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Transcript
Rost: Bullerwell lecture Seismic constraints on Ear The small-scale structure of the Earth’s interior is more difficult to access than the big picture. In the Bullerwell Lecture 2009, Sebastian Rost discusses seismic studies targeting small-scale structure at the core–mantle boundary, including areas containing partially molten mantle material and the remnants of subducted slabs, and assesses how they influence aspects of the Earth’s dynamics. 70˚ (a) 2.26 65˚ 65˚ 60˚ 60˚ 55˚ 55˚ 50˚ 50˚ 45˚ 45˚ 40˚ 40˚ 350˚ 0˚ 10˚ 20˚ (c) 35˚ ˚ 0 4 30˚ 350˚ 0˚ 10˚ 20˚ (d) 70˚ 30˚ 65˚ 60˚ 60˚ 55˚ 55˚ 50˚ 50˚ 45˚ 45˚ 40˚ 40˚ 350˚ 0˚ 10˚ 20˚ 30˚ 35˚ ˚ 0 4 70˚ 65˚ M ost of the Earth’s interior is com pletely inaccessible. Direct measure ments using boreholes have reached depths of no more than 12 km, but compared to the radius of the Earth – about 6371 km – these samples are just scratching the surface. To study the bulk of the Earth we must use remote sensing techniques. Seismology is one of the best remote sensing tools used to study the Earth’s interior, and many major discoveries of structures in the deep Earth (such as the existence of the inner and outer core and the crust–mantle boundary) have been made using seismological techniques. Despite considerable success in imaging the Earth’s interior, many unanswered questions about the structure and dynamics of the interior of our planet remain. Seismology has to play a major role in answering many of these. The Earth’s core–mantle boundary (CMB) lies almost 3000 km beneath the surface of the planet. It is the boundary between two major convective systems with very different physical properties. On the one side, the core consists of a liquid iron alloy, convecting with a typical veloc ity in the range of kilometres per year and having a viscosity, although poorly constrained, in the range of 10 –2 Pas (Mound and Buffett 2007) at the CMB – comparable to the viscosity of water (10 –3 Pas) at room temperature. On the other hand, the mantle contains silicate rocks with a viscosity on the order of 1021 Pas (Moucha et al. 2007) and convects with velocities of a few centimetres per year. The CMB affects many processes active in the Earth: coupling between mantle and core influences the generation of the magnetic field; it represents the lower thermal boundary layer of mantle convection and is a major driver of plate tectonics; thermal instab ilities from this boundary might form mantle plumes generating intraplate volcanism such as seen in Hawaii and Iceland. The chemical 70˚ (b) 35˚ ˚ 0 4 350˚ 0˚ 10˚ 20˚ 30˚ 35˚ ˚ 0 4 1: Topographical maps of Europe. Political boundaries are marked. Topography from the Etopo5 grid from the US National Geophysical Data Center (NGDC). (In: Data Announcement 88-MGG02, Digital relief of the Surface of the Earth. NOAA, National Geophysical Data Center, Boulder, Colorado, 1988.) (a) Original dataset. The resolution of the 5-minute gridded dataset is about 9 km. (b) Same dataset but filtered with a 100 km lowpass. Topographical features seem blurred, but in general the nature of the topography is preserved. (c): Same dataset but filtered with a 500 km lowpass. Main topographical features are changed, such as Britain being an independent island. (d) Same dataset but filtered with a 1000 km lowpass. Major features of Europe such as Britain and Ireland are lost in this filtered topography version. differences across the boundary mean that little mass transfer between core and mantle is likely and the CMB might be the final resting place for slabs making their long way down from the surface. Therefore the CMB region is crucial for our understanding of the dynamics and evolu tion of our planet. As can be expected from such a dominant feature, the CMB is not a simple boundary between mantle and core. Indeed, seismology resolves many structures within the CMB region (for recent reviews see Garnero 2000 and Lay 2007). Seismic tomography resolves large-scale variations in seismic velocities in the deep Earth such as the broad regions with reduced S-wave velocities at the CMB beneath Africa and the central Pacific, dubbed large low shear velocity provinces (LLSVPs). The so-called D″ (“dee-dou ble-prime”) region, ranging from the CMB itself to a few hundred kilometres above the CMB, shows changes in seismic velocity gradients that have been related to the lower thermal boundary of the mantle. The upper boundary of the D″ region is often marked by a sharp increase in velocity, the D″ discontinuity, for S-waves, and a weaker discontinuity for P-waves. There is also evidence for strong seismic velocity anisotropy within D″. These observations can be explained by the phase transition from perovskite to postperovskite that has been detected in the