An earthquake at 5 km depth and one at 600 km depth can have identical magnitudes but utterly different consequences. Depth is the most misunderstood number in any earthquake report — and the most important one to read correctly. Here is the complete guide.
USGS LIVE DEPTH DATA — EVERY EARTHQUAKE INCLUDES DEPTH IN km
FEED ACTIVE
Open any USGS earthquake alert. You will see magnitude — the number almost everyone focuses on. And below it, almost always ignored: depth in kilometres. That number changes everything. A M6.0 at 5 km depth can level a city. A M7.5 at 600 km depth may produce a gentle wobble that most people on the surface sleep through. Depth determines how seismic energy dissipates before reaching the surface, how broad an area feels the shaking, and whether a tsunami is possible at all.
The transparent Earth simulation above places each earthquake dot inside the sphere at its actual depth — making the three-dimensional structure of global seismicity visible for the first time in an intuitive form. The Wadati-Benioff zone — the geometry of subducting slabs — is visible as inclined planes of dots descending into the mantle. This article teaches you to read every depth value in every earthquake report with full understanding.
WHY DEPTH CHANGES EVERYTHING
When an earthquake occurs, seismic energy radiates outward in all directions from the hypocenter — the actual point of rupture underground. The epicenter is the point on the surface directly above it. The deeper the hypocenter, the further the seismic waves must travel to reach the surface, and the more they attenuate (weaken) along the way. But depth does more than attenuate energy — it fundamentally changes the geometry of how that energy reaches the surface.
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SHALLOW = INTENSE BUT LOCAL
A shallow earthquake concentrates its energy over a small surface area directly above the hypocenter. Peak ground acceleration (PGA) near the epicenter can be extremely high. Damage is catastrophic at the epicenter but falls off rapidly with distance. The 1999 Izmit earthquake (M7.6, 17 km depth) destroyed 100,000 buildings within 20 km but was barely felt 300 km away.
▸ HIGH PGA NEAR EPICENTER · RAPID FALL-OFF · TSUNAMI RISK IF OFFSHORE
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DEEP = BROAD BUT DILUTE
A deep earthquake spreads its energy over a very wide surface area — the circle of felt shaking can be thousands of kilometres across. But peak ground acceleration at any given point is much lower. The 1994 Bolivia earthquake (M8.2, 631 km depth) was felt across most of South America and even into parts of North America — but caused no structural damage anywhere.
▸ LOW PGA · HUGE FELT AREA · NO TSUNAMI POSSIBLE
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DEPTH AND TSUNAMIS
Tsunamis require vertical displacement of the ocean floor. Only shallow earthquakes (generally less than 70 km depth) beneath the ocean can generate tsunamis — the rupture must be close enough to the seafloor to physically move it. Deep earthquakes cannot generate tsunamis regardless of their magnitude. The first question after any submarine earthquake: what is the depth?
▸ >70 km = NO TSUNAMI POSSIBLE · <35 km OFFSHORE = HIGHEST RISK
THE OFFICIAL DEPTH CLASSIFICATION SYSTEM
The USGS and International Seismological Centre (ISC) classify earthquakes into three depth categories based on where in Earth's structure the rupture occurs. Each category has distinct physics, distinct causes, and distinct hazard implications:
▸ 0 – 70 KM
SHALLOW FOCUS EARTHQUAKES
The most dangerous category — accounting for approximately 70% of all M6+ events globally and nearly all earthquake fatalities in recorded history. Shallow earthquakes occur in the brittle upper crust and upper mantle, where rock is cold enough to fracture rather than flow. The category includes crustal earthquakes (0–35 km, within the continental or oceanic crust) and the shallowest subduction zone events where the slab has just begun to descend. The 2023 Turkey-Syria earthquake sequence (M7.8 and M7.5, 10–20 km depth), the 2010 Haiti earthquake (M7.0, 13 km depth), and the 1906 San Francisco earthquake (M7.9, estimated 8–10 km depth) were all in this category.
▸ HIGHEST HAZARD · ALL MAJOR TSUNAMI SOURCES · ALL HISTORIC URBAN DISASTERS
▸ 70 – 300 KM
INTERMEDIATE FOCUS EARTHQUAKES
Intermediate earthquakes occur in the upper part of the descending subducting slab — below the continental Moho (crust-mantle boundary) but above the depth where the slab material becomes too warm and ductile to fracture. The mechanisms of faulting at these depths are different from shallow earthquakes — they involve dehydration embrittlement (water released from hydrated minerals lowers the effective pressure, allowing fracture) and possibly transformational faulting. These earthquakes can still cause significant damage if the source region is directly beneath a populated area, but peak intensities are lower than for equivalent shallow events. The 1970 Ancash earthquake in Peru (M7.9, ~50 km depth) triggered the deadliest landslide in recorded history, killing 70,000 people.
