Can you tell me how scientists read seismometers and Richter scales, and what happened when Mt.St.Helens erupted in 1988

rocky Sara Smith

seismometer Hi Sara,

Scientists use seismometers to determine the location and strength of earthquakes. The seismometer is the device that actually measures the ground shaking. The signal is transfered to a seismograph which plots the data as a squiggly line called a seismogram. Depending on how far away you are and on how strong the earthquake is, the squiggly line can be jagged or barely noticeable. The richter scale was developed so that earthquakes could be compared in an objective way. For example, a medium-sized earthquake might be really devastating if it occurrs near a city while a big earthquake might be no big deal if it happens out in the desert. The richter scale ignores all the "human" effects and allows scientists to determine the amount of energy released by an earthquake. You probably know that it is not a linear scale. For esample, a 4 on the richter scale is not twice as strong as a 2. For each step on the richter scale there is a multiple of 30x the energy released. Therefore a magnitude 3 releases 30 times as much energy as a magnitude 2. A magnitude 4 releases 30 times as much as a magnitude 3 therefore a magnitude 4 is 30 x 30 as strong as a magnitude 2 or 900 times as strong as a magnitude 2!

There is a scale that takes into account the human side of earthquakes and it is called the Mercalli scale. It has categories such as "barely felt", "dishes broken" all the way up to "buildings collapsing". Since whether or not a building collapses depends on things like how tall it is or how well it is built, this Mercalli scale is much more subjective.

The Mount Saint Helens eruption of 1988? The big Mount Saint Helens eruption was in 1980. That was when the north side of the volcano collapsed away and there was a big explosive eruption and lots of flooding. Since then a lava dome has grown up in the caldera that formed. Every once in a while the dome is destroyed by an explosion, or a landslide carries part of the dome away. My list of eruptions doesn't have anything specifically listed for 1988 but there may have been one of these small dome explosions.

I hope this answers some of your questions.

Sincerely,

Scott Rowland, University of Hawaii 

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