High-Density Macroseismic Survey in Urban Areas. Part 2 ... - CiteSeerX

Bulletin of the Seismological Society of America, 90, 2, pp. 298–311, April 2000

High-Density Macroseismic Survey in Urban Areas. Part 2: Results for the City of Rome, Italy by F. Cifelli, S. Donati, F. Funiciello, and A. Tertulliani

Abstract High-density macroseismic surveys were carried out in Rome, following both the 14 October 1997 (Mw 4 5.6; I0 4 VIII MCS), and the 26 March 1998 (Mw 4 5.3; I0 4 VI–VII MCS), Umbria-Marche (Central Italy) earthquakes. The aim of the work was to investigate the relationship between intensity and local geology for the modern urban area of Rome, an issue yet to be examined in contemporary times. Intensity data for both earthquakes were correlated with near-surface geology. The results show a statistically significant amplification in Holocene alluvial deposits. In particular, we found one degree of difference in MCS intensity between hard rock and recent alluvium formations. Furthermore, the distribution of the earthquake effects in alluvium-filled valleys was studied as a function of the shape ratio (h/l) of the alluvial basins. In the Tiber River valley, higher effects were found to be concentrated within a 150-m-wide band along the bedrock-alluvium surface contact, suggesting the occurrence of an edge effect. Surprising results were observed in the minor alluvial valleys, which displayed the strongest effects in terms of shaking. The narrowest valleys (width , 150 m) revealed the highest intensities, particularly where values of the shape–ratio range within 0.4 and 0.8. This behavior is in agreement with expected 2D resonance in small-sized alluvial basins. For areas of Rome urbanized after the damaging 13 January 1915 Apennine earthquake, these results contribute to a precise hazard assessment of areas located above recent alluvial deposits. Such a detailed analysis has the advantage of being fast and cheap and could be easily applied to other large cities, particularly where instrumental measurements are difficult to obtain or not available. Introduction that spectral amplification and damage distribution were strongly correlated with the underlying lithologic unit. Kawase (1996) and Pitarka et al. (1996) proposed the presence of an edge effect to explain the heaviest damage in the city of Kobe after the 17 January 1995 earthquake (Mw 4 6.9; I0 4 X). In this instance, a focusing amplification phenomenon may have been due to both the presence of a deep underground basin structure and shallow surface layers (Motosaka and Nagano, 1996). In all of these examples, sharp variability of the shaking was important. Intensity effects can be correlated with strong motion recordings and mapped geology on a very fine scale. Nevertheless, strong motion data are not always available, especially in large urban areas. In these cases, the use of macroseismic data in site effects studies can be important, particularly in the identification of sites where amplifications occur. Case histories include San Francisco, California (Seekins and Boatwright, 1994), Christchurch, New Zealand (Toshinawa et al., 1997), and Rome, Italy (Cifelli et al., 1999a).

Many urbanized areas display a geological complexity often responsible for locally significant amplification of ground motion during an earthquake. In regions where the near-surface geology presents lateral hard-rock to soft-soil transitions, site effects are observed to vary sharply and significantly, producing locally important increases of intensity. Recent disastrous earthquakes show high damage in cities in which the presence of alluvial deposits is known. For example, strong amplification of ground motion caused the greater part of damage occurring in Mexico City, located about 300 km from the epicenter of the 19 September 1985 earthquake (Ms 4 8.1) (Sa`nchez-Sesma et al., 1988; Singh et al., 1988). Unexpectedly large destruction occurred during the December 7, 1988, Armenia earthquake (Ms 4 6.8; I0 4 X), mainly due to the unusual amplification of ground motion in lakebed clays (Hadjian, 1993). The 11 October 1989 Loma Prieta earthquake (Ms 4 7.0; I0 4 IX), caused severe damage in San Francisco, about 90 km from the epicenter. In analyzing Modified Mercalli intensities and earthquake recordings, Seekins and Boatwright (1994) concluded 298

High-Density Macroseismic Survey in Urban Areas. Part 2: Results for the City of Rome, Italy

