Residual gravity variations in volcanic areas: Constraints to the interpretation of uplift episodes at Campi Flegrei, Italy

Phys. Chem. Earth (A), Vol. 24, No. 11-12, pp. 963-961, 1999 0 1999 Elsevier Science Ltd

All rights reserved 1464- 1895/99/$ - see front matter PII: S1464-1895(99)00143-X

Residual Gravity Variations in Volcanic Areas: Constraints to the Interpretation of Uplift Episodes at Campi Flegrei, Italy M. Bonafede and M. Mazzanti Department of Physics, University of Bologna, Viale Berti-Pichat 8,40127 Bologna, Italy Received 25 April 1997; accepted 0.5July 1999

Abstract.

An elastically

homogeneous,

compressible

half space model with vertical density stratification is employed to compute the displacement field and the gravity variations produced by the inflation of a spher‘ically symmetric deformat,ion source. Contributions to

1

Two episodes of ground deformation (170 cm of maximum uplift for the 1970-72 crisis and 180 cm for the 1982-84 crisis) took place recently in It,aly at Campi Flegrei (C.F.), accompanied by gravity changes (Berrino, 1994), seismic activity (De Natale et. al., 1995) and gee

gravit,y variations are produced by (1) the displacements of the free surface and of subsurface layers, (2) dilation/contraction of the medium. (3) t,he displacement, of source boundaries and, possibly, by (4) new mass input. from remote distances into the source volume. Two ext,reme

case5 were examined

in detail.

chemical anomalies (Martini et al., 1991). During the uplift phase in 1982-84, gravity variations were measured amounting t,o -213 f 6 /&al per meter of uplift

in which the

magma chamber is identified as the deformation source: in the first model the volume and pressure increase is due to input of magma with the same density of t,hc magma already resident, in the magma chamber (deformation source wit,h constant density), model it is due to magma differentiation

Introduction

(average over all stations).

In field measurements

of t,he

free air gravity correction yield a value -29Of5 pGal/m (Berrino et al., 1992). Residual gravity variations (free

in the second (deformation

air corrected gravity variations) accordingly amount to f77 f 8 /rGal/m. Aft.er 1984 a slower subsidence phase began (which is st,ill ongoing) during which gravity residuals were completely different from the previous ones.

source with constant mass). In recent years (1970-72 and 1982-84) two inflation episodes took place in t,he Campi Flegrei caldera (Italy), characterized by significant ground uplift and st.rongly corrdated gravity varia-

(measured variations f130 f 20 PGal per meter of sub sidence, yielding a residual gravit,? change of -160 =t 21 PGal per meter of subsidence.

tions. From the comparison between measured and computed gravity residuals (free-air-corrected gravit,y vari-

LJplift, episodes at C.F. have been described in terms of

ations) we can assess that an inflation source with const ant density would predict gravit,.y residuals consistent with those measured during the phase of uplift (within

volume or pressure ber, enclosed in an and Decker 1975, 1984, Rundle and

experimental errors), while an expansion at constant mass would predict gravity residuals much lower t,han observed. However, during the subsidence phase, which followed after the maximum uplift in 1984, gravity residuals at most sites were completely different from those measured during the uplift phase, suggesting that more mass left the system during deflation than entered during inflation. An alternative model is then proposed, in

increases in a shallow magma charnelastic medium (Mogi 1958, Dieterich Corrado et al. 1976, Berrino et al. Withcomb 1984) or in a viscoelastic

medium (Rundle 1978, Bonafede et al. 1986). However, an implausibly high overpressure (more t.han 50 MPa) is required to produce the observed uplift. Furthermore: seismic and magnetot,elluric surveys. eart,hquake foci and drilling of geothermal wells indicate that the top of the magma chamber must be deeper than 4 km (Ort,iz et al. 1984, De Natale and Pingue 1988. AGIP

which the deformation source is ascribed t.o fluid pressure increase within a geothermal s.ystem, close to the critical point,, shallower than the magma chamber.

report 1987), while geodetic data suggest, for t,he source cent,re a dept,h of 3 km only. Since C.F. are a regiou of high permeability and hy-

0 1999 Elsevier Science Ltd. All rights reserved

drothermal circulation of hot and pressurized fluids, (hydrological data from AGIP report, 19871, interaction be-

Correspondence

to: M. Bonafede 963

M. Bonafede and M. Mazzanti: Residual Gravity Variations in Volcanic Areas

964

tween the magma chamber

and the geothermal

system

free surface

0

above it may provide the source of deformation as a shallow aquifer is heated and pressurized from below. Bonafede (1990, 1991) shows that hot fluid migration has great influence on ground deformation: this process

