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Journal of Volcanology and Geothermal Research 182 (2009) 23–33
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j v o l g e o r e s
Characterising unrest during the reawakening of the central volcanic complex on
Tenerife, Canary Islands, 2004–2005, and implications for assessing hazards and
risk mitigation
J. Martí a,⁎, R. Ortiz b, J. Gottsmann c, A. Garcia b, S. De La Cruz-Reyna d
a
Institute of Earth Sciences “Jaume Almera”, CSIC, Lluís Solé Sabarís s/n, Barcelona 08028, Spain
Department of Volcanology, Museo Nacional de Ciencias Naturales, CSIC, C/ José Gutiérrez Abascal, 2, 28006 Madrid, Spain
c
Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, United Kingdom
d
Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, México 04510 D.F., México
b
a r t i c l e
i n f o
Article history:
Received 13 June 2007
Accepted 30 January 2009
Available online 15 February 2009
Keywords:
Tenerife
central complex
unrest
volcano monitoring
hazard assessment
a b s t r a c t
Increased onshore seismic activity in April 2004 marked the first documented renewal of tectonic unrest on
Tenerife, Canary Islands, Spain, since the island's last volcanic eruption in 1909. Events included tremors, felt
earthquakes and the occasional emission of a visible gas plume from the central 3718 m high Teide volcano,
and an increased diffuse emission of CO2. Here, we evaluate results from seismic and microgravimetric
observations in addition to other available data obtained between April 2004 and July 2005, in order to shed
light on the source of these events. We discuss the information to assess whether collectively the phenomena
qualify to be termed “volcanic unrest”, and the socio-economic implications of the phenomena, and critically
examine the ensuing scientific response. We also evaluate the potential volcanic-eruption precursory
character of the data. Suggestions for the establishment of improved volcano monitoring programmes, early
warning systems and civil response protocols for volcanic crises on Tenerife are proposed.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Episodes of unrest are inherent components of the lifecycle of a
volcano. In a number of recent cases including Soufrière Hills (Sparks
and Young, 2002) and Mt. St. Helens (Endo et al., 1981), unrest
preceded eruptions and must hence be seen as an important eruption
precursor (Sandri et al., 2004). However, there are also examples of
unrest waning-off after months or years of restlessness without any
eruptive volcanic activity. Most prominent cases include the volcanic
calderas of Long Valley (Battaglia et al., 2003a,b) and the Campi
Flegrei (Dvorak, and Berrino, 1991).
Volcanic unrest is the manifestation of complex sub-surface
processes leading to detectable signals at the ground surface. Processes
such as magma migration and emplacement, tectonic and hydrothermal
activity can trigger seismicity, ground deformation, thermal variations
and changes in the potential fields around a volcano. Seismicity and
ground deformation may be induced by brittle failure of surrounding
rocks due to the pressure increase accompanying the replenishment of
magma reservoirs and the exsolution of a gas phase, a process regarded
as a key trigger for volcanic eruptions (Murphy et al., 1998).
Alternatively, volume/pressure increases within a sub-surface hydro-
⁎ Corresponding author.
E-mail address: [email protected] (J. Martí).
0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2009.01.028
thermal system may equally produce ground deformation and seismicity (Sturtevant et al., 1996; Bianco et al., 2004; Tikku et al., 2006;
Gottsmann et al., 2007). As a consequence, the beginning of unrest at a
dormant volcano presents investigators with the intrinsic dilemma as to
whether the unrest will culminate in an eruptive phase, hence posing a
direct threat to life and property around a volcano, or whether the
“unusual” behaviour will eventually dissipate, causing little disruption
to communities and hence little socio-economic damage. This uncertainty is even more challenging and extends to the basic question of how
to define unrest when the dormant volcano has not erupted in historical
times or has not previously shown signs of unrest, either witnessed by a
local population or recorded by a monitoring network. This situation
was dramatically illustrated, for example, by the eruptions of El Chichón
(1982) and Popocatépetl, (1994) (México) (De La Cruz-Reyna and
Tilling, 2007), Pinatubo (Philippines) (1991) (Newhall and Punongbayan, 1996), or Soufrière Hills (Montserrat) (1995–present) (Kilburn and
Voight, 1998; Sparks and Young, 2002).
