Overnight Japan had a 7.3 earthquake followed by several smaller quakes (M4+) near the epicenter of last year’s massive 9.0 Tohoku quake and tsunami, but thankfully there haven’t been any reports of deaths or serious damage. And TEPCO has found no problems with the Fukushima Daiichi nuclear plant from any of these aftershocks either, according to several reports. (As fyi, since these tremors occurred in the same area, they are technically considered aftershocks.)
Although 2012 quake activity has been about average according to USGS many people continue to wonder how and why earthquakes happen and if there is an increase in the number of quakes lately.
First let’s look at some numbers. The U.S. Geological Survey estimates over 3 million earthquakes occur globally each year. That’s about 8,000 seismic events every day or 1 every 11 seconds, but most of them are very small. And with the 10 to 20-fold increase of seismograph stations operating around the world (from 350 in 1931 to 4,000 or 8,000 [reports vary]) combined with the Internet and 24-hour news sources, the numbers are up due to more accurate data and reporting methods.
But if you analyze global earthquake records over the past century (which is a tiny sliver in time compared to our planet’s history), the averages of large events (6.0 and higher) are fairly constant especially during the past three decades.
Also … let’s look at a few statistics about Japan.
Did you know…
…Japan accounts for about 20 percent of the world’s earthquakes of magnitude 6.0 or greater?!
…Tokyo, with a population of 12 million, sits on the junction of four tectonic plates: the Eurasian, North American, Philippine and Pacific?! The sudden bending or breaking of any plate can trigger an earthquake.
So … how and why do earthquakes happen?
There are many factors involved but one key reason is our planet’s surface is made up of slowly-moving sections called tectonic plates that can build up friction or stress in the crust as they creep around. The fastest plate races along at 6 inches (15 cm) per year while the slowest plates crawl at less than 1 inch (2.5 cm) per year according to USGS.
These plates slide over the lubricating athenosphere layer of the lithosphere (the surface layer of our planet) and have been crawling around the planet for billions of years. The plates have edges called the plate boundaries that are made up of many faults (cracks or fractures in the crust). Since the edges of the plates are rough, they can get stuck while the rest of the plate keeps moving. When the force finally unsticks, all that stored up energy is released and radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond.
Sometimes part of the crust dives (or subducts) under another plate sinking into the earth’s mantle and these areas are often busy with volcanic activity and earthquakes. In fact, nine out of the ten largest quakes to occur in the last 100 years were subduction zone events.
Also, some parts of the planet have faults and fractures in rifts where there is a weakness or a split in the crust like the New Madrid Seismic Zone in Central U.S. This rift zone is a bit unusual since it’s in the middle of a plate, but it was created about 600 hundred million years ago then weakened 200 hundred million years ago when Pangea broke up. (Pangea graphic from New World Encyclopedia)
Some U.S. and Canadian fault systems
There are three basic fault types: the normal fault, where one block of rock drops down relative to the other; the strike-slip fault, where the fault blocks slide horizontally past each other; and the reverse fault, where one fault block moves upward relative to the other.
An example of a strike-slip fault system is the San Andreas Fault in California. The San Andreas fault is NOT a single, continuous fault, but rather is actually a fault zone made up of many segments. The fault system is more than 800 miles (1300 km) long, and in some spots is as much 10 miles as (16 km) deep. Also .. since the plates are moving horizontally past one another, California will not fall into the ocean … but … someday Los Angeles and San Francisco will be adjacent to one another!
Canada’s Queen Charlotte fault in B.C. was the site of the country’s largest historical earthquake with a magnitude 8.1 in 1949. The most seismically active areas in Canada are Yukon Territory and northwest British Columbia but historically all Provinces have had tremblers as seen here.
The Cascadia Subduction Zone is a very long sloping fault that stretches from mid-Vancouver Island to Northern California and could produce a very large earthquake, magnitude 9.0 or greater. The last known great earthquake there was in 1700 and geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years or about every 400 to 600 years.
The New Madrid Seismic Zone (extending from northeast Arkansas, through southeast Missouri, western Tennessee, western Kentucky to southern Illinois) has repeatedly produced major earthquakes, including several magnitude 7 and 8 quakes, over the past 4,500 years. The last major occurrence there was the 1811-12 earthquake sequence (mag 7s and 8) that struck Arkansas and Missouri with such intensity it temporarily reversed the Mississippi River flow, created a new lake and caused massive landslides and damage across multiple states.
Waves and Liquefaction
Most destruction from earthquakes is caused by the seismic waves (ground motion) and soil liquefaction (where soil behaves like a liquid). There are two major types of seismic waves — body waves and surface waves.
Body waves (pressure or primary or P waves and shear or secondary or S waves) are short, sharp motions moving at high speeds that move with an up-and-down [P] and side-to-side [S] motion.
Surface waves (Rayleigh and Love waves) travel along the surface causing the most destruction. Rayleigh waves are similar to ocean waves; whereas Love waves displace earth in a snake-like motion. Both types of surface waves can demolish buildings and trigger landslides and avalanches far from the epicenter.
Liquefaction is a process by which water-saturated sediment temporarily loses strength and acts as a fluid, like when you wiggle your toes in the wet sand near the water at the beach. In other words, the shaking of an earthquake jiggles the sand and squeezes the water trapped between grains so much that the layer begins to act like a muddy liquid. Because liquefaction occurs in saturated soil, its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans. (liquefaction graphic from Univ of MD EDCI)
So what do we do?
Unfortunately, scientists cannot predict earthquakes but there are technologies like GPS and LIDAR (Light Detection And Ranging) that are helping experts locate faults, rifts and vulnerable areas of our planet. But, as stated above, we live on a violent planet and thousands of earthquakes happen every single day. (Click here to see a cool NASA animation of cumulative global earthquake occurrences from 1960 – 1995. Earthquakes are shown as yellow dots.)
We don’t need to worry or panic, but something we CAN do is learn about different types of risks and disasters and how to mitigate or reduce the damage to yourself, your loved ones and your property. Knowledge is power and the more we learn and prepare for the unexpected, the better off we’ll be as a society.
A way to get started is to download some safety tips from our IT’S A DISASTER! book, and read through some of the resources below for more information.
HowStuffWorks Earthquake Facts
Kidzworld Earthquakes 101
Natural Resources Canada
San Diego State University “Notes on Planet Earth version 3.0”
USGS Earthquake Hazards Program