appropri ate temperature and pressure range (Murakami et al. 2004, Oganov and Ono 2004) although other explanations are discussed. Further seis mic observations indicate increased small-scale structures within the D″ region, which have been related to the existence of chemical heteroge neities. Also, there is evidence for very thin lay ers (between 5 and 25 km thick) with strongly reduced seismic velocities, called ultra-low veloc ity zones (ULVZs), atop the CMB (Garnero and Helmberger 1995) and for layering at the top of an otherwise well mixed core (Buffett et al. 2000, Rost and Revenaugh 2001). For many of these observations a generally accepted model A&G • April 2010 • Vol. 51 Rost: Bullerwell lecture th’s small-scale structure for their generation and influence on larger scale mantle processes is still lacking. The structures discussed above are detected over many lengthscales – from the D″ discontinu ity being detected over thousands of kilometres, to ULVZs being detected in areas as small as 100 km, and scattering indicating structure of scales smaller than 10 km. This is similar to geo logical observations at the surface of our planet showing heterogeneities on many scale lengths from the difference between oceanic and conti nental crust to the crystal scale. The importance of knowing the full spectrum of Earth heteroge neity is highlighted in figure 1: using only infor mation on the large-scale structure might lead to overlooking some important information, while only looking at small-scale structure might blur our vision of the large-scale picture. The origin, either thermal or chemical, of many structures in the deep Earth remains unclear. Large-scale structures such as LLSVP have been explained as purely thermal features or regions chemically distinct from the rest of the mantle, but are likely to be a mixture of these two end members, i.e. they will be of thermo-chemical origin. Small-scale heteroge neities are more likely to have a chemical origin due to their small size; a thermal structure on this scale would equilibrate with its surround ings relatively quickly. We expect only chemi cal heterogeneities to survive in the convecting mantle for long periods of time. Chemical het erogeneities require mechanical mixing to be assimilated because chemical diffusion rates in the lower mantle are very small. The extent and the details of this mixing process are unknown. Using estimates from geodynamical models of the volume of chemical heterogeneity introduced into the mantle and an estimate of the volume of chemical heterogeneities in the mantle we might be able to understand this process. The major process that can introduce chemical A&G • April 2010 • Vol. 51 (a) R VS (b) VP 2870 mantle 5 to 40km remnants of basal magma ocean ULVZ depth (km) 2: (a) ULVZs are characterized by strong velocity reductions of the order of 10 and 30% for P- and S-waves in thin layers (<30 km) atop the CMB. Several studies indicate a density increase of the order of 10% in these layers. (b) Possible models for ULVZs. The most common explanation for ULVZs is the existence of partial melt. Alternative models invoke iron enrichment of the perovskite or postperovskite mantle material either through material from the mantle, surface (banded iron formations, BIFs) or core–mantle reactions. It has also been speculated that ULVZs might be the last remnant of the basal magma ocean that existed during Earth’s accretion. BIF partial melt CMB 2900 Fe-enriched pPv 0 4 8 12 velocity (km/s) density (g/cm3) 16 heterogeneity into the mantle is plate tecton ics. Oceanic crust is constantly formed at mid-oceanic ridges by partially melting the uppermost mantle beneath the ridge. There fore, the crust is rich in the incompatible ele ments that partition into the melt, while the upper part of the mantle from which the crust was extracted is depleted. The oceanic crust and lithosphere is then recycled into the mantle through subduction. Therefore, the subduction process constantly introduces chemical hetero geneities into the mantle. This process is imaged both in geophysics and geochemistry. Geochemistry and geophysics have very differ ent approaches to the study of the Earth. While geochemistry tries to balance the interaction of different geochemical reservoirs, geophysicists try to understand the processes that lead to the interaction from physical arguments (Helffrich 2006). Information from both disciplines is necessary to understand processes in the deep Earth, but comparing results is difficult due to the different lengthscales of sampling. Compar ing tomography results sampling the Earth on scales of a few hundred to thousand kilometres with geochemical results sampling the Earth at much shorter scales is challenging. Seismic information about the small-scale structure of the Earth