atenuación

ATENUACIÓN
ATENUACIÓN INTRÍNSECA
¿Qué es atenuación?
¿Cómo describimos la atenuación?
¿Cuáles son las causas de la atenuación?
¿Cómo podemos estimar la atenuación?
¿Dónde en la tierra vemos la atenuación?
CAMBIOS EN LA AMPLITUD
DE ONDAS CAUSADOS POR:
•
Difusión geométrica (geometric spreading)
- el frente de onda se expande y la energía se extiende sobre una área más
grande (o más pequeña) y la amplitud de la onda se disminuye (aumenta).
•
Esparcimiento (scattering)
•
- la onda interactúa con cuerpos más pequeños de la longitud de onda, con
velocidad de onda diferente que el medio circundante y se esparce.
•
Atenuación intrínseca (intrinsic attenuation)
•
- el movimiento de la onda activa procesos que convierten la energía de la
onda a otras formas de energía (calor), por fricción interna (internal friction).
•
En este clase nos enfocamos en la atenuación intrínseca.
DESCRIPCIÓN DE ATENUACIÓN OSCILADOR ARMÓNICO AMORTIGUADO
OAA (DAMPED HARMONIC OSCILLATOR)
• Para
analizar el comportamiento de un sistema amortiguado
mecánico, simple, estudiamos el OAA.
La ecuación de movimiento (balance de fuerzas)
γ - factor de amortiguamiento
k
↓ u(t)
Vamos a ver que es útil usar las siguientes variables
m
Q - factor de calidad
DESCRIPCIÓN DE ATENUACIÓN OSCILADOR ARMÓNICO AMORTIGUADO
OAA (DAMPED HARMONIC OSCILLATOR)
• Supongo
una solución en la forma
k
↓ u(t)
• Solución:
m
término oscilatorio
término decaimiento
• Los
dos términos contienen Q!!
Stein & Wysession
ATENUACIÓN - FACTOR DE
CALIDAD, Q (QUALITY FACTOR)
Usamos el factor de calidad para
describir el decaimiento de la amplitud
de un movimiento oscilatorio
¿Cómo entender Q?
Q-1 describe la relación entre la parte
imaginaria y real de la frecuencia
(1)
(2)
(3)
e-folding time
Q es proporcional al tiempo necesario para bajar
la amplitud al valor 1/e de la amplitud original
número de ciclos para bajar la amplitud a 4%
de la amplitud original
ATENUACIÓN - FACTOR DE CALIDAD
EN SISMOLOGÍA, Q (QUALITY FACTOR)
CUIDADO! En sismología
usamos Q para describir el
decaimiento de las ondas, pero
también para describir el medio,
Qα y Qβ o QK y Qμ
El factor de calidad de corte Qμ
es mucho menor que el factor
de calidad volumétrico QK. Los
factores de calidad de ondas P y
de ondas S ambos dependen del
factor de calidad de corte. El
factor de calidad de ondas P es
más de dos veces el factor de
calidad de ondas S.
Q-1K << Q-1μ
OPERADOR DE ATENUACIÓN
- T*
Para simular atenuación para ondas de cuerpo
podemos convolucionar sínteticos calculados sin
atenuación con la función en la figura a la derecha.
Esta función se llama operador de atenuación.
Parametrizado con t* = t/Q,
t = tiempo de viaje
En la Tierra, para ondas de cuerpo de periodos más
de 1 segundo:
tα* = 1 seg y tβ* = 4 seg.
ATENUACIÓN LA DEPENDENCIA DE FRECUENCIA
(FREQUENCY DEPENDENCE)
OBSERVACIÓN:
Q casi constante sobre un rango grande de
frecuencias.
¿Cómo puede ser? Misterio!!
¿el factor de amortiguamiento depende de
frecuencia?
ATENUACIÓN - DISPERSIÓN FÍSICA
(PHYSICAL DISPERSION)
Dispersión física - significa que la velocidad de ondas depende de la frecuencia
Ejemplo: Una función delta viajando en un medio con velocidad de onda c.
