Seismic velocity structure is the distribution and variation of seismic wave speeds within Earth's and other planetary bodies' subsurface. It is reflective of subsurface properties such as material composition, density, porosity, and temperature. Geophysicists rely on the analysis and interpretation of the velocity structure to develop refined models of the subsurface geology, which are essential in resource exploration, earthquake seismology, and advancing our understanding of Earth's geological development.
History The understanding of the Earth's seismic velocity structure has developed significantly since the advent of modern seismology. The invention of the seismogram in the 19th-century catalyzed the systematic study of seismic velocity structure by enabling the recording and analysis of seismic waves.
20th century
The field of seismology achieved significant breakthroughs in the 20th century.In 1909, Andrija Mohorovičić identified a significant boundary within the Earth known as the Mohorovičić discontinuity, which demarcates the transition between the Earth's crust and mantle with a notable increase in seismic wave speeds. This work was furthered by Beno Gutenberg, who identified the boundary at the core-mantle layer in the early to mid-20th century. The 1960s introduction of the World Wide Standardized Seismograph Network dramatically improved the collection and understanding of seismic data, contributing to the broader acceptance of plate tectonics theory by illustrating variations in seismic velocities.
Later, seismic tomography, a technique used to create detailed images of the Earth's interior by analyzing seismic waves, was propelled by the contributions of Keiiti Aki and Adam Dziewonski in the 1970s and 1980s, enabling a deeper understanding of the Earth's velocity structure. Their work laid the foundation for the Preliminary Reference Earth Model in 1981, a significant step toward modeling the Earth's internal velocities. The establishment of the Global Seismic Network in 1984 by Incorporated Research Institutions for Seismology further enhanced seismic monitoring capabilities, continuing the legacy of the WWSSN.
21st century The advancement in seismic tomography and the expansion of the Global Seismic Network, alongside greater computational power, have enabled more accurate modeling of the Earth's internal velocity structure. Recent progress focuses on the inner core's velocity features and applying methods like ambient noise tomography for improved imaging.
Principle of seismic velocity structure
The study of seismic velocity structure, using the principles of seismic wave propagation, offers critical insights into the Earth's internal structure, material composition, and physical states. Variations in wave speed, influenced by differences in material density and state (solid, liquid, or gas), alter wave paths through refraction and reflection, as described by Snell's Law. P-waves, which can move through all states of matter and provide data on a range of depths, change speed based on the material's properties, such as type, density, and temperature. S-waves, in contrast, are constrained to solids and reveal information about the Earth's rigidity and internal composition, including the discovery of the outer core's liquid state since they cannot pass through it. The study of these waves' travel times and reflections offers a reconstructive view of the Earth's layered velocity structure.
Average velocity structure of planetary bodies LayerEarthMoonMarsCrustP-wave: 6.0–7.0 km/s (continental) P-wave: 5.0–7.0 km/s (oceanic)
S-wave: 3.5–4.0 km/sP-wave: 5.1–6.8 km/s S-wave: 2.96–3.9 km/sP-wave: 3.5–5 km/s S-wave: 2–3 km/sMantleUpper Mantle: P-wave: 7.5–8.5 km/s
S-wave: 4.5–5.0 km/sP-wave: 7.7 km/s S-wave: 4.5 km/sUpper Mantle: P-wave: 8 km/s
S-wave: 4.5 km/sLower Mantle: P-wave: 10–13 km/s
S-wave: 5.5–7.0 km/sLower Mantle: P-wave: 5.5 km/s
S-waves: Not applicable (liquid)CoreOuter Core: P-wave: 8.0–10 km/s
S-waves: Not applicable (liquid)Outer core: P-wave: 4 km/s
S-waves: Not applicable (liquid)P-wave: 5 km/s S-waves: Not applicable (liquid)Inner Core: P-wave: ~11 km/s
S-wave: ~3.5 km/sInner core: P-wave: 4.4 km/s
S-wave: 2.4 km/s
Velocity structure of Earth
Seismic waves traverse the Earth's layers at speeds that differ according to each layer's unique properties, with their velocities shaped by the respective temperature, composition, and pressure. The Earth's structure features distinct seismic discontinuities where these velocities shift abruptly, signifying changes in mineral composition or physical state.
Crust
Average P-wave velocity: 6.0–7.0 km/s (continental); 5.0–7.0 km/s (oceanic) Average S-wave velocity: 3.5–4.0 km/s
Within the Earth's crust, seismic velocities increase with depth, mainly due to rising pressure, which makes materials denser. The relationship between crustal depth and pressure is direct; as the overlying rock exerts weight, it compacts underlying layers, reduces rock porosity, increases density, and can alter crystalline structures, thus accelerating seismic waves.
Crustal composition varies, affecting seismic velocities. The upper crust typically contains sedimentary rocks with lower velocities (2.0–5.5 km/s), while the lower crust consists of denser basaltic and gabbroic rocks, leading to higher velocities.
Although geothermal gradient, which refers to the increase in temperature with depth in the Earth's interior, can decrease seismic velocities, this effect is usually outweighed by the velocity-boosting impact of increased pressure.
