Since 2004 a research project has been developed to
monitor subsurface deformation of Italian volcanoes using borehole
strainmeters and long-baseline tiltmeters. Six Sacks-Evertson dilatometers
were installed around Campi Flegrei caldera and Vesuvius during 2004–2005
(Scarpa et al., 2007), and in 2008 these instruments were supplemented by
two arrays of 28–280 m long water-tube tiltmeters in underground tunnels.
Relevant strainmeter and tiltmeter data have been collected and analysed
from the instruments installed near Campi Flegrei caldera during the recent
unrest episodes. In the period 2004–2005 strain, tilt and GPS data from
Campi Flegrei indicate the onset of surface deformation that accompanied a
low rate of vertical displacement that continued to 2006, corresponding to
an increase of CO2 emission. This strain episode preceded caldera
microseismic activity by a few months, as was observed also during a
significant inflation episode in 1982. Other transient strain episodes
occurred in October 2006, which were accompanied by a swarm of VT
(Volcano-Tectonic) and LP (Long Period) events, in 2009, at the time of
renewed gas emission activity at Solfatara, and again in March 2010, several
minutes before a seismic swarm. The time scale of these transient strain
events ranges from some hours to several days, putting tight constraints on
the origin of ground uplifts at Campi Flegrei. Their location is compatible
with a source inferred from long term deformation signals, located about 4 km beneath Pozzuoli. A proposed mechanism for these aseismic strain episodes
is that they are associated with magma growth in reservoirs with occasional
pressure relief associated with the leakage of gas.
Introduction
Campi Flegrei is an active caldera, with a diameter exceeding 10 km, located
close to the city of Naples in southern Italy. The northern and western
parts of the caldera are located inland and are characterized by the
presence of numerous volcanic cones and craters, whereas its southern part
is submarine and lies beneath the Gulf of Pozzuoli. The region is famous for
the slow subsidence of Roman columns of the Serapeo market in Pozzuoli until
their emergence prior to the volcanic eruption in 1538 that formed Monte
Nuovo, a small cinder cone, characterized by a volcanic explosivity index VEI = 3. The 1538
eruption was preceded several decades before its occurrence by ground uplift
and seismicity that peaked a few days before the eruptive event. After this
eruption, the Pozzuoli region continued to subside at a rate of 14–15 mm yr-1 until 1950. Approximately 74 cm of uplift occurred between 1950 and
1952 (Del Gaudio et al., 2010), but without any report or record of
associated seismic activity. Slow subsidence followed this period of uplift
interrupted in 1970–1972 and in 1982–1984 by rapid (≈1.5 m) inflation
episodes, the first accompanied by moderate low seismicity (Corrado et al.,
1977) with only few events felt by residents, whereas the second was
accompanied by relatively intense swarms of volcano tectonic (VT)
earthquakes (Barberi et al., 1986). This seismic activity
caused alarm in the population and a spontaneous partial evacuation of the
city of Pozzuoli (44 000 residents). Since this last episode, subsidence has
been recorded for several years, interrupted by some small mini-uplift
episodes, each with a duration of several weeks, all accompanied by seismic
swarms of low magnitude VT events. The last mini-uplift episode had an
unusual duration, starting from 2004 and continuing until 2013, with a total
uplift amounting to about 25 cm, accompanied by swarms of low magnitude VT
and LP (long period) earthquakes (M<2), most noticeably in October
2006. Cumulative uplift since 1950 amounts to ≈2.5 m, similar to
pre-eruptive uplift before 1538: both episodes are characterized by
an average rate of uplift of the order of 4–5 cm yr-1. It is noteworthy that
the strain linked to the recent uplift episodes has accumulated almost
aseismically, as firstly evidenced by Yokoyama (1971), which makes this
caldera quite different from many other volcanic areas or calderas in the
world experiencing unrest episodes.
