced hazard scenarios within the Castelfranci municipal territory aimed at microzonation of level 2, by estimating the slope permanent displacements comparable to those caused by the strongest historical seismic event that hit this area: the 1980 Irpinia earthquake. To this aim, geotechnical characterization of local soils collected over the last 25 yr by local technicians have been used to predict possible permanent displacements by means of Newmark's sliding block approach. Two simplified relationships relating peak ground acceleration and Arias intensity to permanent displacements have been used and compared. Although similar results are drawn, the two analyses point out the most hazardous sectors of the Castelfranci urban area.
Highlights
Castelfranci is located in the southern sector of the Apennine chain, in an area affected by the strong seismicity of the seismogenic zone 927, according to the latest Italian Seismogenic zonation ZS9 (MPS Working Group, 2004), where the 1980 Irpinia earthquake was generated with a 6.9 Moment Magnitude
Hereafter, the Irpinia earthquake has been considered as the reference seismic shock for calculating the highest seismic action in terms of peak ground acceleration (PGA) and Arias intensity (IA) for drawing the permanent displacement maps within the study area
Assessment of the permanent displacements induced by seismic action within unstable areas can be accomplished by means of three different approaches: (1) the pseudostatic approach, by means of the use of the Pseudostatic Safety Factor calculated as the ratio between the dynamic shear resistance of the slope and the pseudo-dynamic action; (2) permanent displacement estimation, according to the Newmark “sliding block model” (Newmark, 1965); (3) advanced numerical simulations, that can be carried out whenever single slopes characterized by ad hoc investigation campaigns are monitored
Summary
This paper shows possible seismically induced hazard sce- ing scale, with increasinHg yqudarnotitlaotigveychaarnacdterization of the narios within the Castelfranci municipal territory aimed at microzonation of level 2, by estimating the slope permanent ground response co-seismic effect sfhroomwslesEpveealci1rfitcthocrlSietivcyealsl 3pt.eoiNnmotsntehtahtehleasvse, each to be managed by means of scientific tSooclsiebenfocreebseing formalized. – The most common microzonation maps of seismicallyinduced landslides rely on the Newmark sliding block approach that needs initial stable conditions and translative slope movements to be applied. This study tries to tackle the microzonation study at level 2 of the active unstable territory at the Castelfranci municipality (Campania, southern Italy), a site with high seismic hazard (maximum expected peak horizontal acceleration ag is 0.246 g, with return period of 475 yr) where relevant instability phenomena are in progress. The two major landslides in the area are indicated (A is the Chianiello landslide, B the Lago landslide)
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