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PECULIAR CHARACTERS OF NEAR AND FAR FIELD MEASURED PARAMETERS FROM RECENT ITALIAN STRONG MOTION EVENTS

Authors:

Abstract

Italy is one of the highest seismic risk country in the Mediterranean, due to the frequency of earthquakes that have recently and historically affected its territory and the intensity that some of them reached, with a significant social and economic impact. Italian strong-motion wave form data with Magnitude ranging between 5 and 7 Mw have been drawn from ESM database (Luzi et al., 2016; http://esm.mi.ingv.it). Peak accelerations in horizontal and vertical direction, Arias intensity, Trifunac duration and Spectral accelerations have been extracted and used in this study to identify the Italian boundary between the so called: near-field and far-field distances. Fifteen seismic events have been selected with a moment magnitude Mw ≥ 5.0. The analysis of the data in the first 30 km far from the epicentre (defined as near-field) shows that all characteristic data of the wave forms are highly scattered and here there are the highest values of acceleration, velocity, displacement, and intensity. Hence, to define the ground motion prediction equations GMPEs in that range of epicentral distances is almost uncertain although these equations are well calibrated for the far field distances. Furthermore, the PGA values recorded along the three directions within near field distances (<30 km) show similar values and sometimes the vertical PGA component is even higher than the horizontal ones. It is worth noticing that the horizontal and vertical peaks ground acceleration show different values in relation to the seismic soil categories (Eurocode 8 classification) especially in near field. From an Engineering Geological standpoint, the authors try to characterize the near field zone according to the recorded Italian seismic wave forms in order to improve definition of the input parameters in the Local Seismic Response Numerical Analyses.
PECULIAR CHARACTERS OF NEAR AND FAR FIELD MEASURED
PARAMETERS FROM RECENT ITALIAN STRONG MOTION EVENTS
Mancini F.1, Placentino E.1, Vessia G.1
mancini.francesca.92@gmail.com; placentinoenrica@gmail.com; g.vessia@unich.it
1 University “G.d’Annunzio” of Chieti-Pescara, Via dei Vestini 31, Italy
ABSTRACT
Italy is one of the highest seismic risk country in the Mediterranean, due to the frequency of earthquakes that have recently and historically affected its territory and the intensity that some of them reached, with a significant social and economic impact.
Italian strong-motion wave form data with Magnitude ranging between 5 and 7 Mw have been drawn from ESM database (Luzi et al., 2016; http://esm.mi.ingv.it). Peak accelerations in horizontal and vertical direction, Arias intensity, Trifunac duration and Spectral accelerations have been extracted and used in this study to
identify the Italian boundary between the so called: near-field and far-field distances. Fifteen seismic events have been selected with a moment magnitude Mw ≥ 5.0.
The analysis of the data in the first 30 km far from the epicentre (defined as near-field) shows that all characteristic data of the wave forms are highly scattered and here there are the highest values of acceleration, velocity, displacement, and intensity. Hence, to define the ground motion prediction equations GMPEs in that
range of epicentral distances is almost uncertain although these equations are well calibrated for the far field distances.
Furthermore, the PGA values recorded along the three directions within near field distances (<30 km) show similar values and sometimes the vertical PGA component is even higher than the horizontal ones. It is worth noticing that the horizontal and vertical peaks ground acceleration show different values in relation to the
seismic soil categories (Eurocode 8 classification) especially in near field. From an Engineering Geological standpoint, the authors try to characterize the near field zone according to the recorded Italian seismic wave forms in order to improve definition of the input parameters in the Local Seismic Response Numerical
Analyses.
INTRODUCTION AND AIMS
Italy shows a medium-high seismic hazard level characterized by peculiar tectonic mechanism: the normal faulting mostly observed alongside the central-southern axial zone of the Apennine chain and the
compressive faulting prevalent in the North part of Italy (Fig.1). Focusing on the wave shapes of the recorded seismic events through the Italian peninsula different characters can be evidenced comparing
near and far field records.