The Spanish volcanological community was confronted with this
dilemma in early 2004, during a period of increased seismic activity
and manifestations of activity that was potentially volcanic began on
the volcanic island after a quiescence of almost 100 years. The lack of
both previous data and a volcano monitoring network, created a
particular situation in which the available information was assessed
and interpreted based on comparison to other volcanic systems, not
necessarily to Tenerife, and on the expertise of the scientists involved
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
in the investigation. However, there was no general consensus on
either the volcanic evolution of Tenerife or on its current state of
activity to use as a basis for judging the future outlook. This gave rise
to a rather confusing situation in which some scientists defined the
anomalous behaviour as volcanic unrest (García et al., 2006;
Gottsmann et al., 2006), while others claimed that there was no
clear indication for unrest (Carracedo and Troll, 2006; Carracedo et al.,
2006). This controversy, which is in part due to the lack of knowledge
on the Tenerife volcanology, was particularly dramatic in the light of
a possible reawakening of the Teide-Pico Viejo volcanic complex, as
there is clear disagreement concerning whether or not this is still
an active volcanic complex (Ablay and Marti, 2000; Carracedo
et al., 2003; García et al., 2006; Carracedo et al., 2007; Marti et al.,
2008).
Unfortunately, the situation experienced in Tenerife is not exclusive to this region, but has occurred previously in other volcanic
areas (e.g., the Guadeloupe crisis in 1976, Tazieff, 1979) and it will
most certainly occur again somewhere else in the future. In an attempt
to address the hazards and risk implications of a poorly known
volcanic system, we analyse the particular case of Tenerife. We present
data collected prior to and (with an emphasis) during the crisis from
early 2004 to late 2005 from geophysical investigations including
seismic, gravimetric and geodetic observations, and discuss whether
or not these data allow to suggest a change in the behaviour of
volcanic system on Tenerife, i.e. the occurrence of volcanic unrest. We
also discuss historic volcanic activity on Tenerife in the light of theses
new investigations and examine whether or not Teide should still be
considered an active volcano. Finally, we discuss implications for risk
mitigation on the island given the different interpretations of the
observed phenomena by various authors with respect to the abnormal
activity on Tenerife.
2. Volcanological background information
The Canary Islands form a volcanic archipelago with a long-standing
history of volcanic activity that began more than 40 million years ago
(Araña and Ortiz, 1991; Anguita and Hernan, 2000). More than a dozen
eruptions have occurred on the islands of Tenerife, Lanzarote, and La
Palma since the 16th century. Tenerife, the largest of the Canary Islands,
has an eruptive history of over 12 million years including a shield
building phase followed by the construction of a central volcanic
structure, the Las Cañadas edifice (Marti et al., 1994) (Fig. 1). The
volcanic evolution of Tenerife comprises both constructive and
destructive phases including vertical and lateral collapses on the order
of several km3 (Marti et al., 1997). At least three vertical collapses
resulted in the formation of the 16 km-wide Las Cañadas caldera, into
which the prominent Teide-Pico Viejo volcanic complex was emplaced
during predominantly effusive and also occasional explosive activity
over the past 170–190 ka (Marti et al., 1994; Marti and Gudmundsson,
2000; Ablay and Marti, 2000). This complex appears to be fed by both
shallow-level (b5 km) phonolitic magma reservoirs and deeper-seated
basaltic magma patches (Ablay et al.,1998; Ablay and Marti, 2000; Martí
et al., 2008). Recent (b0.5 ka) volcanic activity was located on the TeidePico Viejo complex (explosive and effusive phonolitic eruptions) as well
as along a NW–SE and NE–SW oriented extensional structural
lineaments, referred to as the Santiago del Teide Rift and the Dorsal
rift, respectively (dominantly monogenetic mafic eruptions) (Fig. 1).
Historic eruptions from the Teide-Pico Viejo complex and the rifts
Fig. 1. Simplified geological map of Tenerife (after Ablay and Marti, 2000) indicating the location of the IGN seismic station CCAN and the benchmarks used in microgravimetic
surveys. T, Teide; PV, Pico Viejo; MB, Montaña Blanca; RG, Roques de García; G, Guajara; SRZ, Santiago del Teide rift zone; DRZ, Dorsal rift zone; SVZ, Southern volcanic zone. Vents:
black symbols: mafic and intermediate vents; white symbols, felsic vents; stars: historic and sub-historic; circles: other vents; Other symbols: solid square, CCAN; open squares,
benchmarks.
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
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Fig. 2. Seismic network of the Canary Islands. White symbols: seismic stations installed before 2004. Black symbols: seismic stations installed after May 2004. Diamonds: Shortperiod seismic stations from CSIC. Circles: Short-period seismic stations from IGN. Squares: Broad Band seismic stations from IGN.
complex were of basaltic composition and occurred in 1704, 1706, 1798,
and 1909.