will help to reconcile these datasets. I will discuss below two examples of imaging small-scale structure close to the CMB. I will show evidence for the existence of partially mol ten material in the deep Earth and of piles of the remnants of subducted oceanic lithosphere. Ultra-low velocity zones ULVZs are among the most enigmatic structures found at the CMB. ULVZs are characterized by strong decreases in P-wave and S-wave velocities in the range of 10% to 30% relative to global 1D Earth models in a thin veneer ranging in thick ness from a few kilometres to about 20 km (figure core CMB core– mantle reactants 2a). There is also growing evidence for a density increase on the order of 10% in these thin lay ers (Thorne and Garnero 2007). One puzzling aspect of ULVZs is their distribution at the CMB. Seismic studies detect areas of the CMB that are apparently devoid of ULVZ structure in close proximity to clear ULVZ detections (Thorne et al. 2004, Rost et al. 2005). There seems to be no ubiquitous ULVZ layer at the CMB within the resolution of seismology (Persh and Vidale 2004), which would be able to detect ULVZs as thin as 3 to 5 km with some studies report ing higher vertical resolution (Rost et al. 2009). ULVZs seem to be strongest in regions that are correlated with intraplate volcanism in the form of hot spots (Williams et al. 1998, Helmberger et al. 1998, Rost et al. 2005) and seem to prefer the edges of large regions of low seismic velocities as detected in tomographic studies (Garnero and McNamara 2008) and strong lateral gradients in the lowermost mantle (Thorne et al. 2004). Several models to explain ULVZs have been proposed (figure 2b). The most commonly used explanation is the existence of partial melt in thin layers (Williams and Garnero 1996, Rost et al. 2005). The existence of partial melts within ULVZs is supported by the observation that S-wave velocity reduction is three times greater than P-wave velocity reduction in ULVZs. This can be explained by rocks containing between 5 and 30% of melt depending upon whether the melt forms well-connected or poorly con nected melt pockets and the configuration of the melt (Williams and Garnero 1996, Berryman 2000). The melt could either percolate down from the overlying mantle (Rost et al. 2005), be generated at the CMB, or be the remnant of a global magma ocean (Labrosse et al. 2007). Other models for ULVZs include accumulated silicate sediments from the core (Buffett et al. 2000), iron-rich remnants of subduction (Dob son and Brodholt 2005), core–mantle reaction 2.27 Rost: Bullerwell lecture 50 KS SKS P Ps c ULVZ P 10 relative time (sec) –1 0 PcpupP PcP PuP 20 110° PscP ScP 50° ScsP SdP SPcP outer core SPKS CMB outer core 60° ULVZ ULVZ CMB relative time (sec) +1 0 SPdKS + SpupKS P Sc SpupKS P Pc SKS S cs P P Pu SPdKS CMB surface SPd SPc P Sd 0 110 1 12 15 0 (c) SPdKS 70 (b) PcP 60 35 45 ICB 2.28 (a) ScP 55 products (Knittle and Jeanloz 1991) and local ized zones of iron-enriched post-perovskite (Mao et al. 2006). If the ULVZs are partially molten mantle material, they should exist wherever the solidus of the mantle material is reached since the CMB is isothermal. Assuming an iso-chemical lower mantle, this would predict a ubiquitous ULVZ layer, although its thickness might vary laterally and the ULVZs might be below the resolution level for our current seismic probes. Several seismic probes for ULVZs are used regularly. They can be roughly divided into probes using diffracted paths (e.g. SPdiff KS, PKKPdiff [Garnero and Helmberger 1995, Rost and Garnero 2004]) or core-reflected phases (e.g. ScS, PcP, ScP: Reasoner and Revenaugh 2000, Rost et al. 2005, Mori and Helmberger 1995, Avants et al. 2006). Other probes using scattering and atypical diffractions have also been used (Xu and Koper 2009, Thomas et al. 2009). Ray paths for these probes are shown in figure 3. The largest CMB areas can be sampled for ULVZs by SPdiff KS, while the core-reflected phases such as PcP and ScP give high lateral resolution but are restricted to small lateral areas. Overall, less than half of the CMB area has been sampled for ULVZ structure. ULVZs can be detected in the waveforms of these phases with the interaction of SPdiff KS with a ULVZ leading to a distinct post-cursor (a wave arriving after the reference arrival) to SKS while the interaction of core-reflected phases leads to dominant precursors (seismic energy arriving before the reference arrival) to ScS, PcP and ScP phases. For the core reflections, post-cursors also exist but are complicated to analyse due to the interference with coda waves generated mainly in the crust beneath the station. Here I will focus on some studies using seismic array data to study waveforms of ScP. Figure 4 shows ScP waveforms from the Tonga–Fiji subduction zone as recorded at the