Measuring earthquakes
Measuring earthquakes involves assessing the energy released during seismic events, which occur due to the movement of the Earth's tectonic plates. These movements produce seismic waves that can range in impact from being barely perceptible to causing significant destruction and loss of life. Key concepts include the "hypocenter," the point of release beneath the surface, and the "epicenter," which is directly above it. Various methods have developed over history for measuring and understanding these natural phenomena, from early philosophical theories to modern scientific tools.
Seismographs are essential instruments used to detect and record the intensity of earthquakes, while the Richter scale provides a logarithmic measurement of the seismic energy released. This scale allows for a standardized method to understand the magnitude of earthquakes. Additionally, the Modified Mercalli Intensity Scale offers a qualitative ranking of earthquake effects on people and structures, encompassing 12 gradations from "feeble" to "catastrophic." Understanding these measurements is vital for earthquake preparedness and risk management in seismically active areas.
Measuring earthquakes
Summary: Earthquakes are measured in several ways, the most famous of which is the logarithmic Richter scale.
Earthquakes are the movements of Earth’s crust resulting from tectonic plates colliding against each other. This sudden release in energy causes seismic waves that cause destruction. Depending on their severity, earthquakes range from being barely noticeable to causing permanent damage to infrastructure along with a significant loss of life. Most earthquakes are caused by the action of geological faults but they can also be caused by mine blasts, volcanic activity, and subterrestrial activity, such as injecting high-pressure water for geothermal heat capture. The focal point of the earthquake is called the “hypocenter.” The point on the ground directly above the hypocenter is known as the “epicenter” of the earthquake. Philosophers, mathematicians, and scientists have long attempted to understand earthquakes. Thales of Miletus thought that earthquakes occurred because Earth rested on water. Mathematician, astronomer, and geographer Zhang Heng invented the first seismograph for measuring earthquakes in the second century. Mathematician Harold Jeffrey theorized that Earth’s core is liquid after analyzing earthquake waves. Geologists use statistical methods to try to predict earthquakes.
Seismic Waves
A tremendous amount of energy is released from the epicenter radially outward. As the energy spreads, it is manifested in three forms: compression waves (P waves), shear waves (S waves), and surface waves.
P waves are felt first and do minimal damage. S waves follow the P waves and do minimal damage. It is the slower surface waves (also known as “Love waves”) that cause the majority of the damage.
Measurement
The goal of earthquake measurement has been to quantify the energy released. Seismographs are highly sensitive instruments employed to record earthquakes. Conventionally, earthquake magnitudes are reported in the Richter scale. The Modified Mercalli Intensity Scale is commonly used to ordinally quantify (or rank) the effects of an earthquake on humans and infrastructure. Body wave or surface wave magnitudes are also used to measure earthquakes.
Richter Scale
The Richter scale quantifies the amount of seismic energy released during a quake. It is a base-10 logarithmic scale, which means that the difference between an earthquake of rating 2.0 on the Richter scale and 3.0 correlates to a tenfold increase in measured amplitude. Specifically, the Richter scale is defined as
ML = log10 A + B
where A is the peak value of the displacement of the Wood-Anderson seismograph (mm) and B is the correction factor. The wave intensity measurements are also logarithmic functions, using variables such as the ground displacement in microns, the wave’s period in seconds, and distance from the earthquake’s epicenter.
Modified Mercalli Intensity Scale
The Modified Mercalli Intensity Scale has 12 gradations: instrumental, feeble, slight, moderate, rather strong, strong, very strong, destructive, ruinous, disastrous, very disastrous, and catastrophic.
Bibliography
Brune, James. Tectonic Stress and the Spectra of Seismic Shear Waves From Earthquakes. La Jolla, CA: Institute of Geophysics and Planetary Physics, UCSD, 1969.
Gutenberg, B., and C. F. Richter. Earthquake Magnitude, Intensity, Energy and Acceleration. Pasadena, CA: Bulleting of the Seismological Society of America, 1956.
Hough, Susan. Predicting the Unpredictable: The Tumultuous Science of Earthquake Prediction. Princeton, NJ: Princeton University Press, 2009.