Continental Mantle Earthquakes
Continental Mantle Earthquakes
Context
Stanford University (published in the journal Science) unveiled the first global map of rare continental mantle earthquakes. By analyzing over 46,000 seismic events recorded since 1990, researchers identified 459 specific events that occurred deep within the mantle beneath continental landmasses, challenging the long-held belief that the mantle is too ductile to "snap" and cause quakes.
About Continental Mantle Earthquakes
What are they? Most earthquakes occur in the Earth’s brittle crust (the top 10–29 km). Continental mantle earthquakes are anomalies that originate far deeper, often more than 80 km below the Mohorovičić discontinuity (Moho), which is the boundary between the crust and the mantle.
Global Distribution: While these events occur worldwide, they are not random. The study identified two primary clusters:
- The Himalayan Collision Zone: Where the Indian Plate subducts beneath the Eurasian Plate. The Tibetan Plateau is almost "ringed" by these mantle quakes.
- The Bering Strait: The region between Asia and North America, south of the Arctic Circle.
- Other Noted Areas: The Alpine-Himalayan belt, Romania’s Vrancea zone, and Iran’s Zagros mountains.
How They Originate
For decades, scientists debated if the mantle could support earthquakes because high heat and pressure usually make rocks flow like plastic (ductile) rather than break (brittle).
- Thermal Variations: Localized "cool" zones within the mantle (often from subducting plates) may remain brittle enough to fracture even at great depths.
- Stress Transfer: Intense tectonic stress from crustal collisions can penetrate the Moho, triggering ruptures in the upper mantle.
- Mantle Convection: Internal heat-driven "rivers of rock" recycle old crustal slabs, which can crack as they descend.
Comparison: Crustal vs. Mantle Earthquakes
|
Feature |
Crustal Earthquakes |
Continental Mantle Earthquakes |
|
Origin Depth |
Typically 10–29 km |
>80 km below the Moho |
|
Material State |
Brittle rock |
Generally ductile (quakes occur in rare brittle pockets) |
|
Seismic Wave Ratio |
High Lg waves (crustal-travelling) |
High Sn waves (mantle-travelling) |
|
Surface Impact |
Can be highly destructive |
Minimal; shaking is rarely felt at the surface |
|
Frequency |
Very common |
Rare (approx. 3-4% of identified deep events) |
Detection Technology: The "Waveform Signature"
The Stanford team used a "game-changer" method to identify these quakes by comparing the ratio of two specific seismic waves:
- Lg Waves: High-frequency waves that bounce through the crust.
- Sn (Lid) Waves: Shear waves that travel through the top layer of the mantle (the "lid").
A high Sn/Lg amplitude ratio serves as a "fingerprint," confirming that the earthquake's energy originated below the crustal boundary.
Significance
- Earth’s Interior "Sonogram": These quakes act like a natural ultrasound, providing data on the stress and temperature of the mantle that we cannot reach by drilling.
- Mountain Building: In the Himalayas, mantle quakes offer clues into how deep tectonic processes drive orogeny (mountain formation).
- Interconnected Cycle: The study suggests that the crust and mantle are a single "interconnected earthquake cycle," where deep breaks may influence future shallow, destructive quakes.