Cumulative Moment Mag. Part of the Century of Earthquakes Poster. Between 1906 and 2006 about half of all global moment release occurred in just three great earthquakes: Sumatra (2004), Alaska (1964) and Chile (1960). GREAT EARTHQUAKES
Charles Richter introduced the first earthquake magnitude scale in 1935 for small California earthquakes. However, the Richter and similar magnitude scales are inadequate for characterizing extremely large earthquakes. Seismologists now use a magnitude scale designed to blend with the original Richter scale, but based on the seismic moment, which relates directly to three key physical properties of the fault: stiffness of rock, fault area, and fault slip. Seismic moment is a quantity used by seismologists to measure the amount of energy released by an earthquake. The seismic moment generated by a slipping fault is:
Mo = rigidity x fault area x fault slip
The rigidity is a number that characterizes the stiffness of rocks near the fault. Fault area and fault slip can be estimated from the analysis of seismograms. For a seismic moment expressed in units of Newton-meters (which is a unit of force times distance), the corresponding moment magnitude, Mw, is:
Mw = (2/3) x (log10Mo - 9.1)
The constants in the equation allow the moment magnitude scale to describe great earthquakes while matching other magnitude scales at smaller magnitudes. An earthquake with Mw greater than or equal to 8.0, which on average occurs about every 1.5 years, is classified as a great earthquake. The total energy release on Earth is dominated by the largest of the great earthquakes; between 1906 and 2006 nearly half of all global moment release occurred in just three great earthquakes: Sumatra (2004), Alaska (1964) and Chile (1960).
Compared to the 1906 San Francisco earthquake, the 2004 Sumatra earthquake had roughly three times the average slip on a fault with about 60 times the area, which corresponds to an increase in moment magnitude of about 1.4. As seen in the formula above, fault area, slip and rigidity of the rock constrain how large an earthquake can be. Rock type and depth account for how rigid the rocks are and how much stress can build up. The more stress that builds up, the more energy can be released when the rocks slip. In California, rocks that are deeper than 20 km are weak and do not let stress build up over time. In contrast, earthquakes in subduction zones have large surface areas in zones of stronger rocks, which allow more stress to be built up over time. Because of these factors, about 80% of the global seismic moment is released by subduction zone earthquakes.
We need to understand great earthquakes and live with them. In fact, we can’t live without them. Great earthquakes are a result of plate tectonics, which keeps the Earth habitable by maintaining our atmosphere. We have not yet learned to predict when great earthquakes will happen. The best we can do is build our societies to minimize the damage from earthquakes. This can be done by earthquake resistant construction, systems to warn of imminent tsunami waves, and other measures. Deciding how to do this is a challenging and important task.