Tokyo earthquake: 5.8, 5.9 or 6.0?

ExactTrack Radar detects Japan Earthquake Tuesday afternoon (Wednesday morning their time)

You may have heard about a strong Earthquake off Japan’s coast yesterday at 3:22pm CDT (5:22am Japan Time). Our KPRC 2 Anchors in Tokyo, Keith Garvin and Christine Noel, reported that the minor shaking lasted less than 30 seconds and felt a bit like standing on a rocking boat. ExactTrack radar detected the quake 130 miles from Tokyo as you can see in my cover graphic.

This is hardly the first quake of Magnitude 5 or greater in Japan! In fact, there have been twelve reports of 5+ in just the last 30 days, according to

That’s not a bad number when you consider that the Incorporated Research Institutions for Seismology (IRIS) counts 100,000 Earthquakes in Japan every year!

Fortunately, most of these quakes are hardly even felt. Japan rests in the delicate position between four tectonic plates and they are all moving one way or the other and that puts the country in an unfortunate position. Notice the Philippine Plate is moving 4.5 cm (1 and 3/4 inches) per year!

Map courtesy Incorporated Research Institutions for Seismology

Obviously, Japan getting an earthquake is not unusual, except to say that Tokyo’s Olympics have been besought by the “unusual” since its inception. What I did find interesting were the different Magnitude Measurements reported for this earthquake. Look at what the USGS (United States Geological Survey) reported...a 5.8 magnitude:

I even saw on Twitter someone saying 5.9M and then Lester Holt reported that the Japan Meteorological Agency declared this a 6.0M and sure enough they did:

What is going on here? A 5.8 or a 5.9 or a 6.0! This isn’t a gymnastics competition where we judge the same earthquake like a tumbling Olympian!

Or is it??

Why the differences in data?

I did a little digging on the USGS website and found that, in fact, reporting different magnitude measurements for different earthquakes happens often, especially measurements differing by .2 or .3, but can be as different as a full 0.5 measurement! That seems like a lot...why the discrepancy?

Accordingly, “Obtaining an accurate measure of an earthquake’s size is difficult. Earthquakes are complex processes that occur below the Earth’s surface away from direct observation and measurement. Determining a single number to represent an earthquake’s size has inherent uncertainties due to our assumptions about the material in which they occur and our inability to fully reconstruct the hidden process,” USGS says.

Different agencies use different calculation methods for magnitude because no single method is available. Some calculations are designed to be very fast (we like instant in our world) while others require time for a trained seismologist to go over the data. And not all agencies use the same modeling of the Earth itself. Earthquakes are tricky business and understanding them is challenging (thus, forecasting them is beyond our ability with any real-time skill). If you’d like the full explanation from the USGS, it’s right here. It’s only a few paragraphs long and I found it absolutely fascinating!

I suppose the upside is that while yesterday’s earthquake was a bit rattling, at least it happened in the early morning hours before any Olympians were competing. Can you imagine trying to pole vault or strut a balance beam during even a minor earthquake? How would those judges score that?


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About the Authors:

KPRC 2's chief meteorologist with three decades of experience forecasting Houston's weather.