A "horst earthquake" is not a specific type of earthquake, but rather an earthquake that occurs in or is associated with a horst geological structure. Horsts are significant landforms that result from specific tectonic forces.
Understanding a Horst
A horst is an upthrown block of crustal rock that lies next to a downdropped block known as a graben. Both horsts and grabens are typically found together in an extensional environment, which means an area where the Earth's crust is being pulled apart or stretched.
Key Characteristics of Horsts and Grabens:
| Feature | Horst | Graben |
|---|---|---|
| Description | An upthrown, elevated block of land | A downdropped, depressed block of land |
| Formation | Bounded by two normal faults that dip away from each other | Bounded by two normal faults that dip towards each other |
| Environment | Forms in extensional tectonic settings, where the crust is being stretched apart | Forms in extensional tectonic settings, where the crust is being stretched apart |
| Appearance | Often appear as ridges or elevated plateaus | Often appear as valleys or basins |
Earthquakes Associated with Horsts
Since horsts and grabens form in extensional environments, the earthquakes that occur in these regions are primarily caused by normal faulting. Normal faults are a type of dip-slip fault where the hanging wall (the block of crust above the fault) moves downward relative to the footwall (the block below the fault). This movement is a direct result of the tensional forces pulling the crust apart.
When movement occurs along these normal faults, it releases seismic energy, generating earthquakes. Therefore, an earthquake occurring within a horst-graben system, or an earthquake that contributes to the formation or further displacement of these structures, could be colloquially referred to as being "related to a horst."
Types of Earthquakes in Extensional Settings:
- Tectonic Earthquakes: The vast majority of earthquakes associated with horsts are tectonic, meaning they result from the sudden release of stress accumulated along fault lines due to the movement of tectonic plates.
- Shallow Earthquakes: Earthquakes in extensional environments tend to be relatively shallow because the crust is thinner and more brittle where it is being stretched, allowing for more frequent, but generally less deep, ruptures.
Examples of Horst and Graben Systems
Horst and graben structures, and the earthquakes associated with them, are common features in many active rift zones around the world. These include:
- The East African Rift Valley: A prime example of a continental rift where extensive horst and graben systems are forming, accompanied by frequent shallow earthquakes.
- The Basin and Range Province (Western United States): Characterized by numerous parallel mountain ranges (horsts) and valleys (grabens) formed by crustal extension and normal faulting.
- Rift Valleys on Mid-Ocean Ridges: Though not typically landforms, the central valleys along mid-ocean ridges are essentially grabens formed by the spreading of the seafloor, and they experience many earthquakes.
Understanding horsts provides insight into the type of tectonic activity and associated earthquakes expected in regions undergoing crustal extension.