2=200

“I

z=l km

provides a steady uplift rate at constant overpressure, while elastic (Mogi) models can only produce increasing deformation

if the overpressure

at the source increases

too. If the deformation magma chamber,

is due to a pressure

there are two possible

could be responsible

increase processes

in a that

for it: (i) pressure increase is gener-

ated by the input of new magma from the mantle (case I), or (ii) differentiation of magma a1read.y resident is accompanied by the release of volatiles (case II). We shall employ an elastic half space model of deformation with vertical density stratifications to consider

n

P=lXOOkg/m?

first the problem in which the source is described in terms of a pressurevolume increase at constant density

(1

p-2000 kg/n?

()p=2200

(case I) and then case II, in which deformation and gravity changes are produced by source dilatation at constant mass. The gravitational deformation

of density contrast

effects resulting boundaries

from the

will be com-

puted at 4 benchmarks within the caldera rim: Serapeo (close to the maximum uplift site), Solfatara (- 2 km toward NNE), Arco Felice (- 2 km toward WNW) and Bagnoli (- 2 km toward E). Computed residual gravity variations will be compared with measured values.

2

Contributions

The total gravity

to gravity

changes

change observed

at a benchmark

can

be split into two terms: Ag = Ag, + Ag, where Ag,

(1)

is the gravity

variation

dependent

on eleva-

tion change of the benchmark (removed through the free air correction) and Ag, is the gravity change due to the mass redistribution. The term Ag, includes contributions arising from the displacement of density layers Ag, (including the free surface), from the change of density related to medium compressibility Ag, and the contributions of the source Ass:

4,

= &, + 4,

+ Ag,

(2)

The density structure pertinent to C.F. is taken from Cassano & La Torre (1987) and is shown in Fig. 1. 2.1

Gravity

contributions

due to the deformed layers

The deformation of a density contrast interface, at depth H, below the surface of an elastic half-space, can be computed from Bonafede (1990). The following gravity

kg/n?

-1

p=2400 kg/m’

n

P=2600 kg/m” J

Fig. 1. Density stratification at Campi Flegrei according to Cassane & La Tome (1987).

change is expected

to be found at the ground surface Ag,

= Ci Ag,,

where G is the gravitation constant, Ap, is the difference between the density in the (i + I)-th layer and the i-th layer, h, up,z are cylindrical coordinates around the vertical z-axis through the benchmark, uf’ is the vertical component of the displacement at depth H, (rewritten from Bonafede (1990) in terms of h, 0, z) and f(z) = Hi +u: - .z (~9 is the ground uplift at the benchmark, z = h = 0). In evaluating A, = Ag,/us, we shall restrict the range of source depth values from 2.5 km to 5 km since this is the plausible range of magma chamber depths at C.F., as discussed in the introduction. The first density interface to be considered is the airground surface; its gravity effect can be easily computed from the Bouguer formula (the same result is obtained by using eq.n (3) with Apr = pl). The Bouguer correction is computed as 75 pGal/m. It is significantly higher than the contributions of other subsurface layers, which were computed from eqn (3) at 4 different benchmarks located within the uplift area. The overall layer contribution A, is shown in Fig.2. It appears that the effect of the buried layers is not negligible, amounting to N 25% of the total layer contribution. The steps in Fig. 2 are due to the presence of a density interface at 3 km in the density model: if the source is shallower than 3 km this interface deforms

M. Bonafede and M. Mazzanti: Residual Gravity Variations in Volcanic Areas

SERAPE0

110

Fig.

965

SERAPE0

SOLFATARA

a

2.

function

Computed

contributions

of the source depth

A, to

the gravity

~0 at 4 selected

changes

as

benchmarks.

Fig.

3.

Computed

at 4 selected

contributions

benchmarks,

A, +

A,

to the gravity

vs source depth 20 (ordinate)

changes

and source

radius a (abscissa).

downward while, if the source is deeper, it deforms up ward. In any case, the most important result is that the gravity corrections due to uplift of the ground and of the buried layers (Ag,) are too large to remove the observed 77 f 8 PGal residual for each meter of uplift. 2.2

Contribution

due to medium compressibility.

