Abstract. Teleseismic data recorded by temporary and permanent stations located in the Northern Victoria Land region are analysed in order to identify the presence and location of seismic anisotropy. We work on data recorded by 24 temporary seismographic stations deployed between 1993 and 2000 in different zones of the Northern Victoria Land, and by the permanent very broad-band stations TNV located near the Italian Base M. Zucchelli. The temporary networks monitored an area extending from Terra Nova Bay towards the South beyond the David Glacier and up to the Indian Ocean northward. To better constrain our study, we also provide an analysis of data recorded by TNV in the same period of time and we take into account also SKS shear wave splitting measurements performed by Barruol and Hoffman (1999) on data recorded by DRV. This study, to be considered as preliminary, reveals the presence of seismic anisotropy below the study region, with a mainly NW-SE fast velocity direction below the Terra Nova Bay area and rather large delay times, that mean a deep rooted anisotropic layer

Danesi, S.

Margheriti, L.

Pondrelli, S.

Earth-prints Repository

3.8
From: Fütterer DK, Damaske D, Kleinschmidt G, Miller H, Tessensohn F (eds) (2005) Antarctica:
Contributions to global earth sciences. Springer-Verlag, Berlin Heidelberg New York, pp 153–160
Abstract. Teleseismic data recorded by temporary and permanent
stations located in the Northern Victoria Land region are analysed
in order to identify the presence and location of seismic anisotropy.
We work on data recorded by 24 temporary seismographic stations
deployed between 1993 and 2000 in different zones of the North-
ern Victoria Land, and by the permanent very broad-band stations
TNV located near the Italian Base “M. Zucchelli”.  The temporary net-
works monitored an area extending from Terra Nova Bay towards
the South beyond the David Glacier and up to the Indian Ocean
northward. To better constrain our study, we also provide an analy-
sis of data recorded by TNV in the same period of time and we take
into account also SKS shear wave splitting measurements per-
formed by Barruol and Hoffman (1999) on data recorded by DRV.
This study, to be considered as preliminary, reveals the presence of
seismic anisotropy below the study region, with a mainly NW-SE
fast velocity direction below the Terra Nova Bay area and rather large
delay times, that mean a deep rooted anisotropic layer.
Introduction
The study region, the northern Victoria Land, is mainly
characterized by the presence of the Transantarctic Moun-
tains, that border the Ross Sea all along this region. From
the tectonic point of view, this high-elevated belt, extend-
ing for more than 2 500 km, is considered an asymmetric
rift shoulder segmented by several transverse fault sys-
tems (Fig. 3.8-1; Behrent et al. 1991; Salvini et al. 1997;
Wilson 1999). The origin of the West Antarctic Rift, mainly
constitued by the Ross Embayment, is still object of de-
bate. The first hypothesis was to relate it to an active plume
centered below Marie Byrd Land (Behrendt et al. 1991),
but recently some investigations revealed a complex Ceno-
zoic geodynamic, mainly due to the activation of intraplate
right-lateral strike-slip structures, inducing a strong ob-
lique component in the rifting process. The examination
of these major tectonic structures may support a
transtension-related source for the extension which de-
signed the Ross Sea Embayment and the volcanism re-
lated to it (Salvini et al. 1997; Rocchi et al. 2002).
The transition from the Ross Sea extensional basin to
the Transantarctic Mountains is abrupt, with a Moho de-
tected at less than 20 km of depth in the Ross Sea and
increasing up to more than 40 km of depth beneath the
mountain chain (Di Bona et al. 1997 and references
therein; Pondrelli et al. 1997; Bannister et al. 2003). This
sharp variation marks the presence of a strongly hetero-
geneous crustal and lithospheric structure. Surface wave
tomography as well clearly indicates that the northern
Victoria Land region is on the boundary of the East Ant-
arctica craton and the West Antarctic Rift System
(Ritzwoller et al. 2003; Danesi and Morelli 2001). A dra-
Chapter 3.8
Seismic Anisotropy beneath Northern Victoria Land
from SKS Splitting Analysis
Silvia Pondrelli1  ·  Lucia Margheriti
2
  ·  Stefania Danesi
2
1 Istituto Nazionale di Geofisica e Vulcanologia, Via D. Creti 12, 40128 Bologna, Italy, <pondrelli@bo.ingv.it>
2 Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy
Fig. 3.8-1. Tectonic sketch map of Victoria Land (modified from
Salvini et al. 1997). Legend 1, 2, 3 …???
