Seismic actions on structures in the near-source region of the 2016 central Italy sequence

Abstract

The central Italy seismic sequence began in the latter half of 2016 and continued well into 2017, causing severe damage in the villages close to the source and causing hundreds of casualties. It is a sequence especially interesting to study, from the perspective of seismic actions experienced by structures, because it saw nine M C 5.0 earthquakes within a period of 5 months, rupturing parts of the complex central Apennine mountain range fault system. Consequently, some of the main earthquake engineering issues that arose are the multiple locations where the code-mandated seismic actions were exceeded in more than one of the main events of the sequence and the number of pre- and low-code existing buildings that suffered heavy damage or collapse due to the intensity of individual earthquakes and the cumulative effect of repeated damaging shocks. The present article picks up on these topics and uses probabilistic seismic hazard, as well as the multitude of strong ground motion recordings available from the sequence, to provide a discussion on certain issues, that are all related to the topical subject of seismic actions. These issues are: (1) the unsurprising exceedance of code spectra in the epicentral areas of strong earthquakes; (2) the particular spectral shape and damaging potential of near-source, pulse-like, ground motions, possibly related to rupture directivity; and (3) structural non-linear behaviour in the wake of a sequence that produces repeated strong shaking without the necessary respite for repair and retrofit operations.

Full text

ORIGINAL RESEARCH PAPER
Seismic actions on structures in the near-source region
of the 2016 central Italy sequence
Iunio Iervolino
1
•Georgios Baltzopoulos
2
•Eugenio Chioccarelli
3
•
Akiko Suzuki
1
Received: 25 July 2017 / Accepted: 4 December 2017 / Published online: 9 December 2017
ÓSpringer Science+Business Media B.V., part of Springer Nature 2017
Abstract The central Italy seismic sequence began in the latter half of 2016 and continued
well into 2017, causing severe damage in the villages close to the source and causing
hundreds of casualties. It is a sequence especially interesting to study, from the perspective
of seismic actions experienced by structures, because it saw nine M C5.0 earthquakes
within a period of 5 months, rupturing parts of the complex central Apennine mountain
range fault system. Consequently, some of the main earthquake engineering issues that
arose are the multiple locations where the code-mandated seismic actions were exceeded in
more than one of the main events of the sequence and the number of pre- and low-code
existing buildings that suffered heavy damage or collapse due to the intensity of individual
earthquakes and the cumulative effect of repeated damaging shocks. The present article
picks up on these topics and uses probabilistic seismic hazard, as well as the multitude of
strong ground motion recordings available from the sequence, to provide a discussion on
certain issues, that are all related to the topical subject of seismic actions. These issues are:
(1) the unsurprising exceedance of code spectra in the epicentral areas of strong earth-
quakes; (2) the particular spectral shape and damaging potential of near-source, pulse-like,
ground motions, possibly related to rupture directivity; and (3) structural non-linear
behaviour in the wake of a sequence that produces repeated strong shaking without the
necessary respite for repair and retrofit operations.
Keywords Seismic hazard Pulse-like ground motion Cumulative damage
&Iunio Iervolino
iunio.iervolino@unina.it
1
Dipartimento di Strutture per l’Ingegneria e l’Architettura, Universita
`degli Studi di Napoli
Federico II, Naples, Italy
2
Istituto per le Tecnologie della Costruzione, Consiglio Nazionale delle Ricerche (ITC-CNR),
Naples, Italy
3
Universita
`Telematica Pegaso, Naples, Italy
123
Bull Earthquake Eng (2019) 17:5429–5447
https://doi.org/10.1007/s10518-017-0295-3