Borehole strainmeters permit subsurface deformation processes associated
with aseismic events to be quantified. Several processes can be identified,
some of which precede seismic activity: slow earthquakes, episodic tremors
and slip (ETS) and aseismic creep. Strainmeters, like the Sacks-Evertson
dilatometers treated in the current paper, are installed in boreholes at
depth of hundreds of meters below the surface thereby obtaining high signal
to noise ratio and useful measurement sensitivities of 10-10 strain
(Agnew, 1986). Long baseline tiltmeters achieve similar (10-10 radian) measurement accuracy at long periods when installed in the
subsurface and when their lengths exceed tens of meters.
One objective of the strain and tilt monitoring program is devoted to
isolating and identifying the response to subsurface mass movement. The main
aims of these experiments are to discriminate between magma inflow and other
mass transport processes (e.g. hydrothermal activity). In contrast to
seismic records of volcanic tremor, borehole strainmeter data are less
sensitive to propagating waves associated with nucleation and impulsive
fluid mobility, thus providing more direct information on the structure of
the conduit, the physical properties of magma and the dynamics of magma
transport (Scarpa et al., 2000; Scarpa, 2001). The observations performed by
using the few borehole strainmeters and long-baseline tiltmeters operating
near volcanic systems elsewhere in the world (see e.g. Ishihara, 1988;
Linde and Sacks, 1995; Fukao et al., 1998; Mattioli et al., 2004; Voight et
al., 2006; Bilham et al., 1982) have shown clear signals related to
eruptions with significantly higher sensitivity than GPS, and permit the
details of the gas-magma transport mechanisms linked to strain transients
with durations from minutes to weeks to be quantified. Moreover, the
subsurface installation of strainmeters and tiltmeters reduces high
frequency seismic surface noise, mostly due to anthropogenic activities or
weather conditions, which further enhances the signal-to-noise ratio,
helping in the detection of LP, VLP (very-long period) or ULP (ultra-long
period) events, which are currently considered among the most reliable
precursors of volcanic activity (Chouet, 1996).
Overview of ground deformation at Campi Flegrei caldera
Following the deflation after the 1538 Monte Nuovo eruption, the uplift
started in 1950 and reached a maximum of ≈4 m in 1985 near the
town of Pozzuoli (Del Gaudio et al., 2010). Subsequent to this episode, slow
subsidence of the area started, with mini-uplifts, superimposed on the
general trend, occurring in 1989, 1994, 2000 and 2006 (Amoruso et al.,
2007). Since 2005 this subsidence ceased and the region is now uplifting
erratically at an accelerating rate (http://www.ov.ingv.it/ov/campi-flegrei.html, last access: 14 May 2020).
The Solfatara crater shows many active fumaroles and the presence of a
significant geothermal field. Its three main fumaroles (Bocca Grande, Bocca
Nova and Pisciarelli) are subject to diffuse emissions of
volcanic-hydrothermal gases (mainly H and CO2) (Chiodini et al., 2012),
which are routinely analysed for their chemical composition. Increased gas
emission is attributed to the injection of magmatic fluids into the surface
hydrothermal system. Analysis of SBAS-DInSAR data (Manconi et al., 2010)
during the period 1992–2008 showed that the deformation can be related to
the activity in a small, shallow (<3 km) source located beneath the
centre of the caldera. The ground uplift and seismicity episodes occurred
during the 1989–2010 period were correlated (D'Auria et al., 2011) to hot
fluids injections, with a significant magmatic component, in the geothermal
reservoir.
The inflation episodes of 2000 and 2006 have been interpreted as caused by
the volumetric expansion of strain sources at about 5 km below Campi
Flegrei, migrating upward, giving rise to complex spatial and temporal
patterns (D'Auria et al., 2012). Both events have been interpreted as hot
fluid batches injected at the bottom of the geothermal reservoir, which
reached the surface. These injections were found to occur at different
points along the margins of the caldera, in different points, before
migrating to its centre.
