Image copyright SPL Image caption Scientists say huge earthquakes like the one that struck Japan in 2011 appear less frequently before 1900 
 
 The Earth could have been struck by many more huge earthquakes in its recent history than was previously thought, scientists say. 
 
 Research suggests that half of all quakes measuring more than 8.5 in magnitude that hit in the 19th Century are missing from records. 
 
 Scientists are scanning historical documents for the lost tremors. 
 
 The findings are presented at the American Geophysical Union Fall meeting in San Francisco. 
 
 Dr Susan Hough from the US Geological Survey said: "If you try to make a statistical case there are too few earthquakes in the 19th Century." 
 
 People shouldn't have been surprised by Haiti - there was a similar quake in the 18th Century Dr Roger Musson, British Geological Survey Earthquakes on BBC Science 
 
 Earthquakes measuring more than M8.5 cause immense devastation. 
 
 Recent examples include the 2004 quake in the Indian Ocean that unleashed a deadly tsunami, Chile's massive 2010 earthquake, and the 2011 event in Japan. 
 
 But records before the 20th Century are strangely devoid of natural disasters on this scale. 
 
 'Warnings from history' 
 
 "Seismometers were developed around 1900. As soon as we had them, earthquakes started to look bigger," explained Dr Hough. 
 
 Researchers use historical documents to track down seismic events that occurred before this and assess their magnitude. 
 
 Dr Hough believes that many large earthquakes in the 18th and 19th Century have been missed. 
 
 Image copyright AFP Image caption The earthquake in Haiti in 2010 killed more than 100,000 people 
 
 One reason for this is because there is a general assumption that earthquakes measuring M8.5 and above generate significant tsunamis. 
 
 "But this isn't always the case, and the magnitudes for some of these earthquakes has been underestimated," said Dr Hough. 
 
 A candidate is a quake that hit Kamchatka in Russia in 1841. Its magnitude had been thought to be 8.3, but now Dr Hough believes it should be upgraded to M9.2. 
 
 Another is a quake that struck in the Lesser Antilles in 1843. 
 
 Dr Hough said: "This was catalogued at a low magnitude 8. It turns out it was felt by a quarter of the globe." 
 
 The researchers say finding these lost earthquakes is vital to help them assess where and when these deadly events could strike next. 
 
 In another piece of research presented at the AGU meeting, scientists say they have created a database of earthquakes that occurred in the years 1000 to 1900. 
 
 Again the team has been trawling through historical documents to catalogue the events. 
 
 Dr Roger Musson from the British Geological Survey said the database provided a "warning from history". 
 
 "For example, with the Fukushima disaster - people were surprised that there was such a large tsunami," he told BBC News. 
 
 "But it happened before. There was a historical earthquake in the 9th Century that was very similar." 
 
 The catastrophic earthquake that hit Port-Au-Prince in Haiti in 2010 was another example, he said. 
 
 "People shouldn't have been surprised that an earthquake happened there. There was a very similar earthquake in the 18th Century," Dr Musson added. 
 
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 More than 100 years of earthquakes glow on a world map. Credit: John Nelson, IDV Solutions. 
 
 SAN FRANCISCO — The number of great earthquakes experienced in the past may be higher than previously thought, one researcher said here today (Dec. 11) at the annual meeting of the American Geophysical Union. 
 
 As a result, the global community may be underestimating the risk of the next big one, said Susan Hough, of the U.S. Geological Survey. 
 
 "There's very compelling evidence that we have underestimated the magnitude of earthquakes in the 19th century and possibly in the first half of 20th century," Hough said. 
 
 Prior to about 1900, scientists didn't have an easy way to measure the strength of an earthquake. When seismologists try to recreate historical temblors, they typically look to see whether a tsunami is generated or how far away people felt the quake to get an idea of how strong it was. 
 
 But those are imprecise measures. Hough wondered whether many of the past big earthquakes — such as those now classified between about a magnitude-8.0 and a magnitude-8.5 — were underestimated. In the 19th century, for instance, most records say there were just three big earthquakes larger than magnitude-8.5, but 12 in the 20th century. At first glance, that seemed suspicious, Hough said. [The 10 Biggest Earthquakes in History] 
 
 "In 1900, we invented seismometers and earthquakes got bigger," she said. 
 
 So Hough performed a statistical analysis to see whether there really was a sharp change between the 19th and 20th centuries' rates of large earthquakes. Unfortunately, there weren't enough large earthquakes to draw a firm conclusion, she said. 
 
 Instead, she decided to look at modern earthquakes larger than magnitude-8.5 and see how they compared with earthquakes in the history books, and whether they would have been missed. 
 
 The Lesser Antilles earthquake of 1843, for instance, was listed as a magnitude-8.3 earthquake by the National Earthquake Information Center, but was felt all along the Atlantic coast, thousands of miles away. In comparison, the 2012, magnitude-8.6 earthquake in Sumatra was similarly felt from an equivalent distance, according to the USGS' "Did You Feel It" Sensor network, which asks people to report when they feel an earthquake. 
 
 An 1841 earthquake in Kamchatka in Russia was listed as a magnitude-8.3 temblor, yet it generated a 15-foot (4.6 meters) tsunami in Hilo, Hawaii. Follow-up research by others suggests it was actually a magnitude-9.2 quake. 
 
 Hough also looked at how modern earthquakes would have been classified based on their characteristics. For instance, another earthquake in Sumatra in 2005, a magnitude-8.7 temblor known as the Nias Earthquake, didn't generate a tsunami, and so would have been classified in the past as being approximately a magnitude-7.8 in the catalog. 
 
 If these historical big ones are being missed, then the global assessment of seismic hazard, which relies on historical earthquake rates, may be underestimating the biggest possible temblors that could happen, Hough said. 
 
 Follow Tia Ghose on Twitter and Google+. Follow us @livescience, Facebook & Google+. Original article on LiveScience.
– The Earth might have a more turbulent past—and hence a more turbulent future—than we thought. In a presentation yesterday, Dr. Susan Hough of the US Geological Survey argued that hidden throughout history are a glut of giant earthquakes that either went unrecorded or have been underestimated. "Seismometers were developed around 1900. As soon as we had them, earthquakes started to look bigger," Hough said. To wit, LiveScience reports that records show that three temblors registered above magnitude-8.5 in the 19th century; the 20th century claims a dozen. Her theory is that earlier earthquakes in the 8.0 to 8.5 magnitude range were actually bigger than that. "If you try to make a statistical case, there are too few earthquakes in the 19th century," she says. She found evidence that many 19th-century quakes were likely bigger than we thought, the BBC reports. One 1843 quake, for example, was recorded as magnitude-8.3. "It turns out it was felt by a quarter of the globe." This matters, because these historical quakes can help us predict future ones. The upshot being that the risk of future "big one" quakes is likely higher than we've been assuming.