Residents of Rome were aware of many of the shocks occurring during the September 1997–April 1998 UmbriaMarche (Central Italy) seismic sequence. In parts of the city the earthquakes generated alarm, reaching an intensity degree V–VI Mercalli-Cancani-Sieberg (MCS). The Istituto Nazionale di Geofisica (ING) and the Department of Geology of University “Roma Tre” undertook a research project using macroseismic surveys to determine intensity levels in the capital following two aftershocks of the sequence: the 14 October 1997 (15:23 GMT; Mw 4 5.6; I0 4 VIII MCS; h ' 10 km), and the 26 March 1998 (15:25 GMT; Mw 4 5.3; I0 4 VI–VII MCS; h ' 50 km), earthquakes, located 115 km and 145 km North of Rome, respectively (Fig. 1). The greater city zone was considered appropriate due to the detailed knowledge of its near-surface geology (e.g., Marra et al., 1998), the large number of historical seismology studies (above all Ambrosini et al., 1986) and to the numerical modeling of the seismic response of its historic center (e.g., Fah et al., 1993; Rovelli et al., 1995). With more than three million inhabitants, a priceless artistic and archaeological heritage, and a historic center characterized by vulnerable ancient architecture, the city of Rome displays a significant exposure to seismic risk. Chronological records from 1349, 1703, and 1915 reveal strong Apennine earthquakes that have caused diffuse damage within the city. In the period following the most recent large event (13 January 1915), the city has under gone considerable and quite chaotic expansion (Fig. 2). Hence, there is a need for a careful analysis of Rome’s seismic response, extending from the historic center to the greater suburban area developed since the beginning of this century. The aim of this project was to analyze the intensity distribution in Rome from the 14 October 1997 and 26 March 1998 earthquakes in relation to the near surface geology. In order to investigate the presence of edge effects and 2D resonance phenomena in small alluvial basins, we correlated intensity to the shape and dimensions of the sediment-filled valleys of the Tiber River and its tributaries. In the first part of the project (see Cifelli et al., 1999b), we developed an original methodology for high-density questionnaire surveys in urban areas, being aware that human response, when averaged over a large number of samples, is useful for assessing the level of intensity (Kayano, 1990). We also checked sample reliability by considering the data distribution as a function of the urban framework, nonhomogeneous, and the percentage distribution of the outcropping geological units.

Seismicity of the Rome Area The city of Rome is prone to moderate seismicity. Since 461 B.C., historical sources and macroseismic surveys indicate the occurrence of more than 450 earthquakes felt by the population, of which approximately 20 events causing widespread effects and in some cases, severe damage to the

Figure 1.

Map of Central Italy indicating the epicenter and the focal mechanism for each of the 14 October 1997 (15:23 GMT; Mw 4 5.6; I0 4 VIII MCS; h ' 10 km), and the 26 March 1998 (15:25 GMT; Mw 4 5.3; I0 4 VII MCS; h ' 50 km), earthquakes (Morelli et al., 1999). The first event occurred 115 km north of Rome, and the second one occurred 145 km north of the city. The two earthquakes occurred at the same time of day, with similar magnitudes, but with different depths.

Figure 2.

Sketch of the urbanized area of Rome within the local ring-road (GRA). The dark gray area represents the current urban situation. The black area represents the urban setting after the 13 January 1915 Central Italy earthquake, the last strong event causing damage to Rome. More than 80% of the modern city has developed during the twentieth century.

299

300

F. Cifelli, S. Donati, F. Funiciello, and A. Tertulliani

cultural heritage of the city (Molin et al., 1995, see also Donati et al., 1999). Earthquakes associated with three different seismogenic districts (Fig. 3) mainly affect Rome: the Central Apennines area (“regional seismicity”), the Alban Hills volcanic area (“local seismicity”) and the Rome area (“urban seismicity”). Regional seismicity, represented by Central Apennine seismogenic sources located between 60 and 150 km from Rome, has produced the strongest intensities in the city (VII– VIII MCS for the 1349 and 1703 earthquake sequences). In particular, events generated in the Aquilano district exhibit the greatest magnitude (nearly 7), with a hypocentral depth of between 10 and 15 km. The 13 January 1915 Avezzano earthquake (Ambrosini et al., 1986, see also Rovelli et al., 1995), and the recent Umbria-Marche seismic sequence belongs to the Apenninic Chain regional activity. Local seismicity is mainly due to activity of the Alban Hills volcanic district, characterized by very frequent earthquakes with an intensity felt in the city of around V–VI MCS, a maximum magnitude of about 5 (Amato et al., 1994) and with a hypocentral depth of between 5 and 10 km. The most recent event due to this seismogenic district causing actual damage in Rome (up to the VI degree) was the earthquake of 19 July 1899. Urban seismicity, located within a radius of 20 km from the center of Rome, is characterized by a low frequency of occurrence, with a maximum intensity of around VI–VII MCS; magnitude values have probably never exceeded 4, and hypocentral depths are estimated at approximately 10 km. The best documented of these events occurred on 1 November 1895 (Tertulliani and Riguzzi, 1995), and on 12 June 1995 (Basili et al., 1996; Tertulliani et al., 1996). Furthermore, seismic catalogs indicate the city of Rome as being the epicenter of large historical earthquakes (in 443 and 801, among others), whose origins remain uncertain.

