Seismicity, seismotectonics and crustal velocity structure of the Messina Strait

Istituto Nazionale di L. Scarfì, H. Langer, A. Scaltrito Geofisica e Vulcanologia Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Italy. Sezione di Catania

contacting e-mail: [email protected]; [email protected]; [email protected]

14.00 Long. E 17.00 3 3

3 0 10 3

. N . A )

8 km 8

Tyrrhenian Sea 3 3 ea Alpine S -A an MSR Introduction p ni en e MSC n rrh in Aeolian Islands hy ic T It's now 100 years ago that a disastrous earthquake on December 28, 1908, one of the largest events ever recorded in the O r o MSI g S e ou n th GMB ern c central Mediterranean, destroyed Messina, Reggio Calabria and the adjacent areas. In addition to the damage due to the Fa Ty r ul rr A MRC t S hen n ATN yst ia ta MPN em n ri impact of the seismic waves, a great deal of losses was due to a Tsunami which developed as a consequence of the lo i e N N an P it - . . r ro t t 0 lo b

e a a a earthquake. In the last decades plenty of material has been collected in the Messina Strait, both concerning seismic data as 0 P al . . C ts L L 8 M 3 MAL MTT MMB well as the tectonic structure. From topography and the bathymetry we infer marked escarpments which separates the Chain s ghrebian it nnine - Ma n Mt. Etna Ionian Sea Ape U n sicilian and calabrian mainlands (Fig. 1). From geological investigations at the surface, however, we have no proof whether ia b MPAZ re N h g . a the important topographical elements around the Messina Strait form main fault systems, perhaps cutting the whole crust t M

a - n M

L a i a l l i t Etnean volcanics and reaching the upper mantle. ic a s S E

c Calabro-Peloritan Units 1 1 a

r 8 8 0 p . . In the present study we present a synoptic view on seismotectonics, seismicity and 3D velocity structure. The analyses are m 0 ian Sea Hyblean Foreland 7 Ion 7 . e Maghrebian Units

n 3 3 7 t 3 Hyblean Foreland 15.35 Long. E 16.01 0 10 Depth (km) 30 40 based on seismic data collected in the time span from 1999 to 2007 (Fig. 2). Fault plane solutions and seismicity patterns Main thrust front 0 0 25 50 Normal faults ( strike-slip help to identify regimes of tectonic motion. Besides this, 3D seismic imaging permits to identify the presence of lateral a km component shown by arrows) velocity contrasts along major fault systems.

0 A 10 5

. e

o ) 8 l 3 i a m

n Tyrrhenian Sea k (

Tectonic settings I s l h a t

n p d e a The investigated area belongs to the Calabro-Peloritan Arc which is a part of the Apennine-Maghrebian orogenic belt, along s D 0 the Africa-Europe plate boundary (see Fig. 1). The Messina Strait appears to be the most important of the structural 30

i discontinuities cutting the southern part of the Arc, being a narrow fan-shaped basin that links the Ionian sea to the c 0 t 20 ia ra i no m u a a fe 10 c r l u ) r t So E u S S. Tyrrhenian sea. It is bounded by high angle normal faults with prevailing N-S to NE-SW orientation, detected on land and C a a- 40 m a n n k N i i n t i a ( t r s l a . a e s C t P d e f r h offshore, active during Pliocene and Pleistocene times (Ghisetti, 1992). The complementary NW striking elements are

o a a t a M i a lf L r v

b p L u o 20 G la u a e C n ia particularly evident in the Eolian Islands and NE Sicily, contributing to a rather complex structural picture. o l D a i e Fig. 2 - Map view, N-S and E-W cross sections of the t g t g D le e a R c S o studied area with earthquakes located from 1999 to 2007. 30 rm A 30 I llo o e o Black/white boxes indicate seismic stations. 20 n eb til i nt at 10 0 a o tid Y 0 n M n . e 0 S P a 30 8 e x 20 3 a is -10 10 ( 0 Cape S. Alessio km -20 -10 B) 0 ) 0 ) -20 (km 0 0 -30 is 1 0 ax 4 X Data Mt. Etna Cape Taormina 1 0 10 20 b km We analyzed seismicity located in the area of the Messina Strait by using the data recorded by a local network deployed by 15.00 Long. E 16.00 the Istituto Nazionale di Geofisica e Vulcanologia in eastern Sicily and southern Calabria (Figs. 1 and 2) since the 1990's. Our N b 0 data set consists of more than 300 events occurring in the years from 1999 to 2007 and having a magnitude range from 1.0 Fig. 1 - Simplified tectonic map (a) and structural to 3.8 (Fig. 2). sketch map (b) of Sicily and southern Calabria 10 ) m