In presence of an inflation source, density changes may take place, relat,ed to finite rock compressibility. The contribution to gravit,y variations due to the relative volume change of the i-th layer contained between the depths Hi~_l and Hi is,

where pi is the density of the i-th layer, L; is the domain of the i-th layer and V . u can be evaluated from E%onafcde (1990). The contribution

A,

= C Agv, /ui has been evalu-

ated for different values of the radius a and the depth ~0 of the deformation source. Different behaviours appear at different stations, but a nearly uniform value of A,

(- -30 pGal/m) is obtained, independent from source depth .zo and source radius a for all the 4 benchmarks considered. In Fig. 3 the sum (A, -t A,) is plotted. Values N 70 pGal/m are obtained at all 4 benchmarks, nearly independent on zo and u. The value above compares favourably with the average experimental residual 77 f 8 pGal/m. Accordingly, most of the gravity variation during uplift can be explained by deformation of the layers and rock compressibility, without any explicit contribution from the source itself. More specifically, no contribution would be provided by the source if it expands at constant density and its density is the same as the surrounding medium.

It is however worthwhile

studying source contribu-

tions in order to asses if any between cases I and II can be safely discarded, taking into account t,he experimental accuracy. 2.3

Contribution

due to the inflation source

The contribution to gravity variations at a benchmark, due to the deformation Ag,=G

[J$Lv-

of the source is

p$Le]

V'

where primed variables refer to the deformed configuration and 0 is the angle between the vertical and the line joining the benchmark to the volume element dV of the source volume V. Employing an expansion in spherical harmonics and neglecting 3-rd order terms, eq. (5) becomes:

ag,

+(+)V+

where d” = r~/cos&

(++j

(6)

(ro is the distance between the

centre of mass of the source and the benchmark, &J the angle between the vertical and line joining the benclmark to the centre of mass), pm is the density of the source, pc is the density of t,he surrounding medium, V is the volume of the source. Again, primed variables refer to the configuration after the expansion and AV = L” - V. From Bonafcde (1990) the average vertical displacement of the source boundary is computed to be shifted up wards by less than 30 - 40 cm (for a surface uplift U: = 1 m) and this shift would produce a gravity change amounting to a fraction of 1 /IGal only. Accordingly, we assume d’ 2: d, so that eq. (6) becomes:

aS,=$(pin-pn)V t(dn-pc)AV

1

M. Bonafede and M. Mazzanti: Residual Gravity Variations in Volcanic Areas

966 SERAPE0

BACNOLI

Fig.

4.

Gravity

contributions

of the source at constant depth

zo (ordinate)

p,=lOO

kg/m3

AS

density

(in pGal/m)

due to the inflation

for different

values of the source

and source radius a (abscissa).

A value pm -

and (7) becomes:

;(~m- P,)AV

(case I)

(8)

that can be interpreted BS the gravity contribution pr+ duced by a volume AV that was filled with a rock of density pc before the expansion, replaced by a rock of density pm after the expansion. This contribution was evaluated for different values of the radius and the depth of the source, assuming pm pe = 100 kg/m3 (for reference); it is found to be = 3 PGal (Fig. 4 shows this contribution at the 4 selected benchmarks). For an expansion

at constant

mass (case II),

p,V =

p&V' and (7) becomes: Ag,

5.

Gravity

contributions

A,

(in pGal/m)

due to the inflation

mass for different valuea of the depth and

the radius of the source.

is assumed.

Expansion at constant density (case I) requires p& = pm

Ag, =

Fig.

of the source at constant

pcAV = -GcF

(case II)

which can be interpreted as the contribution of a volume AV which was filled with a rock of density pc before the expansion and becomes “empty” after the expansion. This contribution is shown in Fig. 5: it is negative, of course, and its absolute value (70 - 80 p,Gal/m for each meter of uplift) is significantly higher than it was for a deformation at constant density. The source contribution A, = Ag,/ut can now be added to the contribution AL + A, due to deformation of the medium surrounding the magma chamber. In the previous section, a difference between measured residuals and computed residuals (taking into account the deformation of density boundaries and medium compressibility), was found to be at most N 10 PGal for 1 m uplift. An expansion of the source at constant density (PA = pm) may decrease further this difference by

N 3pGal/m for a density contrast pm- pc = 100 kg/m3. An expansion at constant mass, on the contrary prcF duces too large a negative contribution. In conclusion, case I predicts gravity residuals compatible with measurements taken during the uplift phase, while case II would predict vanishing or negative residuals, incompatible with the observations.

