Double-Difference relocation and focal mechanisms of Lucanian Apennine (southern Italy) seismicity.

EGU2010-4962

Cosmiana Maggi (1,2)*, Alberto Frepoli (1), Giovanni Battista Cimini (1), Rodolfo Console (1,2), Massimo Chiappini (1)

(1) - Istituto Nazionale di Geofisica e Vulcanologia (INGV), via di Vigna Murata, 605, 00143, Rome, Italy; (2) – Centro di Geomorfologia Integrata per l’Area del Mediterraneo (CGIAM), Potenza, Italy.

T

P

T

P

T

P

Irpinia 1 Irpinia 2 Irpinia 3

EV: 050917 0044Z= 17.09 km

EV: 070607 1716Z= 5.61 km

EV: 070718 1041Z= 10.57 km

15.1 15.2 15.3 15.4 15.540.6

40.7

40.8

40.9

41

1864

1980

HypoEllipse

Lon E

1853

Lat N

15.1 15.2 15.3 15.4 15.540.6

40.7

40.8

40.9

41

1980

18531864

HypoDD

Lon E

Lat N

15.1 15.2 15.3 15.4 15.5

-30

-20

-10

0

1980

18641853

HypoEllipse

Lon E

Dep

th (k

m)

15.1 15.2 15.3 15.4 15.5-40

-30

-20

-10

0

1980

1853 1864HypoDD

Dep

th (k

m)

15.1 15.2 15.3 15.4 15.540.6

40.841

-40

-30

-20

-10

0

Lon E

1980

HypoDD

18641853

Lat N

Dep

th (k

m)

15.1 15.2 15.3 15.4 15.5 40.640.8

41-40

-30

-20

-10

0

Lat N

1864

1980

Lon E

HypoDD

1853

Dep

th (k

m)

a) b)

c)

d)

e) f )

23

1

32

1

TP

TP

T

PT

PT

P

Potentino 4 Potentino 5

Potentino 3Potentino 2Potentino 1

EV: 081108 0924Z= 10.38 km

EV: 080721 1223Z= 19.61 km

EV: 080514 1629Z= 18.33 km

EV: 040904 0236Z= 12.48 km

EV: 080225 0542Z= 18.77 km

P

T

a) b)

P T

a) b)

Fig.9: Composite focal mechanisms computed for seven clusters located in the Potentino area (see Fig.8b, d).

15.5 15.6 15.7 15.8 15.9 16-40

-30

-20

-10

0

HypoDD

Dep

th (k

m)

Lon E15.5 15.6 15.7 15.8 15.9 16

-40

-30

-20

-10

0

HypoEllipse

Dep

th (k

m)

Lon E

15.5 15.6 15.7 15.8 15.9 1640.35

40.4

40.45

40.5

40.55

40.6HypoEllipse

Lon E

Lat N

15.5 15.6 15.7 15.8 15.9 1640.35

40.4

40.45

40.5

40.55

40.6

HypoDD

Lon E

Lat N

15.6 15.8 1640.440.540.6

-40

-30

-20

-10

0

Lon E

HypoDD

Lat N

Dep

th (k

m)

15.615.8

16 40.4 40.5 40.6

-40

-30

-20

-10

0

Lat N

HypoDD

Lon E

Dep

th (k

m)

Abriola-Pietrapertosa

Abriola-Pietrapertosa

a) b)

c)d)

e)f )

1

1

2

2

T

P

Pietrapertosa 1T

P

T

P

T

P

a) b)

c) d)EV: 081120 2125

Z= 13.77 km

TP

Pietrapertosa 2

EV: 0111121 0621Z= 11.54 km

Fig.12: Composite focal mechanisms computed for five clusters located in the Pietrapertosa area (see Fig.11b,d).

Moliterno 1

P

T

P

T

TPTP

Moliterno 2 Moliterno 3

a) b)EV: 020531 1632

Z= 10.18 km

EV: 070727 0304Z= 9.99 km

EV: 080405 1657Z= 12.94 km

Fig.15: Composite focal mechanisms computed for four clusters located in the Moliterno area (see Fig.14b,d).