(from Ghisetti, 1992; Lentini et al., 2000; Monaco k (

h and Tortorici, 2000). Red boxes represent the area t Tomography Method

20 p e

studied in this paper. Permanent seismic network is D The data set was exploited in a local earthquake tomography, carrying out a simultaneous inversion of both the three- also reported. 30 dimensional velocity structure and the distribution of seismic foci. We applied the “tomoADD” algorithm (Zhang and 30 20 Thurber, 2005), which use a combination of absolute and differential arrival times and a self-adaptative mesh of nodes, 10 Y a 0 30 x 20 based on the ray density. This method is able to produce more accurate event locations and velocity structure near the is -10 10 ( 0 km -20 -10 source region than standard tomography. ) -20 m) -30 is (k X ax E We started the inversion from a regular horizontal grid, with 5x5 km node spacing covering an area of 60x60 km, based on the P and S ray paths (Fig. 3). Unlike the most common tomography methods which use regular three-dimensional grid Fig. 4 - Irregular mesh nodes approaches, the adaptive mesh method, based on tetrahedral and Voronoi diagrams, is able to automatically adapt the Fig. 3 - 3D sketch of P-wave ray paths traced (triangles) for P- (a) and S-waves inversion mesh to match with the data distribution, such that the ray sampling densities on inversion mesh nodes are more in the minimum 1D model (Langer et al., (b) at the final iteration. On the uniform than the regular grid case (Fig. 4). 2007). Earthquakes and seismic stations are bottom, the projection of the mesh indicated by red circles and blue triangles (crosses) and the map of the area respectively. (red contour line) are shown. 2 2

0 10 3 3 a . . N Se

8 8 n km S2 cluster

a 3 S1 cluster i 3 i C) n c a o e r n 1000 rh u la ia (a) (b) r c o y r S em h u uf T C C. Piale 1000 N Revisited Seismicity . E (a) (b) -S na 750 an N al C ia r 750 Applying the new 3D velocity model to well locatable events we obtain a seismicity pattern e d a 500 a a v Lr ri o

u ) b n S1 l a aa li 500

S1 m with a much higher degree of clustering than before (Fig. 5). In particular, we recognize C e

( D

o ) S2 i g 250 a t g S2 e

t e m c le R ( N

N a 250 n

c

epicenters nicely aligned along the Reggio Calabria-Calanna-S. Eufemia fault and the Armo- e o .

. S a

m t t t r c 0 A s a n a i a L d L

t 0 Delianuova fault systems in SW Calabria. We noticed a limiting depth of ca. 20 km for foci in s e o i -250

l v

l d e i

b lo t te ti

t e n a -250 a o id l the area of the Messina Strait. Depth of up to 40 km are found for the events south of the t v

M n i

e e P t -500 a R l calabrian coast. e -500 R Cape S. Alessio -750 S Well defined clusters of multiplet events (S1 and S2) (Fig 5) are found in the Messina Strait. -750 S -1000 -1000 -750 -500 -250 0 250 500 750 1000 Cape Taormina Ionian Sea -1000 -1000 -750 -500 -250 0 250 500 750 1000

High precision relative location, with the master-event technique, permits to learn more 8 8 7 -1000 -750 -500 -250 0 250 500 750 1000 -1000 -750 -500 -250 0 250 500 750 1000 Relative distance (m) Relative depth (m) . 7 . 7

7 -1000 about the clusters geometry. For S2 we infer a cluster forming a N 30° E striking planar 3 Relative distance (m) Relative depth (m) 3 0 10 30 40 -1000 W E 15.28 Long. E 15.97 Depth (km) element, whereas S1 forms a more stocky body in N-S direction (Figs 6a and 6b). W E -750 -500 -1000 0 -750 -500 -500 -250

