Earth Science Chapter on Lithosphere and Asthenosphere

Earth's Lithosphere and Asthenosphere

• The lithosphere is a rigid layer formed from the crust and upper mantle, derived from the Greek word "lithos" meaning "stone."
• The asthenosphere is a soft, weak layer beneath the lithosphere, approximately 80 to 150 km thick. Its base can be found up to 250 km deep.

Earthquake Distribution

• Earthquakes are not uniformly distributed; they mainly occur in discrete belts.
• Notable earthquake zones include the western coasts of North and South America and mid-ocean ridges.
• Regions of high seismic activity include the Middle East, China, and Tibet, while abyssal plains often have few earthquakes.
• Large earthquakes can create tsunamis, as evidenced by disasters in the Indian Ocean (2004) and Japan (2011).

Tectonic Activity and Plate Convergence

• Tectonic activity, which includes earthquakes and volcanic eruptions, occurs primarily along plate boundaries with active deformation.
• Oceanic-oceanic convergence forms ocean trenches and subduction zones known for large earthquakes.
• When an oceanic plate converges with a continental plate, it results in crust compression and uplift.

Pacific Ring of Fire

• The Pacific Ring of Fire comprises volcanoes surrounding the Pacific Ocean, linked to subduction activities on both sides.
• It extends from the southwestern Pacific, through the Philippines, Japan, Alaska, and down western coasts of the Americas.

Continental Collisions and Transform Boundaries

• Continental collision occurs at continent-continent convergent boundaries, generating major mountain ranges.
• Transform boundaries allow plates to slip horizontally past each other, creating transform faults typically associated with mid-ocean ridges.

Earthquake Mechanisms and Volcanic Activity

• Steep volcanic slopes can lead to catastrophic landslides, causing ground shaking and smaller precursor earthquakes.
• Magma movement beneath a volcano can fracture rocks, leading to volcanic tremors and minor earthquakes, alerting geologists to potential eruptions.

Other Sources of Seismic Waves

• Landslides, both terrestrial and underwater, can cause seismic waves, often detected by seismometers.
• Human activities such as mine blasts and nuclear detonations also generate detectable seismic waves, important for monitoring compliance with test-ban treaties.

Earthquake Processes and Aftershocks

• Prior to faulting, rocks undergo minor deformation until sudden slippage occurs.
• Aftershocks, smaller earthquakes following a major event, help reduce stress and allow rocks to return to their elastic behavior.

Notable Earthquakes

• The 2010 New Zealand earthquake (magnitude 7.1) was shallow and caused moderate damage, while the 2011 Christchurch quake, though smaller, led to significant destruction due to its proximity to the city.
• The 2004 Indian Ocean earthquake (magnitude 9.1) triggered a massive tsunami, affecting multiple countries with waves reaching heights of 20-30 meters off the coasts.

Seismic Wave Behavior

• Seismic waves refract when transitioning between materials of different densities, with the direction of refraction depending on whether they slow down or speed up.
• Seismic waves effectively travel through diverse geological materials in Earth's crust and mantle.

GeoHazard Assessments in the Philippines

• The Philippine government established a long-term strategy for geological hazard protection, mandating preliminary geo-hazard assessments for land development projects.