What Caused the ‘Shamrock Shake’?

SCIENCE

This week, a small crack unzipped under the Santa Monica Mountains north of Los Angeles, waking millions of people with a magnitude 4.4 earthquake. (LiveScience)

Feel the Earth move under your feet? Watch “Earthquakes 101″ to understand why.

The USGS makes som pretty maps, don't they? The St. Patrick's Day "Shamrock Shake" jolted the Southern California coast just north of Encino, colored green on this lovely map. The red lines are, of course, California's jigsaw of (geologic) faults. The famous San Andreas fault is the long diagonal just south of Lancaster. Map by USGS

The USGS makes som pretty maps, don’t they? The St. Patrick’s Day “Shamrock Shake” jolted the Southern California coast just north of Encino, colored green on this lovely map. The red lines are, of course, California’s jigsaw of (geologic) faults. The famous San Andreas fault is the long diagonal just south of Lancaster.
Map by USGS

Discussion Ideas

  • Read through the short, great LiveScience article, then take a quick look at the overview of earthquakes in our “Forces of Nature” interactive. (Earthquakes are the final “force of nature”, with the seismograph icon in the upper left.) Click on slide four. It lists the four major types of faults: normal, reverse, strike-slip, and dip-slip. What type of fault was responsible for Southern California’s “Shamrock Shake”?
    • Trick question! The answer isn’t on there. Coastal California is a mess of all kinds of geologic faults. (My favorite? The Mendocino Triple Junction!) The most prominent faults, such as the San Andreas and the Hayward, are strike-slip faults. However, the Shamrock Shake probably wasn’t a direct result of the San Andreas. It was more likely the result of transpression activity. To quote the geologist in the LiveScience article, the Shamrock Shake “might . . . have alleviated squeezing caused by the San Andreas Fault’s big bend . . . Where the San Andreas Fault kinks, the Pacific Plate and the North America Plate push together instead of sliding past one another, as they do elsewhere along the major fault. The compression creates hundreds of faults in Southern California and pushes up the spectacular mountain ranges that ring Los Angeles, including the Santa Monica Mountains and the San Gabriel Mountains.”
    • Geology is awesome.
  • Why are Angelenos and geologists calling this week’s earthquake the “Shamrock Shake”?

2 responses to “What Caused the ‘Shamrock Shake’?

  1. Reblogged this on Shouts from the Abyss and commented:
    I thought a Shamrock Shake was a chemical concoction invented by McDonalds to quench our thirst to quaff something green. Who knew? Here’s some info on the St. Patrick’s Day earthquake. The epicenter is indicated on the map with a star. As it happens, I’ve been to that exact spot many times. I believe there was a taco stand there. Good carne asada burritos. I have to believe that the location of this quake was no accident. It has to mean something!

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