) -500 s

i -250 m

3D Imaging x 10 ( ) S2 A

S1 -250 0 h m t Z s ) i ( p

0 0 x e

From the pattern of high- and low-velocity anomalies visible in the layers at different depths h m A

t d k Z

p

( 0 250 e

e v d

h 250 i

as well as in the crossections (Figs 7 and 8) we identify a NE-SW striking strip with negative t t e

a 500 p l v 250

i 500 e e t

a 500 R 1000 D velocity anomalies. On the calabrian mainland this strip coincides essentially with the zone l -500

e 500 500 0 R -250 North 1000 30 X 750 -500 delimited by the Reggio Calabria - Calanna - S Eufemia (northern border) and Armo - 0 1000 A 500 th -1000 -500 0 x Nor -1000

750 i 250 s 1000 X Axis Delianuova faults (southern border, see Fig. 8, crossections A-A'', B-B”, C-C” and also F-F”). 500 (c) (d) 1000 -500 -250 0 250 500 The continuation of this trend in the Ionian Sea is well identifiable in the southernmost 40 (c) Y Axis (d) profiles. Clear positive P-wave velocity anomalies are found in the northern part of the Messina Strait and along the eastern coast of Sicily. The positive anomaly in the Peloritani Fig. 5 - Final event locations in map and vertical sections. In Fig. 6a - Relative locations of S1 cluster using the master- Fig. 6b - Relative locations of S2 cluster using the master- Mountains is separated from the one of the Strait by a narrow but evident strip of relatively the map, the main fault system are also shown. event-technique; map view (a), vertical cross-sections (b, event-technique; map view (a), vertical cross-sections (b, low velocities. The picture of the deeper layers (12, 15 and 18 km) are characterized by a c) and 3 D sketch (d). c) and 3 D sketch (d). ridge of positive anomalies starting in the area of Cape Taormina, then bending in ENE direction. The ridge crosses the Ionian Sea and is also found in Calabria. 15.29 15.97 2 3 . 8 15.97 For the sake of clarity, in figure 9 we show the topography of the 6.5 km/s VP isosurface 3 which summarize the main features of the obtained velocity structure. In the figure, we also

plot the epicenters of the earthquakes located above the isosurface. 15.28 Long. E 15.97 2

3 ea . N 2 Tyrrh S 3 A N enian Sea 8 n . ia 4 3 8 n 1 i e

3 c a o h 9 1 r it n r 4 B u a la ia r 8 r o rc t m y 7 S S fe h . u u C a E T 7 6 km in C. Piale . 3 s S s a- 1 C e n 4 M an al D) a C ri e d D r a a 19 Fault plane solutions a ri v L b A’ o a u 1 l n 9 a a 4 C li 1 On the base of the event locations and of the fault plane solutions we have distinguished the e E io g D a g t e B’ t R 4 focal mechanism in four groups: (i) the Messina Strait, (ii) the Aspromonte complex, (iii) the le a 1 c o

N S rm 1 14

C’ A N 4 Ionian Sea adjacent to the southernmost coast of Calabria, (iv) the offshore region of Cape .

t F’’ .

a F t

Taormina and Cape S. Alessio ( see Fig. 10). L D’ lo a el b B’’ o L te til A’’ In the Messina Strait area, focal mechanism confirm the characteristics encountered by 8 km n at 4 o id 1 E’ M nt C’’ Pe Schick for the 1908 Messina earthquake, with normal faulting along NNE striking elements. 14 This type is typical also for the Peloritani mountains, adjacent to the Messina Strait. Towards D’’ 14 Cape S. Alessio south, in the Taormina area, horizontal strike slip movement with NE-striking P-axes become 14 9 E’’ 1

important. In the context of the stress field they could be understood as transform faults in 4 Cape Taormina Legend 1

the context of an extension perpendicular to a NE axis. The earthquakes falling in the Ionian 8 main faults

7 0 10 . M < 3p.5rofiles Sea, south to the calabrian coast, are affected by the change of the global stress field. In fact, 7 Ionian Sea km 9 1 accordingly in their fault plane solutions, P-axes striking in NW direction prevail. 3 C. Taormina

