Shear-wave velocity based seismic microzonation of ...

3 downloads 183 Views 7MB Size Report
observed outcropping at northern and western parts of the city. At the centre of Lorca ..... map of EC8 (or NEHRP soil classification) Vs30 site classes in. Lorca city. .... technical conditions of the soil is relevant as the buildings from that district .... Conference Proceedings, 20–24 February 2010, West Palm Beach, FL. Dobry R.
Near Surface Geophysics, 2014, 12, xxx-xxx 

doi:10.3997/1873-0604.2014032

Shear-wave velocity based seismic microzonation of Lorca city (SE Spain) from MASW analysis P. Martínez-Pagán1,*, M. Navarro2, J. Pérez-Cuevas1, F.J. Alcalá3, A. García-Jeréz4 and S. Sandoval-Castaño5 Departamento de Ingeniería Minera, Geológica y Cartográfica, Universidad Politécnica de Cartagena, 30203 Cartagena, Murcia, Spain 2 Departamento de Química y Física, Universidad de Almería, 04120 Almería, Spain 3 Geo-Systems Centre (CVRM), Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal 4 Instituto Andaluz de Geofísica, Universidad de Granada, 18070 Granada, Spain 5 Geofisica Aplicada Consultores SL, 28250 Madrid, Spain 1

Received October 2013, revision accepted April 2014 ABSTRACT Many populated areas located in moderate seismic risk regions have been hit by earthquakes of moderate magnitude which, surprisingly, have caused very serious damage to buildings and led to the loss of human life. The city of Lorca (SE Spain) is a clear example. On 11 May 2011, two main shocks occurred in the vicinity of Lorca city with a maximum magnitude of 5.1 Mw causing some casualties and serious widespread damage in the city and its surroundings. Most of the damage was concentrated in certain districts in the city, while other parts remained intact. Actually, in Lorca, investigators detected both a clear example of seismic site effect and a seismic wave amplification occurrence due to the type of geological materials on which the city is located. For this reason, several studies are being carried out in Lorca city to assess the actual contribution of the soil conditions to the seismic amplification phenomena. In these studies, the shear wave velocity plays a key role as a parameter for evaluating the dynamic behaviour of the shallowest geological materials. Consequently, site characterization applied to calculating seismic hazard is usually based on the near surface shear-wave velocity distribution. This study looked at the average shear-wave velocity for the uppermost 30 m of ground, which is referred to as Vs30. The Vs30 values obtained from multichannel analysis of surface waves (MASW) were used to create a new soil classification map of Lorca city. Thus, the derived Vs30 map was transformed into the NEHRP and Eurocode 8 (EC8) standards. In Lorca city, the softness and the thickness of shallow geological formations have been observed as two important factors that affected the level of ground shaking and the degree of damage. The results show that there is a significant correlation between the Vs30 values and the damage distribution within the city. INTRODUCTION On 11 May 2011, two seismic events with a maximum magnitude of 5.2 (Mw) hit the city of Lorca located in the south-eastern corner of the Iberian Peninsula (Fig. 1). The strongest earthquake damaged around 890 buildings and killed 8 people. Despite the moderate size of the Lorca earthquake, its effects were surprisingly severe. It is worth noting that in the recent history of Spain it was the first time that an earthquake of 5.2 Mw had occurred near a city of medium-size. Also, it should be noted that an important percentage of the severely damaged buildings comes from those termed as technological buildings. The term “techno*

[email protected]

© 2014 European Association of Geoscientists & Engineers

logical building” is used here to mention those modern reinforced-concrete (RC) buildings that were constructed in accordance with the newest Spanish seismic building codes (IGN, 2011). The level of damage in those types of building may mean the local soil properties played an important role in those harmful results and that those seismic building codes should be revised and updated for this region. In recent decades, the Lorca city has extended into areas where the soil is mostly composed of anthropogenic fillings, alluvial terraces, and colluvials. This paper deals with the use of multichannel analysis of surface waves (MASW) method as an important tool in finding a satisfactory relationship between shear-wave velocity (Vs) distribution maps and local geology. These data give insight into the 1

2

P. Martínez-Pagán et al.

actual influence of the local soil properties on the damage distribution of Lorca city. This investigation has provided a preliminary seismic microzonation map as a critical step for the accurate code-based site classification and optimal seismic design of Lorca city as the rebuilding process continues to progress. The multichannel analysis of surface waves (MASW) method has been shown to be a promising tool for detecting shallow voids and tunnels, mapping the surface of bedrock, locating remnants of underground mines, and delineating fracture systems (Miller et al. 1999a,b; Park et al. 1999c; Xia et al. 1999; Steeples 2005). In fact, surface wave (SW) analysis is often preferred for the estimation of shear properties of sediments (Socco et al. 2008). Particularly, the MASW method has been used extensively over the past decade to determine near-surface Vs profiles for engineering applications (e.g., Socco and Strobbia 2004; Foti 2005; Kanli et al. 2006; Cox and Wood 2010; Park and Carnevale 2010).