15.6 15.7 15.8 15.9 16 16.1 16.2 16.3

39.7

39.8

39.9

40

40.11998

HypoDD

Lon E

Lat N

15.7 15.8 15.9 16 16.1 16.2 16.3

-50

-40

-30

-20

-10

0HypoEllipse

1998

Dep

th (k

m)

Lon E15.6 15.7 15.8 15.9 16 16.1 16.2 16.3

-50

-40

-30

-20

-10

0HypoDD

1998

Dep

th (k

m)

Lon E

15.6 15.8 1616.2 39.8

40

-50

-40

-30

-20

-10

0

Lat N

1998

HypoDD

Lon E

Dep

th (k

m)

15.6 15.8 16 16.239.840

-50

-40

-30

-20

-10

0

Lon E

HypoDD

1998

Lat N

Dep

th (k

m)

15.7 15.8 15.9 16 16.1 16.2 16.3

39.7

39.8

39.9

40

40.1

1998

HypoEllipse

Lon E

Lat N

Mercure

Valley

PollinoRange

Mercure

Valley

PollinoRange

PolicastroGulf

PolicastroGulf

a) b)

c) d)

e) f )

1

1

15.7 15.8 15.9 16 16.1

-30

-20

-10

0

HypoDD

1857

Dep

th (k

m)

Lon E

15.7 15.8 15.9 16 16.140.2

40.4

-30

-20

-10

0

Lat N

1857

HypoDD

Lon E

Dep

th (k

m)

15.715.815.916 16.140.140.240.340.4

-30

-20

-10

0

Lon E

HypoDD

1857

Lat N

Dep

th (k

m)

15.7 15.8 15.9 16

40.1

40.2

40.3

40.4

1857

HypoEllipse

Lon E

Lat N

15.7 15.8 15.9 16 16.1

40.1

40.2

40.3

40.4

1857

HypoDD

Lon E

Lat N

15.7 15.8 15.9 16

-30

-20

-10

0

HypoEllipse

1857

Dep

th (k

m)

Lon E

Val d’Agri

Vallo di Diano

Val d’AgriVallo di Diano

a) b)

c) d)

e) f )

Auletta BasinAuletta Basin

1

1

2

2

3

3

T

P

Castelluccio 1

EV: 020713 0557Z= 19.61 kmFig.6: Composite focal mechanisms computed for three clusters located in the Irpinia

area (see Fig.5b, d).

Introduction

The Lucanian Apennine belongs to the southern Apenninic chain which is located within the complex, articulated central Mediterranean plate boundary zone between the Eurasian and African plates. The geodynamic setting of this region is characterized by the NNW-SSE relative plate convergence between Eurasia and Africa. The tectonics of the Lucanian Apennine is accommodated by the collision between the Adria microplate and the Apenninic belt. Recent geodetic data suggest that the Adria is a microplate separated from the Eurasian and African plates, while its southern border is ambiguously defined. The eastward migration of the extension-compression system of the Apenninic chain is related to the subduction process of the Adria microplate lithosphere and the opening of the Tyrrhenian back arc basin. Since the Early Pleistocene, the active extension within the chain produced a broad and complex system of NW-trending normal faults and related basins. Seismological data and recent geodetic studies reveal that the Apenninic chain is undergoing a NE-trending extension, with seismic deformation rates higher in the southern portion. The southern Apenninic chain is one of the main seismically active regions of Italy (Fig.1). In the last four centuries, this area was characterized by moderate to strong events. Among the strongest earthquakes supported by historical records, the 1694 events (Me = 6.9) hit the Irpinia area and the 1857 Basilicata earthquake (Me = 6.9) hit the upper Val d’Agri and Vallo di Diano area, both with effects of the XI degree on the Mercalli–Cancani–Sieberg (MCS) scale. The least strong earthquake in this region hit the Irpinia area in 1980 (Ms = 6.9) with effects of the X degree MCS and normal mechanism of rupture.