Upper mantle
Average P-wave velocity: 7.5–8.5 km/s Average S-wave velocity: 4.5–5.0 km/s
Seismic velocity in the upper mantle rises primarily due to increased pressure, similar to the crust but with a more pronounced effect on velocity. Additionally, pressure-induced mineral phase changes, where minerals rearrange their structures, in the upper mantle contribute to this acceleration. For example, olivine transforms into its denser polymorphs, wadsleyite and ringwoodite, at depths of approximately respectively, resulting in a more compact structure that facilitates faster seismic wave propagation in the transition zone.
Lower mantle
Average P-wave velocity: 10–13 km/s Average S-wave velocity: 5.5–7.0 km/s In the lower mantle, the rise in seismic velocity is driven by increasing pressure, which is greater here than in the upper layers, resulting in denser rock and faster seismic wave travel. Although thermal effects may lessen seismic velocity by softening the rock, the predominant factor in the lower mantle remains the increase in pressure.
Outer Core
Average P-wave velocity: 8.0–10 km/s S-waves: Do not propagate as the outer core is liquid In the outer core, seismic velocity significantly decreases due to its liquid state, which impedes the speed of seismic waves despite the high pressure. This sharp decline is observed at the core-mantle boundary, also referred to as the D'' region or Gutenberg discontinuity.
Furthermore, the reduction in seismic velocity in the outer core suggests the presence of lighter elements like oxygen, silicon, sulfur, and hydrogen, which lower the density of the outer core.
Inner core
Average P-wave velocity: ~11 km/s Average S-wave velocity: ~3.5 km/s The solid, high-density composition of the inner core, predominantly iron and nickel, results in increased seismic velocity compared to the liquid outer core. While light elements also present in the inner core modulate this velocity, their impact is relatively contained.
Anisotropy of inner core The inner core is anisotropic, causing seismic waves to vary in speed depending on their direction of travel. P-waves, in particular, move more quickly along the inner core's rotational axis than across the equatorial plane. This suggests that Earth's rotation affects the alignment of iron crystals during the core's solidification.
There is also evidence suggesting a distinct transition zone ("inner" inner core), with a hypothesized transition zone some beneath the inner core boundary. This is inferred from anomalies in travel times for P-wave that travels through the inner core. This transition zone, perhaps thick, may provide insights into the alignment of iron crystals, the distribution of light elements, or Earth's accretion history.
Studying the inner core poses significant challenges for seismologists and geophysicists, given that it accounts for less than 1% of Earth's volume and is difficult for seismic waves to penetrate. Moreover, S-wave detection is challenging due to minimal compressional-shear wave conversion at the boundary and substantial attenuation within the inner core, leaving S-wave velocity uncertain and an area for future research.
Lateral variation of velocity structure Lateral variation in seismic velocity is a horizontal change in seismic wave speeds across the Earth's crust due to differences in geological structures like rock types, temperature, and fluids presence, affecting seismic wave travel speed. This variation helps delineate tectonic plates and geological features and is key to resource exploration and understanding the Earth's internal heat flow.
Discontinuity Discontinuities are zones or surfaces within the Earth that lead to abrupt changes in seismic velocity, revealing the composition and demarcating the boundaries between the Earth's layers.
The following are key discontinuities within the Earth: Mohorovičić discontinuity: the boundary between the crust and the mantle, located approximately below the continental crust and beneath the oceanic crust. discontinuity: a phase transition where olivine becomes wadsleyite. discontinuity: a phase transition where wadsleyite becomes ringwoodite. discontinuity: a phase transition where of ringwoodite to bridgmanite and ferropericlase. Gutenberg discontinuity: the core-mantle boundary, at approximately depth. Lehmann discontinuity: marking the inner core boundary, at approximately depth.
Velocity structure of the Moon
Knowledge of the Moon's seismic velocity primarily stems from seismic records obtained by Apollo missions' Passive Seismic Experiment (PSE) stations. Between 1969 and 1972, five PSE stations were deployed on the lunar surface, with four operational until 1977. These four stations created a network on the near side of the moon, configured as an equilateral triangle with two stations at one vertex. This network recorded over 13,000 seismic events, and the gathered data remains a subject of ongoing study. The analysis has revealed four moonquake mechanisms: shallow, deep, thermal, and those caused by meteoroid impacts.
Crust
Average P-wave velocity: 5.1–6.8 km/s Average S-wave velocity: 2.96–3.9 km/s
The seismic velocity on the Moon varies within its roughly 60 km thick crust, presenting a low seismic velocity at the surface. Velocity readings increase from 100 m/s near the surface to 4 km/s at a depth of 5 km and rise to 6 km/s at 25 km depth. At 25 km depth, a discontinuity presence, at which the seismic velocity increases abruptly to 7 km/s. This velocity then stabilizes, reflecting the consistent composition and hydrostatic pressure conditions at greater depths.
Seismic velocities within the Moon's approximately 60 km thick crust exhibit an initial low of 100 m/s at the surface, which escalates to 4 km/s at 5 km depth, and then to 6 km/s at 25 km depth where velocities sharply increase to 7 km/s and stabilize, revealing a consistent composition and pressure conditions in deeper layers.
Surface velocities are low due to the loose, porous nature of the regolith. Deeper, compaction increases velocities, with the region beyond 25 km depth characterized by dense, sealed anorthosite and gabbro layers, suggesting a crust with hydrostatic pressure. The Moon's geothermal gradient minimally reduces velocities by 0.1-0.2 km/s.
Mantle