En el dominio de frecuencia:
Sin dispersión, la amplitud disminuye con distancia y con constante Q la
atenuación depende fuertemente de la frecuencia. El cambio de amplitud con
distancia es A(w):
con t=x/c
Solución:
?
non-causal
ATENUACIÓN - DISPERSIÓN FÍSICA
(PHYSICAL DISPERSION)
¡Dispersión física es necesaria para conservar causalidad!
Una solución (La ley de atenuación de Azimi):
Para una onda vertical de ScS, el tiempo de viaje
es 5 segundos más para T = 40 segundos que
de T = 1 segundo.
Causa discrepancia entre modelos de ondas de
cuerpo y los de modos normales, si no es
tomado en cuenta.
REPASO: OSCILADOR ARMÓNICO
AMORTIGUADO FORZADO
(Forced damped harmonic oscillator)
Oscilador forzado con frecuencia ω
Solución de prueba:
Solución:
CAUSAS DE LA ATENUACIÓN MODELOS FÍSICOS DE ANELASTICIDAD
Banda de absorción:
Respuesta del “standard linear solid” de una
deformación armónica, con frecuencia w
La atenuación:
La velocidad:
τ = η/k2
“Relaxation time constant”
“Standard linear solid”
CAUSAS DE LA ATENUACIÓN MODELOS FÍSICOS DE ANELASTICIDAD
Q casi constante sobre un rango grande de
frecuencias. ¿Cómo puede ser?
Por la superposición de bandas de absorción
de diferentes mecanismos. (?)
CAUSAS DE ATENUACIÓN PROCESOS MICROSCÓPICOS
•
Thermally Activated Processes: Processes that are more effective with
increasing temperature and follow (τ - relaxation time, E* - activation
energy, R - Gas constant): τ=τ0 eE*/RT
•
Diffusion creep: Deformation of crystalline solids by the diffusion of
vacancies through their crystal lattice
•
Dislocation creep: Involves the movement of dislocations through the
crystal lattice
•
Grain boundary mobility: se mueve la frontera entre granos
•
Partial melting: very effective in lowering Q, even for small melt
fractions
•
Grain boundary friction: if the material is not completely glued
together, the motion can cause grains sliding against each other,
resulting in friction.
Rango sísmico
IMPACTO DE ALGUNOS VARIABLES EN LA
ATENUACIÓN Y LA VELOCIDAD DE ONDA
•
Temperatura: La temperatura más alta aumenta la atenuación y disminuye las
velocidades de onda.
•
Composición: La atenuación no tiene mucha sensibilidad, pero las velocidades
de onda sí.
•
Fusión parcial: Fusión parcial aumenta la atenuación de corte (Q-1β) y
disminuyen las velocidades.
•
Contenido de agua: Contenido de agua aumenta mucho la atenuación pero no
tiene un gran impacto en las velocidades de onda.
•
Tamaño de granos: Algunos procesos de atenuación son más eficientes en las
fronteras entre granos, y por eso la atenuación disminuye cuando el tamaño de
granos aumenta.
e to thoseto
in displacement (Figure 2). Signal-to-noise
erted
bally [Eiler et
2.1. Calculating t*
h-temperature
tios (SNRs)
were determined using the noise
[ ] The spectra of P and S waveforms from local
ght occur in
events recorded
by theaTUCAN
array were anapectra
calculated
from
3-s window
before each
. Thus, taken
lyzed using the vertical and transverse components,
geophysical,
the
signals.
COMO
EVALUAMOS
ATENUACIÓN
respectively,
following the
method of Stachnik et
reater hydra-
nd magmas.
13
al. [2004]. P and S arrivals were picked on waveform time series [Syracuse et al., 2008], then
multitaper spectra [Park et al., 1987] were calculated in 3-s windows, starting 0.5 s before the
arrival, corrected for instrument gain, and converted to displacement (Figure 2). Signal-to-noise
ratios (SNRs) were determined using the noise
spectra calculated from a 3-s window before each
of the signals.
