Quaternary Active Tectonic Structures in the Offshore Bajo Segura Basin (SE Iberian Peninsula – Mediterranean Sea)

Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Natural Hazards doi:10.5194/nhess-12-3151-2012 and Earth © Author(s) 2012. CC Attribution 3.0 License. System Sciences

Quaternary active tectonic structures in the offshore Bajo Segura basin (SE Iberian Peninsula – Mediterranean Sea)

H. Perea1,2, E. Gracia` 2, P. Alfaro3, R. Bartolome´2, C. Lo Iacono2, X. Moreno2, E. Masana4, and EVENT-SHELF Team* 1LATTEX – IDL, GeoFCUL, Universidade de Lisboa, Ed.C6, Campo Grande 1749-016 Lisbon, Portugal 2Unitat de Tecnologia Marina – CSIC, Centre Mediterrani d’Investigacions Marines i Ambientals, Psg. Mar´ıtim de la Barceloneta, 37–49, 08003, Barcelona, Spain 3Dept. de Ciencias de la Tierra y del Medio Ambiente, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain 4RISKNAT, Dept. Geodinamica` i Geof´ısica, Universitat de Barcelona, Mart´ı Franques,´ s/n, 08028 Barcelona, Spain *M. Farran (ICM-CSIC), E. Andara (IGME), S. Perez´ and M. Roman´ Alpiste (UG)

Correspondence to: H. Perea ([email protected])

Received: 23 March 2012 – Revised: 19 July 2012 – Accepted: 6 August 2012 – Published: 23 October 2012

Abstract. The Bajo Segura fault zone (BSFZ) is the north- 1 Introduction ern terminal splay of the Eastern Betic shear zone (EBSZ), a large left-lateral strike-slip fault system of sigmoid geometry stretching more than 450 km from Alicante to Almer´ıa. The present-day crustal deformation of the southeastern The BSFZ extends from the onshore Bajo Segura basin fur- Iberian margin is driven mainly by the NW–SE convergence −1 ther into the Mediterranean Sea and shows a moderate instru- (4–5 mm yr ) between the African and Eurasian plates (Ar- mental seismic activity characterized by small earthquakes. gus et al., 1989; DeMets et al., 1990; Serpelloni et al., Nevertheless, the zone was affected by large historical earth- 2007; Vernant et al., 2010; Koulali et al., 2011). This con- quakes of which the largest was the 1829 Torrevieja earth- vergence is accommodated over a wide deformation zone with significant seismic activity south of the Iberian Penin- quake (IEMS98 X). The onshore area of the BSFZ is marked by active transpressive structures (faults and folds), whereas sula (Buforn et al., 1995; Grimison and Cheng, 1986; Morel the offshore area has been scarcely explored from the tec- and Meghraoui, 1996). In the southeastern Iberian margin tonic point of view. During the EVENT-SHELF cruise, a to- (Fig. 1), the Neogene and Quaternary faulting activity is tal of 10 high-resolution single-channel seismic sparker pro- dominated by a large left-lateral strike-slip fault system of files were obtained along and across the offshore Bajo Segura sigmoid geometry known as the Eastern Betic Shear Zone basin. Analysis of these profiles resulted in (a) the identi- (EBSZ) (De Larouziere` et al., 1988; Silva et al., 1993). The fication of 6 Quaternary seismo-stratigraphic units bounded EBSZ stretches more than 450 km from Alicante to Almer´ıa, by five horizons corresponding to regional erosional surfaces and includes, from north to south, the Bajo Segura, Carras- related to global sea level lowstands; and (b) the mapping of coy, Alhama de Murcia, Palomares and Carboneras faults the active sub-seafloor structures and their correlation with (e.g. Bousquet, 1979; Silva et al., 1993). Instrumental seis- those described onshore. Moreover, the results suggest that micity is mainly characterized by low to moderate magnitude the Bajo Segura blind thrust fault or the Torrevieja left-lateral events. Nevertheless, historically, major destructive events strike-slip fault, with prolongation offshore, could be consid- such as the 1522 Almer´ıa (IEMS98 IX), the 1829 Torrevieja ered as the source of the 1829 Torrevieja earthquake. These (IEMS98 IX-X) and the 1910 Adra (IEMS98 VIII) earthquakes data improve our understanding of present deformation along occurred in the area (IGN, 2011). the BSFZ and provide new insights into the seismic hazard In this paper, we focus on the northern termination of in the area. the EBSZ, the Bajo Segura fault zone (BSFZ). This area is marked by moderate instrumental seismic activity and low magnitude earthquakes (Figs. 1 and 2). Nevertheless, this

Published by Copernicus Publications on behalf of the European Geosciences Union. 3152 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin

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Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin 3153

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Fig. 2. Bathymetric (dashed blue lines with contour interval 100 m) and geological (Alfaro et al., 2002a, b) map of the Bajo Segura basin. Stars and dots show the historical and instrumental epicenters, respectivelyFigure 2 (IGN, 2011). Orange lines correspond to commercial MCS proﬁles (survey S-81B 25/81; Archivo Tecnico´ de Hidrocarburos, 2012) and pink lines to SCS sparker proﬁles obtained during the EVENT- SHELF cruise. Fault kinematics refers to Upper Neogene. Faults: BSFZ: Bajo Segura fault zone; GF: Guardamar fault; SMF: San Miguel de Salinas fault; TF: Torrevieja fault. Folds: BA: Benejuzar´ anticline; BSS: Bajo Segura syncline; CBS: Cala del Bosque syncline; CCA: Cabo Cervera anticline; CRA: Cabo Roig anticline; GA: Guardamar anticline; HA: Hurchillo anticline; LJA: Lomas de la Juliana anticline; LMA: La Marina anticline; LMS: La Mata syncline; PHS: Pino Hermoso syncline; PPA: Punta Prima anticline; SSS: Salinas de Santa Pola syncline; TA: Torremendo anticline; TS: Torrevieja syncline.