Specifically, the 2004–2006 uplift (about 4 cm of total displacement) has
been studied with episodic vertical levelling data and continuous horizontal
GPS data (Troise et al., 2007), with the ratio between maximum horizontal
and maximum vertical displacement equalling that observed during the 2000
uplift, and with spatial similarities to significant vertical deformation
that accompanied the 1982–1984 uplift episode. The use of a layered medium
(Amoruso et al., 2007) permits all the surface deformation data to be
emulated theoretically as inflation of a circular horizontal crack source.
Borehole deformation network at Campi Flegrei
The Campi Flegrei area and Mt. Vesuvius are currently monitored by the
Osservatorio Vesuviano – Istituto Nazionale di Geofisica e Vulcanologia (see
web site http://www.ov.ingv.it/ov/, last access: 14 May 2020). Volcano monitoring has been improved in the last
forty years by distance change measurements (originally EDM surveys and
since 1995 GPS), levelling (referenced to a benchmark located in Naples),
gravity and broad band seismic data acquisition.
In 2004, the DINEV project started in the Campi Flegrei – Vesuvius volcanic
areas (Scarpa et al., 2007). This research program belongs to the Centro
Regionale di Competenza AMRA, supported by PON funds, Regione Campania. The
project was designed to complement the seismic and geodetic monitoring
system of the Osservatorio Vesuviano through the installation of a small
network of seven borehole stations, each installed at a depth up to 200 m and
instrumented with a broad-band 3-component Teledyne Geotech KS2000BH
borehole seismometer and a Sacks-Evertson volumetric strainmeter. At the
time of writing, six borehole Sacks-Evertson strainmeters have been
installed within the DINEV project (three near Mt. Vesuvius, along with the
three dilatometers installed in the proximity of the Campi Flegrei caldera),
and two sites (Quarto and Mt. S. Angelo, a few kilometres east of the
caldera) were instrumented with borehole seismometers. In addition, two
Guralp CMG 3-ESP broad band seismometers were installed at surface sites.
Each borehole station is separated by several kilometres from other
stations, and the network covers many strategic distances from Campi
Flegrei and Vesuvius volcanoes.
Subsequently, starting in 2008, the UNREST project was developed, supported
by a convention between INGV and DPC (Civil Protection Department). This
project aimed to install in the same region two multiple-sensor,
high-sensitivity long-baseline Michelson water pipe tiltmeters.
In the present work, data acquired by three Sacks-Evertson dilatometers and
the two long-baseline tiltmeters installed near Campi Flegrei caldera (Fig. 1) have been used.
Location of the borehole dilatometers (red dots) and long baseline
tiltmeters (red crosses) in the Campi Flegrei region. RT = Rione Toiano,
MR = Monteruscello, QU = Quarto. The submarine inflation source is indicated
with a black star. Blue dots represent the location of Pozzuoli town, as
well as the two interest areas of highest seismic and degassing activity:
Solfatara and Agnano-Pisciarelli fumarole system.
Sacks-Evertson strainmeters
The installation of a borehole strainmeter involves it being lowered into a
borehole that has been cored or rotary drilled slightly larger than the
strainmeter diameter. The strainmeter is then coupled to the rock by
injection of a special expansive grout between the instrument and borehole
wall, used to couple it to the wellbore in slight compression. Installation
sites can be equipped with multiple sensors, and, in this way, it is
possible to monitor multiple parameters at the installation site, which
allows the removal of any spurious, non-strain, signals that are recorded.
In the current work, each installation site is equipped with a barometric
pressure transducer, used in order to remove from the recorded data the
dilatational strain signal arising from an atmospheric-pressure loading.
Most sites are equipped with broadband seismometers (both surface
accelerometers or borehole seismometers), in order to correctly calibrate
the dilatometers to higher frequencies. Calibration in the lower frequency
bands is carried out by using the theoretical tides calculated at the
installation sites. Changes in the aquifer caused by surface rainfall also
influence the contractional strain measured by the dilatometers, and it has
been proved (Segall, 2003) that the contractional strain coincides with an
increase in the fluid pressure. Monitoring the rainfall therefore allows the
removal of a possible spurious signal overlapping the strain measured by the
instruments.