3
ONDAS DE CUERPO - T*
Geochemistry
Geophysics
Geosystems
Geochemistry
Geophysics
Geosystems
G3
4an
] along-arc
Path-averaged attenuation was parameterized
G
rychert et al.: attenuation beneath nicaragua and costa rica
rychert et al.: attenuation beneath nicaragua and costa rica
10.1029/2008GC002040
10.1029/2008GC002040
terms of an attenuation operator, t* = t/Q, for
avel time t assuming a displacement spectrum
(f ) as follows:
rc changes in
ay also play a
esses. Preving-arc varia1990, 2003,
ket al.,
i 1994;
, 2005]. In
port of mand [Herrstrom
1; Hoernle et
dge material
has a longer
plate and a
[14] Path-averaged attenuation was parameterized
*
in terms of an attenuation
operator,
t* = t/Q, for2
jk =ð1 þ
Ajktravel
ðfi Þ time
¼ Ctjkassuming
M0k e$pfa i tdisplacement
ðfspectrum
i =fck Þ Þ
Ajk(fi) as follows:
ð1Þ
.g., Anderson and
Hough,* =ð11984]
for the
kth event
A ðf Þ ¼ C M e
þ ðf =f Þ Þ
ð1Þ
corded at[e.g.,
theAnderson
jth station
for each frequency (fi),
and Hough, 1984] for the kth event
for greater
at the jth station
for each frequency
(f ),frequencyis a constant
accounting
for
here
wedge,C
the
jk(r)recorded
where C (r) is a constant accounting for frequencyd magmatic
dependent
effectseffects
of ofeach
path,
independent
each path,
such assuch
geome- as geomevariations in
trical spreading, free surface interaction, and the
ith a greater
ical
spreading,
free ofsurface
and the
spherical average
the radiation interaction,
pattern [Aki and
n Costa Rica,
Richards, 1980]. M /(1 + (f /f ) ) is a simple
pressures of average
pherical
of the radiation pattern [Aki and
source spectrum [Brune, 1970] where M and f
gua as well
2
are the seismic
moment
and (f
corner
frequency,
/(1
+
/f
)
)corner
is a simple
ichards,
M
muir et al., 1980].
0k
i
ck
allowing
a
different
seismic
moment
and
t al., 2008].
ource
spectrum [Brune, 1970] where M0k and fck
5 of 26
e the seismic moment and corner
frequency,
lowing a different seismic
and corner
Rychert moment
et al 2008
jk
i
jk
0k
$pfi tjk
i
ck
2
i
jk
0k
i ck
2
0k
ck
Figure 2. Waveform and spectrum examples. (a, b, e, f) Waveforms and (c, d, g, h) corresponding spectra of both
the
P wave
2a–
2d)spectrum
and the examples.
S wave (Figures
2h) are compared
station
in the forespectra
arc, MANS
Figure
2. (Figures
Waveform
and
(a, b, e, 2e–
f) Waveforms
and (c, d,at g,a h)
corresponding
of both
(Figures
2a, 2c,
2e, and2a
2g),
andand
a station
the back
arc, B4
2d, 2f, and
The hypocenter
theMANS
event
the P wave
(Figures
– 2d)
the Sinwave
(Figures
2e (Figures
– 2h) are2b,
compared
at a2h).
station
in the fore of
arc,
was
at 1092a,km
i.e.,
the arc,
slab,B4
beneath
the2b,
arc2d,
near
Rica
line of theofTUCAN
(Figures
2c,depth,
2e, and10.6!N,
2g), and$84.8!W,
a station in
theinback
(Figures
2f, the
andCosta
2h). The
hypocenter
the event
array.
In 109
the waveform
the $84.8!W,
blue lines represent
signal,
and the
redthe
lines
are Rica
the picks
on the
the TUCAN
signals.
was at
km depth, plots
10.6!N,
i.e., in thetheslab,
beneath
thedashed
arc near
Costa
line of
Inarray.
the spectrum
plots, blue
lines
spectra,the
redsignal,
lines represent
noise, red
andlines
cyanare
lines
fitting
In the waveform
plots
therepresent
blue linesthe
represent
and the dashed
theshow
picksthe
on best
the signals.
spectra
from the t*
inversions.
