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Up-to-fivefold reverberating waves through the Earth’s center and distinctly anisotropic innermost inner core
Nature Communications volume 14, Article number: 754 (2023) Cite this article
In the first several decades after its discovery7, seismological investigations of the IC focused mainly on the characterization of its isotropic structure and boundary with the liquid outer core8,9. However, since the 1980s, the studies of its anisotropic structures have complemented the existing knowledge. P-wave transverse isotropy, specifically the IC bulk’s depth-independent cylindrical anisotropy, was the first proposed conjecture to explain the travel times of compressional body waves traversing the IC (PKIKP waves)10,11 and Earth’s normal-mode splitting functions12. However, that conjecture was soon updated by the discoveries of anisotropy’s hemispherical dichotomy13 and radial variations1,14,15. Recent studies tend to introduce more complex structures, including variations in P-wave anisotropy16,17 and attenuation18. Additionally, S-wave anisotropy has also been recently observed19,20. The seismological observations have provided essential constraints on the mineralogical properties of stable crystallographic structures of iron in the IC. However, it remains an open debate whether the hexagonal close-packed (hcp)21,22 or body-centered cubic (bcc)23 phase of iron stabilizes at the IC temperature-pressure conditions.
Despite the expanding number of studies, the IC remains enigmatic, particularly in its innermost part. That is because of the inherent limitation in a volumetric sampling of the existing seismological probes and the fact that this Earth’s volume is buried beneath other layers. On the one hand, travel times and amplitudes of PKIKP waves have been the primary short-period tools to obtain inferences on spatially-distributed properties such as anisotropy and attenuation24,25,26. However, to probe the centermost ball of the IC, seismic stations and earthquakes must be positioned at near antipodal distances, which is challenging in practice due to the confinement of large subduction-zone earthquakes in the quasi-equatorial belt and the limited seismic deployments in the oceans and remote areas. On the other hand, normal modes have limited lateral and radial resolution because of their long-period nature, and their sensitivity approaches zero in the Earth’s center.
To improve the spatial sampling of the Earth’s deep interior, coda correlation studies27,28,29, which exploit correlated features lasting in long earthquake recordings, have emerged as promising tools to probe the Earth’s interiors. The correlation wavefield that exploits the similarity of weak signals samples the IC differently from the previous techniques19,30 (for a recent review, see ref. 31). Most recently, the correlation feature I2* has been suggested as another class of observations to probe the P-wave anisotropy of the IC32. The challenges in proceeding with this correlation approach include the overwhelmingly complex correlation features kernels and require future investigation.
The innermost inner core (IMIC) was initially hypothesized as a central ball within the Earth’s IC characterized by distinctive anisotropic properties from the outer shell1,33. The original studies1,33 suggested the 300-km-radius IMIC with a slow direction at ~45° from the fast axis, aligning with the Earth’s rotation axis. This hypothesis has been corroborated by subsequent studies using the International Seismological Center (ISC) datasets via robust non-linear searches34, dedicated picking of the antipodal PKIKP waves35,36,37, and normal mode analysis15. However, there are still significant unknowns related to the IMIC radius, the nature of the transition to the outer IC, and its precise anisotropic properties, such as the strength and the fast and slow directions. These topics keep inspiring further investigations.
This study reports a previously unobserved and unutilized class of seismological observations of reverberating waves through the bulk of the Earth along its diameter up to five times, later referred to as PKIKP multiples. To our knowledge, reverberations from more than two passages are hitherto unreported in the seismological literature. Simultaneous observations of these exotic arrivals at regionally dense seismic networks opportunistically provide tools to constrain the IMIC properties because they sample the IMIC in an unprecedented fashion. Measurements from these observations, in agreement with recent independent findings32,34,37,38, confirm an anisotropically distinctive IMIC from the less anisotropic outer shell.
