Seismic magnitude scales

Seismic magnitude scales are essential tools used to describe the overall strength of an earthquake. It is important to distinguish magnitude from seismic intensity. Intensity describes the severity of ground shaking at a specific location. Magnitude, however, is an estimate of the relative size or strength of the quake. It is approximately related to the amount of seismic energy released during the event. Scientists determine these values by measuring seismic waves on a seismogram. A seismogram is a record of the ground's motion over time.

Earthquakes occur when tectonic forces stress the Earth's crust. When this stress overcomes friction, a rupture occurs in the crust. This sudden slip releases energy in the form of seismic waves. These waves travel through the Earth in different ways. First, P waves, or primary waves, arrive. These are compressive waves that act like sound passing through rock. They are the fastest waves and arrive first. Next, S waves, or secondary waves, arrive. These cause the ground to shake sideways. S waves travel at about half the speed of P waves. Finally, surface waves like Love and Rayleigh waves travel along the Earth's surface. Surface waves carry most of the energy and cause the most damage.

Different types of scales exist because earthquakes vary in depth and location. Some scales focus on body-waves, which pass directly through the Earth's crust. The mb scale is a short-period body-wave scale. It uses P waves measured in the first few seconds to detect small events. The mB scale is another body-wave scale designed to handle larger, distant earthquakes. It does not reach saturation until around magnitude 8.0. Other scales, like the mbLg scale, focus on specific wave types for certain regions. The mbLg scale uses Lg waves, which are a complex form of Love waves. These waves travel well through the hard granitic crust of North America.

History shows how our understanding of these scales has evolved. In 1935, Charles F. Richter developed the first magnitude scale. This is often called the "Richter" scale, though its technical name is the local magnitude scale, or ML. Richter established that the scale is logarithmic. This means each unit increase represents a ten-fold increase in wave amplitude. Because energy is proportional to amplitude to the 1.5 power, each unit represents about a 32-fold increase in energy. Richter set a zero point based on specific horizontal displacement on a seismograph. However, the ML scale had limitations. It was designed for the crust in Southern California and Nevada.

As scientists traveled further, they found the original Richter scale was inaccurate for other areas. For example, the crust east of the Rocky Mountains is a stable mass of granite. This continental crust has different seismic characteristics than the oceanic crust in California. The ML scale gave anomalous results in these eastern regions. To fix this, scientists developed the moment magnitude scale, or Mw. Most authorities, such as the USGS, now report magnitudes above 4.0 using this scale. In Japan, the Japan Meteorological Agency uses the JMA scale for shallow earthquakes. Their JMA magnitudes agree well with seismic moment magnitudes between 4.5 and 7.5.

Local conditions can change how an earthquake feels, even if the magnitude is the same. Soil conditions play a massive role in ground shaking. Thick layers of soft soil can amplify seismic waves. This can happen even far away from the earthquake's source. Sedimentary basins can also cause resonance, which increases the duration of shaking. During the 1989 Loma Prieta earthquake, the Marina district of San Francisco suffered heavy damage. This was because it was nearly 100 km from the epicenter. Geological structures can also channel waves. In San Francisco, waves reflected off the base of the crust to reach Oakland.

Understanding these measurements helps us map the power of the Earth. Scientists use isoseismal maps to show the distribution of observed intensities. These maps show how shaking varies across a region due to geology. By looking at the maximum intensity and the area where shaking was felt, scientists can estimate magnitude. This process connects the mathematical waves on a seismogram to the real-world impact on the surface. Using the correct scale ensures that we accurately measure the energy moving beneath our feet.