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What We Know About Earthquakes
Grest mega quakes have left landmarks for us to see.
We have learned a lot about earthquakes, but thre is still much that we don't know, like how to accurately forecast them.
The Earth is a dynamic planet within a dynamic solar system undergoing unceasing change. The Earth evolved from a long history of accretion accumulation of all the known elements. But there was competition and this was the beginning of catastrophic changes. Many more planetoids existed in the early eras of the solar system and this was reduced to what we see by way of planets, moons, asteroids and comets. Earthquakes have always been part of the evolution of the Earth, where the planet undergoes surface and subsurface shaking. They come in many intensities, from different depths and causes. The basic intensity of an earthquake is measured by the amount of surface ground shaking and there are a few systems for measuring them, the Richter scale of intensity being the one popular in this part of the world. Depths of Earthquakes are important, especially where tectonic plates are involved. Causes range from fracturing faults, impacts, underground atomic detonations, planetary influences and volcanic activity.
The Richter scale is measured as a logarithmic function where each increase of one point indicates a tenfold increase in earthquake intensity. So a Richter scale of four is a hundred times more intense than one measured at two. But this is only a measure of intensity and does not describe how the ground shakes, which is what makes up the complexity of an earthquake and the real cause of destruction. The ground shakes in three principle manners during an earthquake. There are compression waves, shear waves and vertical displacement waves. When a fault ruptures, compression waves between the sides are created causing contracting and expanding waves that radiate along the line of the fault. In a subduction zone, a slip releases compression waves perpendicular to the fault line. Shear waves occur when the ground waves from side to side and they originate from strike slip faults. Vertical displacement waves cause the land to bounce up and down and they originate from subduction events. All of them are destructive based on a direct relationship of intensity.
All of them are destructive in a logarithmic function according to intensity. Depending on what type of building exists, a low intensity quake can have a significant or almost no impact. Typically in the developed world, a quake of Richter five would make you sit up and take notice with cracked windows and dislodged objects. A Richter nine quake would collapse infrastructure and large buildings. In the developing world, where mud brickwork is the norm, a Richter six would flatten whole towns. The Earth has around a thousand quakes a day, almost all of them of such low intensity, that only Seismometers can detect them. Quakes have three main types of waves, each of them destructive in different ways. There is the up and down motion, the side to side motion and compression waves. They have as there cause, the movements of massive tectonic plates that grind, lurch, compress and ride over one another due to fluid motion inside the earth. As the earth's interior is in a state of turbulence, the pattern of earthquake activity can change and show up in regions thought to be earthquake free. Usually they will occur near fault lines or volcanoes. Faults come in strike-slip, spreading-collapsing and in subduction varieties. Though they behave differently, the cause is essentially the same, building stress, friction and sudden, catastrophic release.
The Earth exists in the presence of other planets and the changing gravitational influence of these planets cause changes in the Earth through tidal flexing. The most influential of these are the moon and the sun. There are tidal influences upon the ocean that we experience almost twice daily. There are land tides as well, where the land flexes by as much as a meter. The Earth is mostly a viscous liquid with a thin crust that floats on the surface. The crust is a patchwork of pieces called tectonic plates, which are in motion primarily due to the currents within the mantle and core. At the edges, often called faults, they will push up mountains; trigger volcanic activity, spread apart, grind past one another and one will push under another. All of these are flashpoints for earthquake activity. Some are worse than others, such as the subduction zones and slip-strike zones, but none are immune from earthquakes. This was proven by a spreading fault near Malaysia that caused a seafloor drop, a great earthquake and a tsunami that was felt over a sixth of the globe.
Earthquake information and preparedness
Stresses accumulate at fault boundaries and when they reach the breaking point, the stresses are suddenly released in a sudden adjustment resulting in an earthquake as the two side of the fault lurch into a less stressful position. The fracture often does not happen in just one instance, but occurs in clusters that make up the aftershocks, some of which can be almost as intense as the original quake. There may or may not be forewarning quakes or events. Forewarning may come in the form of tremors or earth lights, ground lightning and the like. The more stress that is released, the greater the quake. Nor are quakes limited to plate boundaries. Volcanoes often serve as a focus, as was described by Pliny the Elder concerning Vesuvius. At a certain point in the massive eruption, there was a sudden massive shaking of the ground far from the volcano and then the volcanic column collapsed resulting in a pyroclastic flow that buried Pompeii and Herculaneum. The explanation is that the magma chamber had emptied and the ground collapsed over the void.
If you look at geological formations you can appreciate that there were huge earthquakes at some points in Earth's history that far surpass anything ever recorded by our instruments. As a rough measure, a ten meter change or subsidence between plate boundaries happens with a Richter nine earthquake. Thus a hundred meter displacement would be caused by a quake of Richter ten, something never witnessed by humanity in recorded history, yet there are scarps that approach such displacements. A hint of the power of such a quake is the Niagara escarpment that stretches for hundreds of miles and averages some 40 meters in displacement from top to bottom. Thus a quake measuring about 9.5 would be required to cause such a displacement. The escarpment is only 12,600 years old as measured by the wear of the falls from the beginning and where they are now. 12,600 years ago, a catastrophic event occurred that was focused in a direction roughly to the northeast of the escarpment. The catastrophe was an impact that cracked the earth at the escarpment and resulted in a massive and perhaps long lasting shaking. Other massive changes occurred such as flooding on a Biblical scale and a huge release of trapped methane gas from frozen methane hydrate. An Earthquake of this magnitude would demolish anything built by humanity, leaving nothing but rubble. But a Richter 9.5 or even a ten is not the upper limit. There are hints that quakes measuring up to eleven or more have occurred in geological history. What would trigger such a massive event?
A major impact from an asteroid or large comet of a few or several kilometers would be enough to trigger world wide earthquakes of varying magnitude. Evidence exists on Earth and Mercury where impacts literally were responsible for building mountains on the far side of the planet in question. The evidence of the Barranger crater in Arizona suggests a local quake induced by impact on a level of 9.5. There are greater craters on Earth and the impacts that created them must have caused huge earthquakes. The Chicxulub event 65 million years ago would have caused a Richter eleven quake that reverberated planet wide and triggered an age of volcanic activity and mountain building.
We can build to adapt to earthquakes up to 9.2, but we have never been tested in recorded history by anything greater. Buildings today in earthquake prone areas are now being constructed upon huge shock absorbers that can change under the influence of the three main types of earthquake shock waves. Using a combination of inertia and flexibility, the shock absorbers flex while the building remains relatively still and the ground moves. Building techniques are also being used to give more flexibility in structures to accommodate shaking from the ground up to absorb the energy and dissipate it harmlessly. But such methods are good only to the limits of our experience. Lately we have learned that buildings between four and twenty storeys are susceptible to earthquake resonance and can topple as a result of a long shake. Buildings lower than four storeys collapse for different regions, as do those of twenty or more storeys. Today, tall buildings flex and they can stand even major events with little damage except to glass exteriors. Most earthquakes can't be felt, but the odd one that does, will have its effect on us to a lesser or greater degree. Earthquake prediction is an emerging science that is being successfully pioneered in Italy, Japan and China. One of the precursors found is an increase of radon gas just before a quake of significant force.