How to measure a quake (Part 1): How big was that one?
By Jenny Chandler, Research Assistant, Centre for Sustainability, University of Otago

“A 4.5 Magnitude earthquake struck 50km south of Renwick and caused moderate shaking”

New Zealanders are used to seeing and hearing earthquake reports – our country experiences around 20,000 earthquakes a year – but what do the terms and numbers actually mean?

In the first of this two-part article, we’ll explore WHAT we measure when it comes to earthquakes and explain the terminology and scales used. In the second part we’ll look at HOW we measure them and introduce the tools of the trade.

Let’s start with a quick reminder of what an earthquake is.

Inner Workings

Our planet is a lot less steady and unchanging than it seems– the seemingly solid ground we live on is just a thin crust moving on the Earth’s surface, above a molten, toffee-like core.

The Earth’s crust is in pieces called tectonic plates and the edges of these (tectonic plate boundaries) is where most of the action happens. At some plate boundaries, molten rock oozes up and creates new crust, while in other places, a plate may be pushed under, along or up against another plate. All the worlds tectonic plates are moving all the time and this isn’t always smooth and steady.

Our planet has a solid crust and a molten core with a viscous layer in between (the consistency of gooey caramel) called the mantle. The crust moves around on the mantle in pieces called tectonic plates. The locations where these tectonic plates move over, under or alongside each other are called convergent, divergent or transform tectonic plate boundaries. Source: AF8

Sometimes friction will cause one tectonic plate to snag and grab against another, locking them together for years at a time. The tension builds up like a loaded spring and when things finally move – BAM! – a whole lot of energy is released at once. That energy releases along a fault and causes ruptures, cracks and movement. That is an earthquake.

Cause and Effect

Like a bomb going off underground, the energy from an earthquake ripples out in all directions. The size of an earthquake can be described and measured in two ways – the amount of energy released and the shaking caused by that energy release at different places on the surface – cause and effect.

Cause: Energy release – this is described with MAGNITUDE. Magnitude is a single fixed value for each earthquake, regardless of where it is felt or measured, making it useful for comparing and studying earthquakes.

Effect: Ground movement – this is known as INTENSITY. Intensity describes the damage caused by ground shaking on buildings and other infrastructure, and the experience of people in a particular location. Intensity is most useful for understanding earthquake impacts.

Magnitude - Energy Released at the Focus

Magnitude expresses the amount of energy released at the earthquake's focus. The focus is the starting point of all the seismic waves and the point exactly above it on the Earth’s surface is called the epicentre.

When an earthquake happens, seismic waves travel out from the focus in all directions. The epicentre of an earthquake is the point directly above the focus on the ground.

Magnitude measurements initially used the Richter scale, but these days scientists use the Moment Magnitude scale, because it works better across all regions and different sizes of earthquakes.

The scale goes from 1-10 and is logarithmic. As you can see from the illustration below, a magnitude 3 earthquake is approximately the equivalent energy release as 1800kg of dynamite, whereas a magnitude 8earthquake would be like 56 million tons! And by the way, we’ve never had a magnitude 10 earthquake – the largest was 9.6 in Chile in 1960!

Each step up the scale represents 10x more ground displacement and about 32x more seismic energy released! [link to formulas here for the geo-nerds]

Earthquakes release a huge amount of energy – this diagram compares the energy release of some large earthquakes (on the left)with other natural and man-made phenomenon (on the right). Note: this diagram is intended as an illustrative tool only as there is no exact formula for converting magnitude or volcanic force to megatons of TNT.

Intensity - Ground Movement at a Specific Location

We have all watched enough action movies to understand that how an explosion impacts the surface depends on more than just how powerful it is –the ground conditions, the depth and the hero’s distance away will all have an effect.  

In the same way, magnitude isn’t necessarily a good indicator of how intense the shaking or surface motion will be in an earthquake or how that level of shaking will impact us.  

Intensity and surface motion measurements describe the actual ground movement at a specific location and can be measured in a few different ways – by human experience and by scientific measurements.  

Human Experience - Modified Mercalli Intensity Scale (MMI)

The MMI Scale is used globally to express intensity and ranges from 1-12. Have you ever felt an earthquake, gone onto the GeoNet website and submitted a ‘Felt Report’? If so, you’ve used the modified Mercalli Intensity scale (MMI) to tell scientists about the intensity of shaking you felt in your location. This data helps them estimate the impacts of the earthquake across the motu. As well as numbers, the MMI scale uses set terminology for the intensity of shaking – if you look at the descriptions below you can see what is meant by the ‘moderate shaking’ described in the headline at the start of this article.

In Aotearoa New Zealand the Modified Mercalli scale is used to describe earthquake intensity in 12 steps with one representing the weakest shaking and 12 being almost total destruction.

Scientific Measurements – Peak Ground Velocity (PGV) and Peak Ground Acceleration (PGA):

Intensity can also be measured using instruments that measure ground movement – seismometers and strong motion sensors. These give us Peak Ground velocity (PGV) or Peak Ground acceleration (PGA) at a given point. These objective measurements are useful for building design, infrastructure planning and understanding the way different materials react to seismic waves.

Comparing Scientific Measurements with Human Experience

Human experience measurements (like the Felt reports) give areal-time indication of how an earthquake feels to those on the ground. However, in a strong earthquake, people are not going to spend time filling ina form on their phone (if phones are even working).

A screenshot of the GeoNet ‘Felt report’ that captures the earthquake intensities recorded by members of the public for the 2016 Kaikoura Earthquake. You can see a lack of reports from around the epicentre, (where there was the highest intensity shaking happening) as people focused on more important things. Source: GeoNet]

This means that immediately after a big quake, scientists are reliant on the scientific measurements coming in from ground motion sensors to work out what is happening to people and structures on the ground.

But is there a perfect formula for converting ground motion measurements such as PGV and PGA into numbers that reflect the human experience on the ground (like MMI)?

Not exactly. Around the world, many studies have compared data from ground motion sensors and their countries version of the ‘felt reports’ for sets of earthquakes.  In January 2021scientists published the latest ground motion to intensity conversion equations for New Zealand. The graphs below compare their findings with those of previous studies – the results are similar but not identical.

Comparison of the relationship between ground motion and intensity from different studies (Peak Ground Velocity PGV on the left, Peak Ground Acceleration PGA on the right). Source: Moratalla et al 2021]

Our collective knowledge of the relationship between magnitude, ground motion and intensity is improving all the time, but there are no perfect equations yet.

Continuing the Learning

So now you understand the details of that news headline about the latest earthquake – where the focus was, how deep, how much energy was released and how intense the shaking was. Tell all your friends!

Then, continue the learning with part two of ‘how to measure an earthquake’ to find out about the tools scientists use to calculate these measurements.

This article is a part of series developed in collaboration with AF8[Alpine Fault magnitude 8] aimed at explaining earthquake science and increasing understanding of earthquake risk and resilience. With thanks to our science partners for their contributions: QuakeCoRE: New Zealand Centre for Earthquake Resilience, Resilience to Nature’s Challenges, University of Otago and GNS Science.