Stramondo (1) , S., M. Tesauro(2) , F. Doumaz(1) , L. Colini(1) , A. Pesci(1) , A. Galvani(1)
(1) Istituto Nazionale di Geofisica, Via di Vigna Murata 605, Rome. Italy
(2) CNR-IRECE, Naples, Italy
MODELING CO-SEISMIC SURFACE DEFORMATION OF THE
SEPTEMBER 26, 1997, CENTRAL ITALY, SEISMIC SEQUENCE FROM
DIFFERENTIAL SAR INTERFEROMETRY (DINSAR) AND GPS.
Abstract. On September 26, 1997 the Umbria-Marche area was struck by two earthquakes of Mw=5.7
(00:33 GMT) and Mw=6.0 (09:40 GMT) that were felt in the whole central Italy. Severe damages and ground
cracks were reported in a wide area around the epicenters. Combining data provided by ERS-SAR
interferometry and GPS the total surface displacement field related to the two main shocks was revealed.. In
the differential interferogram nine fringes are clearly visible in and around the Colfiorito basin, corresponding
to about 25 cm of surface displacement in the sensor line of sight. GPS data detected a maximum
horizontal displacement of 14±1.8 cm and a maximum subsidence of 243 cm We used these geodetic data
and the seismological parameters from CMT (Complex Moment Tensor) solutions to estimate the
geometrical parameters and the slip of the fault planes. Modeled fault maximum depths and slip amplitudes
are 6.5 km and 47 cm for the first event and 7 km and 72 cm for the second one. The seismic moments are
in good agreement with those derived from the seismological data.
MODELLAZIONE CO-SISMICA DELLA DEFORMAZIONE SUPERFICIALE DEL 26 SETTEMBRE 1997,
ITALIA CENTRALE; CONFRONTO DELLA SEQUENZA SISMICA; DETERMINATA CON
INTERFONOGRAMMA; SAR (DINSAR) E GPS
Riassunto. Il 26 Settembre 1997 alle 00:33 e 09:40 GMT due eventi sismici di media magnitudo (M w 5.7 e
6.0) furono registrati nei pressi della piana di Colfiorito, al confine tra Umbria e Marche. Il terzo evento
principale della sequenza ebbe luogo il 14 Ottobre (Mw 5.6) più a sud, nei pressi di Sellano. Per lo studio del
campo di deformazione superficiale co-sismico è stata utilizzata la tecnica della Interferometria SAR
Differenziale (DInSAR). A partire da un set di dati ERS-SAR sono state selezionate le coppie
interferometriche aventi il miglior grado di coerenza. Per sottrarre il contributo topografico è stato utilizzato
un Modello Digitale del Terreno fornito dall’Istituto Geografico Militare (IGM) a 20m di risoluzione.
L’interferogramma differenziale mostrato individua una serie di frange di spostamento (ciascuna pari a 28
mm) nella direzione di vista del satellite. Sono così presenti due massimi di subsidenza, nella piana di
Colfiorito (14 cm ) e presso Annifo (25.2 cm). Una serie di dati GPS sono stati registrati il 9-10 Ottobre e
confrontati con quelli della campagna IGM del ’95. Il confronto tra dati interferometrici e GPS evidenzia un
notevole accordo, confermando la bontà dei dati SAR. L’area epicentrale è orograficamente complessa e
caratterizzata da una fitta copertura vegetale, l’interferogramma presenta aree di layover o prive di
coerenza. Dati SAR e GPS sono stati quindi utilizzati assieme ai dati sismologici per ricostruire il
meccanismo della sorgente sismica attraverso la realizzazione di modelli di sorgente. I dati GPS individuano
uno spostamento orizzontale massimo di (14±1.8)cm e un massimo di subsidenza di (243)cm. Abbiamo
utilizzato tali dati geodetici e i parametri sismologici ottenuti dalla soluzione CMT (Complex Moment Tensor)
per realizzare una stima dei parametri geometrici e la distribuzione dello slip sui piani di faglia. La profondità
massima della faglia modellata è 6.5 km e lo slip massimo 47 cm per il primo evento, mentre 7 km e 72 cm
per il secondo. I risultati della nostra modellazione sono in buon accordo con quelli derivati dai dati
sismologici
INTRODUCTION
Starting from September 1997 a large area of the Umbria-Marche Apennines in
Central Italy was struck by a strong seismic sequence. Earlier activity began on
September 3 with a Mw= 4.5 earthquake followed by several smaller aftershocks whose
frequency decreased to almost zero in a few days. On September 26 at 00:33 GMT, a Mw
5.7 earthquake occurred near the village of Colfiorito followed at 09:40 GMT by a large
event of Mw 6.0 (mainshock) whose epicentral location was only within 3 km of the
previous one (Amato et al., 1998). On October 14 at 15:23 GMT, a third large event (Mw
5.7) occurred 15 km south-eastward of the mainshock, near the village of Sellano. The
focal mechanisms for these three events are all consistent with purely normal faulting with
a NE-SW T axis (CMT solutions, Ekström et al., 1998). Seismic stations of the italian
national seismic network together with local stations deployed just after the mainshocks
located most of the aftershocks at depths ranging between 10 and 3 km, suggesting SWdipping fault planes (Amato et al., 1998). Geomorphic observations and surface geological
data are consistent with this interpretation and also indicate that the youngest (NW-SE)
tectonic structures in the area do not show structural continuity for more than 10-15 km
(Salvi et al., 1997). Field observations carried out shortly after the mainshock (Cinti et al.,
1997), allowed the identification of several surface deformation features (mostly ground
breaks with centimetric throw), along a 10 km wide belt coincident with the epicentral area.
