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.

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”?
  • It happened on St. Patrick’s Day, of course! They could have also called it the Holi Roller . . . (Holi fell on March 17 this year.)

• According to “Forces of Nature,” both earthquakes and volcanoes are associated with plate tectonics, the movement of tectonic plates—Earth’s giant slabs of crustal rock. (The Shamrock Shake was the result of a minor interaction between the Pacific and North American plates.) Take a look at our MapMaker Interactive, displaying tectonic plates, earthquakes, and volcanoes. There is a lot of overlap! However, while the California Coast has experienced dozens of earthquakes (blue circles on the map), it has not experienced any recent volcanic activity. (That lone red triangle up north is the 1980 Mount St. Helens eruption in Washington.) Why aren’t there any volcanoes in California?
  • Well, there are. Mount Shasta is a big, beautiful volcano. Lassen even erupted in the 20th century. Oh, you mean really active volcanoes . . .
  • Most big faults in California are transform, strike-slip faults. Volcanoes are usually associated with other fault types. One such volcano-prone fault is what “Forces of Nature” calls a normal fault, where tectonic plates are ripping apart from one another. The mid-Atlantic Ridge is one such normal fault, and volcanoes are spectacularly common where the ridge divide the island nation of Iceland. Volcanoes are also associated with reverse faults, where tectonic plates are crashing together in my favorite geologic process, subduction. Beautiful Mount Fuji, Japan, and Krakatoa, Indonesia, are volcanoes in subduction zones.
  • Along the California coast, “there is no ripping apart or subduction taking place,” so “there isn’t any magma formation to lead to volcanoes.”