This study uses the ever-growing global seismograph network (Fig. 1A) to produce global stacks for some significant seismic events individually (Fig. 2). We retrieve seismic waveforms from several international data centers (i.e., IRIS, ORFEUS, GFZ, ETHZ, and INGV) to construct global seismic stacks with 1-degree distance bins for all large earthquakes (Mw 6.0+ ) in the last decade (see Methods section). To avoid possible misalignment of phases due to heterogeneities of the Earth, all waveforms are bandpass filtered at long periods between 10–100 s. For example, Fig. 2 shows a global stack of a thrust-faulting earthquake (22 Jan 2017, Mw 7.9) in the Solomon Islands (see Fig. 1A for a location map). In the stack, the horizontal axis spans epicentral distances from 0°, meaning earthquake and station are nearly collocated, to 180°, meaning they are antipodal.
Fig. 1: Location map and schematic ray paths of PKIKP multiples.
A The location map of the 22 Jan 2017, Mw 7.9 Solomon Islands earthquake and stations that contribute to the global stack (see Fig. 2). Black inverted triangles denote the seismic stations, and beachball marks the location and mechanism of the earthquake. The contours show the epicentral distances from the event. B Schematic ray paths of the second and fourth PKIKP multiples, i.e., PKIKP2 and PKIKP4, reverberating along the Earth’s diameter twice and four times. ξ′±Δξ′
is the representative sampling direction angle relative to the Earth’s rotation axis (ERA). The red dashed-dotted circle denotes the tentative innermost inner core (IMIC) boundary with a radius of 650 km. C Similar to panel (B) but for PKIKP and the third multiples, i.e., PKIKP3 reverberating along the Earth’s diameter three times.
Full size image
Fig. 2: Global stack for the 22 Jan 2017, Mw = 7.9 Solomon Islands earthquake.
A Histogram of seismic waveforms as a function of epicentral distance in 1-degree bins. B and C Global stacks spanning 0–55 min and 55–110 min. Exotic podal and antipodal reverberations up to five multiples (with near-horizontal slope) are labeled by red fonts.
Full size image
Perhaps the most eye-catching features observed in the global stack of the 2017 Solomon Islands event (Fig. 2) are relatively flat arrivals indicating that the associated seismic phases arrive steeply to the Earth’s surface. Their arrival times and slowness properties suggest they are seismic phases reverberating along the entire Earth’s diameter, including the inner core, multiple times. Thus, we adopt an abbreviated nomenclature, similar to previous counterparts in the correlation wavefield29: PKIKP (I), PKIKP2 (I2), PKIKP3 (I3), PKIKP4 (I4), and PKIKP5 (I5), in which the last digits represent the number of passages reverberating the entire Earth’s diameter as compressional waves (see their schematic ray paths Fig. 1B, C). Observations of such exotic arrivals in several other single-event stacks are included in the supplementary material (Fig. S1). They present similar quality exotic phases of three- or fourfold reverberations that can be routinely observed. Fivefold reverberations (as in Fig. 2) are seen clearly on only a few global stacks.