The Sacks-Evertson strainmeters (7 cm diameter, 4 m long cylinders filled
with degassed silicone oil) provide two signal outputs, obtained from two
different hydro-mechanical amplification systems. The high-sensitivity
output integrates the volumetric change in the strained reservoir. The
low-sensitivity one is connected to the strained reservoir only when the
instrument is sensing a rapid and strong strain change, and thus measuring
strain. Usually the low-sensitivity channel measures the pressure in a
closed cell, that is proportional to local temperature. The temperature
resolution is a few microdegrees. Air pressure is also measured at the
surface, in order to correct recorded data for dilatational strain resulting
from atmospheric pressure loading. The nominal resolution of the
Sacks-Evertson strainmeter is about 10-12 strain units, with a nominal
dynamic range of 10-11–10-3. Low-frequency calibration of
installed strainmeters is obtained by comparison with Earth tides (Hart et
al., 1996; Amoruso et al., 2000), while surface waves caused by earthquakes
can be used to calibrate the sensors in higher frequency bands (Currenti et
al., 2017). High- and low-sensitivity strain signals are continuously
recorded, and sampled at 50 Hz by a six channels, 24 bit Kinemetrics
Quanterra Q330 digitiser.
Long-baseline tiltmeters
Tiltmeters monitor ground deformation relative to a gravitational
equipotential surface. The lower half of the 12 cm diameter horizontal pipes
of the Michelson tiltmeters installed near Campi Flegrei caldera are filled
with water and terminated by 20 cm diameter reservoirs, fastened to concrete
floors by concrete-filled steel pillars (Fig. 2). Heated floating sensors
support the core of a linear variable displacement transducer (LVDT) whose
output is recorded to a 16 bit precision once per minute with a resolution
of approximately 0.02 µm. The tilt is given by the ratio of the
difference in height change divided by the total length of the pipes. The
tilt resolution in the longest water-pipes attains 80 picoradians (0.08 nrad), approximately three orders of magnitude more sensitive than the
electrolytic bubble sensors hitherto operating near Pozzuoli. Depending on
the length of each pipe, the range obtainable without mechanical adjustment
is ±9 to ±100µrad, a range that can be extended by a
factor of 7 by mechanical adjustment. Thermal noise in the tunnel
environment increasingly dominates the signal at sensitivities of 1 nrad.
(a) Schematic section through water level sensor. The float is
heated to inhibit condensation. (b) Michelson tiltmeter pipe installation in
sloping NW/SE tunnel. (c) Map view of pipe geometries in the North and South
Pozzuoli tunnels. Filled circles indicate the location of water level
sensors. In both tunnels co-linear tiltmeters were installed to enhance
noise discrimination resulting from sub-micron thermal perturbations to
water level and mounts. The calibration of each of the sensors is of the
order of 1 %, and since the instruments are long compared to the width of
the tunnels, strain-tilt coupling is negligible.
The tiltmeters were installed in early 20th century tunnels that were driven
through a weak volcanic ash and lined with tephra blocks or concrete. The
tunnels are inclined at low gradients to permit drainage, but because the
half-filled water pipes must be installed to within a few mm of horizontal,
the lengths of the tiltmeters in certain azimuths are restricted by these
gradients and by the floor-to-ceiling height of the tunnels. Pipes with
lengths of 28–285 m were installed at various azimuths in the two tunnel
systems. The tunnels are ventilated to the atmosphere and, although they
afford good stability and immunity from surface-induced thermoelastic noise
and precipitation, they are environmentally less stable than a sealed
tunnel. Air currents in the tunnels result in thermal fluctuations of up to
3 ∘C at periods of minutes, especially when atmospheric pressure
fronts pass above the area. In the 285 m long water pipe the fundamental
mode is approximately 8 min and we note that it is rarely stimulated,
damping being close to critical.
