Paleoearthquake evidence in Tenerife (Canary Islands) and possible seismotectonic sources

Paleoearthquake evidence in Tenerife (Canary Islands) and possible seismotectonic sources l 1 2 1 3 L.I. Gonzalez de Vallejo , R. Capote , L. Cabrera ...
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Paleoearthquake evidence in Tenerife (Canary Islands) and possible seismotectonic sources l 1 2 1 3 L.I. Gonzalez de Vallejo , R. Capote , L. Cabrera , I.M. Insua and I. Acosta 1 Dpto. de Geodinamica, Universidad Complutense, 28040 Madrid, Spain. ([email protected]) 2 Laboratorio COAC, 38509 Guimar, Tenerife, Spain

3Instituto Espafwl de Oceanografla, Cl Coraz6n de Mada 8, 28002 Madrid, Spain

Key words: Canary Islands, Paleoliquefaction, Paleosismicity, Seismites, Seismotectonic, Tenerife

Abstract A series of clastic dikes and tubular vents were identified in soutbern Tenerife (Canary Islands). These fea­ tures are the result of seismic liquefaction of a Holocene sand deposit, as the consequence of a high intensity paleoeartbquake. The peak ground acceleration (pga) and magnitude of tbe paleoeartbquake generating tbese lique­ faction features were estimated by back calculation analysis. A representative value of 0.30 ± 0.05 g was obtained for tbe pga. From tbis, an eartbquake intensity of IX was estimated for tbe liquefaction site. Magnitude bound methods and energy based approaches were used to determine the magnitude of the paleoearthquake, providing a moment magnitude M 6.8. The zone in which the liquefaction structures are found has undergone tectonic uplift and is affected by two faults. One of tbese faults was responsible for displacing Holocene materials. Dating of tbe uplifted sand formation indicates an age of 10,081 ± 933 years, tbe liquefaction features ranging from tbis age to 3490 ± 473 years BP. This paleoeartbquake was of much greater magnitude tban tbose known historically. Faults with neotectonic activity are significant features that should be borne in mind when assessing the seismic hazards of the Canary Islands, presently considered as low and mainly of volcanic origin. =

Introduction and regional seismicity Several structures attributed to liquefaction phenom­ ena of seismic origin have been identified in exposed sand deposits near El Medano, on tbe soutb coast of Tenerife, Canary Islands (Figure 1). These findings prompted subsequent tectonic investigations including the geoteclmical characterization of soils, geochrono­ logical analysis, and tbe analysis of geophysical, seis­ micity, and neotectonic data which we report here. In these investigations, we were able to characterize a Holocene sand formation and analyze the liquefac­ tion structures. Possible formation mechanisms and the origin and age of these structures were evaluated. In tbe same area, we identified two faults tbat af­ fected the Holocene deposits. Estimates were made of the acceleration and magnitude of the paleoearthquake that produced these structures, and possible seismic sources were characterized

Based on eartbquake information, tbe Canary Is­ lands have been generally tbought to experience low to very low seismicity, with earthquakes always as­ sociated with volcanic activity. During the historical period (Figure 2A), which starts in tbe XN cen­ tury with the first references to volcanic eruptions, the most intense earthquakes on the archipelago took place in Yaiza (Lanzarote) in 1730 (intensity X), in Fuencaliente and Cumbrevieja (La Palma) in 1677 and 1920, respectively (botb VII), in Ingenio (Gran Ca­ naria) in 1913 (VII), and in Fuerteventura in 1915 and 1917 (botb VII). On tbe island of Tenerife, tbe maxi­ mum intensity recorded was VI for the earthquakes on 1910/03/15 in Ieod, 1909/01/4 in Puerto de la Cruz, 1909/09123 in La Orotava, 1909/11121 in Vilaflor and 1937/06121 in Garachico. In tbis historic record, six earthquakes of intensity VI were registered on Tener­ ife, all in tbe 20th century and mostly affecting tbe nortb side of tbe island or its capital city Santa Cruz.

Canary Islands """-I La Palma

,

list e

Gran C

La Gomera

Fuerteventura.J -'

ana

Figure 1. General location of the study area.