FIGURE 1 Location of Lorca city, Murcia region, in SE Spain.

GEOLOGICAL SETTING The geological structure in the vicinity of Lorca city, and the resulting geometry of sedimentary formations, are controlled by a set of faults with ENE-WSW directions (Fig. 2), which belong to the main Alhama de Murcia Faults System (AMF), and by a set of faults with NW-SE and SW-NE directions (Martínez-Díaz et al. 2012; Navarro et al. 2013). Figure 3 shows the new geological map of Lorca city at scale 1:10,000 (Alcalá et al. 2012), accomplished by merging the information derived from 14 field-ground tests, 40 mechanical drillings, 27 vertical electrical sounding (VES) arrays, and 10 shallow refraction profiles with a detecting depth ranging from 10 to 50 m depth (Alcalá et al. 2012). Besides, this information was completed by aerial photointerpretation and numerous field observation trips to outline every significant rock outcrop. In Lorca city, 17 geological formations have been identified (Fig. 3). These 17 geological formations have been grouped into five main geological-seismic formations in accordance with the geological, geotechnical, and geophysical information provided. This type of grouping is described as follows: (1) Palaeozoic to Triassic pre-orogenic metamorphosed hard-rock (bedrock), which includes schist, phyllites, quartzite, and dolomitic limestone from the Alpujárride and Maláguide Complexes; (2) Middle-Upper Tortonian pre-orogenic medium-hard bedrock, which includes conglomerates, marls, gypsum, sandstones and Pliocene consolidated glacis; (3) Pleistocene unconsolidated glacis and colluvials; (4) Holocene unconsolidated colluvials and alluvial terraces; (5) croplands and anthropogenic fillings. In this classification the basement (bedrock) was considered as “medium-hard” for the Miocene materials, and as “hardest” for the pre-Triassic materials. These two types of bedrock can be observed outcropping at northern and western parts of the city. At the centre of Lorca city, these materials are located at a depth ranging from 10 to 50 m. At the southeast part of the city, the FIGURE 2 Geostructural map of the Alhama de Murcia fault in the epicentral area of the Lorca’ 2011 earthquake. Mapped structures affecting the upper Miocene deposits are also shown: dotted lines: fold axis; continuous lines: normal faults; lines with arrows: reversestrike slip faults. To the left four transversal and longitudinal topographic profiles are shown (Martínez-Díaz et al. 2012).

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

Shear-wave velocity for seismic microzonation analysis

FIGURE 3 Geological map of Lorca city at scale 1:10.000 after Alcalá et al. (2012). Legend for geological formation: 1 Anthropogenic fillings (dashed black lines), 2 Alluvial terraces, 3 Colluvials, 4 Glacis III, 5 Glacis II, 6 Glacis I, 7 Sandy marls and breccias, 8 Gypsum and marls, 9 Marls, 10 Breccias and marls, 11 Conglomerates, sandstones and marls, 12 Marls, gypsum and sandstone, 13 Polygenic conglomerates, 14 Dolomitic limestone, 15 Red clays, slates and quartzite, 16 Phyllites, schist and quartzite, 17 Schist, phyllites, and quartzite; (a) undifferentiated geological contact; (b) normal fault; (c) thrust fault; (d) inferred fault; (e) geological crosssection A-A’ (Refer to Navarro et al. 2008; Alcalá et al. 2012); (f) urban boundary (blue bold line); (g) main roads. SPAC array locations: SP1 to SP11 (refer to Navarro et al. 2012).

same geological materials have been collected at 100 m depth. The thickness of Plio-Quaternary materials decreases steadily, disappearing towards the north and northwest parts of the city. The bedrock will get deeper to the southeast part of the town due to a set of ENE-WSW faults. However, the bedrock will get deeper or shallower in turn by the set of ENE-WSW faults.

