6.1: What is an Earthquake?

What Is An Earthquake?

An earthquake Links to an external site. is a sudden shaking of the ground primarily as a result of fault movement. However, volcanic activity, landslides, impacts, or even human-induced activity can trigger an earthquake. Earthquakes have been experienced by humans throughout our short history. Most ancient cultures developed myths to explain them, including envisioning large creatures - gods or goddesses - within Earth moving to create the quake.

On July 5, 2019, a 7.1 magnitude earthquake hit Ridgecrest, California. This was the main shock of the sequence; the previous day, a 6.4 magnitude foreshock Links to an external site. (smaller earthquakes that precede the main earthquake) occurred. Both earthquakes were felt over a wide area of Southern California. Thousands of aftershocks Links to an external site. (smaller earthquakes that follow a larger earthquake) have occurred since (Figure 6.1).

Map with aerial image overlay illustrating the fault surface rupture (red, gray, blue) during the 2019 Ridgecrest earthquake sequence. The star indicates the earthquake epicenter.

Figure 6.1: General surface rupture based on field mapping and satellite data as of July 15, 2019. Ruptures from the magnitude 6.4 event trend northeast to southwest, and ruptures from the magnitude 7.1 event trend northwest to southeast. Circles indicate where scientists have visited the fault surface (Public Domain, USGS Links to an external site.).

California is no stranger to earthquakes. We have many faults and fault systems throughout the state, including the infamous San Andreas Fault. Many of our faults are a consequence of California’s position on a tectonic plate boundary. Most earthquakes (95%) occur along plate boundaries, while the remaining 5% occur in the interior of a tectonic plate (intraplate). In North America, most earthquakes occur along the West Coast in association with plate boundaries, but intraplate earthquakes occasionally occur in places like the New Madrid, Missouri area (Figure 6.2).

Map of the US illustrating earthquake hazard potential. Red signifies higher hazard, blue and gray lower hazard.

Figure 6.2: Earthquake hazard map showing peak ground accelerations having a 2 percent probability of being exceeded in 50 years, for a firm rock site. The map is based on the most recent USGS models for the conterminous U.S. (2018), Hawaii (1998), and Alaska (2007). The models are based on seismicity and fault-slip rates and consider the frequency of earthquakes of various magnitudes. Locally, the hazard may be greater than shown, because site geology map amplify ground motions (Public Domain, USGS Links to an external site.).

Earthquakes provide valuable information about conditions in Earth’s interior and at the surface. And, unlike previous topics we’ve explored, earthquakes are a hazard Links to an external site. with inherent risks to human society. Those of us in California likely have experienced at least one earthquake in our lifetime (maybe more!). Therefore, we are aware of the need to consider hazard mitigation, including planning, preparation, and communication, all commonly supported by the science of seismology Links to an external site..

Box 1: When an earthquake hits, people flood the internet with posts about it – some within 20 or 30 seconds. Stick figure 1, “ROBM163 says “Huge earthquake here!”. Box 2: Damaging seismic waves travel at 3-5 km/s. Fiber signals move at 200,000 km/s (minus network lag). Box 3: This means when the seismic waves are about 100 km out, they begin to be overtaken by the waves of posts about them. Image shows “quake” line and “tweets” line. Box 4: People outside this radius may get word of the quake via Twitter, IRC or SMS before the shaking hits. Stick figure 2: “Whoa! Earthquake!”. Box 5: Sadly, a Twitter’s first instinct is not to find shelter. Stick Figure 2 and 3, “RT ROBM163 Huge earthquake here!”

Figure 6.3: "Seismic Waves" (CC-BY-NC 2.5, xkcd Links to an external site.).

Seismologists and geophysicists are the primary investigators of earthquakes; however, other geoscientists may study them, as well. Like many other geoscientists, working with other disciplines is common, with a heavy influence from both math and technology. Many are employed by universities where they teach and/or do research, and state and federal agencies, including geological surveys, like the California Geological Survey Links to an external site. or United State Geological Survey (USGS) Links to an external site.. Additional career pathways are available in the private sector including in mining and natural resource extraction or in hazard mitigation and assessment. Many of these career options require a college degree and postgraduate work. If you are interested, talk to your geology instructor for advice. We recommend completing as many math and science courses as possible (chemistry is incredibly important for mineralogy). Also, visit National Parks Links to an external site., CA State Parks Links to an external site., museums, gem & mineral shows, or join a local rock and mineral club. Typically, natural history museums will have wonderful displays of rocks, including those from your local region. Here in California, there are a number of large collections, including the San Diego Natural History Museum Links to an external site., Natural History Museum of Los Angeles County Links to an external site., Santa Barbara Museum of Natural History Links to an external site., and Kimball Natural History Museum Links to an external site.. Many colleges and universities also have their own collections/museums.