Istituto Nazionale di Geofisica e Vulcanologia, Sezione Milano-Pavia, via Bassini 15, 20133, Milano, Italy.

Transcription

1 RAIS: a real time strong-motion network in Northern Italy Paolo Augliera (1), Marco Massa (1), Ezio D Alema (2) and Simone Marzorati (2) (1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione Milano-Pavia, via Bassini 15, 20133, Milano, Italy. (2) Istituto Nazionale di Geofisica e Vulcanologia, Centro Nazionale Terremoti, c/o strada Cameranense 1, 60131, Passo Varano (Ancona), Italy. Subject classification: Surveys, measurements, and monitoring Key words : Measurements and monitoring, strong motion network, data acquisition system, data processing, North Italy Corresponding author: Paolo Augliera, Istituto Nazionale di Geofisica e Vulcanologia, Sezione Milano-Pavia, via Bassini, 15, Milano, Italy: paolo.augliera@mi.ingv.it 1

2 Abstract When compared to more seismically active regions, damaging earthquakes appear occasionally in the central part of Northern Italy. Nevertheless, the lack of a dense strongmotion network in this area was highlighted by the occurrence of the 24 th November 2004, M L 5.2, Salò earthquake. Since 2006, the INGV (Italian National Institute for Geophysics and Volcanology) department of Milano-Pavia (hereinafter INGV MI), begun the installation of new strong motion stations in Central- Northern Italy. At present the RAIS (Strong-Motion Network in Northern Italy) includes 22 stations with an average inter-distance of about 20 km. All stations are equipped with Kinemetrics FBA ES-T coupled with 24-bits digital recorders. Starting from 2009, 14 strong-motion stations send data to the INGV acquisition center of Milan in real time using TCP/IP over Wi- Fi links. Another 8 stations still work in dial-up mode and send data through GSM modems. The real time connection allow to use strong motion data recorded by RAIS for generation of the ShakeMaps for Italy, the implementation of which represented one of the main task of the last agreement between INGV and Italian Civil Protection (DPC). The RAIS data storage, directly performed at INGV MI, is realized by using the MiniSEED format; the data management and exchange are realized by the SeisComP package with the SeedLink protocol. The metadata dissemination is achieved through the website where the strong motion parameters related to each recorded and processed waveforms are available. Introduction Strong-motion monitoring in Italy started in the early 70 s, when a national strong motion network was designed and installed by ENEA (Italian energy and environment organization) and ENEL (Italian electricity company). Since 1997 the Italian strong motion network (Rete Accelerometrica Nazionale, RAN) is run by the Italian civil protection (DPC). In spite of the relevant number of RAN stations (at present about 250 digital stations and 120 analogue stations, they do not homogeneously cover the Italian territory. In 2

3 particular the great number of stations are installed in the areas characterized by a high level of seismicity, such as the central and southern Apennines, the Friuli and Sicily regions. This problem was pointed up after the occurrence of the 24 th November 2004, M L 5.2 (Mw 5.0) Salò earthquake (Augliera et al., 2006). This event was one of the strongest earthquake that shocked the Northern Italy region in the last 30 years. On the basis of the official data provided by the Lombardia Region authorities, this earthquake, felt in Northern Italy, strongly affected about 70 municipalities close to the epicentral area, damaging about 3500 buildings and 300 churches, for an approximate damage evaluation of 200 million euros. In the epicentral area only the Gavardo analogue strong-motion station (GVD, see triggered (on the S-phase) during the mainshock. The peak ground horizontal acceleration, recorded at an epicentral distance of 14 km, was 71 cm/s 2. Due to the lack of others strong-motion stations installed in the surroundings of the epicenter, no data were available for distances lower than 90 km. With the aim to assure high quality nearsource recordings in the case of future earthquakes in Northern Italy, since June 2006, the INGV MI started up with the installation of a dense strong-motion network (RAIS, in Italian Rete Accelerometrica in Italia Settentrionale, in the area surroundings the 24 th November 2004 epicenter (Figure 1). Even if Northern-Central Italy is a region characterized by low seismicity (concerning both the energy release and the occurrence of the events) the importance and the necessity of the strong-motion monitoring in this areas is demonstrated by the occurrence of some relevant historical earthquakes (Figure 2, bottom panel), such as the 1117 Mw 6.49 Verona earthquake, the 1222 Mw 6.05 Brescia earthquake, the 1695 Mw 6.61 Asolo earthquake and the more recent 1901 Mw 5.67 Salò earthquake (CPTI04 catalogue, Gruppo di lavoro CPTI 2004). At present, after 4 years of recordings, the data storage performed at the acquisition center of Milan, allow us to collect a high-quality dataset (Figure 2 top panel) composed by 103 events (about components waveforms) with M L ranging from 0.7 to 5.1 (the 23 th December 2008 Parma earthquake). 3

