Deep-focus earthquakes

Deep-focus earthquakes occur at depths ranging from 70 to 700 kilometers below the earth's surface. This range of depths represents the zone from the base of the earth's crust to approximately one-fourth of the distance into the earth's mantle. Deep-focus earthquakes provide scientists with information about the planet's interior structure, composition, and seismicity. Observation of deep-focus earthquakes has played a fundamental role in the discovery and understanding of plate tectonics.

Earthquakes and Earth's Interior

Because direct physical access to the earth's interior is restricted, most knowledge about it is derived from the study of earthquake waves that travel through the planet and vibrate the earth's surface at some distant point. Earthquakes recorded on seismograms allow scientists to accurately measure the time required for earthquake waves (seismic waves) to travel from the focus of an earthquake to a seismographic station. Primary waves (P waves) travel the fastest, and this information is used to determine distances to earthquake epicenters. The time required for P waves and secondary waves (S waves) to travel through the earth depends on the physical properties encountered as the waves pass through the subsurface. Seismologists, therefore, search for variations in travel times that cannot be explained by differences in the distance traveled. These differences correspond to changes in the properties of the subsurface earth material encountered. Changes in rock properties indicate that the earth has four major layers: the crust, a thin outer layer; the mantle, a rocky layer beneath the crust with a depth of 2,885 kilometers; a 2,770-kilometer outer core exhibiting characteristics of a mobile liquid composition; and a solid inner core, a metallic sphere with a radius of 1,216 kilometers.

Earthquake Zones

Most earthquakes occur in narrow zones that globally connect to form a continuous seismic network. Characteristic surface features of seismic zones are rift valleys, oceanic ridges, mountain belts, volcanic chains, and deep-ocean trenches. These global seismic zones represent the boundaries of the lithospheric plates. The interior regions of the lithospheric plates are largely free of earthquakes. The lithosphere is made up of twelve rigid plates that cover the entire globe. The depths of the lithospheric plates range between 60 and 100 kilometers. They are composed of either the entire continental crust and a portion of the upper mantle or the entire oceanic crust and a portion of the upper mantle. The lithospheric plates are in constant motion relative to each other and can diverge away from each other, forming ridge axes and new oceanic crust material; converge toward each other, where one plate subducts under the leading edge of its neighbor plate; or transform, where plates slide horizontally past each other.

Within the global seismic network, there are four types of seismic areas that are recognized by their form, structure, and geology. The first type is represented by narrow zones of high surface-heat flow and basaltic volcanic activity along the axes of mid-ocean ridges where the earthquake focus is shallow (70 kilometers deep or less). Here, molten rock material is welling up from the mantle area and is emplaced on either side of the ridge, adding to the ocean crust. Mid-ocean ridges are active areas of seafloor spreading and are found in all ocean basins. A primary example is the Mid-Atlantic Ridge trending north to south in the Atlantic Ocean basin. This ridge rises above sea level in Iceland and delineates the boundary between the South American and African lithospheric plates.

The second type of seismic zone is identified by large surface displacements occurring parallel to the fault, shallow-focus earthquakes, and absence of volcanoes. Excellent examples of this seismic zone are the San Andreas fault in California and the Anatolian fault in Turkey, both of which demonstrate strike-slip movement between plate boundaries.

The third type of seismic zone is a widespread continental area ranging from the Mediterranean Sea to Myanmar; it is associated with high mountain ranges created by converging plate margins. Although this zone is usually characterized by shallow-focus earthquakes, earthquakes of intermediate focus (70 to 300 kilometers deep) have occurred in the Hindu Kush and Romania; deep-focus earthquakes (300 to 700 kilometers deep) have been recorded in a few places north of Sicily under the Tyrrhenian volcanoes.

The fourth type of seismic zone is physically connected to volcanic island-arc-trench systems, such as the Japan Trench and the Kermadec-Tonga Trench in the South Pacific Ocean. Earthquakes associated with trenches can be shallow, intermediate, or deep focus depending on where they are located in the steeply converging lithospheric plate adjacent to the trench. The earthquake foci define the plate being carried into the earth's interior and away from the trench. These inclined earthquake zones, called Wadati-Benioff zones, underlie active volcanic island arcs and have an assortment of complex shapes. The Wadati-Benioff zone also marks the downflowing portion of the convection cell as identified by geophysicists through calculations of moving oceanic plates.

