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This lesson examines conservative (transform) plate boundaries and intra-plate tectonic activity — two settings that produce significant seismic hazards without the volcanic activity typically associated with divergent and convergent margins. These settings are essential for answering Edexcel A-Level Geography Enquiry Question 1 (EQ1): Why are some locations more at risk from tectonic hazards?
At conservative boundaries (also called transform boundaries), two plates slide laterally past one another. There is no creation of new crust and no destruction of existing crust — hence the term "conservative." However, friction between the plates causes stress to accumulate, which is periodically released as earthquakes, sometimes of very high magnitude.
Transform faults involve strike-slip motion — horizontal displacement along a vertical or near-vertical fault plane. The two plates do not slide smoothly past each other. Instead, friction locks the fault, and stress builds up over years or decades. When the accumulated stress exceeds the frictional resistance of the rock, the fault ruptures in an earthquake.
This process is described by the elastic rebound theory (Reid, 1910), developed after the 1906 San Francisco earthquake:
graph TD
A["Stress Accumulation<br/>Plates locked by friction"] --> B["Elastic Strain<br/>Rock deforms adjacent to fault"]
B --> C["Rupture<br/>Stress exceeds friction"]
C --> D["Elastic Rebound<br/>Rock snaps back, releasing seismic energy"]
D --> E["Earthquake<br/>Seismic waves radiate from focus"]
E --> A
Transform faults can be classified by the types of boundary they connect:
| Type | Description | Example |
|---|---|---|
| Continental transform | Connects two continental plate segments; occurs on land | San Andreas Fault, North Anatolian Fault |
| Oceanic transform | Offsets segments of a mid-ocean ridge; occurs on the ocean floor | Numerous faults offsetting the Mid-Atlantic Ridge |
| Ridge-trench transform | Connects a spreading ridge to a subduction zone | Queen Charlotte Fault (Pacific-North American) |
Oceanic transform faults are by far the most numerous — they offset mid-ocean ridges into segments and accommodate differences in spreading rates along the ridge. However, they rarely produce damaging earthquakes because they occur far from populated areas.
The San Andreas Fault is the most studied and most famous continental transform fault. It marks the boundary between the Pacific Plate (moving NW) and the North American Plate (moving W-SW), extending approximately 1,300 km through California from the Salton Sea to Cape Mendocino.
| Feature | Detail |
|---|---|
| Length | ~1,300 km |
| Type | Right-lateral (dextral) strike-slip |
| Slip rate | ~46 mm/year (~33–37 mm/year on the main fault; remainder on parallel faults) |
| Depth of seismicity | Mostly < 15–20 km (within the brittle upper crust) |
| Notable earthquakes | 1906 San Francisco (Mw ~7.9), 1989 Loma Prieta (Mw 6.9), 1857 Fort Tejon (Mw ~7.9) |
| Locked segments | Southern section has not ruptured since 1857 — accumulating significant strain |
The San Andreas Fault system is not a single fault but a fault zone comprising multiple parallel and subsidiary faults, including the Hayward Fault, Calaveras Fault and San Jacinto Fault. Greater Los Angeles and the San Francisco Bay Area — home to over 25 million people combined — sit directly on or adjacent to these faults.
A seismic gap is a segment of a fault that has not experienced a significant earthquake for an unusually long time relative to other segments. The southern section of the San Andreas Fault (from Parkfield to the Salton Sea) has not ruptured since 1857 — over 165 years of accumulated strain. The USGS estimates a 75% probability of a magnitude 7.0+ earthquake in Southern California within the next 30 years (UCERF3 model, 2015).
Exam Tip: When discussing conservative boundaries, avoid stating that they produce "no hazards other than earthquakes." While volcanism is absent, secondary hazards include landslides, liquefaction, fire (from ruptured gas mains), and — in coastal settings — localised tsunamis from submarine landslides triggered by the earthquake.
The North Anatolian Fault (NAF) is a 1,500 km right-lateral transform fault running across northern Turkey, marking the boundary between the Anatolian Plate (moving westward) and the Eurasian Plate. Since 1939, a remarkable sequence of earthquakes has propagated westward along the fault:
| Year | Location | Magnitude (Mw) | Deaths |
|---|---|---|---|
| 1939 | Erzincan | 7.8 | ~33,000 |
| 1942 | Erbaa-Niksar | 7.0 | ~3,000 |
| 1943 | Tosya | 7.6 | ~4,000 |
| 1944 | Bolu-Gerede | 7.2 | ~3,900 |
| 1957 | Abant | 7.1 | ~52 |
| 1967 | Mudurnu | 7.1 | ~89 |
| 1999 | Izmit | 7.6 | ~17,000 |
| 1999 | Duzce | 7.2 | ~894 |
| 2023 | Kahramanmaras* | 7.8 | ~60,000 |
*Note: The 2023 earthquakes occurred on the East Anatolian Fault, a related structure.
This westward migration of rupture segments — called stress triggering or Coulomb stress transfer — occurs because each earthquake transfers stress to the adjacent unruptured section, bringing it closer to failure. Istanbul, a city of over 15 million people, sits directly on the westernmost unruptured segment of the NAF.
The Alpine Fault extends approximately 600 km along the western coast of the South Island of New Zealand, marking the boundary between the Pacific and Australian plates. It is a right-lateral transform fault with a significant dip-slip component, producing both horizontal displacement (~27 mm/year) and uplift (~10 mm/year). This uplift has created the Southern Alps, with peaks exceeding 3,000 m.
Paleoseismic studies reveal that the Alpine Fault ruptures in major earthquakes (Mw ~8) approximately every 300 years, with the last major rupture occurring in 1717 CE. The fault is therefore considered overdue for a significant earthquake.
Intra-plate tectonic activity occurs within the interior of tectonic plates, away from plate boundaries. While less frequent than boundary activity, intra-plate events can be highly destructive because they are unexpected and populations are unprepared.
Intra-plate earthquakes are attributed to several mechanisms:
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