共用题干 Eastern Quakes can Trigger Big ShakesIn the first week of November 2011,people in central Oklahoma experienced more than two dozen earthquakes.The largest,a magnitude 5.6 quake,shook thousands of fans in a college football itadium, caused cracks in a

共用题干
Eastern Quakes can Trigger Big Shakes

In the first week of November 2011,people in central Oklahoma experienced more than two dozen
earthquakes.The largest,a magnitude 5.6 quake,shook thousands of fans in a college football itadium,
caused cracks in a few buildings and rattled the nerves of many people who had never felt a quake before.
Oklahoma is not an area of the country famous for its quakes.If you watch the news on TV,you will see
reports about all sorts of natural disasters一hurricanes,tornadoes,flooding and wildfires,to name a few.But
the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U.S.Geological Survey estimate that several million earthquakes rattle the globe each
year. That mnay sound scary,but people don'I feel many of the tremors because they happen in remote and
unpopulated regions.Many quakes happen under the ocean,and othlers have a very small magnitude,or
shaking intensity..
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23,2011,was felt
from central Georgia to southeastern Canada.In iilany urbanl areas,including Washington,D.C.,dnd New
York City(Wall Street shown),people crowded the streets while engineer inspected buildings.Credit:
Wikimnedia/Alex Tahak.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seis-
mometers.These devices detect and measure the size of ground vibrations produced by earthquakes.
Altogether,USGS researchers use seismornetero to identify and locate about 20,000 earthquaks
each year.
Although earthquakes can happen anywhere in the world,really big quakeg occur only in。erttiin areas.
The largest ones register a magnitude S or higher and happen, on average,only once each year. Such big
ones typically occur along the edges of Earth’s tectonic plates.
Tectonic plates are huge pieces of Earth’s crust,sornetinies many kilometers thick.These plates cover
our planet's
surface ike a jigsaw puzzle.Often,jagged edges of these plates temporarily lock together. When
riates jostle and serape past each other earthquakes occur. On。、crage,tectonic plates move very slowly一
about the same speed as sour fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate'S edges.Although
less cxpcctcd,these"mid-plate"tremors can do substontaI damage,Some of the biggest known examples
rattled the eastern haif of the United States two centuries ago.Today,scientists are still puzzling over why the
quakes occurred and when similar ones nught occur.

Few earthquakes happen without people’s awareness.
A:Right
B:Wrong
C:Not mentioned