What Happened
- Researchers at Stanford University published the first comprehensive global map of rare continental mantle earthquakes in the journal Science (February 2026), based on analysis of over 46,000 earthquake recordings since 1990.
- The study identified and confirmed 459 continental mantle earthquakes occurring below the Mohorovičić discontinuity (the crust-mantle boundary), at depths exceeding 80 kilometres.
- Significant clusters were found beneath the Himalayas and the Bering Strait — two geologically active zones with complex tectonic histories.
- The breakthrough methodology compares two seismic wave types — Sn waves (travelling through the mantle) and Lg waves (moving through the crust) — to distinguish mantle-generated earthquakes from crustal ones using waveform signatures alone.
- Lead researcher Shiqi Wang noted: "With this new dataset, we can start to probe at the various ways these rare mantle earthquakes initiate."
- Future research will investigate whether these deep events result from stress transfer from surface earthquakes, or from heat-driven mantle processes.
Static Topic Bridges
Earth's Internal Structure: Crust, Mantle and the Moho
Earth's interior is divided into concentric layers based on composition and physical state, inferred primarily from seismic wave behaviour. The boundary between the crust and mantle — the Mohorovičić discontinuity (Moho) — is where this new research is most significant.
- Crust: Oceanic crust is 5-10 km thick (basaltic composition); continental crust averages 35 km (granitic/felsic). Under mountain ranges like the Himalayas, it can be 60-70 km thick.
- Mohorovičić Discontinuity (Moho): Named after Croatian geophysicist Andrija Mohorovičić who discovered it in 1909. Marks the boundary where P-wave velocities jump from ~6.7-7.2 km/s (crust) to ~7.6-8.6 km/s (mantle).
- Upper Mantle: Composed primarily of peridotite (olivine-rich). The asthenosphere (partially molten zone, ~100-350 km depth) enables tectonic plate movement.
- Lower Mantle: Extends to ~2,900 km; solid, high-pressure silicate minerals.
- Outer Core: Liquid iron-nickel, ~2,900-5,100 km depth; generates Earth's magnetic field through convection (dynamo effect).
- Inner Core: Solid iron-nickel, ~5,100-6,371 km depth.
Connection to this news: Mantle earthquakes occur below the Moho — a zone previously assumed to be aseismic (non-earthquake-producing) in continental regions. The Stanford study challenges this assumption and establishes the Moho as a more dynamic boundary than previously understood.
Seismic Waves: Types, Properties and Diagnostic Use
Seismic waves are the primary tool for studying Earth's interior. They are generated by earthquakes or artificial sources and travel through Earth's layers, changing velocity and direction at boundaries.
- P-waves (Primary/Compressional): Travel through solids, liquids, and gases. Fastest seismic waves (6-8 km/s in crust). Alternately compress and expand material in the direction of travel. First to arrive at seismographs.
- S-waves (Secondary/Shear): Travel only through solids (not liquids). Slower than P-waves (~3.5-4.5 km/s in crust). Oscillate perpendicular to direction of travel. Cannot pass through Earth's liquid outer core — creating the "shadow zone."
- Surface Waves (Rayleigh and Love): Travel along Earth's surface; slower than body waves but cause most earthquake damage due to large amplitude.
- Sn waves: Mantle P-type waves that travel just below the Moho in the uppermost mantle — key diagnostic tool used in this study.
- Lg waves: Crustal guided waves trapped in the continental crust — used in contrast with Sn waves to identify earthquake source depth.
- Shadow Zone: Region on Earth's surface (~103°-142° from epicentre) where neither P nor S waves arrive directly — evidence for the liquid outer core.
Connection to this news: The Stanford team's innovation was using the contrast between Sn (mantle) and Lg (crustal) wave signatures to definitively classify earthquake depth and origin — enabling the first systematic identification of mantle quakes at global scale.
Himalayan Tectonics and Seismicity
The Himalayas are among the most seismically active and geologically complex regions on Earth, formed by the ongoing collision of the Indian Plate with the Eurasian Plate — a process that began ~50 million years ago.
- The Himalayan collision zone has thickened the continental crust to 60-70 km, creating exceptional conditions where mantle depths are reached at shallower absolute elevations.
- India experiences frequent earthquakes (Zones III, IV, V under BIS seismic zonation), with high-magnitude events along the Main Boundary Thrust and Main Central Thrust.
- Significant Himalayan earthquakes include: Bihar-Nepal 1934 (Mw 8.0), Uttarkashi 1991, Chamoli 1999, Nepal 2015 (Mw 7.8).
- The Himalayas also host the planet's highest peaks, with topographic loading creating stress regimes that extend deep into the lithosphere.
- India's National Centre for Seismology (NCS) under MoES monitors seismic activity and operates a national seismograph network.
Connection to this news: The detection of mantle earthquake clusters beneath the Himalayas suggests that the collision-zone stress field extends deeper than previously mapped — with implications for understanding deep seismic hazard and the long-term evolution of the Himalayan orogen.
Key Facts & Data
- Study published in Science journal, February 2026, by Stanford University (Doerr School of Sustainability).
- Dataset: 46,000+ earthquake recordings since 1990; confirmed 459 continental mantle earthquakes.
- Mantle earthquakes occur below 80 km depth (below the Moho boundary).
- Typical crustal earthquakes: 10-29 km depth.
- Moho depth: 5-10 km under oceans; 20-90 km under continents (avg 35 km); 60-70 km under mountain ranges.
- Identification method: Sn wave (mantle) vs. Lg wave (crustal) waveform comparison.
- Key clusters found: beneath the Himalayas and the Bering Strait.
- P-wave velocity jump at Moho: ~6.7-7.2 km/s (crust) → 7.6-8.6 km/s (mantle).
- Moho discovered in 1909 by Andrija Mohorovičić using Zagreb earthquake data.