18
The CMT scalar seismic moments calculated for the two Colfiorito shocks are 1.2 x 10
18
Nm for the 09:40 event and 0.4 x 10
Nm for the 00:33 event (Ekström et al., 1998).
Even if the strain release during the Colfiorito shocks did not originate clear tectonic
surface ruptures, the seismic moment values, the shallow depth seismicity and the
aftershock distribution, all suggested that a diffuse zone of surface displacement had
occurred. We sought to investigate the displacement field related to the largest events of
the sequence by means of GPS measurements and SAR interferometry. In this paper we
present preliminary data and the resulting fault model for the two September 26 main
shocks
THE SAR INTERFEROMETRIC FRINGES
The DInSAR (Differential Interferometry-Synthetic Aperture Radar) technique has
been demonstrated to be an important tool for coseismic crustal displacement analysis
(Massonnet et al., 1993a) and observation of dynamic deformation of volcanoes
(Massonnet et al., 1995; Lanari et al., 1998).
For the Colfiorito area we obtained a good final interferogram using an accurate ERS
image selection and a high resolution DEM (IGM, 1998) to eliminate the effect of the
topography. We selected a set of 30 ERS-1/ERS-2 images (from ascending and
descending orbits) acquired within a time window of 4 years. Our best (most coherent)
interferogram was obtained from a 35-day ERS-2 pair, the pre-seismic image was
acquired on September 7, 1997 (orbit 12458) and the post-seismic one on October 12
(orbit 12959). For these two images the ambiguity height is 79m, the orthogonal baseline
130 m and the parallel baseline 70 m; according to the estimated accuracy of the DEM,
the maximum expected error due to the topography is thus about 1/4 of a fringe, (i.e. less
than 7mm).
Although noisy, the differential interferogram clearly shows (Figure 1) concentric
fringes in two distinct areas: near the Colfiorito plain and in the adjacent area of the Annifo
plain, corresponding to the areas of maximum damage (Gasparini et al., 1997). We
interpreted the interferogram using our multilayer GIS where ground breakage,
topography, earthquakes, vegetation distribution and other themes can be compared
simultaneosly to the fringes. We finally obtained the fringes plotted in Figure 2. Three
significant features are visible in figures 1 and 2: the displacement field elongation agrees
with the strike of the two September 26 fault planes as determined from the CMT solutions
(140-150ƒ, Ekström et al., 1998); the deformation shows a marked asymmetry, with a
higher displacement gradient towards the NE border of the Colfiorito and Annifo basins;
two superimposed fringe patterns are present, with two clear relative minima. The global
along-strike extension of the fringe structure is about 20 km. Given the limited temporal
baseline (35 days), it likely represents the total coseismic displacement (in slant range
view) of both the 00:33 and 9:40 GMT, September 26 main-shocks. This is also confirmed
by the presence of two minima in the interferogram. Nine fringes can be counted in Figure
2 with the outermost being the limit of the zero displacement. Considering that each fringe
describes a range path difference of 28 mm, there is a total of 252 mm of surface
displacement in the SAR line of sight.
THE GPS DATA
Shortly after the main shocks of the seismic sequence we re-surveyed a subset of
the Italian Istituto Geografico Militare IGM95 GPS network (Surace, 1993; 1997). We
obtained 3-D coordinates with a mean accuracy of 10 mm and 20 mm in the horizontal
and vertical components respectively (95% confidence level). To compare IGMI-1995 and
our 1997 coordinates we applied a seven-parameter conformal transformation which
eliminates any systematic effect due to differences in reference systems. Considering as
coseismic displacement the result of the comparison which minimises the coordinate
residuals of the stations located farther than 15km from the epicenters, only Collecroce
(CROC), Pennino (PENN), Colfiorito (COLF) and Capannacce (CAPA) detected a
significant displacement (Figure 2). The GPS displacements, when projected onto the
SAR line of sight, show a very good agreement with the SAR displacements (Figure 2).
