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Physics of the Earth and Planetary Interiors 170 (2008) 89–94
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Physics of the Earth and Planetary Interiors
journal homepage: www.elsevier.com/locate/pepi
Bulldozing the core–mantle boundary: Localized seismic scatterers
beneath the Caribbean Sea
Meghan S. Miller ∗ , Fenglin Niu
Department of Earth Science, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USA
a r t i c l e
i n f o
Article history:
Received 2 November 2007
Received in revised form 27 May 2008
Accepted 24 July 2008
Keywords:
Core–mantle boundary
PKIKP precursors
Seismic scatterers
Farallon slab
a b s t r a c t
Images of the scattering field near the core–mantle boundary (CMB) beneath the Caribbean were obtained
from migration processing of PKIKP precursors recorded by a broadband seismic array in the southern
Caribbean from earthquakes in the Western Pacific. Strong seismic scatterers were found to be concentrated near the CMB north of Colombia and are surrounded by high velocity anomalies associated with
the subducted Farallon plate. The relative location between the imaged scatterers and the surrounding
high velocity anomalies leads us to speculate that the observed seismic scatterers are remnants of the
subducted slab. As the Farallon plate penetrated into the lower mantle, segregation allowed its basaltic
crust to descend more quickly and to pool in front of the major portion of the subducting slab. The ponding
oceanic crustal material could be further collected (or bulldozed) by the residual motion of the subducting
plate, forming a localized region with strong seismic scatterers.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
The nature of the core–mantle boundary (CMB) region, the D
layer, has been a focus of research for decades because it is crucial in understanding the Earth’s dynamic structure. Many studies
have found that the D region is extremely heterogeneous and has
complicated seismic structures that require involvements of both
chemical and thermal processes (e.g., Garnero et al., 1998; Lay and
Garnero, 2004). One representative and well-studied area is Central America and the Caribbean where the D region is featured
by a well-defined seismic discontinuity underlain by anisotropic
and higher velocity materials (e.g., Garnero and Lay, 2003; Hung
et al., 2005; Hutko et al., 2006). Prior studies have primarily used
S waves from earthquakes in South America recorded at North
American stations. Here we present scattering images of the D
region beneath the Caribbean using precursors to the PKIKP phase
recorded by a broadband seismic array in northern South America
and the Caribbean from earthquakes in the Western Pacific.
Scattering of P waves in the lower mantle can be detected as
precursors to the compressional waves that traverse through the
Earth’s inner core at the distance range of 130–143◦ (PKIKP, Fig. 1)
(e.g., Cleary and Haddon, 1972; Haddon and Cleary, 1974; Husebye
et al., 1976; Hedlin et al., 1997; Hedlin and Shearer, 2000). Arrival
times of the precursors and their amplitude are directly related to
∗ Corresponding author. Fax: +1 713 348 5214.
E-mail address: [email protected] (M.S. Miller).
0031-9201/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.pepi.2008.07.044
location, geometry, density and magnitude of the seismic heterogeneities in the lower mantle. Previous studies found that PKIKP
precursors can generally be explained by a random scattering field
uniformly distributed in the bottom 200 km (Haddon and Cleary,
1974), and perhaps in a broader depth range (Hedlin et al., 1997)
of the lower mantle. There are a few exceptions that are interpreted as isolated scatterers (Vidale and Hedlin, 1998; Wen and
Helmberger, 1998; Thomas et al., 1999; Wen, 2000; Niu and Wen,
2001; Cao and Romanowicz, 2007). In most case, these isolated
scatterers are found to be related to partial melt. Here we seek to
understand the features and causes of PKIKP precursors recorded
at the BOLIVAR/GEONINOS array.
2. PKIKP precursors
We selected 192 seismograms with high signal-to-noise ratios
(SNR) from a total of 19 earthquakes that occurred in the Western
Pacific during 2003–2005 recorded by an array of 82 broadband
seismometers deployed under the U.S. BOLIVAR (Broadband OceanLand Investigations of Venezuela and the Antilles arc Region) and
Venezuelan GEODINOS (Geodinámica Reciente del Límite Norte de
la Placa Sudamericana—Recent Geodynamics of the Northern Limit
of the South American Plate) projects (Levander et al., 2006) (Fig. 1).
