Study the following loads that are acting
on a tree. Classify them as “live” or “dead.”
• wind blowing against a tree
• the weight of the tree
• the weight of a bird in the tree

A diamond is able to cut glass due to its:

How do we know there has not been an increase in the number of earthquakes? Officials from the U.S. Geological Survey, the government agency that keeps track of earthquakes, point to statistics. On average, there have been no more earthquakes so far in 2010 than there were in previous years.

What Happens During Earthquakes?

In reality, it's not the number of quakes that has gained our attention, but the devastating impact quakes can have. Earthquakes are very natural—it's inevitable that they'll happen because of the way the planet works. The earth's surface is made up of tectonic plates that fit together like puzzle pieces and are always moving past each other. When two of the plates' edges get caught, stress builds up at the fault line—the crack between the plates. When the plates are finally able to break free of each other, the stress is released, and an earthquake occurs. Earthquakes of varying strengths happen every day, usually at fault lines. Some of these quakes have a high magnitude, while others are relatively weak. But it's often where an earthquake takes place that determines whether it will be something that goes largely unnoticed, or a disaster that affects thousands of people.

What Is Changing?

In the past, earthquakes were not as much of a problem because Earth's population was smaller, and cities were less populated.

"Look at some of the big [earthquakes] recently," said Debarati Guha-Sapir of the World Health Organization. Guha-Sapir cited some earthquakes that took place in areas where the population had increased in recent years, making the quakes more serious. "Had the Izmit or Bhuj quakes [of 1999 and 2001] happened 30 years ago, the events would have been relatively insignificant, as the [populations] of these cities were a third of what [they were when the earthquakes happened]. Increasing population density makes a small event into a big one."

Some of the earthquakes that took place in early 2010 occurred in highly populated areas, as well. More people are moving into megacities that happen to be built on fault lines—and many cities are on fault lines. Of the 130 cities with more than one million people, more than half are in earthquake zones, making them more prone to earthquakes.

In many of these cities, officials have drawn up laws requiring that buildings be designed and constructed to endure earthquakes. But some cities, particularly those in developing countries, lack the resources to do this. There, people are rapidly putting up substandard buildings that can't withstand the force of the shaking earth. For example, the Haiti earthquake took place near the nation's crowded capital. There was a great deal of destruction because there were "too many people crammed into a city that wasn't meant to have that many people and have an earthquake," said University of Miami geologist Tim Dixon.

Another reason quakes seem worse is that we're paying more attention. Everyone noticed when the Haiti earthquake was quickly followed by the Chile earthquake. Many people watched the news and the Internet for updates on these disasters, unable to believe that they were seeing two major earthquakes in such a short period of time. But eventually, the earth will have a period when earthquakes are minor or in unpopulated areas. Then people won't notice quite so much anymore.

"People are paying attention to the violent planet we've always lived in," said Dennis Mileti, a seismic researcher. "Come back [at a time when] there have been no [noticeable] earthquakes, [and] most people will have forgotten [about them] again."

According to the article, what is one reason why it seems as if the world is experiencing more earthquakes than usual?