In this study, a number of strong motion events from recent Italian strong earthquakes have been analyzed due to their disastrous effects in terms of several hundred of casualties and damages to dwellings and
infrastructures, with millions of euros losses. They are: Umbria-Marche seismic sequence (26 September 1997 (00:33:11 UTC, Mw 5.7); 26 September 1997 (09:40:24 UTC, Mw 6.0); 14 October 1997,(15:23:09 UTC,
Mw 5.6), 6 April 2009 mainshock L’Aquila Earthquake (01:32:40 UTC, Mw 6.1), 21 June 2013 Fivizzano Earthquake (10:33:56 UTC, Mw 5.1); the central Italy seismic sequence 2016-2017 (24 August 2016
(01:36:32 UTC, Mw 6.0); 26 October 2016 (19:18:05 UTC, Mw 5.9), 30 October 2016 (06:40:17 UTC, Mw 6.5) e 18 January 2017 (09:25:42, Mw 5.1; 10:14:09, Mw 5.5; 10:25:26 Mw 5.4; 13:33:37 Mw 5.0),
31 October 2002 San Giuliano di Puglia Earthquake (10:33:00 UTC, Mw 5.7), 01 November 2002 San Giuliano di Puglia Earthquake (15:09:02 UTC Mw 5.7) and 29 May 2012 Emilia Earthquake (07:00:02 UTC,
Mw 6.0). In the aftermath of these events, different site effects have been observed and surveyed in urban areas and in the outskirts (GEER Reconnaissance 2017, 2018; EMERGEO GL 2002, 2009, 2012, 2016).
The main objective of this comprehensive study is to calculate some strong motion parameters that represent the recorded signals in time and frequency domain measured from seismic stations located both near
(within 30km) and far field (up to 100km). The parameters taken into account are the peak ground acceleration PGA, the peak ground velocity PGV, the peak ground displacement PGD, the Arias intensity IA, the
spectral acceleration SA at 0.1s, 0.3s and 1s, the Housner Intensity IH and the Trifunac duration Df.
The records obtained from the ESM database (Luzi et al., 2016; http://esm.mi.ingv.it) were analysed for these fifteen earthquake events. The strong motion parameters of the stations within 100 kilometers from the
epicenter were considered; the selected dataset contains parameters from: 62 stations for the Umbria-Marche seismic sequence, 13 stations for San Giuliano di Puglia Earthquakes, 23 stations for the mainshock
L’Aquila Earthquake, 91 stations for Emilia Earthquake, 42 stations for Fivizzano Earthquake, 362 stations for the central Italy seismic sequence and 386 stations for 2017 Campotosto Earthquakes.
Figure 1: Map of the seismic events localizations and focal mechanisms (http://cnt.rm.ingv.it/tdmt)
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TREND OF THE PGA (SOIL TYPE OF THE STATIONS "E")
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TREND OF THE PGA (SOIL TYPE OF THE STATIONS "D" AND "D*")
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TREND OF THE PGA (SOIL TYPE OF THE STATIONS "C" AND "C*")
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TREND OF THE PGA (SOIL TYPE OF THE STATIONS "B*)
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TREND OF THE PGA (SOIL TYPE OF THE STATIONS "B")
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TREND OF THE PGA (SOIL TYPE OF THE STATIONS "A*")
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TREND OF THE PGA (SOIL TYPE OF THE STATIONS "A")
TREND OF THE SPECTRAL ACCELERATION AT 1,0 S VERSUS EPICENTRAL DISTANCE
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TREND OF THE SPECTRAL ACCELERATION AT 0,3 S VERSUS EPICENTRAL DISTANCE
TREND OF THE SPECTRAL ACCELERATION AT 0,1 S VERSUS EPICENTRAL DISTANCE
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TREND OF THE TRIFUNAC DURATION VERSUS EPICENTRAL DISTANCE
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TREND OF THE HOUSNER INTENSITY VERSUS EPICENTRAL DISTANCE
Housner intensity (cm)
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TREND OF THE ARIAS INTENSITY VERSUS EPICENTRAL DISTANCE
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TREND OF THE MAX PGD VERSUS EPICENTRAL DISTANCE
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TREND OF THE MAX PGV VERSUS EPICENTRAL DISTANCE
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TREND OF THE MAX PGA VERSUS EPICENTRAL DISTANCE
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TREND OF THE PGA_Z VERSUS EPICENTRAL DISTANCE
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TREND OF THE PGA_N VERSUS EPICENTRAL DISTANCE
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TREND OF THE PGA_E VERSUS EPICENTRAL DISTANCE
STRONG MOTION PARAMETERS
Figure 2: Trends of the strong motion parameters (the peak ground acceleration PGA, the peak ground velocity PGV, the peak ground displacement PGD, the Arias intensity IA, the Housner Intensity IH and the Trifunac duration Df) versus the epicentral distance.