Australian Warramunga (WRA) array (Rost and Revenaugh 2003, Rost et al. 2005). WRA is a small-aperture array (approximately 20 km) with 20 vertical component, short-period stations deployed along two roughly perpendicular legs. Each trace represents the summation of the time series of the 20 seismic stations located at WRA recording the elastic waves from one earthquake to increase the amplitude of the coherent signal relative to that of the incoherent noise (SNR). The time differences of the arrivals due to the incidence angle of the seismic waves and the distribution of the stations have been taken into account in a process called beam-forming (Rost and Thomas 2002). The array processing allows very high signal quality for these shortperiod signals. All arrivals are aligned on the ScP arrival time and sorted by source latitude which represents a roughly north–south profile along the CMB in a region east of Australia. The ScP 10 20 30 relative time (sec) 3: Raypaths for different phases used to sample the CMB for ULVZs. P-waves are marked as solid lines and S-waves are marked as dashed lines. (a) Core-reflected phase ScP, an S-wave that converts to a P-wave upon CMB reflection. Reflections and conversions at the top of the ULVZ will lead to additional pre- and post-cursors of the ScP waveform including SdP (ULVZ top reflection), ScsP (conversion on receiver side) and SPcP (conversion of source side). (b) Core reflection PcP. A dominant precursor (PuP) is generated through a reflection at the ULVZ top. Other post-cursory phases include PscP and multiples within the ULVZ. (c) SPdiffKS which is generated when SKS reaches the critical ray parameter for P waves at the CMB and forms a distinct post-cursor to SKS. Other rays influencing the SKS waveform include SPKS and multiples in the ULVZ. (Courtesy of Ed Garnero, garnero.asu.edu) waveforms show a distinct precursor from the reflection from the top side of the ULVZ (SdP) in a small CMB region. The precursor is very coherent within this distance range (figure 4b), indicating a laterally constant structure over approximately 100 km of the CMB. ScP wave forms sampling outside of this region do not show the distinct precursor (figure 4b). Modelling the CMB Using waveform forward modelling it is possible to find a model of the CMB region that pro duces seismic waveforms in agreement with the data (figure 4c). The best fitting model for the ULVZ region sampled here shows a thickness of 8.5 ± 1 km, with P-wave and S-wave velocity reductions of 8 ± 2.5% and 25 ± 4% (Rost et al. 2005), respectively, relative to a 1D reference Earth model. Higher frequency data allow a resolution of possible internal structure in the ULVZs (Rost et al. 2006). The thickness of the ULVZ in this area seems constant. More complicated waveforms sampling the edges of the ULVZ indicate multi-pathing at the edges of the ULVZ. Unfortunately, the seismic data cannot resolve whether the ULVZ is indeed dis appearing at this edge or is thinning below the resolution of the seismic probe. Nonetheless, the observation of multi-pathing indicates that the ULVZ is restricted to this small area. The region studied by the data shown in fig ure 4 lies at the edge of a large area of reduced seismic shear velocities, the LLSVP resolved in several tomographic studies. This is in good agreement with previous work observing a correlation between ULVZ material and the edges of the large low-shear velocity provinces (Thorne and Garnero 2004). This correlation can be understood from geodynamical model ling. Models including chemical and thermal convection (McNamara and Zhong 2005) are able to explain the distribution of the LLSVP as anomalously dense lower mantle material piled up by the convective flow driven by subduction of oceanic crust (McNamara and Zhong 2005, Garnero and McNamara 2008). Geodynamic simulations show that these piles are internally convecting and heat up due to a lack of convec tion across their boundaries and enrichment in radioactive elements. The hottest regions of these dense thermo-chemical piles (DTCP) can be found near the edges and underneath upwell ings. These observations are in good agreement with a partial melting model of ULVZs, both in their location close to the edges of the LLSVP and the necessary energy to invoke partial melt ing of the mantle material. Some ULVZs show a P-wave to S-wave reduc tion ratio of 1:3, possibly indicative of partial melt, but some ULVZs do not show this feature (Hutko et al. 2009). This might indicate that there are several mechanisms active at the CMB that result in ULVZs. Unfortunately, as discussed before, our coverage of the CMB is incomplete so a full interpretation of ULVZ properties and their connection to other mantle processes is currently A&G • April 2010 • Vol. 51 Rost: Bullerwell lecture