3

MATERIAL AND METHODS The site characterization in calculating seismic hazards is usually based on the near surface shear-wave velocity values. Actually, Vs30 is accepted for site classification, such as the National Earthquake Hazards Reduction Program (NEHRP 2001) in the USA and the Eurocode 8 (EN 1998) in Europe. The International Building Code (IBC) published the same classification designations in 2000 as one of the parameters that should be accounted for in structural design. In fact, the methodology chosen in this study, which is based on shear-wave velocity (Vs) ranges as described below, is based on those considerations. Through the streets of Lorca city, eight kilometres of linear transects of multichannel analysis of surface waves (MASW) method were acquired, a technique developed and discussed by Park et al. (1999a,b) and Xia et al. (1999). More details about the theoretical background of the MASW method can be found in Roma (2010). The MASW method has been employed extensively over the past decade to determine near-surface shear-wave velocity (Vs) profiles for engineering applications. MASW allowed us to differentiate and separate coherent sources of noise (i.e., direct, refracted, and reflected P- and S- waves). One of the main steps of MASW data processing is to identify trace-to-trace coherent arrivals that are not fundamental-mode Rayleigh waves and to remove them from the data (Steeples 2005; Socco and Strobbia 2004; Roma 2010). Then, the dispersion properties of those surface waves or ground roll can be analysed, providing useful information for geotechnical applications (Socco et al. 2008; Roma 2010; Park 2013) as is discussed in this paper. From all versions of MASW method that might be chosen we decided to employ the active MASW method (Park et al. 1999c; Styles 2012). It is the conventional mode of survey using an active seismic source (e.g., a sledge hammer) and a linear receiver array, collecting data in a roll-along mode (Fig. 4a). Nevertheless, in order to perform measurements in the urban area of Lorca city a towed land streamer was developed to collect seismic data with high productivity. This land streamer was constructed with a heavy-duty fire hose. Into that rubber hose 12  28Hz- and 12 4.5Hz-geophones were interleaved, mounted and screwed on metal plated with a spacing of 2 m to protect them from any damage and improve their coupling. In fact, using 4.5Hz-geophones improved the capabilities of the land streamer as they were able to obtain not only dispersion curves with a better signal-to-noise ratio but also to increase the investigation depth (Lima Júnior et al. 2012). In any case, a more powerful seismic source than that of a sledge hammer was necessary to obtain seismic data down to 30 m depth with a high signal-tonoise ratio (SNR) in the dispersion image. For that purpose a self-propelled penetrometer wagon (Fig. 4b) was used as nonexplosive seismic source which generated a seismic energy that allowed us obtain seismic information from 35 m to 45 m depth. It is worth noting that in civil works the use of this type of penetrometer wagon is widespread so it makes this equipment very accessible, affordable and suitable for seismic studies as an

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

4

P. Martínez-Pagán et al.

FIGURE 4 (a) Land streamer and recording unit. (b) Seismic source used during the Lorca city MASW surveys.

FIGURE 5 (a) MASW profiles through the urban area of Lorca city. (b) Damage distribution in Lorca city due to 11 May 2011 Lorca earthquake (Navarro et al. 2012).

appropriate seismic source. The offset to the first geophone was 4 metres. As discussed by Park (2013), that offset falls into the optimum offset range which is required for a high SNR due to the strong energy of surface waves. The recording unit (Fig. 4a) was a SUMMIT II Compact unit, a commercially available seismograph from DMT, Germany. The acquisition array of 46 m length was displaced every 10 metres allowing us to collect enough data to map the urban area of Lorca quickly and efficiently. Actually, the total length of MASW profiles carried out in Lorca city with this land streamer was 9 kilometres. It should be pointed out that never before has a city in Spain been surveyed with this detail using MASW method for seismic microzonation studies. It should also be remarked that this land streamer had a pace of 1 kilometre per day even though some measurements were done in the middle of heavy traffic. The software used to process the seismic data was the “SurfSeis” package from the Kansas Geological Survey, USA. The aim of that process consisted of extracting the fundamental-mode dispersion curves of Rayleigh waves from each set of records (or

shot gathers) and inverting them to obtain one-dimensional (1D) Vs profiles (one profile from each curve). Then, by placing each 1D Vs profile at a surface location corresponding to the middle of the receiver line, a 2D (surface and depth) Vs map was constructed by means of the “SurfSeis” package mapping tools. For additional details about surface-wave analysis, inversion and attenuation, readers can be referred to Strobbia et al. (2011), Xia et al. (1999), Park et al. (1999c), and Socco and Strobbia (2004). It should be mentioned that the MASW Vs-maps have been correlated with the Vs data obtained through the spatial autocorrelation (SPAC) method (Martínez-Pagán et al. 2012; Navarro et al. 2012). Besides, the completion of the urban shallow geology and its geometry were based on previous geological and geotechnical data (Alcalá et al. 2012). RESULTS In 2012, we conducted the MASW method in a continuous way so as to obtain non-intrusive Vs profiles in Lorca town. A total of 22 MASW profiles covering 9 kilometres of linear transects were

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

Shear-wave velocity for seismic microzonation analysis

finally conducted, giving the largest MASW survey displayed in Spain to date. Figure 5a provides the location of the MASW profiles. These profiles were laid out according to areas of interest where strong site effects were suspected based on previous works, the local geology, and the analysis of building damage (Fig. 5b). Actually, most of the MASW profiles were carried out on urban areas where the shallow geology is composed of anthropogenic filling, alluvial terraces, and colluvials. In fact, the most severely damaged RC technological buildings are located in those urban areas. These strongly affected urban areas are La Viña, La Alberca, and La Alameda districts (Fig. 5b). Figures 6 and 7 depict the sort of results obtained with the different MASW profiles in Lorca city. Due to the large number of MASW profiles performed in Lorca city, only two representative profiles are described below by way of example to show the methodology for data treatment and mapping, and the geological