3. Background volcanic activity and chronology of events (April
2004–July 2005)
The background level of volcanic activity on Tenerife has been
manifested as weak fumarolic activity, with average temperatures of
around 86 °C on the summit of Teide, as well as gas emissions along a
spur known as the Roques del Garcia, which divides the Las Cañadas
caldera into a western and an eastern sector (Hernández et al., 1998,
2000; Galindo, 2005) (Fig. 1). In addition, there were diffuse gas
emissions (CO2, H) above background level (as defined by average
values elsewhere on the island) around the Teide crater, the caldera
border and the rift zones (Hernández et al., 1998, 2000; Galindo,
2005). Epicentres location of seismic events recorded by the regional
monitoring network (Fig. 2) can be obtained on-line from the official
seismic catalogue of the National Geographic Institute (IGN) at www.
ign.es. This catalogue has been used as the main data source in this
paper. Earthquake epicentres of background seismicity clustered in
two off-shore areas located to the north and to the south of Tenerife
(Figs. 2–5). These (deep) earthquakes were most likely associated
with tectonic movement along crustal heterogeneities. Onshore
seismicity also occurred, but was much less frequent.
“Unusual” manifestations commenced in early 2004, when a series
of earthquakes marked a change from the usual “background” activity
on Tenerife. This phenomena was unusual insofar, as: (i) the epicentres were located mainly onland (Fig. 4), (ii) the number of events
was significantly higher than during background activity (Fig. 5),
(iii) most of the events where located at shallow depth and (iv) four
earthquakes were felt, representing the first account of such phenomena since the last eruption on the island in 1909. During 2004
Fig. 3. Seismicity in and around Canary Island from 1993 to 2005. Data from the public catalogue of the Instituto Geográfico Nacional (IGN, www.ign.es). The activity is concentrated
around Tenerife and after 2004 in the North-West sector of the island.
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
Fig. 4. Evolution of seismicity in and around Tenerife from 1993 to 2005. Data from the public catalogue of the Instituto Geográfico Nacional (IGN, www.ign.es). The high level of
activity appear in the first moths of 2004.
epicentres propagated with time from the northern and northwestern part of the island (Fig. 6) southward, towards the Teie-Pico
Viejo stratovolcanoes into the westernmost depression (Llano de
Ucanca) of the Las Canadas caldera. Inland seismicity continued
throughout 2005, 2006, and 2007 albeit at a lower rate (249 in 2004,
207 in 2005, 138 in 2006 and 141 in 2007) up to the time of this
submission.
Simultaneous with the seismic events, an increase in the flux of
CO2 in diffuse degassing was detected along the Santiago del Teide Rift
(Galindo, 2005). A fumarolic plume at the summit of Teide (3718 m a.
m.s.l) was visible to the naked eye for a few hours on the 20 October
2004, in addition to an increase in the seismic noise (tremor) at Teide
(García et al., 2006). On the 5 December 2004, a new fracture (few
tens of meters long) appeared in the Orotava valley, with associated
gas emission. It was preceded nine days before by a significant
increase in low frequency seismic energy events and associated with
some seismic event in the area (1, 8, and 9 December, 2004) (García
et al., 2006).
4. Scientific responses to unrest and data analysis
Seismic monitoring has been performed on Tenerife for ca. 20 years
by the National Geographical Institute of Spain (IGN), with a 7 station
seismic network in the Canary Islands originally designed to monitor
regional tectonic seismicity, with similar characteristics to those of the
National Seismic Network deployed in the mainland. At present, the
increment in the monitoring tasks conducted by several national and
local institutions (IGN, CSIC, UCA, ITER) have resulted in a significant
increase of the number of seismic stations on Tenerife (Fig. 2), as well
as the installation of permanent GPS and magnetic networks. Gas
monitoring has been conducted since 1997 by the Tenerife-based
Institute of Technology and Renewable Energies (ITER) (www.iter.es).
With the advent of unusual seismic activity, a scientific discussion
ensued as to whether Tenerife is preparing for a renewal of eruptive
volcanic activity. This was mirrored by a public and political debate as to
the associated risks and the consequences for the socio-economic
stability of the region. The lack of an integrated monitoring database
Fig. 5. Number of seismic events recorded from 1999 and 2005 in and around Tenerife at the CCAN (IGN) reference seismic station, unchanged from 1992. The figure shows the high
increment of activity in April–May 2004.
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Fig. 6. Located seismic events during 2004 in the northwest sector of Tenerife (IGN 2004 catalogue). The epicentres show a moderate migration from north to south during this
period.
for Tenerife had important consequences for regional hazard assessment,
risk mitigation, scenario planning and the development of response
protocols:
i.) The assessment of the status quo of the volcanic system was
difficult as crucial background data was either missing or not
available.
ii.) Changes in geodynamic processes were hence difficult to
interpret in terms of the degree of deviation from background
behaviour.
iii.) The reliability of existing data, though of crucial importance,
was difficult to assess.
An additional complexity in the scientific response to the emerging
unrest was the existence of three different levels of political
administration (federal, regional, local) with different responsibilities
for civil protection and consequently differences in the response to a
potential volcanic crisis, and an unclear line of command.