Data observations and analysis
The network of borehole strainmeters and long-baseline tiltmeters in the
Pozzuoli region permits us to examine subsurface magma movements and
pressures associated with magma ascent, storage, and the recharge of shallow
magma reservoirs, and with the response of surface faults to these changes.
Strainmeters and tiltmeters are both recognized as the potential best short-
and middle-term instruments for eruption forecasting. They are optimally applied to the measurement of deformation signals with durations of
hours to weeks, providing quantitative constraints on the depth of the
pressure source in case of inflation or deflation, and the possible
detection of ground deformation accompanying small pressure variations due
to increased bubble formation, hydrothermal fluid motions, and/or magmatic
ascent in prior to inflation episodes.
A recent uplift episode was initiated in Campi Flegrei in November 2004,
shortly after the installation of the borehole strainmeters. The uplift
begun relatively slowly, but after two well recognised transient
accelerations (in October 2006 and March 2010, Fig. 3) was recorded by GPS
data at an increasing rate after 2011 (Pingue et al., 2006; Troise et al.,
2007).
Uplift of the vertical component of the GPS station RITE near
Pozzuoli harbour (compiled from Vesuvius Observatory data) showing times of
transient accelerations and reversals in inflation rate discussed in text.
Time period covered by the datasets of the two arrays used in the present
paper is shown in the figure.
The time series of the strainmeter signals, starting from the date of
installation, are shown in Fig. 4: data, acquired at 50 samples s-1, have been
downsampled at 60 s. They show overall stable trends, apart during the first
few months, where they are heavily affected by cement-curing effects. Minor
interruptions to the data occurred in the first months of operations due to
problems in the electronics (battery charger, data logger, etc.), mainly at
the Quarto and Monterusciello sites.
The tiltmeter data were interrupted several times by vandalism resulting in
the loss of data loggers, electrical cables, and consequent loss of power.
These interruptions have resulted in incomplete capture of some of the
transient events by the entire array.
Time series of the raw, unfiltered, 60 s downsampled strain and
temperature signals. Data are represented from installation up to 30 June
2010. The upper panel shows the strain (dimensionless and plotted relative to an arbitrary datum) recorded by the
three instruments, while the lower panel represents the change in
temperature recorded by the low-sensitivity output, expressed in Celsius
degrees.
Due to their intrinsic resolution and stability, both strainmeters and
tiltmeters record Earth tides and ocean loading tides, which can largely be
removed by predictive filtering prior deformation analysis. Moreover, other
spurious signals overlap the trend sought, these can be related to the
atmospheric pressure load on the area, local rainfall or earthquakes. While
higher frequency spurious signals don't modify the overall strain trend,
lower frequency signals (as is the case of the atmospheric pressure loading)
can, so it is necessary to suppress their effects in order to observe the
true strain or tilt deformation. To do so, a Bayesian approach (Hart et al.,
1996; Tamura et al., 1991) has been used in the following of the current paper. Despite this
processing technique, the Quarto strainmeter (QU) shows a residual pressure
effect, possibly due to the non-linear dependence of strain on atmospheric
pressure.
Many of the residual transients are correlated with imperfectly supressed
atmospheric pressure variations. There are, however, some interesting
transients observed occasionally related to other
phenomena. Strain data show typical strain trends characterized by their
duration and/or their occurrence, and noticeable correlate with different
kind of events happening in the Campi Flegrei caldera. Specifically, from
the observations of recorded strain data, we distinguish distinct
deformational patterns related to exogenous or endogenous events in the
region. Exogenous events are typically caused by rainfall episodes occurring
in the area. Conversely, endogenous events originate within the caldera, and
are due to genuine magmatic ascent, hot fluid injections in the geothermal
reservoir and/or temperature variation in the hydrothermal system. As an
example, Fig. 5 depicts the strain data from the QU strainmeter during the
period 17 October–6 November 2006. Raw data have been cleaned by
removing the tidal signal and atmospheric pressure loading, then filtered in
the 0.0005–0.005 Hz frequency band: larger signals are due to rainfall
episodes that occurred in the area, while pre-swarm strain changes can be
distinctly discerned. Although background variations in filtered strain data
remain, a pronounced strain-rate change occurs just before LP swarms are
recorded by nearby seismic stations.