FAULT MODELING AND DISCUSSION
From the CMT solutions and the aftershock distribution (Ekström et al., 1998, Amato
et al., 1998) we know that the September 26 fault planes are aligned along a 140-150ƒ
direction and that the two events originated within a distance of 3 km with opposite rupture
propagation directions (Pino et al., 1998). This suggests that they belong to the same
tectonic structure. To invert the ground displacement data in order to determine the slip
distribution along this structure, we assumed a single fault plane of length 12.5 km, dip
45ƒ to the SW and strike 144ƒ, these values averaged from the corresponding values of
the two CMTs, weighting more the 09:40 event.
To account for the reported complexity (Pino et al., 1998; Olivieri and Ekström, 1998)
of the sources we divided the fault plane in 5 segments of 2.5 km length, each one in turn
divided in three patches located at different dephts (see figure 3 and the values of Hmax
and Hmin in Table 1). The fault segments F1 and F2 relate to the earthquake of 00:33 and
the segments F3, F4, F5 to the 09:40 event. From the fringes in Figure 2 we sampled one
data point every 0.5 km along each line. The total number of displacement points is 258.
Since the vertical GPS components are in good agreement with the fringe displacements
(Figure 2), we introduced in the model only the horizontal GPS displacements. The
uncertainties used in the data inversion were 20mm for the InSAR data and 10mm for the
GPS. An inversion program (Briole et al., 1986) based on a least square minimisation
algorithm developed by Tarantola and Valette (1982) was used, representing the source
as a rectangular fault in a uniform elastic half-space, computed using the Okada (1985)
formulation. Table 1 presents the results of the inversion. The synthetic interferogram and
the projected fault plane are shown in figure 3, the r.m.s. residual after the inversion is
16mm.
The maximum modeled slip is 47cm for the 00:33 event and decreases to 5 cm in
the last km, the maximum slip is located in the SE part of the fault plane. The 09:40 event
has a maximum slip at shallow depth of 24 cm in the central segment; this large value
near the surface is required to correctly fit the horizontal displacement at the GPS point
PENN. The maximum slip for this event is about 70 cm at depth in the south-central part
and decreases towards the north. For both earthquakes our values of maximum slip (47
and 72 cm) are in good agreement (within 10%) with those calculated by broadband
waveform modeling (52 and 65 cm, Pino et al., 1998). Hunstad et al., 1998, have obtained
by forward modelling of the GPS displacements a similar slip for the 09:40 event (65 cm)
and a smaller value for the 00:33 event (36 cm). These values are calculated using the
same 4 data points that we use in this work plus one extra point (FOLI) which is out of the
epicentral area; slip on the 00:33 event (33 cm) is well constrained by only one GPS point,
COLF (Figure 2).
The total fault length in our model (12.5 km) is smaller than that modelled by
Hunstad et al., 1998 (6+12 km) and that calculated by Pino et al., 1998 (10+14 km). We
also tested a model with 7 segments for a total length of 17.5 km but it led to unstable
solutions at the fault edges. This is apparently due to the fact that there are too few data
points to correctly constrain the terminations of the rupture.
The seismic moments retrieved from our best model are consistent with the values
found by Ekström et al., 1998 for both earthquakes (we used a value of 3x10^10 Nm-2 for
the rigidity of the seismogenic layer), the differences being about 5% for the larger event
and 15% for the other.
CONCLUSIONS
For the first time, SAR Interferometry has been successfully used in Italy for the
detection of coseismic surface displacements during a seismic sequence. Even if the area
of the September 1997 sequence is far from an ideal site for SAR Interferometry because
of vegetation and strong topography (Massonnet et al., 1993b), we show that using a
number of SAR images and selecting the best interferometric pairs it is possible to obtain
reasonably good results. Nine fringes are visible in the interferogram, and the location of
the maximum subsidence is in good agreement with the GPS measurements.
We modeled the DInSAR and GPS data using a dislocation model in an elastic half
space and allowing for variable slip on a single fault plane comprising both events. Even
with this approximation the fit with the DInSAR data is good, while some variations are still
present with the observed GPS measurements. The maximum depth of the faults is well
constrained and stable at 6.5-7 km for all the models calculated. The pattern of slip
distribution is well defined and shows that for the 09:40 event most of the energy was
released from an area of 5x7 km in the southern part of the fault.