Green
vertical
delimit
the frequencies
that are
the inversion
In the spectrum
plots,
blue lines
represent
thelines
spectra,
red lines
represent noise,
andused
cyaninlines
show the for
bestt*.
fitting
spectra from the t* inversions. Green vertical lines delimit the frequencies that are used in the inversion for t*.
frequency for the P and S wave of each earthquake.
The
equation
wasPrearranged
the earthquake.
following
frequency
for the
and S waveinto
of each
[Stachnik et al., 2004]. Equation (2) was then
solved
for aetsingle
corner frequency
and was
moment
[Stachnik
al., 2004].
Equation (2)
then
the source and receiver. We argue in the Appendix that
integrating along the great circle path instead of the true ray
path is valid for the length scales in which we are interested.
[10] The perturbation in phase velocity, dc/c0(w), is expanded in spherical harmonics,
she
stud
And
COMO EVALUAMOS ATENUACIÓN
X X
dc
ðw; q; fÞ ¼
C ðwÞY ðq; fÞ;
ONDAS DE cSUPERFICIE
wh
ð4Þ k(r
lm
lm
sen
0
l¼0 m¼&l
B05317
DALTON AND EKSTRÖM: SURFACE
(Fig
where Ylm(q, f) are the fully normalized surface spherical obt
the effect
maximum
harmonics
degree l 2000].
and order
Selby and of
Woodhouse,
Here,m,weLtreat
on
c is this
freq
are the resu
degree
of the
phase
expansion,
and Cray
amplitude
using
an velocity
expression
from linearized
theory,
lm(w)
coefficients to be determined. The focusing depends linearly
[1
dcj0
dcj
1
onlnthe
AF ðwphase
Þ ¼ velocity,
ðwÞ þ Dand
ðwÞ we
þ write
cosec D
ith
2c0
2c0
2
Z D
sim
!
dc
Lc
• Se
l
usa que las ondas de
superficie disminuyen su
X& cosðD & 2fÞ' ðwÞdf; ð3Þ
sinðD & fÞX
sin f@
%
ln A ðwÞ ¼
C ðwÞF ; c
ð5Þ
amplitud con distancia, where D is the epicentral distance, f is the along-path
coordinate, q is the path-perpendicular coordinate, dc/c is
debido a la atenuación. where
F represents
theinimplementation
of equation
the relative
perturbation
surface wave phase
velocity, (3)
andin
0
i;j
F
Lc
l2
q
lm
i;j
lm
0
l¼0 m¼&l
i,j
lm
•
•
A(ω) = AS(ω) AI(ω)
AF(ω) AQ(ω)
0
spherical
pathvelocity
connecting
the ithatearthdcjD indicatefor
thethe
phase
perturbation
the
dcj0 and harmonics
quake
and
the
jth
receiver.
source and receiver, respectively [Dahlen and Tromp,
11] The
effect
of attenuation
on the modified
wave amplitude,
AQ,
[1998].
This
expression
is slightly
from the
is original
expressed
oneasprovided by Woodhouse and Wong [1986], as
wh
ave
var
it includes a term "with sensitivity
to the phase velocity# at the
Z
w
receiver. The wave amplitude
due&1 to focusing depends
A
dQ
ðw; q; fÞdsðq; fÞ ;
ð6Þ
Q ðwÞ ¼ exp &
primarily on the second
2U ðderivative
wÞ path of velocity perpendicular
to the ray path. Waves traveling through a low-velocity
wh
trough
are focused
and amplified,
and therespectively,
opposite is true
where
q and
f are latitude
and longitude,
U(w) dQ&
l
for propagation along a channel of fast velocity. Implicit in
is group velocity, and dQ&1(w, q, f) is the perturbation in
[1
equation (3) is the assumption of an infinite frequency wave
Figur
surface
wave
attenuation
away
from
the
value
predicted
by
atte
Dalton
al 2006
that does not deviate from
the et
great
circle path connecting
&1
funda
(w,
q,
f)
is
related
to
PREM.