▸ MODERATE HAZARD · CAN CAUSE DAMAGE NEAR SOURCE · NO TSUNAMI BELOW 70 km
▸ 300 – 700 KM
DEEP FOCUS EARTHQUAKES
Scientifically extraordinary, operationally benign. Deep earthquakes occur in the cold interior of the descending subducting slab — at depths where the ambient mantle temperature is hot enough that rock should be ductile and unable to store elastic strain energy. Yet deep earthquakes happen. The leading explanation: phase transformational faulting — when olivine in the subducting slab transforms to spinel at around 410–520 km depth, the volumetric change can nucleate a rupture. Deep earthquakes have anomalous seismic signatures: they lack the S-wave shadow zone expected of shallow events and generate unusual radiation patterns. The deepest instrumentally recorded earthquake was a M7.9 at 751 km depth beneath the Sea of Okhotsk in 2013.
▸ LOWEST HAZARD · FELT OVER HUGE AREA · NO STRUCTURAL DAMAGE · NO TSUNAMI
DEPTH RANGES AND WHAT EACH MEANS FOR YOU
DEPTH
CATEGORY
LOCATION IN EARTH
FELT AREA
TSUNAMI RISK
DOMINANT CAUSE
0–10 km
Very Shallow
Upper continental crust
Small (~100 km radius)
High if offshore
Strike-slip, normal, reverse faults
10–35 km
Shallow Crustal
Lower crust / Moho
Moderate (~200 km)
Moderate if offshore
Crustal faults, volcanic systems
35–70 km
Shallow-Mantle
Uppermost mantle
Large (~400 km)
Lower; possible if <70 km
Subduction initiation, subcrustal
70–150 km
Intermediate
Upper subducting slab
Very large (~600 km)
None
Dehydration embrittlement
150–300 km
Intermediate-Deep
Mid-slab
Continent-wide
None
Dehydration, thermal cracking
300–500 km
Deep
Transition zone
Multi-continent
None
Phase transformation (olivine→spinel)
500–700 km
Very Deep
Lower mantle boundary
Global (large events)
None
Phase transformation, unclear mechanics
WHERE IN THE EARTH EACH DEPTH SITS
To understand why different depths produce different behaviours, it helps to know exactly what material the earthquake is rupturing through — and what the temperature and pressure conditions are at that depth. The planet is not uniform from surface to core.
0–7 km
OCEANIC CRUST
The thin basaltic crust beneath the ocean floors — densest and oldest at subduction zones where it begins its descent. Oceanic crust ranges from 6–10 km thick. The very shallowest submarine earthquakes — including the source zones of most major tsunamis — occur here, at the interface between the descending oceanic slab and the overriding continental crust.
▸ MAJOR TSUNAMI SOURCES: 2004 INDIAN OCEAN, 2011 TŌHOKU — BOTH AT 5–30 km DEPTH
0–70 km
CONTINENTAL CRUST
Continental crust averages 35 km thick but ranges from 25 km (thin continental margins) to 70 km (beneath the Tibetan Plateau, thickened by the India-Eurasia collision). Most continental earthquakes occur in the upper 20 km — in active fault zones within the brittle upper crust. The lower crust (35–70 km, the ductile lower crust) generates fewer earthquakes because at these temperatures, rock deforms plastically rather than fracturing.
▸ MOST URBAN DISASTERS OCCUR 5–20 km DEPTH IN THIS LAYER
70–410 km
UPPER MANTLE
Below the Mohorovičić discontinuity (Moho), the composition changes from felsic/mafic rock to peridotite (olivine-rich). The upper mantle is predominantly solid but hot enough to deform slowly over geological timescales. Within subducting slabs, temperatures are anomalously cold — cold enough to maintain brittle behaviour and generate intermediate earthquakes. The Wadati-Benioff zone — the inclined plane of seismicity marking a descending slab — descends through this region.
▸ WADATI-BENIOFF ZONE VISIBLE ON 3D TRANSPARENT GLOBE IN THIS LAYER
410–660 km
TRANSITION ZONE
Two major seismic discontinuities define this layer: the 410 km discontinuity (olivine → wadsleyite phase transition) and the 660 km discontinuity (wadsleyite → perovskite, the major upper-lower mantle boundary). Deep-focus earthquakes are concentrated at and just above these phase transitions. The volumetric change during phase transformation generates stress that can nucleate shear ruptures — the leading explanation for why earthquakes occur at all at these extreme pressures and temperatures.
▸ DEEPEST EARTHQUAKES CONCENTRATE HERE · PHASE TRANSFORMATION MECHANICS
>660 km
LOWER MANTLE & CORE
Below 660 km, earthquakes effectively cease. The lower mantle is perovskite (bridgmanite) — a mineral phase stable only at extreme pressure. The mantle here is thought to be too viscous and hot to support brittle fracture. The outer core (2,900–5,100 km) is liquid iron — no earthquakes possible. The inner core (5,100–6,370 km) is solid iron under extreme pressure — also seismically quiet, though it does transmit seismic waves from surface events as a probe of its structure.
