Academic Reading — Test 66

Reading passage

The Pacific Ring of Fire is a vast horseshoe-shaped belt that traces the margins of the Pacific Ocean, passing through the western coasts of the Americas, the islands of Japan and the Philippines, and the chains of New Zealand and Indonesia. Along this boundary, the rigid plates that form the outer shell of the planet grind past, plunge beneath, or pull away from one another. The result is a concentration of geological violence found nowhere else on Earth: roughly four-fifths of the world's largest earthquakes and a great many of its active volcanoes are clustered along this single arc. Because so many millions of people live within reach of the hazard, the region has become the proving ground for the most ambitious seismic monitoring efforts ever attempted. At the heart of any monitoring system lies the seismometer, an instrument that records the trembling of the ground as waves of energy spread outward from a rupture. Modern broadband seismometers are sensitive enough to detect motion far smaller than the width of a human hair, and they capture a wide range of frequencies, from the slow heave of a distant great quake to the sharp jolt of a nearby tremor. These instruments are usually buried in sealed vaults or boreholes to shield them from wind, traffic and temperature swings, all of which would otherwise drown out the faint signals the operators wish to study. Each station transmits its readings continuously to a central processing facility, where computers sift the incoming stream for the distinctive signature of an earthquake. Detecting a quake is only the first task; locating it quickly is the harder one. When the ground ruptures, it releases two principal kinds of body wave. The faster of the two, known as the primary or P-wave, races through rock as a push-and-pull motion and arrives first at any given station. The slower secondary or S-wave follows behind, shaking the ground from side to side and causing much of the damage. By measuring the gap in arrival times between these two waves at several stations, analysts can triangulate the position of the source and estimate its depth. The denser the network of stations, the more precisely this can be done, which is why agencies around the Pacific have invested heavily in extending coverage into remote terrain and across the ocean floor. The ocean presents a particular difficulty, because the most dangerous ruptures often occur far offshore where no land-based instrument can sit directly above them. To close this gap, engineers have laid cables of sensors along the seabed and anchored buoys above pressure recorders that sit on the bottom. These deep-water gauges can sense the subtle rise of the water column as a tsunami passes overhead, sending a warning that is independent of the seismic signal itself. Japan operates one of the most extensive of these submarine arrays, a dense lattice of instruments wired to the shore that feeds data to forecasters within seconds. Such systems are expensive to install and to maintain, yet the cost is modest beside the losses that a single unanticipated wave can inflict. Speed is the currency of early warning. A well-designed system exploits the simple fact that the electronic signal carrying the alarm travels far faster than the destructive shaking itself. When sensors nearest the epicentre detect the first faint P-waves, software can estimate the likely strength of the event and issue an alert to communities further away before the slower, stronger waves reach them. The warning may amount to only a handful of seconds, but that interval is enough for trains to brake, for surgeons to pause, and for automated valves to shut off gas and electricity. Mexico and Japan have both built public warning networks of this kind, and their alerts now reach millions of telephones at once. The systems are not infallible, however; a quake directly beneath a city offers almost no warning at all, and occasional false alarms remain an unavoidable price of caution. For all their sophistication, these networks cannot yet predict when an earthquake will strike. Decades of research have failed to identify any reliable precursor that consistently announces an impending rupture, and most scientists now regard short-term prediction as beyond present reach. What the monitoring network does instead is measure, locate and warn, turning the unavoidable into the survivable. Each event recorded along the Ring of Fire also adds to a growing archive that helps researchers refine their maps of hazard and improve the building codes that ultimately save the most lives. In this sense the network is as much a long-term scientific instrument as an emergency tool, patiently listening to a restless ocean and translating its movements into knowledge.