SIF 2010, 20-24 September 2010 1

Interaction between deformation sources and seismicity in a volcanic region: the case of the 1982-1984 bradyseism at the Campi Flegrei caldera (Italy).

M. E. Belardinelli (1), A. Bizzarri (2), G. Berrino (3) and G. P. Ricciardi (3),

(1) Università di Bologna(2) INGV, Bologna (3) INGV-OV, Napoli

SIF 2010, 20-24 September 2010 2

Mainshocks affect subsequent seismicity (e.g. mainshock-aftershock sequences).

The Omori law is an empirical law to express the aftershock rate (AR) as a function from time since the mainshock.

Dieterich (1994) proposed a physical model for the Omori law.

He assumed a rate- and state- (R&S) dependent friction on aftershock faults.

Background

t ( )

SIF 2010, 20-24 September 2010 3

R&S friction:Dieterich – Ruina law

( )

parameters lrheologica L, , , variablestate

velocitysliding v *stressnormaleffective

tdd

L v1

tdd

L

vln

vvln v,

αΨ

σ

σσΨαΨΨ

σΨ

μσΨμτ

ba

b

b a

eff

effeff

eff*

**efff

⎟⎟⎠

⎞⎜⎜⎝

⎛−−=

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛+

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛+==

resistancefrictionalon theeffect direct effaA σ=

variablestateaandvelocityslidingtheondependsresistancefrictional fτ

* (normal stress decreased by pore pressure)

SIF 2010, 20-24 September 2010 4

Let us consider a population of faults subjected to τ (t) as a shear stress and σeff (t) as an effective normal stress(Dieterich ’94) seismicity rate R(t)

where

If a localized source of deformation/stress is present:

shearinrateloadingregional rate, background , rr

rrR ττγ

&&

=

Background

)()(

)(1)(

1⎥⎥⎦

⎤

⎢⎢⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−+−= ttt

ta effeffeff

σασ

τγτγσ

γ &&&),(tγγ =

the seismicity depends on the related perturbation of stress on neighboring faults.

E.g.: 1976-1983 intrusion of magma into the rift zones of Kilauea.

SIF 2010, 20-24 September 2010 5

In seismic areas, the seismicity rate (SR) depends on the stress change due to the mainshocks (coseismic stress-changes). E.g. Catalli et al., 2008:

1997, Umbria-Marche, Central Italy, seismic sequence

In volcanic areas, SR is shown to be also affected by moderate* stressing rates.

The latter can be provided by deformation sources typical of volcanic areas (e.g. evolving dykes, sills or reservoirs of magma).

Following Dieterich ’94

*compared to coseismic ones.

Toda et al. (2003): seismicity associated to the 2000, Izu Islands dike intrusion,

the aftershock decaydepends on the distance from the dyke

data model

SIF 2010, 20-24 September 2010 6

The Campi Flegrei case

A correlation between seismicity rate and rate of ground uplift can be noted at Campi Flegrei (CF) caldera (e.g. Berrino and Gasperini, 1995).

Berrino and Gasperini, 1995

/ month

SIF 2010, 20-24 September 2010 7

The 1982-1984 unrest

During the 1982-1984 unrest episode seismicity interested a restricted area in the Campi Flegrei (CF) caldera. Most of earthquakes have normal or normal-strike-slip mechanisms (e.g. Orsi et al. 1999).

modified from R. Scandone et al., 2009 (personal communication) Miseno Nisida

Solfatara

Miseno Nisida

SIF 2010, 20-24 September 2010 8

Summary1. Static stress changes caused by the inflating source at CF on OOPS

(optimally oriented planes for shear failure).

2. Modeling of stress dependence on time from uplift history recorded by a tide gauge at Pozzuoli.

3. Estimate of SR as a function of time from a R&S model proposed by Dieterich (1994).

SIF 2010, 20-24 September 2010 9

σ T chosen in order to have a bounded area with

Static stress changes

In each location, OOPS have the same total stress of CoulombRegional stress: vertical compressive axis, N36°E trending tensional axis,

fliteff

effTOTc

pp −+=

−+−=00

00

σσσΔμτΔμστσ

Source: vertical, penny-shaped spheroid (Amoruso et al., 2006) in a layered half space at 5.2 km depth.Computation depth: 3 km

) chydrostati ( MPa 20 fT p=σ

Region of interest: 80% of 1982-84 seismic events

0>TOTcσ

03

Tσσ −=

SIF 2010, 20-24 September 2010 10

One plane in the couple of OOPS

Static stress changes: OOPS

The conjugate plane in the couple of OOPS

Normal faults with oblique components near the source

σΔμτΔσΔ ×−=C

Δσ=0 :

mean value between the two OOPS in the same location;

for each OOP, average value within the region of interest.

SIF 2010, 20-24 September 2010 11

In the region of interest there are two main, almost equivalent, configurations with different sign in normal stress : PP and PN.In order to keep changes in normal stress, we keep separate the two configurations in averaging stress changes and prestress within the region of interest.