1 Introduction
Since the end of August 2016, an extended region of central Italy has experienced a long
lasting seismic sequence (Luzi et al. 2017). The initiating event was the Amatrice earth-
quake that occurred on August 24th, 2016 at 1:36:32 UTC. The event was characterized by
a moment magnitude (M) equal to 6.0 and heavily damaged the villages of Amatrice and
Accumoli. It caused about three-hundred fatalities, resulting from the collapse of several
buildings in the area closest to the source. During the coming months, and until June 2017,
nine seismic events surpassing M5.0 occurred in the area. Notable among these were two
events that occurred on October 26th, one at 17:10:36 UTC (M5.4, epicenter near the
village of Castelsantangelo sul Nera) and another at 19:18:06 UTC (M5.9 near Ussita). The
largest event of the sequence (M6.5) occurred on October 30th at 06:40:18 UTC with the
epicenter located in the vicinity of the town of Norcia;
1
this event will be hereafter
identified as the mainshock. The sequence continued into 2017, with four more events with
M between 5.0 and 5.4 occurring on the same day, January 18th, in the area between the
villages of Amatrice and Pizzoli. In Fig. 1the sequence is represented in terms of number
of earthquakes with magnitude larger than 2 in cells 5 km by 5 km wide and the corre-
sponding released cumulative seismic moment, from the beginning of the sequence up to
February 2017.
This long-duration seismic sequence came in the wake of the 2009 L’Aquila earthquake
and the 2012 Emilia sequence to rekindle scientific debate on, among other topics, the
seismic actions considered for structural design (regarding the earlier events see also
Chioccarelli and Iervolino 2010; Iervolino et al. 2012b). In fact, during this central Italy
sequence, many communities found themselves near the source of different seismic events,
sustaining considerable damage, especially to old constructions, not built according to
current standards or even to any seismic provision at all. What is more, several settlements
were found in that near-source situation more than once. In those cases, the extent of the
damage suffered by the building stock was, at least partly, attributable to the cumulative
M 5.9
Fermo
Ascoli
Piceno
Marcerata Ter amo
L'Aquila
0
Perugia
13°5
43°0
Longitude
Latitude
Rieti
13°0
200
Terni
42°5
400
600
No. < 50
No. 100
No. 200
No. 300
No. 400
No. 500
No. 600
Fermo
Ascoli
Piceno
Marcerata Ter amo
L'Aquila
0
Perugia
13°5
43°0
Longitude
Latitude
Rieti
13°0
Terni
42°5
5
10
25
10
M
o
< 1x10
20
M
o
< 1x10
25
M
o
1x10
25
Fig. 1 Number of earthquakes with M C2 recorded in 5 km by 5 km cells during the Aug. 24th 2016–Feb.
24th 2017 period (left) and corresponding released cumulative seismic moment: M0dyne cm½(right).
Data from http://cnt.rm.ingv.it/, last accessed February 2017
1
Ground motion data and source information about on these events can be found at http://esm.mi.ingv.it/,
via the following Event ID codes: EMSC-20160824_0000006 (Amatrice M6.0), EMSC-20161026_0000095
(Ussita M5.9), EMSC-20161026_0000077 (Castelstantagelo sul Nera M5.4), EMSC-20161030_0000029
(Norcia M6.5).
5430 Bull Earthquake Eng (2019) 17:5429–5447
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effect of being subjected to repeated strong motion shocks and the peculiar features of
shaking close to the seismic rupture.
From a scientific, earthquake engineering and engineering seismology, point of view
this sequence is unique in the Italian history of instrumental seismicity so far. This is
because of the number of large earthquakes recorded in a relatively short time, and the
acquisition of about ten-thousand recorded ground motions which have been made
available by the Italian Accelerometric Network (RAN, Presidency of the Council of
Ministers 1972;http://ran.protezionecivile.it/), managed by the Dipartimento della Pro-
tezione Civile (DPC), and the Italian seismic network (RSN), managed by the Istituto
Nazionale di Geofisica e Vulcanologia or INGV (INGV Seismological Data Centre 1997;
http://cnt.rm.ingv.it/instruments/network/IV). Furthermore, the progressively increasing
density of a temporary accelerometric network that was deployed as the sequence unfolded
meant that a large number of near-source ground motions were recorded.
This bulk of near-source ground motions forms the base material for the present study,
by virtue of being the most significant part of the overall recordings from a structural point
of view. The ensuing discussion focuses on three specific issues generally pertaining to the
topic of seismic actions for seismic design and assessment, addressing them in light of the
specific features of such a strong sequence. In the following, attention is first given to the
fact that the pseudo-acceleration spectra (or simply acceleration spectra from here on) of
ground motions recorded in areas close to the seismic sources of the strongest events of the
sequence, exceeded the design actions provided by the national code for new constructions.
Then, the identification of near-source pulse-like records in the sequence and their effect
on simplified structural response is addressed; this is a topical issue, which is still not
explicitly or appropriately accounted for by even state-of-the art seismic codes (the Italian
code included). Finally, the damage accumulation effect due to repeated seismic shocks is
analysed for some locations that have been subjected to at least five structurally damaging
shakings.
2 Should code spectra be questioned because they are exceeded close
to the source?
During the central Italy sequence, elastic spectra at the basis of design of ordinary con-
structions in Italy have been systematically exceeded, in areas relatively close to the
seismic source, by recordings of more than one earthquake. To picture this observation, the
maps in Fig. 2report the source surface projections (dashed lines) for the three main events
as well as the locations of the stations (depicted as triangles) which have recorded these
earthquakes in an area of about 8800 km
2
. Black triangles indicate locations where the
code spectra for life-safety limit state design of ordinary new constructions in the most
recent Italian seismic code (C.S.LL.PP. 2008, NTC hereafter) have been exceeded at least
in one spectral ordinate in the range 0–2 s and for at least one of the two horizontal
recording directions: east–west (EW) and north–south (NS).
It is apparent that code spectra have been systematically exceeded and this raised a
question that has been also asked due to similar observations in preceding events in Italy,
such as the 2009 L’Aquila earthquake and the 2012 Emilia sequence (see, for example,
Akinci et al. 2010; Meletti et al. 2012): are code spectra underestimated as we see sys-
tematic exceedance in major seismic events? The rest of this section demonstrates that
exceedance of code spectra close to the source is quite expected by the very definition of
Bull Earthquake Eng (2019) 17:5429–5447 5431
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code spectra and, as a consequence, underestimation of design seismic actions cannot be
claimed based on these observations alone.
Code spectra in Italy are based on probabilistic seismic hazard analysis (PSHA). PSHA
allows one to compute the rate kim
ðÞof earthquakes exceeding a given ground motion
intensity measure (IM) threshold imðÞat a site of interest (McGuire 2004), Eq. (1). In the
equation, nis the number of seismic sources relevant for the hazard at the site, miis the
annual rate of earthquake occurrence on source i,fM;Rm;rðÞis the joint probability density
function of magnitude and source-to-site distance RðÞand, finally, PIM [im m;rj½is the
probability of exceeding the intensity measure threshold, given magnitude and distance.
The latter probability is provided by a ground motion prediction equation (GMPE).
kim ¼X
n
i¼1
miZ
M
Z
R
PIM [im m;rj½fM;Rm;rðÞdm dr ð1Þ
If the IM is the elastic spectral pseudo-acceleration SaðÞat different natural oscillation
periods TðÞ, it is possible to build the uniform hazard spectrum (UHS). All the ordinates of
the UHS are characterized by the same exceedance rate or, equivalently, are exceeded once
every so many years on average; i.e., their exceedance has the same return period Tr
ðÞ.
According to the NTC, design elastic response spectra are close approximations of UHS’
0 0.5 1 1.5 2
T[s]
NTC T = 475 yr
r
M5.9 Oct 26.
th
NRC EW
M5.9 Oct 26.
th
NRC NS
0 10 km10
0 0.5 1 1.5 2
T[s]
NTC T = 475 yr
r
M6.5 Oct 30.
th
NRC EW
M6.5 Oct 30.
th
NRC NS
Ascoli Piceno
Macerata
Perugia
Rieti
Teramo
L’Aquila
Fermo
Ascoli Piceno
Macerata
Perugia
Rieti
Teramo
L’Aquila
Fermo
Ascoli Piceno
Macerata
Perugia
Rieti
Teramo
L’Aquila
Fermo
0 0.5 1 1.5 2
T[s]
0
0.5
1
1.5
2
Sa(T) [g]
NTC T = 475 yr
r
M6.0 Aug 24.
th
NRC EW
M6.0 Aug 24.
th
NRC NS
M6.0 Aug. 24 2016
th M5.9 Oct. 2 20166th M6.5 Oct. 201630th
Fig. 2 Top: maps of stations recording the main events of the sequence. Black triangles are the stations
where horizontal code spectra were exceeded at least in one ordinate between 0 and 2 s. Bottom: code
(adjusted for site conditions) and recorded spectra at NRC station (Norcia). Left column Aug. 24th 2016
M6.0, center column Oct. 26th 2016 M5.9, right column Oct. 30th 2016 M6.5
5432 Bull Earthquake Eng (2019) 17:5429–5447
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determined via the study described in Stucchi et al. (2011), which computed uniform
hazard spectra over a grid of more than ten thousand points for nine return periods from 30
to 2475 years (http://esse1.mi.ingv.it/, last access July 2017) all-over the country, con-
sidering rock site conditions.
At this point, more details on Fig. 2can be given. In NTC, the return period of the
spectrum to be used in design or assessment depends on the limit-state considered and on a
reference period proportional to the design life of the structure in question. For ordinary
(e.g., residential or office) constructions, the return period of the code spectrum for the life-
safety limit state is 475 years. Figure 2refers to the exceedance of at least one spectral
ordinate in the range 0–2 s of such spectra in the area of the sequence. Code spectra were
adjusted for local site conditions according to the prescription of the code and the geo-
logical information of the recording sites retrieved by the Engineering Strong Motion
database (ESM, http://esm.mi.ingv.it, last access July 2017). To provide a more quanti-
tative measure of the issues, Table 1counts the number of exceeding stations and the
corresponding percentage with respect to the total number of stations recording the events
for bins of distance from the source. The metric used for distance is the minimum distance
from the surface projection of the source that is usually termed Joyner and Boore (1981)
distance Rjb

