many collaborators: vadim levin (rutgers) nikolai shapiro (univ colorado) michael ritzwoller (univ...

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 lateCenozoic plate tectonic history of Kamchatka:

Former ideas e.g. step-back of subduction

Triple-junction migration hypothesis

Data needed -- volcanic history of Kamchatkavolcanics, focussed on igneous rocks since 30 Ma

Subduction step-back would imply a synchronouschange in volcanism in Sredinny range.Triple junction migration predict an age progressionas the coastal volcanoes strart, and Sredinny volcanoes lose steam

Seismological evidence for mantle strain around

KamchatkaMethod 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 trench-parallel 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

Fast axis

Need 2 anisotropic layers to fit T component

data blue, synthetic red

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

flow-like regime (in terms of anisotropic indicators)

• Others do not, especially the northern edge of the Pacific slab

Final word• By using multiple lines of evidence we stand a good chance of constraining anisotropic properties at depth.

• We can still be wrong, of course…..

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 long-period 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

no

no

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

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”