Seismic hazard maps at regional scale outline the reference ground shaking in terms of peak ground acceleration (PGA), peak ground velocity (PGV), and/or peak ground displacement (PGD); the multi-disciplinary analysis necessary to retrieve such data can be carried out in a deterministic or probabilistic approach, or a combination of these. 

The assessment of seismic hazard at regional scale is carried out on vast areas often assuming attenuation laws for a reference bedrock and not including any local and secondary effects induced by the earthquakes. Hence, this causes a partial ineffectiveness of the results to be employed for detailed development or management plans of sensitive areas or functions. Under earthquake loading, soft soils can amplify the motion up to ten times from the bedrock; further, landslide, earthquake-induced effects, such as instability, permanent displacement and liquefaction, can be recorded. Seismic micro zoning is aimed at identifying areas of the reference territory where a homogeneous seismic response of the soil is expected. 

The objective of the analysis is to distinguish stable areas from those more prone to local amplification phenomena and areas where earthquake-induced soil deformation are possible. From a policy-making perspective, micro-zonation is aimed at defining sub-areas of a region where different protection strategies must be applied to reduce, and/or prevent damages, loss of life and societal disruptions in case a large devastating earthquake strikes the region. 

From an operational point of view, it consists of predictions at much bigger scales – thus involving an enhanced resolution and greater precision (e.g. from 1:25’000, 1:10’000, to 1:5’000 scales).

The driving idea is to define the actual layout of soils and to connect to them a sufficiently robust mechanical model able to predict permanent deformation effects or frequencies typical of the soil ply in relation with those expected at the bedrock. 

In the framework of micro zoning, both the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE) and the Italian Society of Geophysics and Volcanology together with the Italian Civil Protection Department (INGV and DPC) in their technical guidelines (ISSMGE, 1993; MS, 2008) classify three grade levels in relation with the deepening of the instrumental/analytical investigations. 

Grade-1 is the crudest and lowest-cost approach, used to analyse a wide region. The first level is based on compilation and interpretation of existing information and provides a qualitative zoning of the areas where amplifications or secondary geological deformations can occur. Grade-1 also identifies areas where additional information is required. 

The quality of Grade-1 zonation can be relevantly improved with moderate costs, by making use of additional information. In Grade-2 phase, simplified geological models are employed and additional instrumental investigations are carried out to define expected thresholds of acceleration and velocity amplification. Grade-3 is intended for smaller areas and in all those cases where more refined and detailed geological models are necessary. This last level of zonation, requiring specific information, is generally expensive, but it may be necessary where the hazard potential is very high, or if the existing built asset (such as for monumental buildings) is regarded as critical. 

 

 

 

GRADE LEVEL 1 – MAP OF THE UNIFORM-RESPONSE AREAS

The deepening level generally zooms in up to 1:50000 – 1:10000 definition scale and the objective is the shaping of those areas where the same local response is reasonable to occur. This deepening level prompts the target level (2 or 3), is essential to the definition of the required further analyses and, more importantly, to the placing of further instrumental investigations. 

The state of the knowledge on the selected area is the prerequisite, for which all the already available cartographic information are gathered, systematised and put in relation:

• - Geologic map
• - Lithologic map
• - Geotechnical map
• - Geomorphological map
• - Instability areas
• - Regional/national Seismic hazard maps

 Analysis of the State of the Knowledge maps will form the Instrumental Investigations map, where the following information is collected:

• - Placing of the available instrumental outcomes
• - Kinds/Nature of the future instrumental investigations
• - Areas where further information are indispensable 

The crossed linking and comparison between the prerequisite information and the instrumental investigation map forms the map of the uniform-response areas. The map identifies, with a specific perimeter, the areas where different localization effects (amplification, landslide and liquefaction) are to be expected. It consists of the superposition of the instrumental investigation map, the available geological and geomorphological maps (1:10000 definition level), the lito-technical map, the outcomes of the available litostratigraphy investigations, the geological sections (deduced from available maps) and the hydrogeological risk map (instability areas).

The map of the uniform-response areas will identify three groups: 

• - STABLE AREAS, where any local effects are hypothesised, i.e. superficial bedrock, flat or moderate slope soil morphology and hillsides with slopes lower than 15°.
• - LOCALLY AMPLIFICATION-PRONE AREAS, where amplifications of ground shaking are expected, due to the morphological and lithostratigraphical layout. 
• - LOCALLY INSTABILITY-PRONE AREAS, where the dominant expected effects of the seismic action manifest as landscape permanent deformations. (these areas can superpose to locally amplification-prone areas). The four classes of instability phenomena are: hill-side instability, liquefaction, presence of active and responsive faults, differential dislocation (occurring in the neighbourhood of contact areas between formations with highly different mechanical and lithological characteristics).

If any of the listed information is not available, new dedicated investigations should be foreseen, at this stage, however, any quantitative assessment of the effects is considered. 

