Measuring earthquakes is complicated and the initial magnitude is done through an automated program before seismologists look at the data and do their own calculations
According to Natural Resources Canada, the official magnitude of the quake landed at 4.7.
Differing numbers are common in the first hours after an earthquake is felt, said Brent Ward, a professor in the Earth Sciences department at SFU and co-director at the centre for Natural Hazards Research.
In the case of Friday’s quake, the initial differing numbers may have had something to do with its remote location, an unpopulated mountainous area in the Tetrahedron Provincial Park on the Sunshine Coast.
“The first numbers are calculated through an automated program,” said Ward. “The computer gets information from different seismic stations and does a very basic calculation to determine the magnitude.”
Seismologists then look at the data and do their own calculations. “There would be several people doing the calculations to make sure it’s correct,” said Ward.
“It’s complicated,” said John Cassidy, a seismologist with Natural Resources Canada, who was in a Zoom meeting at his home in Cordova Bay on Vancouver Island when he felt the quake.
Larger earthquakes — 4.5 and upwards — are more complicated to measure, because they cover a larger area and generate different types of waves, said Cassidy.
“Large earthquakes generate rapid, high-frequency shaking, the really rapid up and down shaking people feel, and more long period energy, which is felt as slow, rolling shaking.”
Initial magnitudes may be based on data from a relatively small number of stations, said Cassidy.
“For this recent earthquake on the Sunshine Coast, the very initial magnitude estimate based on those P-waves was a 5.1. Over the initial few hours, as you add in more data from different distances, folding in more information, and the magnitude may change.”
There are different magnitude scales used to measure earthquakes, ML (local magnitude, up to 600 kilometres away), MB (body wave magnitude) and MS (surface wave magnitude), and a more uniformly applicable measure called MW (moment magnitude).
“For this earthquake, the local magnitude was 5.1, but the magnitude we typically use as seismologists is moment magnitude, or MW, based on longer period energy in addition to high frequency shaking, using the entire wave form to get a better estimate on how much energy is released during an earthquake,” said Cassidy.
MW can differ from ML and is considered the most reliable measure. The MW, or moment magnitude, of Friday’s earthquake was 4.7, said Cassidy.
First measurements of magnitude are an estimate calculated by the online system within minutes. Then analysts look at the wave forms, the data from seismic stations further away, and consider “felt reports” to calculate and refine the magnitude.
“Reports on how you felt it and where you live are really, really useful to scientists,” said Cassidy. Shaking across the region is varied, depending on whether you live on solid ground, or areas built on deep layers of sediment and silt.
“We have many more people feeling the shaking than we have seismometers measuring it, and it can help give us a more detailed pattern of how waves travel to different areas, to Vancouver or Richmond or Surrey,” said Cassidy.
Friday’s earthquake was a “relatively shallow crustal earthquake within the North American Plate,” said Cassidy, with a fault of approximately one kilometre long, and movement of only a few centimetres.