5

knowledge for a 2D calibration: Curtidores street MASW profile and Granada street MASW profile. In both profiles, the detecting depth for the average Vs was slightly below 30  m. The term “detecting depth” is meant here as the actual depth from which come back the seismic energy to the geophones in order to obtain valuable information from those deep layers. As commented above, the integrated average Vs value for 30 m depth is denoted Vs30. Figure 6a shows the average Vs30 value through the Curtidores Street, near La Viña district (see Fig. 5a). This Vs30 curve ranges slightly above 400 m/s which means that this subsurface is classified as B2 site class (soft rock or very dense soil) according to EC8 site classification. Figure 6b shows the shearwave velocity (Vs) section of the Curtidores Street, examination of which explains the reason for the sort of Vs30 values range provided. The Vs section reflects a simple Vs values distribution that does not display significant lateral variations. From Fig. 6b, FIGURE 6 (a) Vs30 MASW profile no. 10 obtained through the Curtidores Street (La Viña district), (b) S-wave velocity section from the MASW survey conducted along the Curtidores Street.

FIGURE 7 (a) Vs30 MASW profile no. 10 obtained through the Granada Street, (b) S-wave velocity section from the MASW survey conducted along the Granada Street.

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

6

P. Martínez-Pagán et al.

it can be seen that these S-wave velocity values are below 500 m/s to a depth of around 23 metres which is referred to as the Pleistocene glacis. Below 23 m depth, the Vs values increase to reach S-wave velocities above 800 m/s due to the Miocene medium-hard bedrock. Figure 7a shows the Vs30 curve of a partial section from the MASW profile no. 10 through the Granada Street, which is of 450 metres length. In this case, the Vs30 values range from 360  m/s to 460 m/s. Based on the EC8 site classification, this surveyed area falls into a B2 site class (soft rock or very dense soil). Despite the section being located very close to the Curtidores Street, which was also classified as B2 site class, the Vs30 curve shows values slightly lower due to local geological heterogeneities (see Figs 3, 7b, and 8). The S-wave velocity section in Fig. 7b exhibits this fact where the S-wave velocity values distribution is not utterly homogeneous and where it presents areas of low S-wave velocity values (600–700 m/s) between areas of higher S-wave velocity values (800–900 m/s) at a depth of 30 metres. This is due to the fact that, in La Viña district area, a 20-m-thick Pleistocene continental glacis formation is underlain by a Miocene medium-hard bedrock and a Pre-Triassic hard bedrock. Continental glacis is formed by a high-energy succession of piled colluvials with erosive, cemented channels at the bottom and silty-carbonated levels with greater lateral continuity at the top of each sequence. This results in lateral and vertical changes in Vs as shown in Fig. 7b. In each sequence, the decreasing trend of Vs toward the top gives the alternating appearance of high- and low-Vs values, while the lateral changes are due to the contact between erosive and not erosive tracts. Nevertheless, it is necessary to highlight that in both S-wave velocity sections

(Figs 6b and 7b) the material considered as stiff soil has a thickness of about 23 metres which means it has had a significant influence on the provided Vs30 curves falling into a B2 Site Class. The validity of the MASW data was confirmed by an independent seismic method, such as the Spatial Autocorrelation (SPAC) method (Navarro et al. 2008, 2012) which consisted of ambient noise observations. Figure 9 reports the results obtained from an SPAC array labelled as SP11 (refer to Fig. 5a). This SPAC profile SP11 classifies the area as a B2 site class, which is in absolute agreement with the results obtained through the MASW method. In fact, this validation was done, where it was possible, by the comparison of Vs30 models from close SPAC arrays and MASW profiles and the matching was absolutely satisfactory. Actually, at the array SP7 position (Fig. 3), the obtained value of Vs30 was of 536 m/s according to Navarro et al. (2012; 2013), which the EC8 site classification gives a site code of B1 (490–620 m/s) and where the Vs30 result obtained with MASW method was analogous. In order to support that in Fig. 10 two S-wave velocity profiles from SPAC (SP7 array) and MASW (profile no. 21) measurements are compared. Also, in Fig. 10 the lithological column provided by borehole Bh3 is compared with the S-wave velocity profiles (Fig. 5a). It is essential to point out that for the seismic design of a code-compliant structure, the Vs30 beneath the structure determines the appropriate short- and mid-period amplification factors to be applied to modify the reference earthquake spectra (e.g., Dobry et al. 2000; Navarro et al. 2013). One relevant goal in our study was the generation of Fig. 11 as a result of an exhaustive gathering of Vs30 data from all MASW profiles conducted through the urban area of Lorca city. Figure 11 updated a

FIGURE 8 Lithological columns from boreholes (a) S-1, (b) Bh10, and (c) Bh9.

