Five earthquakes Friday morning including a negative magnitude

Five earthquakes Friday morning including a negative magnitude

Credit: KING

A seismograph registers a 2.6 magnitude earthquake near Duvall, Wash., July 12, 2013.

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by KING 5 News

KING5.com

Posted on July 12, 2013 at 8:07 AM

Updated Friday, Jul 12 at 8:57 AM

Five small earthquakes struck Western Washington Friday morning including one registered as a negative magnitude.

The first was at 2:37 a.m. about 25 miles east of Eatonville. It had a magnitude of negative 0.3.

The second, a magnitude 2.6, was at 6:56 a.m. about five miles northwest of Kirkland. That was followed by a magnitude 1.5 quake at 7:25 a.m. about 10 miles north of Sedro-Woolley.

About 9 minutes later, a magnitude 2.3 earthquake struck south of Duvall and then there was a magnitude 1.2 quake at 7:43 a.m. about 20 miles north of Friday Harbor. That last quake was technically in Canadian waters near Vancouver Island.

There were no reports of damage or injuries.

How do you get a negative magnitude reading? It’s pretty confusing, so we’ll let this blog post by the Pacific Northwest Seismograph Network explain it.


Back in the day (1930s), magnitudes started as simply the logarithm of the amplitude of the largest oscillations written on a specific seismograph at CalTech (a Wood-Anderson torsion seismograph measuring horizontal motion, if you must know!). The instrument Charles Richter used was the standard. Really for no other reason than that he had it and so he used it. He used the logarithm of the amplitude because there was such a large range of amplitudes that the compression of scale you get from taking logs made things easier to plot. Some darned earthquake that was 100 km away, made a trace with a peak displacement of 1 mm on his seismograph and he called that the standard earthquake--magnitude 1. An earthquake at the same distance that made a displacement of 10 mm was a magnitude 2, 100 mm was magnitude 3, and so on. "Smaller" earthquakes closer to the lab or "larger" earthquakes more distant might produce the same peak amplitude, so the formula includes a distance correction for earthquakes not at the standard distance. This explains an enduring mystery to many folks--negative magnitudes. An earthquake that wrote a record with a peak displacement of 0.1 mm would be a magnitude 0, right? And if that earthquake were fartther away than 100 km, well ... it would have a negative magnitude. Remember: Magnitudes are a relative size estimate. Nothing absolute about them (but, perhaps, Charlie Richter's arbitrary "standard" earthquake and seismograph); no physical units ascribed to them.

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