154
matic discontinuity in shear wave velocity pattern (see
Fig. 4 in Morelli and Danesi, 2004) marks the limit be-
tween the Archean shield and the Ross Sea lithosphere,
the former showing deep cold continental roots down to
250–300 km (Morelli and Danesi 2004), the latter anoma-
lously warm and stretched. The depth to which seismic
tomography can reliably image the Antarctic region is
however hampered to 350–400 km, which is not enough
to undoubtedly confirm or disprove the presence of a
mantle plume head beneath West Antarctica.
Shear wave splitting is generally considered as caused
by lattice preferred orientation of anisotropic minerals of
the mantle, as olivine (Vinnik et al. 1989; Silver 1996; Sav-
age 1999). Measurements from teleseismic SKS phases are
a powerfull instrument to constrain the strain pattern in
the mantle, extending the geological analysis at depth and
helping in the interpretation of lithosphere kinematics.
When shear-waves travel in an anisotropic material, they
split into two polarized waves travelling at different veloci-
ties. Polarization direction φ  and time delay δt between
these two phases characterise the anisotropy. To detect the
presence of seismic anisotropy beneath Northern Victoria
Land and to study its relation with the geodynamic of this
region, we analyze seismographic data recorded during tem-
porary deployments and by TNV; moreover, we take into
account the results of SKS measurements obtained by
Barruol and Hoffman (1999) from DRV data records.
Sations and Dataset
Since 1993 several temporary geophysical campaigns have
been performed in the Terra Nova Bay region (Cimini et al.
1995; Pondrelli et al. 1997; Della Vedova et al. 1997). On
the total we have data to analyze from 24 temporary sta-
tions (Table 3.8-1). Most of them were located in the area
Silvia Pondrelli  ·  Lucia Margheriti  ·  Stefania Danesi
155Chapter 3.8  ·  Seismic Anisotropy beneath Northern Victoria Land from SKS Splitting Analysis
156
around the Italian M. Zucchelli Station and had multiple
reoccupations (up to five campaigns, from 1993 to 2000),
recording the larger amount of data. On the contrary,
northernmost and southernmost sites, respectively related
to the last campaign, in 2000, and to the ACRUP1 experi-
ment performed in 1994 (Della Vedova et al. 1997), are
those for which only one occupation was done and con-
sequently scarce data were recorded. In the region, the
permanent Italian very broad-band seismographic sta-
tion TNV has been recording continuously since 1989 to
present and data availability is rather large.
We selected from NEIS Catalog (http://www.neic.cr.usgs.
gov/neis/epic/epic.html) all earthquakes with a magni-
tude greater than 5.5 occurred during the recording times
(from 1993 to 2000), located at a distance between 85° and
120° from our seismographic stations, to collect SKS
phases of sufficient energy. The collected dataset includes
more than 100 teleseisms, and more than a half of them
gave good results (Table 3.8-2). However, only for TNV
permanent station the azimuthal coverage was good, while
for the other sites the NW and SE quadrants only were
well sampled (Table 3.8-2).