. As shown, all stations within 10 km from the source exceeded life safety
design actions during both the M6.0 and M6.5 events, while 78% did so during the M5.9
event. As expected, these percentages rapidly decrease with increasing distance: 20%, 37%
and 62% of the stations within 30 km from the source exceeded design actions during the
M6.0, M5.9 and M6.5 events, respectively.
Red circles in the maps of Fig. 2identify one of the two stations that exceeded code
spectra in all the three considered events; one such of stations is Norcia (NRC) while the
other, not considered in the following, is FOC (Foligno Colfiorito, located on the
boundaries between Macerata and Perugia provinces). For each event, recorded NRC
response spectra are reported in the figure below the corresponding map, along with the
Tr¼475 years code spectrum for the same site. The exceedance in the range of periods of
about 0–0.25 s is common to the three events.
2
Table 1 Statistics of Tr¼475 years code spectra exceedance per bin of distance from the sources
Rjb M6.0—August 24th 2016 M5.9—October 26th 2016 M6.5—October 30th 2016
No.
exceedances
No. exceedance/
no. stations (%)
No.
exceedances
No. exceedance/
no. stations (%)
No.
exceedances
No. exceedance/
no. stations (%)
\10 2 100 7 78 14 100
\20 4 36 12 50 22 79
\30 5 20 13 37 23 62
2
Although there are several recording stations outside the boundaries of the maps, none of them has
exceeded the life-safety code spectrum for ordinary structures. Exceptions are the stations AQK (L’Aquila)
and MMUR (Monte Murano). AQK experienced exceedances during the M6.0 and M6.5 events, being
distant from the source 34 and 43 km, respectively. These exceedances are due to the unusual shape of
recorded ground motions with increment of spectral ordinates in a narrow range of periods around 1.5 s,
probably due to local effects that have been discussed, among others, in Monaco et al. (2009). Exceedance at
MMUR occurred during the M5.9 event at 49 km distance. Exceedance is slight (i.e., recorded spectrum is
5% higher than code’s) at 0.1 s vibration period.
Bull Earthquake Eng (2019) 17:5429–5447 5433
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The observed exceedances of design actions triggered a scientific debate about the
possible inadequacy of the hazard assessments derived from PSHA. It has been discussed
elsewhere (e.g., Iervolino 2013), that data acquired in the epicentral area of a single event
cannot be sufficient to substantiate an alleged underestimation of code spectra derived from
hazard analysis. This is because a time-span of many years is necessary to validate the
frequency associated with exceedance of a certain ground-motion intensity at a site.
3
Conversely, it is well expected by the nature of code spectra that they are exceeded in the
epicentral area of relatively high-magnitude earthquakes, even if these earthquakes are
fully considered by the hazard analysis used to build the spectra. In fact, PSHA, in
assessing seismic hazard for a specific site, accounts for ground motions from all possible
earthquake locations and magnitudes (building a spectrum which does not represent a
specific event). Because ground motion intensity (at least when spectral acceleration is
concerned) tends to decay with distance from the source, the greatest effects of any given
event are necessarily observed close to the source. In other words, in the case of large
seismogenic zones, as those of the model by Meletti et al. (2008) that were used to build
code spectra in Italy, significant contributions to probabilistic seismic hazard are almost
exclusively due to possible earthquake locations closest to the site (exception to this
statement may occur in specific cases when multiple zones are concerned; see Iervolino
et al. 2011). Moreover, the more frequent earthquakes among those considered in PSHA
are, typically, those with comparatively lower magnitude; conversely, the largest magni-
tude events are relatively rare. It follows that, given a UHS for a medium-long return
period (say, for example 475 years), its ordinates are unlikely to be exceeded by an
earthquake that occurs at the site quite frequently, while they are very likely going to be
surpassed in the case of an earthquake with rare magnitude occurring close to the site.
To substantiate this discussion, in the following figure it is quantitatively illustrated that
such exceedances should have been well expected within the areas close to the seismic
sources. To this aim, one should first recall that the PSHA behind the NTC spectra was
performed via a logic tree comprising sixteen branches. The results of ‘‘branch 921’’ are
claimed to be the closest to the hazard estimate provided by the full logic tree (Stucchi
et al. 2011). This branch considers the ground motion prediction equation (GMPE) of
Ambraseys et al. (1996) and the style-of-faulting correction factors proposed by Bommer
et al. (2003). These models are also considered herein for consistency. It is also worth
recalling that the seismogenic source model at the basis of PSHA used to develop NTC
spectra considers maximum magnitude larger than 7 for the zone where the central Italy
sequence occurred, thus observed magnitudes are accounted for by code spectra, in terms
of possible earthquakes.
In order to identify the areas in which the exceedance of design seismic actions should
have been expected upon occurrence of the events considered in Fig. 2, one has to consult
Fig. 3. In the figure, the surface projection of the ruptures and the provinces’ administrative
boundaries are shown. The background colours of the maps represent, for each site, the
values of code spectra with 475 years return period (on rock). For representation needs,
two spectral ordinates are considered, peak ground acceleration (PGA) and Sa 1s
ðÞ
, both
indicated as im475 years. Their values are from http://esse1.mi.ingv.it/d2.html (last accessed
in May 2017). Solid lines are the contours of the exceedance probability pðÞof im475 years,
which are drawn plugging in the GMPE of Ambraseys et al. (1996) the actual magnitude of
3
One may argue that multiple exceedances have been observed in this sequence, yet it should be recalled
that PSHA, and therefore code spectra in Italy, refers to exceedance due to mainshocks and does not
explicitly account for exceedances caused by aftershocks or foreshocks to the main event.
5434 Bull Earthquake Eng (2019) 17:5429–5447
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each event and considering, for each site in the map, the actual distance from the source
surface projection, Eq. (2). With this information, the GMPE provides the probability that
the considered code spectra ordinates are exceeded by an event of the kind as each of those
occurred.
p¼PIM [im475 years m;rjb


