11.2
Chapter 11: Earthquakes
11.2 Earthquakes and Plate Tectonics
Distribution of Earthquakes:
- Earthquake occurrences correlate strongly with plate boundaries, as illustrated in Figure 11.7.
- Divergent Boundaries:
- Examples include:
- Mid-Atlantic Ridge
- East Pacific Rise
- Characteristics:
- Earthquakes occur in a narrow zone near the ridge.
- Depth: Consistently less than 30 km.
- Transform Faults:
- Example: San Andreas Fault
- Characteristics:
- Shallow earthquakes common; active but less intense than subduction zones.
- Subduction Zones:
- Defined by a high abundance of earthquakes.
- Characteristics:
- Earthquakes deepen landward from the trench, following the subducting plate's path.Intraplate Earthquakes:
- Some seismic activity occurs away from plate boundaries.
- Causes may include:
- Buildup of stress due to continental rifting.
- Stress transfer from other regions.
- Examples of intraplate earthquake-prone areas:
- Great Rift Valley (Africa)
- Tibet (China)
- Lake Baikal (Russia)
Earthquakes at Divergent and Transform Boundaries
Mid-Atlantic Ridge Earthquake Activity:
- Figure 11.8 showcases M4 and larger earthquakes from 1990 to 1996 in the mid-Atlantic region near the equator.
- Characteristics:
- Segments of the mid-Atlantic ridge offset by transform faults.
- Earthquakes primarily on transform faults, not spreading segments.
- Infrequent small earthquakes on spreading ridges due to high rock temperatures in those areas.
Earthquakes at Convergent Boundaries
Caribbean and Central America Earthquake Activity
- Figure 11.9 illustrates earthquake distribution and depths in this region from 1990 to 1996.
- Plate interactions:
- Cocos Plate subducting beneath North America and Caribbean Plates (ocean-continent convergence).
- South and North America Plates subducting beneath Caribbean Plate (ocean-ocean convergence).
- Characteristics:
- Earthquakes become deeper with increased distance from the trench.
- South America Plate's subduction is questioned due to minimal earthquake activity along this zone.Kuril Islands Earthquake Activity:
- Figure 11.10 provides insights into the distribution of earthquakes in the northwest Pacific ocean-ocean convergent boundary.
- Notable events:
- April 2009 M6.9 earthquake associated with a larger rupture zone between 60 km and 140 km inland from the trench.
- Characteristics:
- Background seismicity indicates greater frequency of earthquakes near the upper side of the subducting plate extending to at least 400 km depth.
- Significant activity in the overriding North America Plate.India-Eurasia Plate Boundary Activity:
- Figure 11.11 depicts the distribution of earthquakes in this continent-continent convergent boundary from 1990 to 1996.
- Characteristics:
- While the India Plate continues moving north towards the Asia Plate, there is no current subduction.
- Active transform faults are present on either side of the India Plate.
- Seismic activity concentration across northern India, Nepal, Bhutan, Bangladesh, parts of China, Pakistan, and Afghanistan.
- Influences provoking earthquakes:
- Transform faults and tectonic squeezing from India and Asia Plate convergence.
- Himalayan uplift linked to significant earthquake activity.Thrust Faults and Earthquake Focus:
- Figure 11.12 illustrates earthquake-related thrust faults as associated with India-Asia Plate interactions.
- Example:
- Nepal earthquake (May 2015) occurred along one of these thrust faults.Seismic Activity in Afghanistan:
- Significant concentration of both shallow and deep earthquakes (depths >70 km).
- Interpretation:
- Deep earthquakes attributed to the northwestern subduction of the India Plate beneath the Asia Plate in this region.