Depth Magnitude

Hystorical seismicity

XI X - IX VIII

Thrust front

Strike-slip faults

Normal faults

BR

AD

AN

O F O

RE D

E E P

Pollino Range

L UC

AN

I AN

AP E N

NI N

E

Vallo di Diano

Agri Valley

Irpinia

TYRRHENIANSEA

IONIAN SEA

ADRIATICSEA

Gargano

Sila

TARANTOGULF

Salento

Ofanto Graben

1638

1743

1832

1638

1836

18701854

1184

1836

1857

1694

1851

14561731

1561

1980

989

12231646

1627

193017321702

1456

1456

1688

346

1805

Policastro Gulf

Tavoliere

A P U L I A F O R E L A N D

Murge

MurgeTarantine

SibariPlain

1560

Salerno Gulf

Molise

Potentino

CaF

CiF

PF

MeF

MaF

MAIF

VAFVDF

AlF

AnIrF

SGF

IrFVF

UFMAF

MMFS

1349

Mattinata fault

Foggia

Barletta-Bisceglie

Canosa

Taranto

BariOfanto River

Bradano River

Otranto ChannelFrancavillaFontana

Nardo’

Napoli

Cilento

Campania

15˚ 16˚ 17˚

40˚

18˚

40˚

41˚

17˚ 18˚15˚ 16˚

41˚

19˚42˚

39˚19˚14˚

39˚

42˚14˚

Fig.1: Southern Italy historical and instrumental seismicity map. Historical seismicity from the Catalogo parametrico dei terremoti italiani (CPTI Working Group, 2004). Instrumental seismicity from the Seismicity map of Italy, 2000-2007, INGV-CNT, Roma, (Castello et al., 2008). Active faults from Galadini et al. (2000), Maschio et al. (2005) and Papanikolaou and Roberts (2007) (from Frepoli et al., 2010).

Data and analysisWe studied the seismicity occurred in the period 2001-2008 analyzing a high-quality waveform dataset collected during two recent passive seismic experiments carried out in the study region (Fig. 2). The SAPTEX array (green circles in Fig.2) was a long-term tomography experiment lasted from June 2001 to December 2004 (Cimini et al., 2006), while the SeSCAL array (magenta triangles in Fig.2) has operated in the period between December 2007 and March 2009. From the continuous recordings of the temporary stations we extracted over 15600 waveforms of events with local magnitude range 1.5 ≤ ML ≤ 4.1. These were hand-picked along with those recorded by the permanent stations of the Italian National Seismic Network (RSNC; white squares in Fig.2) obtaining a dataset of 15666 P- and 9228 S-arrival times. We located 566 events out of 1047 recorded earthquakes using the Hypoellipse code (Lahr, 1989), the 1D velocity model and Vp/Vs ratio of 1.83 computed for the study region by Maggi et al. (2009) (Table I). Taking into account the hypocentral distribution, we subdivided the studied region into 6 sub-areas (Fig. 3). We relocated the seismicity of each sub-area using the double-difference (DD) technique (HypoDD code, Waldhauser and Ellsworth, 2000; Waldhauser, 2001). This algorithm can be used when the hypocentral distance between two earthquakes is small compared to the source receiver distance and the scale length of velocity heterogeneity. Therefore the ray paths between the events and common station are similar and the difference in the travel times for two events recorded by the same station can be attributed only to the spatial offset between the events. We computed composite focal mechanisms using the FPFIT code (Reasenberg and Oppenheimer, 1985) only for the relocated events grouped in clusters characterized by a maximum of 2 km of distance between the hypocenters.

15˚

15˚

16˚

16˚

17˚

17˚

40˚ 40˚

41˚ 41˚

0 50

km

Potentino

Pietrapertosa

Bradano foredeep

Irpinia

Pollino Range

Moliterno

Castelluccio

PolicastroGulf

Murge

Cilento

Tyrrhenian Sea

Adriatic Sea

Salerno Gulf

Taranto Gulf

Apulia foreland

Lucanian Appenines

Ionian Sea

Tavoliere

Murge

tarantine

Sibari Plain

Fig.2: Seismic stations used in this study. White squares indicate the permanent stations of the Italian national network, green circles indicate the temporary stations deployed for the SAPTEX array (June 2001 – December 2004) and magenta triangles indicate the temporary stations deployed for the SeSCAL project (December 2007 – March 2009) (from Frepoli et al., 2010).

Fig.3: Map with the Hypoellipse location of the 566 earthquakes. Colours are indicating the six groups of events analyzed with HypoDD.

Top of layer

(km)

Velocity of model

Test8 (km/s)

0 4.27

-2 5.52

-11 6.1

-23 6.5

-35 7.31

-45 7.9

Table I: Lucanian Apennines velocity model computed with VELEST code (Maggi et al., 2009).

