Supplementary Information on S receiver function technique to
"Rejuvenation of the lithosphere by the Hawaiian plume"
Xueqing Li, Rainer Kind, Xiaohui Yuan, Ingo Wölbern & Winfried Hanka
While P receiver function analysis has become a routine method for studying the Moho and
upper mantle boundaries, observations of S-to-P conversions (Sp) are still much more rare.
An Sp conversion is generated when an incoming S wave penetrates a discontinuity beneath a
seismic station. The Sp converted wave arrives at the station earlier than the main incoming
phase S. The Sp converted phases at a shallow discontinuity such as the lithosphereasthenosphere boundary are best identified at epicentral distances of 55-85°. At distances
larger than 85°, SKS arrivals are earlier than S (or Sdiff). Precursors of SKS may also be
studied like S precursors. At present there are not enough SKS data available at stations
MAUI and POHA.
The S receiver function analysis is practically identical to the P receiver function analysis. To
separate the weak Sp from S, we rotate the Z-, N- and E-components into the local P, SV and
SH system. After deconvolution of the P-component by the SV-component, we obtained the S
receiver functions. The coefficients of Sp and Ps conversions have different signs. To make
both datasets directly comparable, we reversed the polarity of S receiver function amplitudes.
Like P receiver functions, positive (negative) amplitudes indicate velocity contrasts with
velocity increasing (decreasing) downwards. The time difference between Sp and S can be
measured in a single trace, if the data quality is good enough. Otherwise we use summation.
The time difference depends on the ray parameter (distance moveout), which can be corrected
for by using a reference ray parameter (moveout correction). To enable easy comparison of
Sp and Ps times at a station, we applied the same reference slowness of 6.4 s deg-1 to both
types of data. Times may be converted into the depth domain using a reference velocity model
(e.g. the IASP91).
The ray paths and conversion (piercing) points of P and S receiver functions differ
significantly (see Figs. S1 and S2). Piercing points of S receiver functions occur much further
away from the stations. S receiver functions are usually much noisier than P receiver
functions, because S waves are later arrivals. However, Sp data have one big advantage in
that they are free of disturbing multiples. Sp conversions are precursors of S and multiples
arrive later than S. This advantage is very important for studying the lithosphere1
asthenosphere boundary, since this boundary is almost invisible in P receiver functions due to
crustal multiples arriving at nearly the same time. Moveout corrected Sp and Ps data of each
Hawaiian station are stacked and shown in Fig. S3. The S receiver functions are plotted in
reversed time to make them directly comparable with the P receiver functions. A closely
correlated negative phase (velocity reduction downwards) is marked in the S and P receiver
function traces at each station and is interpreted as the lithosphere-asthenosphere boundary.
Because of different locations of the piercing points and interference with the crustal
multiples in the P receiver functions, the marked phases are not completely identical.Figures
Fig. S1: Ray paths of P and S receiver functions.
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Fig. S2: Location map of piercing points of P (blue circles) and S (red crosses) receiver
functions at 100 km depth for each of the three broadband stations, showing the sampling
regions of the two kinds of data.
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Fig. S3: Summation of P (blue) and S (red) receiver functions at each station. Moveout
corrections are made for a reference slowness of 6.4 s deg-1. Time scale, as well as the polarity
of amplitudes of the S receiver functions is reversed in order to be directly comparable with
the P receiver functions. The large amplitudes of S receiver functions at time 0 s are
remaining S wave energy resulted from imperfect coordinate rotation. The S waves, as well as
the multiple phases, appear at later time and therefore do not interfere the Sp conversions.
Arrows mark the average time of conversion from the lithosphere-asthenosphere boundary.
Both types of receiver function agree reasonably well, considering the different locations of
their piercing points.
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