Subduction of the Lithosphere

New lithosphere is created at mid-ocean ridges by upwelling and cooling of magma from the earth's mantle. With new lithospheric material constantly created and no measurable expansion of the earth occurring, some of the lithosphere must be removed globally. Older oceanic lithosphere is removed by subduction of the oceanic plate into the earth's mantle at island-arc-trench systems. The originally rigid plate slowly descends and heats; over millions of years, it may be fully absorbed into the earth's mantle. The subduction of lithospheric plates accounts for many of the processes that shape the earth's surface, such as volcanoes and earthquakes, including nearly all of the earthquakes with deep and intermediate foci. Deep earthquakes cannot occur except in subducting plates, so the presence of a deep-focus earthquake implies the presence of subducting oceanic plate material.

The proposed driving force for movement of the lithospheric plates is a convection cell arrangement where molten mantle material rises into a crustal rift zone from the subsurface mantle zone. Portions of the molten mass that do not move into the central rift zone move to either side of the rift and travel away from it, below and parallel to the lithospheric plate. The convection current travels beneath the plate for some distance. It is in this zone that the convection current is thought to move and carry the lithospheric plate with it. As the convection cell cools because of its interaction with the cool oceanic plate, the outer area of the cell sinks into the mantle to be rewarmed by mantle convection currents. Again, the downflowing portion of the convection cell is represented by the subducting oceanic plate.

Island-Arc-Trench Systems

Major island chains are known to geologists as island-arc-trench systems. The island chains are a surface expression of the oceanic subduction process, and the associated deep trenches are a surface expression of the seaward boundary of subduction zones. Examples of deep trenches appearing in connection with island arcs are the Java and Tonga Trenches.

When foci of earthquakes near island arcs and ocean trenches are compared, a notable pattern emerges that is well illustrated in the Tonga island arc in the South Pacific. To the east of the volcanic islands of Tonga lies the Tonga Trench, which is approximately 10 kilometers deep. Beginning at the Tonga Trench and moving from east to west, the earthquake foci lie in a narrow but well-defined zone, which slopes from east to west from the Tonga Trench to beneath the Tonga Island Arc at an angle of about 45 degrees. The earthquake record in the arc-trench zone reveals that the earthquake foci are extremely shallow at the trench; however, moving to the west away from the trench, the earthquake foci register at a depth of 400 kilometers. An additional move to the west reveals deep-focus earthquakes at below 600 kilometers. In other regions of deep earthquake activity, some variation in the angle of dip and distribution of foci is recorded, but the common feature, that of a sloping seismic zone, is characteristic of island arcs and deep-ocean trenches. Other regions associated with island arc and deep-ocean trenches are the Kurile Ridge and Trench and the Mariana Islands and Trench.

What the deepening earthquake foci are defining at island-arc-trench systems, and especially the Tonga Trench and Island Arc system, is the movement of the descending oceanic plate (the downgoing slab) into the inclined seismic zone, known to scientists as the Wadati-Benioff zone. In the process of the plate's gravity-driven downward movement, additional force is created, causing further deformation and fracturing and deep-focus earthquakes. At mantle depths of 650 to 680 kilometers, either the plate may have been absorbed into the mantle interior, or its properties may have been altered to the extent that earthquake energy cannot be released.

Oceanic and Continental Crust Systems

Major mountain belts, such as the Andes in South America, have been raised by the convergence and subduction of oceanic lithospheric plates beneath continental lithospheric plates. As the two plates converge, the denser oceanic plate is subducted under the less-dense continental plate (South America). The zone of subduction is identified by an oceanic trench, the Peru-Chile Trench. Initially, in this setting, the angle of descent of the plate into the subduction zone is low, but the angle gradually steepens into a downward curve as revealed by intermediate- and deep-focus earthquakes. On June 8, 1994, a magnitude 8.2 deep-focus earthquake occurred 600 kilometers below Bolivia in the Andes Mountains. This earthquake released a tremendous amount of energy, but because of its deep focus, the seismic waves were slow to reach the earth's surface, preventing damage to populations and structures.

