Transcript DYKE INTRUSION

Seismic monitoring of
volcano processes
Paola Traversa, Jean-Robert Grasso
Chambery, 17 octobre, 2008
Main objective
• To study the seismic response of a volcano to
different magmatic processes
Work axes
• To identify the portion of seismicity directly related to
volcano processes
• To detect specific patterns of volcano tectonic seismicity
with respect to tectonic seismicity and for different phases
of volcanic activity, in particular during dyke propagation
• Mechanical implications for basaltic intrusion dynamics:
• Laboratory experiments
• Numerical modeling
• Monitoring of the stress history created by the dyke
intrusion responsible for the nucleated seismicity
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1. Volcano vs Tectonic seismicity
Model of
earthquake
occurrence

N  0   i (t )
t t i
Modified Omori’s law
[Utsu et al., 1995; Helmstetter and
Sornette, 2002]
Uncorrelated Seismicity
• Background activity, modeled by
a homogeneous Poisson process
• Generated by an external loading
• Tectonic case: driven by platetectonic loading
• Volcanic case: driven by a
volcanic process (pressure
change, mass transfer...)
Correlated Seismicity
• Sequence of events following
a “mainshock” (“Aftershock
Cascade”)
• Triggered by earthquake
interactions
• Average patterns reproducible
by ETAS model
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Volcano vs tectonic seismicity
• Observations:
• Tectonic case and quiescent volcanoes:

N  0   i (t )
t t i
EQ occurrence => =
and
• During magma intrusions:

N  0   i (t )
t t i
Magma rising =>
and
Most portion
of volcano
seismicity
during
intrusion is
related to
volcano
process (little
noise)
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Volcano vs Tectonic seismicity
Vesuvius –dormant (Feb. 1972 – Aug. 2006)
t0 ≡ mainshock occurrence, defined as any
event (independently of its magnitude) not
preceded by another one within a time equal
to the median of the Δt.
t > t0 seismicity following the mainshock
(aftershock cascade)
Correlated
Uncorrelated
Quiescent volcano:
Volcano seismicity = Typical tectonic
seismicity pattern
Tectonic seismicity / ETAS
model:
Power law decrease of the
seismicity rate following a
“mainshock”,
Go back to the background
rate.
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Volcano vs Tectonic Seismicity
During dyke intrusions: Piton de la Fournaise 1988-1992 (7
intrusions) – Etna 2002 intrusion – Miyakejima 2000 intrusion
Piton de la
Fournaise
Etna
intrusion
Peculiar dyke
seismicity patterns:
Miyakejima
Etna non-intrusion
weak, if any, Omori’s
law pattern following
mainshocks.
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Volcano vs Tectonic seismicity
• Why seismicity during dyke injection looses
its capacity of producing aftershocks?
HP: Forcing rate generated by the intruding
dyke is too high to allow for stress
redistribution and full aftershock development
within the solid matrix
Analysis of the seismicity related to the
2000 Miyakejima dyke injection (the largest
seismic swarm ever recorded)
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2000 Miyakejima dyke intrusion
Seismic swarm accompanying the intrusion: June 26 – August 29
Coulomb stress loading:
CFS   S   '  n 
The overall change in
normal stress
generated by the
dyke opening
explains 80% of the
total swarm seismicity
(the near-dyke one)
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2000 Miyakejima dyke intrusion
Toda et al., [2002] explains
the overall seismicity by the
total change in shear
stressing rate generated by
the intrusion and a magma
chamber inflation.
( stressing rate ↔
relaxation time)
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2000 Miyakejima dyke intrusion
Time monitoring of the stressing history generating the 2000 seismicity
Deterministic approach:
RATE- and STATEdependent friction law
[Dieterich, 1994]
Magmatic intrusion
Non-linear relationship
between the stress state and
the seismicity rate.
We follow the evolution of the
earthquake nucleation
sources subjected to some
stressing history.








γ is a state variable evolving with time and


1

stressing history. It determines the
d

dt

d


d




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distribution of slip speeds preceding
A



instabilities
2000 Miyakejima dyke intrusion
Time monitoring of the characteristics of the seismic swarm
accompanying the magmatic intrusion
Stochastic approach:
Epidemic Type
Aftershock Sequence
modeling (ETAS)
Seismicity driven by
magma movement -->
Background activity and
earthquake-interaction
driven activity are nonstationary.
N t   0 t   
ti t
K 0 t 
e mi mc 
p
t  ti  c 
Non-stationary ETAS
formulation in λ0(t) and K0
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2000 Miyakejima dyke intrusion
HIGHER stressing rate
higher component of events generated by the dyke intrusion
lower component generated by earthquake interaction.

