19-05-2012, 12:19 PM
BASIC EARTHQUAKE PRINCIPLES
BASIC EARTHQUAKE PRINCIPLES.ppt (Size: 2.02 MB / Downloads: 56)
Interior on the Earth
The top cover is the crust, thickness ranging from 30 to 50 km, consists of rock forming silicates such as granites and basalts. Followed by Mantle, separated by Moho discontinuity, consists of much denser material.
The Mantle is about 28.50 km thick and divided into the upper mantle and lower mantle. The outer core or liquid core is around 2260 km thick and S-waves cannot transmit. The inner core or solid core is very dense with Ni-Fe material.
The outer and inner cores are separated by Gutenberg discontinuity. Temperature and pressure increase tremendously towards the centre of the Earth.
Convection Currents
Convention currents develop in the viscous mantle because of high temperature and pressure gradients between core and crust.
These convection currents facilitate the circulation of mass: hot molten lava comes out and the cold rock mass goes into the earth.
Many such local circulations are taking place at different regions underneath the Earth’s surface, leading to different portions of the earth undergoing different directions of movements along the surface.
Continental Drift
In 1915, the German geologist and meteorologist Alfred Wegener first proposed the theory of continental drift, which states that parts of the Earth's crust slowly drift atop a liquid core.
Wegener hypothesized that there was an original, gigantic super continent 200 million years ago, which he named Pangaea, meaning "All-earth".
Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents.
PLATE TECTONICS
According to the plate tectonic theory, the earth’s surface contains tectonic plates, also known as lithosphere plates, with each plate consisting of the crust and the more rigid part of the upper mantle
Divergent Boundary
This occurs when the relative movement of two plates is away from each other.
The upwelling of hot magma that cools and solidifies as the tectonic plates move away from each other forms spreading ridges.
Figure 2.3 illustrates seafloor spreading and the development of a mid-ocean ridge. An example of a spreading ridge is the Pacific Ocean ridge.
Earthquakes on spreading ridges are limited to the ridge crest, where new crust is being formed. These earthquakes tend to be relatively small and occur at shallow depths.
Transform Boundary
A transform boundary, or transform fault, involves the plates sliding past each other, without the construction or destruction of the earth’s crust.
When the relative movement of two plates is parallel to each other, strike-slip fault zones can develop at the plate boundaries.
Strike-slip faults are defined as faults on which the movement is parallel to the strike of the fault; or in other words, there is horizontal movement that is parallel to the direction of the fault.
California has numerous strike-slip faults, with the most prominent being the San Andreas fault. Fig. 2.8 presents an example of the horizontal movement along this fault (1906 San Francisco earthquake).
Since a boundary between two plates occurs in California, it has numerous earthquakes and the highest seismic hazard rating in the continental United States.
Theory of Plate Tectonics
The theory of plate tectonics helps to explain the location and nature of earthquakes.
Once a fault has formed at a plate boundary, the shearing resistance for continued movement of the fault is less than the shearing resistance required to fracture new intact rock.
Thus faults at the plate boundaries that have generated earthquakes in the recent past are likely to produce earthquakes in the future. This principle is the basis for the development of seismic hazard maps.
The theory of plate tectonics also helps explain such geologic features as the islands of Hawaii. The islands are essentially large volcanoes that have risen from the ocean floor.
The volcanoes are believed to be the result of a thermal plume or “hot spot” within the mantle, which forces magma to the surface and creates the islands.
Predicting Where Earthquakes Will Occur
Segments of active faults that have not moved recently are known as seismic gaps. These are locations where stress is building up and where earthquakes can be expected in the future.
In fact, the 1989 Loma Prieta earthquake occurred in along a portion of the San Andreas Fault identified as a seismic gap