A misfit betweeen modeled and observed interferograms (figure 1 and 3) remains in
the shape of the fringes. This can be due to local complexities such as: cumulation of
surface deformation occurred during the 3/10, Mw=5.2, the 6/10, Mw=5.4, and other
moderate aftershocks occurred before October 12 (date of the second ERS SAR
acquisition), which are not considered in our model; release of deformation on old, shallow
fault planes. This in particular might be the case for the 7-8cm ground displacement
described along the Costa-Corgneto alignment (Cello et al., 1998), which corresponds to a
southeastern shifting of two fringes (figure 2). This ground displacement occurred during
the 00:33 event (Cello et al., 1998) and is parallel to, but several kilometers SW of the
modeled fault plane. Some other local complexities in the fringe pattern can be seen in
figure 1 and in other interferograms. We are presently carrying on a more detailed
interpretation of the SAR data to complete the picture of ground deformation in the area.
The faults apparently do not break the entire brittle upper-crust, nevertheless the
similarity between the displacement pattern and the topographic low comprising the plains
of Colfiorito and Annifo (figure 2) and the damming of the drainage from the Colfiorito
basin towards the Chienti river valley to the NE, suggest that the present geomorphic
setting of the area may be a direct consequence of repeated diffuse, coseismic
deformation events occurred during the Upper Quaternary.
Figure 1
SAR differential interferogram of the Colfiorito area showing the total surface
displacement occurred between September 7 and October 12, 1997. Each fringe
represents a full phase cycle (+p,-p) caused by a displacement increment of 28 mm in the
slant range; the direction of the increment is from the outermost fringe inward. Also shown
are the epicentral location of the 00:33 Mw=5.7, and the 09:40 Mw=6.0 shocks. The image
is not geocoded (SAR geometry), the geocoded version would be more noisy and the
fringe pattern would be less clear.
Figure 2
Geocoded displacement contours retrieved from the interferogram (epicentral
locations same as in figure 1). Each contour line represents a displacement increment of 28 mm. GPS stations (in lowercase) and planar displacement vectors (red arrows) are
shown. A and B are the traces of the bedrock fault plane reactivations noted by Cello et
al., 1998. C is the location of the saddle damming the eastward drainage out of the
Colfiorito basin.
Figure 3
Fault model resulting from the inversion of GPS horizontal and DInSAR
displacements. Also shown are: the projection at the surface of the fault plane used in the
model (strike=144ƒ, dip=45ƒ, rake = 270ƒ) and the CMT focal mechanisms for the 26
September mainshocks (after Ekstrom et al., 1998). The size and slip values of the
various patches on the fault plane are reported in Table 1.
Table 1
Fault segment Event
F1
F2
F3
F4
F5
lower
medium
upper
lower
medium
upper
lower
medium
upper
lower
medium
upper
lower
medium
upper
GMT
0:33
9:40
East
km
330.3
331.1
331.9
328.8
329.6
330.4
327.3
328.1
328.9
325.8
326.6
327.4
324.3
325.1
325.9
North
km
4765.6
4766.1
4766.7
4767.6
4768.1
4768.7
4769.6
4770.1
4770.7
4771.6
4772.1
4772.7
4773.6
4774.1
4774.7
Hmax Hmin Width Slip
km
6.9
2.0
1.0
6.7
2.0
1.0
7.0
2.0
1.0
6.9
2.0
1.0
6.5
2.0
1.0
km
2
1
0
2
1
0
2
1
0
2
1
0
2
1
0
km
6.8
1.4
1.4
6.6
1.4
1.4
7
1.4
1.4
6.8
1.4
1.4
6.3
1.4
1.4
cm
47
19
6
30
5
5
69
29
16
72
29
24
41
12
13
Moment
Total
Seismic moment
16
*10 Nm
16
*10 Nm
*1016 Nm
24.0
2.0
0.6
14.9
0.5
0.5
36.2
3.0
1.7
36.7
3.0
2.5
19.4
1.3
1.4
42.5
40
105.2
120
Parameters of the fault plane segments used in our model: coordinates (UTM) refer
to the surface projection of the midpoint of the upper edge of the fault; strike=144ƒ and
dip=45ƒ for all segments; Hmin is the depth at the upper edge and Hmax the depth at the
lower edge of the segement; seismic moment after Ekstrom et al., 1998.
ACKNOWLEDGEMENTS
We thank ESA (European Space Agency) and Telespazio s.p.a. for providing the
ERS images, the Italian Civil Protection for financial support in obtaining the DEM and
other important data, the Italian Istituto Geografico Militare (IGM), several people at ING
who encouraged this work in its early stages. Part of the research has been funded by the
Italian Space Agency, contract No. RS 96-87 and Telespazio.
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