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An Earth-Shaking Talk

Writer's picture: Mishkat BhattacharyaMishkat Bhattacharya

The setting


I am currently attending a workshop in Spain, in the town of Benasque. The town is in the center of the Pyrenees, a mountain range at the border of France and Spain.


The location was likely chosen so as to prevent the scientists from getting distracted by big city attractions and to get them to talk more with each other: the building in which the workshop is being held has hallways lined with blackboards, small rooms designed to nucleate discussions, and large halls for talks, poster presentations and coffee sessions.


The surprise


Things became exciting in the morning session of the second day: in the middle of the talk the seminar room started to shake - an earthquake! It turns out - I was not told this prior to committing my attendance - that Benasque is in a seismically active area.


I should have guessed though, as it is in a mountainous region (see below). As if to confirm, a second tremor, which I felt clearly in my hotel room, came at about 1 am the same night.


Prior to this, I had experienced mild earthquakes in my home in Delhi, and have been fascinated by earthquakes for a while (some would say due to my inclination towards damage prevention and control). Below, I address some of the basic questions I have encountered in my study of earthquakes.


Some questions and answers


i) Why do earthquakes occur?


The Earth's uppermost land layer is the lithosphere, which is about 100 km thick. The crust of the lithosphere is made of chunks called tectonic plates. The lines where these plates meet are called faults. When the plates move against each other; elastic strain is first built up at the fault and then released in the form of kinetic energy. This release of energy is what constitutes an earthquake.


Mountains also form when plates collide and their boundary crumples (the Pyrenees were formed by the collision of the Iberian and Eurasian plates; the Himalayas formed where the Eurasian plate collided with the Indian plate). This is why mountainous areas are seismically active.


ii) What is the epicenter?


The earthquake originates from a location inside the Earth's crust; this point is called the hypocenter. The point on the Earth's surface directly above the hypocenter is called the epicenter.


iii) What kind of physics describes earthquakes?


The transport of kinetic energy along the Earth's crust basically takes the form of acoustic wave propagation. These waves obey Huygen's principle: every point on the wavefront serves as a source of waves itself.


Seismic waves are of two types: body waves that travel within the earth; and surface waves, that travel, well, on the surface of the earth.


Body waves compress and expand the rock (these are called P or primary waves, as they are the first to arrive during an earthquake); or they can shear the rock (S or secondary waves; these arrive later). By measuring the time delay between the P and S waves the distance from the seismograph to the epicenter can be calculated.


Surface waves come after the body waves. They can move the rock side to side as they travel (Love waves) or move the ground in an elliptical pattern (Raleigh waves). For those interested in more, mathematical modeling of earthquakes is described here.


iv) What is the Richter scale?


This scale measures the logarithm of the amplitude of the largest wave in an earthquake recorded by a seismograph. Typically notable earthquakes are strong (6.0-6.9), major (7.0-7.9), great (8.0-8.9) or extreme (9.0-9.9).


Although in principle the Richter scale has no upper limit, the maximum value ever recorded is 8.6 (Chile, 1960). The tremors I experienced in Benasque and Delhi were likely 'slight' (3.0-3.9). Apparently there are 100,00 of these events per year, worldwide.


v) Can earthquakes be predicted?


Not with certainty, only through probabilities, like weather forecasts. In this context, it is interesting to recall how some scientists were initially convicted of manslaughter for not giving out severe enough warnings before the 2009 earthquake in L'Aquila, Italy. They were subsequently acquitted.


vii) Are earthquakes ever beneficial?


Maybe if you are out bowling and its your turn to bowl. More seriously, earthquakes seem to have benefits for soil enrichment (by bringing fresh nutrients to the surface), and for concentration of ores (gold!).


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