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Physics of the Earth and Planetary Interiors 170 (2008) 89–94 Contents lists available at ScienceDirect 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 90 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 M.S. Miller, F. Niu / Physics of the Earth and Planetary Interiors 170 (2008) 89–94 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 92 M.S. Miller, F. Niu / Physics of the Earth and Planetary Interiors 170 (2008) 89–94 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- M.S. Miller, F. Niu / Physics of the Earth and Planetary Interiors 170 (2008) 89–94 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 94 M.S. Miller, F. Niu / Physics of the Earth and Planetary Interiors 170 (2008) 89–94 References Fig. 7. Cartoon of the subducting Farallon slab and the features relevant to the study. The oceanic crust may have subducted separately and pooled on the core–mantle boundary in front of the subducted oceanic lithosphere (not to scale). and continue moving in its east–northeast direction. The former basaltic crust on the upper portion of the slab may have subducted more quickly and pooled in front of the major portion of the subducting Farallon plate. The descended oceanic crustal material could have been localized (and bulldozed) by the residual motion of the subducting plate and therefore become concentrated into the area shown in Figs. 4 and 6. Lee and Chen (2007) showed with a simple force balance equation that the subducted oceanic lithosphere can become segregated into two parts, the eclogitized crust and the lithospheric mantle, and then journey to different resting grounds. We believe that the combined scattering and tomographic images provide arguable evidence for this scenario. According to the calculation of Lee and Chen (2007), the eclogitized oceanic crust slips at a velocity slightly different from the subduction of the Farallon plate. As the result, it pools at the base of mantle “ahead” of the large volume oceanic lithospheric materials which are imaged as fast velocity anomalies by seismic tomography (Fig. 7) (e.g., Grand, 2002; Hung et al., 2005; Ren et al., 2007). Other models using triplicated S waveforms and migration methods have found anomalous areas to the east of the mapped PKP precursors (Fig. 4), which have suggested to be the presence of folded slabs in the lowermost mantle (Hutko et al., 2006; Sun et al., 2006; van der Hilst et al., 2007) or fast velocity anomalies (Cao and Romanowicz, 2007), which is in agreement with our interpretation. While the segregated oceanic crust seems to be a viable explanation for the observed seismic scatterers, other interpretations still exist. The intensity of seismic scattering observed here is comparable to those observed in the CMB west of Mexico (Niu and Wen, 2001) and requires a very large P-wave velocity perturbation (≥6%, based on the forward modeling of Niu and Wen, 2001), suggesting possible involvement of melts. Tan et al. (2002) found that plumes preferentially develop on the edge of slabs. 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