Geologic Setting Three different geologic domains characterize the Rome area: the Plio-Pleistocene regional sedimentary bedrock, the Middle-Late Pleistocene volcanic plateau, and the Holocene alluvial plain of the Tiber River. The Plio-Pleistocene sedimentary units outcrop in the western part of the city along the M. Mario-Gianicolo structural high. Late Pliocene and Early Pleistocene marly-clayey marine deposits represent the actual bedrock of the Rome area and compose the lower part of the sequence; their outcrops cover around 4% of the whole area. Marine deposits are covered by a complex transgressive cycle, characterized by the alternation of depositional and erosive phases during the Early and Middle Pleistocene. The fluvio-deltaic clastic deposits of the PaleoTiber, the ancient river of the Rome area, complete this complex continental sequence (Marra et al., 1998). Continental deposits outcrop on almost 10% of the urban area. During the Middle and Upper Pleistocene, from 0.6 Ma

Perugia Central Apennines Seismicity

43

Magnituded 4.0 L’Aquila

Rietr

5.0

42

Rome

6.0

Avezzano

Urban Seismicity Ty r

7.0

rh

en

ia

n

Se

a

Alban Hills Seismicity

41 12

13

14

Figure 3.

Sketch map of the seismicity of interest for the city of Rome, from the Italian Seismic Catalogue from 461 A.D. up to 1998. Rectangles represent the three main seismogenic districts affecting the city.

the Sabatini Mountains (25 km northwest from Rome) and the Alban hills (20 km southeast) volcanic districts began to deposit their eruption products. This mainly explosive volcanic activity was linked to the opening of the Tyrrhenian Sea basin. A thick cover of ignimbrites, pyroclastic, and airfall products blanketed the Rome area, interbedding with continental deposits and confining the Tiber River to its present riverbed. Volcanic products are largely predominant, representing more than 60% of this area. In the last glacial low-stand (Wu¨rm), the Tiber River and its tributaries excavated the Plio-Pleistocene bedrock down to a maximum depth of 50 m below the present sea level. During the Holocene, in the last 10,000 years, this articulated hydrographic network was back-filled with up to 60 meters of coarse clayey-sandy alluvial deposits, subsequently blanketed by a cover of man-made fill accreted from the early history of the town. Holocene deposits, mostly unconsolidated and water-saturated, cover more than 25% of the area. Laboratory analyses in cores drilled through the Tiber River valley and its minor tributary streams (e.g., Bozzano et al., 1999) indicate a lower density for these recent alluvial deposits as compared to the pre-Holocene sedimentary and volcanic formations. A sharp transition between the Holocene alluvium and Plio-Pleistocene bedrock is also shown in the shear strength parameters and in the overconsolidation pressure values. Also, alluvial deposits of the minor tributaries present a lower density with respect to the Tiber River valley alluvium (Bozzano et al., 1999). Unfortunately, Vs in situ measurements are still not widely available for the city of Rome, and most authors make use of laboratory analyses

301


and Vs estimations. Nevertheless, the different mechanical properties reported here may imply significant variations in the seismic impedance between the geological units outcropping in Rome. As part of this study, an original 1:20,000 scale lithologic map of the urban area of Rome was compiled (see Figure 7). Main objectives were to correlate earthquake effects and geology in a more refined scale. Outcropping units were gathered into four principal lithologic units according to their key geotechnical properties (Cifelli et al., 1999b).

Questionnaire Survey and Data Distribution A macroseismic network was established in cooperation with 27 public high schools in the city of Rome. Students between the ages of 14–19 years were requested to complete the questionnaire routinely used by ING (enclosed as Appendix 1). High schools represent a larger catchment area than primary schools and can be considered more representative of the city population. Each student was asked to fill in a form, trying to summarize the effects experienced by different people in the same place. The number of interviewed people, used as an index of the involved population, and the precise address of the building must be indicated. On the basis of their geographical location, 10 high schools were selected for the survey performed following the 14 October 1997 earthquake, collecting 1222 questionnaires within two weeks of the event. These data were integrated with our direct investigation in zones from which questionnaires were not available. Following the 26 March 1998 earthquake, our macroseismic network was extended to cover 27 high schools. Within seven days of the event, 1643 questionnaires were collected.