Figure 3: Trends of the spectral acceleration at 0,1 - 0,3 - 1,0 second versus the epicentral distance.
Figure 4: Trends of the vertical PGA versus the epicentral distance.
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In this study, different strong motion parameters were considered (Fig.2), with the focus on the peak ground acceleration values considering all three components (two horizontals and one vertical), in order to highlight the contribution of the vertical PGA especially in near field. The PGA values of the events considered
don’t exceed 1 g. Afterwards, the highest values of acceleration are shown for each seismic events. The 1997 Umbria-Marche earthquake with Mw 5.7 has the highest PGA_E and PGA_Z at 2,8 km epicentral distance, recorded at the IT.CLF station (seismic soil type EC8 D) with -0,252 g and 0,180 g respectively; instead,
the maximum PGA_N has recorded at the IT.NCR station (seismic soil type EC8 E) at 13,2 from the source with 0,387 g. This station has recorded the highest peaks ground motion (horizontals and vertical) for the 1997 Umbria-Marche seismic event with Mw 6.0, located at 10,9 km epicentral distance: -0,414960 g (PGA_E),
-0,492165 g (PGA_N) and 0,398234 g (PGA_Z). The last event of the 1997 Umbria-Marche seismic sequence with Mw 5.6 shows: -0,138813 g (PGA_E) at 23,5 km from the epicentre in IT.NCR2 station (EC8 E), -0,175507 g (PGA_N) at 8,7 km from the source in IT.CESM station (seismic soil type EC8 A*), 0,067947 g
(PGA_Z) at 20,4 km epicentral distance in IT.NRC (seismic soil type EC8 E). No stations were installed in near field during the San Giuliano di Puglia earthquakes and for this reason the highest accelerations were recorded located in far field. IT.AQV station (seismic soil type EC8 B) shows the highest acceleration peaks of
ground motion (horizontals and vertical) for the 2009 Aquila earthquake, located at 4,9 km epicentral distance: 0,644247 g (PGA_E), -0,535203 g (PGA_N), 0,486651 g (PGA_Z). The 2012 Emilia earthquake with Mw 6.0 has the highest peak horizontal acceleration at 9,3 km from the epicentre, in IV.T0814 station (seismic
soil type EC8 C*): 0,435682 g and -0,495194 g for the E-W and N-S component respectively; instead, another station closer to the epicentre IT.MRN at 4,1 km (seismic soil type EC8 C) has recorded the maximum PGA_Z with a value of -0,840739 g. The 2013 Fivizzano earthquake with Mw 5.1 has the highest peak horizontal
acceleration at 11,9 km from the epicentre, in IT.FVZ station (seismic soil type EC8 E): 0,227246 g and -0,139853 g for the E-W and N-S component respectively; instead the highest vertical acceleration has recorded in GU.FIVI station (seismic soil type EC8 C*) at 12,2 km epicentral distance and it is equal to 0,146380 g.
