Table of Contents
Passage
A In early November 2000 the Big Island of Hawaii experienced its largest earthquake in more than a decade. Some 2,000 cubic kilometers of the southern slope of Kilauea volcano lurched toward the ocean, releasing the energy of a magnitude 5.7 shock. Part of that motion took place under an area where thousands of people stop every day to catch a glimpse of one of the island’s most spectacular lava flows. Yet when the earthquake struck, no one noticed not even seismologists.
B How could such a notable event be overlooked? As it turns out, quaking is not an intrinsic part of all earthquakes. The event on Kilauea was one of the first unambiguous records of a so-called silent earthquake, a type of massive earth movement unknown to science until just a few years ago. Indeed, I would never have discovered this quake if my colleagues at the U.S. Geological Survey’s Hawaiian Volcano Observatory had not already been using a network of sensitive instruments to monitor the volcano’s activity. When I finally noticed that Kilauea’s south flank had shifted 10 centimeters along an underground fault, I also saw that this movement had taken nearly 36 hours— a turtle’s pace for an earthquake. In a typical tremor, opposite sides of the fault rocket past each other in a matter of seconds—quickly enough to create the seismic waves that cause the ground to rumble and shake.
C But just because an earthquake happens slowly and quietly does not make it insignificant. My co-investigators and I realized immediately that Kilauea’s silent earthquake could be a harbinger of disaster. If that same large body of rock and debris were to gain momentum and take the form of a gigantic landslide—separating itself from the rest of the volcano and sliding rapidly into the sea—the consequences would be devastating. The collapsing material would push seawater into towering tsunami waves that could threaten coastal cities along the entire Pacific Rim. Such catastrophic flank failure, as geologists call it, is a potential threat around many island volcanoes worldwide.
D Fortunately, the discovery of silent earthquakes is revealing more good news than bad. The chances of catastrophic flank failure are slim, and the instruments that record silent earthquakes might make early warnings possible. New evidence for conditions that might trigger silent slip suggests bold strategies for preventing flank collapse. Occurrences of silent earthquakes are also being reported in areas where flank failure is not an issue. There silent earthquakes are inspiring ways to improve forecasts of their ground-shaking counterparts.
E The discovery of silent earthquakes and their link to catastrophic flank collapse was a by-product of efforts to study other potential natural hazards. Destructive earthquakes and volcanoes are a concern in Japan and the U.S. Pacific Northwest, where tectonic plates constantly plunge deep into the earth along what are called subduction zones. Beginning in the early 1990s, geologists began deploying large networks of continuously recording Global Positioning System (GPS) receivers in these regions and along the slopes of active volcanoes, such as Kilauea. By receiving signals from a constellation of more than 30 navigational satellites, these instruments can measure their own positions on the planet’s surface at any given time to within a few millimeters.
F The scientists who deployed these GPS receivers expected to see both the slow, relentless motion of the planet’s shell of tectonic plates and the relatively quick movements that earthquakes and volcanoes trigger. It came as some surprise when these instruments detected small ground movements that were not associated with any known earthquake or eruption. When researchers plotted the ground movements on a map, the pattern that resulted very much resembled one characteristic of fault movement. In other words, all the GPS stations on one side of a given fault moved several centimeters in the same general direction. This pattern would have been no surprise if it had taken a year or longer to form. In that case, scientists would have known that a slow and steady process called fault creep was responsible. But at rates of up to centimeters a day, the mystery events were hundreds of times as fast as that. Beyond their relative speediness, these silent earthquakes shared another attribute with their noisy counterparts that distinguished them from fault creep: they are not steady processes but instead are discrete events that begin and end suddenly.
G That sudden beginning, when it takes place on the slopes of a volcanic island, creates concern about a possible catastrophic flank event. Most typical earthquakes happen along faults that have built-in brakes: motion stops once the stress is relieved between the two chunks of earth that are trying to move past each other. But activity may not stop if gravity becomes the primary driver. In the worst-case scenario, the section of the volcano lying above the fault becomes so unstable that once slip starts, gravity pulls the entire mountainside downhill until it disintegrates into a pile of debris on the ocean floor.
H The slopes of volcanoes such as Kilauea become steep and vulnerable to this kind of collapse when the lava from repeated eruptions builds them up more rapidly than they can erode away. Discovering the silent earthquake on Kilauea suggests that the volcano’s south flank is on the move perhaps on its way to eventual obliteration.
I For now, friction along the fault is acting like an emergency brake. But gravity has won out in many other instances in the past. Scientists have long seen evidence of ancient collapses in sonar images of giant debris fields in the shallow waters surrounding volcanic islands around the world, including Majorca in the Mediterranean Sea and the Canary Islands in the Atlantic Ocean. In the Hawaiian Islands, geologists have found more than 25 individual collapses that have occurred over the past five million years—the blink of an eye in geologic time.
J In a typical slide, the volume of material that enters the ocean is hundreds of times as great as the section of Mount St. Helens that blew apart during the 1980 eruption“more than enough to have triggered immense tsunamis. On the Hawaiian island of Lanai, for instance, geologists discovered evidence of wave action, including abundant marine shell fragments, at elevations of 325 meters. Gary M. McMurtry of the University of Hawaii at Manoa and his colleagues conclude that the most likely way the shells could have reached such a lofty location was within the waves of a tsunami that attained the astonishing height of 300 meters along some Hawaiian coastlines. Most of the tallest waves recorded in modern times were no more than one tenth that size.
Questions
Questions 1-5 Do the following statements agree with the information given in Reading Passage 1?
In boxes 1-5 on your answer sheet, write
TRUE if the sataement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if there is no information on this
1 It takes a quite fast interaction caused by certain parts of the fault zone to produce a representative earthquake.
2 Flank failure is a prerequisite which is followed by a silent earthquake.
3 The silent earthquake can be used to forecast any forms of earthquake.
4 Kilauea falls into a category of the stirring volcanoes.
5 In some islands, no less than 25 independent dilapidations period from the perspective of geology.
Questions 6-10 The reading Passage has seven paragraphs A-H. Which paragraph contains the following information?
Write the correct letter A-H, in boxes 6-10 on your answer sheet.
NB You may use any letter more than once.
6 the main characteristic to differentiate fault creep from earthquakes
7 occurrence of landslide in water areas near volcanoes in archaic times
8 catastrophe caused by silent earthquake under certain circumstances
9 a metaphor to describe how slow a silent earthquake takes place
10 the possible ending for the south slope of Kilauea are noticed in a short
Questions 11-13 Complete the following summary of the paragraphs of Reading Passage, using no more than three words from the Reading Passage for each answer. Write your answers in boxes 11-13 on your answer sheet.
Summary
When a model slide happens, the amount of the parts flowing into the sea is so huge that it might bring about 11 ……………….. Ample shell debris included in 12 ……………….. is a good example because they might be moved to the high area by currents of 13 ……………….. This height of the waves is much taller than that documented in the the contemporary era
Answers
