The Averroes Fault: a main tsunamigenic structure in the westernmost Mediterranean

AGU2018- NH41C-0979

Abstract

*González-Vida1 J.M., Estrada, F.2, Peláez, J.A.3, Ercilla, G.2, Galindo-Zaldívar, J.4,5, Ortega1 S., Vázquez, J.T.6, D’Acremont, E.7, Tendero, V.4

1Dpt. Applied Mathematics, University of Málaga, Spain; 2Institut de Ciències del Mar, ICM-CSIC, Barcelona, Spain; 3Dpt. Física, University of Jaén, Spain; 4Dpto. Geodinámica, University of Granada, Spain; 5Instituto Andaluz de

Ciencias de la Tierra, Granada, Spain; 6Intituto Español de Oceanografía, Centro Ocenográfico de Málaga, Fuengirola, Spain; 7Sorbonne Université, UPMC Université Paris 06, Paris, France.

*jgv@uma.es

The Alboran Sea, surrounded by the Gibraltar Arc, is deformed by the active NNW-SSE Eurasian-

African convergence in the westernmost Mediterranean. An indentation tectonics active system of

conjugated strike-slip faults deforms its central and eastern seafloor. There, the Averroes fault

seems to have a seismogenic and tsunamigenic potential hazard. It is a NW-SE vertical dextral fault

that at the northwestern tip produces an upthrown southwestern block and a downthrown

northeastern block. Considering the 22 km fault length of this northernwestern segment, a

characteristic earthquake of magnitude 6.3 may produce a vertical throw of up to 1.07 m. High and

very high-resolution seismic profiles evidence recent vertical displacements of up to 5 m in each

event, supporting maximum magnitudes close to 7. The modeling of sea bottom coseismic

displacements of the Averroes fault allows to advance their tsunamigenic effects with a realistic

source.

Acknowledgements

This work has been funded by the project DAMAGE (AEI/FEDER CGL2016-80687-R) and MTM2015-70490-C2-1-R of FEDER andthe SpanishGovernment.

Main tectonic structures in the central Alboran

Sea

Numerical Results

Numerical modeling

The central Alboran Sea is a region undergoing the initial stages of continental collision between Eurasian and

African crusts of similar buoyancy, deformed by laterally unlocked indentation tectonics. The indenter structures are

the main deformations accommodating Eurasian-African plate motion in the central Alboran Sea and highlights this

area with a high seismically and tsunamigenic risks.

Conclusions

WNW–ESE dextral conjugate fault

sets forming a 75° angle. The

Alboran Sea constitutes a

Neogene–Quaternary basin of the

Betic–Rif Cordillera, which has

been deformed since the Late

Miocene during the collision

between the Eurasian and African

plates in the westernmost

Mediterranean. NNE–SSW sinistral

and WNW–ESE dextral conjugate

fault sets forming a 75° angle surround a rigid basement spur of the

African plate, and are the origin of most of the shallow seismicity of

the central Alboran Sea. Northward, the faults decrease their

transcurrent slip, becoming normal close to the tip point, while

NNW–SSE normal and sparse ENE–WSW reverse to transcurrent

faults are developed. The uplifting of the Alboran Ridge ENE–WSW

antiform above a detachment level was favoured by the crustal

layering. Despite the recent anticlockwise rotation of the Eurasian–

African convergence trend in the westernmost Mediterranean, these

recent deformations— consistent with indenter tectonics

characterised by a N164°E trend of maximum compression—entail

the highest seismic hazard of the Alboran Sea.

Main tectonic features of the central Alboran Basin.

(a) Detailed tectonic map. AC, Alboran Channel. AdR, Adra Ridge. ALF, Al-Idrisi Fault. AR, Alboran Ridge. CD, Campo de Dalías. DB, Djibouti Bank. FPB, Francesc Pagès

Bank. S-CF, Serrata-Carboneras Fault. YF, Yusuf Fault. WAB, Western Alboran Basin. EAB, Eastern Alboran Basin. SAB, Southern Alboran Basin.