For each event, the resulting information was ultimately related to 669 and 926 observation points, respectively. Responses relating to localities outside the urban area or affected by error (contradictory answers; incomplete) were eliminated. The remaining forms, as reported in Table 1 (Cifelli et al., 1999b), were useful for our aim. A macroseismic survey granting such a large amount of data in urban areas is uncommon (see Dengler and Dewey, 1998). The form used in the survey was a revised version of the ING macroseismic questionnaire (Gasparini et al., 1992), based on the MCS scale and devoted to effects experienced by humans and objects. An algorithm that makes use of the weighted mean of the number of given answers concerning each degree, computes intensity as follows: n

¯I 4

o Wi Xi i41 n

o

(1)

,

Wi

i41

where Xi is the intensity degree and Wi is the weight, computed as Wi 4

Pi

o

(2)

,

k

Pi

i41

where Pi is the probability assigned to an attained degree, as Pi 4

i11

Ri Di

p

j4i

11 1 DR 2. j

(3)

j

Table 1 Details for the 14 October 1997 and the 26 March 1998 Surveys Survey

Elapsed Time

Number of Schools

Collected Forms

Useful Collected Forms (UCF)

Direct Investigation Forms (DIF)

Used forms (UCF ` DIF)

Number of Interviews

Intensity Points

14 October 1997

2 weeks

10

1222

605 (49.5%)

344

949

1842

669

26 March 1998

1 week

27

1643

1083 (65.0%)

0

1083

2529

926

Figure 4.

(a) Percentage distribution of the outcropping units in the urbanized area of Rome; (b) distribution of the intensity points of the October 14, 1997, survey; (c) distribution of the intensity points of the 26 March 1998 survey. The data distribution on the different lithological units is consistent with the percentage distribution of the same units in the urbanized area of Rome.

302


Ri is the number of answers for each degree and Di is the number related to the ith degree. Every questionnaire refers to a definite geographic location, which has a circular 100-m-diameter influence area. When the influence areas of two or more questionnaires overlap, their intensities are averaged as a unique observation point (Cifelli et al., 1999b). Computed intensities are approximated to the nearest integer value and then gathered in four distinct classes, ranging from not felt (NF), up to V degree of the MCS scale. The same weight is assigned to single as is to averaged surveys. In a 2500-year-old city, such as Rome, the urban setting is particularly irregular (see Figure 2) and can therefore affect the geographical distribution of the sample. Hence, it was necessary to check the reliability of the data set as a function of the urban framework (Cifelli et al., 1999b). From the October 1997 survey, the mean density within the 200 km2 urbanized area inside the local ring road (hereafter referred to as GRA) was 3.4 data/km2. In the March 1998 survey, the mean density improved to 4.7 data/km2. Such a parameter could not yet be considered sufficient to evaluate if the data distribution was uniform. Consequently, in Cifelli et al. (1999b), we analyzed the density distribution for both surveys, resulting in a range of 1 to 25 data/km2 in the urbanized area consistent with the irregularities of the urban setting. In particular, a remarkable portion of territory showed a density higher than the mean values (see Figures 4 and 5 in Cifelli et al., 1999b). Another check of the sample distribution was also needed to compare macroseismic and geological information at the same scale (Tables 2 and 3). Such a comparison is useful in the geological setting of Rome, which is characterized by sharp lateral variations in near surface geology (see Figure 7). This peculiar geological frame involves the presence of very narrow outcrops of PlioPleistocene sedimentary formations along the gullies excavated by the Tiber River and its tributaries. To evaluate if the data distribution over Plio-Pleistocene deposits was statistically significant, we compared the percentage distribution of the outcropping units in Rome (Fig. 4a) with the percentage distribution of our intensity points over each lithological unit (Fig. 4b, c). A good correspondence is shown between the outcropping units and the intensity points for the two surveys, especially for that of March, 1998. Therefore, the data sample can be considered representative, es-

Figure 5.

Frequency (a) and normalized intensity distribution (b) in Rome for the 14 October 1997 and 26 March 1998 Umbria-Marche (Central Italy) earthquakes.

pecially when taking into account that each intensity point is the synthesis of several observations.

Intensity Distribution and Local Geology Distribution of the effects in Rome for both the October 14, 1997 (Mw 4 5.6), and the March 26, 1998 (Mw 4 5.3), Umbria-Marche earthquakes reveals a large variability in macroseismic intensity, ranging from NF to V–VI MCS. The intensity distribution of the first event (Fig. 5), which occurred 115 km north of Rome and was largely felt in the city, is quite discontinuous, displaying a sharp peak around the IV degree of the MCS scale and with a noteworthy contribution of degree V. Point distribution of the second event (Fig. 5), which occurred 145 km north and was felt with lower intensity, shows a minor peak around the IV degree and a remarkable contribution of both degree III and NF. Considering the epicentral distance for both earthquakes (between 115 and 145 km) and the relevant width of the

Table 2 Intensity Points vs. Lithological Units for the 14 October 1997, Survey Lithological Unit

V

IV

III

NF

Total

%

Recent alluvium Volcanic products Continental deposits Marine deposits

129 57 11 0

94 247 27 13

5 38 7 1

5 27 4 4

233 369 49 18

34.8 55.2 7.3 2.7

Total

197

381

51

40

669

100.0

Pr

2.2 2 10118 2.7 2 10110 0.12 5.7 2 1014 —

The Pr is the probability of rejection of the breakdown category with a confidence level of 95%, computed from the t-test (Davis, 1986).