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

Shear-wave velocity for seismic microzonation analysis

7

FIGURE 9 Example of the analyses carried out by using seismic arrays at La Viña district: (1) SPAC coefficients for circular array of radius 20 m (mean, mean-plus-one-sigma and mean-minus-one sigma curves are represented); (2) Rg-wave phase velocities. Blue colour line represents smoothed fundamental-mode Rg dispersion curve, red colour line represents the theoretical dispersion curve obtained from shear-wave velocity model and dispersion curves for radius measured for different radii R (20 m; 10 m; 5 m; and 2.5  m); (3) Shear-wave velocity model (red colour) derived from inversion of phase velocities, initial model represented by yellow colour, and blue circles which represent the S-wave velocity values obtained from the l/3 criterion; (4) H/V spectral ratio (Navarro et al. 2012).

more detailed map of EC8 Vs30 site classes (or NEHRP soil classification). This Vs30 map of Lorca city represents a new map of EC8 (or NEHRP soil classification) Vs30 site classes in Lorca city. Besides, it is worth noting that in Spain this sort of detailed Vs30 map from 1D S-wave velocity profiles for seismic microzonation studies, which are conducted every 10 metres, had never before been obtained. In fact, 1D S-wave velocity profiles carried out every 10 metres means S-wave velocity sections of huge detail (i.e. one hundred 1D S-wave velocity profiles every thousand metres of MASW profile). Soil classification of Lorca city by Eurocode 8 standards In Fig. 11, the Vs30 boundaries are contoured and their correspondence to NEHRP and EC8 site class codes are represented by different colours to make them easily identifiable on the map. For simplicity, the results are nevertheless described only in terms of EC8 standards. The northernmost part of the city shows Vs30 values ranging from 490 m/s to 760 m/s which are within the Vs30 bounds of EC8 Site Class B1 and therefore correspond to a soil profile of soft rock. That part of the city did not reveal significant occurrence of structural damage to the buildings. Actually, most of

FIGURE 10 (a) Lithological column from borehole Bh3, (b) S-wave velocity profiles from SPAC array (SP7) and MASW profile no. 21.

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

8

P. Martínez-Pagán et al.

FIGURE 11 Vs30 map of Lorca city following criteria from the EC8 and NERRP site class codes (Holzer et al. 2005; NEHRP 2001; EN 1998).

those damage patterns to buildings could be categorized as shear cracks in non-structural walls, which is classified as slight damage according to Okada and Takai (2000). The Vs30 values comprised between 180 m/s and 360 m/s in the dry watercourse of Guadalentin river, classify this soil as stiff (EC8 Site Class C). Another dry watercourse which flows out of the Guadalentin river (refer to Fig. 5a) shows Vs30 values in the 180–360 m/s range (yellow colour representing Site Class C in Fig. 11). The westernmost part of Lorca city exhibits areas with Vs30 values above 500 m/s with some zones of the city having Vs30 values within the Vs30 bounds of EC8 Site Class A. In that part of the city, close to the Lorca castle, there are Miocene formations with better geotechnical conditions outcrops (Fig. 11). Actually, that district did not display the occurrence of severe damage in its buildings. The results from the easternmost area of Lorca city depict Vs30 values falling into the Vs30 bounds of EC8 Site Class B2 (Vs30 values from 360 m/s to 490 m/s). In the district named La Alameda, which is located in this part of the Lorca city (Fig. 5a), the soil falls into EC8 Site Class C (Fig. 11). The district of La Alameda is located between a dry watercourse and the Guadalentin river. That fact affects the geotechnical characteristics of its soil which is in poorer condition than that of the surrounding area. In fact, La Alameda was one of the most affected districts by the earthquake as part of its buildings had to be demolished due to severe structural damage. The results obtained through the centre of Lorca city, where the main streets are located, mostly show Vs30 values that are

within the Vs30 bounds of EC8 Site Class B2 except for small areas where the Vs30 values range from 180 m/s to 360 m/s (Fig.  11). Particularly, two small areas classified as EC8 Site Class C were identified close to La Viña and La Alberca districts (Fig. 5a). Those districts also had an important number of buildings severely damaged: some of them ended up collapsing just after the earthquake’s occurrence or being demolished for safety at the beginning of the reconstruction tasks which the city is currently being subjected to. DISCUSSION Comparison of EC8 site class boundaries with surficial geology The Vs30 values calculated with MASW have been used to create a new EC8 seismic site classification map for Lorca city. This is an important breakthrough in facilitating a code-based soil classification which updates with a greater resolution other preliminary maps (Navarro et al. 2008; 2012). Figure 11 shows contour lines for Vs30 with lines of 180, 360, 500, and 800 m/s related to the boundaries between the EC8 zones. The EC8 zones for the Lorca area, as shown in Fig. 11, are directly related to shallow geological formations (Alcalá et al. 2012). These are discussed as follows. The Vs30 values of unconsolidated Holocene colluvial and alluvial terraces are comprised between 180 and 360 m/s so these geological materials are classified as EC8 Class C (or D (NEHRP)). The Holocene colluvials and alluvial terraces belong to the Guadalentin river valley (geological material keyed as (2)