An initial response to the developments in early 2004 was the
installation of a joint deformation and micro-gravity network (Gottsmann
et al., 2006), the deployment of additional seismic stations by CSIC and IGN
and an increase in the frequency of gas sampling by ITER. Other research
projects carried out by the CSIC and supported by Spanish Ministry of
Education and Science, started one year later (2005) in order to investigate
the origin and temporal evolution of the “anomalous” situation. The
Fig. 7. Characteristic waveform and its spectrogram of the tremor recorded on Tenerife: examples from 23 May 2004. Note the two frequencies of the tremor. Tremor was recorded at,
CSIC short period seismic station, BODE, placed at the northern side of Tenerife in the Icod Valley.
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
Fig. 8. During 2004 some seismic events preceded tremor by some minutes. The plot shows a one hour waveform and its spectrogram on 7th of October 2004. Arrow S indicates the
occurrence of the seismic event and arrow T the tremor.
motivation to respond on a scientific level originated from the indication
that epicentre locations appeared to migrate from the northern coastline
towards the central Teide-Pico Viejo complex. Combining these observations with results from petrological investigations (Ablay et al., 1998;
Triebold et al., 2006; Andujar, 2007), which indicate the existence of
shallow phonolitic magma reservoirs beneath the complex, evoked the
possibility that migrating magma or hot fluids might disrupt the thermodynamic balance of these reservoirs. Early warning of mass and density
changes at depth coupled with seismic information on source location and
evolution were therefore seen as essential means to assess the potential
for hazards associated with the unusual situation on the island.
4.1. Seismicity
Prior to the increase in seismic activity in 2004, most of the
seismicity recorded on and around Tenerife corresponded to relatively
deep (several tens of km depth) seismicity generally concentrated
offshore, both to the north and to the south of the island. This pattern
of seimicity has been traditionally attributed to a regional tectonic
origin (Mezcua et al., 1992). Activity has been continuously monitored
on Tenerife since 1992 with an IGN permanent seismic station, CCAN,
an analogue short period seismometer with UHF telemetry located in
the Las Cañadas caldera. Another two short period seismic stations on
Tenerife, located in the Güimar Valley (S) and Anaga (NE) together
with CCAN and few other stations on the remaining islands comprised
the regional IGN seismic network.
In 2000, IGN replaced some of their seismic stations with broad
band instruments while at the same time expanding the existing
network from 7 seismometers to a total of 12. By 2004 the IGN
operated a total of three broad-band and five new short period seismic
stations. Before, during and after network amendments, station CCAN
acted as the control reference (Fig. 2). As a result of the denser network
Fig. 9. Example of a seismogram from CCAN showing the three components recorded on the 29th of August 2004. The amplitude of P and S waves indicates that the incidence is
nearly horizontal, i.e. the waves come from shallow depths. This fact could be also results from a particular relations between the seismic station and the focal mechanism of the
seismic event. Anyway, the time difference between the arrivals of P and S waves is of the order of 1 s (shown by the double arrow). This suggest a depth of less than 7 km even in the
case that the seismic event was located exactly below the station.
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
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Fig. 10. Time interval histograms showing the different statistical behaviour of two populations of earthquakes: tectonic occurring offshore and those mainly concentrating onland.
Two radii of 25 km and 75 km, respectively, have been fixed arbitrarily around Teide, although the results do not show significant variations between 20 and 30 km for the inner limit
and between 60 and 80 km for the outer. 60% of the seismic events occurring onland are clustered in the following 24 h, while the earthquakes from offshore occur isolated except for
the appearance of pairs (two seismic events in the same zone within a few hours of each other), which contribute to the value of 25% in the next 24 h.
with higher precision instruments, an increase in seismicity was
detected along the entire Canarian Archipelago and also on and around
Tenerife. There, in addition to the previously known seismo-tectonic
zone located offshore to the southeast of the island, a previously
undetected seismo-tectonic zone (offshore) to the north was revealed
(Fig. 4). In addition, a lineament of epicentre locations oriented NW–SE
was clearly detectable, joining the two offshore zones by crossing the
La Orotava and Güimar valleys on the island (Fig. 4).
Since April 2004, several seismic swarms occurred (including four
felt earthquakes with magnitudes of 2.5 to 3.5 Mw) and continued to
Fig. 11. Gutenberg-Richter b values of the seismic events recorded on Tenerife for the period 2004–2005. Dots indicate earthquakes located at less than 25 km, diamonds represent
earthquakes located between 25 and 75 km. The histograms showing the magnitude distribution of earthquakes located at less than 25 km (left) and between 25 and 75 km (right),
respectively, are used to define the minimum magnitude in the compute the b value. The b-histograms show the bimodal character of the distribution of the seismic event near Teide
volcano, and unimodal for the seismicity around the island.