303


Table 3 Intensity Points v. Lithological Units for the 26 March 1998, Survey Lithological Unit

V

IV

III

NF

Total

Recent alluvium Volcanic products Continental deposits Marine deposits

60 28 0 0

102 202 22 8

38 177 24 17

29 172 25 22

229 579 71 47

24.7 62.5 7.7 5.1

Total

88

334

256

248

926

100.0

investigated area (20 km long), the potential contribution of regional attenuation to the macroseismic field was investigated. The intensity distribution and epicentral distances for both earthquakes were plotted (Fig. 6). Distance does not seem to affect the intensity of the shaking within the city, thus a regional attenuation trend can be considered negligible. As the variation of the human perception is generally limited to around 1 degree in intensity (e.g., Dengler and Dewey, 1998), such a widely varying response is most probably due to site conditions. Intensity variations of the two events within the city were then correlated with near-surface geology. The t-test (Davis, 1986) was used to check the statistical significance of the data with a confidence level of 95% (see the Pr column in Tables 2 and 3). The test establishes the likelihood that the mean of a given breakdown category is different from the mean of the rest of the population. Highest effects occur preferentially on the recent alluvial deposits. In particular, the intensity distribution of the 14 October 1997 earthquake (Figs. 7 and 8) shows a correspondence between the V degree and Holocene alluvium in both the Tiber River valley and the tributary valleys. Alluvial deposits were prone to higher intensity, on average equal to one degree of difference in MCS scale with respect to volcanic and sedimentary bedrock formations (see Figure 8a). Almost 60% of the intensity points on Holocene alluvium were V (see Figure 8b). Moderate and low effects (IV, III, and NF) mainly affected preHolocene units; it is noteworthy that no V intensity occurred on Pliocene bedrock formations. Results for the 26 March 1998 earthquake (Figs. 9 and 10) confirm the influence of the local geology on the seismic response of the city. The highest effects (IV and V MCS), even though less diffuse with respect to the previous event, show a preferential concentration on recent alluvium (see Figure 10). Lower intensities were distributed all over the bedrock formations. One degree of difference in MCS intensity is shown (Fig. 10a) between alluvial deposits and remnant formations (IV MCS versus II–III MCS, on average). The normalized MCS intensity shows a trend, which seems to be linked to the geomechanical properties of each lithologic unit (Fig. 10b).

Intensity Distribution in Alluvial Valleys Having focused on the strongest response on recent alluvium, a further investigation of the effects on Tiber and

%

Pr

1.6 2 10120 1.6 2 1015 0.008 4.2 2 1015 —

tributary valleys was undertaken by considering shape and dimension of the alluvial deposits. Intensity points on Holocene alluvial deposits for the 14 October 1997 and the 26 March 1998 earthquakes were 232 and 229, respectively. For every intensity point, we measured the width of the alluvial

Figure 6.

Distribution of the macroseismic intensity in the urban area as a function of the epicentral distance for the 14 October 1997 (a) and 26 March 1998 (b) earthquakes.

304


Figure 7.

Lithologic map of the urban area of Rome and distribution of seismic effects for the October 14, 1997 (Mw 4 5.6; I0 4 VIII MCS), Umbria-Marche earthquake. Highest effects (black dots) tend to concentrate on Holocene alluvium, filling the valleys of the Tiber River and its tributaries.

valley and the distance between that point and the edge of the valley. Specifically, the distance was determined by projecting every point onto the nearest bedrock-alluvium surface-contact. The intensity distribution within the main valley of the Tiber River for both the surveys (97 and 99 intensity points, respectively) was analyzed. The valley has a mean width of about 2200 m (ranging from about 1000 m, within the historic center of Rome, to more than 3500 m, in its lower reaches). Each side of the valley was divided into

four longitudinal bands, characterized by different width, in order to consider the lateral variations of both the alluvium thickness and the shape of the basin. Intensity points from both sides were then combined, obtaining a quite homogeneous distribution of data for each band (see percentages in Tables 4 and 5). The intensity distribution of the 14 October 1997 survey, as a function of the distance from the edge of the valley (Table 4), shows a concentration of highest effects (V MCS