1992) has provided some

This record only reflects earthquakes felt in the most

earthquake (Mezcua et aI.,

highly populated areas or those associated with vol­

ideas regarding the seismotectonic setting of the Ca­

canic eruptions. Knowledge of events occurring on

naries, which could help explain the paleoearthquake

the islands of El Hierro, La Gomera or

the south of

Tenerife is practically non-existent.

1958 that a seismological station the Canaries. Two further stations were built in 1975, and over the past few years a more It was not until

was installed in

extensive network is being set up, with plans for sta­ tions over all the islands (there were seven stations in

2002). This will allow greater precision in locating and characterizing earthquakes. The distribution of epicen­ ters recorded over the instrumented period is shown in Figure

2B.

The largest instrumented earthquake had a mo­ ment magnitude M

=

5.2, its epicenter being in the

that caused

the paleoliquefaction discussed here. The

distribution of aftershocks recorded by a terrporary station set up on the south coast of Tenerife between

9 and June 17, 1989 indicates concentrated af­ 80 km long band aligned N33° (Figure 2C). This earthquake corresponds to a fault of around 30 km length. The hypocenter depth of the 5.2 magnitude event has been calculated as 50 km by Mezcua et al. (1992) and as 15 km by Dziewonski et al. (1 m). The analysis of the focal rnxhanism and that of the aftershocks points to a NNE-SSW alignment and ioclination close to the vertical for the fault. This May

tershocks along an

sea between the islands of Tenerife and Gran Canaria

fault could also have been responsible for seismicity

(27°56.8' N and 16°12.a' W). Its maxiIlllm inten­

of greater magnitude and not related to processes of

sity was even felt on Tenerife. An analysis of this

volcanic activity. Further more, geophysical marine

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15;'0'1

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P,fara 1917 VII

VI

p.1"ara 1915 VII

In enio 1913 VII

LaGO!t

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,

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.

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28'N • •

1975

=

El

., .

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aooSite j� ,

I

''''''K''": oIth11Main Shock

L _ Km _SO/ _ - - ---= ,---� -11'W l S"W

Granf)inaria

2002 between Thnerife and Gran Canaria. mechanism: A 33°_71° SE;B 298°_77° NE. to



,..,

Figure 2. Seismicity of the Canary Islands. A: Historical seismicity until from

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. " '1': .)... 7r. ',:....�

.

, ...

'1'W

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jt'li"

< 15 1 5-J� ·J.H� • )J,� ·



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Gran Gamma

1975 f eT earthquakes of intensity I:o: VI. B Earthquake epicenters 1989 earthquake and its aftershocks. Nodal planes from focal

C: Epicenters of the

=

investigations have revealed the occurrence of sig­ nificant tectonic events (Llanes et aI., this volume) associated with epicenters in the sea. There is an obvious need for investigations that fo­ cus on paleoseismicity and neotectonics in regions for which earthquake information is scarce. This is defi­ nitely the case for the Canary Islands, whose instru­ mental period is shorter than 30 years and historical record is iocomplete.

Geology of the Study Area The area iINestigated is found in El Medano close to Leocadio Machado Beach (Figure 3). This beach is bounded inshore by a 40--50 meter wide range of coastal dunes orientated in a NE--S W direction. Some

small lagoons have forrnxl between the dunes and a coastal platform. This platform overlies a formation of vokanic tuffs of acid corq:lOsition, and descends from the volcanic central part of the island. Towards the SW, a minor volcanic strlK:ture, the Montaf1a Roja, is corq:lOsed of pyroclastic basaltic materials that over­ lie the tuff formation. These materials are overlain by a foonation corqxising beach sands that rises 2 to 15 m above sea level and shows several liquefaction strlK:tures. The tuffs correspond to a set of pyroclastic units related to a phase of explosive salic eruptions be­ tween 0.7 and 0.13 Ma. The material is corrposed of pumice lapilli, lithic fragments and sanidine crys­ tals. The Montaf1a Roja volcano lies at the southern margin of the study area (Figure 3). The vokano is a breached cone, open towards the ESE. Its altitude

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