How do we know just how big or strong an earthquake is? Scientists have developed a number of instruments and scales over time to accurately and precisely describe the magnitude of these events.
The main tool for land-movement measurement is a seismometer, from the Greek words for earthquake and measure. A seismometer comprises weights suspended on springs from frames. Because of inertia, the weight remains at rest even as the frame, which is connected to the ground, moves. The difference between the movement of the frame and the weights allows us to calculate the magnitude at which the ground is shaking. With the evolution of information and communications technologies, digital seismometers with more sensitive electronic detection and more robust and detailed record-keeping have replaced traditional stylus and paper cylinder equipment.
Because any one seismometer described above can measure waves in only one direction, seismic stations use at least three directions: one to measure north-south ground movement, one to measure east-west ground movement, and one to measure vertical ground movement. A seismogram is a graphical representation of this information.
A phrase you may have heard in relation to an earthquake is that it measured X “on the Richter scale.” Charles F. Richter developed his local magnitude scale (denoted ML) in the 1930s to describe earthquakes in California. The Richter magnitude scale uses a logarithmic scale and is governed by wave amplitude and distance of the seismometer to the earthquake. Richter devised the scale specifically for crust movement in Southern California, and analysis indicates that this scale is less accurate for earthquakes that occur outside of that geographic area. Also, because seismologists measure at a particular range of frequencies, signals from large earthquakes aren’t adequately represented. Thus, the maximum reliable magnitude for this scale is around 7. Because of these limitations, the Richter Magnitude Scale is no longer commonly used by scientists.
Today, seismologists are able to look at a broader range of seismic signals than just amplitude, and the Moment Magnitude Scale (MMS and denoted Mw) is most commonly used. In the news, readers will see earthquakes reported, for example, as a Mw 6.9 earthquake. Developed in the 1970s, MMS describes earthquakes using three measurements: how far a fault moves (fault slip), how much rock breaks (fault area) and how rigid the broken rocks are (shear strength). Multiplying slip times area times rigidity yields a numeric representation of the seismic moment. At smaller magnitudes, MMS matches the Richter Magnitude Scale. Both MMS and the Richter Magnitude Scale are logarithmic, meaning that each number going up along the scale represents a magnitude ten times greater than the number below it.
A subjective measure of earthquake vigor is the Modified Mercalli Intensity Scale. This measure relies on qualitative descriptions of surface-level effects of an earthquake. On this scale, designated by Roman numerals I-XII, an intensity of III describes weak shaking characterized as being noticeable by people on upper floors of buildings but possibly mistaken for a truck passing on the street outside. An intensity of VIII describes severe shaking that might topple chimneys or flip over heavy furniture.
Images: “Seismograph” by Belish via Shutterstock