The data were selected based on the signal-to-noise ratio (SNR ≥ 5)
of the PKIKP phase. 118 seismograms from 14 events exhibited
clear and strong precursors to the PKIKP phase while 74 seismograms from 5 earthquakes showed very weak or no precursor at
all (Table 1). The observed PKIKP precursors have clear onsets and
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M.S. Miller, F. Niu / Physics of the Earth and Planetary Interiors 170 (2008) 89–94
Table 1
Event list
Date
Fig. 1. (A) Location of 14 events with strong precursors to PKIKP and 5 events with
no precursors in the Western Pacific. (B) Distribution of seismic stations (temporary
and permanent) in the BOLIVAR deployment. (C) Ray paths of PKIKP phase and
PKIKP precursors at distance of 135◦ . Earthquake hypocenter represented as an X
and seismic station as an inverted triangle. The shaded regions indicate locations
of seismic scatterers, which would produce seismic waves arriving earlier than the
PKIKP phase.
relatively large amplitudes. One example of seismograms with a
strong and isolated precursor is shown in Fig. 2A. These are recordings from the deep focus event that occurred at the Izu-Bonin
arc on April 19, 2005. In comparison, traces for the Oct-08-2004
event near the Solomon Islands are shown in Fig. 2B. Although
being at roughly the same epicentral distance range, these seismograms show no significant precursory arrivals before the PKIKP
phase. Here the waveform data were preprocessed in two steps. We
first removed the instrument response from the broadband data
and then convolved them with the WWSSN (WorldWide Standardized Seismograph Network) short-period instrument response. The
Time (GMT)
Latitude
(◦ )
Longitude
(◦ )
Depth
(km)
Magnitude
(Mw )
Events with precursors
Feb-22-2004
23:17
Mar-27-2004
06:19
Jul-22-2004
09:45
Sep-05-2004
10:07
Sep-06-2004
23:29
Sep-08-2004
14:58
Oct-04-2004
19:20
Oct-15-2004
04:08
Dec-11-2004
01:55
Jan-16-2005
20:17
Jan-17-2005
10:51
Feb-05-2005
03:34
Apr-19-2005
01:46
Aug-13-2005
07:36
18.505
−6.256
26.489
33.070
33.205
33.140
14.546
24.530
−4.613
10.934
10.986
16.011
29.642
20.130
145.387
154.760
128.894
136.618
137.227
137.200
146.993
122.694
149.779
140.842
140.682
145.867
138.891
145.800
197.0
48.0
20.0
14.0
10.0
21.0
7.0
94.0
541.0
24.0
12.0
142.0
425.0
48.0
5.7
5.9
6.1
7.2
6.6
6.1
6.0
6.7
5.5
6.6
6.1
6.6
5.9
6.0
Events without precursor
Mar-06-2004
06:26
May-13-2004
09:58
Oct-08-2004
08:27
Nov-02-2004
08:45
Nov-11-2004
17:34
−5.112
−3.584
−10.951
28.702
−11.128
152.588
150.730
162.161
143.214
162.208
44.0
10.0
36.0
10.0
10.0
5.5
6.4
6.8
5.7
6.7
WWSSN short-period data appeared to possess the best SNR for the
PKIKP phase and its precursors.
Generally, amplitudes of PKIKP precursors increase with epicentral distance. The observed difference in precursor amplitude
between the two groups of earthquakes, however, does not seem
to result from a systematic difference in epicentral distance. Fig. 3
shows the amplitude ratios between the precursor and PKIKP as
a function of epicentral distance. For the group with clear precursors, we used the time window between the onsets of the precursor
and PKIKP phase to calculate the amplitude ratio, while for the
weak precursor group, the predicted arrival time window based
on homogeneous random scattering models was employed. The
amplitude ratios are further averaged over a 1◦ binning distance.
As shown in Fig. 3, the amplitude ratios for the weak/no precursor
group remain very low through all the distance range of 130–138◦ ,
while those of the strong precursor group are significantly higher.
In general, scatterers located at both the source and receiver
sides can produce PKIKP precursors (Fig. 1C). Due to the wide range
in location of the 14 events in the Western Pacific (Fig. 1A and C),
here we argue that the observed precursors are most likely due to
receiver-side scattering. The earthquakes are distributed roughly
in two broad regions: the western Pacific and southwest Pacific.