Stressing rate
Background fraction
Aftershock
productivity
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2000 Miyakejima dyke intrusion
Power law relaxation of seismicity following the last eruption --> eruption
behaves as a mainshock in generating the seismicity.
Result confirmed by
the seismicity pattern
following the Etna
2002 eruption.
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Intrusion Data
• Piton de la Fournaise (1988-1992), 7
seismic crises (durations: 0.5-4.5 h), 6 of
them ending up into an eruption. Temporal
histories extracted from analog signals (time,
Md, no location).
• Etna (2002), crisis preceding the 2002-2003
eruption, duration: 6.3 h
• Miyakejima (2000), seismic swarm
accompanying the June-July 2000 intrusion,
duration: 281.2h
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Seismic activity during intrusion
• No clear accelerating/decelerating patterns
of seismicity
Constant occurrence rate is
repruducible by a
homogeneous Poisson
process (constant mean)
P( x) 
x e  
(m≥mc)
N
58
153
199
34
50
44
x!
Random draws with the
same dimension as
seismic series
Fluctuations dues to the
undersampling of the
Poisson process
N (m≥mc)
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2145
97
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Energy rate during intrusion
• Fluctuating around a mean value, without any
accelerating/decelerating pattern.
Fluctuations compatibles with
those of a Gutenberg-Richter
law with constant b value.
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Start injection
End injection
Seismicity during dyke intrusion
• Stationary seismicity rate
• Stationary energy rate
Independently of:
• Intrusion
duration SCALE
(0.5 h to 11
days)
DYKE
INTRUSION:
INDEPENDENT
STATIONARY
• Maximum magnitude (1.6
to 5.6)
PROCESS
• Dyke size (~1km to 15 km)
SMALL SCALE HETEROGENEITIES, STEP-WISE
• Emitted lava volume
PROPAGATION (lab simulations), AND TWO-PHASES
INTRUSION NOT SOLVED
Constant damage rate ~ Constant injection
flux
Seismicity during dyke intrusion is a response of
the edifice on its all
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Generic model for the intrusion
• Dyke propagation: STATIONARY
• At lab scales: stationarity reproducible
by strain driven experiments 
variable loading:
• Strain driven peeling
Tesile mode I on paper
• StrainDYKE
drivenPROPAGATION
tensile
~ STRAIN-DRIVEN
Sec. Creep
(mode
I)
PROCESS
(VARIABLE LOADING) → SECONDARY
• Secondary creep
CREEP
Secondary creep on rocks
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Dynamics of magma injection
• Observed stationary seismicity prevent from distinguishing
the recurrently observed two-phase intrusions → HP:
constant magma flux at the dyke entry
• Verify that the constant flux model is compatible with the
observations. vertical injection → effect of a lithological
discontinuity (?) → change to a horizontal magma
injection near the surface
z
- Same flux injection as
the vertical “conduit”
observed velocity scaling
between vertical and
horizontal dyke
propagation?
- ¿ How to model the
change in direction?
Volcanic edifice
h
- ¿ Can we recover the
Dyke
Magma
reservoir
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Dynamics of magma injection
Numerical model of stationary volumetric magma flux into a dyke
rising vertically from a superficial reservoir (test on Piton de la
Fournaise, input data from previous studies):
- Rising time
- Volume of magma injected into the dyke
- Average propagation velocity
RESULTS
COMPATIBLE
WITH
OBSERVATIONS
Vertical magma rising from a
superficial reservoir. Processes:
z
h
Volcanic edifice
- Buoyancy
- Varying pressure at the dyke inlet
- Lithostatic loading
Horizontal migration. Processes
Dyke
Magma
reservoir
- Effect of a density discontinuity
- Topography
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- Flux at the entry
2000 Miyakejima dyke intrusion
¿Any correlation between the dyke propagation velocity and the
observed seismicity rate?
Time since June 26
Dyke propagation
period: June 26 to July
8 [Ueda et al., 2005]
Migration of
hypocenters
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2000 Miyakejima dyke intrusion
Migration of earthquake center of mass during dyke propagation
~ propagation velocity of the dyke.
200 event (m ≥ mc)
non-overlapping
windows.
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2000 Miyakejima dyke intrusion
Correlation between the dyke propagation velocity and the observed
seismicity
Cumulative seismicity
~ constant
propagation
velocity
Dyke propagation speed
correlation
between the
observed
seismicity rate.
Inversion of sense
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Conclusions
• On volcanoes, ↑ in seismicity are primarily due to
background seismicity rate changes (strongest during
dyke intrusions).
• Dyke intrusions: external forcing rate > tectonic loading
rate. This prevents earthquakes interactions (Omori’s
law) to fully develop.
• Magma intrusions can be modeled as “silent” or “slow”
earthquakes in terms of stress history.
• Constant seismicity rate → Dyke intrusion is a stationary
process → Constant volumetric flux at the dyke inlet
• Stationarity → Dyke intrusion ~ strain driven process
with variable loading (i.e. secondary creep) → ¿
pressure decrease at the base of the dyke? → FINITE
SIZE OF THE RESERVOIR
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