10 km F C’ F’’ A A’ A’’ 8

0 0 7 ian Sea Earthquakes located in the Calabrian mainland belong to a regime with an extension . Ion ) ) 7 m m 3 k k

perpendicular to NE striking P-axis. On the whole, the picture here resembles to the one ( (

h Long. E 10 h 10 t

t 15.29 15.97 p obtained for the Peloritani mountains and the Messina Strait. p e e D D 20 20 Conclusion Fig. 9 - Topography of the 6.5 km/s P-wave velocity isosurface. Darker colour indicate a greater depth. Contours In conclusion, the various seismological evidences - 3D velocity structure, seismicity B B’ B’’ patterns, fault plane solutions - confirm that important tectonic elements visible at the 0 are at an interval of 2.5 km and the numbers indicate the )

12 km m corresponding depth. Relocated earthquakes, above the k (

surface reach a considerable depth, at least down to the levels illuminated by earthquake

h 10 3 4 5 6 7 8 t isosurface, are plotted as red circles. p

activity. The graben-like structure well identifiable in the calabrian inland, continues e

P-wave velocity (km/s) D underneath the Messina Strait. The northeastern coastline of Sicily from Taormina to Messina 20 is accompanied by considerable velocity contrasts well identifiable down to a depth of 12 to 15 km. The Messina Strait as well as the adjacent Perloritani Mountains are characterized by C C’ C’’ extension perpendicular to the NNE striking elements. Both hypocenter locations as well as 0 ) m

focal mechanisms give hints that in the southern part of the area some features change. In k T

( y rrh

h 10 e 15 km t nia particular, focal depth of the events increase and the principal direction of major horizontal p n S e ea D stress turns from NNE to NW. it Cape Milazzo a 20 tr S i a o c n an ia ra i ol m u s S fe c u atti r s E Gulf of P u e C. Piale . C M -S D D’ D’’ na an 0 al C

) a i a r v m e S. Lucia del Mela d o k r u ( a n a L li h 10 e t D p ta

e t le

D a Castroreale c o S 18 km rm 20 A

Novara di Sicilia llo e o b til te at on tid M n E E’ E’’ Pe References 3 4 5 6 7 8 0 P-wave velocity (km/s) ) m

k (

h 10 Ghisetti, F., 1992. Fault parameters in the Messina Strait (southern Italy) and relations with the seismogenic source. t p e

Tectonophysics 210, 117-133. D Taormina 20 Langer, H., Raffaele, R., Scaltrito, A., Scarfi, L., 2007. Estimation of an optimum Velocity Model in the Peloritani Fig. 7 - P-wave velocity model for Mountains Assessment of the variance of model parameters and variability of earthquake locations. Geophys. J. Int. six representative layers resulting 170, 3, 1151-1164, doi: 10.1111/j.1365-246X.2007.03459.x. ian Sea from the 3D inversion. Contour Ion 0 10 20 Lentini, F., Catalano, S. & Carbone, S., 2000. Carta geologica della provincia di Messina, Provincia Regionale di Messina. lines are at interval of 0.2 km/s. On Fig. 8 - Vertical sections through the P-wave velocity model. The km Assessorato TerritorioServizio geologico, SELCA, Firenze. the 6 km layer the main structural traces of sections are reported in the sketch map (A-A”, ... F-F”). Monaco, C. & Tortorici, L., 2000. Active faulting in the Calabrian Arc and eastern Sicily, J. Geodyn., 29, 407-424. features are reported with white Contour lines are at an interval of 0.4 km/s. White curves contour the lines. Red circles represent the zones with DWS>100. Relocated earthquakes, within ± 4 km from the Fig. 10 - Focal Mechanisms of the major events of the Zhang, H., Thurber, C.H., 2005. Adaptive mesh seismic tomography based on tetrahedral and Voronoi diagrams: relocated earthquakes within half northeastern Sicily and southern Calabria. application to Parkfield, California. J. Geophys. Res. 110, B04303, doi:10.1029/2004JB003186. sections, are plotted as red circles. the grid size of the slice. The zones with DWS>100 are circumscribed by yellow contour lines.