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
occur at the time of writing, though with less intensity. This increase
in the number of seismic events is evident if we only consider the
records from the CCAN station, which has remained invariable since
1992 (Fig. 5). This activity included the well-known deep offshore
tectonic seismicity as well as previously known seismic sources whose
epicentres concentrated mostly onland. These new events included
volcanotectonic and purely volcanic signals such as harmonic tremor
and few long period events (Tárraga et al., 2006; Almendros et al.,
2007; Tárraga, 2007) (Figs. 7 and 8). Compared to the tectonic seismicity, this new volcano-tectonic and volcanic activity was characterised by shallow-seated (few km depth) hypocentres, as shown in
Fig. 9. It is important to note that the new seismicity was initially
located beneath the northern and north-western flanks of the TeidePico Viejo complex along the western portion of the Icod valley but
subsequently migrated towards the interior of the island along the
western sector of Las Cañadas caldera.
It is also worth mentioning that the statistical behaviour of the two
populations of earthquakes (off-shore tectonic as opposed to onshore
volcanic and volcano-tectonic) is markedly different (Fig. 10). There
appears to be a relationship between the offshore and onshore seismicity
in the sense that the tectonic and volcanic seismic events, respectively,
show predictive patterns (Tárraga et al., 2006; Tárraga, 2007).
Finally, many onshore events are followed by periods of tremor
that in some cases exceed one hour in duration. (Carniel et al., 2005;
Tárraga et al., 2006; Vila et al., 2006; Carniel et al., 2008). Detailed
analysis of seismic signals show that N45% of the events recorded
during the period of maximum activity (from July to September of
2004), were related to a tremor-type seismic signal (Tárraga et al.,
2006). Moreover, the study of the system memory from the seismic
signal (Carniel et al., 2008) shows the presence of long duration
tremor periods associated with the occurrence of seismic events. In
general, the low amplitude of this signal makes it difficult to detect
directly from the seismogram (Carniel et al., 2008) Also, the existence
of some LP events has been reported in the area of Pico Viejo
(Almendros et al., 2007).
those from between 25 and 75 km (Fig. 11). The b-values obtained
clearly indicate that by October 2004 seismicity changed from an
earlier, mainly tectonic origin to bi-modal b-value seismicity, largely
influenced by a preponderance of volcano-seismic events (Fig. 11).
4.2. Time-lapse microgravity and ground deformation
As a further response to the unusual activity the first joint ground
deformation/microgravity network was installed on the island in May
2004, approximately two weeks after the start of increased seismicity
(Gottsmann et al., 2006). Ground deformation data allow the quantification of sub-surface volume changes which, combined with subsurface mass variations obtained from micro-gravity data, enables an
assessment to be made of the nature of the source causing the unrest, be
it magma, hydrothermal fluids or a mixture of both. Prior to 2004 a
comprehensive network around the Teide-Pico Viejo complex was
lacking. The new network aimed at providing i) important data on the
sub-surface dynamics and ii) critical baseline data for future
developments.
The network consists of 15 benchmarks, which are positioned to
provide coverage of the central volcanic system, including the TeidePico Viejo complex, the Las Cañadas caldera, as well as the Santiago
Rift (Fig. 12). The first reoccupation of the network was performed in
July 2004, followed by campaigns in April 2005 and July 2005.
Detailed descriptions of the network and resulting data are given in
Gottsmann et al. (2006). Here, we summarise the key findings.
Gravity changes across the area under investigation were smallest
in the central and eastern depression of Las Cañadas caldera, where
cumulative changes over the 14-month period where only slightly
higher than the instrument precision level (±0.015 mGal on average;
1 mGal = 10 µm/s2) average. A marked gravity anomaly with a
maximum gravity increase of around 0.4 mGal was found in the
4.1.1. Temporal variation of the Gutenberg-Richter b parameter
In order to demonstrate the difference between purely tectonic
and volcanic or volcano-tectonic seismicity we calculated the
Gutenberg-Richter b-value from the IGN seismic catalogue considering the data available from 2000. The Gutenberg-Richter law states
that the distribution of seismic events of a certain magnitude occurs in
a particular region during a particular time interval corresponding to:
log ðN Þ = b4M + a
where N is the cumulative number of events exceeding a given
magnitude M, and a, b are constants. The b-value strongly depends on
the properties of the seismic medium and on the nature of the stress
regime in which the earthquakes occur. Tectonic areas where
seismicity is caused by regional stresses usually show b-values near
1.0 (Frohlich and Davis, 1993). Stress concentration leading to
clustering of seismicity and fracture over distances of a few kilometers
seems to have a strong effect on the value of b (Ogata and Katsura,
1993; Wiemer and Mc Nutt, 1997; Wiemer and Wyss, 1997, NoveloCasanovaa et al., 2006).