We found mixed observations from both regions, making it hard to
argue for the source-side contribution of our observation. On the
other hand, the core exits of the PKIKP rays of the two groups are
localized into two separate groups (Fig. 4a). The exit point locations
from the events with strong precursors appear to be constrained to
the central Caribbean region, while the exit points with no precursors are limited laterally to northwest South America and southern
most Central America.
As shown in Fig. 1C, source- and receiver-side scattered waves
are received in different incident angle ranges. Thus it is in principle possible to overcome the ambiguity in source- and receiver-side
scattering with array analysis techniques. Fig. 1C shows that waves
generated from the source-side scattering (P to PKPbc or P to PKPab)
have larger incident angles than those from receiver-side scattering (PKPbc to P or PKPab to P). We used the beam-forming or
slowness-azimuth stacking technique (Aki and Richards, 1980; Niu
and Kawakatsu, 1997; Kaneshima and Helffrich, 1998) to measure
the slowness and back azimuth of the isolated precursor shown
in Fig. 2A. In a beam-forming analysis, all the seismograms are
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91
Fig. 2. (A) An example of PKIKP precursors and PKIKP/PKiKP phases observed at northern South America and the southeastern Caribbean. Each trace is aligned to manually
picked PKIKP (t = 0). Squares indicate the first arrival of the PKIKP precursors estimated from a random scattering model. (B) An example of events that show no precursors
to the PKIKP phase.
Fig. 3. Averaged amplitude ratio between the precursor and the PKIKP is plotted
as a function of epicentral distances. Squares and circles indicate the strong- and
no/weak-precursor group, respectively.
nth-root stacked (Muirhead, 1968) after a time correction calculated from the assumed slowness and back azimuth before stacking.
The best slowness and back azimuth were determined when the
nth-root stacking amplitude reaches to a maximum (Fig. 5). We
varied slowness from 0.5 to 5 s/deg at increments of 0.05 s/deg,
and searched the back azimuth deviation from the great circle
path within a range of ±20◦ at increments of 2◦ . The measured
slownesses of precursors are less than that of PKIKP (∼1.92 s/deg),
suggesting that receiver-side PKPbc to P scattering is the most likely
origin of the observed PKIKP precursors.
3. Mapping the seismic scatterers
PKIKP precursor onsets were manually picked and were found to
arrive approximately 3–6 s after the earliest precursory arrivals pre-
dicted by the random scattering models (Haddon and Cleary, 1974;
Husebye et al., 1976). We adopted a migration technique (Wen,
2000) to calculate the scatterer probability and hit count for discretized regions at the core–mantle boundary and six other depths
(50, 100, 150, 200, 250, and 300 km above the CMB). This method
is based on the hypothesis that PKIKP precursor energy arriving
at a certain time can be attributed to scatterers along isochronal
shells in both the source and receiver sides. As described above,
from beam-forming analysis, we were able to constrain the scattering mode as the receiver-side PKPbc to P scattering. We used
the 1D iasp91 model (Kennett and Engdahl, 1991) to compute the
isochrones at each assumed scattering depth. To account for 3D
effects in the migration moveouts, we manually picked the PKIKP
onsets by tracking the relative polarity and moveout of the PKIKP
and PKiKP phases recorded from deep focus and intermediate depth
earthquakes. Residual times calculated from these events were then
used as the 3D travel time correction for earthquakes that occurred
in the same region.
The scatterer probability (Fig. 6A–G) at one grid cell (0.1◦ by 0.1◦
cell) is computed from the ratio of the two numbers: number of
observations that can be explained by scattering at the grid within
a 0.5 s window and the total number of seismic observations used
in this study. In addition, the hit count at one grid is simply the total
number of PKIKP precursors sampling the grid cell (Fig. 6H). Therefore, the grid cells with a large hit count and high probability can
be interpreted as the origins of the PKIKP precursor onset energy.
We only used the onset energy to locate the seismic scatterers
here since later arrivals could be the result of scattering of the first
arrival by other heterogeneities at shallower depths, and using the
energy following the onsets requires a complete knowledge of the
source-time function (which is unknown). The observed precursors are best explained by seismic scattering near the core–mantle
boundary north of Colombia (Fig. 6A–G). This region has both the
highest probability and significant hit count. It also does not overlap
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Fig. 5. Normalized energy of the stacked PKIKP (A) and the precursor shown in
Fig. 2B (B) is plotted as a function of back azimuth and slowness. A time window of
4 s is used in calculating energy. Note that precursors are received in steep incident
angles, indicating that they are generated by receiver-side scattering.