realistic mantle materials at high pressures and temperatures from experimental or computa tional mineral physics; ● understanding of the behaviour of melt at high pressures; ● h igh-resolution geodynamical modelling to understand the dynamics of ULVZs. There is strong progress in all of these fields. ScP (a) –24.00 source latitude (deg) –24.25 –24.50 CMB scattering –24.75 –25.00 –25.25 –25.50 –25.75 5s (b) precursor ScP (c) synthetic no precursors 5s observed 5s 4: (a) Source longitude sorted ScP beam traces of Tonga/Fiji earthquakes recorded at the Australian Warramunga array. This sorting is equivalent to a north–south profile along the CMB south of New Caledonia. Blue traces show a coherent and distinct precursor to ScP indicative of ULVZ structure at the ScP reflection point. This precursor is absent in purple traces. Green traces show complicated waveforms indicative of multi-pathing at sharp ULVZ edges. (b) Overlay of ScP precursor traces (top) illustrates the coherency of the precursor. Red trace indicates the summation of all ScP precursor traces (also shown in the middle). Bottom trace shows the summation trace of all ScP waveforms not showing a precursor. (c) Synthetic ScP waveform for the best-fitting ULVZ model (top black line). The thick grey line shows the ScP summation trace for the precursor traces which is the trace to be fitted – also shown at the bottom. The ScP waveform can be fitted very accurately by a simple 8.5 km thick ULVZ with 8% and 25% P-wave and S-wave reduction, respectively, and 10% density increase. not possible. New dense station deployments and new probes for ULVZ structure might give better CMB coverage in the future. Another problem arises from the need to explain the apparent density increase in ULVZs and how to keep dense partially molten ULVZs stable. The required melt fraction to explain the seismic velocities ranges from 5% to 30% (Wil liams and Garnero 1996, Berryman 2000) and might be very close to the percolation threshold (Rost et al. 2005). Dense interconnected melt could therefore drain out quickly and spread out at the CMB, forming a thin extended layer below the seismic resolution (Hernlund and Tackley 2007). Several mechanisms have been invoked to prevent this, including interstitial crystals blocking the flow of material towards the CMB (Rost et al. 2005). A further challenge involves the observed densities in a pure partial A&G • April 2010 • Vol. 51 melt. Although silicic melt at lower mantle pres sures might be denser than the solid (Karato and Karki 2001, Stixrude et al. 2009), it is not clear whether this can explain the 10% density increase observed. It seems likely that a combi nation of melt and enrichment (e.g. of iron) is responsible for the density increase. Whether the necessary iron for enrichment drains out of the mantle (Rost et al. 2005) or is the product of reaction products with the core (Knittle and Jeanloz 1991) remains unclear. ULVZs are enigmatic features detected at the CMB which have so far eluded any comprehen sive explanation. Progress in several fields will be necessary to solve this puzzle: ● b etter seismic resolution of ULVZ regions and ULVZ properties and an understanding of where ULVZs are not present; ● characterization of the material properties of The high-frequency teleseismic wavefield at fre quencies around 1 Hz is dominated by extended codas accompanying each arrival. These codas are mainly generated through elastic scatter ing at small-scale heterogeneities with scales on the order of a wavelength (~10 km in the lower mantle). Therefore, the short-period seis mic wavefield contains information about the small-scale structure of the Earth which is not routinely used in seismology. Most of the coda is produced when the seismic energy travels through the crust to the receiver or in an area close to the source. Nonetheless, several studies indicate that scattered waves are also generated in the deep Earth (Cleary and Haddon 1972, Haddon and Buchbinder 1987). This scattered wavefield has been used to resolve small-scale structure in the mantle and core (Vidale and Hedlin 1998, Hedlin and Shearer 2000, Vidale and Earle 2000, Helffrich 2006). The scattered seismic wavefield is sensitive to velocity and density heterogeneities on the scale of the wavelength of the sampling seismic wave. This means that short-period teleseismic data with a dominant frequency of about 1 Hz are able to resolve heterogeneities on the order of a few kilometres. This small scale makes a chemi cal origin of these heterogeneities likely. Thermal disturbances would quickly equilibrate to ambi ent temperatures, while chemical heterogeneities can survive much longer because of incomplete mechanical mixing during convection (Becker et al. 1999, Albarede 2005). Therefore the scat tered wavefield