QU strainmeter data recorded during the period 17 October–6 November 2006. Raw data have been corrected by removing earth tides and
the effects of atmospheric pressure variations before applying a band-pass
filter in the 0.0005–0.005 Hz frequency band. The units of strain are dimensionless, relative to an arbitrary datum. (c) shows the overall strain in the period: crosses
and circles show VT and LP events, respectively. (a) and (b) show a
magnified comparison of strain transient recorded at the three sites (RT, MR
and QU sites depicted in black, red and blue, respectively), corresponding
to the two dashed lines depicted in (c).
In the following, we show the characteristic signals for these different
event types, recorded by the strainmeters and long-baseline tiltmeters
network installed near Campi Flegrei caldera.
Rainfall and precipitation events
This class of events recorded by the sensors installed at various radial
distances from the centre of the deformation area, is characterized by
abrupt strain changes in the dataset, not correlated with changes in the
atmospheric pressure ensuing at the time of recording, but occurring from
several minutes up to a few hours after heavy rain in the caldera. In Fig. 6 we show typical 1–2 h duration strain signals caused by rainfall.
Exogenous events, caused by three different rainfall episodes. In
(a) it is depicted the rain in mm/day from 1 January 2006, up to
28 January 2008 recorded by the Agnano rain sensor (data from CAR – Regione
Campania). (b), (c), and (d) represent RT, MR and QU strainmeters data
for the three most significant rainfall events, the first of them is
accompanied by the VT swarm. Strainmeter data for other intense rainfalls
are missing. Strain data are plotted relative to an arbitrary datum, and in (b) and (c)
QU data have been multiplied by a factor 2.
Strain transients caused by magma movements and subsurface
thermoelastic expansion
Endogenous events are characteristic of Campi Flegrei caldera. As shown by
D'Auria et al. (2011), although a quantitative relationship between ground
uplift and seismicity is not easily found, deformation of the caldera is
related to the injection of magmatic fluids in the geothermal reservoir
beneath the Campi Flegrei area. Based on these findings, the LP swarm that
occurred in October 2006, when more than 870 LP events were recorded within
7 d, has been interpreted (Saccorotti et al., 2007)
as evidence of a transient upward migration of fluids, exciting the
resonance of fluid-filled fractures, linked to the fluid transfer from a
deeper to a shallower geothermal reservoir. Continuous GPS data at RITE and
ACAE sites (De Martino et al., 2014), however, show how the deformation
process starts several months before the time that the LP swarm initiates,
as evidenced also by geochemical data (Chiodini et al., 2015).
In addition, the strain data appear to be sensitive to thermoelastic
deformation that is caused by the intense heating processes that affect the
deeper parts of the hydrothermal system, as well as the magmatic gas zone
(Chiodini et al., 2015). The impulsive 10-7 strain changes with
decaying time constants of the order of several weeks recorded during June
2008 by the MR strainmeter (Fig. 7) are attributed to this effect.
Endogenous events recorded by the three strainmeters. (a) shows the strain recorded by the three strainmeters at RT, MR and QU
from January 2006 until March 2010. Strain data are plotted relative to an arbitrary datum. (b) and (c) show a magnified view of
the two most significant endogenous events, occurred during 2006 (b) and
2008 (c).
Increased volcanic tremor activity in 26 October 2006, corresponded to a
transient strain contraction at the RT strainmeter with a duration of
approximately 1 h (Fig. 8).
Strainmeter signal observed at RT. Crosses and circles on the zero
axis show the time of occurrence of VT and LP events recorded near
Solfatara. Tides and atmospheric pressure have been removed from the raw
strainmeter data. An arbitrary datum has been used for strain.
