Wednesday, August 24, 2011

Earthquakes and the East Coast

As you've probably heard by now, there was a 5.8 earthquake that struck in rural Virginia, southwest of Washington DC, on August 23rd. You can read more details about the event here, via Yahoo! News.

Here in California, a 5.8 might only be cause for some dinner conversation. Back East, though, earthquakes work a little differently....

Naturally, we see a lot more earthquakes through California than they see elsewhere in the country. This is because we sit astride the San Andreas fault system, one of the largest and most active tectonic boundaries in the world. That doesnt mean that other parts of the country are "earthquake-free", though. Fault systems of various sizes exist pretty much throughout the United States. In the Midwest, for example, the New Madrid fault system runs through southern Missouri, and had a major earthquake in the early 19th century.

Many fault systems run along the eastern seaboard. Though not as active today as the faults along the West Coast, hundreds of millions of years ago these faults were created when what is now Africa collided with North America, pushing up the Appalachian mountains, which at the time may have rivaled the Himalays in height. Though those mountains have eroded over time to the modest heights they have today, some of those faults created still remain, and do still occasionally trigger earthquakes.

When an earthquake occurs along the East Coast, it moves through the ground differently than it does here. The ground in California is riddled with fault lines of all sizes, cracked like a broken windshield. The bedrock along the East Coast is much less broken-up, with time and heat and pressure sealing many old fault lines and annealing the bedrock together.

When a vibration, like an earthquake, travels through the ground in California, it's like trying to ring a cracked bell; the vibration doesnt travel very well, or very far. Along the East Coast, however, the bedrock is more like a solid bell, and when an earthquake "rings" that bell, the vibration can travel much farther, which is why such a relatively small quake was still felt from Georgia to Toronto.

The Easter quake of 2010 near El Centro (magnitude 7.2) released about 25 times as much energy as yesterday's Virginia quake. The Japanese quake (9.0) released over 1500 times as much energy. Fortunately, though, it looks like the most that the Virginia quake did was rattle the nerves of some Easterners who arent used to a little temblor now and then :)

Friday, March 11, 2011

Earthquake and Tsunami Facts

Last night, a magnitude 8.9 earthquake hit off the northeast coast of Japan, causing widespread damage and triggering a tsunami reported up to 23 feet in height. You can read more about the earthquake and its effects on Japan in this article from Yahoo! News.

While California seems to have avoided any significant tsunami threat from this earthquake, you may be wondering if something like this could ever happen somewhere near us. Here are some common questions about earthquakes and tsunamis that you might like to know:

What is a tsunami?
Tsunami is japanese for "Harbor Wave". Tsunami is the preferred term over "tidal wave", as tsunamis have nothing to do with the daily cycle of tides. However, while many people might still envision a tsunami as a giant, cresting breaker crashing onto the shore, when it hits land it behaves much more like a very high "tide", where the water level rises significantly and rushes inland, then ebbs back out. Tsunamis are also immediately preceded by an extreme drop in the ocean level, much like a very low "tide".

Tsunamis are caused when the sea floor shifts upwards, like during an earthquake, and raises up all the water above it. This "bump" of water spreads outwards in all directions and becomes a tsunami. Normal waves are caused by wind and only flow a few dozen feet deep, while tsunamis move through the entire depth of the ocean. They may only be a few inches in height in the open ocean, but as they approach shore and the ocean shallows, the energy gets concentrated and the height of the wave increases.

How do tsunami detectors work?
Since only tsunami waves can be felt at the bottom of the ocean, tsunami sensors are sunk down to the sea floor. Any time they notice a change in pressure, they send a signal to monitoring centers. If it's just a fish swimming too close, then no other sensors will go off - but if many sensors all start detecting the same thing, the tsunami can be tracked as it progresses through the ocean.

Tsunamis can travel very fast through the open water, as fast as a commercial jetliner flies. Seismologists tracking a tsunami can make fairly accurate predictions as to when it will hit land at any given point, but predicting the intensity of a tsunami is less accurate, since many factors such as the shape and topography of the coastline can play a role in tsunami behavior once it hits land.

How big was the quake?
The magnitude is currently being reported at 8.9, which places it among the top 5 magnitude earthquakes since 1900. The magnitude measures the amount of energy released by an earthquake, similar to how we measure atomic blasts. Each full point on the scale represents a tenfold increase in the energy of an earthquake. The energy of the Japanese quake was over 100 times that of the Northridge quake, measured at magnitude 6.8.

Many people commonly believe that the magnitude given is measured on the Richter scale, but this is no longer true; sesimologists now use the similar but more accurate moment magnitude scale. The Richter scale becomes less accurate when measuring extremely powerful earthquakes like the one experienced in Japan.

Can we have a similar earthquake in California?
Based on our current understanding of seismology, the biggest quake we ever expect along the San Andreas and related faults is somewhere in the low 8 magnitude range, significantly less energetic than the Japanese quake. California's fault system is a strike-slip system, where two tectonic plates move side-by-side. Japan, and other areas like the west coast of South America and Indonesia, lie over subduction zone systems, where one plate lies on top of another. This "double-stacked" arrangement of tectonic plates places much more weight and pressure along the faultline, allowing much deeper and more powerful earthquakes to build up.

Strike-slip systems also have a lower risk of triggering catastrophic tsunamis. Since the plates are moving side-to-side, not up-and-down, submarine earthquakes will disturb the ocean less.

The biggest earthquakes ever recorded - 9.5 in Chile, 9.3 in Sumatra, 9.2 in Alaska, and other similar earthquakes - all occur within subduction zone systems.

What happens when a big earthquake does hit us?
Despite the death and destruction this earthquake and tsunami did cause along the Japanese coastline, things could have been a lot worse. Japan is one of the most densely populated areas of the world, over 3.5 times as densely populated as California. The current death toll numbers being reported are over 10,000; compare this to the 2004 Indian Ocean tsunamis that claimed over 200,000 lives, or the 2010 Haiti earthquake that claimed over 300,000. Sadly, the greatest danger from major seismic events, it turns out, is poverty and ignorance.

Building codes in developed countries such as Japan, the US or Mexico ensure that buildings can safely withstand most earthquakes they might experience. These codes and standards are updated with new knowledge gained in every major quake, and have led to a general decrease in the fatality rate for every major earthquake, from over 3000 in the 1906 San Francisco quake to just 4 in the 2010 Easter Sunday quake. Countries without well-enforced building codes, such as Haiti or Indonesia, suffer much greater loss of property and life.

Tsunamis - incredibly destructive and impossible to stop - are much more dangerous due to the dual threat of flooding and physical damage. Much like a tornado, most of the damage is not caused by the water itself, but by all the churning debris it carries. Loose objects such as cars, or weak buildings like sheds and garages, can be easily swept up, but stronger buildings with good foundations can withstand the tsunami. The destruction is limited to the height of the tsunami, so anyone caught in it can make it to safety just by getting above the top of the flood, by heading for high ground or to the second or third floor of a building.