Analysis and Discussion
The analysis performed here is done on teleseismic
SKS phases, which travel as an S phase in the crust and
mantle, as a P phase in the liquid core and convert to an
S phase at the core-mantle boundary at the receiver side,
polarized in the vertical plane of propagation. If SKS
phases travel across an isotropic medium, all the energy
propagates on the radial component only. In practice, al-
most all our data give splitted SKS phases, pointing out
that shear waves encountered anisotropic material
(Fig. 3.8-2, upper panel). The fast velocity direction φ ,
measured clockwise from the north, corresponds to the
direction along which the strain aligns highly anisotro-
pic cristals in the mantle; the delay time δ t, measured
between the fast and the slow components, is a quantifi-
cation of the thickness of the anisotropic layer transversed
by SKS phases. To determine the fast velocity direction ϕ
and the delay time δ t, we use the method of Silver and
Chan (1991), that assumes that shear waves traverse a
single homogeneous anisotropic layer. The method is
Fig. 3.8-2.
Example of performed analysis;
event occurred in Japan on
November 15, 1997 and was
recorded by NAN9 station.
Upper panel: Radial and trans-
verse components of seismo-
graphic recordings before and
after the removal of the effect of
the anisotropy. Lower left panels:
Selected SKS phase (above) and
its particle motion (below). Fast
(dotted line) and slow (continu-
ous line) phases are superim-
posed by the removal of the
anisotropy effect. Lower right
panel: Contour plot of the mini-
mized energy on the corrected
transverse component; the star is
the minimum value, first contour
is the 95% confidence region
Silvia Pondrelli  ·  Lucia Margheriti  ·  Stefania Danesi
157
based on a grid search over the possible splitting param-
eters space to find the pair of ϕ  and δ t that, when used to
remove the anisotropy effect, most successfully removes
its (Silver and Chan 1988). This is done by minimizing
the energy on the reconstructed transverse component
(upper part of Fig. 3.8-2, indicated as Corrected Radial
and Transverse components). The error bounds for the
estimated splitting parameters are obtained through an
F-test analysis, as described by Silver and Chan (1991).
When the absence of splitting is detected on the analysed
waveform we have a “null” measurement, that doesn’t nec-
essarily mean that anisotropy is absent. In fact, a “null”
from a single measurement also occur when the initial
polarization of the shear wave is parallel to the fast or
slow polarization direction of the anisotropic media. For
this reason null measurements are characterized by the
event back-azimuth (Table 3.8-2).
Results of our analysis reveal the presence of seismic
anisotropy below the study region (Fig.3.8-3 and 3.8-4),
with different directions moving from the northern to the
southern part. Below DRV (Dumont D’Urville, French
base) the anisotropy shows an E-W average direction
(Barruol and Hoffman 1999). Going eastward, the few data
available at stations BT01 to BT08 (deployed only during
the 1999/2000 campaign) give a couple of N-S directions.
The scarcity of data do not allow any large scale interpre-
tation. Going southward, around the Terra Nova Bay area,
our results are better constrained due to the larger amount
of data recorded by multiple deployed temporary stations
and by the TNV permanent station. In Fig. 3.8-4 data are
mapped at the piercing point at 150 km of depth, to iden-
tify the presence of different fast velocity direction re-
spect to different back-azimuths. It is evident that the NE-
SW fast direction dominates, with also null measurements
in agreement. Only few data show a different pattern, as
the NW-SE trend obtained for a pair of events recorded
at MEL9 or the E-W trend shown by another pair of event
recorded at ESK9 (Table 3.8-2). Delay times are 1.6 s on
average, and some examples show larger values with re-
spect to world wide evaluation, indicating therefore that
the anisotropy layer which splits SKS is deeply rooted. The
NE-SW dominant fast velocity direction is in agreement
with the Transantarctic Mountains trend, an usual obser-
vation along mountain chains, where fast direction is in
general parallel to the belt axis (Savage 1999). We there-
fore consider that this anisotropy is related to the strain
induced in the mantle by the formation of the chain; con-
sidering the deep rooted structure of the mountains belt,
this hypothesis is also supported by the rather large val-
ues for the delay times. Concerning data reporting EW
and NW-SE fast velocity directions, we can suggest two
possible explanations: the pattern could be due to the ex-
tensional trend regime that produced the opening of the
Ross Sea or, alternatively, the paths could have sampled
Fig. 3.8-3.