Surface
wave
attenuation
Q
qua
the source and receiver. We argue in the Appendix
&1 that
shear
(r, ray
q, f) tion
theintegrating
Earth’s intrinsic
bulk
attenuation,
Qm true
along theshear
greatand
circle
path
instead of the
&1
study.
(S - source, I - receiver,
F- geometric spreading,
Q - attenuation)
plane. Figure 1, from Geller and Stein (1977), shows the spectra of the spheroidal
400
'
r'l'
'
_oS:
'1'
300
COMO EVALUAMOS ATENUACIÓN MODOS NORMALES
14_
÷2
200
I I
c
o7
o
-2 0+2
IO0
I
i
J
i
i
00180
00185
00190
Frequency,
cycles/rain
200
'
'
'
'
_oS3
o
+a
I00
-
5
~
•
•
Se usa que los modos normales disminuyen su
amplitud con tiempo, debido a la atenuación.
Para modos radiales podemos ajustar (“fit”) el
logaritmo de la amplitud, A(t), a una línea recta.
•
Difícil porque la división de los modos causa
pulsación, menos para los modos radiales que no
tienen división (“splitting”).
•
Q para el modo fundamental radial 0S0 es 5700
(!), 1S0 - 2000, 1S0 - 1200
]
'
'
'
-5.
'
I
'
'
I
/I
+i
0
A
II
+5
I llll
-2 0+2
-
ATTENUATION M E A S U R E M E N T S - - C H I L E A N AND ALASKAN EARTHQUAKES
C
i
i
i
i
I
I
i
i
i
i
i
1597
i
The solution for the
modes
and Geller, 1977) is obtained
0 0 2amplitudes
75
0 0of
2 8split
0
0 0 2(Stein
85
by transforming the spherical
expansion
F r eharmonic
quency,
c y c l e s / r a m of the excitation from the frame
of reference
of the source
into
geographic
singlet
amplitudes
FIG.
1. Split spheroidal
mode
spectra
for 0S2coordinates.
(top) and 0SaThe
(bottom)
excited
by the are
Chilean earthquake,
written so
there are separate
factors
for source
(latitude separation
and longitude),
as observed
onthat
a strainmeter
at Isabella,
California.
Thelocation
Eigenfrequency
is taken from Dahlen
source
fault frequency
geometry has
(strike,
and slip
direction),
location,
and peaks. Synthetic
(1968),
but depth,
the central
beendip,
chosen
to yield
a best receiver
fit with the
observed
relative
spectra for the
finite
of Kanamori and Cipar (1974) are given for each mode. The
the normalized
energy
of fault
each geometry
mode.
amplitudes
normalized
and plotted
with regular
spacing.
For a are
spheroidal
or torsional
multiplet
and for
a step function dislocation with
multiplets 0,92 and
by the Chilean earthquake, as observed on a strain0S2 0Ss excited
T=53.8 min,
meter (strike 38.4 W of N) at Isabella, California by Benioff et al. (1961). The singlet
pair with m = _+1 has much larger amplitudes than the rest of the o,92multiplet and,
similarly, 0S3-+2 stands out from its multiplet. We also show synthetic relative
spectral amplitudes computed for the finite fault and long-period precursor determined by Kanamori and Cipar (1974} from long-period surface waves. The spectral
amplitudes do not depend on the precise frequency separation, so for convenience
the theoretical amplitudes are plotted with regular spacing.
L
0
L
,
L
,
i
50
L
L
hr
i
,
I
I00
,
i
i
,
I
150
FIG. 2. Data and synthetics for 0S2. The top trace is filtered (tide-removed) data from the first 150 hr
of Isabella strain record of the Chilean earthquake. The lower trace is the synthetic seismogram,
including the effects of splitting. The synthetic was tapered and filtered in the same way as the data.
unit moment, the displacement or strain component, summing over modes with
angular order 1 and azimuthal order m, is given to zeroeth order by
MODELOS GLOBALES DE ATENUACIÓN PREM
•Q
en un modelo esférico:
• alto
en la corteza
• bajo
en el manto superior
• bajo
en el núcleo interno
MODELOS GLOBALES DE ATENUACIÓN (DE ONDAS SUPERFICIALES)
• Una
162
correlación
entre anomalías
en atenuación
(1/Q) y
velocidad.