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

Shear-wave velocity for seismic microzonation analysis

and (3) according to the legend in Fig. 3). Moreover, the MASW method (MASW profile 3, 4, 11, 13 and 18) highlighted an area classified as Class C which is located close to the dry watercourse. For this area, Fig. 3 depicts a shallow geology characterized by unconsolidated Pleistocene glacis and dispersed colluvials (geological material keyed as (4) and (5) according to the legend). However, the intensive activity of erosion, sediment transport, and fine particles settling on the dry watercourse has contributed to the establishment of an area characterized by MASW Vs30 values which are within Site Class C boundaries. Besides, in the same area, investigators have found some anthropogenic fillings (contoured by a dashed black line in Fig. 3) that have also contributed to a general decreasing of the Vs30 values. Furthermore, there are small parts of the city that also have soil conditions corresponding to site class C, like that located in the district La Alberca (Figs 5 and 11). This finding about the geotechnical conditions of the soil is relevant as the buildings from that district were especially damaged. As seen in the site classification map of Lorca city and its surroundings (Fig. 11), a large part of the city belongs to the B2 category according to EC8 and the C1 category according to the NEHRP standards. This important urban area consists of unconsolidated Pleistocene glacis (geological material keyed as (4) and (5)) with Vs30 values ranging from 360 to 490 m/s). The La Viña district, which was also strongly damaged, is located on this EC8 category B2 site. The historic district (Fig. 5) is different: it belongs to the B2 and B1 categories and was the location of some severe building damages although its buildings cannot be classified as “technological buildings” because they were built before any modern seismic building code. Nevertheless, a second urban MASW measurements campaign is planned, to be conducted through the narrow streets of the historic district to support the conclusions of other studies already carried out there (Navarro et al. 2012). These planned MASW profiles could provide some useful insight into potential reasons others than the poor quality of buildings that, if so, should also be taken into account. The south-westernmost part of Lorca city falls into the EC8 category A: this part of the city is located on a hillside which belongs to the Sierra de Las Estancias hill (Fig. 2). This hillside is composed of Palaeozoic to Triassic pre-orogenic hardest bedrock from the Alpujárride and Maláguide Complexes (geological materials keyed as (14) to (17) in Fig. 3). The Vs30 values range from 800 to more than 1000 m/s. This urban area has not reported significant building damage. The northernmost part of Lorca city, which is partly located in the margin of the Sierra de La Tercia hill, mainly belongs to the EC8 category of B1. This northern part consists of consolidated Pliocene glacis and lower-to-upper Tortonian post-orogenic medium-hard bedrock (geological materials keyed as (6) to (13) in Fig. 3). This part of the city suffered irrelevant building damage (Fig. 5). Consequently, this new Eurocode 8 site classification map

9

points out the best soil conditions for those districts located either at the south-westernmost part or at the northernmost part of Lorca city. In those zones, the structural damage to technological buildings have been non-existent or irrelevant. Those areas are characterized by thin sedimentary layers as these districts are located near the Sierra de Las Estancias and Sierra de La Tercia hills, respectively (Fig. 2). The eastward and southeastward spread of Lorca city means an impoverishing of the EC8 site class quality as those areas are located over the Guadalentin river basin with higher thickness of Holocene colluvials, alluvial, and anthropogenic fillings. The highly damaged district of La Alameda is located there. Actually, Lorca city is spreading to these areas, which are presently occupied mainly by small crops, vegetable gardens, and farmsteads. As a consequence, the updated EC8 Site Class in the designing of those new settlements should be considered. Comparison of average S-wave velocity and the damage distribution On the other hand, the average shear-wave velocity distribution at different depths, 5, 10, 15, 20, 25 and 30 m, respectively (Figs 12a, 12b, 12c) has been constructed as another interesting approach of showing the S-wave velocity results. In fact, Kanli et al. (2006) already pointed out the usefulness of Vs distribution maps for different depths in earthquake hazard studies. The colour-coded scale for the S-wave velocity values is blocked every 150 m/s. The set of average Vs values to 5 m depth (Vs5) interestingly shows that the three most damaged districts fall into Vs5 values ranging from 150 m/s to 300 m/s (Fig. 12a). Actually, moving downwards to 25 metres depth, the average Vs25 values in La Alameda district vary in the 150–300 m/s range. This range must be taken into account for future settlement planning in the surroundings since the city is growing towards that area. Also, the colour-coded scale at depths of 5, 10, 15, and 20 metres clearly reveals velocity values between 150 m/s and 300 m/s near the dry watercourse as a result of the thick unconsolidated sediments and anthropogenic fillings found (Fig. 12b). At the northern and western zones of Lorca city, where the structural building damage was irrelevant, the average S-wave velocity depicts an increasing pattern of its values ranging from 450 m/s to 600 m/s due to the influence of the bedrock formations. In fact, near the Lorca castle area and in a small area located at the northernmost district of Lorca city, the average Vs30 values were above 600 m/s due to the pre-Triassic preorogenic hardest and the Tortonian medium-hard bedrocks, respectively (Fig. 12c). CONCLUSIONS This paper demonstrates the usefulness of the MASW method to conduct land streamer shear-wave profiles through the Lorca city. This seismic land streamer system has proved to be a useful tool for gathering a huge amount of seismic data down to 30 m depth at a very quick pace. This last point is significant as it