 ð2Þ
For the three considered events, when PGA is of concern, the probabilities of exceeding
design actions are large: maximum exceedance probability is 0.76, 0.77 and 0.90 for the
M6.0, M5.9 and M6.5 event, respectively. This means that it was almost certain that design
PGA were going to be exceeded at sites close to the rupture. Such exceedance probabilities
rapidly decrease when the source-to-site distance increases. On the other hand, when
Sa 1sðÞis the selected IM, maximum exceedance probability is 0.78 in the case of M6.5
while, for the M6.0 and M5.9 events, maximum probabilities are 0.48 and 0.46,
respectively.
These calculations confirm the initial premise of this section, that the high likelihood of
exceeding code spectral ordinates close to the source of earthquakes of this magnitude is
nothing unexpected and does not warrant surprise.
Fig. 3 Probability of exceedance of NTC design actions for Tr¼475 years in the near-source areas of the
three main events of the sequence—top: PGA; bottom: Sa 1sðÞ. Left column: Aug. 24th 2016 M6.0; center
column: Oct. 26th 2016 M5.9; right column: Oct. 30th 2016 M6.5. (Upper-left corner panels are nationwide
maps of im475 years)
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3 Pulse-like versus ordinary records: spectral shape and inelastic
response
As discussed in the previous section, the exceedance of code spectra in near-source areas
does not constitute, per se, a proof of flawed derivation of the spectra. However, it is well
known that ground motion in near-source regions is affected by systematic spatial vari-
ability that, for a number of reasons, is neither captured nor appropriately represented by
classical PSHA. Pulse-like records constitute one of the manifestations of such spatial
variability. Impulsive features in ground motions have been identified in earthquakes since
quite some time, yet have been relatively less studied in the case of normal-faulting
earthquakes (Chioccarelli and Iervolino 2010), which constitute the dominant focal
mechanism in the area of the central Italy seismic sequence.
Near-source, pulse-like, ground motions constitute a special category of seismic input,
whose engineering relevance has been long recognized (Bertero et al. 1978). The most
prominent causal mechanism of impulsive ground motion is rupture directivity: at sites
located along the propagation direction of shear dislocation on the fault, shear wave fronts
emitted at distinct times may arrive almost simultaneously. This can lead to a constructive
wave interference effect, observable on the ground velocity trace as a coherent, double-
sided pulse that caries most of the seismic energy (Somerville et al. 1997). A consequence
of this feature is that such ground motions can subject ductile structures to greater inelastic
displacements, on average, with respect to non-impulsive seismic input (e.g., Iervolino
et al. 2012a), thus attracting the research interest of earthquake engineers.
It is worth noting that the emergence of pulse-like ground motions is never guaranteed
at all near-source sites and that the probability of observing them depends, among other
factors, on site-to-source geometry and focal mechanism (Iervolino and Cornell 2008;
Iervolino et al. 2016a). This nuance partly explains the relative scarcity of pulse-like
records found in ground motion databases in the past, as it is hard to capture this phe-
nomenon without a dense accelerometric network spanning the epicentral area. In recent
years, seismic events nucleating in the vicinity of denser, modern seismic networks (e.g.,
Parkfield, California, 2004; L’Aquila, Italy, 2009) have provided more empirical evidence
in terms of impulsive recordings, further spurring research into the topic. In this respect,
the central Italy sequence of 2016 stands out for having provided a significant number of
high quality, near-source ground motion recordings, thanks to the multitude of temporary
accelerometric stations deployed to closely monitor seismic activity, following the August
24th 2016, M6.0 initiating event.
In Luzi et al. (2017), these near-source records were investigated for pulse-like char-
acteristics using the continuous wavelet transform algorithm proposed by Baker (2007). As
a side-note, it should be underlined that such methods of identifying pulse-like charac-
teristics in recorded ground motion rely on data that is tractable directly and exclusively
from the velocity time-history. For this reason, rupture directivity can be considered a
likely causal mechanism of such features but in the absence of direct links with the
physical process of fault dislocation, a verdict in that direction cannot be reached with
certainty. Having made this premise, that operation resulted in a set of eighteen ground
motions being identified as pulse-like. These ground motions were recorded during three of
the events comprising the 2016 sequence, namely the August 24th M6.0 shock, the October
26th M5.4 shock and the October 30th M6.5 (main)shock. Figure 4shows maps of the
positioning of accelerometric stations around the rupture plane’s horizontal projection of
the 24th Aug. and the 30th Oct. 2016 events, with distinction made among those that
5436 Bull Earthquake Eng (2019) 17:5429–5447
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recorded pulse-like and non-pulse-like (i.e., ordinary) motions. The increase in network
density during the time interval between the two events is evident. The same figure shows
the most prominent impulsive velocity traces identified, as well as the velocity traces of
ACC
PCB
CLO
T1213
T1214
CSC T1201
13.0 °E
13.5°E
42.5 °N
43.0°N
Aug. 24 2016 M6.0
AMT
NRC
NOR
MNF
RM33
FEMA
42.5 °N
43.0 °N
Oct. 30 2016 5M6.
Epicentre
Rupture projection
Ordinary recorded motion
Pulse-like recorded motion
Main pulse orientation
AMT
AMT
48.9 cm/s
10 s
NOR
NRC
27.6 cm/s
25.1 cm/s
CLO
67.2 cm/s
T1214
53.7 cm/s
T1213
61.9 cm/s
CSC 17.5 cm/s
ACC
52.6 cm/s
PCB
FOC PCB
7.1 cm/s
FOC
8.0 cm/s
CIT
NOR
PRE
NOR
46.0 cm/s
CIT 15.3 cm/s
PRE 15.2 cm/s
CNE
CNE
41.8 cm/s
24/08
30/10
Pulse-like velocity signal
Non-pulse-like velocity signal
Fig. 4 Position of accelerometric stations that recorded pulse-like or non-pulse-like ground motions
(according to Luzi et al. 2017) relative to the horizontal projection of the rupture plane, for the 24th Aug.
2016 M6.0 and 30th Oct. 2016 M6.5 events. Velocity traces of pulse-like (black lines) and non-pulse-like
(grey lines) records shown for comparison (station codes reported close to the signals); amplitude scale and
duration (20 s) is common to all depicted velocity time-histories. Orientation of pulse-like components is
shown on the maps; non-pulse-like components are always shown in the fault-normal (i.e., strike-normal)
direction
Bull Earthquake Eng (2019) 17:5429–5447 5437
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some nearby ordinary recordings (fault-normal, FN, component) for comparison. The
orientation of each impulsive component depicted is indicated on the maps (this infor-
mation is also available on http://esm.mi.ingv.it).
This ground motion set is used in the present article to discuss the differences between
pulse-like and ordinary (i.e., non-impulsive) records in terms of spectral shape and
structural seismic demand, the two being closely related as the former can have important
influence on the latter (e.g., Baker and Cornell 2005). For the benefit of this comparison, a
set of ordinary ground motions is also assembled, from within those recorded during the
2016 seismic sequence. Thus, the ordinary set contains sixty-two ground motions recorded
at sites with subsoil classified as A, B or C according to NTC (avoiding known soft soil
sites), exhibiting PGA in excess of 0.10 g and belonging to the four highest moment
magnitude events of the 2016 sequence.
A well-documented fact, regarding the response spectra of pulse-like ground motions, is
the dominant role of the impulsive waveform in the determination of the spectral shape in
terms of pseudo-velocity. An example of this effect is provided in Fig. 5, where the
velocity trace of the impulsive horizontal component is shown for the recording obtained at
station CLO (Castelluccio di Norcia), along with the impulsive waveform extracted by the
identification algorithm. The pseudo-spectral velocity (PSV) of this impulsive signal is
plotted in Fig. 5(right) against PSV due to the extracted pulse alone; one notes the broad
peaks appearing around a period equal to the pulse period (or pulse duration) TP.
4
The imprint of the pulse can also be found on the pseudo-acceleration spectrum, even if
it is less pronounced than in terms of PSV. While Somerville et al. (1997) initially sug-
gested a broadband amplification model to account for the emergence of pulses on Sa
GMPEs, modern consensus has gradually shifted towards narrowband amplification
schemes centered around TP(e.g., Shahi and Baker 2011). Put in different words, for a
given magnitude of causal event, site-to-rupture distance and site conditions, larger-than-
average Sa ordinates are expected for pulse-like horizontal ground motion components
than for ordinary ones, at least for periods around the vicinity of TP. Typically, when
observing the effect of this narrowband amplification on average spectral shape over a
large set of pulse-like motions, the result appears broadband. This is due to the variability
that TPtends to exhibit even within a single event and can be observed here as well, since
the sequence provided enough pulse-like records for considerations on average spectral
shape to be meaningful. Spectral shape can be represented by normalizing the Sa ordinates
10 15 20 25 30 35
−50
0
50
Vt
icoley [
m
c/s
]
0 1 2 3 4
0
50
100
150
200
PSV m
c/s][
original signal
extracted pulse
original signal
extracted pulse
T =1.39s
p
Time [s] T [s]
Fig. 5 Velocity time history and extracted pulse (left) and corresponding pseudo-spectral velocity (right)
for the CLO (Castelluccio di Norcia) record of the October 30th 2016 M6.5 shock
4
In fact, the vibration period for which PSV attains its maximum value has been used in the past as a proxy
for TP.
5438 Bull Earthquake Eng (2019) 17:5429–5447
123

