AnewstudyrevealedthattheformationoftheEarth,SunandMoont...

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A new study revealed that the formation of the Earth, Sun and Moon that cause changes in Earth's tides can also trigger a special kind of Earthquake along the California's San Andreas Fault(断层)．

The study, published in the journal Proceedings of the National Academy of Science，is based on previous researcher suggesting that the gravitational tug of sun and moon is not only affecting the sea levels of Earth, but can also stretch and compress that Earth's crust, producing tremors and quakes. Furthermore, the new study showed that small tremors deep underground were more likely to occur during certain periods of the tidal cycle.

For the study, the researchers looked at about 81，000 lowfrequency earthquakes from 2008 to 2015 along the Parkfield section of the San Andreas Fault and compared it to the twoweek tidal cycle known as the “fortnightly tide”. Lowfrequency earthquakes are named due to the rumbling sound they make and not by the rate of its occurrence. These kinds of earthquake only occur about 15 to 30 kilometers below ground and are no larger than magnitude 1 in the Richter scale.

The researchers discovered that lowfrequency earthquakes tend to occur during the waxing period of the tidal cycle, when the tides are getting little by little. It is already known that the strongest Earth tides occur when the sun and moon are in a straight line, while the tide is at its weakest state when the sun and moon are 90 degrees apart.

According to the researchers, lowfrequency earthquakes show that some part of the San Andreas Fault is creeping, or slowly moving, almost all the time. With the help of the tidal forces, lowfrequency earthquakes could provide more information about the deeper part of the fault.

“They tell us that the fault continues down below where the regular or typical earthquakes stop on the San Andreas, about 10 or 12 km (about 6 to 7 miles)，” said David Shelly, a seismologist at USGS and helped write the study, in a statement. “And they tell us a lot of things about that deep part of the fault that we had no idea existed at all before.”

“It's almost like having a lot of little creep meters embedded in the fault，” Shelly said. “We can use these lowfrequency earthquakes as measurements of, at least in a relative sense, how much slip is happening at each little spot on the deep part of the fault where we see these events. When we don't see them, we don't know what's happening; we don't know whether it is slipping silently or not slipping at all.”

The information is incredibly useful, he added. Whenever the deep part of the fault slips, the stress gets transferred to the shallow part of the fault.

“So if all of a sudden, we saw that the deep part of the fault was slipping a huge amount, it might be an indication that there was an increased chance of having an earthquake come at the shallower part of the fault”， he said.

38. What causes tremors and quakes according to the passage?

A. The formation of the Earth, Sun and Moon.

B. The change of the sea levels of Earth.

C. The effect of the gravitational tug of sun and moon on Earth's crust.

D. The movement of creeping in the deep part of the San Andres Fault.

39. The underlined word “they” in Paragraph 6 refers to ________．

A. the researchers B. the tidal forces

C. the lowfrequency earthquakes D. the typical earthquakes

40. Lowfrequency earthquakes occur when ________．

A. the tides are getting bigger gradually B. the sun and moon are in a straight line

C. the sun and moon are 90 degrees apart D. the San Andreas Fault moves slowly

41. According to Shelly, the most important function of the study is to ________．