Once thought to be immune from major quakes, the so-called creeping segment of the San Andreas Fault can potentially set off a temblor affecting both the northern and southern portions of the state. 
 
 A new analysis of GPS data has revealed previously undiscovered movement along the San Andreas Fault, which forms the tectonic boundary between the Pacific Plate and the North American plate and spans much of California. 
 
 According to the analysis, which is built on records that have been available for over a decade, large sections of land on either side of the fault are moving up or down every year in what, seismically speaking, could be considered "major movement." 
 
 "While the San Andreas GPS data has been publicly available for more than a decade, the vertical component of the measurements had largely been ignored in tectonic investigations because of difficulties in interpreting the noisy data,” said Samuel Howell, the lead author of the study. “Using this technique, we were able to break down the noisy signals to isolate a simple vertical motion pattern that curiously straddled the San Andreas fault.” 
 
 The GPS records, which were studied by a slew of research teams from University of Hawaii at Manoa, University of Washington and the Scripps Institution of Oceanography, also reveal nearly 125-mile long "lobes" of motion that straddles the fault system. Although the motion has been predicted before, it had never been documented. 
 
 To anyone who isn't studying fault lines, the data may not mean much at this point. But many researchers have said that this new insight into how the fault operates could help them predict "The Big One" -- the large earthquake that many believe is overdue to hit California. ||||| Howell said the time span for a large rupture on the southern San Andreas is, on average, once every 150 years or so. But the actual interval can vary over the centuries, and that's why it's hard to say exactly when the next big one will hit. ||||| A new analysis has revealed hidden vertical motion along the San Andreas fault in four huge lobes of ground. The vertical motion is tiny and doesn't contribute much to earthquake risk, but could help researchers better understand motion along the fault. 
 
 For the first time, scientists have revealed the vertical motion that has been hiding in plain sight along the San Andreas Fault. 
 
 Four huge lobes of the Earth on either side of the San Andreas Fault are sinking, while others are rising, a new analysis of GPS data reveals. The simple motion makes it look as if the ground under the fault is doing the wave in slow motion. 
 
 "While the San Andreas GPS data has been publicly available for more than a decade, the vertical component of the measurements had largely been ignored in tectonic investigations because of difficulties in interpreting the noisy data," study lead author Samuel Howell, a doctoral candidate in ocean and earth science and technology at the University of Hawai'i at Mānoa, said in a statement. 
 
 Now, using a new technique, researchers were able to tease out the vertical motion, which "curiously straddled the San Andreas Fault," Howell added. [Photo Journal: The Gorgeous San Andreas Fault] 
 
 For more than a decade, tiny GPS arrays have dotted the land along the San Andreas Fault, the region where the North American and the Pacific plates collide. The 800-mile-long (1,287 kilometers) fault system, which creates a threaded network of many smaller faults, snakes through California from the Mendocino Coast in the north to the Salton Sea in the south. 
 
 In the past, researchers have mainly relied upon the GPS sensor data to determine the horizontal motion of the fault. But the vertical motion of the fault can be affected not only by tectonic motion of the plates, but also by everything from rainfall to the levels of groundwater (which have dropped dramatically as a result of California's drought) to local surface geology. Teasing apart the tectonic motion from these other, more transient factors was a challenging task, the researchers said. 
 
 "You can picture an old radio where you want to listen to your favorite show (vertical motion that the fault causes). You would have to turn the knob through other stations (natural and human-caused ground movements) and through a lot of static," Howell told Live Science in an email. "We wanted to dial-in to the part of the vertical signal that was due to the fault." 
 
 So Howell's team used a statistical technique to extract smooth trends in ground motion from the noisy data. 
 
 They found that there were four large hunks of ground about 125 miles (201 km) wide that were uplifting or subsiding on either side of the fault. What's more, the findings matched theoretical models for how the earth around the fault was moving in response to powerful earthquakes. 
 
 "We were surprised and thrilled when this statistical method produced a coherent velocity field similar to the one predicted by our physical earthquake-cycle models," study co-author Bridget Smith-Konter, a geologist at the University of Hawai'i at Mānoa, said in the statement. 
 
 As for the cause of this wave-like movement, "these vertical motions didn't just now start happening, but have been building up since the last major earthquake, and will begin to go away after the next one," Howell said. "We knew that the ground had to be moving up and down somehow, but this provides a measurement of that motion." 
 
 The creeping vertical motion is tiny, just a few millimeters, or two penny-widths a year, and doesn't add much to the seismic risk in the region, Howell said. 
 
 "The major risks come from the horizontal motions near the fault. The vertical motion comes from the fault being locked, [or] unable to move, between earthquakes," Howell said. 
 
 But the new understanding does create a more detailed understanding of the fault, which could in turn help with estimating the risk of future ruptures along the fault, the researchers reported June 20 in the journal Nature Geoscience . 
 
 Original article on Live Science.
– Scientists have detected previously unknown movement along the San Andreas Fault, although the discovery doesn't change the two most important facts: There will be a major quake along the California fault at some point, and nobody knows just when it will happen. The analysis of GPS data found that large areas of land along the fault, including the Los Angeles Basin, are rising by 2mm to 3mm a year—which NBC San Francisco notes counts as major movement in seismic terms—while other areas, including Santa Barbara and San Luis Obispo counties, are sinking. The motion makes it seem like the huge 125-mile lobes of land along the fault are "doing the wave in slow motion," reports Live Science. Lead study author Sam Howell of the University of Hawaii tells the Los Angeles Times that the movement is the result of a buildup of seismic energy, which will eventually be released in an earthquake. He says they happen along the fault roughly every 150 years, and while the last major one was 159 years ago, stress has been building up in some areas for centuries longer. The movement will help researchers "understand more about how the fault is behaving and the effect it's going to have in the surrounding region," he says, though it won't be any help in predicting an exact date for the next quake. "It's pretty much impossible to say when the next one will happen." (The director of the Southern California Earthquake Center says the southern part of the fault "looks like it's locked, loaded, and ready to go.")