We have used the maximum likelihood method to calculate the
coefficient b by means of the following equation:
b =
logðeÞ
bM N − Mmin
where bMN is the average magnitude of 20 consecutive events and
Mmin is the minimum magnitude that the monitoring network can
detect in the whole area (Fig. 10). We deduce Mmin from the
histograms of events located at less than 25 km from Teide and
Fig. 12. Residual gravity changes between May 2004 and July 2005, corrected for the
effect of ground deformation and water table changes (from Gottsmann et al., 2006).
Residual changes are draped over a DEM of the central volcanic complex (CVC) of
Tenerife. Uncertainty in gravity residuals are on average ± 0.015 mGal
(1 mGal = 10 µm/s2). Stars represent epicentres of seismic events recorded between
May 2004 and July 2005. Both gravity increase and seismicity appear to be spatially and
temporally correlated. Locations of boreholes from which water table data was used for
data reduction are shown as white circles. See Gottsmann et al. (2006) for details.
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
northwestern part of the covered area. With time, this anomaly
appeared to “migrate” in a southward direction, reaching the western
part of Las Cañadas caldera between July 2004 and April 2005
(Fig. 12). The gravity increase along the Santiago del Teide Rift, noted
between the first two campaigns, had disappeared by April 2005. At
the same time, though, gravity, increased significantly along the
northern slopes of Teide, adding to the impression of a spatiotemporal migration of the causative source (Fig. 12). It is thus
pertinent that in this same vicinity on 5 December 2004 a new fissure
with fumarole emission appeared in the Orotava valley (see below). A
gas plume emanating from the summit fumaroles of Teide was
particularly noticeable during October 2004 (García et al., 2006),
between surveys 2 and 3. In summary, significant gravity changes
occurred mainly along the northern flanks of the Teide-Pico Viejo
complex and along a ca. 3 km wide zone along the western side of the
volcanic complex extending into the westernmost sector of the Las
Cañadas caldera between May 2004 and July 2005 (Fig. 12). At the
same time ground deformation over the investigation period
remained largely within measurement errors (2.5 cm vertical). The
significant perturbation of the gravity field was hence not induced by
widespread ground deformation but rather by residual sub-surface
mass/density changes.
4.3. Other observations
In addition to the increase in seismicity and the microgravity
changes described above other signs of anomalous behaviour on
Tenerife were also reported. These include a significant increase in the
diffuse emission of carbon dioxide along the Santiago del Teide Ridge
and around the Las Cañadas caldera (Galindo, 2005; Pérez et al.,
2005), an increase in the CO2 flux of the Teide fumaroles, the
appearance of traces of SO2 in the Teide fumaroles (M. Martini,
personal communication), an increase in the fumarolic emission from
Teide (García et al., 2006), and the opening of a new fissure with gas
emission in La Orotava valley (García et al., 2006).
5. Discussion
The data and information presented in the previous sections are in
our opinion sufficient and reliable enough to regard the unusual
behaviour starting in spring 2004 as unequivocal evidence of volcanic
unrest (see below). However, the existence of unrest has been strictly
ruled out by Carracedo and Troll (2006) and Carracedo et al. (2006),
who suggest that Tenerife has never abandoned its equilibrium state
and argue that the manifestations of geophysical activity can be
attributed to changes in the sensitivity of the monitoring devices and/
or misinterpretation of the observations made. While a diversity of
opinions motivates good scientific debate, in the particular case of
Tenerife (and perhaps in fact in all cases of the reawakening of
volcanoes after significant repose periods) discrepancies in the
interpretation of such signals can have important socio-economic
implications. It is therefore of paramount importance to clarify the
facts on which any scientific interpretation is based.
Regarding whether or not volcanic unrest occurred or is
perhaps still occurring, the data presented in this paper strongly
support the existence of a change from the “normal” background
volcanic activity on Tenerife. In this regard the following facts are
important:
a) The significant increase in seismicity as well as the location of a
considerable number of earthquakes onshore with hypocentres at
shallow depths, contrasts with the previous seismicity pattern
characterised by deep-seated earthquakes mostly located offshore.
b) The presence of volcanic tremor and long period signals suggest
that, at least, part of the new seismicity is non-tectonic in origin.
This is also confirmed by the Gutenberg-Richter b parameter that
c)
d)
e)
f)
g)
31
clearly indicates events related to volcanic processes became a
major feature of seismicity from October 2004 onwards.
Thus, starting from early 2005 onwards, onshore seismicity
appears to correspond to a combination of tectonic and volcanic
processes. However, offshore seismicity kept nearly constant with
values of b ≈ 1. The limited detection capability of the seismic
network at these distances and the relatively small number of
events would explain the observed variations.
The pronounced increase in b values for close events (less than
25 km from Teide) occurred at the end of October 2004, coinciding
with the increase of the fumarolic activity observed on 20 October.
