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

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.

Local S waves Shear waves from events within the slab recorded by a variety of seismic stations in Kamchatka between 1996 and 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: 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 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”