2 Geodynamic and seismicity settings of the Bajo Se- low marine facies (evaporites, sandstones and coral reefs); gura basin (c) Pliocene unit comprising (Montenat, 1977), from bottom to top, the following: Hurchillo marls formation (pelagic ma- 2.1 The onshore Bajo Segura basin rine facies), Rojales sandstone formation (coastal and shallow shelf facies), variegated lutite formation (river floodplain facies), and Segura conglomerates formation (fluvial chan- The Bajo Segura basin is located in the Eastern Betic nelized facies); and (d) Quaternary units, about 30 m thick, Cordillera (SE Spain), in the northern terminal splay of the with the upper 20 m dated between 14 570 yr BP (14C age; EBSZ and extends eastwards into the Mediterranean Sea Soria et al., 1999) and the present, and characterized by iso- (Figs. 1 and 2). The basin is infilled by a succession of Upper lated outcrops of Pleistocene alluvial fans, marine terraces Miocene to Quaternary sedimentary units that can be sum- and a cover of unconsolidated sediments deposited along the marized (Montenat, 1977), from bottom to top, as the fol- present Segura River valley (Fig. 2). The Upper Miocene– lowing: (a) Upper Miocene (Upper Tortonian) shallow ma- Quaternary sedimentary cover overlies a basement forming rine (conglomerates and sandstones), slope and pelagic facies part of the Internal Zone of the Betic Cordillera (Triassic cal- (marls and turbidites); (b) Upper Miocene (Late Tortonian– careous metamorphic rocks of the Alpujarride complex). Messinian) slope and pelagic basin facies topped by shal- www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 3154 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin

From the Upper Miocene to the present, the Bajo Segura already known and described in the offshore is the Tabarca basin area has been distinguished by a transpressive regime anticline ridge (Fig. 3), which extends around 80 km from (Silva et al., 1993; Alfaro et al., 2002a, b), with a NNW– the coast in an ENE–WSW direction and is 20 to 30 km SSE compressive stress field. In this geodynamic setting, the wide (Alfaro et al., 2002b). This ridge is bounded by thrust basement and the Upper Miocene to Quaternary sedimen- faults (GF4 and GF5, to the south, and SPF2, to the north, tary cover of the Bajo Segura basin have been folded and Fig. 3) verging in opposite directions and folding the Up- faulted (Fig. 2). This basin is bordered by two main faults: per Miocene unit. The southern thrusts dip to the north and the Crevillente fault (Focault, 1974) to the north (Fig. 1), and produce the uplift of the Guardamar high, whereas the north- the BSFZ (Montenat, 1977) to the south, both running ap- ern thrusts dip to the south and raise the Santa Pola high. In proximately ENE–WSW. The BSFZ is characterized by sev- the middle of the Tabarca anticline ridge is the BSFZ, which eral ENE–WSW blind thrusts that offset the Triassic base- is composed of thrust faults that dip to the south and fold ment of the Internal Zone and fold the Upper Miocene to the Upper Miocene unit. To the north and south of this an- Quaternary sedimentary cover (Montenat, 1977; Bousquet, ticline ridge, there are the Alicante and Torrevieja growth 1979; Lopez´ Casado et al., 1987; Somoza, 1993; Taboada synclines, respectively. These two synclines have operated et al., 1993; Alfaro, 1995; Alfaro et al., 2002a, 2012). The as subsided zones since the start of the folding (Alfaro et al., activity of these blind thrusts has produced an anticlinorium 2002b). Along the Tabarca anticline ridge are secondary ac- constituted by several north verging folds (Fig. 2), the most tive anticlines and synclines. These synclines are filled with important being the Torremendo, Hurchillo, Benejuzar,´ Lo- syntectonic sediments that reach the Quaternary. mas de la Juliana and Guardamar anticlines, and the Bajo Segura, Torrevieja and La Mata synclines. Other secondary 2.3 Instrumental and historical seismicity folds, hectometric in scale, are also present in the basin. The folds are generally gentle or open with limbs usually dip- The Bajo Segura basin shows moderate seismic activity ping less than 30◦, although strata of the Pliocene Segura marked by earthquakes with magnitudes lower than 4.0 conglomerate formation are sub-vertical in the northern limb (Fig. 2). Nevertheless, there have been some earthquakes of the Hurchillo anticline (Alfaro et al., 2002a). These ac- with higher magnitudes, such as the 1979 San Miguel de tive growth folds are interpreted as the result of several im- Salinas earthquake (mb 4.2), the composed 1919 Jacarilla bricated ENE–WSW blind thrusts limited by NW–SE San earthquake characterized by two events (mb 5.2 and 5.1) and Miguel de Salinas (SMSF), Torrevieja (TF) and Guardamar the 2003 east Torrevieja earthquake (mb 4.0). The seismicity dextral transfer faults (Alfaro et al., 2002a). The age of for- observed in the onshore basin seems to be related to the main mation of the folds was established from the analysis of suc- structures such as BSFZ, SMSF and TF faults. In addition, cessive progressive unconformities in the fold limbs, and it significant seismic activity has been recorded offshore. Given is not synchronous along the entire basin. For instance, the the uncertainty in the location of this seismicity, it is diffi- E–W folds, such as La Marina and Santa Pola anticlines, cult to associate the earthquakes to some specific structures. initiated their activity during the Upper Miocene, whereas However, a group of earthquakes shows an E–W strike coin- the NW–SE folds, such as the Cabo Cervera anticline and ciding with the offshore prolongation of the BSFZ (Fig. 2). the Torrevieja syncline, became active during the Pliocene The historical seismic catalog (IGN, 2011) shows that the (Alfaro et al., 2002a; Gimenez´ et al., 2009). A vertical up- area has been affected by some destructive earthquakes, such lift rate of about 0.2 mm yr−1 has been calculated compar- as the 1048 (IEMS98 = X), the 1482 (IEMS98 VIII) and the ing high-precision leveling measurements obtained in 1976, 1484 (IEMS98 IX) in Orihuela, the 1523 (IEMS98 VIII) in 1997 and 2003 along a 30 km profile cutting across the BSFZ Guardamar del Segura, the 1746 (IEMS98 VII) in Rojales and (Gimenez´ et al., 2009). This result confirms the recent activ- the 1802, 1828, 1837, 1867 and 1909, all of them (IEMS98 ity of the faults and folds in the Bajo Segura basin. VII) in Torrevieja (Bisbal, 1984; Giner et al., 2003; IGN, 2011). Nevertheless, the most destructive earthquake to have 2.2 The offshore study area struck the Bajo Segura basin and the SE Iberian Peninsula was the 1829 Torrevieja earthquake (IEMS98 IX–X), which The offshore Bajo Segura basin is characterized by a wide caused 389 fatalities and devastated Torrevieja and other shelf with an average width of 15 km and a smooth depres- small towns along the Segura River valley (Rodr´ıguez de la sion in its center (Fig. 2). The maximum depth in the area Torre, 1984; Munoz˜ and Ud´ıas, 1991; Albini and Rodr´ıguez is around 700 m and is located to the east of the central de- de la Torre, 2001; Mart´ınez Solares and Mezcua, 2002). pression. Tabarca island is located in the north of the basin where the shelf is wider. The integration of seismic reflection profiles, gravimetry, seismicity, cores and outcrop data showed that the compressive active structures described onshore propagate towards the east into the Mediterranean Sea (Alfaro et al., 2002b; Perea et al., 2010). The main feature

Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin 3155 5260 5260

NE 350 m 5000 5000 4500 4500 5 km 4000 4000 Unit III Basement Alicante Syncline 3500 3500 Unit II (Pliocene) (Upper Miocene) Unit I (Quaternary) 3000 3000 S-81B-20A 2500 2500 2000 2000 S-81B-18B 1500 1500 Alicante trough 1000 1000 S-81B-16B 500 500 8400/1 8400/1 EVS-30 SPF2 S-81B-14 8000 8000 Santa Pola high SPF3 EVS-31 7500 7500 Zone Santa Pola S-81B-12 SPF4 7000 7000 S-81B-07A Figure 3 6500 6500 S-81B-10 6000 6000 La Marina high LMF1 LMF2 5500 5500 Zone La Marina S-81B-08 LMF3 trough 5000 5000 Segura river Anticline Ridge EVS-27 4500 4500 (Fig. 9b) S-81B-06 abarca T GF1a ´ ecnico de Hidrocarburos, 2012) and relative line drawing. The names of the faults depicted in the line drawing are the same of 4000 4000 3500 3500 BSFZ (Fig. 6) EVS-26 3000 3000 Guardamar high GF2a Zone S-81B-04 GF3 2500 2500 Guardamar EVS-28 2000 2000 GF4 1500 1500 GF5 EVS-25 (Fig. 9a) trough La Mata 1000 1000 S-81B-02 500 500 Zone Multiple Commercial MCS proﬁle S-81B-07A (Archivo T orrevieja Syncline T SW 1 1 Cabo Cervera

0.0 0.5 0.0 0.5 2.5 2.5 2.0 2.0 1.0 1.0 1.5 1.5 SP: SP: (s) TWTT (s) TWTT Fig. 3. those in Fig. 8. Thepenetration small between red both squares types in of the profiles. line See drawing profile show location the in penetration Fig. of 2 the and SCS fault sparker location profiles in EVS-25 Fig. (Fig. 8. 9a), EVS-26 (Fig. 6) and EVS-27 (Fig. 9b). Note the different www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 3156 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin

(Figs. 2 and 4). The signal was recorded in a SEG-Y format EVS-24 and later analyzed on SMT Kingdom Suite software. EVS-32 EVS-25 EVS-23 Commercial multichannel seismic (MCS) profiles were Fig.9a EVS-26 obtained and processed by the EXXON Company in 1982 EVS-27 during the S-81B (25/81) survey along the Alicante offshore basin (Archivo Tecnico´ de Hidrocarburos, 2012). The source Fig.6 used comprised a tuned array of 6 clusters of 6 airguns, towed Fig.9b at a depth of 9.1 m (30 ft) and fired every 22.5 m (73.8 ft). EVS-31B Seismic data were recorded at 4 ms sampling interval, record Torrevieja Marina C1 (proj.) length of 5 s TWTT, using a 203-channel Teledyne streamer Fig.5 towed at a depth of 10.6 m (35 ft). The seismic processing flow included exponential gain correction and pre-stack de- convolution (160 ms operator length). Later, CMP sorting, N velocity analysis, NMO correction and post-stack deconvo- lution (256 ms operator length) were applied. Then, the profiles were post-stack wave equation time migrated. Finally, MCS sections were time variant band-pass and time scaled Fig. 4. 3-D view from the SE of the SCS sparker profiles ob- filtered. tained during the EVENT-SHELFFigure cruise4 on the Bajo Segura off- The multibeam swath-bathymetry was obtained at the shore basin. Note the shallowness of the Neogene basement (almost same time as the SCS sparker profiles. The used multibeam- transparent seismic facies) to the north of the basin and how the echosounder was an Elac Nautik SeaBeam 1050D, emitting Quaternary sedimentary basin (continuous reflector facies) thick- a frequency of 180 kHz. Given that the objective of the cruise ens to the east and onlaps the basement to the north. Figures 5, was the acquisition of SCS sparker profiles, the bathymetric 6, 9a and 9b are located in this figure. Lower-left inset; red lines: coverage was limited to the areas crossed during the seismic displayed profiles; dotted red lines: not displayed profiles; yellow survey. Thus, about 276 km with an average width of 300 m symbol: point of view of the 3-D view. See location of profiles in Fig. 2. were recorded during the cruise. Nevertheless, small-scale morpho-structures such as sloping ramps and rocky reliefs up to 4 m high were found in the study area. However, a high- resolution bathymetric map is necessary to understand the 3 Data and methods significance of these morpho-structures and the active pro- cesses that occur in the area. In the offshore Bajo Segura basin during the EVENT-SHELF cruise, we sought to explore the prolongation towards the continental shelf of the onshore structures (faults and folds) 4 Seismo-stratigraphy and structures of the offshore active in the Quaternary and to provide evidence of their Bajo Segura basin recent activity. To this end, a SCS sparker system was installed onboard the R/V Garc´ıa del Cid (CISC) in or- 4.1 Seismo-stratigraphic units der to obtain high-resolution profiles to investigate seafloor ruptures and the shallow sub-surface structure. The source The commercial MCS and SCS sparker profiles enabled us to was a sparker GEO-SPARK 6 kJ manufactured by GEO- distinguish seismo-stratigraphic units at different resolutions RESOURCES BV, specially designed to favor high frequen- and depth intervals. cies. The triggering was every 2 s and the source used ranged In the commercial MCS profiles three seismo-stratigraphic between 4 and 6 kJ. The receiver consisted of a 9-m-long, 24- units were identified (Fig. 3), units I, II and III. Unit I, from hydrophone single-channel streamer. The seismic signal was top to bottom, is mainly preserved in the depressed areas with recorded by the Geo-trace acquisition system with a sam- a maximum thickness of 0.2 s TWTT. It has sheet drape ge- pling rate of 100 µs and a length of the record between 1.5 ometry and does not show internal reflectors at this resolu- and 2.0 s TWTT, allowing up to 30 cm of vertical resolution tion. Unit II has regional distribution and sheet drape geom- and 400 m of penetration below the seafloor surface. Post- etry. Internal reflectors are roughly continuous and parallel processing consisted of change of polarity (the system uses with medium-to-high amplitude and low frequency. Diver- a negative electric discharge instead of a positive pulse), de- gent reflections can be observed close to the active zones. In bias, high-pass filter of 300 Hz and a low-pass band filter of the uplifted areas, reflectors from the top of unit II are eroded 1400 Hz, AGC (10 ms window applied to the whole trace), and onlap the lower unit III. Unit III is recognized in all the gain, spherical divergence correction and a swell filter. Ten profiles. It shows sheet to sheet drape geometry with paral- SCS sparker profiles (EVS-23 to EVS-32) were obtained, lel and roughly continuous reflectors of high-to-medium am- most of them being perpendicular to the onshore structures plitude and medium-to-low frequency. Comparison with the

Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin 3157

W EVS-32 E Torrevieja EVS-24 EVS-25 EVS-26 EVS-27 EVS-31B (Fig. 10a) (Fig.7) Marina C1 (proj.) S-81B-09A S-81B-02 SP: 3580 3500 3000 2500 2000 1500 1000 500 1 0.025