(b) Magnetic anomaly map (nT) and bathymetric contours. Note the dipole bounding the northern side of the Alboran Ridge.

(c) Fault set orientation. Blue arrows indicate palaeoplate convergence and black arrows indicate present-day convergence.

(d) General structural sketch.

(e) Map of main tectonic structures and seismicity locations since 2000

Seismic profiles showing the Averroes strike-slip fault in the central zone of the Alboran Basin.

(a) The northern end is characterised by a remarkable normal component, while the southern reach

(b) displays a right lateral component.

(c) Tectonic sketch of the Averroes fault and profile location (note the 164°N palaeostress vector); positive

and negative signs represent uplifted and sunken areas respectively. Black line represents the base of the Pliocene; dashed line represents the base of the Quaternary. Scale: vertical in seconds (twoway

travel time) and horizontal in km.

Averroes Fault

Considering the previous analysis, two scenarios, with 1.07 m and

5.4 m vertical throw respectively, have been modeled using

Tsunami-HySEA, a non-linear hydrostatic shallow-water model

implemented in GPU by the EDANYA Group (Differential

Equations, Numerical Analysis and Applications) at the University of

Málaga.

A high-resolution ambient grid (~50m) is used to study the global

tsunamigenic potential risk in the Alboran Sea. Propagation map

and maximal water heights reached by the tsunami triggered by the

Averroes Fault, has heterogeneous propagation pattern, affecting

mainly to local sectors of the Spanish coast. Propagation features

of tsunami waves seems to be controlled by the interplay between

the fault orientation and seafloor morphology.

The NW–SE Averroes Fault (see figures) is considered

representative of the behavior of most of the tectonic structures in

this area. Its strike-slip component, well marked by the displacement

of the northern slopes of the Alboran Channel, decreases northward

until the fault becomes normal towards the tip line. At this area, the

Averroes fault seems to have a seismogenic and therefore a

tsunamigenic potential hazard. Considering the 22 km fault length of

this northernwestern segment, a characteristic earthquake of

magnitude 6.3 may produce a vertical throw of up to 1.07 m. On

other hand, a high and very high-resolution seismic profiles

evidence recent vertical displacements of up to 5 m in each event,

supporting maximum magnitudes close to 7. The modeling of sea

bottom coseismic displacements of the Averroes fault allows toadvance their tsunamigenic effects with realistic sources.

The Alboran Sea constitutes a Neogene–Quaternary

basin of the Betic–Rif Cordillera, which has been

deformed since the Late Miocene during the collision

between the Eurasian and African plates in the

westernmost Mediterranean. NNE–SSW sinistral and

Simulation 1 data:

• Total time simulated: 3600 s

• Wallclock in 2 Nvidia Tesla P100: 6497s

• Maximum vertical displacemnt: 1.07m

Simulation 2 data:

• Total time simulated: 3600 s

• Wallclock in 2 Nvidia Tesla P100: 6497s

• Maximum vertical displacement: 5.4m

In order to study the potential inundation risk along coasts we have considered two nested meshes, the ambient grid with

the same resolution considered before and a finer nested grid (~25m resolution) covering the area of Malaga bay. The

scenario simulated is the one with 5.4m vertical displacements. We can observe that large inundated areas along the

coast, especially in the northern area. The port is also affected while more inland inundated areas are located around the

Guadalhorce river mouth.

Ambient grid data:

• Cells: 14998x7498 = 112.455.004

• Longitude: [-5.9992, 0]

• Latitude: [35, 37.9992]

Level 0 grid data:

• Cells: 7071x4000 = 28.284.000

• Longitude: [-4.9997, -1.81827]

• Latitude: [35.0502, 36.8497]

Level 1 grid data:

• Cells: 1490x374 = 557.260

• Longitude: [-4.52153, -4.1865]

• Latitude: [36.666,36.7499]