4 This paper is focused on the RAIS technological upgrade performed in the last two years. The first phase of installations, the main features of the first-generation RAIS network (mainly based on a dial-up transmission system) and the detection capability were already described in a previous paper (Augliera et al. 2009). In this paper the last developments and the actual configuration of the network are presented, with particular attention on site selection, acquisition system and data transmission. The final section of the paper is focused on data processing and real-time dissemination, in particular with the aim to improve the generation of ShakeMaps for events occurred in Northern Italy. Site selection and its characterization RAIS arose in the framework of the INGV-DPC agreement. The first installation were founded by the project Stazioni Accelerometriche (in English Strong-Motion stations, while the further developments were made thank to the last INGV-DPC S3 project Fast evaluation of the parameters and effect of strong earthquake in Italy and in the Mediterranean areas. At the beginning, two of the main goals of the Stazioni Accelerometriche project were to improve the earthquake detection in the area of North-Central Italy and to assure a tool for strong-motion monitoring able to provide highquality records in the case of strong events. The first phase of the mentioned project concerned the selection of sites suitable for seismic installations. In this way three main aspects were considered: the locations of the seismic stations managed by other organizations (Figure 3), in order to avoid too high or too low density of stations (both weak and strong-motion) in correspondence of a particular area, the location of instrumental seismicity recorded in the region under study from 1981 ( and and the municipality of the area characterized by the highest values of seismic hazard (Gruppo di Lavoro MPS04, expressed in term of maximum peak ground horizontal acceleration with a 10% of probability to be exceeded in the next 50 years (return period of 475 years). 4

5 In Figure 3 only RAIS and national network are reported, but others local and regional networks operated in this area has been considered, for example RAF (Friuli Venezia Giulia Accelerometric Network), the short-period seismometric regional networks of Friuli Venezia Giulia (RSFVG) and of Veneto (RSV) managed by Centro di Ricerche Sismologiche of the OGS, the Regional Seismic Network of Northwestern Italy (managed by Dip.Te.Ris., Genoa University). For each considered site, the final selection represented generally a compromise between the network geometry, which depends on the monitoring purposes, and the characteristics that a given site presents in order to be suitable for an installation. Considering the high degree of urbanization and industrialization in the considered area, all installations were preceded by microtremors analysis (Nakamura 1989). Taking into account that a low level of noise (Peterson, 1993) is difficult to achieve in this area, the measures was in particular carried out with the aim to avoid very unfavourable situations. For each site quick horizontal to vertical spectral ratio (HVSR) and additional probability density functions (McNamara and Buland 2004) were computed by following the procedure presented in Marzorati and Bindi (2006). Concerning the processing of the seismic noise, time series with a duration of at least 30 minutes, recorded with a broad band sensor (sampling frequency 100 Hz), were considered. The time series were divided into segments of 60 seconds with an overlapping of 75% in order to reduce the variance in the power spectral densities (PSD) computation. For each window of noise the mean and the linear trend were removed and a digital Butterworth filter, in the frequency range Hz, were applied. At present all site where a RAIS station is installed are characterized both from a geological ( link caratterizzazione siti ) and a geophysical point of view. Concerning geology, all information come from 1: geological maps provided by Lombardia region (CARG project, 2003) or 1: Italian geological maps (SGI, 1984). From a geophysical point of view, for each site an averaged horizontal to vertical spectral ratio is available considering the earthquake recordings from June