▸ NO EARTHQUAKES BELOW ~700 km · OUTER CORE IS LIQUID · INNER CORE IS SOLID
THE 10 KM DEFAULT — WHAT IT REALLY MEANS
If you look at the USGS earthquake catalog regularly, you will notice a striking pattern: an enormous number of earthquakes have a reported depth of exactly 10 km. More than any other value. This is not a coincidence — it is a data artifact that every regular earthquake watcher needs to understand.
// WHY SO MANY EARTHQUAKES ARE LISTED AT EXACTLY 10.0 km DEPTH
When the USGS automated detection algorithm cannot reliably constrain an earthquake's depth — because not enough seismic stations recorded the event, the station geometry was poor, or the event was too small — it assigns a default depth of 10.0 km. This default is a placeholder, not a measurement. It means: "we detected this earthquake but cannot determine its depth with confidence."
For small, local events in poorly instrumented regions, default depths are common. For significant events (M4.5+) in well-instrumented regions, depth is typically well-constrained within ±5 km. The USGS adds a "depth fixed by location program" note to events where the default was assigned. On Pandita Data's earthquake globe, you will see clusters of events at exactly 10 km in regions with sparse seismic networks — particularly in parts of Africa, Central Asia, and remote ocean areas. These are real earthquakes with uncertain depths, not shallow events masquerading as 10 km. A second clue: events listed as exactly 35 km depth in oceanic regions also frequently indicate a fixed-depth placeholder — 35 km being the standard USGS default for oceanic events where depth is unconstrained.
READING THE SEE-THROUGH EARTH SIMULATION
The Pandita Data transparent Earth simulation is designed specifically to make depth visible. Unlike the standard earthquake globe where all dots appear on the surface, the transparent view renders each earthquake inside the sphere at its actual depth — allowing you to see the three-dimensional structure of global seismicity.
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SURFACE CLUSTER = SHALLOW
Earthquakes clustered close to the Earth's surface (visible on the outer shell) are shallow events — the category responsible for almost all earthquake fatalities. These are the red and orange dots on the standard globe, and they appear near the surface on the transparent view. High density clusters mark active fault systems: Japan, Indonesia, Chile, Turkey, California.
▸ 0–70 km · SURFACE LEVEL ON TRANSPARENT GLOBE
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INCLINED PLANES = SUBDUCTING SLABS
The most striking feature of the transparent globe: in subduction zones, earthquake dots form clear inclined planes descending from the surface into the mantle — the Wadati-Benioff zone. The angle of descent, the depth extent, and the density of events all reveal the geometry of the subducting slab in each region. Japan's slab descends steeply. Chile's slab is shallower. The Tonga trench plunges to nearly 700 km.
▸ WADATI-BENIOFF ZONE · SLAB GEOMETRY VISIBLE · 70–700 km
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DEEP ISOLATED DOTS = DEEP FOCUS
Deep blue and purple dots deep inside the sphere — particularly beneath South America, the western Pacific, and the Sea of Okhotsk — are deep-focus earthquakes in the cold interior of deeply subducted slabs. Their positions map the oldest, coldest parts of slabs that have descended furthest. These events confirm that subducted material penetrates the 660 km discontinuity into the lower mantle in some regions.
▸ 300–700 km · DEEP INSIDE SPHERE · SCIENTIFICALLY SIGNIFICANT
// DEPTH IN PRACTICE — THREE CONTRASTING REAL EARTHQUAKES
2010 Haiti M7.0 — 13 km depth: Killed approximately 230,000 people. The shallow crustal rupture directly beneath Port-au-Prince concentrated peak ground acceleration values exceeding 0.5g in a densely populated city built almost entirely of unreinforced concrete. The shallow depth meant the energy had almost no distance to attenuate before reaching the surface. A comparable magnitude at 150 km depth would have caused zero casualties.
2013 Sea of Okhotsk M8.3 — 608 km depth: One of the largest deep-focus earthquakes ever recorded. Felt across Russia, Japan, and parts of China — a felt area of roughly 15 million km². Zero fatalities, zero structural damage. The energy, though enormous at the source, was spread across such a vast area by the time it reached any surface that peak intensities nowhere exceeded MMI IV (light shaking). No tsunami advisory was issued.
2011 Tōhoku M9.1 — 29 km depth: The earthquake itself killed fewer people than might be expected for a M9.1 — Japan's seismic engineering absorbed most of the ground motion. What killed ~20,000 people was the tsunami generated by 29 km of shallow seafloor displacement raising a column of ocean water 50–60 m above normal sea level. The shallow depth — just 29 km beneath the seafloor in the Japan Trench — was the direct cause of the tsunami's destructive power.