305


Subsequently, for both earthquakes, the response of all the valleys (232 and 229 intensity points, respectively), was analyzed as a function of width and thickness (h) of the alluvial deposits. In the sediment-filled basins of Rome studied, the thickness of Holocene alluvium and man-made fills range from a maximum of 60–70 m in the Tiber valley, to a minimum of 30–40 m in the minor alluvial basins (Funiciello et al., 1995; Marra et al., 1998). We classified the sediment-filled alluvial valleys into five different categories, according to width (Tables 6 and 7): type A (width , 150 m), type B (width between 150 and 300 m), type C (width between 300 and 600 m), type D (width . 600 m) and lastly, the main Tiber River valley. The distribution of intensity values on the different alluvial basins for the 14 October 1997 earthquake (Table 6 and Fig. 12a) shows that the response of the Tiber valley is remarkably lower with respect to other minor sedimentary basins. On minor streams (type A, B, and C), intensity seems to be roughly proportional to the narrowness of the basin. The narrowest streams (type A), for instance, present more than 80% of highest effects (V MCS). Secondary valleys (type D) also show a much higher response than that of the Tiber valley. Results for the March 1998 survey (Table 7 and Fig. 12b) confirmed the lower response of the Tiber valley with respect to the minor basins. Higher effects (IV and V) exceeded 95% in the narrowest streams (type A). Also the peculiar seismic behavior of the secondary valleys (type D) was confirmed.

Figure 8.

(a) Distribution of the intensity points in Rome of the 14 October 1997 earthquake as a function of the lithologic units. (b) Normalized MCS intensity distribution versus lithologic units. Alluvial deposits are prone to a higher ground motion (one MCS degree, on average) with respect to bedrock formations.

exceeds 60%) in the 150-m-wide band near the bedrockalluvium limit (Fig. 11a). The other bands, toward the center of the basin, show more moderate and homogeneous effects. Similar results are partially confirmed by the 26 March 1998 survey, (Table 5 and Fig. 11b): IV and V account for almost 80% of the intensity values on the edge band and decrease to around 45% toward the center of the valley.

Discussion and Conclusions High-density macroseismic surveys carried out for the 14 October 1997 (Mw 4 5.6; I0 4 VIII MCS), and the 26 March 1998 (Mw 4 5.3; I0 4 VI–VII MCS), earthquakes (Cifelli et al., 1999b) provided very similar and homoge-

Table 4 Intensity Points vs. Distance from the Edge of the Tiber Valley for the 14 October, 1997 Survey Distance (m)

V

IV

III

NF

Total

%

Pr

d , 150 150 , d , 300 300 , d , 600 d . 600

16 6 7 11

9 11 12 18

0 2 1 1

1 0 1 1

26 19 21 31

26.8 19.6 21.6 32.0

0.10 0.40 0.39 0.66

Total

40

50

4

3

97

100.0

—

Table 5 Intensity Points vs. Distance from the Edge of the Tiber Valley for the 26 March 1998 Survey Distance (m)

d , 150 150 , d , 300 300 , d , 600 d . 600 Total

V

IV

III

NF

Total

%

Pr

4 4 7 7

9 3 10 9

0 2 12 11

4 3 5 9

17 12 34 36

17.2 12.1 34.3 36.4

0.53 0.82 0.68 0.29

22

31

25

21

99

100.0

—

306


Figure 9.

Lithologic map of the urban area of Rome and distribution of seismic effects for the 26 March 1998 (Mw 4 5.3; I0 4 VII MCS), Umbria-Marche earthquake. Highest effects (black and dark gray dots) tend to concentrate preferentially on alluvial deposits. Lower effects (white and shallow gray dots) are well distributed over the bedrock volcanic and sedimentary formations.

neous results. The analysis reveals the occurrence of significant site effects in the Rome urban area. Higher effects (V MCS) are well correlated with lateral variations of the local geology. Recent alluvial deposits, which are mainly coarse grained and water-saturated, and generally present poor geomechanical properties, are characterized by a marked difference in intensity (one MCS degree, on average) with respect to the pre-Holocene sedimentary and volcanic formations. Such a difference is statistically significant even for the less

diffuse marine and continental deposits in Rome, and noteworthy in a low intensity macroseismic field. The different response of the outcropping units seems to be related to their relevant contrast in seismic impedance between bedrock units and Holocene deposits. The latter is estimated to be around 4.1 (e.g., Bozzano et al., 1999) and can be responsible for local amplification phenomena. All the data suggest that Holocene deposits in Rome present a higher level of hazard than Plio-Pleistocene bedrock units. Previous authors

307


Figure 11.

Normalized, intensity distribution of the 14 October 1997 (a), and 26 March 1998 (b), earthquakes within the Tiber River alluvial valley. In the first survey, the edge band (d , 150 m) is affected by higher effects (V MCS); in the second one, this higher response is less evident even though the NF and III degree increase in the central part of the basin.