6 For each recorded waveform an automatic procedure allow to update step by step the actual averaged amplification function calculated for a single site. The processing of the recorded strong-motion data (sampling frequency of 100 Hz) included the following steps: removal of the mean and linear trends, 5% cosine tapering, application of an acasual 4-pole Butterworth band-pass filter between 0.2 Hz and 25 Hz. For each event, the Fourier spectra were computed considering different time windows (5 s and 10 s starting 0.5 s before the S-phase onset). To investigate the differences between differently polarized horizontal components, directional spectral ratios were obtained by applying different rotation angles. In particular, for computing directional spectral ratios, the NS and EW components were rotated clockwise from the North, by between 0 and 175, in angular steps of 5, which provided for computation of 36 rotated components, which were representative of 36 directions of horizontal ground shaking. A resume of the products available for each site is presented in figure 4 for the Vobarno station (VOBA in table 1). Finally, taking into account both the geological and geophysical remarks, all stations were classified following the provision of the Italian seismic code for building (NTC 2008). Instrumentation and installation At present the RAIS network is composed by 22 stations (table 1): the strong motion sensors are s FBA ES-T (http//: characterized by a dynamic range of 155 db. Generally the full-scale is set to ±2.0 g. In figure 5 is shown a typical frame of installation. In each site the accelerometer is housed in ad-hoc built concrete basement and anchored to it (top right panel). Close to the concrete basement a plastic-box, with dimension of about 100 cm long, 80 cm width and 50 cm high is located. As shown in figure 5 (bottom left panel) the box contains the digital recorder (the GAIA-2 recorder in this picture), a router for data transmission (TCP/IP protocol) and a stabilized power supply connected to a 12V battery in order to avoid possible lacking of electricity. 6

7 The time synchronization of the recorded signal is assured by a GPS antenna installed near the station. At present the network is characterized by 2 different type of digital recorders: 11 strong-motion sensors are coupled with 24 bits Reftek digital recorder ( while the other 11 sensors are coupled with 24 bits GAIA-2 (Rao et al. 2010). This type of digital recorder was designed and produced directly by the laboratory of INGV-CNT ( At present all stations record signals in-continuous-mode with a sampling frequency of 100 Hz. Acquisition system and data transmission: a brief history and the present Since June 2006 until the end of the 2008, the installed s were equipped with 20 bits Lennartz Mars88 Modem Control ( or 24 bits Reftek 130 digital recorders. In both cases the remote stations transmitted data to INGV MI acquisition center GSM modems (D Alema and Marzorati 2004; D Alema 2007). The GSM (Global System for Mobile communication) data transmission did not allow real-time determinations of the engineering parameters that actually represents one of the main goal for a strong-motion network. The GSM data transmission represented in the first period a relevant technological limitation, that forced the on-line data dissemination to have a time-delay of the order of hours. In the framework of the recent INGV-DCP S3 project, many efforts were made to replace the highest possible number of GSM stations with a system able to record data in continuous mode and at the same time to transmit data in real-time. This important evolution of the network started at the beginning of the 2009 and led to the replacement of the Mars88 Modem Control digital recorder (without a TCP/IP support and thereby not suitable to transmit data in continuous mode) with 24 bits digital recorders. At the same time a contemporary phase (at present still in progress) that led the replacement of the GSM technology with TCP-IP connections started. At present 14 stations on a total of 22 are connected to the acquisition center of Milano in real-time. 11 of this 14 stations are now equipped with the GAIA2 digital recorders, while 7

