Without precedent or warning, a loud boom sounding like a major piece of artillery frightens your normally quiet neighborhood. Houses shake and dishes rattle. The jolt is singular, percussive -- and ominous. Later the TV news reports that the boom was heard over many miles, but nothing exploded. No supersonic aircraft flew by. Someone saw yellow light in the sky.
Residents of New York's Rockland and Westchester Counties, not far from New York City, experienced this in March 2009. It could have been a rare, beach ball sized meteor that disintegrated before it hit the ground. Meteors are certainly supersonic and have been known to make loud sonic booms. A bounty hunter offered $10,000 for a piece of the meteorite.
But the meteor theory blew up a couple days later. Another loud boom in the same area jolted people awake at 5:15 am. Nanuet resident Keith Wallenstein said of the second boom. "The house was shaking. It sounded like someone had flown an F-16 over the house. If it was thunder, it had to be right on the house. [But] I know a bunch of people who heard it within 3 to 4 or 5 miles away."
By now you may be thinking the military was up to something after all. They'd be mum about it, wouldn't they?
In James Fenimore Cooper's day there were no supersonic aircraft. As he recounted in 1850:
The 'Lake Gun' is a mystery. It is a sound resembling the explosion of a heavy piece of artillery, that can be accounted for by none of the known laws of nature. The report is deep, hollow, distant, and imposing. The lake seems to be speaking to the surrounding hills, which send back the echoes of its voice in accurate reply. No satisfactory theory has ever been broached to explain these noises. Conjectures have been hazarded about chasms, and the escape of compressed air by the sudden admission of water; but all this is talking at random, and has probably no foundation in truth. The most that can be said is, that such sounds are heard, though at long intervals, and that no one as yet has succeeded in ascertaining their cause.
Cooper was talking about Lake Seneca, one of the Finger Lakes in upper New York State. The Lake Gun was the name given to the booms by local settlers. The Native Americans said it was their god talking.
Moodus, Connecticut is another hot spot for loud booms, and other noises too. The Native Americans there called the area Machemodus, or Place-of-Noises, and warned the early settlers about them. The Moodus noises ceased in the 1980's but sprang back to life in 2011. In 1979 Boston College's Weston Observatory set up seismometers and measured Moodus quakes producing pops or bangs more than a hundred times too small for people to feel, some as low as minus 2 on the Richter scale. The geologists found that the source of the quakes is in hard bedrock only 1500 meters deep under Moodus, very shallow for an earthquake. They offered no explanation for the sound.
Quakes hundreds or thousands times more powerful occur elsewhere yet are nearly silent. More powerful quakes, ones that start to do damage do make noise, but more like rolling rumbles, not singular explosions. Why should some quakes produce percussive booms so efficiently?
The booms are not caused by an explosion nor any material object moving supersonically. Instead they are launched by the world's largest loudspeaker: the ground or surface all around you, especially if it is hard bedrock or water (water is actually very stiff-try compressing it!).
This is apparently controversial. A website devoted to the Guns of Barisol, India, on the northern shore of the Bay of Bengal, another place of bewildering sonic booms, tries to inoculate readers against the microquake explanation of the booms: "You may read . . . that the Guns of Barisal are supposed to be caused by earth movements too feeble to be felt. Earthquakes can make noises, but not when no movements are felt....'' Actually this statement is quite false: small quakes can produce loud booms.
Oddly, the surface does not need to move very far nor very fast to launch exceedingly loud sound resembling cannon fire or a sonic boom. What it does need is a lot of acceleration. But how can something have huge acceleration, yet not wind up moving very far or very fast?
The answer is the acceleration must be very brief. Suppose the ground accelerates at 1000 G's straight up before recoiling and reversing direction, all in 1000th of a second. (The Tesla Roadster, capable of 0 to 60 in 3.2 seconds, accelerates 1000 times less at under 1 G) If the ground or water surface does that, its speed is never more than a modest 1 meter per second, and it will move less than a millimeter! But one heck of a loud boom will be launched if large surface areas do that.
Acceleration is the agent of sound production. An accelerating surface "surprises" the air next to it and launches a pressure wave moving at the speed of sound, even though the surface and the air itself never comes close to moving that fast. When your fingernail touches a desktop, you hear an audible clack. The surface of the desk clearly never moves very far, and the desktop certainly doesn't move any faster than your fingernail was moving, but the sudden contact of desk with fingernail causes a significant (many G's ) but short lived acceleration of the desktop, which launches the sound.
A sudden breaking of a large piece of rock under great tension sends out a sharp compression wave moving fast in the rock -- like a sound wave, only in rock. When the wave reaches the surface, the surface is very suddenly pushed up over a large area- a huge but short lived acceleration -- and a boom in born.
Sharp waves traveling in rock tend to quickly round off, so the pulse and the acceleration will be reduced unless the quake is very close to the surface, as it is in Moodus, and presumably under Rockland and Westchester Counties, Seneca Lake, etc. A Richter 1 quake is plenty to launch an ear splitting boom if it occurs close to the surface, yet at Richter 1, the geologists won't dignify the quake with a mention.
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