Shockwaves under

Shockwaves under
Although earthquakes are not an uncommon occurrence in the world, devastating ones like the one that happened in Nepal recently are rather rare. On an average, there are three quakes every day, most of them very mild. Despite decades of research, no one can predict earthquakes with reliability.

Vast temporal seismic fluctuations make accurate predictions at a given location almost impossible. More than 90 per cent of the world’s earthquakes are caused by movements between tectonic plates which float (like icebergs on water) on the denser magma of the mantle at speeds of several centimetres each year.

As they move, the compacted rocks grind against each other, possibly becoming locked together, causing pressure to gradually build up till the forces involved exceed the strength of the rocks. This makes them rupture, causing the earth’s crust to shift suddenly, which in turn causes the ground to literally tremble. Seismic waves spread out from the ruptured region at speeds of several thousand km per hour, potentially leading to damage in places hundreds of kilometres from the direct quake centre.

Earthquakes can also be caused by factors other than just shifting tectonic plates. Volcanic eruptions, collapsing mine shafts and perhaps even collapse of subterranean caverns excavated of vast oil and gas deposits can also produce quakes. Geoscientists try to understand all factors causing quakes with the help of a network of seismic sensors. They estimate how far below a quake is formed, where there is friction between rocks and the direction in which it is likely to move.

Recent progress in space-based geodesy made possible by GPS and satellite interferometry has provided a clear pattern of crustal movement and strain
accumulation. Relative plate motion, on an average, is two to seven cm per year, implying a fraction strain accumulation of one third of a millionth, along plate boundaries.

In plate interiors, the strain is ten times lesser. As the rigidity of crustal rocks is about 40 billion Pascal (atmospheric pressure is about one lakh Pascal), this implies a stress accumulation rate of one hundredth of a Pascal per year along plate boundaries. When stress at a point in the earth’s crust exceeds a critical value (called local strength), sudden failure occurs.

Stick-slip

The plane along which failure occurs is called a fault plane and the point where the failure is initiated is called the focus. There is a sudden displacement of crust at the fault plane (after a fault failure) and elastic shock waves are radiated. This is an earthquake. In the earth’s crust, there are planes that can support only relatively low stresses before rupturing. These are fault planes. The earthquake is precipitated when stress on a weak fault plane exceeds the static friction stress. Plates on either side of the fault experience a relative displacement or slip. Sliding stops when shear stress drops below the frictional stress. Fault motion does not occur smoothly but in a stop-and-go fashion called stick-slip.

Dynamic stick-slip behaviour including rubbing and scrubbing have been studied extensively. Earthquakes occur only if friction increases rapidly with slip. Inherently large events, like the recent Nepal quake, involve slip displacements as large as 10 m (Indian plate slipped 10 m under Nepal) and velocities up to 3 m/s. For most earthquakes, displacement occurs at an already existing geological fault, i.e. already weak plane.
Strain change for large earthquakes can be as large as 300 parts per million and the static stress drop is about 10 million Pascal. Plate tectonics links geological observations to stresses that generate earthquakes over geological time scales.

Geodesic measurement of crustal motion, heat-flow measurements and observations of old faults at depths of around 10 km (brought to surface by uplifts) add to the information. Catalogues of when and where quakes occur help to identify patterns pointing to common cause and interaction between events.

From the archives

Much has been learned from seismic records and detailed rupture history of many recent events. Most earthquakes occur at depths of 50 to 70 km (including the recent Nepal quake which was relatively shallow) but many as deep as 700 km have been observed, like the 1994 deep Bolivian earthquake (having the largest deep focus). Elastic waves excited by sudden crustal motion travel through the earth and are observable at several seismic stations.

The waves carry information about movements at source but extraction of information from signals is complicated by the complex structure of Earth between source and receiver. Short term triggers remain obscure, making earthquakes unpredictable. Dividing seismic strain drop by strain accumulation rate suggests repeat times of major earthquakes at a given location are about 100 – 300 years on plate boundaries (on
average) and ten times less frequent within plates. These values are consistent with what is observed at many plate boundaries.

However, the details are far more complex, stress accumulation is not uniform over time and crust strength not constant either. Presence of migrating fluids can weaken crust significantly (elastohydrodynamic lubricating effects). Large quakes on fault segments can change stress on adjacent segments, accelerating or decelerating seismic activity. Stress drops during quakes can also vary from event to event.

All these complex factors can affect intervals between quakes. Also, a major quake like the recent Nepal one can also be followed by aftershocks. The May 12 quake of magnitude 7.3 that struck close to Mt Everest is an aftershock of the 7.9 magnitude April 25 quake.

In detail

Some explanation about magnitudes is in order. The intensities of earthquakes are generally expressed as magnitudes on the so called Richter scale. This quantifies the energies released, on a logarithmic scale.

A magnitude 7.9 quake is ten times more intense than a 7.3 one. The largest quakes have been of magnitude nine, like the December 2004 quake off the Sumatra coast that caused the killer tsunami waves. It was followed by two aftershocks of 7.2 magnitudes. A magnitude 9 quake releases an energy a few million times that of the Hiroshima atomic bomb. Majority of tsunamis are caused by quakes on the ocean floor (seaquakes).
The so-called Gutenberg-Richter relation (again logarithmic) shows that smaller quakes are far more frequent, for instance, a thousand quakes of fifth magnitude occur annually, compared to ten of seventh magnitude. Seventh magnitude is at least 100 times more powerful than the fifth magnitude.

The basic and challenging questions still confronting geologists are

What exactly happens during an earthquake?

What exactly are the forces and motions during a seismic event?

The recent quake in Nepal (and the accompanying aftershocks) tragically underlined the abruptness with which quakes occur and the vast devastations that follow. Cities built on joint lines  between tectonic plates are at greater risks because of friction between adjoining plates.

So we are far from having reliable forecast for earthquakes in any given region and it is very rare for events to turn out as predicted. The architecture of earthquake resistant structures is now well established (it is the collapse of massive buildings that cause the most deaths). This along with early warning systems can in future vastly reduce the ensuing tragedy when such disasters unexpectedly strike.

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