Tyrrhenian SeaIonian Sea

Adriatic Sea

Group 1 (yellow)

This seismicity is located in the Irpinia region within an area stretching from 40.62º to 40.90º of latitude N and from 15.10º to 15.54º of longitude E (Fig.3). This group consists of 102 events located with Hypoellipse code. Event hypocentres are concentrated between 10 and 18 km of depth (Fig.4g, h). Hypocentral errors and rms are significantly reduced using the double-difference method (Fig.4). In this way hypocenters are more clustered (Fig.5). Analyzing the epicentral distribution we observe that the events are mainly located between the structures of the 1980 Irpinia earthquake (main fault and antithetic; Fig.5a, b). The HypoDD hypocenters show shallower depths in comparison with those obtained with Hypoellipse. Moreover, depth is increasing from NW to SE (Fig.5c, d, e, f).

Fig.4: Rms (seconds), Maximum Horizontal (MaxErr_H) and vertical (Err_Z) errors (kilometres) and depth distribution of the 97 best earthquakes located using Hypoellipse (a, c, e, g) and HypoDD code (b, d, f, h).

Fig.5 (left side): Map view of hypocentral distribution and W-E cross-sections of the 97 events located with Hypoellipse (a, c) and HypoDD code (b, d). 3D plots of events relocated with double-difference method (e, f). Red dotted line trace variation in seismogenic base. Brown lines: active structures observed in this area (DISS Working Group, 2009). Blue stars are epicentres of historical earthquakes and green stars are hypocentres. Numbers indicate clusters used to compute composite focal mechanisms.

Fig.7: Rms residuals (seconds), horizontal and vertical errors (kilometres) and depth distribution of the 84 earthquakes located using Hypoellipse (a, c, e, g) and HypoDD code (b, d, f, h).

Fig.8: Map view of hypocentral distribution and W-E cross-sections of the 84 events located with Hypoellipse (a, c) and HypoDD code (b, d). 3D plots of events relocated with double-difference method (e, f). Green stars hypocentre of hystorical earthquakes. Red dotted line trace variation in seismogenic base. Yellow circle outlines the Savoia di Lucania cluster. Numbers indicate clusters used to compute composite focal mechanisms.

Group 6 (blue)Seismicity of this group is located in the area of the Bradano Foredeep and the Apulia Foreland (40.40°-40.90° N of latitude, 16.00°-16.88°E of longitude). It consists of 56 earthquakes (Fig.3). This area is characterized by a more sparse seismicity. From historical records we know that this area was not hit by strong earthquakes. We relocated 54 events with the double-difference method improving the hypocentral locations (Fig.19, 20). Most of the microseismicity is located in the Bradano foredeep area. To the west, close to the eastern margin of the chain, the seismicity shows hypocentral depths between 9 and 30 km, while in the eastern part of the Bradano foredeep it is characterized by deeper events (9-40 km depth range) (Fig.20). We observe a cluster of shallow events (around 5-8 km of depth) to the east of Matera city. The deep crustal seismicity in the Bradano Foredeep and the Apulia Foreland area indicates the westward flexure of the Adria plate beneath the Apenninic belt.

Fig.19: Rms residuals (seconds), horizontal and vertical errors (kilometres) and depth distribution of the 50 earthquakes located with Hypoellipse (a, c, e, g) and HypoDD code (b, d, f, h).

Fig.20 (right side): Map view of hypocentral distribution and W-E cross-sections of the 50 events located with HypoEllipse (a, c) and HypoDD code (b, d). 3D plots of the events relocated with the double-difference method (e, f). Red dotted line trace variation in seismogenic base. Yellow circle outlines the cluster of Matera.

Conclusions

The accurate earthquake relocations performed with the double-difference method in this study allow us to retrieve a very detailed picture of the microseismicity in the area of the Lucanian Apennine. Earthquakes are mostly located along the Apenninic chain and overlaps the area characterized by the great active normal faults of the Southern Apennines (Group1, Group2). The studied microseismicity beneath the chain could be explained with the post-seismic relaxation process related to the 1980 strong Irpinia earthquake (Group1), the 1990 and 1991 Potentino sequences (Group2) and the 1998 Castelluccio sequence (Group 5). The seismicity beneath the outer margin of the chain, in the Bradano Foredeep area (Group 6), is more scattered. The depth of the seismogenic layer is around 20 km beneath the Apenninic chain and between 30 and 40 km below the outer margin of the chain and the Apulia Foreland. This eastward deepening indicates a deeper boundary between the brittle and ductile crust beneath the external margin of the Lucanian Apennine and the foredeep, compared to that beneath the chain itself. This increasing seismogenic layer depth is associated with the westward flexural bending of the Adriatic continental lithosphere beneath the Apenninic chain. The available composite focal mechanisms computed in this study suggest that the whole Southern Apennines are characterized by a NE-trending extensional stress regime in agreement with previous studies. The more constrained locations of microseismicity examined in this study is a further contribution to the comprehension of the seismogenesis and state of stress of a tectonically complex region as the Southern Apennines characterized by a very high seismic hazard. Moreover, the scarce microseismicity observed in some sectors along the Apenninic chain could be related with active fault segments presently locked where possible large earthquakes might be expected in the future.