of various records by dividing with the corresponding PGA and plotting the resulting
spectral amplification. This normalizing operation was performed for the pulse-like and
ordinary records mentioned earlier and the results are presented in Fig. 6. It should be
noted that, the majority of pulse-like records identified within the sequence by Luzi et al.
(2017) exhibited prominent pulses around the FN orientation (see also Fig. 4). In fact, this
is the direction where directivity-induced pulses are mostly expected, due to the polar-
ization of shear wave radiation patterns.
Figure 6(left) offers a comparison of the average spectral shape of the pulse-like
horizontal components with the spectral shape of the geometric mean (geomean for short)
of the ordinary recordings’ horizontal components. It can be observed that at high fre-
quencies (up to a period of around 0.20 s) ordinary ground motions, on average, slightly
supersede the impulsive ones in terms of spectral amplification factors. On the other hand,
a wide spectral region from 0.40 to 3.0 s sees clear predominance of the pulse-like set’s
average amplification over the geometric mean of ordinary ground motions. This is
actually the period range where the detected pulse periods were found, rendering this result
consistent with previous observations.
An analogous, but less pronounced, difference is observed in Fig. 6(right) between the
average spectral shape of the pulse-like components and the corresponding average of the
transverse components of the same pulse-like records (which, in the vast majority of cases,
are not considered impulsive). Although not shown here, it was observed that average
spectral shapes of the FN and fault-parallel (FP) rotated ordinary components are, by
contrast and as expected, quite similar with each other.
Previous research has suggested that this systematic difference in spectral shape
between pulse-like and ordinary records (affected by TP) is directly related to the larger
average seismic demand imposed on structures by the former when compared to that of the
latter (Tothong and Cornell 2008; Bojo
´rquez and Iervolino 2011). In the present study, the
aspect of inelastic seismic response to pulse-like vs. ordinary records is investigated via
incremental dynamic analysis (IDA, Vamvatsikos and Cornell 2002) and comparison of
the results with the analytical model of Baltzopoulos et al. (2016) is made. This investi-
gation employs two example single-degree of freedom (SDOF) structures that have a
common trilinear (elastic-hardening–softening branch) monotonic backbone and follow a
peak-oriented hysteretic rule that undergoes moderate cyclic strength deterioration (for the
implementation in OpenSEES software; see Altoonash and Deierlein 2004). The two
systems have periods of natural vibration of 0.50 and 1.00 s and a yield force set at 20%
and 12% of gravity loads, respectively. The hysteretic behavior of the T¼0:50 s structure
can be seen in Fig. 7, where the response of the system under cyclic load reversals is
plotted in terms of ductility, l, defined as the ratio of displacement to yield displacement,
l¼ddy. These SDOF systems can be regarded as pushover-based idealizations (e.g.,
Vamvatsikos and Cornell 2005) of low-to-mid-rise, low-code structures, representative of
some structures one might encounter in the zones damaged by the central Italy sequence.
IDA was performed using the eighteen pulse-like component set and both sixty-two
ordinary component sets (FN and FP). For each single record, the spectral acceleration
causing collapse of the structure was calculated, indicated as Sacol, collapse being defined
as reaching the point of zero lateral strength. This result for the pulse-like records, is
plotted in Fig. 7, against pulse period-to-vibration period ratio TpT.
For comparison reasons, two more results are reported on the same panel: the median
Sacol obtained from the ordinary ground motion FN component set (FP result omitted,
being too similar to the FN one) and the analytical prediction provided for the same SDOF
Bull Earthquake Eng (2019) 17:5429–5447 5439
123

