عمق الزلزلة: زلازل ضحلة ومتوسطة وعميقة

Shallow Earthquakes (0-70 km): The Most Destructive

The vast majority of the world's most destructive earthquakes are shallow — their Hypocenter (Focus)The actual point within the Earth where an earthquake rupture initiates. Also called the focus. Depth of the hypocenter significantly affects how an earthquake is felt at the surface.s lie less than 70 kilometres below the surface. This depth category accounts for roughly 75 percent of all seismic energy released globally. Shallow earthquakes are most destructive because the shaking energy has the shortest distance to travel before reaching populated areas at the surface, resulting in concentrated, intense ground motion directly above the rupture zone.

The shallow category encompasses all tectonic environments: strike-slip faults like the San Andreas, normal faults at rift zones, reverse faults at mountain belts, and the shallowest portions of Subduction ZoneA region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest earthquakes (M8.5+) and are associated with deep ocean trenches and volcanic arcs. megathrusts. Shallow crustal earthquakes — those in the 0–35 km range — are typically the most damaging per unit of MagnitudeA single number that quantifies the total energy released by an earthquake. Each whole number increase represents roughly 31.6 times more energy released. because they occur in the brittle, cold upper crust where stress accumulates most efficiently and where the shallow depth maximises surface Seismic IntensityA measure of the strength of shaking at a particular location, determined by observed effects on people, structures, and the natural environment. Decreases with distance from the epicenter.. The 2010 Haiti earthquake (Mw 7.0) at just 13 km depth caused catastrophic destruction; a similarly sized event at 200 km depth would have produced far less damage.

Intermediate Earthquakes (70-300 km): Subduction Zone Activity

Intermediate-depth earthquakes occur almost exclusively within actively descending Subduction ZoneA region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest earthquakes (M8.5+) and are associated with deep ocean trenches and volcanic arcs. slabs — slabs of oceanic lithosphere that have plunged beneath continental or other oceanic plates. As the slab descends, the cold brittle material continues to behave seismically (failing by brittle fracture) until it eventually heats up sufficiently to deform plastically rather than break.

The Wadati-Benioff zone — the inclined band of seismicity that traces the descending slab from the surface down to 700 km — encompasses both the intermediate and deep earthquake categories. Intermediate earthquakes are important sources of shaking in countries overlying active subduction zones. In Chile, Japan, and Indonesia, intermediate-depth events routinely affect populated areas hundreds of kilometres inland. Because the shaking energy must travel upward through the cold slab and surrounding mantle before reaching the AsthenosphereThe partially molten, ductile layer of Earth's upper mantle beneath the lithosphere, extending from about 100-700 km depth. Tectonic plates 'float' and move on the asthenosphere. and LithosphereThe rigid outer layer of Earth, comprising the crust and upper mantle, broken into tectonic plates. The lithosphere averages about 100 km thick under oceans and 150 km under continents., the amplitude may be somewhat attenuated compared to shallow events of the same magnitude, but the very large geographic footprint means many people feel them.

Deep Earthquakes (300-700 km): Mysteries of the Mantle

Deep focus earthquakes, occurring between 300 and approximately 700 km depth, pose a fascinating geophysical puzzle. At these depths and pressures, rock should be unable to fail by brittle fracture — yet deep earthquakes occur with the same sharp, sudden character as shallow ones, indicating some kind of rapid shear failure. Two main mechanisms have been proposed: transformational faulting (mineral phase transitions that produce sudden volume changes) and dehydration embrittlement (water released from subducting sediments enabling faulting). The exact mechanism remains an active area of research.

The deepest confirmed earthquakes occur near 700 km depth, where the slab appears to encounter a phase transition boundary that either absorbs it into the lower mantle or causes it to stagnate. Beyond this depth, seismicity essentially disappears — the Earth below 700 km is aseismic. The 2013 Sea of Okhotsk earthquake (Mw 8.3) at approximately 600 km depth was one of the largest deep-focus earthquakes ever recorded; it was felt across much of Russia but caused no damage because its depth spread the energy over an enormous surface area.

How Depth Affects Intensity at the Surface

The relationship between depth and surface Seismic IntensityA measure of the strength of shaking at a particular location, determined by observed effects on people, structures, and the natural environment. Decreases with distance from the epicenter. is governed by geometry. An earthquake at 10 km depth that produces shaking of MMI VIII directly above the Hypocenter (Focus)The actual point within the Earth where an earthquake rupture initiates. Also called the focus. Depth of the hypocenter significantly affects how an earthquake is felt at the surface. will produce that intensity over a small area. The same magnitude earthquake at 100 km depth might produce only MMI VI at the surface point directly above, but that moderate shaking will be spread over an area ten times larger. Total energy reaching the surface is roughly conserved, but it is redistributed over a much larger footprint.

This depth effect is especially pronounced for very shallow earthquakes. Events at 3–5 km depth can produce catastrophic shaking in a tiny geographic area while causing very little damage just 50 km away. The 2010 Canterbury sequence in New Zealand included the September mainshock at about 10 km depth and the devastating February 2011 Christchurch earthquake at only 5 km depth, which destroyed the city centre despite having a lower magnitude (6.2) than the September event, precisely because its shallower depth concentrated the energy more directly under the city.

Why Subduction Zones Produce Earthquakes at All Depths

Subduction ZoneA region where one tectonic plate dives beneath another into the mantle. Subduction zones produce the world's largest earthquakes (M8.5+) and are associated with deep ocean trenches and volcanic arcs.s are the only tectonic setting where earthquakes occur at depths greater than about 30–35 km — the base of normal continental crust. This is because the descending oceanic slab carries cold, brittle material down to great depths faster than it can be heated by the surrounding mantle. The cold slab maintains sufficient rigidity to fracture seismically well into the transition zone at 400–700 km depth.

The AsthenosphereThe partially molten, ductile layer of Earth's upper mantle beneath the lithosphere, extending from about 100-700 km depth. Tectonic plates 'float' and move on the asthenosphere. and LithosphereThe rigid outer layer of Earth, comprising the crust and upper mantle, broken into tectonic plates. The lithosphere averages about 100 km thick under oceans and 150 km under continents. model helps explain this. The rigid lithosphere (crust plus uppermost mantle) is cold enough to store elastic strain and fail suddenly. The underlying asthenosphere is hot enough to flow plastically. In a subduction zone, a cold slab of lithosphere descends into the hot asthenosphere, but remains cold enough to earthquake for hundreds of kilometres before finally heating and becoming aseismic. This is why the Wadati-Benioff zone can be traced to 700 km depth in the deepest subduction zones, like those beneath Tonga and the Sea of Okhotsk.

The Deepest Earthquakes Ever Recorded

The deepest instrumentally recorded earthquakes have occurred at about 700 km depth, near the base of the upper mantle transition zone. These extreme events require unusually cold, rapidly descending slabs — the conditions found in the western Pacific subduction systems where old, cold oceanic crust plunges steeply into the mantle. At these depths, pressures exceed 200,000 atmospheres and temperatures approach 1,500°C, yet the slab remains cool enough relative to its surroundings to behave seismically.

Use the Felt Radius Calculator to explore how earthquake depth interacts with magnitude to determine how large an area feels significant shaking. For a given magnitude, doubling the depth roughly doubles the radius of felt shaking but halves the maximum Seismic IntensityA measure of the strength of shaking at a particular location, determined by observed effects on people, structures, and the natural environment. Decreases with distance from the epicenter. at the surface — a clear demonstration of the depth-intensity trade-off that makes shallow earthquakes so disproportionately dangerous.