References

Castello, B., Moschillo, R., Pignone, M., Vinci, S., Doumaz, F., Nostro, C., Selvaggi, G., 2008. Seismicity map of Italy, 2000-2007. INGV-CNT, Roma.

Cimini, G.B., De Gori, P. and Frepoli, A., 2006. Passive seismology in Southern Italy: the SAPTEX array. Annals of Geophysics , 49 (2/3), 825-840.

CPTI Working Group, 2004. Catalogo Parametrico dei Terremoti Italiani, versione 2004 (CPTI04), INGV, Bologna. http://emidius.mi.ingv.it/CPTI04/

DISS Working Group, 2009. Database of Individual Seismogenic Sources (DISS), Version 3.1.0: A compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas. http://diss.rm.ingv.it/diss/, © INGV 2009 - Istituto Nazionale di Geofisica e Vulcanologia - All rights reserved.

Frepoli, A., Maggi, C., Cimini, G.B., Marchetti, A., Chiappini, M., 2008. Seismotectonic of Southern Apennines from recent passive seismic experiments, Journal of Geodynamics, doi:10.1016/j.jog.2010.02.007

Galadini, F., Meletti, C. and E. Vittori, 2000. Stato delle conoscenze sulle faglie attive in Italia: elementi geologici di superficie. In: Galadini et al., Editors, Le ricerche del GNDT nel campo della pericolosità sismica (1996–1999), CNR-Gruppo Nazionale per la Difesa dai Terremoti, Roma, pp. 107–136 Also available from http://www.ingv.it/gndt/Pubblicazioni/Meletti_copertina.htm.

Lahr, J.C., HYPOELLIPSE/Version 2.0: a computer program for determining local earthquake hypocentral parameters, magnitude, and first motion pattern. U. S. Geol. Surv. Open File Rep., 95, 89-116, 1989.

Maggi, C., Frepoli, A., Cimini, G. B., Console, R., Chiappini, M., 2009. Recent seismicity and crustal stress field in the Lucanian Apennines and surroundings areas (Southern Italy): seismotectonic implications. Tectonophysics, 463, 130-144, doi:10.1016/j.tecto.2008.09.032.

Maschio, L., Ferranti L. and P. Burrato, 2005. Active extension in Val d’Agri area, southern Apennines, Italy: implications for the geometry of the seismogenic belt, Geophys. J. Int., 162, pp. 591–609

Papanikolaou I.D. and G.P. Roberts, Geometry, 2007. Kinematics and deformation rates along the active normal fault system in the soutern Apennines: implications for fault growth, J. Struct. Geol., 29, pp. 166–188.

Reasenberg, P., and D. Oppenheimer, 1985. FPFIT, FPPLOT and FPPAGE: FORTRAN computer programs for calculating and displaying earthquake fault-plane solutions, U.S. Geol. Surv. Open-File Rep. 85-739, 1985.

Waldhauser, F., 2001. HypoDD: A program to compute double-difference hypocenter locations, U.S. Geol. Surv. Open File Rep., 01-113.

Waldhauser, F., and W.L. Ellsworth, 2000. A double-difference earthquake location algorithm: Method and application to the northern Hayward Fault, California, Bull. Seismol. Soc. Am., 90, 1353-1368.