system by the model of Baltzopoulos et al. (2016). For the latter case, both median and
plus/minus one standard deviation interval are shown (shaded area) assuming a lognormal
distribution of Sacol given impulsive seismic input with fixed TP. In the literature, pulse-
like records causing inelastic demand lower than the average of ordinary records (in Fig. 7,
those appearing above the red dashed line) are sometimes termed as benign, while those
that cause higher demand are termed as aggressive records; this nomenclature is also used
here.
It can be observed from Fig. 7that in the region defined by TpT[2 (a region where
previous studies have shown that impulsive input is typically more aggressive towards
inelastic structures than ordinary seismic input) the central Italy sequence pulse-like
records generally fall around the median prediction of the analytical model and never tread
outside the shaded area that denotes one standard deviation distance from that median.
Overall, in the case of 6 [TpT[2 both the analytical model and the numerical results
from the sequence’s impulsive input confirm the expected increased seismic demand with
respect to the ordinary case: for the T¼0:50 s system, the former provides Sacol average
Sa PGA
10
−2
10
−1
10
0
10
1
10
−
2
10
0
Ts[]
10
−2
10
−1
10
0
10
1
Ts[]
Pulse−like component (average)
Ordinary geomean (average)
Individual ordinary geomean
Pulse-like (average)component
Transverse component (average)
Individual impulsive component
Fig. 6 Left: Spectral amplification factors (individual and average) of the ordinary ground motion set
compared with the average of the pulse-like horizontal components. The ordinary set comprises all non-
pulse like ground motions recorded during the sequence with PGA [0.10 g on stiff-to-firm soil or rock and
the geomean of horizontal components is used. Right: spectral amplification factors of the impulsive
horizontal components (individual and average) compared with the average of the transverse components of
the pulse-like set
Sa [g](0.50 s)
p
TT
loc
Median givenSa T
p
one std. interval givenSa T
p
col
col
Sa from single pulse record IDA
col
Median , ordinary FNSa
col
−10 0 10
−200
−100
0
100
200
µ
F (kN)
Sa [g]
(1.00 s)
l
o
c
Median , ordinary FNSa
col
p
TT
CMI M5.4
NOR M5.4
RM33 6.0M
T1245 M5.4
CMI M5.4
AMT 6 0M.
original backbone
cyclic loading path
1 2 3 4 5
0
1
2
3
4
1 2 3 4 5
0
1
2
3
4
T1214 6 5M.
NRC 6.0M CSC 6 5M.
NRC 6.0M
NOR 6.0M
ACC 6.5M
Fig. 7 Collapse intensity of pulse-like records from the Central Italy sequence estimated via IDA,
compared with collapse intensity given Tpaccording to Baltzopoulos et al. (2016) for a SDOF structure with
period T¼0:50 s (left) and one with period T¼1:00 s (center); trilinear backbone curve and hysteretic
response to cyclic loading common to both SDOF structures used in the analysis (right)
5440 Bull Earthquake Eng (2019) 17:5429–5447
123

values around 1.0 g and the latter slightly above 2.0 g while the corresponding situation for
the T¼1:00 s structure sees a comparison of around 0.85 g (pulse-like) to 1.60 g (ordi-
nary). On the other hand, the shorter-period pulses clustered around TpT¼1 appear to
exhibit a greater-than-average benign effect on this structure, as quantified by Sacol values
exceeding the median of the ordinary set as well as the median-plus-standard-deviation
analytical predictions. Note that the most benign and most aggressive of ground motions
are tagged in the plots by recording station, in order to reveal potentially systematic
culprits of one behavior or the other.
Generally speaking, the sample of pulse-like motions provided by the central Italy
sequence contained enough records to allow for some considerations in terms of average
spectral shape and expected inelastic response. The discussed agreement with previous
studies in these matters of engineering significance is important, since from an engineering
seismology perspective, pulse-like motions nascent from normal faulting mechanisms used
to be, as mentioned, a rarity in the databases and there are hints in the literature suggesting
that the underlying rupture-related mechanisms can differ from those associated with
strike-slip events (Howard et al. 2005; Poiata et al. 2017).
4 Sequence effects on non-linear structural response
Structures designed according to modern seismic codes, are typically expected to cope with
rare ground shaking intensities through dissipating seismic energy by sustaining a certain
amount of damage. Seismic design according to these codes implies possible failure (i.e.,
exceedance of a limit state) due to a single event (which is not directly related but is
coupled with the fact that classical PSHA only accounts for mainshocks). In fact, an
underlying assumption of this concept, is that damaging events will not only be rare, but
also enough far apart in time to allow for a damaged structure to be repaired in the
meantime. This reasoning can clearly fall through in the case of seismic sequences. In fact,
in recent years, the concept that structures damaged by a mainshock earthquake may be
unable to meet performance criteria during the aftershock sequence that follows, due to
deterioration of lateral force resisting mechanisms and seismic energy dissipation capacity,
has been receiving increased attention in earthquake engineering research (e.g., Yeo and
Cornell 2009; Iervolino et al. 2016b). A conceptually similar, but less studied situation is
the case at hand: a series of strong shocks, potentially damaging individually, closely
clustered in time and space.
During the 2016 central Italy seismic sequence, within a course of \90 days, some
sites repeatedly found themselves at close distances to ruptures corresponding to events of
magnitude M C4. The most notable such case is the town of Norcia, that was found at
epicentral distances of 15 km or less during events with M 5:3 five times within a ninety-
day interval. In fact, Norcia was among the locations that, apart from repeatedly experi-
encing strong ground motion, also hosted permanent instrumented stations and thus pro-
vided continuous accelerometric records throughout the sequence (as can be seen in Fig. 2;
see also, ReLUIS-INGV Workgroup 2016; Luzi et al. 2017). Figure 8shows two such
cases as examples of the aforementioned situation, by providing pseudo-acceleration
spectra of five ground motions recorded at the station of Amatrice (AMT) and another five
recorded at one of the stations at Norcia (NRC) between August 24th and October 30th
2016 (EW component shown), details of the causal events can be found in Table 2. These
records were selected on the basis that they exhibited the highest shaking intensities,
Bull Earthquake Eng (2019) 17:5429–5447 5441
123