Map of all sites for which data
have been analyzed. On DRV
the average value obtained by
Barruol and Hoffman (1999) is
mapped. Thick lines are average
fast direction (used here for
DRV and TNV), thin lines are
single measurements, mapped
at the SKS ray piercing point at
150 km of à depth. Length of
lines is proportional to the delay
time. Null measurements are
plotted with two small lines with
directions parallel to the two
allowed fast direction. Inset
shows area enlarged in Fig. 3.8-4
Chapter 3.8  ·  Seismic Anisotropy beneath Northern Victoria Land from SKS Splitting Analysis
158
one of the several large strike-slip NW-SE structures
mapped in the northern Victoria Land (Fig. 3.8-1; Salvini
et al. 1997). However, most of the out-of-trend data are
related to events with a SE backazimuth, that therefore
along the upward path, sample the warm and stretched
lithospehere-astehosphere of the Ross Sea, while on the
contrary, NE-SW fast velocity directions seems to be
mainly related to data sampling the East Antarctica cra-
ton. However, the density and azimuthal distribution of
obtained fast direction only allow preliminary hypoth-
eses, that deep structure heterogeneity shown by tomo-
graphic images beneath the region (Morelli and Danesi
2004) would authorize. Certainly in the future, with a
larger amount of recordings from permanent stations and
from recently going on temporary deployments (e.g.,
Wiens et al. 2003; Bannister et al. 2003), we would better
depict the characteristics of seismic anisotropy beneath
Northern Victoria Land.
Conclusions
We analyzed more than 100 teleseisms recorded by
24 temporary and one, TNV, permanent stations, de-
ployed in northern Victoria Land region. Our study pro-
duced preliminary results, that reveal the presence of
seismic anisotropy beneath all of the study region, with
different fast velocity direction moving from north to
south. However, only around Terra Nova Bay area the data
availability was large enough to better depict the anisot-
ropy distribution. Here the dominant fast velocity direc-
tion is NE-SW, in agreement with the Transantarctic
Mountains trend. Only few examples of E-W and NW-SE
directions are recorded, probably due to the extensional
trend that allowed the opening of Ross Sea or the pres-
ence of large strike-slip structures characterizing the
Northern Victoria Land. Delay times show rather large
values, on average 1.6 s, indicating that the detected
anisotropic layer may be thick and deep rooted. The pre-
liminary character of these results will be solved in the
future with a hopefully larger availability of seismo-
graphic data.
Acknowledgment
We would like to thank all people involved in installation
and maintenance of temporary stations and TNV perma-
nent station. Thanks to D. Wiens and an anonymous re-
viewer for their useful comments.
Reference
Bannister SyuJ, Leitner B, Kennet BLN (2003) Variations in crustal
structure across the transition from West to East Antarctica,
Southern Victoria Land. Geophys J Internat 155:870–884
Barruol G, Hoffman R (1999) Upper mantle anisotropy beneath the
Geoscope stations. J Geophys Res 104(5):10757–10773
Behrendt JC, LeMasurier WE, Cooper AK, Tessensohn F, Tréhu A,
Damaske D (1991) Geophysical studies of the West Antarctic Rift
System. Tectonics 10(6):1257–1273
Cimini GB, Amato A, Cerrone M, Chiappini M, Di Bona M,Pondrelli
S (1995) Passive seismological studies in the Terra Nova Bay Area
(Antarctica): First Results From the 1993–94 Expedition. Terra
Antartica 2:81–98
Di Bona M, Amato A, Azzara R, Cimini GB, Colombo D, Pondrelli S
(1997) Constraints on the lithospheric structure beneath the Terra
Nova Bay area from teleseismic P to S conversion. In: Ricci CA
(ed) The Antarctic region: geological evolution and processes.