Deep-focus earthquakes have proved especially useful to researchers because they do not produce many surface seismic waves, and they provide information about the density of the earth's mantle—a factor that ultimately controls how convection currents of rock inside the planet move the continents around the surface. Because the core and mantle remain hidden from view, researchers interested in these deep regions must wait for large deep earthquakes to provide indirect information about the planet's interior. The 1994 Bolivian earthquake was well recorded because of global seismic detectors placed by scientists some twenty years in advance. The seismic equipment was placed in an effort to fully capture deep-focus earthquake activity as a result of a large deep-focus earthquake under Colombia in 1970. The 2004 earthquake in Vanuatu occurred at a depth of 735.8 kilometers and was considered to be the deepest ever. However, its record was broken in 2024 by the Bonin Islands earthquake, which took place 680 kilometers below Earth's surface. The 2013 Okhotsk Sea earthquake was the strongest in seismic record, with a magnitude of 8.3. It occurred at a depth of 609 kilometers.

Thermal Properties of the Descending Lithosphere

Temperatures near the surface of the earth increase rapidly with depth, reaching about 1,200 degrees Celsius at the depth of 100 kilometers. Here the minerals in peridotite, an olivine-rich major mineral constituent of the upper mantle, begin to melt. The temperatures then increase more gradually to 2,000 degrees Celsius at approximately 500 kilometers depth.

As the lithospheric plate descends into the mantle, it is heated primarily by heat flowing into the cooler lithospheric plate from the enclosing hotter mantle. Since the conductivity of the rock increases with temperature, conductive heating becomes more efficient with increasing depth, further warming the subducting plate. Heat within the earth is generated by the energy released when minerals in the mantle change to denser phases or more compact crystalline structures with the higher pressures present at depth. Additional heat sources in the mantle are radioactivity and the heat of compression, which is activated by increasing pressure at depth.

Despite the elevated temperatures at depth, the interior of the descending plate remains cooler than the surrounding mantle until the plate reaches a depth of about 600 kilometers. As the plate continues to subside into the deep mantle interior, it may heat rapidly because of efficient heat transfer by conduction. At 700 kilometers, however, the lithosphere plate is difficult to decipher as a separate structural unit. Complicating the detection of the plate properties is the low number of earthquakes at or below the 700-kilometer depth, which could possibly reveal information about any remaining plate material. The subduction zone under the Japanese island of Honshu, under the Kuriles, and under the Tonga-Kermadec area (north of New Zealand) represent almost ideal subduction zones without major complications caused by the age or thermal properties of the plate.

However, not all subduction zones behave ideally. The descending oceanic lithospheric material can be assimilated before reaching deep mantle zones: A slow-moving plate may achieve thermal assimilation before reaching 700 kilometers, such as the Mediterranean plate under the Aegean Sea. In younger subduction zones, as found in the Aleutian Islands and Trench, the descending plate may have penetrated far less than 700 kilometers owing to the warmer, more buoyant oceanic plate.

Study of Deep-Focus Earthquakes

The deep-focus earthquake problem has been one of the leading scientific problems of solid-earth geophysics since the 1920s, when Kiyoo Wadati, a Japanese seismologist, demonstrated that some earthquakes occur hundreds of kilometers beneath the earth's crust. Laboratory experiments attempting to replicate the temperatures and pressures of the earth's interior have confirmed that rock under stress at the higher temperatures and pressures of 70 kilometers fails suddenly by brittle fracture—that is, shallow-focus earthquakes fail by brittle fracture. At even higher temperatures and pressures, similar to what is present in the deep interior of the earth (300 to 700 kilometers), shear stress should deform rocks by ductile flow, even in the colder regions of the mantle beneath subduction zones. Yet seismic data indicate that rocks at these depths are apparently also failing by brittle fracture.

Research has been directed toward understanding the triggering mechanism for brittle fracture in deep-focus earthquakes. The first area of research concerns the mechanism for brittle failure on intermediate-depth earthquakes. Research seems to indicate that the brittle failure is driven by water subducted along with the oceanic lithospheric plate or by water released from the molecular structure of oceanic crustal minerals. Thus, down to a depth of 350 kilometers or so (many scientists consider any earthquake below 70 kilometers a deep-focus earthquake), the triggering mechanism is much like that of shallow earthquakes, with the available water reducing the normal stresses on faults at depth and allowing failure (an earthquake) to occur.