primarily in terms of PGA but also considering spectral ordinates up to a period of 0.50 s,
recorded at those sites during the first ninety days of the sequence.
In this context of recurring shocks repeatedly producing high-to-moderate shaking
intensity at certain sites within a relatively short span of time that practically precludes
intermediate retrofit operations, emerges one of the principal features of the central Italy
sequence; i.e., damage accumulation during a seismic sequence. Field reconnaissance
missions undertaken shortly after the initiating M6.0 event of August 24th and also after
the M6.5 shock of October 30th (e.g., GEER Workgroup 2017) highlighted the fact that
there were many structures left apparently undamaged (or only slightly damaged) after the
initial event but were brought to a state of severe damage or near-collapse due to the
cumulative degrading effect of the ensuing events. The phenomenon of damage accu-
mulation during this sequence has been already touched upon by ReLUIS-INGV Work-
group (2016) and is showcased here as well.
In order to undertake an analytical study, illustrating the aforementioned issue of
structural damage accumulation during the course of the sequence, a set of SDOF inelastic
structures were considered. These structures follow the same peak-oriented hysteretic rule
with moderate cyclic strength degradation as the one described in the preceding section,
0 0.5 1 1.5 2
0
0.5
1
1.5
2
2.5
T[s]
Sa [g]
AMT − EW
M6.0 ’16Aug 24 01:36:32
M4.3 01 56 03’16Aug 24 : :
M4.8 ’16Aug 26 04:28:25
M4.4 ’16Aug 25 12:36:06
M6.5 ’16 Oct 30 06:40:18
00.5 1 1.5 2
T[s]
NRC − EW
M6.0 ’16Aug 24 01:36:32
M5.3 ’16Aug 24 02:33:29
M5.4 ’16 Oct 26 17:10:36
M5.9 ’16 Oct 26 19:18:06
M6.5 ’16 Oct 30 06:40:18
−10 −5 0 5 10
−1
0.5
0
0.5
1
FF
/
y
original backbone
cyclic loading path
μ
Fig. 8 Pseudo-acceleration response spectra of the five shocks considered in the illustrative analysis of
cumulative damage during the sequence at Amatrice (left) and Norcia (center). Backbone curve and cyclic
loading hysteresis of the case-study SDOF systems (right)
Table 2 Event details corresponding to the response spectra shown in Fig. 8
Station Date and time
(UTC)
ESM event ID M Epicentral
distance (km)
PGA EW
(cm/s
2
)
PGA NS
(cm/s
2
)
AMT 2016/08/24 01:36:32 EMSC-20160824_0000006 6.0 8.5 850.8 368.4
2016/08/24 01:56:03 EMSC-20160824_0000007 4.3 3.6 190.4 152.7
2016/08/25 12:36:06 EMSC-20160825_0000096 4.4 3.6 228.6 200.9
2016/08/26 04:28:25 EMSC-20160826_0000013 4.8 3.1 318.7 329.8
2016/10/30 06:40:18 EMSC-20161030_0000029 6.5 26.4 521.6 393.6
NRC 2016/08/24 01:36:32 EMSC-20160824_0000006 6.0 15.3 352.9 366.8
2016/08/24 02:33:29 EMSC-20160824_0000013 5.3 4.4 167.0 190.8
2016/10/26 17:10:36 EMSC-20161026_0000077 5.4 10.1 294.7 258.2
2016/10/26 19:18:06 EMSC-20161026_0000095 5.9 13.2 248.3 366.4
2016/10/30 06:40:18 EMSC-20161030_0000029 6.5 4.6 476.4 365.1
5442 Bull Earthquake Eng (2019) 17:5429–5447
123

while the vibration period varies between 0.30 and 0.40 s. Yield strength Fyis set to
correspond to 20% of each structure’s weight and the quadrilinear backbone (see Fig. 8)is
representative of the static pushovers of low-rise, low-code reinforced concrete buildings
in central Italy.
The observation of the hysteretic response and evolution of a structural system
throughout the sequence is used to provide some initial insights. Figure 9shows the
hysteretic response of two SDOF structures, one assumed at the AMT site and the other at
NRC, during sequential dynamic excitation by the records whose spectra are shown in
Fig. 8. This response is then compared to that of the same system subjected to the October
30th M6.5 shock alone (on the left-hand side of the figure). At the end of each individual
shock, the residual displacement is registered and plotted on the graphs and static pushover
is carried out in both directions, providing the shape of the monotonic backbone’s evo-
lution during the sequence (post-EQ in the figure). Residual displacement is an engineering
demand parameter that has seen extensive use as a proxy for the post-earthquake damage
state of a building in seismic loss assessment (e.g., Ruiz-Garcı
´a and Miranda 2006). The
post-earthquake pushover on the other hand, offers additional information such as loss of
stiffness (often termed period elongation) and loss of peak strength. Such information
could be important when evolutionary hysteretic rules with strength degradation are
considered, in merit of being more representative of the actual behavior of, among others,
reinforced concrete structures (for a discussion of the effect of such hysteretic rules on
residual displacements, see Liossatou and Fardis 2015).
-10 0 10
/
y
-1.0
0
1.0
F/F
y
-10 0 10
/
y
-1.0
0
1.0
F
/F
y
-10 0 10
/
y
-10 0 10
/
y
-10 0 10
/
y
-10 0 10
/
y
040
Time [s]
-0.8
0
0.8
.c
c
A [g
]
M6.5 Oct 30 '16
028
-0.8
0
0.8
.
c
c
A [g]
M6.0 Aug 24 '16
043
M4.3 Aug 24 '16
027
Time [s]
M4.4 Aug 25 '16
033
M4.8 Aug 26 '16
040
M6.5 Oct 30 '16
tnenopmocWE)TMA(ECIRTAMA,ecneuqescimsieSylnokcohsniaM
-10 0 10
/
y
-1.0
0
1.0
F
/F
y
hysteretic response up to previous shock hysteretic response in current shock
-10 0 10
/
y
-1.0
0
1.0
F/F
y
-10 0 10
/
y
-10 0 10
/
y
-10 0 10
/
y
-10 0 10
/
y
050
Time [s]
-0.5
0
0.5
.
ccA [g]
M6.5 Oct 30 '16
040
-0.5
0
0.5
.cc
A [g]
M6.0 Aug 24 '16
047
M5.3 Aug 24 '16
053
Time [ s]
M5.4 Oct 26 '16
058
M5.9 Oct 26 '16
050
M6.5 Oct 30 '16
post-EQ pushover curve residual deformation collapse
tnenopmocWE)CRN(AICRON,ecneuqescimsieSylnokcohsniaM
Fig. 9 Hysteretic response and post-excitation static pushover considering single-shock versus sequential
excitation for a T¼0:30 s SDOF structure at AMT (top) and a T¼0:40 s structure at NRC (bottom)
Bull Earthquake Eng (2019) 17:5429–5447 5443
123

