Ratih Fitria Putri

Thursday, May 13, 2010

What is the most likely geologic hazard in your part of the country?

The most likely geologic hazard in Indonesia???
a. earthquake
b. volcanic hazard

Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. In the case of transform or convergent type plate boundaries, which form the largest fault surfaces on earth, they will move past each other smoothly and aseismically only if there are no irregularities or asperities along the boundary that increase the frictional resistance.

Most boundaries do have such asperities and this leads to a form of stick-slip behaviour. Once the boundary has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.

Plate tectonicists who accept the evidence for deep continental roots have proposed that plates may extend to and glide along the 400-km or even 670-km seismic discontinuity (Seyfert, 1998; Jordan, 1975, 1978, 1979). Jordan, for instance, suggested that the oceanic lithosphere moves on the classical low-velocity zone, while the continental lithosphere moves along the 400-km discontinuity. However, there is no certainty that a superplastic zone exists at this discontinuity, and no evidence has been found of a shear zone connecting the two decoupling layers along the trailing edge of continents (Lowman, 1985). Moreover, even under the oceans there appears to be no continuous asthenosphere. Finally, the movement of such thick "plates" poses an even greater problem than that of thin lithospheric plates.

The driving force of plate movements was initially claimed to be mantle-deep convection currents welling up beneath midocean ridges, with downwelling occurring beneath ocean trenches. Since the existence of layering in the mantle was considered to render whole-mantle convection unlikely, two-layer convection models were also proposed. Jeffreys (1974) argued that convection cannot take place because it is a self-damping process, as described by the Lomnitz law. Plate tectonicists expected seismic tomography to provide clear evidence of a well-organized convection-cell pattern, but it has actually provided strong evidence against the existence of large, plate-propelling convection cells in the upper mantle (Anderson, Tanimoto, and Zhang, 1992). Many geologists now think that mantle convection is a result of plate motion rather than its cause, and that it is shallow rather than mantle deep (McGeary and Plummer, 1998).

The favored plate-driving mechanisms at present are "ridge-push" and "slab-pull," though their adequacy is very much in doubt. Slab-pull is believed to be the dominant mechanism, and refers to the gravitational subsidence of subducted slabs. However, it will not work for plates that are largely continental, or that have leading edges that are continental, because continental crust cannot be bodily subducted due to its low density, and it seems utterly unrealistic to imagine that ridge-push from the Mid-Atlantic Ridge alone could move the 120°-wide Eurasian plate (Lowman, 1986). Moreover, evidence for the long-term weakness of large rock masses casts doubt on the idea that edge forces can be transmitted from one margin of a "plate" to its interior or opposite margin (Keith, 1993).

Where plate boundaries adjoin continents, matters often become very complex and have demanded an ever denser thicket of ad hoc modifications and amendments to the theory and practice of plate tectonics in the form of microplates, obscure plate boundaries, and exotic terranes. A good example is the Mediterranean, where the collisions between Africa and a swarm of microcontinents have produced a tectonic nightmare that is far from resolved. More disturbingly, some of the present plate boundaries, especially in the eastern Mediterranean, appear to be so diffuse and so anomalous that they cannot be compared to the three types of plate boundaries of the basic theory.

Plate boundaries are identified and defined mainly on the basis of earthquake and volcanic activity. The close correspondence between plate edges and belts of earthquakes and volcanoes is therefore to be expected and can hardly be regarded as one of the "successes" of plate tectonics (McGeary and Plummer, 1998). Moreover, the simple pattern of earthquakes around the Pacific Basin on which plate-tectonics models have hitherto been based has been seriously undermined by more recent studies showing a surprisingly large number of earthquakes in deep-sea regions previously thought to be aseismic (Storetvedt, 1997). Another major problem is that several "plate boundaries" are purely theoretical and appear to be nonexistent, including the northwest Pacific boundary of the Pacific, North American, and Eurasian plates, the southern boundary of the Philippine plate, part of the southern boundary of the Pacific plate, and most of the northern and southern boundaries of the South American plate (Stanley, 1989).
The theory states that the Earth's lithosphere is divided into plates (about 100 km thick) that move around on top of the asthenosphere. Continental crust is embedded within the lithospheric plates. The Plates move in different directions, and meet each other at plate boundaries.

Three types of plate boundaries occur:
a. Divergent Plate Boundaries-
These are boundaries where plates move away from each other, and where new oceanic crust and lithosphere are created. Magmas rising from the underlying asthenosphere intrude and erupt beneath and at an oceanic ridge to create new seafloor. This pushes the plates on either side away from each other in opposite directions.

The margin itself becomes uplifted to form oceanic ridges, which are also called spreading centers, because oceanic lithosphere spreads away on each side of the boundary. While most diverging plate boundaries occur at the oceanic ridges, sometimes continents are split apart along zones called rift zones, where new oceanic lithosphere may eventually form. Volcanism and earthquakes are common along diverging plate boundaries

b. Convergent Plate Boundaries :

These are boundaries where two plates move toward each other. Atsuch boundaries one of the plates must sink below the other in a process called subduction. Two types of convergent boundaries are known.

c. Subduction Boundaries:

These occur where either oceanic lithosphere subducts beneath oceanic lithosphere (ocean-ocean convergence), or where oceanic lithosphere subducts beneath continental lithosphere (ocean-continent convergence). Where the two plates meet, an oceanic trench is formed on the seafloor, and this trench marks the plate boundary.

When two plates of oceanic lithosphere run into one another the subducting plate is pushed to depths where it causes melting to occur. These melts (magmas) rise to the surface to produce chains of islands known as island arcs. A good example of an island arc is the Caribbean islands. When a plate made of oceanic lithosphere runs into a plate with continental lithosphere, the plate with oceanic lithosphere subducts because it has a higher density than continental lithosphere. Again, the subducted lithosphere is pushed to depths where magmas are generated, and these magmas rise to the surface to produce, in this case, a volcanic arc, on the continental margin. Good examples of this type of volcanic arc are the Cascade mountains of the northwestern U.S. and the Andes mountains of South America. Squeezing together and uplifting the continental crust on both plates. The Himalayan mountains between India and China where formed in this way, as were the Appalachian Mountains about 300 million years ago.

All convergent boundaries are zones of frequent and powerful earthquakes. Transform Fault Boundaries - When two plates slide past one another, the type of boundary occurs along a transform fault. These are also zones of frequent and powerful earthquakes, but generally not zones of volcanism. The famous San Andreas Fault of California is an example of a transform fault, forming one part of the boundary between the Pacific Plate and the North American Plate.

By: RATIH FITRIA PUTRI (Double Degree Master Programe MPPDAS UGM - Remote Sensing Chiba University,Japan)

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