University of Florida
Department of Geological Sciences

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GLY 2010

GLY 6932


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R. M. Russo
Assistant Professor
223 Williamson Hall
phone: 2-6766

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MWF: 11:40-12:40
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Mantle Flow Beneath Pacific Lithosphere Subducted at Kamchatka

If Pacific oceanic lithosphere slides over an asthenosphere that largely decouples it from underlying mantle circulation, then what happens to the asthenospheric decoupling layer at subduction zones?  Co-Author Jonathan Lees and I have used earthquakes that occurred within the subducted Pacific plate to characterize the upper mantle flow field beneath the slab.  S waves from these earthquakes leave the slab and become split into orthogonally-polarized fast and slow quasi-S waves in the seismically anisotropic upper mantle region below the slab.  They then cross the isotropic region of the lower mantle and become split again by anisotropy beneath recording stations in Eurasia and North America.  We correct for the known shear wave splitting at these stations to isolate the splitting from the sub-slab region. 

Map of Kamchatka study region

           Figure 1, above, shows the topography and bathymetry of the Kamchatka
           Peninsula and environs.  Pacific oceanic lithosphere subducts beneath the
           continental landmass along the Kurile-Kamchatka Trench.  Note, though,
           that the Trench ends abruptly at its northeastern intersection with the Aleutian
           Trench.  The latter is a strike-slip fault which cuts through the entire Pacific
           plate lithosphere, so the subducted slab actually has a lateral termination.

Seismicity of Kamchatka region

           Figure 2.  Seismicity of the Kamchatka subduction zone, 1963-1995 NEIC,
           plotted atop topography and free-air marine gravity anomalies [Sandwell and
           Smith, 1997].  Earthquake depths as per key, lower right.  Isodepth contours
           of seismicity constructed by us also shown, depths as labeled.  Heavy lines
           mark the Kamchatka and Aleutians morphological trenches.

Peyton et al. (2002) station splitting

          Figure 3.  Results of shear-wave splitting measurements at the SEKS network
          stations [Peyton et al., 2001].  Squares are station locations.   Bars parallel to
          fast polarization directions of SK(K)S and PKS phases, length of bars scaled
          according to splitting delay times, see key uppe left.  Note 90° change in
          splitting fast trends from trench-parallel along most of Kamchatka Peninsula
          to trench-normal between 55-56°N, interpreted by Peyton et al. [2001] as
          indicating flow around the edge of the Pacific lithosphere subducting beneath

 Schematic splitting correction

   Figure 4.  Schematic of source-receiver ray paths used in this study.  Earthquakes in
   the Kamchatka slab are recorded at distant stations where upper mantle shear wave
   splitting parameters are known.  Corrections for shear-wave splitting on the receiver
   side of the ray paths allow isolation of splitting engendered in the region beneath the
   Kamchatka slab.  Note that the upper mantle wedge above the slab is not sampled at
   all by these waves.

              Shear wave splitting measurement:  seismograms

              Figure 5a.  Seismogram of S wave from event 97136 recorded at station KEV,
              Finland.  S wave used in our measurement (gray boxed areas) is clearly isolated from
              expected arrival times of other shear phases, marked by vertical dashed lines.   Top
              two traces rotated into calculated fast and slow reference frame.  Bottom two traces
              show effect of linearization via energy minimization of component corresponding to
              minimum eigenvalue of the polarization matrix.

                        Shear wave splitting merasurment:  particle motion

                        Figure 5b.  Waveform coherence for split \fIS\fR of event 97136 shown
                        in Fig. 5a.  Good correspondence of fast and slow waves shown in
                        fast-slow frame (top left) and with delay corrected (top right).  Elliptical
                        particle motion diagnostic of shear-wave splitting seen in bottom left
                        plot, corresponding to waves shown in top left.  Linear particle motion
                        after correction for splitting shown in bottom right.

                                      Shear wave splitting measurement:  grid search

                       Figure 5c.  Result of shear-wave splitting parameter grid search over
                      180° azimuth and 0-4 s delay time.  Contours of energy on the
                      seismogram corresponding to the minimum eigenvalue component of
                      the polarization matrix shown.  Star indicates best splitting parameters,
                      fast polarization azimuth and delay time.

Source-side splitting results

           Figure 6.  Shear-wave splitting results (red bars) for our study.  Results of
           Peyton et al. [2001] also shown (blue bars) for comparison.  Note the delay time
           scales for the two data sets are different:  for source-side S wave splitting key
           is at bottom right; for SK(K)S splitting key is at top left.

Rose diagram

              Figure 7.  Polar histogram of source-side shear-wave splitting fast trends.
              Note predominant trends are parallel to local Kamchatka trench strike (NE)
              and E-W.

Source side null measurements

        Figure 8.  Null splitting results from our study.  These represent fast anisotropy
        in one of either two orthogonal directions, as shown, or possibly no anisotropy
        is present.

3-D flow diagram

                        Figure 9.  Schematic block diagram of upper mantle flow beneath Pacific
                        lithosphere subducting at Kamchatka.  Pacific plate lithosphere shown in
                        light blue.  Orange arrow shows upper mantle flow around the slab edge.
                        View is from the NE looking SW.

            Schematic showing limit of flow field

                  Figure 10.  Vertical section across Kamchatka slab edge from 53.5°N,
                  168.0°E to 57.5°N, 159.0°E.  Seismicity within 100 km of the section
                  line plotted as small black squares.  Large star is location of event
                  98027, and large triangle marks location of SEKS network station on
                  Bering Island.  SK(K)S and S waves ray paths shown schematically
                  as dashed lines from station and 98027 hypocenter.  Splitting along
                  these paths is not trench parallel, as it is beneath Kamchatka proper,
                  hence inferred anisotropy differs between the locales:  double headed
                  arrows parallel to Pacific plate motion indicate anisotropy within the
                  unsubducted Pacific asthenospheric channel; vector arrow heads
                  (bulls-eyes) indicate trench-parallel anisotropy and flow beneath Kam-
                  chatka.  The width of the trench-parallel flow field is limited by the Bering
                  Island and 98027 measurements.  Assuming the asthenospheric channel
                  and its underlying mesosphere maintain their layered form into the sub-
                  duction zone, the maximum width of the trench-parallel flow field is around
                 180 km.

This Project has been supported by the Geophysics Program of the U.S. National Science Foundation  We also gratefully acknowledge data and support from the IRIS Consortium.

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