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

10

P. Martínez-Pagán et al.

most part of Lorca city, close to the Sierra La Tercia hill, the highest Vs30 values were recorded. The soil condition belongs to the EC8 category of B1. Those two predominant bedrock areas in Lorca city were not exposed to high levels of damage. The MASW data provide that a large area of Lorca city is classified as EC8 site class B2 – in which is located the main damaged district of La Viña. Also, the seismic site classification map points out that the other severely damaged districts, such as La Alberca and La Alameda districts, are located in areas with unconsolidated Holocene materials, corresponding to the EC8 site class C. This detailed soil condition map based on the shearwave velocity distribution being within the NEHRP and EC8 standards is going to be very important for not only the future settlement of the urban areas but also for the site selection and the site safety evaluation studies. It is worth noting that the soil site conditions are not the only reasons for damage distribution even though this paper highlights their important role. Indeed, other reasons need to be taken into account for the damage distribution, although their discussion lies outwith the scope of this paper, such as the type of buildings, their natural periods, and the predominant period of each site. In fact, the authors are obtaining promising findings related to those factors in regards to the degree of damage and its distribution in Lorca city (Navarro et al. 2013). ACKNOWLEDGEMENTS The authors gratefully acknowledge the support provided by the Civil Protection Staff and the Local Police of Lorca city. The authors wish to express their sincere gratitude to all those who helped during the field measurements. This research is being funded by the Seneca Foundation (Murcia, Spain) Research Project 15322/PI/10 and the Spanish Ministry of Science and Innovation Research Project CGL201130187-C02-02. The fourth author is also grateful to the Portuguese FCT for the ‘Ciência 2008’ Programme Contract C2008-IST/CVRM.1. REFERENCES

FIGURE 12 Average shear-wave velocity distribution maps of Lorca city down to: (a) 5 (Vs5) and 10 (Vs10) metres depth, (b) 15 (Vs15) and 20 (Vs25) metres depth, and (c) 25 (Vs25) and 30 (Vs30) metres depth.

enabled us to obtain a more exhaustive and consistent EC8 soil classification map of Lorca city. The seismic site classification map for Lorca city highlights that the areas located on the south-westernmost part of Lorca city, close to the Sierra de Las Estancias hill, have the best soil conditions corresponding to EC8 site class A. In the northern-

Alcalá F.J., Navarro M., García-Jerez A., Vidal F., Creus C. and Enomoto T. 2012. Geology of Lorca town (Murcia, Spain). A basis for assessing seismic hazard. Proceedings of the 7th Portuguese-Spanish Assembly of Geodesy and Geophysics, San Sebastian, Spain, pp. 779-785. Cox B.R. and Wood C.M. 2010. A comparison of linear-array surface wave methods at soft soil site in the Mississippi Embayment. GeoFlorida 2010: Advances in Analysis, Modeling and Design. ASCE Conference Proceedings, 20–24 February 2010, West Palm Beach, FL. Dobry R., Borcherdt R.D., Crouse C.B., Idriss I.M., Joyner W.B., Martin G.R. et al. 2000. New site coefficients and site classification system used in recent building code provisions. Earthquake Spectra 16(1), 41–68. EN. 1998. Eurocode 8. 1998. Design of structures for earthquakes resistance- Parte 1: General rules, seismic actions and rules for buildings. EN 1998, European Committee for Standardization (CEN). Foti S. 2005. Surface wave testing for geotechnical characterization. In: Surface Waves in Geomechanics: Direct and Inverse Modeling for Soils and Rocks, CISM Series, Number 481, (eds C.G. Lai and K. Wilmanski), pp. 47–71. Springer.

