The image above is a depiction of the fault zones in the Bay Area, California. The black star indicates where I live in California.
(Source: http://www.ebsinstitute.com/EBS.EQ2_RS_DA.html)
Life in California often evokes images of palm trees, sunny-blue skies, and sandy beaches. As a resident of California, I can attest that all those stereotypes are often correct. But sometimes, people overlook the idea of California as “Earthquake Country.” Living 0.5 miles away from the Hayward Fault and a few more miles away from the San Andreas Fault, I’ve become used to waking up at odd times during the night to a swaying bed frame or watching pendulums move seemingly on their own.
While most natural disasters - including hurricanes, tornadoes, blizzards, volcanos, and tsunamis - can be predicted ahead of time, earthquakes lack that luxury. They are hard to predict because it’s difficult to tell when exactly the pressure built between two plates will release1. Prediction is crucial for saving lives because it can give people time to get to a safe place. Several methods have been created to predict earthquakes, including the analysis of the patterns of previous tremors, the use of animal behavior, changes in the concentration of radon gas, and even variations in electromagnetic behavior1. But these methods all share a common problem - accuracy.
However, we Californians don’t need to fear as much, because there has emerged a new system recently, known as earthquake early warning.
What is earthquake early warning and how do earthquakes work?
In a nutshell, earthquake early warning is a system that sends a warning that an earthquake will strike in a certain area within a few seconds. When an earthquake occurs, one of three main things can happen: two plates may pass each other on a strike-slip fault (one example is the San Andreas Fault), a plate on top of the fault moves down relative to the plate on the bottom on a normal fault, or a plate on the bottom of the fault moves up relative to the plate on the top on a reverse fault2. This movement leads pressure to build up between two pieces of land. As this pressure builds up, reaching a point that’s too large to handle by the pieces of land, it’s released in an earthquake. This release of energy transforms into waves that emanate outward from the epicenter of the earthquake. There are two main types of waves: P-waves and S-waves. P waves, which tend not to cause lots of damage, move faster than S-waves, which can cause lots of damage2. The variety of waves is useful in detecting earthquakes, because earthquake stations (also known as sensors, placed throughout California) detect P-waves first. P-waves are then used to determine the magnitude and location of the earthquake2. These P-waves allow the warning to be sent to people via text messages, computers, radios, and TVs, warning them of the upcoming S-waves, which are dangerous2. This warning normally includes the shaking intensity and estimated time of arrival of the earthquake2.
The image above shows how earthquake early warning functions. (Source: https://earthquake.usgs.gov/research/earlywarning/overview.php)
How do you find the warning time and how long is it?
The main factor determining the warning time, the amount of time it takes for the S-waves to reach a specific location, depends on the distance between that location and the earthquake’s epicenter. The farther away the area is from the epicenter, the greater warning time it receives, because the waves take longer to reach that area3. This extended time decreases the intensity of the wave as well. Therefore, a warning may not be useful for people far away from the epicenter. With this in mind, the warning and detection time change based on several factors including the amount of distance between the epicenter and closest station, the absorption of earthquake data by regional networks, and the accurate “diagnosis” of an earthquake’s magnitude and intensity3. Based on this, a warning time can range between 0-60 seconds.
Is it perfect?
0 seconds? This seems pointless. But this is one of the pitfalls of the earthquake early warning system. Around the epicenter of the earthquake, there is a “blind zone,” a radius of places around the earthquake that receive no warning at all. When the earthquake first happens, the P and S-waves start at the same place. Imagine two cars are racing - Car A, which has an average speed of 100 miles per hour, and Car B, which has an average speed of 75 miles per hour. Car A is analogous to P-waves, while Car B is analogous to S-waves. Car A and B start at the same position; for a while, they remain next to each other, until Car A inevitably overtakes Car B permanently. During the time these cars, or waves, remain together, there is no warning, because the S-waves, which can cause damage, arrive at the same time as the harmless P-waves. This is an issue of great magnitude- excuse my pun- because areas closest to the epicenter are the places that have the highest shaking intensity. In other words, these are the areas that need warning!
Although earthquake early warning systems can offer some people fair warning before an earthquake strikes, earthquake prediction still needs work. But at least we can take comfort knowing there are systems being developed to help warn of earthquakes in areas around the world.
References:
- http://www.bbc.co.uk/guides/zxyd2p3
- https://www.usgs.gov/faqs/what-a-fault-and-what-are-different-types
- https://earthquake.usgs.gov/research/earlywarning/background.php