Numerical investigation of the November 17, 2015 anomaly in the harbor of Crotone, Ionian Sea

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1 Numerical investigation of the November 17, 2015 anomaly in the harbor of Crotone, Ionian Sea F. Zaniboni, A. Armigliato, G. Pagnoni, M. A. Paparo, S. Tinti Dipartimento di Fisica e Astronomia, Alma Mater Studiorum - Università di Bologna, Bologna, Italy

2 The November 17, 2015 Lefkada earthquake From Armigliato et al. (2015), EGU 2015 At 07:10:08 GMT of November 17, 2015, an earthquake (M w = 6.4) occurred in the Greek island of Lefkada. Near-vertical strike-slip fault, running along Lefkada W coast. Two persons died and eight people were injured. Some villages damaged. Cephalonia IONIAN SEA Lefkada

3 The November 17, 2015 Lefkada earthquake Environmental effects No surface expression of the fault. Small landslides and rockfalls, especially in the western and central part of the island. Small scale liquefaction events in some coastal areas. The ground shaking was clearly felt also in the far field, especially in southern Italy (southern Apulia, eastern Calabria, eastern Sicily). No relevant tsunami generated Anomalous sea level oscillations in the harbor of Crotone (Calabria, Italy) From Ganas A. et al. (2015), Earthquake preliminary report

4 CROTONE tide gauge Original record 15 sec, provided by JRC - EU Detided marigram 23 cm 60 min Approximated tsunami travel time: 60 min Peak to peak amplitude: 23 cm Oscillation period: 7 min 7 min

5 OTRANTO tide gauge Original record 1 min, provided by ISPRA Detided marigram

6 TARANTO tide gauge Original record 1 min, provided by ISPRA Detided marigram

7 Strike-slip mechanism (low tsunamigenic potential) About 3 hours perturbation 7 minutes period Oscillations measured only in Crotone harbor RESONANCE EFFECTS directly induced by the earthquake-tsunami? LOCAL SOURCE (such as a small submarine landslide?) A COMBINATION of the two? INVESTIGATION BY NUMERICAL CODES

8 Tsunami simulation COMPUTATIONAL GRIDS Bathymetry from GEBCO and EMODNET databases, topography from SRTM G1 G G23 Nested grids G0: 800 m step (preliminary) G1: 300 m step G2: 60 m step G3: 12 m step G4: 4 m step Numerical simulation run by means of UBO-TSUFD code G0

9 Tsunami simulation INITIAL CONDITION: EARTHQUAKE Seismic Fault Parameter Value Length (km) 23 Width (km) 10 Dip angle ( ) 71 Strike angle ( ) 18 Rake angle ( ) 180 Slip (m) 1.2 Centre of upper edge (UTM) 458,200 4,272,500 Top of the fault (m) 150 Ganas A. et al. (2016), Tectonophysics

10 Grid G0 EQ Tsunami simulation MAXIMUM WATER ELEVATION

11 Grid G0 EQ Tsunami simulation CROTONE HARBOR

12 EQ Tsunami simulation CROTONE HARBOR Grid G3

13 Tsunami simulation INITIAL CONDITION: LANDSLIDE The Ionian Calabrian Margin is characterized by many scars and canyons, which testifies a marked attitude to mass mobilization (Ceramicola et al., 2013, Subm. Mov. and Their Conseq.). In occasion of the 1908 Messina earthquake, anomalous waves were observed in the Ionian Calabria before the arrival of the tsunami from the Straits, which could have been caused by slides (see Tinti and Giuliani, 1983, Il Nuovo Cimento). Many hypothetical landslide sources have been tested, evaluating their generated tsunamis on the Crotone harbor

14 Landslide simulation Landslide Simulation with code UBO-BLOCK1 Volume assumed of about 8 millions m 3, area covered of about 2 km 2

15 LS Tsunami simulation PROPAGATION

16 LS Tsunami simulation CROTONE HARBOR

17 LS Tsunami simulation CROTONE HARBOR

18 Discussion: WHO S GUILTY? Earthquake Landslide PROS No further sources need to be invoked Oscillations amplitude and period fit the observation Perturbation seems to excite resonance in the harbor CONS Tsunami travel time not in agreement Simulated wave is smaller than observed No resonance effects No effects in Apulia Unlikely landslide triggering by the Lefkada earthquake (At the moment) No evidences of slide occurrence

19 Conclusions The 17 th November 2015 Lefkada earthquake produced some small perturbation into the sea surface. No relevant effects are visible on tide gauges in Apulia (Otranto and Taranto), while in Crotone an unexplained enhancement of the sea level oscillations is observed for at least 2 hours. Numerical modelling of the EQ-generated tsunami doesn t match the observations satisfactorily: the simulated tsunami waves seem to anticipate considerably the observations, and the following basin excitation is not accounted for. Resonance effects in the Crotone harbour can be invoked. But the question is: why only there and not in other sites or harbours? A local source would account for the effects observed only in Crotone, simulation of small landslide produce a good agreement in the first oscillations and the perturbation affects the basin for more than one hour. On the other side, it is very difficult to associate a local submarine landslide to the Lefkada earthquake. Important consequences on tsunami warning system issues.

20 Future improvements... Detailed study of resonance effects in the Crotone harbor: WHAT KIND OF SEA LEVEL PERTURBATION COULD EXCITE THE TYPICAL OSCILLATION MODES OF THE BASIN? Study of DISPERSION of tsunami propagation: Boussinesq effects can produce delay on high-frequency signal propagation, spreading the wave train and maybe producing resonance. More instrumental data in other harbors would have helped characterize and better describe the anomaly. SUGGESTIONS?

21 Thanks for your attention! Contacts: