( CNN ) -- Hundreds of small earthquakes have rumbled under northwestern Nevada like a seismic drumroll since midsummer , and in recent days , they have built to a crescendo . This does not necessarily mean a big one will come , state seismologists said , but they added that it 's good to be prepared , just in case . Seismologists refer to such quake groupings as swarms , and the U.S. Geological Survey has detected them regularly . They can produce thousands of small tremors . In September , for example , about 500 miniquakes struck southeastern California in about two days . The largest topped out at a magnitude 3.8 -- a strength that could be felt but was not dangerous . For months . But the Nevada swarm buzzing in and around the Sheldon National Wildlife Refuge has gone on since July 12 . In recent days -- since October 30 -- they have grown stronger , with three over magnitude 4.0 . The strongest one weighed in Tuesday at magnitude 4.6 -- approaching the threshold of a lightly damaging quake . `` There is a slightly elevated risk of a larger earthquake while the swarm is active , '' said seismologist Ian Madin from neighboring Oregon . This swarm looks much like two previous swarms that produced somewhat strong earthquakes , the Nevada Seismological Laboratory says . Similar swarms . In 1968 , in nearby Adel , Oregon , a swarm lasting several months produced three quakes of about magnitude 5 . `` The Adel swarm caused moderate damage , '' the NSL said . Another , shorter swarm near Reno , Nevada , in 2008 led to a magnitude 5 quake and caused moderate local damage . The vast majority of the current Nevada swarm 's quakes have been undetectable to people walking around on top of them . Since they started four months ago , Nevada Seismological Laboratory has recorded around 550 quakes of magnitude 2 or higher , and 42 with intensities above 3.0 . The center of the activity is in a sparsely populated area , the USGS says . There are two towns about 40 miles away , each with a population of about 2,500 . How earthquakes are measured .
one of mainly low-magnitude quakes have been recorded . Such groups of quakes are called swarms . They are happening in and around a national wildlife refuge in California . Seismologists record swarms on a regular basis ; in 1968 there was Hundreds in Nevada .