how do we measure earthquakes

Earthquakes are measured using sensitive instruments called seismometers and by using standardized scales for magnitude (energy released) and intensity (how strong the shaking feels and how much damage it causes).

What actually gets measured?

• Seismometers detect tiny ground vibrations and convert them into electrical signals that are plotted as seismograms (wavy lines over time).

• From these seismograms, scientists measure the amplitude (height) and frequency of seismic waves and use their arrival times to locate the earthquake and estimate its size.

Magnitude: how “big” the quake is

Magnitude is a single number that represents the total energy released at the source of the earthquake.

• Modern measurements use the moment magnitude scale (Mw) , which is based on the fault area that slipped, how far it slipped, and rock strength.

• This “seismic moment” is calculated from seismograms and sometimes field measurements, and then converted to a magnitude number that is comparable to older scales like Richter for smaller quakes.

• Older scales (local magnitude/Richter, body-wave MbM_bMb​, surface-wave MSM_SMS​) use formulas involving the logarithm of wave amplitude, period, distance, and calibration terms; these are still used in some contexts but are less reliable for very large events.

Intensity: how strong it feels at a place

While magnitude is global, intensity varies from place to place.

• Intensity describes effects: what people felt, how objects moved, and what damage occurred in each area.

• Scales like the Modified Mercalli Intensity (MMI) or the European Macroseismic Scale (EMS) use Roman numerals (I–XII) to categorize shaking from “not felt” to “devastating.”

• Today, intensity is estimated using both human reports and instrumental data (measured ground acceleration and velocity), which can be combined into maps such as ShakeMaps.

From waves to location and maps

• By comparing arrival times of primary (P) and secondary (S) waves at multiple stations, seismologists triangulate the earthquake’s epicenter and depth.

• Networks of seismometers continuously record ground motion, allowing rapid automatic magnitude estimates and real-time shaking maps used for emergency response and public alerts.

Why you still hear “Richter scale”

• The Richter scale was the first widely known magnitude scale and is still mentioned in news and forum discussions, even though large modern earthquakes are now reported in moment magnitude (Mw).

• For small, local events, Richter-like local magnitude values and Mw are often similar, which helps the older term persist in media coverage.

Information gathered from public forums or data available on the internet and portrayed here.