Terra Antartica Publication, Siena, pp 1087–1093
Danesi S, Morelli A (2001) Structure of the upper mantle under the
Antarctic Plate from surface wave tomography. Geophys Res Let-
ters 28:4395–4398
Della Vedova, et al. (1997) Crustal structure of the Transantarctic
Mountains, western Ross Sea. In: Ricci CA (ed) The Antarctic re-
gion: geological evolution and processes. Terra Antartica Publi-
cation, Siena
Lawver L, Gahagan L (2003) Evolution of Cenozoic seaways in the
Circum-Antarctic Region. Paleogrogr Paleoclimatol Paleoecol (in
press)
Morelli A, Danesi S (2004) Seismological imaging of the Antarctic con-
tinental lithosphere: a review. Global Planet Change 42:155–165
Pondrelli S, Amato A, Chiappini M, Cimini GB, Colombo D, Di Bona
M (1997) ACRUP1 Geotraverse: contribution of teleseismic sata
recorded on land. In: Ricci CA (eds) The Antarctic region: geo-
logical evolution and processes. Terra Antartica Publication,
Siena, pp 632–635
Fig. 3.8-4. Map of fast velocity direction mapped at 150 km depth
piercing point for single measurements. For symbols see Fig. 3.8-3
Silvia Pondrelli  ·  Lucia Margheriti  ·  Stefania Danesi
159
Ritzwoller MH, Shapiro NM, Leahy GM (2003) A resolved mantle
anomaly as the cause of the Australian-Antarctic Discordance. J
Geophys Re 108(7):2353
Rocchi S, Armienti P, D’Orazio M, Tonarini S, Wijbrans J, Di
Vincenzo G (2002) Cenozoic magmatism in the western Ross
embayment: role of mantle plume vs. plate dynamics in the de-
velopment of the West Antarctic Rift System. J Geophys Res
107(9):2195
Silver PG (1996) Seismic anisotropy beneath the continents: prob-
ing the depth of geology. Ann Rev Earth Planet Sci 24:385–432
Silver PG, Chan WW (1988) Implication for continental structure
and evolution from seismic anisotropy Nature 335:34–39
Silver PG, Chan WW (1991) Shear wave splitting and sub-continen-
tal mantle deformation. J Geophys Res 96:16429–16454
Savage MK (1999) Seismic anisotropy and mantle deformation:
what have we learned from shear wave splitting? Rev Geophys
37:65–106
Salvini F, Storti F (1999) Cenozoic tectonic lineaments of the Terra
Nova Bay region, Ross embayment, Antarctica. Global Planet
Change 23:129–144
Salvini F, Brancolini G, Busetti M, Storti F, Mazzarini F, Coren F (1997)
Cenozoic geodynamics of the Ross Sea region, Antarctica: crustal
extension, intraplate strike-slip faulting, and tectonic inheritance.
J Geophys Res 102(11):24669–24696
Vinnik LP, Farra V, Romanowicz B (1989) Azimuthal anisotropy in
the Earth from observations of SKS at Geoscope and NARS broad-
band stations. Bull Seism Soc Amer 79:1542–1558
Wiens DA, Anandakrishnan S, Nyblade A, Fisher JL, Pozgay S, Shore
PJ, Voigt D (2003) Preliminary results from the Trans-Antarctic
Mountains Seismic Experiment (TAMSEIS). Terra Nostra 2003(4),
Abstracts
Wilson TJ (1999) Cenozoic structural segmentation of the Trans-
antarctic Mounatins rift flank in southern Victoria Land. Global
Planet Change 23:105–127
Chapter 3.8  ·  Seismic Anisotropy beneath Northern Victoria Land from SKS Splitting Analysis



Seismic Anisotropy beneath Northern Victoria Land from SKS Splitting Analysis

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Seismic Anisotropy beneath Northern Victoria Land from SKS Splitting Analysis

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