3

Discussion

and conclusions

Ground inflation in recent years at Campi Flegrei (C.F.) has been accompanied by residual gravity changes, attributed to the input of new maSS beneath the central part of the caldera (Berrino, 1994). Three main causes of gravity variations have been investigated: the deformation of density layers, the dilatation/contraction of the medium, the mass redistribution within the source. The overpressure

within

the source of deformation

(assumed to be a shallow magma chamber), was atr tributed to two different physical processes: inflation of the source at constant density (case I) and inflation at constant mass (case II). The main difference between the two examined cases is in the input of new mass. The previous

computations

show that the deforma-

tion field and the gravity anomalies produced by magma emplacement within a magma chamber must be modelled simultaneously: gravity anomalies cannot be interpreted without taking into account the displacement of the density boundaries in the region affected by the deformation. A different rationale to interpret the deformation and gravity measurements at C.F. would consider an i&ation mechanism based on the interaction between the magma chamber and the geothermal system above it (Case III). In Bonafede (1991) the deformation at C.F.

M. Bonafede and M. Mamnti: was modelled in which

in terms

of a thermo-porwelastic

fluids are affected

10 MPa.

by a pressure

with t,he geothermal

source

increase

In this case? the source of deformation

identified

Residual Gravity Variations in Volcanic Areas

point

would be

system above the magma

and porosity

values

range from 25%

(at 2 km depth) to 0.5% (at 2.7 km depth). An increase of pressure of 10 MPa in water at subcritical conditions produces significant (up to hundreds of kg/m3) density variations Apf (e.g. Haar et al. 1984). These density variations

are pervasive throughout> the aquifers

(i.e. they take place all over V’), and all the pressurized geothermal system can host, new fluid mass from below. When t,he geot,hermal sgst,em expands in undrained conditions,

no porosity

variations

and t,he voids expand variations

produced

are produced

at the same rate),

by pore pressure

at. C.F.

may be

due to the expansion of the geothermal system, triggered by high pressure and high temperature volatiles released from the magma chamber (which induce a pervasive change of density in the fluids present. in the system and a fluid mass input, in t,he increased void volume), seems to be supported by the behaviour of the deformation field during t,he 1970-72 and 1982-84 uplift, episodes (fast uplift phase followed by slow subsidence, see Bonafede 1990). Moreover, as it was mentioned in the introduction, gravity residuals during the subsidence phase are - 160 f 20 pGal per meter of subsidence (averaged over all gravivity stat,ions), showing that more mass left the source

during

subsidence

than entered

during

uplift. The behaviour during subsidence might be qualitatively int,erpreted in terms of pressure decrease in the aquifers, accompanied by density decrease in the fluids and by an upward migrat,ion of the geotherms, shifting the vapour-liquid transition to shallower levels. A more detailed description of the models proposed above can be found in Bonafede and Mazzanti (1998). Acknow&&ements.

Work

from

N&on&

ropean

CNR

(Gruppo

Commission

under

performed per contract

with

MMESG,

financial

la Vulcanologia) ENV4CT960259.

contributions and

the

Eu-

e

Geofisica

Servizi

formation

de1

Central,

San Don&o,

G., Corrado.

Sistema

per

Geotrrmico

I’Esplorazionr.

dsi SERG-

19 pp.

G., Luongo,

and gravity

G., Two.

changes

B.. 1984. Ground

accompanying

de-

the 1982 Pozzuoli

But/. Volcano/., 47. 188-200.

uplift; Berrino,

G.,

Rymer,

Gravity-height

H..

Berrino.

G.,

ations

Brown,

G.C.,

for

unrest

correlations

rand.Gcothen.Res..

53,

1994. Gravity

at. Campi

(‘orrado. at

G..

caldera;

1992.

Vu/-

.I.

11-26. change

Flsgrci

induced

caldera;

.I.

by height-mass

varl-

Gex~thrm..Res.,

I%/canol.

61,293.309. Bon&de,

M..

Dragon’.

RI., Quareni,

st,rrss tieIds produced source

F..

bv a centre

in a viscoelastx

1986. 1)isplac~ment

of dilatation

and

and by a pressure

Geophys. .I R. ost~. Sm..

half-space;

87,455.485. elastic

(Rice and Clearly,

Geologia

Flegrei;

Berrino,

Ronafede.

(5) p,,, = pc and a density increment within the volume L” of the deformation source given by pA - pc : @API, a value of @ 2r 10% was obtained, which is compat.ible with the porosity values mentioned above. The residuals

1987.

Campi

(t,he solid

were taken from the International St,earn Tables (Haar et, al., 1984), considering the actual temperature pre files at CF., an initially hydrostatic pore pressure pre file and a pressure increase of 10 MPa. Considering in

that, gravity

Agip.

while por0sit.y

1976) are expected t,o be negligible. With these assump tions, we evaluated (for a hemispheric source region with constant, porosity Q, radius 1 km and base at 3 km depth) the value of porosity needed to produce gravity residuals equal to those measured. Values for Apf

int,erpretat.ion

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