B09303
C.A. Dalton, U.H. Faul / Lithos 120 (2010) 160–172
QRFSI12
S362ANI
100 km
-0.010
-0.005
DALTON ET AL.: GLOBAL UPPER-MANTLE ATTENUATION
0.000
0.005
dQ-1
B09303
0.010
-6
-4
-2
0
2
4
6
dv/v (%)
400 km
-0.005
0.000
0.005
dQ-1
-2
0
2
dv/v (%)
Fig. 1. Comparison
global
andGTR1
shear-velocity (right) models at 100-km and 400-km depth. The attenuation model QRFSI12 is plotted as the deviation awa
Figure 5. Regionally averaged attenuation
profiles forof the
sixshear-attenuation
tectonic regions (left)
of the
−1
1
regionalization scheme. (a) Results correspond
to the reference
model
1 ofat Figure
4. (b)1-D
Results
from the globally
averaged Q
value
each depth;
Q−
is 0.0126 and 0.00577 at 100 and 400 km, respectively. Isotropic velocity from model S362ANI (Kustowski et al., 2008
μ
μ
correspond to reference model 2 of Figure
4.
is shown
here expanded in spherical harmonics up to degree 12.
decrease in attenuation down to !450 km. Strong radial two regions, although the absolute Q"1 values of our study
gradients in attenuation are penalized by the regularization
slightly
lower than in the earlier
At 150 km,arethe
old-continental
pointswork.
maintain a shallower-than-
sufficient for oceanic regions at 100-km depth and continenta
glob
(sph
from
0.05
be a
the
0.04
poin
loca
0.03
and
A
0.02
exp
than
0.01
regi
resp
0
of t
800 900 1000 1100 1200 1300 1400 1500 1600
C.A. Dalton, U.H. Faul / Lithos 120 (2010) 160–172
con
oC)
Temperature (
gen
(a) 5
poin
(c) 0.08anomalously high-velocity points are generally associated
oceanic crust older than 70 Myr and old-continental areas. adjo
At 150 km (Fig. 5), almost all of the oceanic points fall withi
con
0.07
loca
experimental range, with only ∼25% of the young-oceanic
p
4.8
characterized by lower-than-experimental velocities and/orocea
lo
0.06
loca
than-experimental attenuation. In contrast, 72% of the old-contin
equ
4.6
points have larger velocity and/or higher attenuation than
cenw
0.05
experimental range. This trend persists at 200-km depth,
tion
∼50% of old-continental points fall to the right of the experim
4.4
A
0.04
range while nearly all of the oceanic points are located within
beca
experimental range. At 250 km, a handful of old-continental
p
you
0.03
(15%) are located to the right of the range, but the vast majority
4.2
200
points agree with the mineral-physics model.
Paci
0.02 At 100 km, outliers located to the left of the experimental rang
Ridg
almost
all
located
along
the
global
mid-ocean-ridge
system.
Th
4
exp
0.01
especially true for the East Pacific Rise, the Pacific Antarctic Ridge, an
Southeast Indian Ridge. Areas of the Mid-Atlantic Ridge near the A
0
hotspot
and centered on the equator also exhibit anomalously
3.8
3.8
4
4.2
4.4
4.6
4.8
800 900 1000 1100 1200 1300 1400 1500 1600
velocity
and/or
low
attenuation.