8 other three are equipped with Reftek-130 digital recorders. The other eight, are still dial-up stations and send data via GSM modem. For the management of the real time stations the SeedLink protocol is adopted. The format of the recorded data (previously recorded both in binary Reftek or Lennartz format and then stored in sac format) was upgraded introducing the MiniSEED format. We use the SeedLink system for real time communication using TCP/IP protocol and a SeisComP platform (Hanka et al 2000) for monitor plotting and disk recording. For data acquisition a procedure for events detection was developed. The locations are provided by the Italian National Earthquake Center (INGV-CNT, In our case a new event location represents a warning for RAIS data acquisition and consequently the trigger for the fully automatic system. In particular every five minutes the file of event locations (revised by the seismologist which is in charge for the seismic surveillance activity in Rome) is automatically downloaded. Every time a new event is present, the theoretic spectral amplitude of the earthquake is compared (in a fixed frequency band) with the average noise levels for each station of the network. The synthetic spectrum is computed by considering the omega-square source model (Brune 1970) and the source spectrum is propagated to each station considering the 1/R geometrical spreading term. In this case some bias (however negligible considering our scope) could be introduced by the not correspondence between moment (on which the theoretical scheme is based) and local (recorded) magnitude. The procedure is however conservative (Augliera et al., 2009). At the end of the procedure, if the theoretical signal to noise ratio exceed a fixed threshold for al least 3 stations, the INGV-CNT event-id is stored and the procedure for the RAIS data automatic download is going to start. The event-id represents a necessary information in order to merge in a consistent way the data previously processed by different seismic network and then sent to the common server of INGV-CNT. 8

9 At the end of the process the RAIS waveforms are available in our acquisition center and ready to be processed by the automatic system. A resume of the procedure is shown in the flow-chart of figure 6. Data processing and dissemination At the end of the acquisition system the RAIS data are available in the workstation of the acquisition center of Milan. The last step is represented by the data processing and their dissemination. The analyses on recorded data are automatically made using a series of code, expressly developed both in fortran77 and C languages and in bash-shell. Each strong-motion waveform is processed using a standard procedure described in Massa et al. (2009) that include the baseline correction, performed by a least square regression, the mean removal considering the whole signal, the application of a cosine-taper function (usually 5%) and a filtering performed by an acasual 4 th order Butterworth digital filter ( Hz). For all processed waveforms PGA (peak ground acceleration) and SA (acceleration response spectra, 5% damped) for periods up to 4 s were calculated. Moreover the automatic system provide also PSV (pseudo-velocity response spectra), Sd (displacement response spectra), IA (Arias Intensity; Arias 1970) and IH (Housner intensity; Housner 1952) values. Finally, after the integration of acceleration time series, also the peak ground velocity (PGV) is evaluated. Concerning the site response, as discussed in a previous paragraph, for each recorded event at each site, the EHVSR (Earthquake Horizontal to Vertical Spectral Ratio) is calculated considering both 5 s and 10 s of S phase. The metadata are collected in the web site of the RAIS network that is while all waveforms related to each event with M L equal or higher than 2.5 are included in the ITalian ACcelerometric Archive (ITACA) and downloadable at the web site Moreover, since 2009, for each event with M L higher or equal than 3.0, PGA, PGV and SA values (for periods of 0.3 s, 1.0 s and 3.0 s) are sent to INGV-CNT acquisition center, where they are merged together all available strong-motion and velocimetric data in order to improve the calculation 9

10 of ShakeMaps ( for events occurred in Northern Italy RAIS data set In the period June 2006-June 2010 the RAIS network allowed us to collect a relevant data set, at least in term of number of earthquakes for this area with a low seismicity level. In the considered period 103 events (for a total of about component waveforms) with M L ranging from 0.7 to 5.1 were recorded in the epicentral distance range km (Figure 7). Concerning the magnitude, the most relevant earthquake was recorded on 23 th December 2008 at UTC (M L 5.1, Parma earthquake, /HTML/main.html). Concerning the acceleration, the earthquake that produced the highest recorded peak was the 14 th July 2008 M L 3.5 Salò event. This relatively weak event was able to produce at Vobarno station (VOBA, see table 1), located about 5 km West of the epicenter, a horizontal acceleration peak of 33 cm/s 2. In general, as shown in figure 7 (left top panel), the highest number of recorded events are included in the magnitude range In this interval the records homogeneously cover the hypocentral distance range between 10 and 200 km. For magnitude between 0.7 (the minimum recorded value) and 2.5 the hypocentral distances range between 50 and 75 km. The same consideration is made if the right panel of figure 7 is considered. In particular in the right bottom panel, where PGA versus hypocentral distance is shown for events with M L higher or equal than 3.0, it is possible to observe the presence of anomalous peaks for hypocentral distances closed around 100 km. This phenomenon is due, as demonstrated in Castro et al. (2008) and Bragato et al. (2009) to the not negligible contribution of the Moho reflection in the areas located in the East region of the Po Plain. However it is not possible to exclude also the contribution of local site effects for stations located in B and C (see table 1) lithological classes as described in the Italian seismic code for building (NTC 2008). 10