Figure 10.

(a) Distribution of the intensity points in Rome of the 26 March 1998 event as a function of the lithologic units. (b) Normalized MCS intensity distribution versus lithologic units. The higher shaking on Holocene alluvium, with respect to pre-Holocene formations (IV MCS versus II–III MCS, on average), is apparent.

Table 6 Intensity Points vs. Alluvial Valley Categories for the October 14, 1997, Survey Category

Width (m)

A w , 150 B 150 , w , 300 C 300 , w , 600 D .600 Tiber Valley Total

V

IV

III

NF

Total

%

Pr

21 18 23 27 40

5 7 18 13 50

0 0 1 0 4

0 2 0 0 3

26 27 42 40 97

11.2 11.6 18.1 17.3 41.8

0.01 0.85 0.60 0.05 8 2 1014

129

93

5

5

232

100.0

—

%

Pr

Table 7 Intensity Points vs. Alluvial Valley Categories for the March 26, 1998, Survey Category

Width (m)

V

IV

III

NF

Total

A w , 150 B 150 , w , 300 C 300 , w , 600 D .600 Tiber Valley

11 3 14 10 22

21 17 19 14 31

1 4 4 4 25

1 2 3 2 21

34 26 40 30 99

14.9 11.3 17.5 13.1 43.2

0.01 0.94 0.07 0.17 2.6 2 1015

Total

60

102

38

29

229

100.0

—

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suggested the occurrence of site effects in alluvium deposits of the Tiber valley, based on the damage distribution of the main historical and the more recent earthquakes which have occurred in different seismogenic zones (i.e., Ambrosini et al., 1986; Molin and Guidoboni, 1989; Tertulliani and Riguzzi, 1995; Donati et al., 1999). Our surveys compare well with these previous studies. Nevertheless, the previous studies refer to a very different urban context, largely confined with respect to the present situation. Our work largely confirms the seismic response of the main alluvial valley. In particular, our results are very consistent with the study carried out by Ambrosini et al. (1986; see also Rovelli et al., 1995) on the damage distribution in the historic center of Rome after the strong Avezzano earthquake (13 January 1915). This correspondence suggests that the occurrence of site effects in the sediment-filled valley of the Tiber River is independent of the earthquake azimuth. The concentration of higher effects in the narrow band (, 150 m) close to the bedrock to alluvium transition (see Figure 11) suggests the presence of a characteristic edge effect in the alluvial basin of the Tiber River valley. This observation is in agreement with the edge effects described by previous historical earthquake observations (Tertulliani and Riguzzi, 1995), and numerical modeling of the seismic response (Rovelli et al., 1995), for the city of Rome. In the

October 1997 survey, the statistical significance of the data in the edge band is satisfied with a confidence level of 90% (see Table 4); yet, this condition is not satisfied in the March 1998 survey (see Table 5). This different behavior can be considered due to the low level of shaking in Rome during the 26 March 1998 event. An unexpectedly large seismic response of recent alluvium occurs in the minor alluvial basins of suburban Rome, an effect never before investigated in such detail. Statistical tests confirmed the reliability of data coming from different valley categories (especially for the type A and the Tiber valley, as reported in Tables 6 and 7). In particular, the occurrence of highest effects correspond to the narrowest alluvial streams (type A, in Figure 12), while wider basins, especially the Tiber valley, show a relatively lower shaking. This observation can be explained by considering the shape ratio (h/l) of the alluvial basins as the ratio of the maximum sediment thickness to the half-width of the valley, (introduced by King and Tucker, 1984). For the very narrow sediment-filled streams of Rome, the h/l ratio ranges between 0.4 and 0.8. According to Bard and Bouchon (1985), these values of h/l, if coupled with a contrast impedance of around 4, can induce a 2D resonance in the valley (Fig. 13). Such amplification of the ground motion in the minor alluvial streams was also predicted by numerical modeling of the seismic response in the Fosso Labicano val-

Figure 13. Figure 12.

Normalized intensity distribution of the 14 October 1997 (a) and the 26 March, 1998 (b) earthquakes on the different alluvium-filled valleys of Rome, as a function of their width and shape ratio (h/ l ). Narrowest streams (type A) and Tiber valley display a very different seismic response with respect to other alluvial basins; in particular, the formers are prone to the highest effects, while the latter to the lowest.