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Fig. 5. SCS sparker profile EVS-32 and relative line drawing showing the identified seismo-stratigraphic units and structures. Note that the Figure 5 CCF2 (Fig. 8) fault zone is offsetting the seafloor surface, thus demonstrating the recent activity on this fault zone. Around shot point (SP) 1000, there is an acoustic masking facies that could be related to authigenic carbonates (Lo Iacono et al., 2011). The position of the Torrevieja Marina C-1 well is projected. See profile and well location in Figs. 2 and 4. work of Alfaro et al. (2002b) shows that units I and II are On the SCS sparker profiles (Figs. 2 and 4), up to six units equivalent to their Pliocene–Quaternary unit, and unit III to (I1 to I6) have been identified, bounded by five horizons (H1 their Upper Miocene unit. These authors established the age to H5) that correspond to regional erosion surfaces (Figs. 5, 6 of these units based on a correlation with commercial wells and 7). Units I1 to I6 are divided into several subunits limited located in the area (one of them the Torrevieja Marina C-1 by minor discordances (gray lines in Figs. 5, 6 and 7). A to- described below and located in Fig. 2) and also with the ge- tal of fifteen subunits were differentiated. Commonly, reflec- ological units observed in the onshore basin and explained tors are continuous and parallel with a decreasing amplitude briefly in Sect. 2.1. and frequency in depth. In general, the deeper zones of the basin show concordant geometry and the shallow areas reveal www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 3158 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin

EVS-26 SSW NNE S N

EVS-32 (Fig. 6) EVS-28 S-81B-02 S-81B-09A S-81B-04 S-81B-07A (Fig. 5) SP: 1 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5480 0.025

0.050

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0.300

Fig. 6. SCS sparker profile EVS-26 and relative line drawing showingFigure the 6 seismo-stratigraphic units and structures. Note how the folding affects unit I3 and some reflectors at the base of unit I2 in the Torrevieja zone and that some faults affect unit I1 in Guardamar and La Marina zones. See location of profile in Figs. 2 and 4. erosional truncation, although toplaps can occur (Figs. 4, 5, profile EVS-32 above the multiple (0.152 s TWTT) on the 6 and 7). The base of the units is generally distinguished projection of Torrevieja Marina C-1 well (shot point 360) is by onlapping geometry in the uplifted zones. This geome- around 120 m (velocity of seismic waves between 1500 and try progressively becomes concordant and, finally, downlaps 1600 m s−1) and corresponds to the Quaternary unit in the towards the center of the basin. well. Therefore, units I1 to I5, and most probably I6, must The Torrevieja Marina C-1 commercial exploration well be Quaternary in age. Comparison of the SCS sparker pro- is located close to the EVS-32 SCS sparker profile (Figs. 2, files with commercial MCS profiles shows that units I1 to I6 4 and 5). This well was operated by ENIEPSA in Novem- (Figs. 5, 6 and 7) coincide to the upper unit I (Fig. 3). Thus, ber 1978, and the recovered core was 578 m long (Lanaja et unit I must be Quaternary in age. al., 1987). Three units were recognized in the well (Lanaja Unit I1 is the upper unit distinguished in the offshore et al., 1987), from top to bottom: (a) 386 m of calcareous basin. This unit becomes thinner and discontinuous from the clay unit with fine sand layers, Quaternary in age; (b) 109 m coastline towards the basin, and its lower boundary corre- of clay unit with fine layers of sand and limestone at the sponds to horizon H1, which is a regional erosional surface base, Upper Pliocene to Quaternary in age; and (c) 83 m (Figs. 5, 6 and 7). Close to the coast, unit I1 attains a thick- of dolomite and dolomite limestone unit, Upper Pliocene in ness of 15 m. Onshore, the last 20 m of the Quaternary unit age. The sedimentary thickness imaged in the SCS sparker were deposited during the last 14 570 yr BP. Thus, unit I1

Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin 3159

W EVS-28A E

EVS-31B SP: 1860 2000 2500 3000 3500 4000 4500 4640 0.075

0.100

Left inset

0.150 75 m (s) TWTT

0.200

0.250

2 km 0.300

SP: 1860 2000 2500 3000 3500 4000 4500 4640 0.075

Paleoshoreline I1 0.100 position H1 ? H1 Paleoshoreline H2 position H3 ? Left inset H3 Paleoshoreline 0.150 I1 position H2 H5 (s) I6 I5 H4

TWTT I3 Multiple 0.200 I2 H1 I4 I1 I1 Paleoshoreline H1 position H5 (?) H2 0.250 I2 Paleoshoreline position H4 (?)

Paleoshoreline deposits 0.300

Fig. 7. SCS sparker profile EVS-28A and relative line drawing showing the seismo-stratigraphic units. Note the presence of large erosional Figure 7 surfaces that in some cases end in ancient paleoshorelines indicated by the change in reflector slope and by the probable presence of shoreline deposits (detail in the left inset). These surfaces were used to determine the age of the seismic horizons H1 to H5. See profile location in Fig. 2. must be the continuation of the Quaternary onshore unit and results are in agreement with the observations made in the it may be the offshore sedimentary unit deposited since the Bajo Segura basin and support the hypothesis that H1 is the last glacial maximum (LGM). Accordingly, H1 may be re- erosional surface developed during the LGM lowstand. lated to the last global sea level lowstand, which is associated Given that horizon H1 could be attributed to the LGM sea with Marine Isotopic Stage 2 (MIS2) and dates 20 ± 5 ka. level lowstand, it is reasonable to assume that the other hori- In some SCS sparker profiles, this erosional surface ends zons, H2 to H5, correspond to other global sea level low- in an abrupt change in slope (Fig. 8). This might corre- stands and could therefore be associated with different MIS spond to the position of the LGM paleoshoreline, which at (e.g. Rabineau et al., 2006; Ridente et al., 2008). Neverthe- present is located approximately at 106 m below the present less, it should be noted that the present evolution of the Bajo sea level (b.p.s.l.) (considering that the velocity of seismic Segura basin is controlled by Quaternary active structures. waves propagation on sea water is 1500 m s−1 and in the first Comparison with other zones located in tectonically passive hundreds of meters of the sedimentary layer 1600 m s−1). In environments is therefore not straightforward. To avoid the the Gulf of Lions, the LGM paleoshoreline was placed be- effects of tectonic deformation, the following depth calcula- tween 102 and 107 m b.p.s.l. (Rabineau et al., 2006). These tions of probable ancient paleoshorelines were made in the www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012

3160 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin

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Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin 3161