Time dependence of stress

max0

max0

/)()(

shearin rate loading /)()(

utut

ututt

effeff

r

r

ΔσΔσσ

τΔτΔτττ

+=

++=&

&

16.521.3(MPa)

3.848.28(MPa)

1.70-2.68(MPa)

10.76.76(MPa)

PPPNParameter

τΔσΔ0τ0effσ

We consider a piecewise linear approximation, u(t), of the uplift history recorded by a tide gage at Pozzuoli.We express time-dependent components of stress

as

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Sep-80 Jun-83 Mar-86 Dec-88

time

uplif

t (m

)

1.6 m Δumax

stresschanges

pre-stress

Averages in the region of interest

SIF 2010, 20-24 September 2010 12

(Dieterich ’94)

We analytically integrate an approximation of the differential equation for γ (t) in month interval of constant

We consider different values of both:

We choose the values that best reproduce the initial stage of seismicity at CF, which is delayed of some months with respect to ground uplift.

SR estimatesmonths 4each event seismic 1rate backgroud , ≈= r

rR

rτγ &

effστ && and

.shearinrateloadingregional ,

and

resistance frictionalon effect direct the of value initial ,0

0

r

effaA

τ

σ

&

=

SIF 2010, 20-24 September 2010 13

SR estimates: role of free parametersPN config.

Time scale Amplitude

Effect of A0

0

300

600

900

1200

1500

0 500 1000 1500days since 1 Jan 1982

SR (e

arth

quak

es/m

onth

)

data

0.01 MPa case 1(Toda et al., 2002)

0.04 MPa (Catalliet al.)

0.27 MPa

0.45 MPa(Dieterich et al.,2000 )

, 2008)

Effect of dΤr/dt

0.01

1

100

10000

0 500 1000 1500days since 1 Jan 1982

SR (e

arth

quak

es/m

onth

)

data

1.e-3 MPa/yr,Catalli et al., 2008

1.e-2 MPa/yr(Toda et al.,2002)

0.3 MPa/yr case 1(Dieterich et al.,2000 )

Dieterich et al. (2000): seismicity in a volcanic province, 7 km depth (1976-1990, south flank of Kilauea, dyke intrusions and mainshocks)Toda et al. (2003) seismicity in a volcanic province (2000, Izu Islands dike intrusion and mainshocks) Catalli et al. (2008) seismic sequence (1997, Umbria-Marche, Central Italy)

SIF 2010, 20-24 September 2010 14

Results

-200

300

800

1300

Apr-82 Oct-82 Apr-83 Oct-83 Apr-84 Oct-84 Mar-85 Sep-85

time

SR (n

. eqs

/mon

th)

-8

-6

-4

-2

0

2

4

uplif

t rat

e (m

m/d

ay)

observedmodel PPmodel PNuplift rate

General agreement concerning the duration and maximum values of SR data at CF.

SR are damped bynegative stressing rates (subsidence)

End of uplift

Subsidence

SIF 2010, 20-24 September 2010 15

conditions alhydrotherm (PP) 0.023 sexperimentfriction in standard (PN) 0.013

→==

aa

2008) al.,et (CatalliItaly Centralin seismicityMPa/yr 10 -3 ←≈rτ&

00 /σAa =Estimate of

Estimate of

1.0 ×10-30.27PN

1.3 ×10-30.38PP

Config. (MPa/yr)rτ&(MPa) 0A

Parameter estimates

SIF 2010, 20-24 September 2010 16

Conclusive remarks

In order to simulate a region of potential failure resembling that one affected by most of seismic events, a suitable magnitude of a regional extensional stress can be chosen.

Such a regional stress can explain normal/oblique faulting above the inflating source, as observed.

According to our model, near the end of the uplift, stressing rates useful for Coulomb failure are slowly* decreasing, while they are negative during subsequent subsidence.

In comparison with SR data as a function of time, suitable values of parameters of a R&S model are either consistent with values observed in laboratory (a) or previously proposed for Italy (dτr /dt).

To conclude, seismicity rates can be affected by either slowly* increasing or decreasing the Coulomb stressing rate on potential faults. The last effect is remarkable in the case of CF unrest.

*compared to coseismic stressing rates

We focus on the temporal distribution of seismicity during the 1982-1984 unrest at Campi Flegrei caldera.

SIF 2010, 20-24 September 2010 17

Thank you for your attention References:

Amoruso, A., L. Crescentini and G. Berrino (2008), Simultaneous inversion of deformation and gravity changes in a horizontally layered half-space: evidence for magma intrusion during the 1982-1984 unrest at Campi Flegrei caldera (Italy), Earth. Planet. Sci. Lett., 272, 181-188.

Berrino G., and P. Gasparini (1995), Ground deformation and caldera unrest, Cahiers du Centre Européen de Geodynamique et de Séismologie, 8, 41-55.

Catalli, F., M. Cocco, R. Console, and L. Chiaraluce (2008), Modeling seismicity rate changes during the 1997 Umbria-Marche sequence (central Italy) through a rate- and state-dependent model, J. Geophys. Res., 113, B11301, doi:10.1029/2007JB005356.

Dieterich, J., Cayol, V., and P. Okubo (2000), The use of earthquake rate changes as a stress meter at Kilauea volcano, Nature, 408, 457-460.

Orsi, G., L. Civetta, C. Del Gaudio, S. De Vita, M. A. Di Vito, R. Isaia, S. M. Petrazzuoli, G. P. Ricciardi, and C. Ricco (1999), Short-term ground deformations and seismicity in the resurgent Campi Flegrei caldera (Italy): An example of active block-resurgence in a densely populated area, J. Volcanol. Geotherm. Res., 91, 415–451.

Toda, S., R. S. Stein, and T. Sagiya (2002), Evidence from the AD 2000 Izu Islands earthquake swarm that stressing rate governs seismicity, Nature, 419, 58– 61.