The IT.AMT station (seismic soil type EC8 B) has recorded the highest of the three acceleration components for two seismic events: 24 August 2016 Mw 6.0, located at 8,5 km from the source and 18 January 2017 Mw 5.1, located at 9,746 km epicentral distance (-0,850804 g, 0,368387 g and 0,391368 g for the horizontal and
vertical components of PGA respectively of the first event mentioned ; 0,193099 g, -0,354433 g and -0,115139 g for the second event). The 26 October 2016 earthquake has the highest PGA_N and PGA_Z at 14 km from the source, in IT.MCV station (seismic soil type EC8 B*): -0,538790 g and 0,474881 g respectively;
instead the maximum PGA_E has recorded in IT.CMI station (EC8 C*) at 7,1 km epicentral distance and it’s equal to -0,638312 g. The second October event of the Central Italy seismic sequence has the highest PGA_E and PGA_Z at 12 km from the epicentre, in IV.T1213 station (seismic soil type EC8 A*) : 0,779272 g and
0,868888 g respectively; instead the maximum PGA_N was recorded in 3A.MZ51 station (EC8 E) at 25,9 km epicentral distance and it’s equal to 0,947251 g. The IT.PCB station (EC8 B*) has recorded the highest of the three acceleration components for two 2017 Campotosto seismic events with Mw 5.5 and Mw 5.0 (0,400429
g, -0,575004 g, -0,171944 g for the horizontal and vertical PGA components respectively of the first event mentioned; 0,260781 g, -0,288832 g, 0,097444 g for the second event). The 18 January 2017 event with Mw 5.4 has the highest values of the horizontal PGA components at 10,753 km from the source, in IT.PCB station
and they’re equal to -0,380773 g and 0,550443 g, instead the maximum vertical PGA component was recorded in IT.MSCT station (seismic soil type EC8 B*) at 6,606 km epicentral distance with a value of -0,171982 g. The peaks ground velocity and displacement were registered in 3A.MZ04 located at 23,1 km of epicentral
distance with values of 85,393 cm/s and 24,808 cm respectively, both for the 2016 Central Italy seismic sequence with moment magnitude Mw 6.5.The Arias intensity shows higher values (608,667 cm/s) for the 2016 Central Italy sequence with moment magnitude Mw 6.5 recorded at T1213 station located at 12,0 km of
epicentral distance. The highest value of Housner’s intensity is 283,941 cm recorded in 3A.MZ04 station located at 23,1 km of epicentral distance it is also relative at 2016 Central Italy sequence with moment magnitude Mw 6.5. The same event also shows the greatest value of Trifunac duration (102,330 s) recorded at 44,4
km far from the epicentre at the SSM1 station. As expected, the values of PGV, PGD, Arias and Housner Intensity decrease as the epicentral distance increases; different and opposite trend is observed for Trifunac duration. The spectral acceleration has been analyzed considering the characteristic periods at 0,1 s, 0,3 s and
1,0 s. Also in this case the three components of acceleration were considered: as can be drawn from the graphs, the vertical component has non-negligible values in near field reaching its maximum at 0,1 s (Fig.3). The highest values are obtained, also in this case, for the 2016 Central Italy sequence with moment magnitude
Mw 6.5. For distance greater than 30 km the spectral acceleration at 0,1 s has lower values of 1,051 g for each event analyzed. The spectral acceleration at 0,3 s shows values lower than 0,88 g and for the spectral acceleration at 1,0 s the values are lower than 0,171 g both for far field. In Near field the values are more
scattered but the maxima are shown for all three events of the 2016 Central Italy seismic sequence analysed. Finally, the PGA has been plotted according to the seismic soil categories (Eurocode 8 classification). The subsoil categories B*, C and C* show the greatest amplifications (Fig.4).
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Umbria-Marche 26/09/1997 Mw 5.7
Umbria-Marche 26/09/1997 Mw 6.0
Umbria-Marche 14/10/1997 Mw 5.6
San Giuliano di Puglia 31/10/2002 Mw 5.7
San Giuliano di Puglia 01/11/2002 Mw 5.7
L'Aquila 06/04/2009 Mw 6.1
Emilia 29/05/2012 Mw 6.0
Fivizzano 21/06/2013 Mw 5.1
Central Italy 24/08/2016 Mw 6.0
Central Italy 26/10/2016 Mw 5.9
Central Italy 30/10/2016 Mw 6.5
Campotosto 18/01/2017 Mw 5.1
Campotosto 18/01/2017 Mw 5.5
Campotosto 18/01/2017 Mw 5.4
Campotosto 18/01/2017 Mw 5.0
Legend Strong Motion Parameters
Results of the analyses confirm that the general trend of PGA, PGV, PGD, IA and IH is a reduction with the distance from the epicenter but such a trend gets clear beyond 30km; within this epicentral distance a trend is not evident and high scattered values can be recognized of the aforementioned parameters. Furthermore,
within 30km from the source, the PGA values recorded along the three directions show similar values and sometimes the vertical PGA component is even higher than the horizontal ones. Thus, near field seismic signals show horizontal and vertical contributions of comparable magnitude and scattered according to the site
response. Instead, far field records show a common 2/3 ratio between vertical and horizontal acceleration components and a more predictable soil response according to regional specific Ground Motion Prediction Equation (GMPE) feasible for different strong motion parameters. Conversely, the Trifunac duration increases
as the epicentral distance increases according to the signal enrichment in surface waves getting far away from the epicenter. Moreover, the seismic soil categories according to the Eurocode 8 classification has been investigated in terms of the influence on the amplitude of the abovementioned strong motion parameters.