1616.2

16.416.6

16.8 40.5 40.7 40.9

-50

-40

-30

-20

-10

0

Lat N

HypoDD

Lon E

Dep

th (k

m)

16 16.2 16.4 16.6 16.840.4

40.5

40.6

40.7

40.8

40.9HypoEllipse

Lon E

Lat N

16 16.2 16.4 16.6 16.840.540.740.9

-50

-40

-30

-20

-10

0

Lon E

HypoDD

Lat N

Dep

th (k

m)

16 16.2 16.4 16.6 16.8

40.5

40.6

40.7

40.8

40.9HypoDD

Lon E

Lat N

16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9-50

-40

-30

-20

-10

0HypoEllipse

Dep

th (k

m)

Lon E16 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9

-50

-40

-30

-20

-10

0HypoDD

Dep

th (k

m)

Lon E

Bradano foredeep

Apulia foreland Bradano foredeep

Apulia foreland

Matera

Matera

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 90

5

10

15

20

25

Err_Z (km)

Num

ber o

f eve

nts

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 90

5

10

15

20

25

MaxErr_H(km)

Num

ber o

f eve

nts

0 5 10 15 20 250

5

10

15

20

25

Depth of events (km)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

2

4

6

8

10

rms (s)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

10

20

30

40

50

60

70

MaxErr_H(km)

Num

ber o

f eve

nts

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

20

40

60

80

100

Err_Z (km)

Num

ber o

f eve

nts

0 0.005 0.01 0.015 0.02 0.025 0.03 0.0350

5

10

15

20

rms (s)

Num

ber o

f eve

nts

0 5 10 15 20 25 30 350

5

10

15

20

25

Depth of events (km)

Num

ber o

f eve

nts

a) b)

c) d)

e) f )

g) h)

Group 3 (brown)

This group is located in the area between 40.44°-40.57°N of latitude and 15.50°-16.00°E of longitude named Abriola-Pietrapertosa and consists of 67 earthquakes (Fig.3). We relocated 64 events with the double-difference method improving the hypocentral locations (Fig.10, 11). Large part of earthquakes show hypocentral depths between 10 and 18 km (Fig.10g, h). These relocated events are elongated in a E-W direction (Fig.11a, b). A new result in this work is given by the seismic sequence of November 2008, which consists of 33 events (1.0 ≤ ML ≤ 2.9). The increased number of seismic stations in the study area, with the temporary array of the passive experiment SeSCAL, give us the occasion to relocate these events with good accuracy. This cluster is oriented along a NW-SE direction (Fig.11a, b) and shows a depth range of 10-17 km (Fig.11c, d).

Fig.10: Rms residuals (seconds), maximum horizontal (MaxErr_H) and vertical (Err_Z) errors (kilometres) and depth distribution of the 64 earthquakes located with Hypoellipse (a, c, e, g) and HypoDD code (b, d, f, h).

Fig.11: Map view of the hypocentral distribution and W-E cross-sections of the 64 events located with Hypoellipse (a, c) and HypoDD code (b, d). 3D plots of events relocated with double-difference method (e, f). Red dotted line trace variation in seismogenic base. Yellow circle outlines the cluster of the Abriola-Pietrapertosa sequence. Numbers indicate clusters used to compute composite focal mechanisms.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 80

5

10

15

20

MaxErr_H(km)

Num

ber o

f eve

nts

0 1 2 3 4 5 6 7 8 9 100

2

4

6

8

10

12

Err_Z (km)

Num

ber o

f eve

nts

5 10 15 20 25 30 35 400

5

10

15

20

25

Depth of events (km)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

1

2

3

4

5

6

7

rms (s)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.40

5

10

15

20

25

30

MaxErr_H(km)

Num

ber o

f eve

nts

0 1 2 3 4 5 6 7 80

5

10

15

20

25

30

35

40

Err_Z (km)

Num

ber o

f eve

nts

5 10 15 20 25 30 350

5

10

15

20

25

30

35

Depth of events (km)

Num

ber o

f eve

nts

0.05 0.1 0.15 0.2 0.25 0.30

2

4

6

8

10

12

rms (s)

Nu

mb

er o

f eve

nts

a) b)

c) d)

e) f )

g) h)

Group 4 (green)

This seismicity is located within the area of Auletta Basin, Vallo di Diano and Val d’Agri (40.10°-40.43° N of latitude and 15.62°-16.10°E of longitude). It consists of 78 earthquakes (Fig.3). We relocated 74 events with the double-difference method improving hypocentral locations (Fig.13, 14). Events are mainly localized within the 9-15 km depth range (Fig.13g, h). Hypocenter are more clustered by using HypoDD code. We can identify one main cluster to the SE of the upper Val d’Agri active faults. This includes events of a significant swarm named Moliterno (February-December 2002; Fig.14b, d, e, f). The relocated events of the Moliterno swarm show hypocenter depths between 6 and 11 km. To the East and the NE of the Moliterno swarm the microseismicity is rather sporadic. It is worth to note that the upper Val d’Agri was hit by the strong 1857 earthquake with ground motion reached intensity XI in the localities of Montemurro, Grumento and Viggiano.