(a) and (b) show strain (RT, continuous line) and southward tilt
(dashed line) preceding and accompanying the Astroni-Pisciarelli seismic
swarm in March 2010. The tilt amplitude unit is µrad and the strain
unit multiplier is 10-8. (b) shows an expanded view of the
deformation, and (c) and (d) shows the tilt vector at one-minute
intervals, compared to the seismic moment release, the bulk of which occurs
15–30 min after the first microearthquake in the swarm. The dashed line
lower right shows the 10–30 min RMS noise-level threshold in the raw
data. The first microearthquake corresponds to the approximate time at which
the tilt signal exceeds the short term noise level, however, the
microseismic energy release starts 15 min after this threshold is
exceeded, suggesting its utility in early warning methods.
One of the most significant transients recorded by the strainmeters and
tiltmeters on 30 March 2010 coincided with a seismic swarm containing about
one hundred volcano-tectonic earthquakes with small magnitudes (Md≤1.2), located close to the Agnano crater (Fig. 1). A small increase in
volumetric strain, as well as in the north component of tilt, occurs during
the day preceding the event, followed by a reversal and accelerated
dilatation and a southward tilt which precedes the swarm by 10–20 min
(Fig. 9). The event, clearly recorded by the whole borehole strainmeters and
long-baseline tiltmeters network, has a tilt “step” approximately equal to
200–400 nanoradians, while the volumetric strain-step is 10-8. The source is compatible with the location of the inflation
and deflation episodes analysed by Amoruso and Crescentini (2011) and
Amoruso et al. (2015). The cumulative magnitude of the swarm was equivalent
to magnitude Mw=1.3 and the epicentral area is the
Astroni-Pisciarelli zone, at a depth of approximately 2 km.
Discussion
We briefly discuss here a possible mechanism to account for observed secular
uplift interrupted by transient subsidence and renewed uplift. We use
insights from the most prominent aseismic episode observed coherently by our
Campi Flegrei long baseline tiltmeter and dilatometer data, which was
followed by a seismic swarm of very small magnitude events, with a duration
of 40 min in March 2010. This episode occurs after a period of inflation
with a duration of a few months, which may be related to an increase in the
pressurized triaxial ellipsoid (Amoruso et al., 2015) located at about 4 km
in depth below Pozzuoli. Inflation accelerates the day before the occurrence
of the deflation. We consider the most likely phenomenon for the deflation
event to be a leak from the deeper (∼ 4 km) source to the
shallower reservoir (∼ 2 km in depth), where magmatic fluids
mix with vaporizing meteoric liquids (Chiodini et al., 2015) (Fig. 10). The
duration of the supposed evacuation event coincides with the 40 min
duration of the observed changes measured by strainmeters and tiltmeters. A
cylindrical conduit with radius ≈10 m, or a 100 m × 5 cm
rectangular cross-section fissure, would permit the transfer of ≈105 m3 of volatile magma/gas mix in 40 min at a flow velocity
of 1 m/s, which we consider reasonable values for conduit flow rates
(Vassalli, 2008; Longo et al., 2012). This volume is comparable to a
Strombolian eruption and it is compatible with the release of gas evaluated
at surface by Chiodini et al. (2012) and modelling of the main source
deflation inferred by Amoruso et al. (2015).
(a) Inferred secular Inflation of deep magma source
leading eventually to fracture of the confining rock. (b) Two
possible mechanisms for deflation (i) leak to shallower chamber via a
hydraulic conduit, or (ii) submarine or subaerial venting of gas. The
transient deflation episode in March 2010 is interpreted as process (i)
(Amoruso et al., 2015).