E–W profiles that are parallel to the main active structures that cross the area enabled us to identify different structures (Figs. 5 and 7). (faults and folds) that produced the uplift of the Tabarca an- Horizon H2 is located at the base of unit I2 (Figs. 5, 6 ticline ridge. Most of the structures trend WSW–ENE to W– and 7) and is interpreted as a regional erosional surface. In E and have a compressive component in their kinematics, line with the assumptions made to date H1, H2 might also which is in agreement with the present regional stress field. be ascribed to a global sea level lowstand. In profile EV- The geological structures were grouped into six deformation 28A, the erosional surface ends in an abrupt change in slope zones coinciding with the main structures observed onshore where some possible paleoshoreline deposits could be iden- and bounded by regional synclines. From north to south, tified (Fig. 7). The depth of this ancient paleoshoreline is these deformation zones are (a) Santa Pola zone, (b) La Ma- 121 m b.p.s.l., 15 m below H1. In the Gulf of Lions, the pale- rina zone, (c) Guardamar zone, (d) Cabo de Cervera zone, oshoreline associated with MIS6 is located 16 m below MIS2 (e) Salinas de Torrevieja zone, and (f) San Miguel de Salinas (Rabineau et al., 2006). Because of the similar values ob- zone. These structures were correlated between the different tained in both areas, it may be assumed that H2 developed commercial MCS and SCS sparker profiles and with the on- during MIS6 (135 ± 5 ka). Minor discontinuities in unit I2 shore geology (Fig. 8). could correspond to (a) minor sea level oscillations similar The Santa Pola zone is located in the north of the study to those in the Adriatic Sea that have been attributed to dif- area (Fig. 8). This deformation zone is bounded to the north ferent stages of MIS5 (Ridente et al., 2008), or (b) tectonic and south by two synclines with an E–W direction that ex- activity. tends at least 50 km from the coastline into the sea. The Horizon H3 is located at the base of unit I3 (Figs. 5, 6 northern syncline, which is a prominent regional fold that and 7). As in the case of the aforementioned horizons, H3 is bounds the Tabarca ridge towards the north, marks the begin- a regional erosional surface with an associated ancient pale- ning of the Alicante syncline (Fig. 3). The southern syncline oshoreline located at 125 m b.p.s.l., 4 m below H2 and closer is a smooth fold that could be correlated with the Salinas de to the present shoreline (Fig. 7). Given its regional character Santa Pola syncline observed onshore (Montenat, 1977; So- and after comparison with H1 and H2, H3 could also be cor- moza, 1993; Alfaro, 1995), although with some uncertainty. related with an older global sea level lowstand. In the Gulf of Between these two synclines is the Santa Pola high (Fig. 3), Lions, the difference between the second and the third dis- an E–W trending smooth ridge comprising narrowly spaced continuities ascribed to sea level drops is also 4 m, the lat- anticlines associated with faults with the same direction. The ter discontinuity being assigned to MIS8.2 (Rabineau et al., faults in the north dip slightly towards the south, whereas in 2006). Considering this and the similarities to the other hori- the center of the zone these are sub-vertical (Figs. 3 and 9c). zons, H3 could correspond to MIS8.2 (247.6 ± 5 ka). The commercial MCS profiles show that these faults affect The fourth horizon is H4 located at the base of unit I4 unit III, but whether they disturb unit II is not clear. Never- (Figs. 5, 6 and 7). This horizon also seems to be a regional theless, the SCS sparker profiles show that the faults clearly erosional surface. The uncertainty is related to the fact that offset reflectors under horizon H2 (fault SPF3 in Fig. 9c), the zones where it can be best observed are uplifted and indicating their Quaternary activity. folded. Nevertheless, an ancient paleoshoreline related to H4 South of the Santa Pola zone, La Marina zone (Fig. 8) has was identified approximately at 144 m b.p.s.l. (Fig. 7). Be- a WSW–ENE direction, extends at least 50 km and is located cause of the similarities to H1, H2 and H3, horizon H4 could between two synclines: the Salinas de Santa Pola to the north, have been formed during a global sea level lowstand corre- and the marine continuation of the onshore Bajo Segura syn- sponding to MIS10 (341 ± 5 ka). cline (Montenat, 1977; Taboada et al., 1993; Alfaro et al., Finally, the last regional discontinuity is horizon H5, 2002a, b; Gimenez´ et al., 2009) to the south (Fig. 3), which which is located at the base of unit I5 (Figs. 5, 6 and 7). continues along the offshore basin. The main faults are local- This is the last and the deepest regional erosional surface ob- ized to the south and north of those synclines, respectively served in the SCS sparker profiles. A probable ancient pa- (Fig. 3). Associated with faults LMF1 and LMF2 is an an- leoshoreline was identified between 130 and 168 m b.p.s.l. ticline which in some areas has a double hinge, producing (Fig. 7). This large range in the depth of the paleoshoreline a small syncline in between. Towards the east, this anticline is attributed to the difficulty of observing it in areas without probably joins the southern anticline of the Santa Pola zone, deformation. Compared with the other horizons, H5 could leading to the disappearance of the Salinas the Santa Pola also be related to a global sea level lowstand and assigned to syncline. The faults dip slightly towards the central part of MIS12 (434 ± 5 ka). the zone. In the commercial MCS profiles, the faults affect unit III, and fault LMF3 affects unit II (Fig. 3). The higher 4.2 Offshore faults and fold systems resolution of the SCS sparker profiles shows that these faults are sealed by horizon H1, although in some places this hori- As stated above, the Tabarca anticline ridge is the main zon might be displaced (Fig. 6). The anticlines fold horizon structure in the offshore Bajo Segura basin (Fig. 3). The H2 and probably fold H1. analysis of the commercial MCS and SCS sparker profiles www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 3162 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin rwnssoigtesim-tairpi nt,fut n od.Nt o h eomto tutrsafc h otrcn utrayuis e oaino rfie nFg.2ad4. and 2 Figs. in profiles of location See units. Quaternary recent most the affect structures deformation the how Note folds. and faults units, seismo-stratigraphic the showing drawings 9. Fig. TWTT (s) a

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H5 I5 I6 EVS-32 I2 2800 2800 S-81B-09A EVS-25 Cabo Cervera anticline I1 S-81B-02 2400 2400 Syncline La Mata I5

Multiple 2000 2000 35.25 m N 0.065 0.100 0.150 0.065 0.100 0.150 b SP: SP: S 1200 1200 S-81B-09A

Multiple (a) 1 km V-7aon h uraa zone Guardamar the around EVS-27 , H2 Basement I3(?) GF2a 1000 1000 Figure 9 H1 Guardamar anticline