CONTINUE HERE: https://www.nature.com/articles/s41467-023-36074-2
- Thanh-Son Phạm &
- Hrvoje Tkalčić
Nature Communications volume 14, Article number: 754 (2023) Cite this article
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Abstract
Probing the Earth’s center is critical for understanding planetary formation and evolution. However, geophysical inferences have been challenging due to the lack of seismological probes sensitive to the Earth’s center. Here, by stacking waveforms recorded by a growing number of global seismic stations, we observe up-to-fivefold reverberating waves from selected earthquakes along the Earth’s diameter. Differential travel times of these exotic arrival pairs, hitherto unreported in seismological literature, complement and improve currently available information. The inferred transversely isotropic inner-core model contains a ~650-km thick innermost ball with P-wave speeds ~4% slower at ~50° from the Earth’s rotation axis. In contrast, the inner core’s outer shell displays much weaker anisotropy with the slowest direction in the equatorial plane. Our findings strengthen the evidence for an anisotropically-distinctive innermost inner core and its transition to a weakly anisotropic outer shell, which could be a fossilized record of a significant global event from the past.Introduction
Earth’s inner core (IC), which accounts for less than 1% of the Earth’s volume, is a time capsule of our planet’s history1,2. As the IC grows, the latent heat and light elements released by the solidification process drive the convection of the liquid outer core3,4, which, in turn, maintains the geodynamo. Although the geomagnetic field might have preceded the IC’s birth5, detectable changes in the IC’s structures with depth could signify shifts in the geomagnetic field’s operation, which could have profoundly influenced the Earth’s evolution and its eco-system1,6. Therefore, probing the innermost part of the IC is critical to further disentangling the time capsule and understanding Earth’s evolution in the distant past.In the first several decades after its discovery7, seismological investigations of the IC focused mainly on the characterization of its isotropic structure and boundary with the liquid outer core8,9. However, since the 1980s, the studies of its anisotropic structures have complemented the existing knowledge. P-wave transverse isotropy, specifically the IC bulk’s depth-independent cylindrical anisotropy, was the first proposed conjecture to explain the travel times of compressional body waves traversing the IC (PKIKP waves)10,11 and Earth’s normal-mode splitting functions12. However, that conjecture was soon updated by the discoveries of anisotropy’s hemispherical dichotomy13 and radial variations1,14,15. Recent studies tend to introduce more complex structures, including variations in P-wave anisotropy16,17 and attenuation18. Additionally, S-wave anisotropy has also been recently observed19,20. The seismological observations have provided essential constraints on the mineralogical properties of stable crystallographic structures of iron in the IC. However, it remains an open debate whether the hexagonal close-packed (hcp)21,22 or body-centered cubic (bcc)23 phase of iron stabilizes at the IC temperature-pressure conditions.
Despite the expanding number of studies, the IC remains enigmatic, particularly in its innermost part. That is because of the inherent limitation in a volumetric sampling of the existing seismological probes and the fact that this Earth’s volume is buried beneath other layers. On the one hand, travel times and amplitudes of PKIKP waves have been the primary short-period tools to obtain inferences on spatially-distributed properties such as anisotropy and attenuation24,25,26. However, to probe the centermost ball of the IC, seismic stations and earthquakes must be positioned at near antipodal distances, which is challenging in practice due to the confinement of large subduction-zone earthquakes in the quasi-equatorial belt and the limited seismic deployments in the oceans and remote areas. On the other hand, normal modes have limited lateral and radial resolution because of their long-period nature, and their sensitivity approaches zero in the Earth’s center.
To improve the spatial sampling of the Earth’s deep interior, coda correlation studies27,28,29, which exploit correlated features lasting in long earthquake recordings, have emerged as promising tools to probe the Earth’s interiors. The correlation wavefield that exploits the similarity of weak signals samples the IC differently from the previous techniques19,30 (for a recent review, see ref. 31). Most recently, the correlation feature I2* has been suggested as another class of observations to probe the P-wave anisotropy of the IC32. The challenges in proceeding with this correlation approach include the overwhelmingly complex correlation features kernels and require future investigation.
The innermost inner core (IMIC) was initially hypothesized as a central ball within the Earth’s IC characterized by distinctive anisotropic properties from the outer shell1,33. The original studies1,33 suggested the 300-km-radius IMIC with a slow direction at ~45° from the fast axis, aligning with the Earth’s rotation axis. This hypothesis has been corroborated by subsequent studies using the International Seismological Center (ISC) datasets via robust non-linear searches34, dedicated picking of the antipodal PKIKP waves35,36,37, and normal mode analysis15. However, there are still significant unknowns related to the IMIC radius, the nature of the transition to the outer IC, and its precise anisotropic properties, such as the strength and the fast and slow directions. These topics keep inspiring further investigations.
This study reports a previously unobserved and unutilized class of seismological observations of reverberating waves through the bulk of the Earth along its diameter up to five times, later referred to as PKIKP multiples. To our knowledge, reverberations from more than two passages are hitherto unreported in the seismological literature. Simultaneous observations of these exotic arrivals at regionally dense seismic networks opportunistically provide tools to constrain the IMIC properties because they sample the IMIC in an unprecedented fashion. Measurements from these observations, in agreement with recent independent findings32,34,37,38, confirm an anisotropically distinctive IMIC from the less anisotropic outer shell.