Small clusters
of5 these points ca
C.A. Dalton, U.H. Faul / Lithos 120 (2010)
160–172
o
Fig. 2
Velocity (km/s)
Temperature ( C)
found in the northeastern Pacific and centered on the Red Sea.100Ben
the
oceans,
outliers
located
to
the
right
of
the
experimental
betw
100 km
(b) 150 km
(b)(a)
0.08
comprise much of the western Atlantic, offshore Africa, and the and
nor
0.025
0.025
value
data
central Pacific. Some of the anomalously high-velocity/high-attenu
curv
1mm, V=12
points that are adjacent to continental areas could result from sme
0.07
1cm, V=12
the e
5cm, V=12
of the continental properties into the oceanic regions, given tha
0.02
0.02
1mm, V=20
global models used for this analysis have a relatively coarse resol
0.06
1cm, V=20
(spherical-harmonic degree 12). However, many of the outliers a
5cm, V=20
oceans < 70 my
from any continental region (e.g., the central Pacific) and are not lik
0.05
oceans > 70 my
0.015
0.015
be artefacts. Within the old continents, 84% of the points are locat
old continents
the right of the experimental range. The remaining 16% of old-contin
0.04
points that fall within or to the left of the experiments are gen
located adjacent to tectonically younger provinces, as is the case for E
0.01
0.01
0.03
and Sudan, which are adjacent to the Red Sea, and for southeast Ch
At 150 km, almost all outliers located to the right of
0.02
experimental range (i.e., higher velocity and/or higher attenu
0.005
0.005
than the mineral-physics model) are found within old-contin
0.01
regions: only 1.3% and 8.1% of young and old oceanic reg
respectively, fall to the right of the experimental trend, whereas
0 0
0 the old-continental points do. As is the case for 100 km, the
of
4.2
4.3
4.4
4.5
4.6
4.7
4.8
800 900 4.210004.311004.412004.513004.614004.715004.81600
continental
pointsVelocity
that fall
within the experimental range
oC)
Velocity ((km/s)
(km/s)
Temperature
generally adjacent to tectonically younger areas. The few oc
points
200 km
(d) 250
kmwith values to the right of the experimental range almo
(c)(c)
0.08
0.025
0.025
adjoin an old continent, perhaps indicating some smearing o
continental seismic properties into the nearby ocean basins. Ou
0.07
located to the left of the experimental trends are found most
oceanic regions b70 Myr. As with 100 km, many of these point
0.02
0.02
0.06
located near the East Pacific Rise, Pacific Antarctic Ridge,
equatorial Mid-Atlantic Ridge. Iceland, the eastern Pacific, and
Attenuation (1/Q)
0.06
•
Hay diferencia entre océanos
jóvenes, océanos viejos y
continentes viejos.
Attenuation (1/Q)
(a) comparación entre los
experimentos y los modelos
globales.
164
Attenuation (1/Q)
•
Velocity (km/s)
Experimentos con olivino seco
en un rango de frecuencias,
temperaturas y tamaño de
granos (a-c)
Attenuation (1/Q)
•
Attenuation (1/Q)
MODELOS GLOBALES DE ATENUACIÓN
Dalton & Faul 2010
MODELOS GLOBALES DE
ATENUACIÓN
C.A. Dalton, U.H. Faul / Lithos 120 (2010) 160–172
(b) after dispersion correction
•
Océanos: “melt squirt”, no “grain
boundary sliding”.
•
Continentes: composición diferente,
empobrecido hasta 200km de
profundidad.
0.025
0.025
0.02
0.02
Attenuation (1/Q)
Los puntos afuera del rango de los
experimentos están en las dorsales
oceánicas jóvenes y en las partes más
viejos de la litósfera.
Attenuation (1/Q)
(a) 100 km
•
0.015
0.01
0.005
0
165
0.015
0.01
0.005
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
Velocity (km/s)
0
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
Velocity (km/s)
Fig. 4. (a) Identifying seismological shear velocity and attenuation points that fall inside and outside of the range defined by the mineral-physics model of Faul and Jackson (2005).
