Transcript ~2.6 MB

Many Collaborators:
Vadim Levin (Rutgers)
Nikolai Shapiro (Univ Colorado)
Michael Ritzwoller (Univ Colorado)
Evgenii Gordeev (EMSD, Petropavlovsk)
Jonathan Lees (Univ N. Carolina)
Valerie Peyton (USGS, Albuquerque)
Mark Brandon (Yale)
Alexei Ozerov (IOV, Petropavlovsk)
Thing to remember:
Role of slab detachment in the terrane
accretion process
Next: why are there two volcanic arcs in
Kamchatka??
So there are testable hypotheses for the late
Cenozoic plate tectonic history of Kamchatka:
Former ideas e.g. step-back of subduction
Triple-junction migration hypothesis
Data needed -- volcanic history of Kamchatka
volcanics, focussed on igneous rocks since 30 Ma
Subduction step-back would imply a synchronous
change in volcanism in Sredinny range.
Triple junction migration predict an age progression
as the coastal volcanoes strart, and Sredinny volcanoes
lose steam
Seismological evidence for
mantle strain around Kamchatka
Method 1: SKS splitting
Main Uncertainty vertical position of the
anisotropic material
SKS splitting results
Same SKS phase observed at two nearby
stations yields different fast directions.
Fast direction of shear
wave speed changes at
the northern edge of
Pacific slab
Interpretation, together with SKS results
•
•
Extra evidence for a change in fabric north of PPK (see clear qLove from the
north)
Weak (if any) anisotropic gradient seaward of the trench (no qLove from NE
and SW)
PET
yes
no
no
Summary:
mantle flow beneath the subducting slab (SKS and qLove)
yes
SKS and qLove data
constrain deeper levels of
fabric, present evidence
for
sub-slab
trenchparallel flow of mantle
material, and for a rapid
reorientation of this flow
at the northern edge of
the Pacific plate.
Method 3: local S wave splitting
Sensitive only to anisotropy above the
source;
Range of source depths offers a way of
constraining depth dependence of
anisotropic properties
Problems:
An integral measure, multiple
observations are needed to
discriminate between vertical, lateral
and temporal variations.
Initial polarization of the S wave is not
known -> uncertain meaning of “null”
splitting.
Shear waves from events within the
slab recorded by a variety of seismic
stations in Kamchatka between 1996
and 2001.
Events selected on the basis of the
catalog compiled by the KEMSD.
Selection criteria:
• relation of depth and distance from the
station - incoming ray steeper then 35°
from vertical;
• the quality of the hypocentral location formal errors < 10 km for both depth
and horizontal position.
Final selection via visual inspection.
Our final dataset includes
~700 S phases.
Result of S wave splitting measurements
Observations are plotted at horizontal positions of mid-points
along rays connecting sources and receivers, and color-coded
by depth: <30 km; 30 - 100 km; > 100 km.
Result of S wave splitting
measurements – averaged
 Rapid reorientation of fast
direction with distance from
volcanic front;
 Fast directions near the northern
edge of the Pacific slab trend
neither towards the trench nor
parallel to it, rather – towards the
“open” side edge of the
subduction zone.
Local S and SKS waves have different splitting patterns
Method 4: Receiver Functions
Sensitive to gradients in
anisotropy within the upper mantle
and the crust.
Offers good vertical resolution.
Restricted to sites with large
volumes of teleseismic data
collected.
Receiver functions example: Esso
data blue, synthetic
red
Fast axis
Need 2 anisotropic
layers to fit T
component
Results: Map of fast anisotropic direction for the uppermost mantle
–Evidence of anisotropy at
crust-mantle transition
throughout the peninsula;
–Evidence for multiple
layers of anisotropy
–Caveat: use of “fast” axes
rather then “slow” in
forward modeling is a
choice not constrained by
observations.
Comparison of RF, local S and SKS results
Teleseismic SKS likely sample a
different anisotropic volume
Summary:
mantle wedge above the subducting slab (local S and RF)
• Highly complex laterally
• Some regions display corner flowlike regime (in terms of anisotropic
indicators)
• Others do not, especially the
northern edge of the Pacific slab
• By using multiple lines of
evidence we stand a
good chance of
constraining anisotropic
properties at depth.
• We can still be wrong, of
course…..
Final word
Evening rush hour, Central Kamchatka
Method 2: quasi-Love waves
Quasi-Love Wave:
A surface wave of the Rayleigh
type (SV polarized) observed
within the Love wave time window.
Most efficiently generated through
mode conversion of the longperiod fundamental Love surface
wave at strong lateral gradients in
upper mantle anisotropy.
Relative timing of the “parent” Love wave and the “daughter” quasi-Love wave
constrains distance to the “scatterer”. Long wavelength limits lateral resolution
of features detected by presence of this phase.
Observations and non-observations of quasi-Love waves
yes
Quasi-Love wave is found
conclusively only for a northern
approach to the GSN station
PET (path 3). Modest time
separation between the parent
Love wave and the daughter
qLove wave imply the region of
conversion within 1000 km from
the station.
PET
no
no
Examples of observed S waves
Range of shear-wave splitting
delays from 0 to 1 s was found in
data from both broad-band and
short-period stations
“NULLS”