As expected, the results confirm that type soil A shows lower amplification than soil type D and E, but B*, C and C* show the highest amplification.
DISCUSSION AND CONCLUSION
- EMERGEO Working Group (2002). Relazione sintetica sull’attività del gruppo Emergeo durante l’emergenza Molise
- EMERGEO Working Group (2009). Rilievi geologici di terreno effettuati nell'area epicentrale della sequenza sismica dell'Aquilano del 6 aprile 2009
- EMERGEO Working Group (2012). Rilievi geologici di terreno effettuati nell’area epicentrale del terremoto del 20 maggio 2012
- EMERGEO Working Group (2016). The 24 August 2016 Amatrice Earthquake: Coseismic Effects
- GEER (2016). Engineering reconnaissance of the 24 August 2016 Central Italy Earthquake: Ver 2,GEER Report 050, Geotechnical Extreme Events Reconnaissance Association,DOI:10.18118/G61S3Z.(Ver 1 preliminary report DOI:10.18118/G65K5W)
- GEER (2017). Engineering Reconnaissance following the October 2016 Central Italy Earthquakes: Ver 2, GEER Report 050D, Geotechnical Extreme Events Reconnaissance Association, DOI:10.18118/G6HS39 (Ver 1 preliminary report DOI:10.18118/G6S88H)
- NTC2008 “Norme Tecniche per le Costruzioni” Decreto 14 gennaio 2008 del Ministero delle Infrastrutture. Supplemento Ordinario della G.U. n. 29 del 4.02.2008
- Luzi L, Puglia R, Russo E & ORFEUS WG5 (2016). “Engineering Strong Motion Database, version 1.0. Istituto Nazionale di Geofisica e Vulcanologia, Observatories & Research Facilities for European Seismology.” DOI: 10.13127/ESM"
BIBLIOGRAPHY
... Highly scattered seismic ground motions in FDZs are strongly dependent on the faulting mechanism, source directivity, and nonlinear soil behavior at the site. Because of these effects, ground motions in the FDZs may exhibit impulsive character (Kalkan and Kunnath, 2006;Mollaioli et al., 2006;Baker, 2007), high-frequency contents, and directivity (Peruzzi and Albarello, 2017;Mancini et al., 2018). FDZ observed data show peculiar characters that are dissimilar to records taken away from the source. ...
... The peak ground acceleration (PGA) values and the response spectra, at fixed periods of 0.1-0.3 and 1.0 s, are highly scattered at FDZs, that is, 30-40 km from the epicenter (Mancini et al., 2018). Owing to the scarcity of observed data recorded within FDZ especially on soil "A" category (shear-wave velocity > 800 m/s, according to NTC18 the numerical simulations of the seismic events are helpful (Hollender et al., 2018). ...
Article
Full-text available
Local seismic response (LSR) studies are considerably conditioned by the seismic input features due to the nonlinear soil behavior under dynamic loading and the subsurface site conditions (e.g., mechanical properties of soils and rocks and geological setting). The selection of the most suitable seismic input is a key point in LSR. Unfortunately, few recordings data are available at seismic stations in near-field areas. Then, synthetic accelerograms can be helpful in LSR analysis in urbanized near-field territories. Synthetic accelerograms are generated by simulation procedures that consider adequately supported hypotheses about the source mechanism at the seismotectonic region and the wave propagation path toward the surface. Hereafter, mainshocks recorded accelerograms at near-field seismic stations during the 2016–2017 Central Italy seismic sequence have been compared with synthetic accelerograms calculated by an extended finite-fault ground-motion simulation algorithm code. The outcomes show that synthetic seismograms can reproduce the high-frequency content of seismic waves at near-field areas. Then, in urbanized near-field areas, synthetic accelerograms can be fruitfully used in microzonation studies.
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