The experimental range is shown by the green polygon and includes the velocity and attenuation values predicted for a range of grain sizes and activation volumes (see Fig. 3). Grey
points are located within the experimental range, red points fall to the left of the range and blue points to the right. The map shows the geographical location of all points. Triangles
indicate old-continental points, squares indicate oceanic regions b 70 Myr, and circles show oceanic regions N70 Myr. For a depth of 100 km. (b) Estimated changes in velocity as a
result of corrections for laterally variable anelastic dispersion. Coloring of points and position of polygon are same as in a. Period = 75 s assumed for the calculations.
discussion of the northeastern and central Pacific and old oceanic
seafloor to a subsequent publication.
We recognize that the mineral-physics model may not be appropriate for the upper mantle, given the conditions of the experiments from
which it was derived (e.g., Jackson et al., 2002). For example,
experimental grain size and pressure are too small, requiring extrap-
could be studied. Also, the experiments were performed only on
samples of pure olivine; effects related to the coexistence of olivine with
other mineral phases (e.g., Sundberg and Cooper, 2007) and majorelement compositional variations were not considered. However, these
experiments represent the best quantitative laboratory constraints
currently available and offer an opportunity for direct comparison with
Dalton & Faul 2010
MODELOS REGIONALES DE ATENUACIÓN UNA ZONA DE SUBDUCCIÓN
G
Geochemistry
Geophysics
Geosystems
•
Xenolitos muestran un
manto con más agua, y
más fusión.
•
Restringido - Q-1 no
negativo.
•
No toman en cuenta
multitrayectoria.
3
rychert et al.: attenuation beneath nicaragua and costa rica
10.1029/2008GC002040
Figure 1. Map of the study region. Shading indicates topography. The black line with triangles marks the trench.
Red triangles mark volcanoes. Yellow circles mark the Tomography Under Costa Rica and Nicaragua (TUCAN)
broadband seismic array. White and black inverted triangles mark permanent stations. White circles mark
Seismogenic Zone Experiment (SEIZE) array. Slab contours are shown with blue line. Black dashed line traces the
Quesada Sharp Contortion (QSC) [Protti et al., 1995]. Purple arrow denotes direction of increases in geochemical
indicators of water content and degree and depth of melting [Plank and Langmuir, 1988; 1993; Leeman et al., 1994;
Reagan et al., 1994; Roggensack et al., 1997; Patino et al., 2000; Carr et al., 2003; Kelley et al., 2006; Sadofsky et
al., 2008].
Rychert et al 2008
MODELOS REGIONALES
DE ATENUACIÓN UNA ZONA DE
SUBDUCCIÓN
•
Más atenuación en Nicaragua.
•
La fusión en Nicaragua ocurre en
condiciones más hidratadas, y en un área
más grande y a profundidades más
grandes.
•
Después de corregir los valores de
atenuación por contenido de agua,
obtienen temperaturas similares entre las
dos regiones, o, la atenuación abajo de
Nicaragua es dominada por el contenido
de agua, no por la temperatura.
Figure 5. Attenuation results. Results of (a – b) unconstrained inversion of P wave data for P attenuation, (c – d)
unconstrained inversion of S data for shear attenuation, (e –f) unconstrained joint inversion of P and S wave data for
bulk and shear attenuation (shear attenuation is shown), (g– h) constrained inversion of P and S wave data for shear
and bulk attenuation (shear attenuation is shown), and (i– j) unconstrained inversion of P and S wave data for shear
and bulk attenuation where damping parameters permit bulk attenuation in the crust and mantle (bulk attenuation is
shown). Constrained inversions enforce nonnegativity in QS!1. Green line represents resolution = 0.4. Yellow circles
indicate seismicity. Red inverted triangles mark station locations. Green triangles mark volcanoes. The projection
width is 50 km on either side of the cross-section line.
Figure 5. Attenuation 11
results.
of 26 Results of (a – b) unconstrained inversion of P wave data for P attenuation, (c – d)
unconstrained inversion of S data for shear attenuation, (e –f) unconstrained joint inversion of P and S wave data for
ATENUACIÓN INTRÍNSECA
¿Qué es atenuación?
¿Cómo describimos la atenuación?
¿Cuáles son las causas de la atenuación?
¿Cómo podemos estimar la atenuación?
¿Dónde en la tierra vemos la atenuación?