allows us to collect valuable information about small-scale chemical differ ences that are most likely introduced into the mantle through the recycling of subducted mate rial (Christensen and Hofmann 1994) or through chemical interaction between mantle and core (Knittle and Jeanloz 1991, Buffett and Seagle 2010). Therefore we can use seismology to gather information about the chemical state of the lower mantle on short scales (Helffrich 2006). Several seismic probes have been used to study scattering in the lower mantle. The most com mon seismic phase used to study scattering from the lowermost mantle is PKP (Cleary and Haddon 1972). The ray configuration of PKP allows the separation of scattering from the deep Earth from scattering in the crust beneath the receiver. The scattered energy from the deep Earth will arrive with a shorter traveltime (i.e. as a precursor) to the reference (unscattered) 2.29 Rost: Bullerwell lecture (a) (b) 2100 B 2000 1900 P time (sec) B` PKKP 1800 A C 1700 (a) P PK•KP 0 KP P•P 1600 50 PK•KP P•KKP 100 distance (deg) 150 Pa•Pa 60 distance (deg) 50 40 30 20 0 1000 2000 3000 time (s) 0 30 330 0 30 60 300 330 300 (b) 60 1.0 0.8 90 270 90 270 0.6 0.4 power 6: (a) Beam traces of PKKP scattering for the Yellowknife array (YKA) in Canada (Rost and Earle 2010). Each trace represents a summation of the output of the seismic sensors at YKA. Traces are distance sorted. The large first arrival is the first arriving P-wave. PK•KP can be identified as an energy increase arriving at the station more than 1720 s after the earthquake. Scattering related to the phase PKPPKP (P′P′) is also marked. (b) PK•KP after array processing. Energy of PK•KP is plotted in a slowness (vertical incidence angle) and backazimuth (horizontal incidence angle) coordinate system. Each lobe represents scattering from one distinct CMB location found by backtracing the energy to the CMB using slowness and backazimuth information. The energy plots show the distinct arrival of energy off great circle path (marked as dashed line) with slownesses between 2.2 s/deg (innermost white circle) and 4.4 s/deg (middle white circle) indicative of the origin of PK•KP at the CMB. Black circles indicate 2 s/deg slowness intervals. Note that for one example the lobes do not show equal strength, which indicates lateral variation in scattering strength of the mantle material. SK•KP KK P• 5: (a) Ray path for PKKP scattering from the CMB (PK•KP). The special ray configuration leads to scattering off the great circle path (the shortest distance between source and receiver). (b) Observation time window for PK•KP. Between 20 and 60° epicentral distance the expected arrival time window of PK•KP is free of any interfering body waves (green travel time curves). The main PKKP arrival is marked in blue. For reference other PKKP scattering paths are also marked (red). 210 phase (in this case PKP). In contrast, the scat tered energy produced at small-scale hetero geneities beneath the station will arrive with a longer traveltime (i.e. as a post-cursor). The PKP studies are able to study small-scale heterogenei ties from the CMB to mid-mantle depths. Mod elling of the precursory scattered energy then allows the extraction of statistical properties such as average velocity and density variation and correlation length of the scattering medium. These studies indicate lateral variations in scat tering strength in the lower mantle (Hedlin and Shearer 2000, 2002). A rarely used probe to lower mantle scatter 2.30 180 6 Dec 1999 23:12 120 240 240 120 0.2 150 ing is PKKP (figure 5 and figure 6a). Several scattering geometries for PKKP exist but only scattering from the CMB will be discussed here. As for PKP, the special ray geometry of PKKP allows the analysis of the scattered energy in the precursory PKKP wavefield (figure 5 and figure 6). PKKP is especially suited to study the CMB region because the scattered energy arrives in a time window which is free from any interfering body waves (figure 5b). The best observation window for PKKP scattering ranges from 0 to approximately 60° epicentral distance from the earthquake. In this distance window the PKKP scattered energy is visible in array recordings 210 180 28 Jan 1999 08:10 0.0 150 as increased seismic energy in an otherwise relatively quiet time window starting at about 1720 s after the earthquake (figure 6a). The lack of interfering major arrivals will allow detecting the low-energy scattered arrivals in the seismic wavefield and separating them from ambient noise. Nonetheless, to separate the coherent scattered wavefield from the incoherent seismic noise, array analysis is necessary (figure 6b). Seismic arrays Seismic arrays were developed in the 1950s and 1960s mainly to monitor underground nuclear explosions. Their concept is similar to arrays A&G • April 2010 • Vol. 51 Rost: Bullerwell lecture −2.5 −1.0 0.0 $VS (%) 1.0 2.5 7: Map of located scatterers from data recorded at arrays in India (orange circles) and Canada (yellow circles) (after Rost and Earle 2010). Array locations are marked by triangles. Each circle represents one distinct scatterer location determined using the slowness and backazimuth information of PK•KP. Background model shows S-wave velocity variations from a tomographic model (Ritsema and van Heijst 2002). Scattered energy arrives dominantly from the edge of the LLSVP beneath southern Africa and the subduction zone area beneath central and southern America. in radio-astronomy. A seismic array is able to extract directivity information from the seismic wavefield as well as increasing the ratio of the amplitude of a coherent signal relative to the amplitude of incoherent noise. The analysis of PKKP will exploit both of these characteristics. Several methods have been developed to that end (for reviews see Rost and Thomas 2002, 2009 and Schweitzer et al. 2002) which follow the same mathematical principles as those applied to arrays in other disciplines (e.g. sonar, radioastronomy, radar, optics, infrasound). The PKKP scattered energy (PK•KP) is detected by time-series stacking over many slownesses (i.e. vertical incidence angle of a plane wavefront) and back-azimuths (horizontal angle against north of the wavefront as measured at the sta tion). PK•KP shows a characteristic peak of energy travelling off great circle path (i.e. not along the shortest path between source and receiver) and arriving with a slowness indicative of energy produced at the CMB (figure 6b). Figure 7 shows scattering areas identified using two small-scale arrays located in Canada (Yellowknife Array – YKA) and India (Gau ribidanur Array – GBA) (Rost and Earle 2010). Both arrays are small-aperture arrays with sta tions deployed along two approximately 20 km long legs. The inter-station spacing is about 2.5 km and the legs are roughly perpendicu lar, in a cross shape for YKA, and L-shaped for GBA. The array elements are equipped with short-period vertical seismometers that are well suited to detect PK•KP, which is strong A&G • April 2010 • Vol. 51 est between 0.9 Hz and 2.1 Hz. For each array approximately 650 earthquakes were analysed. Only a small percentage (about 10% for YKA and 5% for GBA) of the analysed earthquakes show evidence for PK•KP. The array analysis determines both the vertical (slowness) and horizontal (backazimuth) incidence angle for PK•KP. Using this information a back-tracing of the scattered energy to its origin using a 1D velocity model of the Earth using ray tracing is possible. The detected scatterers cluster in specific regions mainly beneath southern Africa and Southern and Central America. Detailed analysis (Rost and Earle 2010) shows that these patches indeed are regions of increased smallscale structure and not regions of the high est resolution of PK•KP or dependent on the source–receiver combinations. A comparison with previous geophysical results of the structure of the deep Earth can reveal potential origins for the small-scale heterogeneities detected by PK•KP. The com parison with tomographic images of the velocity structure at the CMB shows that the scattering cluster beneath southern Africa is located in a region of reduced seismic velocities, while the Southern and Central American clusters are in good agreement with high seismic velocities. Due to the correlation of high seismic velocities at the CMB with the surface location of sub duction zones, these are generally interpreted as material from subducted slabs residing at the CMB. Tomography images sheets of high seismic velocities from the subduction zone to the CMB in these areas (van der Hilst et al. 1997), which have been modelled to reach the bottom of the mantle (Lithgow-Bertelloni and Richards 1998). The interpretation of low seis mic velocities as dense thermo-chemical piles has been discussed earlier. This indicates several mechanisms active in the lower mantle produc ing small-scale heterogeneities (figure 8). Scattering beneath southern Africa might be related to the LLSVP in this region. Indeed, pre vious studies of PKP scattering in this region show increased scattering from a region that has been interpreted as the source for the Comoros hot spot (Wen 2000) and similar observations have been made elsewhere (Thomas et al. 1999). The scattering region is located at the edge of the tomographic LLSVP which, as discussed, shows the highest temperature of the dense thermo-chemical piles. The scattering therefore might be related to the existence of partially molten material in this region and ultimately to ULVZ structure. Subduction is the dominant process introduc ing chemical heterogeneities into the mantle by