The leak is interpreted to have been initiated by small fractures preceding
the subsequent seismic swarm. This observation gains importance since
seismic swarms of VT earthquakes have generally been dominated by normal
faulting, occurring at the time of small uplift episodes associated with
magma chamber inflation. Our quantitative model, based on spatially coherent
strain and tilt data, is the first observation reporting short term
anomalous strain associated with a reduction of magma-chamber volume (Amoruso et al.,
2015), preceding seismicity by tens of minutes. The location of these volume
changes is confined to depth of about 4 km beneath Pozzuoli, coincident with
the locus of inflation and deflation episodes estimated by Amoruso and
Crescentini (2011) and Amoruso et al. (2014a, b) on the basis of non-linear
inversion of geodetic data. Calculations made by Amoruso et al. (2015) have
suggested that the Campi Flegrei microseismicity that occurred in March 2010
was induced by a rapid transient deflation on all, or part of, a previously
known, slowly inflating offshore magma source amounting to 4.5×104 m3. The rate of volume decrease during the 40 min deflation event has
been estimated approximately as 105 m3/h, that is a factor of
500 faster than the mean inflation rate before and after the event, thus
suggesting that stressing rates were a relevant factor to the triggering of
seismicity. Although microseismicity occurred ≈20 min after the
starting of the deflation, the mechanism for the delay is uncertain, but the
suggested leaking is the most likely explanation. It is possible that
poroelasticity delayed the transmission of stress, but it is also possible
that rupture responded either to the attainment of a static strain
threshold, or it may also respond to the maximum strain rate change.
Complexity similar to this has been recently modeled at the active volcano
Sakurajima, Japan, by Yokoo et al. (2013) and has been associated with the
observed anomalies of strain signals before vulcanian explosions at the
active Showa crater. The main difference between these examples and the
Campi Flegrei region, is the 4 km depth from the surface at which intense
degassing or underground fluid migration occurs, with no surface eruption of
magma. Any long-term oscillation due to the refilling of magma chamber have
not been observed up to present in Campi Flegrei: this is also different
from recent observations made by Hautmann et al. (2014) and Di Lieto et al. (2020) for SHV (Montserrat) and Stromboli active volcanoes in correspondence
with paroxysmal activity. The transient ULP signals discussed by Bagagli et
al. (2017) as an evidence of magmatic refilling process are correlated with
barometric and other meteorological data and consequently are of external
origin.
Conclusions
Recent deformation in the Campi Flegrei caldera is dominated by aseismic
inflation, interrupted by transient aseismic reversals in rate. These are
typically below the noise level or are poorly sampled by the low sampling
frequency of most geodetic techniques, but can be quantified relatively
easily using high sensitivity strainmeters and tiltmeters. These instruments
provide coherent views of deformation at several different time scales,
capturing reversals in rate with periods from minutes to months. Monotonic
uplift episodes have been recorded with durations of several weeks to a few
years. The longest of these mini-uplift periods recorded since 1994 appears
not be continuous, but occurred in several small episodes, each lasting not
more than a few weeks, as demonstrated by the inflections in rate evident in
Fig. 3.
The current array provides a glimpse of the potential utility of a dense
array of strainmeters and tiltmeters surrounding the Campi Flegrei region in
providing enhanced details of the inflation/deflation process and associated
seismicity. An expanded array of tiltmeters and strainmeters operating
continuously would permit the details of magma-transfer and the underlying
cause of subsequent seismic activity to be monitored.
Data availability
Data used in the current paper are available at 10.1594/PANGAEA.927603 (Di Lieto et al., 2021).
Author contributions
BDL and PR have collected and analyzed the strainmeter and tiltmeter
data, and contributed to the writing of the manuscript. RB, PR, RS and
BDL installed the tiltmeters. RS and PR installed the strainmeters.
RS and RB wrote the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Understanding volcanic processes through geophysical and volcanological data investigations: some case studies from Italian sites (EGU2019 GMPV5.11 session, COV10 S01.11session)”. It is not associated with a conference.
Acknowledgements
This study was supported by Istituto Nazionale di Geofisica e Vulcanologia
and by the University of Salerno.
Financial support
This research has been supported by CRdC – AMRA and Università degli Studi di Salerno.
Review statement
This paper was edited by Paola Cusano and reviewed by Patrick Smith and one anonymous referee.
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