The second research position holds that olivine, a primary constituent mineral of the earth's upper mantle, transforms under stress to spinel, a denser mineral in which the atoms are more closely packed and display a rearranged crystal structure. The newly formed spinel is deposited in many elongated, beadlike structures that eventually become thin, shear faulting zones. Faulting occurs along planes of greatest stress in these thin, shear zones.

However, olivine could remain cool because of its presence within the cooler subducting plate and not transform until some greater depth. At this greater depth, the newly formed, fine-grained spinel could be the lubricating material that makes deep-focus earthquake faulting possible. As olivine undergoes the transformation to spinel, heat is released that may augment the catastrophic faulting.

Neither of these two positions has been proved conclusively, nor are they accepted by all scientists. Yet most research focuses on exploring the detailed geophysics and geochemistry of each position. For example, thermal calculations suggest that olivine may remain present even at great depths in subduction zones where the plate is old and cold (cooling over time) and, therefore, subducts rapidly. These findings might explain why there are deep-focus earthquakes in Tonga and the Kuriles where the plate material may be older, whereas warmer, younger subducting slabs may manifest only intermediate-depth events as in the Aleutians.

Another factor that may need consideration is that analysis of seismograms from intermediate or deep-focus earthquakes reveals little or no gross differences between the seismic properties for intermediate and deep earthquakes. In fact, the only gross mechanical difference between shallow earthquakes and deeper ones is that aftershocks are much rarer for deep and intermediate earthquakes. Therefore, if the seismograms are similar, then should not the source for intermediate- and deep-focus earthquakes be similar? Thus, one of the two premises may be the triggering mechanism for all earthquakes below 70 kilometers—or perhaps the triggering mechanism is a combination of the two positions.

Significance

Deep earthquakes are significant for at least four reasons. First, they are exceedingly common, constituting almost 25 percent of all earthquakes occurring during the period of 1964 to 1986. Second, deep earthquakes most often occur in association with deep-ocean trenches and volcanic island arcs in subduction zones. One of the great achievements of twentieth-century geophysics was the recognition that the occurrence of deep earthquakes in Wadati-Benioff zones apparently delineates not only the subduction of the lithospheric plate but also the cold downflowing cores of convection cells in the uppermost mantle. Third, scientists use seismograms of deep earthquakes rather than those of shallow earthquakes to investigate core, mantle, and crustal structures. Deep-focus earthquakes are mechanically different from shallow-focus earthquakes because their body wave phases traverse the uppermost mantle only once from the focus to the seismic station, thus producing simpler seismographs. Finally, information on how deep-earth materials process and handle stress can be derived from the seismograms of deep earthquakes.

Principal Terms

aftershock: an earthquake that follows a larger earthquake and originates at or near the focus of the latter; many aftershocks may follow a major earthquake, decreasing in frequency and magnitude with time

brittle fracture: rock that fractures at less than 3 to 5 percent compressional or tensional strain

ductile fracture: rock that is able to sustain, under a given set of conditions, 5 to 10 percent deformation before fracturing or faulting

epicenter: the point on the earth's surface that is directly above the focus of an earthquake

focus: the place within the earth where an earthquake commences and from which the first P waves arrive; also called the hypocenter

lithosphere: the solid portion of the earth used in plate tectonics as a layer of strength relative to the underlying plastic-like asthenosphere; encompassing the earth's crust and part of the upper mantle, it is about 100 kilometers in thickness

primary wave (P wave): the primary or fastest wave traveling away from an earthquake through the solid rock; P waves also are capable of moving through liquids

secondary wave (S wave): the secondary wave that travels more slowly through solid rock than the P wave; S waves cannot penetrate a liquid

shear stress: stress that causes different parts of an object to slide past each other across a plane

strike-slip fault: a fault along which movement is horizontal only; the movement is parallel to the trend of the fault

subduction: the process in which a dense lithospheric plate descends into the mantle beneath another, less dense plate in a subduction zone

Wadati-Benioff zone: a narrow zone of earthquake foci that seismically delineate an inclined subduction zone; they are generally tens of kilometers thick

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