Results
Multiple body-wave reverberations through the Earth’s inner core
Stacking seismic waveforms from multiple seismic stations can enhance weak but coherent seismic signals while suppressing incoherent noise. The stacked waveforms can be gathered in a two-dimensional image of lapse time and epicentral distance to represent the seismic wavefield with spatial coherency. In the 1990s, due to the sparsity of global seismic stations, waveforms from many earthquakes were gathered to produce a global stack covering a complete range of epicentral distances39,40, 0–180°. The stacks have served as a reliable tool for identifying many weak seismic arrivals, for example, those relating to the mantle discontinuities40 and in the sound fixing and ranging (SOFAR) channel41 due to the variation of temperature and salinity of seawater with depths in the ocean.This study uses the ever-growing global seismograph network (Fig. 1A) to produce global stacks for some significant seismic events individually (Fig. 2). We retrieve seismic waveforms from several international data centers (i.e., IRIS, ORFEUS, GFZ, ETHZ, and INGV) to construct global seismic stacks with 1-degree distance bins for all large earthquakes (Mw 6.0+ ) in the last decade (see Methods section). To avoid possible misalignment of phases due to heterogeneities of the Earth, all waveforms are bandpass filtered at long periods between 10–100 s. For example, Fig. 2 shows a global stack of a thrust-faulting earthquake (22 Jan 2017, Mw 7.9) in the Solomon Islands (see Fig. 1A for a location map). In the stack, the horizontal axis spans epicentral distances from 0°, meaning earthquake and station are nearly collocated, to 180°, meaning they are antipodal.
Fig. 1: Location map and schematic ray paths of PKIKP multiples.
A The location map of the 22 Jan 2017, Mw 7.9 Solomon Islands earthquake and stations that contribute to the global stack (see Fig. 2). Black inverted triangles denote the seismic stations, and beachball marks the location and mechanism of the earthquake. The contours show the epicentral distances from the event. B Schematic ray paths of the second and fourth PKIKP multiples, i.e., PKIKP2 and PKIKP4, reverberating along the Earth’s diameter twice and four times. ξ′±Δξ′
is the representative sampling direction angle relative to the Earth’s rotation axis (ERA). The red dashed-dotted circle denotes the tentative innermost inner core (IMIC) boundary with a radius of 650 km. C Similar to panel (B) but for PKIKP and the third multiples, i.e., PKIKP3 reverberating along the Earth’s diameter three times.
Full size image
Fig. 2: Global stack for the 22 Jan 2017, Mw = 7.9 Solomon Islands earthquake.
A Histogram of seismic waveforms as a function of epicentral distance in 1-degree bins. B and C Global stacks spanning 0–55 min and 55–110 min. Exotic podal and antipodal reverberations up to five multiples (with near-horizontal slope) are labeled by red fonts.
Full size image
Perhaps the most eye-catching features observed in the global stack of the 2017 Solomon Islands event (Fig. 2) are relatively flat arrivals indicating that the associated seismic phases arrive steeply to the Earth’s surface. Their arrival times and slowness properties suggest they are seismic phases reverberating along the entire Earth’s diameter, including the inner core, multiple times. Thus, we adopt an abbreviated nomenclature, similar to previous counterparts in the correlation wavefield29: PKIKP (I), PKIKP2 (I2), PKIKP3 (I3), PKIKP4 (I4), and PKIKP5 (I5), in which the last digits represent the number of passages reverberating the entire Earth’s diameter as compressional waves (see their schematic ray paths Fig. 1B, C). Observations of such exotic arrivals in several other single-event stacks are included in the supplementary material (Fig. S1). They present similar quality exotic phases of three- or fourfold reverberations that can be routinely observed. Fivefold reverberations (as in Fig. 2) are seen clearly on only a few global stacks.
CONTINUE HERE: https://www.nature.com/articles/s41467-023-36074-2