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx

Shear-wave velocity for seismic microzonation analysis

Holzer T.L., Eeri M., Padovani A.C., Bennett M.J., Noce T.E. and Tinsley J.C. 2005. Mapping NERRP Vs30 site classes. Earthquake Spectra 21(2), 1–18. IGN (Spanish National Geographic Institute). 2011. Informe del sismo de Lorca del 11 de mayo de 2011 (Lorca’s earthquake of May 11th, 2011 Report). IGN, IGME, UPM, and AEIS (Eds), 138 pp. (in Spanish). Kanli A.I., Tildy P., Prónay Z., Pinar A. and Hermann L. 2006. Vs30 mapping and soil classification for seismic site effect evaluation in Dinar region, SW Turkey. Geophysical Journal International 165, 223–235. Lima Júnior S.B., Prado R.L. and Moreda Mendes R. 2012. Application of multichannel analysis of surface waves method (MASW) in an area susceptible to landslide at Ubatuba City, Brazil. Revista Brasileira de Geofísica t(2), 213–224. Martínez-Díaz J.J., Bejar-Pizarro M., Álvarez-Gómez J.A., de Lis Mancilla F., Stich D., Herrera G. et al. 2012. Tectonic and seismic implications of an intersegment rupture. The damaging May 11th 2011 Mw 5.2 Lorca, Spain, earthquake. Tectonophysics 546–547, 28–37. Martínez-Pagán P., Navarro M., Pérez-Cuevas J., García-Jerez A., Alcalá F.J., Sandoval-Castaño S. et al. 2012. Comparative study of SPAC and MASW methods to obtain the Vs30 for seismic site effect evaluation in Lorca town, SE Spain. Near Surface Geosciences 2012 – 18th European Meeting of Environmental and Engineering Geophysics, Paris, France, 3–5 September 2012, P61. Miller R.D., Xia J., Park C.B. and Ivanov J. 1999a. Using MASW to map bedrock in Olathe, Kansas. Society of Exploration Geophysicists. 69th Annual Meeting, Expanded Abstracts, 433–436. Miller R.D., Xia J., Park C.B., Davis J.C., Shefchik W.T. and Moore L. 1999b. Seismic techniques to delineate dissolution features in the upper 1,000 ft at a power plant site. Society of Exploration Geophysicists. 69th Annual Meeting, Expanded Abstracts, 492–495. Navarro M., García-Jerez J.A., Alcalá F.J., Vidal F., Enomoto T., Luzón F. et al. 2008. Vs30 Structure of Lorca town (SE Spain) from Ambient Noise Array Observations. Paper presented at the 31st General Assembly of the European Seismological Commission (ESC 2008), Hersonissos (Crete, Greece), 7–12 September. Navarro M., García-Jerez A., Alcalá F.J., Vidal F., Aranda C. and Enomoto T. 2012. Analysis of site effects, building response and damage distribution observed due the 2011 Lorca, Spain, Earthquake. Proceedings of the 15th World Conference on Earthquake Engineering (WCEE), 24–28 September 2012, Lisbon, Portugal, Expanded Abstracts.

11

Navarro M., García-Jerez A., Alcalá F.J., Vidal F., and Enomoto C. 2013. Local site effect microzonation of Lorca town (Southern Spain). Bulletin of Earthquake Engineering 4(4). doi: 10.1007/s10518-0139491-y. NEHRP. Building Seismic Safety Council. 2001. National Earthquake Hazards, Reduction Program (NEHRP) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures, Part 1 – Provisions and Part 2 – Commentary, Reports No. FEMA-368 and FEMA-369, prepared by the Building Seismic Safety Council for the Federal Emergency Management Agency, Washington, D.C. Okada S. and Takai N. 2000. Classifications of structural types and damage patterns of buildings for earthquake field investigation. Proceedings of the 12th World Conference on Earthquake Engineering (WCEE), 30 January – 4 February 2000, Auckland, New Zealand, Expanded Abstracts, 0705, 8 pp. Park C.B. 2013. MASW for geotechnical site investigation. The Leading Edge 32(6), 656–662. Park C.B. and Carnevale M. 2010. Optimum MASW survey – Revisited after a decade of use. GeoFlorida 2010: Advances in Analysis, Modeling and Design. ASCE Conference Proceedings, 20–24 February 2010, West Palm Beach, FL. Park C.B., Miller R.D. and Xia J. 1999a. Detection of near-surface voids using surface waves. SAGEEP99, Oakland, California, Expanded Abstracts, 281–286. Park C.B., Miller R.D., Xia J., Hunter J.A. and Harris J.B. 1999b. Higher mode observation by the MASW method. Society of Exploration Geophysicists. 69th Annual Meeting, Expanded Abstracts, 524-527. Park C.B., Miller R.D. and Xia J. 1999c. Multi-channel analysis of surface waves. Geophysics 64, 800–808. Roma V. 2010. Seismic geotechnical site characterization by means of MASW and ReMi Methods. FastTIMES 15(3), 16–28. Socco L.V. and Strobbia C. 2004. Surface-wave method for near-surface characterization: a tutorial. Near Surface Geophysics 2(4), 165–185. Socco L.V., Boiero D., Comina C., Foti S. and Wisén R. 2008. Seismic characterization of an Alpine site. Near Surface Geophysics 6(4), 255–267. Steeples D.W. 2005. Shallow seismic methods. In: Hydrogeophysics, (eds Y. Rubin and S.S. Hubbard), pp. 215–251. Springer. Strobbia C., Laake A., Vermeer P. and Glushchenko A. 2011. Surface waves: use them the lose them. Surface-wave analysis, inversion and attenuation in land reflection seismic surveying. Near Surface Geophysics 9(6), 503–514. Styles P. 2012. Environmental Geophysics: Everything you ever wanted (needed!) to know but were afraid to ask! European Association of Geoscientists and Engineers (EAGE), Educational Tour Series, no. 7, 220 pp. Xia J., Miller R.D. and Park C.B. 1999. Estimation of near-surface shearwave velocity by inversion of Rayleigh waves. Geophysics 64, 691– 700.

© 2014 European Association of Geoscientists & Engineers, Near Surface Geophysics, 2014, 12, xxx-xxx