LIQUEFACTION

Well Now I will stop a little talking about geology and seismology, because I want to talk about a topic that has to do more with engineering and tecnique, I will talk abuot liquefaction in soils, because is a very importat them for geotechnical and structural engineers.

During earth quakes the movement on the ground can make lose stiffness in the soil, and cause to collapse structures of buildings, soil slides or damages in pipes underground, and this is because the liquefaction of the soil, and is associated to non-cohesive soils as the sands and silty sands with the water table enough neared to surface.

The terms liquefaction includes all phenomena where is present an excessive deformation or movements as a result of repetitive transient disturbances in saturated soils and low cohesive.

During the large magnitude of destructive affects in the Nigata, Japan earthquake in 1964 engineers understood that was very important to give all attention to this phenomenon. Since and then there have been significant advances to understand this phenomena and this effects analyzing and evaluating the potential of liquefaction of a soil and developing new technologies to to mitigate earthquakes destroying effects.

The knowledge about process and its effects has been based in three different principles.

1. Observation; during and after an earthquake.
2. Lab experiments an satured soil samples, and in structural models.
3. Theory studies.

Some of the liquefaction effects are catastrophic, as the collapse of large slopes or dams, collapse of bridges, walls and buildings. Other effects are less dangerous as large deformation on the ground surface, or floods in big areas.

TECTONIC OF PLATES THEORY

This was a very important step, for development of the understanding of the drift of continents theory. Many years ago in 1872 a British expedition discovered in the ocean floor a ridge  along of the center of the Atlantic Ocean,  later there where many other expeditions that discovered the same in Atlantic and Indian Ocean.

After the Second World War more intense expedition began, with more sophisticated equipment major economic resources; Richard Field geologist of Princeton University was one of the precursors, he convinced a group of important geologist to join to this research, wich revealed many details about oceanic ridges. At the geophysics year (1957-1958) the investigations took a new impetus, and revealed that all the ridges was connected passing by all the oceans conforming 65000km submarine ridge, also the investigation revealed the existence of a very wide crack in the middle.

At that time the seismic epicenter was located with more precision, and many coincided with the meso-oceanic ridges, indicating that those were instability zones.

Many interpreted the oceanic ridges as a product of the earth expansion,  however new data were obtained, in 1962 was established that both sides of the Atlantic Ocean were joined 200 million years ago, implying an expansion 300 times faster than calculated by Dicke (precursor of the theory of the Earth expansion), the idea of a growing world was abandoned soon. If the earth would be expanding so fast, that implies that the inertia moment would get higher, and to to preserve the energy the angular velocity of the planet would decrease; and that means today the days would last less than that time, and there was not registers supporting that fact was real.

At 1960 Harry Hess, who was a Richard Field’s student, had conceived a new hypothesis; the theory of Hess was that the convection currents in the mantle caused the magma to ascend in the oceanic ridges, which solidify and form a new crust. In 1962 this theory was called “the expansion of the oceanic floor”, and has been generally accepted, though there was an aspect that has not explanation. It was supposed the oceanic ridges had been formed originally as continuous lines and had acquired its discontinuous shape with the expansion process. In this model the sections of the oceanic ridges are separated by trans-current faults, but the problem was explain the absence of seismic activity at the frontier of these faults. The answer was: the ridges had always had a discontinuous shape and the faults that are separating the extension sections are transformation faults, out of the extension zone there is not differential motion between both faces of the fault.

image taken from Google Earth shows the oceanic ridges in the middle Atlantic Ocean

AN ALARMING QUESTION

I learned recently that after the earthquake in Sendai, Japan in March 11, 2011 seismology scheme changed, before that event, researches supposed that the largest earthquake that could occur in subduction zone off the coast of my country would have a magnitude no mayor than 8 degrees in Richter scale.

But after the Japanese earthquake seismologists around the world published that all subduction zone, is capable to generate a 9 degrees in Richter scale earthquake or even larger.

Well I don’t still find any paper in the web that talks about, but I would like to know if some of you had heard something about it; I think is important to know because we must be prepared.

Please let me know what you think and comment about.

THE CONTINENTAL DRIFT THEORY

This was the first theory that claimed that the planet’s surface is in motion, it was proposed by german meteorologist and astronomer Alfred Wegener, at the beginning of last century. At that time the most accepted theory was the world’s surface was only the skin wrapping a melting interior that was cooling down and while cooling it is contracting which distorted the surface in mountains. But Wegener reasoned that if this was the mechanism, the mountains should appear randomly, otherwise the big mountains were concentered in limited zones.

The first evidence for Wegener was the similarity of the physiography of the cost of Brazil and the western Africa; the British philosopher Francis Bacon noted this fact too in 1620.

In 1858 in Paris, Antonio Snider had written about the similarity of fossils found at both sides of the Atlantic Ocean suggesting that some time these shores were together. These facts were the basis of the theories of Wegener, theories the he presented in 1912 to Frankfurt Geological Association. Wegener had to stop his researches due to the World War I, but he was shot and discharged, and returned to his research during his long convalescence, but since and then no longer limited to his field, he used data from: meteorology, geology, oceanography, seismology, geomagnetism, paleontology and the theories of evolution.

In 1915 he published his book, the origin of the continents and oceans; in which he presented theory about the continental drift. Most of the scientifics took Wegener’s thesis with derision, because Wegener had presenter too few evidences to support his view, though later, his ideas would be accepted almost universally.

 Now there are too many evidences about the earth as a dynamic planet, for example the similarly of the fauna in Africa and Brazil, the alignment of the mountain ranges of America and Europe also suggests that once were a single, the Brazilian cost seems to fit perfectly with the western African coast; geographical coincidence between these coast is even greater if performed from the continental shelves

SEISMOLOGY AND STRUCTURE OF THE EARTH

We're going to try to explain the mechanism that represents the main cause of earthquake; this is the movement of the surface of the ground, known as tectonic of plates.

The earth can be seen as a 6350 km radius sphere. To understand the causes of earthquake, we have to understand the interior structure of the earth, this structure has been revealed studying the behavior of seismic waves, but to understand the generation of seismic waves, we have to understand the mechanism of earthquakes, and to understand the mechanism of earthquakes we have to know the structure of the earth, so this can be difficult to study, because this turns in a circle, where what we want to know becomes exactly in the research base, so all the research has to be made in an iterative procedure.

For example, the careful study of the changes experienced by seismic waves when traveling through the crust has revealed its interior structure.

The outermost layer is the crust; there are two types of crust: the oceanic crust, with thickness between 5 and 15 km; and the continental crust, with thickness between 30 and 80 km, under the crust lies the mantle subdivided in concentric layers. The exterior part of the mantle is rigid and with the crust conform what we know as lithosphere, the lithosphere has an average thickness of 60km under the oceans and 100km under the continents. Under the lithosphere is the asthenosphere that extends for about 200km.

Asthenosphere has plastic characteristics, it forms a soft plate in partial melting probably and the lithosphere is floating on the asthenosphere.

The rest of the mantle continues about 2900 km, and then appears a discontinuity that marks the beginning of the inner core.