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H3 1 km

H5 2800 2800 35.25 m N

Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin 3163

The Guardamar zone is a large deformation area with The southernmost zone is the San Miguel de Salinas zone a WSW–ENE direction and a length of up to 50 km. It is (Fig. 8). The structures identified in this zone change from located between the Bajo Segura and La Mata synclines WNW–ESE in the north to E–W in the south, and continue (Gimenez´ et al., 2009), to the north and south, respec- for 15 km offshore. Most of the faults are sub-vertical and tively (Figs. 3 and 8). The La Mata syncline propagates offset horizon H2 and, in some places, also horizon H1. Both around 20 km from the onshore towards the sea, progres- horizons are folded. As a result of the correlation with the sively changing its direction from NW–SE to WSW–ENE. structures described onshore (Montenat, 1977; Alfaro et al., The faults located on both sides of the Guardamar zone pro- 2002a; Gimenez´ et al., 2009), the SMSF, the Punta Prima duced the uplift of the area and a regional antiform. As a anticline and the Cala del Bosque syncline extend offshore. result, the Neogene basement is elevated and, in some areas, exposed on the seafloor (Figs. 6 and 9b). The structures located to the north of the antiform may be correlated with the BSFZ and the Guardamar anticline (Figs. 3, 6 and 9b). 5 Discussion Both structures extend along the study area. The commercial MCS profiles show that the BSFZ is composed of thrust 5.1 Style of deformation faults that dip towards the south (i.e. are folding units III and II). Moreover, the SCS sparker profiles show that the BSFZ As stated above, the present regional stress field in the Bajo separates in some minor sub-vertical faults that offset hori- Segura basin is compressive and trends NW–SE to N–S. Ac- zon H1. The structures located to the south of the Guardamar cordingly, most of the described structures, faults and folds, high are not correlated with specific onshore structures and have a compressive component in their kinematics and trend taper off towards the center of the shelf. In the commercial WSW–ENE to W–E (Figs. 3, 6 and 8). Taboada et al. (1993) MCS profiles, the faults usually dip towards the center of the considered three hypotheses in an attempt to explain the de- antiform and affect units III and II. The analysis of the SCS formation undergone in the Bajo Segura basin and to ac- sparker profiles shows that most of the faults are sealed by count for the geometrical relationships between the main horizon H1. However, some faults could displace reflectors faults (BSFZ, TF and SMSF) and anticlines (the Guardamar, above H1 (Figs. 6 and 9b). The folds clearly deform units III Cabo Cervera and Punta Prima). The first hypothesis consid- and II, which are very close to the seafloor surface. The hori- ers that each anticline was created by an independent blind zons developed above these folds are apparently not folded thrust. By contrast, the second and third hypotheses propose in the SCS sparker profiles. that all the faults branch into a common detachment layer The Cabo Cervera zone (Fig. 8) is located between the La at depth. Moreover, the Guardamar and Cabo Cervera an- Mata syncline to the north (Fig. 9a) and some small synclines ticlines would be associated with a thrust system with flat- to the south (Fig. 6). The structures located along the Cabo ramp geometry. The difference between the two hypotheses Cervera zone have an E–W to WSW–ENE direction and con- is that one considers that both anticlines are related to blind tinue up to 15 km offshore. In the commercial MCS pro- thrusts, whereas the other assumes that only the Guardamar files, the faults are sub-vertical and offset unit III and prob- anticline is produced by a thrust. Analysis of the commer- ably unit II, which is folded (Alfaro et al., 2002b). The SCS cial MCS profiles S-81B-07A (Fig. 3) and S-81B-09 (Alfaro sparker profiles show some faults reaching and deforming et al., 2002b), which have directions similar to those of the the seafloor (Fig. 5). The TF extends offshore with some un- cross section provided by Taboada et al. (1993), shows the certainty. Nevertheless, the CCF2 faults have been recently following: (a) there is no shallow (1 km depth) detachment active (Fig. 5) and could be related to TF. The Cabo Cervera layer that could be identified as the flat zone of flat-ramp anticline also extends offshore. This anticline probably folds thrust geometry; by contrast the BSFZ, TF and SMSF seem horizon H1, but decreases in amplitude before tapering off deeply rooted in the crust since they show high dip angles; around 10 km from the coastline. (b) there is a predominance of thrust kinematics only in the The Salinas de Torrevieja zone is located south of the BSFZ; (c) the TF and SMSF are more vertical and produce Cabo Cervera zone (Fig. 8). A number of synclines and anti- less uplift than the BSFZ, indicating that their thrust kine- clines continue offshore around 15 to 20 km from the coast- matics is much lower and that the strike-slip component is line (Fig. 6). These folds have two different directions. The prevalent. In line with these observations, the second and northeastern ones have an E–W trend, a low amplitude and third hypotheses of Taboada et al. (1993) could be discarded do not deform the reflectors above horizon H2. In contrast, even though these hypotheses offer a better reproduction of the folds identified in the south have a WNW–ESE trend, the deformation at the base of the Pliocene. Furthermore, the a larger amplitude and slightly fold the reflectors above H2 first hypothesis should consider a main strike-slip component and near H1 (Fig. 6). The Torrevieja syncline described on- in the TF and SMSF. From the Bajo Segura fault to the north, shore (Montenat, 1977; Somoza, 1993; Alfaro et al., 2002a; there is a predominance of compressive structures (thrust Gimenez´ et al., 2009) extends offshore in a WNW–ESE di- faults and anticlines) that uplift the La Marina and Santa Pola rection for more than 20 km. highs (Fig. 3). There are few structures in this uplifted area, www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 3164 