recycling of crustal and upper mantle material. The existence of small-scale heterogeneities related to long-lasting subduction might indi cate the final resting place of the slab material that is only incompletely mixed back into the mantle material (Becker et al. 1999, Albarede 2005), which is in agreement with previous work (Krüger et al. 1995, Miller and Niu 2008). The application of a new probe to sample small-scale structure in the deep Earth shows that the scattered wavefield contains valuable information on the interior of the Earth. This information can often be used to understand larger scale dynamical structure such as ULVZs, LLSVP and the subduction process. Using the scattered wavefield to extract material prop erties for the CMB region is part of ongoing efforts to better understand the dynamics and evolution of the Earth’s interior. Conclusions In this article I have summarized some recent attempts to use seismic array data to resolve the small-scale structure of the CMB region about 3000 km beneath the surface. The highresolution images indicate that the interior of the Earth holds a multitude of structures. We can use information from the high-resolution seismic studies to understand large-scale dynamical processes acting in the Earth from the existence of dense thermo-chemical piles to the way subducted crustal material is recycled or stored at the CMB. Ultra-low velocity zones at the CMB are intermittent features resolved by seismologi cal data that show a preference for the edges of large-low shear velocity provinces. Using information from seismology, mineral physics and geodynamics, we will be able to understand 2.31 Rost: Bullerwell lecture Sebastian Rost, the 2009 Bullerwell Lecturer, is Lecturer in the School of Earth and Environment at the University of Leeds. Acknowledgments. The work presented here was possible only through collaboration with colleagues from the USA and UK. I would like to thank Ed Garnero, Justin Revenaugh, Quentin Williams, Paul Earle, Allen McNamara, Michael Thorne and Tine Thomas for ideas and contributions. Part of this work has been supported by a NERC grant (NE/F000898/1). Comments by Hannah Bentham and Jon Mound improved the manuscript. I thank the British Geophysical Association for selecting me as Bullerwell Lecturer 2009. References Albarede F 2005 Earth’s Deep Mantle: Structure, Composition, and Evolution 160 27–46 doi:10.1029/ 160GM04. Avants M et al. 2006 Geophys. Res. Lett. 33 L07314. Becker T et al. 1999 Earth Planet. Sci. Lett. 171 351–365. Berryman J 2000 Geophys. Res. Lett. 27 421–424. Buffett B et al. 2000 Science 290 1338–1342. Buffett B and C Seagle 2010 J. Geophys. Res. in press doi:10.1029/2009JB006751. 2.32 lower mantle slab how these small-scale structures influence the composition, dynamics and evolution of the CMB region, mantle plumes and dense thermochemical piles at the CMB. Special seismic methods have to be applied to resolve the ultra-fine structure of the deep Earth on scales of a few kilometres. Exploit ing the information of scattered arrivals in the short-period seismic wavefield, we can extract velocity and density information for structures on the order of a seismic wavelength (approxi mately 10 km). The study presented here indi cates that several mechanisms of producing small scale heterogeneities in the deep Earth might be active. Strong scattering is recorded from regions likely to contain the remnants of slab material from plates since disappeared from the surface (Lithgow-Bertelloni and Rich ards 1998), but also from CMB areas that might contain partially molten material. Currently, we see improvements in many fields essential to decipher Earth’s structure. Increased seismic station densities in many regions of the Earth will open new areas of the deep Earth for studies including the techniques discussed here. Computational and experimental mineral physics help to model the seismic data by study ing the material properties of Earth materials at pressures and temperatures relevant to the CMB region. Finally, current geodynamical models are able to resolve and model small-scale structures embedded in large-scale mantle flow over the history of the Earth, allowing us to investigate how they influence the structures we resolve now. A collaborative effort between these disciplines is necessary to decipher the great secrets of the Earth’s deep interior. ● LLSVP Pv Dq PPv ULVZ? CMB Central/Southern America nomaly frican a A outer core 8: Sketch of mantle structure leading to scattering of PK•KP (after Garnero et al. 2007). Two different processes seem to be active to create the small-scale structure resolved by PK•KP. The edge of the LLSVP might contain partially molten material within a ULVZ that leads to scattering. 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