The upper panel of Fig. 9corresponds to a short-period structure assumed at the AMT
site. Consideration of the October 30th M6.5 shock alone, leaves the structure still standing
with some modest residual displacement but considerably damaged, as attested to by the
loss of stiffness and peak strength apparent on the post-shock static pushover. Consider-
ation of the entire sequence, however, tells a different story. The initial, pulse-like shock of
the sequence leaves the structure severely damaged, with a large residual displacement
apparently due to a single large inelastic excursion that brought the system into the in-cycle
degradation domain of the descending branch of the backbone (for a discussion of cyclic
versus in-cycle degradation, the interested reader is referred to FEMA-P440A 2009). The
next three shocks considered (low-magnitude shocks that occurred after the August 24th
M6.0 event) make little impression on the damaged and reduced-stiffness system, pro-
ducing modest ductility demands in the direction contrary to the residual displacement,
where adequate residual strength still remains. It can be noted from Fig. 8and the response
spectra of these three low-causal-magnitude shocks that, despite PGA values in the
0.20–0.30 g range, the low-frequency content is too poor to cause significant inelastic
demands, with Sa values rapidly dropping off after T¼0:50 s. Then, the arrival of the fifth
shock, which is in fact the mainshock of the sequence, predictably brings the damaged
structure to almost immediate collapse during the very first inelastic excursion towards the
direction of the residual drift.
The bottom panel of Fig. 9on the other hand, deals with a similar structure allegedly
situated at the NRC site. In this case, direct application of the base acceleration produced
by the October 30th M6.5 event to the undamaged model causes the hitherto intact
structure to collapse after a number of ample inelastic cycles. During the first four shocks
of the sequence considered, hysteresis is each time characterized by only a few important
cycles that leave the structure with a limited reduction in peak strength and mild period
elongation. Residual displacement is not observed to increase monotonically and remains
small. The mainshock leaves the structure severely damaged, but still standing, contrary to
what was observed during its application onto the intact structure, which eventually col-
lapsed. It is interesting to note that this behavior results from a complete reversal of the
situation typically associated with a seismic sequence: instead of a large-amplitude shock
followed by a series of less intense ground motions, the strongest shock in this case is
preceded by the relatively weaker shocks of the sequence. Finally, it can be also observed
that the severe loss of peak strength and significant period elongation resulting after the
fifth consecutive shock, are not accompanied by a significant residual displacement; this
situation is certainly reminiscent of the self-centering tendency of peak-oriented, degrading
systems observed by Liossatou and Fardis (2015).
In overview, these two simple case-study examples highlighted some known issues,
which may be far from novelties, but finding affirmation through a real sequence of
numerous shocks of this intensity is in itself noteworthy. The first issue concerns the
isolating effect of period elongation that can come with serious damage: while it may
shield the affected structure from further damage due to low-magnitude shocks that lack
significant spectral ordinates into the higher-period domain, a sequence of higher-magni-
tude shocks, with corresponding richer spectral shape, may prove to be not-as-forgiving. A
second issue has to do with the tendency of some evolutionary hysteretic systems to exhibit
low residual drifts during a multi-shock sequence, but accompanied by significant strength
deterioration. The third issue emerges from the comparison of both example cases sub-
jected to the entire sequence with the corresponding response to the mainshock alone: the
comparison underlines that it is not only the shaking intensity of the individual shocks that
determines the final damage state, but also the relative order of arrival, which can make the
5444 Bull Earthquake Eng (2019) 17:5429–5447
123

difference between collapse and survival of the structure; i.e., the so-called sequence-
effect.
5 Conclusions
The present article discussed a variety of issues concerning seismic actions for seismic
design and assessment, viewed through the lens of both structural engineering and engi-
neering seismology. The discussion revolved around three main issues that emerged during
the study and elaboration of near-source strong-motion accelerometric data collected
during the 2016 central Italy seismic sequence. These issues are the probability of
exceeding design seismic actions during major seismic events and the spatial disposition of
the exceedance locations, near-source pulse-like seismic input, and the effect of a seismic
sequence on structures expected to dissipate energy via inelastic deformation under a
single strong earthquake event.
It was shown that due to the manner in which moderate-to-large magnitude events are
accounted for in the definition of the code (uniform hazard) spectra in Italy, which is
consistent with their probability of occurrence close to a specific site, when an earthquake
with the same characteristics as the main events of the sequence occurs, the probability that
said design actions will be exceeded in an area near to the source is high, possibly close to
one. This result alone, which is confirmed by recorded ground motion in this and other
recent Italian sequences, does not imply that hazard computations for the code spectrum
underestimate the seismic threat for any specific site.
Pulse-like ground motions, identified among the near-source recordings obtained during
the sequence, were studied from a structural engineering point of view. The anticipated
systematic difference, in terms of average spectral shape, between this type of seismic
input and non-impulsive (ordinary) strong motion was discussed. The well-known effect of
pulse duration on the seismic demand imposed by impulsive records to inelastic structures
was showcased also with respect to existing predictive models. The investigation of this
sequence confirmed the relevance of near-source directivity pulses with respect to struc-
tural response in seismic areas with prevalent normal focal mechanisms, such as those
found along the Apennine mountain chain.
Finally, this article sought to take advantage of those accelerometric stations that
recorded multiple instances of strong motion during the first ninety days of the sequence,
to present a number of case-studies that offered interesting insights into the topic of
damage accumulation in structures. Although research into seismic damage is typically
placed within the context of purely mainshock-followed-by-aftershock sequences, the
central Italy sequence includes multiple moderate-to-high magnitude events in close
temporal succession, that may be more relevant in that respect. Case-study examples
specific to this sequence, confirmed that low-magnitude events occurring during the
sequence at very close distances to an already-damaged structure may exhibit large
amplitudes in the high-frequency range, but may lack the low-frequency richness and
corresponding spectral shape to cause significant inelastic displacement demands and
hence damage accumulation. On the other hand, this sequence was observed to have had
the potential for more severe damage accumulation phenomena, by virtue of the several
moderate-to-high magnitude events comprising it. This was mainly due to ground motions
recorded at specific sites that repeatedly found themselves in near-source conditions during
these main seismic events. Another noteworthy affirmation of a known effect was that,
Bull Earthquake Eng (2019) 17:5429–5447 5445
123

when structural dynamic response is characterized by stiffness and strength degradation of
the constituent materials, the evolutionary nature of hysteretic behavior could in effect
isolate a structure from further damage in subsequent shocks. Finally, as expected for
evolutionary hysteretic behavior, the sequence effect is not additive; i.e., given the indi-
vidual ground motions, their order of arrival determines the structural damage progression.
Acknowledgements The study presented in this paper was developed within the activities of ReLUIS (Rete
dei Laboratori Universitari di Ingegneria Sismica) for the project ReLUIS-DPC 2014–2018, as well as
within the H2020-MSCA-RISE-2015 research project EXCHANGE-Risk (Grant Agreement Number
691213).
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