The fact that some of the tremor episodes can be predicted by the
analysis of purely tectonic signals suggests that there is some
coupling of the Tenerife volcanic or hydrothermal system to the
regional tectonic activity.
The evaluation of all seismic data presented here is exclusively
based on data recorded by seismic station (CCAN) that has been
operating for more than 20 years. Therefore, the differences
observed between the seismicity in the interval considered in our
study and the previous background period, including a significant
increase in the number of the total events recorded, cannot be
attributed to an increased sensitivity of the seismic network since
2000.
Then, there is the perturbation of the gravity field, which occurred
simultaneously with the elevation in seismicity. Both spatial and
temporal variations were detected indicating residual mass
changes beneath the PV–PT complex and extending into the
western sector of the Las Cañadas caldera over the 14 months
observation time. The observed changes are most likely due to
shallow (few kms depth) hydrothermal fluid migrations and not
exclusively due to magma movement (Gottsmann et al., 2006).
Moreover, two other observations made during the studied period
have to be added to the previous list: the increase of fumarolic activity
at Teide's crater on the morning of 20 October 2004, and the opening
of a new fracture with gas emission in the Benijos area on the 5
December 2004.
One of the authors of this paper (JM) witnessed the increased
fumarolic activity at Teide crater from the highest point (Guajara
peak) of the Las Cañadas caldera wall. He observed a pulsating (with a
period of approximately half a minute) emission of a dilute, low
column of steam at the western side of the crater. This column formed
in the absence of any weather cloud or other atmospheric phenomena
and was rapidly dispersed by westerly winds. The phenomenon was
clearly observed from 8.30 h to 10.30 h, local time, when clouds
started to appear and masked the steam emission from the crater.
These characteristics do not accord with any meteorological phenomenon usually observed on Teide (e.g., el sombrerito del Teide). Also,
the atmospheric soundings made by the Tenerife meteorological
station during October 2004 do not show any significant variation in
the atmospheric conditions that could justify any anomalous
meteorological events on top of Teide that day. Moreover, the
characteristics of the seismic tremor changed in the period during
which the fumarolic event was observed (Fig. 13). We therefore
propose that this episode of increased fumarolic activity was
associated with the disturbance of the background activity of the
Teide-Pico Viejo complex.
Anomalous gas emission was detected along a new fracture that
opened in the area of Benijos in the Orotava valley on the 5 December
2004. Carracedo and Troll (2006) proposed that this gas emission was
the residual vapour emissions of a cheese factory located in the
vicinity, while ITER reported the presence of a magmatic component
in the Benijos gas emission (as published in the local magazine “El
Baleo”, Feb. 2005, 25, 9–11). The Benijos' fracture is superimposed on
the tectonic lineament defined by epicentre locations between the
Guimar and Orotava valleys, which suggests a tectonic control for the
Author's personal copy
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
fracture location. In this context it is perhaps important to note that
the gas emission appeared between two earthquakes occurring on 29
November at La Guancha and 9 December, northwest of Puerto de la
Cruz. These locations mark the extreme ends of the La Orotava fracture
system. The tremor signal started to increase in intensity on 29
November and disappeared on the day of the Benijos gas emission
(García et al., 2006).
In summary, we propose that there is enough evidence to support
the argument for an episode of volcanic unrest in Tenerife starting in
spring 2004. Whether this unrest is precursory to an eruptive process
cannot be unambigiously answered here. Deep intrusion of magma
into the Santiago del Teide rift and a subsequent migration of
hydrothermal fluids have been proposed to explain the main changes
observed with different monitoring methods (Gottsmann et al., 2006;
Almendros et al., 2007). The probability for an individual intrusion of
magma to reach the surface and to cause an eruption is generally very
low (Gudmundsson et al., 1999), and as such unrest episodes cannot,
on their own, be regarded as unequivocal evidence for an impending
eruption. However, unrest should remind people that the region is
volcanically active and that measures to mitigate risks have to be
considered. Ignoring unrest as a possible sign of incipient active
volcanism leads to the underestimation of risk from volcanic activity.
In fact, simply dismissing such signs inevitably increases the risk to life
by curtailing preparedness and reduces recognition of threat to
property, giving rise to a false perception of safety in the region.
