The second magnitude scale measures the motion of rock alongside the fault. (Sept. 22, 2017)
We all know an earthquake is extreme when it has a excessive magnitude quantity. The 1985 quake that devastated Mexico Metropolis was an 8. The quake that shook central Mexico this week was a 7.1.
However what precisely is magnitude?
Mainly, magnitude is a quantity representing the whole vitality launched in an earthquake, mentioned seismologist Lucy Jones. The vitality launched is set by how a lot rock strikes and the way far it strikes.
Right here’s an explainer for what magnitudes imply, and the way that pertains to the shaking we really feel. This explains why an 8 could possibly be a lot extra terrifying than a 6.
How does magnitude relate to how a lot vitality was produced in an earthquake?
For every whole-number improve in magnitude, the seismic vitality launched will increase by about 32 occasions. Which means a magnitude 7 earthquake produces 32 occasions extra vitality — or is 32 occasions stronger — than a magnitude 6.
A magnitude 8 releases 1,000 occasions extra vitality than a magnitude 6, however it releases that vitality over a bigger space and for an extended time, Jones mentioned.
Initially, the definition of magnitude associated to seismograms, wherein machines used an ink stylus to document fast motions on a rolling drum of paper that will measure shaking. Magnitude was about how massive the waves had been on a seismogram at a specific distance from the epicenter. On the so-called Richter scale, a magnitude 8 on a seismogram was 10 occasions greater than a magnitude 7.
However the Richter scale was ultimately scrapped in favor of what's often known as the second magnitude scale. The second magnitude scale measures the motion of rock alongside the fault, and precisely measures bigger earthquakes, which might final for minutes and have an effect on a a lot bigger space; the Richter scale didn't precisely document such quakes, Jones mentioned.
The U.S. Geological Survey has a calculator that may assist you to make these calculations. So, as an example, a magnitude 8.2 — most likely the strongest earthquake that would hit Southern California on the San Andreas fault — would produce an astonishing 178 occasions extra vitality than the magnitude 6.7 Northridge earthquake in 1994.
What are the earthquake magnitudes to fret about?
Magnitude is vital, however a key issue is the place a quake strikes. Those most individuals care about strike underneath or close to closely populated areas.
Usually, earthquakes of magnitude 6 and above are those for concern. When close by, they'll trigger shaking intensities that may start to interrupt chimneys and trigger appreciable harm to probably the most seismically weak buildings, equivalent to non-retrofitted brick buildings.
Earthquakes of magnitude 7 and above can overturn heavy furnishings and inflict appreciable harm in strange buildings.
OK, so I perceive that magnitudes 6s, 7s and 8s might be dangerous. What does that imply by way of how dangerous the shaking shall be the place I’m situated?
Let’s think about the 1994 Northridge earthquake.
That earthquake was a magnitude 6.7. However the shaking depth various by the place you had been when it hit.
In case you had been in Northridge, proper on prime of the place the earthquake fault moved, you confronted what’s often known as depth 9 shaking, outlined by the Modified Mercalli Depth scale as “violent” — sufficient to shift a construction off its basis.
But the downtown Los Angeles area obtained far much less intense shaking from the identical earthquake — depth 6 or 7, the place harm is negligible in buildings of excellent design and development.
Let’s say a magnitude 7.8 earthquake strikes the San Andreas fault in Southern California. What depth shaking would Southern California really feel?
Huge swaths of Southern California would really feel depth 10 shaking, or excessive shaking — an earthquake that nobody alive at present has skilled on this area.
I’ve by no means heard of this depth scale. Are you able to describe it extra?
Sure. Right here’s the USGS’ clarification:
Depth 10: Excessive — Some well-built wood buildings destroyed; most masonry and body buildings destroyed with foundations. Rails bent.
Depth 9: Violent — Harm appreciable in specifically designed buildings; well-designed body buildings thrown out of plumb. Harm nice in substantial buildings, with partial collapse. Buildings shifted off foundations.
Depth 8: Extreme — Harm slight in specifically designed buildings; appreciable harm in strange substantial buildings with partial collapse. Harm nice in poorly constructed buildings. Fall of chimneys, manufacturing unit stacks, columns, monuments, partitions. Heavy furnishings overturned.
Depth 7: Very sturdy — Harm negligible in buildings of excellent design and development; slight to average in well-built strange buildings; appreciable harm in poorly constructed or badly designed buildings; some chimneys damaged.
Depth 6: Robust — Felt by all, many frightened. Some heavy furnishings moved; just a few situations of fallen plaster. Harm slight.
Depth 5: Reasonable — Felt by practically everybody; many woke up. Some dishes, home windows damaged. Unstable objects overturned. Pendulum clocks could cease.
Depth 4: Mild — Felt indoors by many, outdoor by few through the day. At night time, some woke up. Dishes, home windows, doorways disturbed; partitions make cracking sound. Sensation like heavy truck hanging constructing. Standing motor vehicles rocked noticeably.
Depth 3: Weak — Felt fairly noticeably by individuals indoors, particularly on higher flooring of buildings. Many individuals don't acknowledge it as an earthquake. Standing motor vehicles could rock barely. Vibrations just like the passing of a truck. Length estimated.
Depth 2: Weak — Felt solely by just a few individuals at relaxation, particularly on higher flooring of buildings.
Depth 1: Not felt — Not felt besides by a only a few underneath particularly favorable circumstances.
Bear in mind when individuals used to say ‘Richter scale’ when describing magnitude? What occurred to the Richter scale?
The USGS switched its definition of magnitude from the Richter scale to the second magnitude scale within the late Nineties, Jones mentioned.
The Richter scale, developed within the Nineteen Thirties, was named after American seismologist and physicist Charles Richter.
Twitter: @ronlin
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