Continental mantle earthquakes

Researchers at Stanford University have produced the first-ever global map of a rare category of earthquakes that occur deep within the Earth’s mantle rather than in its crust. The findings have been published in the journal Science.

The mantle is the thick layer sandwiched between the Earth’s thin outer crust and its molten core. While most earthquakes originate in the cold, brittle crust at depths of about 10 to 29 kilometres, mantle earthquakes occur much deeper — sometimes more than 80 kilometres below the boundary separating the crust and the mantle.

This boundary, known as the Mohorovičić discontinuity, or “Moho,” marks the transition from the rigid crust to the warmer, semi-solid mantle that constitutes most of the planet’s interior.

Regional Clustering Observed

According to the study, continental mantle earthquakes occur worldwide but are regionally clustered. Significant concentrations were identified in:

• The Himalayas in southern Asia
• The Bering Strait region between Asia and North America, south of the Arctic Circle

Surprising Geological Insight

The findings are considered surprising because, at such depths, intense heat and pressure typically cause rocks to deform and flow rather than fracture. Beyond the brittle-ductile transition — the point at which rocks stop snapping and begin to creep — earthquakes are generally expected to be rare.

However, the new global catalog shows that ruptures do occur in the upper portion of the mantle, which extends roughly 1,800 miles in thickness.

Limited Surface Impact, Significant Scientific Value

Although mantle earthquakes are too deep to cause significant shaking at the Earth’s surface, researchers say studying them can provide crucial insights into earthquake mechanics, tectonic processes, and the physical behaviour of rocks under extreme conditions.