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin the most significant being the anticline associated with fault the Bajo Segura Plain onshore (Larramendi, 1829; Rodr´ıguez LMF2, which has been correlated with La Marina anticline. de la Torre, 1984; Alfaro, 1995; Alfaro et al., 1999; So- ria et al., 1999). Earthquake magnitudes between 6.0 and 5.2 Timing of deformation 6.9 and focal depths between 3 and 7 km have been estimated for this event by different authors (Munoz˜ and Ud´ıas, The activity of faults and folds identified in the Bajo Se- 1991; Delgado et al., 1993; Giner, 1996; Mart´ınez Solares gura basin began in the Upper Miocene and continues during and Mezcua, 2002; Garc´ıa-Mayordomo and Mart´ınez-D´ıaz, the Quaternary (Alfaro, 1995; Montenat et al., 1990). The 2006). The question still open is about which fault gener- commercial MCS profiles show that faults and folds deform ated the 1829 Torrevieja earthquake. The most recent com- units III and II, Upper Miocene and Pliocene in age, respec- pilations and reevaluations of intensity data points include tively, but it is not certain whether they affect unit I, which 67 localities where the 1829 Torrevieja earthquake caused corresponds to the Quaternary (Fig. 3). However, the analysis damage to buildings and/or was felt by the population (Al- of the SCS sparker profiles shows that most of the structures bini and Rodr´ıguez de la Torre, 2001; Mart´ınez Solares and that were active during the Pliocene have been active dur- Mezcua, 2002). The highest intensities (IEMS98 IX–X and ing the Quaternary (Figs. 5, 6 and 9). As stated above, the IX) were felt between Benejuzar´ and Guardamar, from west different deformation zones show faults and folds that de- to east, and between Almorad´ı and Torrevieja, from north form horizon H2. Some of these structures also affect hori- to south. This geographical distribution of the highest inten- zon H1, and very few reach the seafloor surface (Figs. 5 sities points to the BSFZ and TF as being the most likely and 9). In line with these observations and with the ages sources of the earthquake (Fig. 10). Moreover, given the lack given to the different horizons, most of the structures were of information about intensity offshore and the location of active until 135 ka and in some cases this activity postdates some of the highest values close to the coastline, the earth- 20 ka. Considering the available slip rates of the main faults quake could have resulted from the rupture of an offshore in the area, which vary between 0.12 and 0.40 mm yr−1 (So- segment of one of these two faults. The analysis of the SCS moza, 1993; Garc´ıa-Mayordomo and Mart´ıniez-D´ıaz, 2006; sparker profiles obtained in this area could provide some new Gimenez´ et al., 2009; Alfaro et al., 2012; Garc´ıa-Mayordomo information about the identification of the earthquake source. et al., 2012), some authors have calculated the recurrence in- The first candidate to be the 1829 earthquake source is tervals of the large earthquakes, which range between 1700 the BSFZ (Figs. 2, 8 and 10). This is a WSW–ENE trend- and 30 000 yr (Alfaro et al., 2012; Garc´ıa-Mayordomo et al., ing blind thrust with the hanging wall directed towards the 2012). Thus, reflectors above horizon H1 were probably de- NW. This is a blind fault that deforms the surface with sev- formed by very few earthquakes. Moreover, the deformation eral anticlines affecting the Plio-Quaternary units (Taboada produced by a single event could be distributed over a wide et al., 1993; Alfaro, 1995). Using different geological mark- area given that the main structures of the zone are anticlines ers and high-precision leveling data, an uplift rate of around associated with blind faults. These two reasons would sug- 0.2 mm yr−1 has been estimated around the BSFZ (Somoza, gest that the accumulated offset in not necessarily visible in 1993; Sanz de Galdeano et al., 1998; Garc´ıa-Mayordomo the resolution of the SCS sparker profiles. In conclusion, fol- and Mart´ınez-D´ıaz, 2006; Gimenez´ et al., 2009). Alfaro et lowing this reasoning, the structures affecting the reflectors al. (2002a) interpreted this structure as the most likely source just below or above horizon H2 can be considered as active fault and located the focus between the Segura River and during the Quaternary and capable of causing large earth- Torrevieja, considering the evidence of Quaternary activity quakes. and the southward dip of the BSFZ. This would explain the considerable damage and high intensities around Torrevieja 5.3 Possible sources of the 1829 Torrevieja earthquake as well as the liquefaction and local site effects observed in the basin villages. Analysis of the SCS sparker profiles On 21 March 1829, the province of Alicante was struck by a shows that some minor faults, related to the BSFZ, offset large earthquake known as the Torrevieja earthquake (IEMS98 horizon H2 and maybe H1 (Figs. 6 and 9b). Thus, the off- IX–X). This is one of the largest historical earthquakes in shore part of the Bajo Segura fault has been active during the Spain and the strongest shock of a seismic sequence that af- Upper Quaternary, with possible deformation events in the fected the province of Alicante during 1828 and 1829 (Lopez´ last 20 ka. The total length of the Bajo Segura fault onshore Marinas, 1976; Mezcua, 1982; Rodr´ıguez de la Torre, 1984; is 27 km, 10 km corresponding to the Hurchillo, 9 km to the Munoz˜ et al., 1984; Albini and Rodr´ıguez de la Torre, 2001; Benejuzar´ and 8 km to the Guardamar segments (Alfaro et Mart´ınez Solares and Mezcua, 2002). According to histori- al., 2012). On the basis of the correlation between the dif- cal records, the Torrevieja earthquake was felt over a large ferent seismic profiles, the BSFZ can be traced for at least area, caused a great deal of damage to the epicentral area 27 km offshore. Considering these different dimensions as (e.g. the villages of Almorad´ı, Benejuzar´ and Torrevieja were the surface rupture length of an earthquake and using empir- completely destroyed) and about 389 casualties (Larramendi, ical relationships for reverse faults (Wells and Coppersmith, 1829; Galbis, 1932). Considerable liquefaction occurred in 1994), a moment magnitude (Mw) ranging between 6.1 and

Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin 3165

0°55'W 0°50'W 0°45'W 0°40'W

San Felipe Neri !H

38°10'N Catral La Granja !H ! Benferri Cox Villa de los Dolores E ! !

Callosa Puebla de Rocamora Redov!an ! San Fulgencio Almoradí La Daya ! E Rafal !H La Daya ! !! San Bartolomé Guardamar !H Algolfa Rojales Orihuela ! !H ! Molins !H Benijofar ! Benejuzar 38°5'N E !H Formentera del Segu!Hra Bigastro Jacarilla ! !H

La Mata !H

38°0'N Torrevieja Campo de Salinas ! !H

Intensity EMS98 !H 9,5 !H 8,5 !H 7,5 !H 6,5 E Felt

! 9 ! 8 ! 7 ! 6

Fig. 10. Geological map of the Bajo Segura basin (Alfaro et al., 2002a, b) showing the distribution of the intensity data points of the 21 March 1829 Torrevieja earthquake (Mart´ınez Solares and Mezcua, 2002). Note that the area where the maximum intensities are located suggests that the BSFZ or the TF are the most likely sources of this earthquake. Figure 10 6.7 is obtained. Thus, these ruptures would yield magnitudes ties. This fault trended NW–SE, extended offshore and may in the range of magnitudes estimated by other authors for this be correlated with the TF. Thus, as in the case of the BSFZ, earthquake. Nevertheless, the highest magnitude (Mw 6.9) is an earthquake along the TF could also have accounted for only obtained when the length of the Guardamar and the off- the destruction in the area. Furthermore, rupture propagation shore segments are considered to be the earthquake rupture from offshore towards the northwest could explain the con- (35 km). This maximum magnitude is exceeded when assum- siderable damage in the Bajo Segura basin. Nevertheless, a ing that the entire fault (onshore and offshore segments) rup- shallow earthquake with an estimated magnitude higher than tures together (54 km, Mw 7.1). 6.0 would produce surface rupture, which has not been de- The second candidate, the TF (Figs. 2 and 8), is a NW–SE scribed in the area. The existing reports concern a large frac- trending right-lateral strike-slip fault with a reverse compo- ture in a coastal and rocky outcrop between Torrevieja and nent. Although the fault is difficult to follow along the zone, La Mata (Larramendi, 1829), which could also be a landslide its present activity is evidenced by the presence of the Cabo scar. As in the onshore, the offshore continuation of the TF is Cervera anticline where the Plio-Pleistocene sedimentary uncertain. However, the Cabo Cervera anticline can be iden- units are folded. Besides, the fault could have produced some tified and extends for around 10 km. The CCF2 faults are as- lateral displacement of the BSFZ, which is active (Fig. 2). sociated with both structures and reach and offset the seafloor The estimated slip rate of the TF is 0.075 mm yr−1 because surface (Fig. 5), indicating recent activity. Assuming that the of the displacement of the Upper Pliocene structural surface Cabo Cervera anticline is directly related to the activity of the (Garc´ıa-Mayordomo and Mart´ınez-D´ıaz, 2006). On the basis fault, the onshore and offshore lengths of the TF are similar, of a statistical study of the intensities assigned to some local- about 12 km each (Garc´ıa-Mayordomo and Mart´ınez-D´ıaz, ities during the 1829 Torrevieja earthquake (Lopez´ Marinas, 2006). Considering the length of these individual segments 1976; Munoz˜ et al., 1984; Lopez´ Casado et al., 1992), and as- to be the maximum surface rupture and using empirical re- suming that the source model is linear, Delgado et al. (1993) lationships for reverse and strike-slip faults (Wells and Cop- obtained a fault source that better reproduced these intensi- persmith, 1994), a Mw 6.3 is obtained. Magnitude attains 6.7 www.nat-hazards-earth-syst-sci.net/12/3151/2012/ Nat. Hazards Earth Syst. Sci., 12, 3151–3168, 2012 3166 H. Perea et al.: Quaternary active tectonic structures in the offshore Bajo Segura basin when both segments are regarded as rupturing together. With The relations between the seismo-stratigraphic discontinu- this fault source, the maximum estimated magnitude for the ities and the faults and folds that deform them provide evi- earthquake (Mw 6.9) is not reached. dence of recent Quaternary activity in the Bajo Segura basin. As stated above, the BSFZ and the TF show Upper Qua- Most of the faults are sealed or offset horizon H2 (135 ka), ternary activity. In some cases, this activity postdates horizon and in some cases their activity postdates the formation of H1 or even implies the deformation of the seafloor surface. horizon H1 (20 ka). The structures that deform horizon H2 Thus, it may well be that the fault rupture or part of the rup- may therefore be considered active and capable of producing ture that caused the 1829 Torrevieja earthquake was located destructive earthquakes. offshore. Nevertheless, no tsunami was reported to be asso- Because of the distribution of the intensity data points ciated with this event. The rupture of individual segments of and of the recent tectonic activity, both the BSFZ and TF these two faults or some combinations of them could pro- could represent the most likely sources for the 1829 Torre- duce an earthquake with magnitudes in the range estimated vieja earthquake. Analysis of the length of both faults and for this event. fault segments onshore and offshore shows that almost all Much work has been done in the area substantially im- of the fault segments forming both BSFZ and TF are ca- proving the imaging and characterization of the active/recent pable of producing a magnitude in the range of the mag- tectonic structures. However, there is no definite evidence nitudes estimated for the 1829 earthquake (Mw between yet about which fault caused the 1829 earthquake. Therefore, 6.0 and 6.9). Nevertheless, the maximum magnitude is at- further studies (e.g. swath-microbathymetry, sub-bottom pro- tained or exceeded in the BSFZ when the rupture involves filing or more SCS sparker profiles offshore) are warranted to the Guardamar and offshore segments (Mw 6.9) or the entire improve our understanding of the recent Quaternary activity fault (Mw 7.1). Further studies are warranted to provide more of both faults and of the seismic hazard in the northern ter- insight into the recent Quaternary activity of both faults. mination of the EBSZ.

Acknowledgements. The authors are indebted to Domenico Ri- 6 Conclusions dente, Juan Jose´ Mart´ınez D´ıaz and an anonymous reviewer whose helpful comments and suggestions have considerably improved The analysis of the SCS sparker profiles obtained in the off- the manuscript. Thanks are due to Daniela Pantosti for editing this manuscript. The authors acknowledge the support of the Spanish shore Bajo Segura basin allowed us to identify six seismo- Ministry of Science and Innovation through National Projects stratigraphic units (I1 to I6) limited by five horizons (H1 to IMPULS (REN2003-05996MAR), EVENT (CGL 2006-12861- H5). These horizons can correspond to regional erosional C02-02) and SHAKE (CGL 2011-30005-C02-02); Acciones surfaces produced during the Quaternary global sea level Complementarias EVENT-SHELF (CTM 2008-03346-E/MAR) lowstands and then can be correlated with marine isotopic and SPARKER (CTM 2008-03208-E/MAR); and ESF TopoEurope stages (MIS). We propose that (a) H1 may be correlated to TOPOMED project (CGL 2008-03474-E/BTE). We are grateful to MIS2 (20 ka), (b) H2 to MIS6 (135 ka), (c) H3 to MIS8.2 the captain, crew and science party onboard the RV Garcia del Cid. (247.6 ka), (d) H4 to MIS10 (341 ka), and (e) H5 to MIS 12 Hector Perea was a researcher at IDL-UL Associated Laboratory (434 ka). under contract no. 3/2010/LAB IDL co-financed by the Portuguese A number of faults and folds affecting the Quaternary sed- FCT and FEDER and at UTM-CSIC under the Juan de la Cierva iments were mapped for the first time in the Bajo Segura off- contract no. JCI-2010-07502 funded by the Spanish Ministry of Science and Innovation. shore basin. Most of these structures trend WSW–ENE to W–E and show a compressive component in their kinemat- Edited by: D. Pantosti ics, which is in agreement with the present regional stress Reviewed by: J. Martinez-Diaz, D. Ridente, and one anonymous field. The main faults described onshore were correlated with referee those identified offshore. Six deformation zones bounded by regional synclines were proposed in line with the main onshore structures: (a) Santa Pola zone; (b) La Marina zone; References (c) Guardamar zone; (d) Cabo Cervera zone; (e) Torrevieja zone; and (f) San Miguel de Salinas zone. Albini, P. and Rodr´ıguez de la Torre, F.: The 1828–1829 earthquake Integrated analysis of the commercial MCS and SCS sequence in the provinces of Alicante and Murcia (S-E Spain): sparker profiles shows that (a) the main faults (e.g. BSFZ, Historical sources and macroseismic intensity assessment, in: The use of historical data in natural hazard assessments, edited TF and SMSF) have high dip angles and are deeply rooted in by: Glade, T., Albini, P., and Frances, F., 35–53, 2001. the crust; (b) the thrust kinematics predominates in the BSFZ Alfaro, P.: Neotectonica´ en la Cuenca del Bajo Segura (Cordillera and to the north, and is underlined by the formation of anti- Betica´ Oriental), PhD Thesis, Universidad de Alicante, Alicante, clines; and (c) the strike-slip kinematics prevails in the TF 217 pp., 1995. and SMSF, despite the fact that these faults formed the Cabo Alfaro, P., Estevez,´ A., Moretti, M., and Soria, J. M.: Structures Cervera and Punta Prima anticlines, respectively. sedimentaires´ de deformation´ interpret´ ees´ comme seismites´ dans

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