Fig.13 (right side): Rms residuals (seconds), horizontal and vertical errors (kilometres) and depth distribution of the 74 earthquakes located using Hypoellipse (a, c, e, g) and HypoDD code (b, d, f, h).

Fig.14 (left side): Map view of the hypocentral distribution and W-E cross-sections of the 74 events located with Hypoellipse (a, c) and HypoDD code (b, d). 3D plots of events relocated with double-difference method (e, f). Red dotted line trace variation in seismogenic base. Brown line shows the active structures observed in this area (DISS Working Group, 2009). Blue star: epicentres of historical earthquakes. Yellow circle outlines cluster of the Moliterno sequence. Numbers indicate clusters used to compute composite focal mechanisms.

0 0.5 1 1.5 2 2.5 3 3.5 40

2

4

6

8

MaxErr_H(km)

Num

ber o

f eve

nts

0 1 2 3 4 5 6 7 8 9 10 11 120

5

10

15

20

Err_Z (km)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

3

4

5

6

7

rms (s)

Num

ber o

f eve

nts

0 5 10 15 20 25 300

5

10

15

20

Depth of events (km)

Num

ber o

f eve

nts

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 60

10

20

30

40

50

60

Err_Z (km)

Num

ber o

f eve

nts

0 0.02 0.04 0.06 0.08 0.1 0.12 0.140

5

10

15

20

rms (s)

Num

ber o

f eve

nts

0 0.5 1 1.5 2 2.50

10

20

30

40

50

60

MaxErr_H(km)

Num

ber o

f eve

nts

5 10 15 20 250

5

10

15

20

25

30

35

Depth of events (km)

Num

ber o

f eve

nts

a) b)

c) d)

e) f )

g) h)

Group 5 (magenta)Seismicity of this group is concentrated between 39.62°-40.14° N of latitude and 15.65°-16.36°E of longitude in the area of the Mercure Valley and the Pollino Range. It consists of 54 earthquakes (Fig.3). The microseismicity of this group is located in the same area of the small 1998 seismic sequence of Castelluccio. We relocated 50 events with the double-difference method improving hypocentral locations (Fig.16, 17). Hypocenters are more concentrated in the 10-15 km depth range (Fig.16g, h). There is a gap of microseismicity in the 23-32 km depth range (Fig.17). Few events are deeper than 30 km. This deep seismicity is located to the SW of the Pollino Range and could be related with the Southern Tyrrhenian Subduction Zone.

Fig.16: Rms residuals (seconds), maximum horizontal (MaxErr_H) and vertical (Err_Z) errors (kilometres) and depth distribution of the 50 earthquakes located using Hypoellipse (a, c, e, g) and HypoDD code (b, d, f, h).

Fig.17: Map view of the hypocentral distribution and W-E cross-sections of the 50 events located with Hypoellipse (a, c) and HypoDD code (b, d). 3D plot of the events located with double-difference method (e, f). Red dotted line trace variation in seismogenic base. Grey line: coastline. Yellow star: epicentres of historical earthquakes. Numbers indicate clusters used to compute composite focal mechanisms.

0 0.5 1 1.5 2 2.5 3 3.5 40

1

2

3

4

5

6

7

MaxErr_H(km)

Num

ber o

f eve

nts

0 1 2 3 4 5 6 7 8 90

2

4

6

8

10

Err_Z (km)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

1

2

3

4

5

6

rms (s)

Num

ber o

f eve

nts

0 5 10 15 20 25 30 35 40 45 500

5

10

15

20

Depth of events (km)

Num

ber o

f eve

nts

0 0.5 1 1.5 2 2.5 30

10

20

30

40

MaxErr_H(km)

Num

ber o

f eve

nts

0 0.5 1 1.5 2 2.5 3 3.50

5

10

15

Err_Z (km)

Num

ber o

f eve

nts

0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.160

2

4

6

8

10

rms (s)

Num

ber o

f eve

nts

0 5 10 15 20 25 30 35 40 45 500

5

10

15

Depth of events (km)

Num

ber o

f eve

nts

a) b)

c) d)

e) f )

g) h)

Corrisponding author. E-mail address: cosmiana.maggi@ingv.it (Cosmiana Maggi).

