Site Effects Assessment Using Ambient Excitations sesame european Commission – Research General Directorate Project No. Evg1-ct-2000-00026 sesame report of the wp04



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Background

Fabriano is a city of approx 30.000 inhabitants stricken by the Umbria-Marche sequence of 1997 (Amato et al., 1998). The anomalous level of damage with respect to the magnitudes and epicentral distances (MW=5.7 and 6.0, epicentral distance greater than 30 km) and the economical importance of the city stimulated the Italian civil protection to issue a microzonation project with the goal to support the reconstruction planning in order to reduce the level of risk (Marcellini et al., 2001).



Figure 1 Geological map of Fabriano after Parroni et al. (2001). The blue rectangle borders the area containing damage data.
This project consisted in the collection, processing and synthesis of a large quantity of seismological, geological, geotechnical and structural information. A team of structural engineers completed a detailed damage database allowing, with a proper organization of data, a comparison with geological and seismological information available.

Geological settings

The historical core of Fabriano (see Figure 1) lies on alluvial deposits of the Giano river and eluvial-colluvial materials produced by the crumbling of slopes (Parroni et al, 2001). Alluvial deposits are mostly gravels and coarse sands locally alternated to silt and organic clays of lacustrine origin. Silty and clayey materials of Schlier and Gessoso Solfiera formation compose the eluvial and colluvial deposits at Serraloggia-La Spina and Borgo. The depth of fluvio-lacustrine deposits reaches 30 meters at most in the center of the valley where it is located the historical part of the town, while the thickness of alluvial terraces on the sides of the valley gently decreases moving toward the borders.

Eluvial and colluvial materials are predominantly thinner than the fluvio-lacustrine deposits reaching a thickness of about 20 meters at most. The mechanical properties of these materials is generally poor; as an example, a down-hole test performed at the contact between fluvio-lacustrine and eluvial colluvial deposits in the zone of Serralogia-La Spina (Crespellani et al., 2001) measured shear waves velocities of 135 m/s between 0 and 4m, 230 m/s until 8m of depth and 340m/s till the top (located at 10.3m) of marls owing to the Gessoso-solfifera formation (VS velocities in the upper weathered part of this formation are between 450 and 650 m/s).


Damage data




Historical Damage data



The occurrence of damaging events for Fabriano is well documented by several historical documents (Castelli and Monachesi, 2001). Figure 3 shows the seismic history of Fabriano as contained in the DOM4.1 database (Monachesi and Stucchi, 1997). Since 1279 the city experienced at least seven events with intensity larger than six while the highest observed intensity corresponds to nine.

Castelli and Monachesi (2001) reconstructed a detailed historical earthquake scenario for the event occurred in 1741 (Is=9.0, estimated magnitude 6.2) using historical reports of reparation costs, prepared for each of the four quartieri (portions of the city). The reconstructed scenario (see Figure 2) points out a differential response corresponding to a difference of one degree of intensity at maximum between the most damaged areas (at E and SW, yellow and green sectors in Figure 2) and the less damaged ones.

Figure 3 Seismic history for the city of Fabriano as described in the DOM4.1 database (Monachesi and Stucchi, 1997).


1997 earthquake damage data

In autumn 1997 (Dolce and Larotonda, 2001) completed a vulnerability and damage survey of the city of Fabriano. The investigated areas were the historical center, and two recently built suburbs where the level of damage was considered particularly anomalous given magnitude and distance of 1997 earthquakes.


Damage database structure

The structure of the database adopted for information collection and storage corresponds to a standardized survey form (“Scheda di I livello di rilevamento del danno, pronto intervento e agibilità per edifici ordinary nell’emergenza post-sismica”, Dipartimento Nazionale di Protezione Civile) where the stored data are gathered into four main sections, corresponding to:




  1. General info;

  2. Major building characteristics;

  3. Building structural properties;

  4. Damage data.

The first part collects general data relative to each building such as the cadastral parcel or the municipality name; the second one groups the most relevant information regarding the building: e.g. number of floors, year of construction. The third is a section dedicated to the description of the structural properties of the construction while the last section contains information relative to the level of damage produced by the main shock and eventually the aftershocks.

Buildings properties in the historical center of Fabriano

In the period immediately after the 26 September 1997, 883 buildings were inspected in the historical center. This zone encompasses ancient buildings with masonry vertical structures and wooden roofs of different quality: from good quality (e.g. masonry structure with iron ties) to poor structural quality (e.g. masonry with bad mortar). The distribution of building typologies is (Dolce and Larotonda, 2001):





R/C

02.50%


Mixed masonry – R/C

00.12%

Masonry

97.40%


Buildings characteristics in Serraloggia- La Spina quarter

This quarter is located east of the historical center on a gentle slope of outcropping eluvial and colluvial materials; the buildings in this area were built generally after 1960. The types of structures are equally divided between masonry structures and reinforced concrete structures (Dolce and Larotonda, 2001) as specified in the table below.




R/C

50.80%

Mixed masonry – R/C

1.7%

Masonry

47.50%


Damage distribution

Figure 4 represents the damage distribution in Fabriano as described in Dolce and Larotonda (2001). The damage description is separated in two parts, one to non structural wrecking and one devoted to structural damage.

Non structural damage in the historical part of the city (Figure 4, top panel) does not show a clear pattern or an evident clustering. Nevertheless, in the western part the level of non structural damage appears higher than in the remnant parts; this observation is in contrast with the content of Figure 2 where the earthquake damage scenario for the event of 1741 evidences the eastern and southwestern parts as the ones with major damages. Data relative to structural damage in the historical center, on the contrary, are less diffused and their low density and sparseness inhibit the identification of possible concentrations.

The suburb of Serraloggia – La Spina provides evidences of severe non structural and structural damages indicating a probable stronger level of shaking for this area. This deduction is well supported by geological and geotechnical investigation that found superficial low velocity materials and, most of all, by the results of standard spectral ratios on weak motion data (Tento et al., 2001) and of site effects computed using inversion techniques (Michelini et al., 2001). It must be pointed out, however, that a factor contributing to this level of damage could be brought back to slope instabilities apparently active even in static conditions.





Figure 4 Fabriano non structural (top panel) and structural damage(bottom panel) according to Dolce and Larotonda (2001). Damage grade in the upper and lower legends: 0 – no damage, 1 – light damage, 2 – moderate to heavy damage, 3 – heavy damage to collapse.



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