11 To strengthen the benefit produced by the RAIS network on the strong-motion monitoring of the area under study, in Augliera et al. (2009) a theoretical example was made considering some of the strongest historical events (CPTI04 catalogue, Gruppo di Lavoro CPTI 2004) occurred in the Northern Italy regions (Figure 2, bottom panel). In the case of a "reoccurrence", the 12 th May 1802 Maw 5.67 Oglio Valley earthquake (maximum macroseismic Intensity, Imax of VIII-IX MCS degrees, yellow square in the bottom panel of figure 2) currently would have been recorded by 5 strong-motion stations (4 belonging to RAIS) in the first 30 km. Similarly, the 7 th June 1891 Maw 5.71 Illasi Valley earthquake (Imax IX MCS, red square in the bottom panel of figure 2) and the 19 th February 1932 Maw 5.01 Mount Baldo earthquake (Imax VIII MCS, green square in the bottom panel of figure 2) would have nonsaturated data from 7 stations (6 belonging to RAIS) and 12 stations (7 belonging to RAIS) for epicentral distances less than 30 km, respectively (for more detailed explanation see Augliera et al., 2009). In particular, considering a hypothetical doublet of the 24 th November 2004 M L 5.2 Salò event (blue star in bottom panel of figure 2), it is possible to suppose that a similar event would be actually recorded by 11 strong-motion stations (6 belonging to RAIS) in the first 30 km. ShakeMap implementation at RAIS ShakeMaps are a very useful tool in the first minutes to hours after an earthquake has occurred, but their relevance progressively decreases as information about actual damage becomes available. For this reason it is fundamental to have data in real-time. At INGV MI we have installed the ShakeMap package, developed by the U.S. Geological Survey (USGS) Earthquake Hazards Program (Wald et al 2006) and we use this tool in agreement with the procedures developed by INGV-CNT for the ShakeMap implementation in Italy (Michelini et al. 2008). A dense and uniform spatial distribution of stations in the field is essential to produce reliable ShakeMaps (Douglas, 2007; Moratto et al. 2009) minimizing the uncertainties associated with ground motion prediction equations (Wald et al. 2008). In order to appreciate the role of the RAIS stations in the calculation of Shakemap, an 11

12 example is reported in figure 8 for the 14th July 2008 ML 3.5 earthquake, occurred near Salò. In Figure 8a is reported the Shakemap in term of instrumental intensity, obtained considering all available data (in yellow the RAIS stations), while in Figure 8b are reported the PGA residual (in term of difference between the values of ground motion parameter) obtained generating the same map with and without considering the RAIS stations. In this panel the green zone indicates positive residuals (real data recorded by RAIS are higher then the synthetic ones calculated by the empirical predictive model). Maximum differences are of the order of 18 %. The magenta areas indicate the negative ones (real data recorded by RAIS are lower then the synthetic ones calculated by the empirical predictive model). Maximum differences are of the order of 29 %. This result is probably due to the fact that the ground motion prediction equation implemented by INGV-CNT Shakemaps package and used for the area under study (Morasca et al., 2006) tends to underestimate the shaking in near source and overestimate its in far field. This evidence points out both the importance of real data availability, to be used as a test, and at the same time the importance of regional variability in the calibration of empirical predictive models. Conclusions Beginning from June 2006 a phase of installation of strong-motion stations in Northern Italy started in the framework of the INGV-DPC agreement ( Stazioni Accelerometriche project). In this last four years subsequent installations led to build a new strong-motion network in Northern Italy, named RAIS (in Italian, Rete Accelerometrica Italia Settentrionale). In the last two years, thank to funds provided by the INGV-DPC agreement (DPC-S3 project), a technological improvement of the new network was possible. At present 22 strong-motion stations are installed, with an average inter-distance of about 20 km, in the central areas of Northern Italy (Figure 1). At present 14 stations, on a total on 22, transmit data in real-time mode to the acquisition center located in Milan. The recorded data, managed by SeedLink server using a TCP/IP protocol for real time data communication 12