Group 2 (red)

This group is located in the Potentino area (15.42-15.88 E, 40.5-40.9 N) and consists of 89 events located with Hypoellipse (Fig.3). 84 are the relocated earthquakes with HypoDD code. Most earthquakes are located within the 7-20 km depth range (Fig.7g, h). Hypocentral locations are improved with the double-difference method: vertical and horizontal errors and rms are reduced (Fig.7); hypocenters are more clustered (Fig.8). Analyzing the epicentral distribution we observe that the earthquakes are mainly located to the SW of the two Potentino sequences of 1990 and 1991 (Fig.8a, b). A small seismic sequence started with the event of April 18, 2002 (ML = 4.1), located at 12.4 km of depth beneath Savoia di Lucania, is placed to the SW of the grouped area (Fig.8a, b). Hypocentral depth of this sequence is ranging between 8 and 13 km (Fig.8c, d). Moreover, this cluster is slightly elongated in a NW-SE direction (Fig.8f).

15.5 15.6 15.7 15.8

-35

-30

-25

-20

-15

-10

-5

0

1990

HypoDD

1991

Dep

th (k

m)

Lon E

15.5 15.6 15.7 15.8

40.5

40.6

40.7

40.8

40.9

1990

1991

HypoDD

Lon ELa

t N

15.5 15.6 15.7 15.840.640.8

-30

-20

-10

0

1990

1991

Lon E

HypoDD

Lat N

Dep

th (k

m)

15.5 15.6 15.7 15.8 40.640.8

-30

-20

-10

0

Lat N

1990

1991

HypoDD

Lon E

Dep

th (k

m)

15.5 15.6 15.7 15.8 15.9

40.5

40.6

40.7

40.8

40.9

19901991

HypoEllipse

Lon E

Lat N

15.5 15.6 15.7 15.8 15.9

-35

-30

-25

-20

-15

-10

-5

0

1990

HypoEllipse

1991

Dep

th (k

m)

Lon E

Potenza Potenza

a) b)

c) d)

e) f )

Savoia di Lucania

Savoia di Lucania

1

1

2

2

3

3

4

4

5

5

Fig.18 (left side): Composite focal mechanism computed for one cluster located in the Castelluccio area (see fig.17b, d).

0 0.5 1 1.5 2 2.5 3 3.5 40

2

4

6

8

10

12

MaxErr_H(km)

Num

ber o

f eve

nts

0 1 2 3 4 5 6 7 8 9 100

5

10

15

20

Err_Z (km)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

1

2

3

4

5

6

7

8

rms (s)

Num

ber o

f eve

nts

0 5 10 15 20 25 300

5

10

15

20

Depth of events (km)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

5

10

15

20

25

30

MaxErr_H(km)

Num

ber o

f eve

nts

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

5

10

15

Err_Z (km)

Num

ber o

f eve

nts

0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.110

5

10

15

20

rms (s)

Num

ber o

f eve

nts

0 5 10 15 20 25 300

5

10

15

20

25

30

Depth of events (km)

Num

ber o

f eve

nts

a) b)

c) d)

e) f )

g) h)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

2

4

6

8

10

12

MaxErr_H(km)

Num

ber o

f eve

nts

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.50

5

10

15

Err_Z (km)

Num

ber o

f eve

nts

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.60

1

2

3

4

5

6

7

8

rms (s)

Num

ber o

f eve

nts

0 5 10 15 20 25 30 35 40 450

2

4

6

8

10

12

Depth of events (km)

Num

ber o

f eve

nts

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.80

5

10

15

20

25

30

MaxErr_H(km)

Num

ber o

f eve

nts

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25 3.5 3.75 40

5

10

15

20

25

Err_Z (km)

Num

ber o

f eve

nts

0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.180

2

4

6

8

10

12

rms (s)

Num

ber o

f eve

nts

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 420

5

10

15

Depth of events (km)

Num

ber o

f eve

nts

a) b)

c) d)

e) f )

g) h)