Measuring earthquakes: the difference between magnitude and intensity

Magnitude measures the amount of seismic energy released at the source - or hypocenter - of an earthquake. An earthquake has only one magnitude determined from measurements on seismographs. The first widely-used measurement was the Richter scale. It is now considered outdated and the Moment Magnitude Scale (Mw) is deemed more accurate - a strong earthquake would be typically described as 8.5 Mw magnitude. However, the media often wrongly refer to Mw measurements as Richter magnitudes.

Magnitude scales do not used express damage and nor do they provide a direct indication of the shaking level on the ground surface. However, Intensity scales, like the Modified Mercalli Intensity Scale or the Japan Meteorological Agency’s Seismic Intensity Scale, do provide an indication of the extent and distribution of the damage caused from ground surface shaking caused by an earthquake.

The intensity value is determined from the observable effects of the shaking on people, on manmade structures and their contents, and on the landscape. Intensity values are variable over the area affected by the earthquake, with high intensities typically near the epicenter and lower values further away.

Measuring ground motion intensity for damage calculations

When it comes to developing predictive models and designing seismic building codes, more scientific measures are used. Peak Ground Acceleration (PGA), measures the maximum ground acceleration during an earthquake. Like intensity scales, PGA measures how hard the earth shakes at a given geographic point. But where intensity scales are subjective using personal reports and observations to report earthquake intensity, PGA is measured by instruments, such as accelerographs.

At Zurich, many factors are taken into account when predicting earthquake damage, including a statistical analysis of past earthquakes and knowledge of the tectonic and geological structure. But the key measurement used in developing predictive models is a measurement called Spectral Acceleration.

“Spectral Acceleration describes the maximum acceleration on an object during an earthquake – and is not solely a measure of ground motion. This means it provides a closer approximation to the motion – and potential damage – of a building during an earthquake than PGA. This enables us to calculate potential losses from earthquakes more accurately,” explains Iwan Stalder, Head of Accumulation Management.

The structural performance of a building during an earthquake depends on many factors, including magnitude, distance to the fault, soil type, building height, and construction quality. All these factors are built into the Spectral Acceleration calculation.

As well as helping develop predictive models, Spectral Acceleration is often used as the basis for seismic building codes and is considered by Zurich’s risk engineers when they assess a customer’s earthquake exposure.

“Measurements like Spectral Acceleration help us understand a customer’s earthquake exposure in a given location, but ultimately the construction quality and earthquake readiness help you to truly understand a customer’s risk,” explains Dr Amar Rahman, Principal Risk Engineer / Global RE Practice Leader Natural Hazards Resilience, Zurich Insurance.

“When we work with customers, we compare the version of the seismic building code when the building was originally designed to the latest earthquake engineering best practice. We also look at any changes to the building, such as extensions, new equipment and layout changes, to determine potential vulnerabilities,” adds Rahman.

“And finally, to provide us with a broad picture of a customer’s level of seismic resilience, we examine their organizational measures, e.g. earthquake emergency response and business continuity plans to see if they understand their risks. From all of that information we can provide a comprehensive plan, which includes physical and organizational measures, to increase resilience.”