For the LOCALLY AMPLIFICATION-PRONE AREAS, in order to evaluate soil amplification effects the following information is necessary:

• - Reference seismic input (Acceleration spectra or compatible accelerograms) 
• - Superficial shape of the investigated area
• - Lithostratigraphy of the reference area 
• - Depth and shape of the bedrock
• - Geotechnical characterisation of the soil
• - Vs30 profiles
• - Fundamental vibration frequencies 
• - Dynamic model of the soil

To gain each item of information the following instrumental/numerical investigation are employed:

• - Instrumental seismicity data and PSHA / DSHA / NDSHA
• - 3d digital model of the area, detailed topographic maps
• - Geological direct survey, sampling
• - Sampling, 2D geological sections, geophysical investigations
• - Laboratory and in situ tests, correlations with SPT and CPT
• - Down—hole, Cross-hole, SASW, MASW, seismic arrays
• - Micro tremors measures
• - Resonant column test, cyclic torsional shear test, cyclic simple shear test (con doppio provino)

 

For the LOCALLY INSTABILITY-PRONE AREAS: 

 Hill-side instability. To this aim, the following data are indispensable:

• - Ground shaking (PGA)
• - Topography and geometry of the geological formation
• - Lithostratigraphy
• - Geotechnical characterisation of the hill-side
• - Instability model (assessment of the quality and quantity of the instable debris, geometry znd depth of the rupture surface, associated mechanism)
• - Hydrogeological 
• - Shear strength

Recognition of susceptibility to liquefaction. The following data need to be gathered: 

• - Ground shaking (PGA)
• - Magnitude of the events expected at the site
• - Lithostratigraphy
• - Granulometry 
• - Depth of the stratum
• - Strength of soils under cyclic loading

Recognition of active and responsive faulting. To do this is necessary to collect:

• - Available historical data on geological maps of the area of interest
• - Update geological maps with newer recorded data from accelerometric stations on shape and activity of faults [Required data (investigation methods)]
• • o Surface projections of faults (on site surveys, aerial photo interpretation)
  • o 3D positioning of fault planes at actual depth
  • o General and detailed lithostratigraphy (specimen collection, geotechnical analyses, paleoseismic analyses)
  • o Quantitative assessment of site dislocations (paleoseismic analyses, geophysical investigations)

Historical dating and evolution of fault movements (radiometry)

 

DEEPENING LEVEL 1 – MAP OF THE UNIFORM-RESPONSE AREAS
Investigation	Gathering available data: geological, geomorphological and geotechnical surveys, instrumental surveys
Processing	Synthesis of available information
Deliverables	Instrumental Investigations map Map of the uniform-response areas Technical report

 

 

GRADE LEVEL 2 – MAP OF THE LOCAL SEISMICITY

This deepening level assessment has two objectives: (i) fill the knowledge gap arose from the deepening level 1 and (ii) provide numerical quantification of the modified seismic action and the permanent deformation effects. At this stage, modifications to the geometrical definition of the areas defined at LEVEL 1 could still occur. 

To fill the knowledge gap, a crossed comparison of the available geological, lithostratigraphical and geotechnical information will be compared with the information gathered and evaluated within this study.  The further instrumental investigations will be precisely located on the area, (MAP OF THE INSTRUMENTAL INVESTIGATIONS), eventually highlighting those areas where uncertainties of the available information are relevant or needing a DEEPENING LEVEL 3.

The MAP OF THE LOCAL SEISMICITY if the final objective of the DEEPENING LEVEL 2; and to mark this objective, a quantitative assessment has to be associated to the effects forecasted in the MAP OF THE UNIFORM-RESPONSE AREAS by means of simplified methods. (maps of stable and amplification prone areas where amplification coefficients for two periods of the free field elastic spectrum are to be indicated).  

DEEPENING LEVEL 2 – MAP OF THE LOCAL SEISMICITY
Investigation	Geophysical investigations (DH, CH), seismic cone, stesa sismica, Vs estimations, microtremors
Processing	Simplified geological models
Deliverables	Instrumental investigation map Map of the local seismicity

 

 

GRADE LEVEL 3 – DETAILED MAP OF THE LOCAL SEISMICITY

This deepening level is carried out when:

• • - On LOCALLY AMPLIFICATION-PRONE AREAS when geological and geotechnical framework has a level of complexity that makes necessary the adoption, instead of simplified methods, of global detailed analyses, or when the a building or a set of buildings on the area have a particular value  (economic, cultural, social)
  • - On LOCALLY INSTABILITY-PRONE AREAS with particularly complex layout for which simplified methods are not completely adequate

The main tool for this deepening level is the exploitation of instrumental analyses aimed at the definition of a specific numerical model of the substrate. The instrumental analyses necessary for this aim are: seismometric data, down-hole tests, surface tests for Vs determination, in situ geotechnical tests (Nspt), laboratory tests (static and dynamic), micro-tremor testing.

Estimations should address 1D and 2D numerical soil models for amplification and complete dynamic analyses to estimate expected permanent deformations.

GRADE LEVEL 3 – DETAILED  MAP OF THE LOCAL SEISMICITY
Investigation	Geophysical investigations (DH, CH), seismic cone, seismic arrays, Vs estimations, microtremors, laboratory testing
Processing	1-D and 2-D numerical analyses, complete dynamic analyses
Deliverables	Instrumental investigation map Detailed map of the local seismicity Technical Report

 

 

last update: 05-July-2019