For the particular case of Tenerife there is the added complexity of
the volcanic systems compared to other active volcanic areas where
information on background volcanic activity is more abundant and
enables a better identification of unrest phenomena. In addition to a
complete lack of monitoring data from previous unrest and/or
eruptive episodes, there is still little knowledge on the properties
and dynamics of Tenerife's active volcanic system. There is certainly
the possibility for a new eruption along the rift zones or the central
volcanic complex. Eruptions over the last 5000 years (Carracedo et al.,
2003, 2007; Marti et al., 2008) have occurred from both settings, with
a higher frequency in the former. Basaltic eruptions through the active
rift zones have occurred in historical times with an average of about
Fig. 13. Variations of parameters from Teide Information Seismic System before and
after the appearance of fumarolic emissions at the summit crater of Teide on 20 October
2004. Each plot corresponds to a daily lower envelope of all 60-minute Power Spectral
Density curves from the 16 to the 22 October 2004. The fumarole emission was
preceeded by a significant increase in seismic energy for spectral components above
1 Hz and followed by a predominant frequency of 2.4 Hz, with a return to normal levels
within a few hours after the emission.
one eruption per 100 years. No historical records exist on eruptions
from the central phonolitic system, but the available geochronological
data (Carracedo et al., 2003, 2007) suggest inter-eruption intervals for
the Teide-Pico Viejo system of the order of hundreds of years to one to
two thousand years, having occurred the last eruption about
1000 years ago (Carracedo et al., 2003, 2007).
An important yet perhaps neglected topic relates to coupling
between deep basaltic injections and shallow phonolitic reservoirs
(Marti et al., 2008). Although not yet fully investigated and
understood, the existing petrological data indicate that the mixing
of the two magmas is a consistent signature of the Teide system
(Ablay et al., 1998; Ablay and Marti, 2000; Triebold et al., 2006;
Andujar, 2007). We have to conclude that basaltic magma migration
and injection can be regarded as a main trigger of phonolitic
volcanism on Tenerife. As such, unrest episodes triggered by the
deep intrusion and ascent of basaltic magmas should be assessed in
the context of the possible (re)-activation of the phonolitic TeidePico Viejo magmatic system. Therefore, care is required in identifying and interpreting unrest signals at a reawakening volcano, such
as the Teide-Pico Viejo complex, where reliable information from
previous unrest and eruption episodes is lacking and knowledge
on the evolution and dynamics of its magmatic system is poorly
constrained.
6. Conclusions
A comprehensive federal volcano monitoring programme, comparable to those of other European nations with active volcanic areas such
as Portugal, Italy or France, is absent in the Canary Islands including
Tenerife.
The societal impact of a renewal of volcanic activity on Tenerife
may be severe. Tenerife has a population of ca. 1 million, increasing
significantly during the summer months due to tourism, which
represents the major economic asset on the island. Population and
tourist centres are predominantly spread along the southern and
northern coastlines, but also around the southern and western flanks
of the island. The island also hosts two major airports representing
vital lifelines for prosperity on the island. Signs of volcanic unrest or
for the renewal of eruptive activity on Tenerife need to be assessed in
the light of their potential effects on socio-economic structures in the
area. A centrally administered database including complementary
geophysical, geological, geodetic, and geochemical investigations,
which could contribute to a comprehensive assessment of the current
state of the volcanic system, is absent. Such a data pool administered,
for example by a governmental body, be it on a regional or federal
level, should be a prerequisite for effective volcano monitoring,
reliable hazard assessment and risk mitigation purposes.
Two important lessons that have been learned during the recent
episode of unrest on Tenerife should be considered:
1) Tenerife, as with the rest of the Canary Islands, is an active volcanic
area and as such its volcanism requires scientific, public and
governmental attention.
2) The effective reduction of volcanic risk requires i) the development
of mitigation programmes including a significant improvement of
the knowledge on the volcanological and geological evolution of the
island, ii) a designated volcano monitoring programme capable of
detect changes in the activity of the volcanic system, iii) public
education programmes on the issue of living in an active volcanic
environment, iv) a territorial infrastructure planning programme
based on the knowledge and assessment of present volcanic risk, and
v) effective emergency and crisis management plans. The role of the
scientific community is crucial in helping all members of the society
to appreciate the strengths and weaknesses of scientific insights, as
well as the opportunities and threats of living in an active volcanic
area. In giving scientific advice it is therefore advisable to critically
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J. Martí et al. / Journal of Volcanology and Geothermal Research 182 (2009) 23–33
and objectively review all available data, and not just present or
discuss selective data taken out of the relevant context.
Acknowledgments
This research has been funded by the EC EXPLORIS (EVR1-200100047) and MEC TEGETEIDE (CGL2004-21643-E) projects. JM is grateful
for the MEC grant PR-2006-0499. JG acknowledges support from an MEC
“Ramon y Cajal” grant, a Royal Society University Research Fellowship
and a Royal Society International Joint Project Grant. We thank the
Instituto Geográfico Nacional (IGN) for allowing us to use their data
from the seismic catalogue and in particular Carmen López and María
José Blanco (IGN) for their helpful discussions on the seismic activity in
Tenerife. We thank Willy Aspinall and an anonymous reviewer for their
helpful and constructive reviews.
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