4. Results and discussion
Fig. 4. (A) Map of the Caribbean region showing the calculated points of ray paths
exiting the core and entering the mantle. Blue circles indicate the events with precursors, and red diamonds indicate the events with no evident precursor. (B–D)
Tomographic images of the lowermost mantle from Grand (2002) are shown with
the core exits. Only regions with a 1.6% shear velocity perturbation and greater
are shown. Red lines (and ages) indicate the estimated Farallon plate boundaries
between 100 and 46 Ma (Pindell et al., 2005). (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of the article.)
with the area in which the exit points of the events with no precursors are concentrated. The area with high scattering probability
decreases steadily for shallower depths (Fig. 6A–G), and becomes
insignificant at scattering depths ≥300 km above the CMB (Fig. 6G).
We thus concluded that most of the scatterers are confined within
the D region.
The cause of heterogeneities above the core–mantle boundary
may be due to the intrinsic heterogeneity within the subducted
oceanic lithosphere (Hirose et al., 1999). For example, basaltic crust
has a different chemistry, density and melting temperature from
its underlying peridotite layer. Hirose et al. (1999) suggests that the
former basaltic crust becomes negatively buoyant when it transforms into a perovskite lithology, allowing it to sink into the deep
mantle. At the base of the mantle the former basaltic crust would
become partially molten, resulting in very low velocity anomalies
that have been speculated to generate the PKIKP precursors (Vidale
and Hedlin, 1998; Wen and Helmberger, 1998; Wen, 2000).
It is likely that these small-scale chemical and thermal heterogeneities above the CMB are the sources of our observed seismic
scatterers. The large variation observed in the precursor amplitude
strongly indicates that these small-scale heterogeneities are not
uniformly distributed in the study region. We found a good correlation between the migrated location of the seismic scatterers
and the calculated exit points (Figs. 4 and 6). A comparison of the
exit point locations with a shear velocity tomographic model of the
lowermost mantle (Grand, 2002) shows that there are high velocity anomalies at both the east and west of the mapped exit points
(Fig. 4B–D). The exit points corresponding to the ray paths of events
without a precursor lie to the south of the major high velocity features. The lateral extent of the scatterers is bounded by the high
velocity anomalies on the east and west and by the non-scattering
region to the south. The high velocity body in the western side
observed in the shear wave model (Grand, 2002) has been inter-
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93
Fig. 6. Maps of the “scatterer probability” at depths: 0 km (A), 50 km (B), 100 km (C), 150 (D), 200 km (E), 250 km (F), and 300 km (G) above the CMB. Hit count at depth
300 km about CMB. Note that the area with high probability (bright yellow) drops steadily with decreasing of scattering depth and become negligible at depth shallower than
300 km above the CMB. Probability and hit count are color coded from pink to blue, with blue being the highest. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of the article.)
preted as the subducted Farallon plate. This high velocity anomaly
is also shown in the P-wave tomographic images (Ren et al., 2007).
Estimated positions of the Farallon plate boundary at the Americas (Pindell et al., 2005) are plotted with the tomographic image in
Fig. 4B–D. This paleogeographic reconstruction demonstrates the
convergence of the Farallon plate between 100 and 46 Ma and its
general relationship with the high velocity anomalies in the lower
mantle associated with the ancient subducted Farallon plate. As
it penetrated into the lower mantle and collided with the CMB,
the subducting Farallon slab appears to bend along the boundary
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Fig. 7. Cartoon of the subducting Farallon slab and the features relevant to the study.
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Acknowledgments
We thank the BOLIVAR team, Alan Levander, the PASSCAL Instrument Center, and FUNVISIS for providing the data. The work
benefits from discussions with Cin-Ty Lee, Daoyuan Sun, and Don
Helmberger. We also thank the two anonymous reviewers for their
critical comments which improved the manuscript significantly.
The BOLIVAR project is funded by the National Science Foundation
and Miller is funded by an NSERC postdoctoral fellowship.
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