(The silence is mainly due to working. pre-GSA I was prepping to get everything done before jetting over to Denver. Once I returned, I had to catch up from GSA. Altogether, that means I’ve been spending the majority of my time grading, writing lecture, prepping field trips, and kicking my pseudosection models. I just gave one midterm in petrology and intro has one on Monday, so the grading is not nearly over. But, I want to start to catch up on blogging.)
Week #8 for the geology news journals was 24. – 30. October. In case you forgot, that was the 7.7 magnitude earthquake off of Indonesia, the resultant tsunami, and an unrelated eruption of Mt Merapi. It was not a good week for Indonesia.
Of the 56 submissions I received (yes, I have 70 students in the intro class…), all but 5 dealt with one, two, or all three of the events in Indonesia. Unfortunately, there were a few “common” misconceptions that either my students borrowed from the news articles or inserted themselves into the summaries:
- misconception #1 – the earthquake caused the eruption of Mt Merapi; Jessica over at Magna Cum Laude dealt with this one very well earlier this week, so I’ll let her present the scientific reasoning
- misconception #2 – the USGS reports all of the earthquake sizes on the Richter scale (as a side note, this has been coming up a few entries per week, but we haven’t gotten to EQs yet in lecture for me to talk to the whole class about it yet); the Richter scale was developed in the 1930’s by Richter & Gutenberg at CalTech to specifically measure by the amount of energy (the local magnitude) released by an earthquake for regions in California by looking at ground movement. Different rocks will react differently to seismic waves depending on what they’re composed of & their current temperature, which means you can’t use the same local energy scale for different places on the Earth. For instance, seismic waves on the west coast go through warmer rocks and therefore are slower & diffuse quicker than seismic waves through the cold east coast rocks. When an earthquake happens in LA, it doesn’t ring church bells in San Francisco (380 miles), but in 1886 an earthquake in Charleston, SC rang church bells in New York City (800 miles). The Richter scale is also only appropriate for earthquakes with a magnitude less than about 7. What we now use in the world is a magnitude moment measurement, which was developed in the 1970s (by Hanks & Kanamori–also from CalTech!). Though moment magnitudes are not appropriate for small earthquakes (<3.5), it has no upper end and will be appropriate for all rock types & temperatures. The calculation is based on properties of the rock where the rupture occurred, the area of the rupture, and the average amount of displacement that happened.
The second point will be part of my discussion of earthquakes in the coming weeks, though it wasn’t something that I even understood well until I started teaching introductory earthquakes & volcanoes. I have no idea whether its even a topic in my Marshak textbook–which is currently at work (I’m at home due to a rather large sudden snowstorm in St. Peter today).