The curve represents the existence conditions of the 2D resonance of sediment-filled valleys, as a function of shape ratio (h/l) and velocity contrast (redrawn from Bard and Bouchon, 1985). The four rectangles correspond to the different types of valleys studied in Rome (type A, B, C, and D). Dashed lines represent the bedrock-alluvium impedance contrast in Rome (estimated around 4). Valleys of type A (width , 150 m), where highest effects occurred, lay entirely in the 2D resonance range.

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ley (Moczo et al., 1995) and in the Palatino area (Rovelli et al., 1994). For other types of alluvial basins in the study area (width . 150 m), whose shape ratio cannot exceed 0.2, 2D resonance phenomena are not expected (see Figure 13). With respect to the main valleys, the poorest geotechnical characterization of small-size alluvium-filled basins (lower density and over-consolidation pressure, see Bozzano et al., 1999), could imply a local amplification of the ground motion. The occurrence of amplification effects, as predicted by Fah et al. (1993, 1995) in the Paleo-Tiber basin by means of numerical simulations, was not confirmed by our findings. This could be due to the different azimuth between the two studied Umbria-Marche earthquakes and the simulated input on which the predictions were based (the same as the 13 January 1915 Avezzano earthquake). It is possible that the choice of the geomechanical parameters and the lateral variations of the near-surface geology used by Fah et al. (1995), are not appropriate to the situation in Rome. Results here prove that our macroseismic survey method can be quickly and successfully performed in a large city, producing a statistically significant high-density of intensity points and yielding useful information about modern suburban areas. Furthermore, the cooperation between research institutes and public high schools can unite scientific results with didactical developments in the field of earthquake preparedness, representing an efficient tool to create a permanent macroseismic network. In consideration of Rome’s priceless historical and monumental inventory together with its vulnerability, this first high-density macroseismic survey in the present setting of the city is very important. The occurrence of site effects at even low intensity levels is noteworthy. By allowing a more detailed assessment of hazard in areas where recent alluvium outcrop, these results can provide a significant contribution to larger event seismic risk evaluation for the urban area. Despite the moderate seismicity of the region, plans for seismic risk mitigation should take into account the seismic behavior of rock to soft-soil transition. This is particularly necessary in the historic center, where vulnerability of ancient buildings and cultural heritage exposure are critical factors, and also in the modern suburban areas, where population density is high and no historical earthquake data are available.

Acknowledgments We wish to thank Enzo Boschi for his encouragement and Calvino Gasparini for his support of our work. We also acknowledge Antonio Rovelli, Valerio De Rubeis, and Angelo De Santis for their helpful suggestions and fruitful comments. The valuable comments and criticisms of Martin Chapman, Lori Dengler, and an anonymous referee improved the article. Appreciation is expressed for Angelo Massucci’s work in the graphic realization. Suzanne Watt is acknowledged for revision of the English text. Finally, we are grateful to Renato Funiciello for his precious contribution and his careful supervision to this research.

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Appendix 1 Macroseismic Questionnaire How to fill the questionnaire: 1. Read all the questions. 2. Interview most number of people present in the same place (home, workplace, etc.) during the occurrence of the earthquake. Questionnaire should be filled only by getting information from people living in highest and lowest levels of the building. 3. Answer the questions synthesizing all the available information. 4. Write the number of interviews done: 5. Write the correct address of the observation point, in order to identify its exact topographic location: street , house number , nearest cross-road 6. Note other possible observations not indicated in the questionnaire (car alarm triggering, damage to buildings, animals disturbed, etc.) and every additional comment. Questions 1 Shock not felt 2 Shock felt only by some people at rest in upper floors of buildings 3 Shock felt by few people and not recognized as an earthquake 4 The shock caused skidding of cars Shock felt indoors (houses, schools, cinemas, churches, etc.) by: 5 few people 6 many people 7 most people Shock felt outdoors (squares, roads, fields, etc.) by: 8 few people 9 many people 10 most people The earthquake has awoken: 11 no one 12 few people 13 many people 14 most people The earthquake has frightened: 15 no one 16 few people 17 many people 18 most people 19 Chandeliers swung on lower floors Slight rattling of doors, windows, furniture; slight vibration of chairs, beds, etc.: 20 on upper floors only

21 on all floors 22 Liquids in full containers disturbed 23 spilled slightly 24 overflowed Rattling of glass in windows and furniture or glassware and crockery: 25 on upper floors only 26 on all floors 27 Creaking of furniture and/or beams and rafters in the ceilings Hanging pictures 28 swung or banged against the wall 29 fell 30 Banging or opening of doors, windows or furniture doors Ringing of: 31 small bells 32 bells in belt-towers or towers Small objects: 33 were displaced 34 fell 35 Falling of crockery, glassware, or books Heavy and stable objects 36 were displaced 37 fell 38 Light furniture were displaced Heavy furniture 39 were displaced 40 fell