13 under a SeiscomP platform for monitor plotting and disk recording, are stored in MiniSEED format. At present a phase of upgrade concerning the last 8 dial-up stations (based on GSM technology) is in progress. In the period June June 2010 RAIS recorded 103 events (about component waveforms, see figure 2) with M L ranging from 0.7 to 5.1. It is worth noting that the collection of high quality strong motion data represents a fundamental tool for earthquake engineering and engineering seismology studies. In the last years, the RAIS dataset represented (and represents nowadays) a useful source for the calibration of empirical attenuation predictive models computed both at local (Massa et al., 2007; Massa et al., 2008) and at national scale (Bindi et al., 2009). Moreover a total of components records (in terms of PGA, PGV and SA for periods of 0.3 s, 1.0 s and 3.0 s), related to 30 events with M L equal or higher than 3.0, were sent to the INGV-CNT acquisition center in order to improve the ShakeMap ( calculation for the earthquakes occurred in Northern Italy. The records coming from few stations installed inside building (e.g. ASO7, BAG8) allow us to compute investigation about the soil-structure interactions (Massa et al., 2010). It is worth noting that high quality strong-motion records represent the input for each advanced structural analyses that combine ground-motion records with detailed structural models. All metadata related to the events recorded by RAIS (together with the information regarding stations and site characterization) are collected on the web site The waveforms of the events with M L 2.5, recorded in the period June December 2007, were included in the ITalian ACcelerometric Archive (ITACA, in the framework of INGV-DPC agreement (S4 project). 13

14 Acknowledgements RAIS arose in the framework of INGV-DPC agreement ("Stazioni Accelerometriche" project). This work was supported by INGV-DPC agreement, in the framework of the DPC-S3 project Valutazione rapida dei parametri e degli effetti dei forti terremoti in Italia e nel Mediterraneo. Scientific papers funded by DPC do not represent its official opinion and policies. The authors thank all people, both institutional and private citizen, that cooperated in the phases of installation. 14

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20 Captions Figure 1 - RAIS strong-motion network. Different colours are used to indicate stations characterized by different recording system or different data-transmission type. The white square indicates the epicenter of the 24 th November 2004, M L 5.2, Salò earthquake. Figure 2 - Top panel: events recorded by RAIS in the period June June 2010 (dark grey circles) and background seismicity occurred in the whole North Italy (light grey circles). Epicenter coordinate derive from the official INGV instrumental bulletin ( The red triangles indicate the RAIS strong-motion stations. Bottom panel: historical seismicity of the area under study (from CPTI04, Gruppo di Lavoro CPTI 2004). The blue star indicates the epicenter of the 24 th November 2004, M L 5.2, Salò earthquake. Figure 3 RAIS and national seismic network present in North Italy and surroundings: in red the RAIS stations ( the little black triangles plotted inside the red triangles indicate a site equipped with both weak and strong motion sensor), in green the velocimetric stations managed by the National Earthquake Center of INGV ( in yellow the INGV-CNT satellite velocimetric stations, in grey the RAN strong motion stations managed by Italian Civil Protection (DPC, in violet the SDS- Net velocimetric stations ( Figure 4 - Example of site characterization of VOBA station (NTC08 B lithological class, see also table 1). Top left panel: geological map (1: scale). Top right panel: Probability Density Function (PDF) of power spectral density (PSD) performed on seismic noise. Bottom left panel: Horizontal to vertical spectral ratio performed considering recorded earthquakes 20