Thread by @tamarahking

There are a handful of news articles suggesting that the #Broome #earthquake is the largest recorded in Australia, or the largest in WA.

Magnitude is a funny thing and the Broome event wasn't the largest in Aus or WA (thread)

To get a good location, depth and magnitude for a quake, we need nearby seismometers

Australia got its first permanent seismometer in Perth in 1901 (WA areleaders in earthquakes!)

By 1955 there were only FIVE permanent seismometers in the whole country!

Before seismometers, our earthquake location and magnitude information come from newspaper articles. We use the intensity of shaking described to get approximate information about the quake. Stronger shaking means closer to the earthquake, and higher magnitude

So any earthquake before about 1950 in Australia has high uncertainty on location and magnitude. Earthquakes after about 1960 have better locations and magnitude values

(Info from Leonard 2008: dx.doi.org/10.1785/012005…)

The next thing to know, is that a seismometer doesn't automatically give us a magnitude value. Seismologists plug data from the seismometer into a variety of equations, and get a variety of different magnitude values. Different earthquake catalogues have different magnitudes

For instance, for @GeoscienceAus's new 2018 seismic hazard map (NSHA18), a lot of effort was put into revising magnitude values across the country, particularly for those older poorly recorded earthquakes

This map shows earthquakes > 6.0 from the NSHA18 catalogue, and the years they occurred in.

So based on this catalogue, the largest is the 1941 Meeberrie event in WA (6.8). The 2019 #Broome earthquake is joint 2nd with the 1988 Tennant Creek NT (6.6) and 1968 Meckering WA (6.6)

If we look at the @GeoscienceAus online database (earthquakes.ga.gov.au), the largest are two events in the Tasman Sea near Hobart (6.9, 6.8) in the 1880's, and the #Broome earthquake is joint 3rd with the 1988 Tennant Creek and an earthquake offshore of Broome in 1929

Keep in mind, earthquakes before 1950 have uncertain magnitudes, and the online catalogue includes unrevised magnitude values. So I tend to prefer the NSHA18 map when looking at magnitude, which actively tried to improve those early events from the 1980s and 1929

That places the #Broome #earthquake tied for 2nd largest historic earthquake nationally, and 2nd largest in WA

At this point it's important to note, our historic written earthquake records only go back to 1840. The much longer geological record shows earthquakes up to ~7.0 - 7.5 magnitude, we thankfully haven't experienced one that big historically

Indigenous culture also goes back tens of thousands of years in this country, and records scientifically useful information (e.g. doi.org/10.1080/000491…). Unfortunately I'm not aware of any Indigenous geologists or geographers looking into that significant scientific record

So in short, the 2019 Broome earthquake is either top three, or top five largest historic earthquakes nationally, but probably not number one