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It wasn't the sex, it was the Love waves.

Major earthquakes routinely cause the ground to shake halfway around the world, a team of U.S. researchers has discovered. The mini-quakes are triggered by Love and Rayleigh waves - surface shock waves that travel the globe.

The most powerful of these tremors could make the Earth move for you, or rattle a few teacups, but the majority would be detectable only by sophisticated monitoring equipment.

"They are short little shakes," said the University of Utah's Kris Pankow, an author of a paper published yesterday in the journal Nature Geoscience.

For years, seismologists dismissed the notion that major earthquakes could spark small ones in far-off locations.

Then, in 1992, a large quake in California's Mojave Desert triggered tremors 1,300 kilometres away in Yellowstone National Park.

After that, many researchers theorized that only areas prone to earthquakes, or that had volcanoes or geothermal activity, would be likely to be jolted by a distant disaster.

But Aaron Velasco, lead author on the paper, and his colleagues say the long-distance quake phenomenon, known as "dynamic triggering," is far more widespread.

They analyzed 15 major earthquakes stronger than a magnitude 7.0, that occurred between 1992 and 2006. At least 12 of them triggered small quakes hundreds and even thousands of kilometres away.

The earthquake that hit Sumatra on Dec. 26, 2004, and generated deadly tsunamis also triggered 14 small quakes near Mount Wrangell, Alaska, 11,000 kilometres away. The tremors in Alaska occurred about an hour after the initial rupture and the same quake also caused a small quake that was detected on an island of the coast of Ecuador.

These aren't aftershocks, which occur fairly close to the main quake, the researchers say. So what are they?

Earthquakes release energy in the form of shock waves that move through the ground. The first waves are called P or pressure waves, and they move at high speed with an up-and-down motion. Next come the S or shear waves, which move from side to side and cause most of the damage, like the flattened towns and cities in China's Sichuan province.

Two kinds of surface waves come next. Love waves move in a shearing fashion. Rayleigh waves have a rolling motion, and it is possible to see the little bumps that they cause on the ground, Dr. Pankow said.

The team analyzed data from more than 500 seismic recording stations before and after the 15 major earthquakes.

There are 600 small seismic events around the Earth every five minutes. But the team found a 37-per-cent increase in the number of small earthquakes detected by the stations in the five hours after the Love waves from a major quake arrived.

In the five hours after the Rayleigh waves passed by, the number of little earthquakes registered by the stations jumped by 60 per cent.

Five stations in northern Canada - in Whitehorse, Inuvik, Yellowknife, Iqaluit and Alert - were among those that doubled their detection rate of earthquakes after the surface waves arrived.

The researchers aren't sure how the Love and Rayleigh waves trigger tremors.

Earthquakes occur along faults, or breaks in the Earth's crust. When one side of the crack moves relative to the other side, you get a quake.

Love waves, which move side to side, may increase the strain on a fault.

Rayleigh waves may affect the water table, Dr. Pankow said, causing water to spill into a fault.

Push your hands together, she said, and try to rub them back and forth. If you add a little water it is much easier.

"Just from the volume of the water you start to move the fault apart, so if you put more water in there, it can change the fluid pressure, and cause the fault to slip. When the fault slips, you get an earthquake," Dr. Pankow said.

The team found that the distant mini-quakes were triggered by both Love and Rayleigh waves.

"We aren't choosing either camp," said Dr. Velasco, a seismologist at the University of Texas.

Quakes felt around the world

These 12 major earthquakes triggered mini-quakes hundreds

or thousands of kilometres away. The strength of the quakes

is measured on the "moment magnitude" scale.

1992

Landers quake in California (7.3)

1998

Balleny Island near Antarctica (8.1)

1999

Izmit, Turkey (7.6)

2000

New Ireland, Papua New Guinea (8.0)

2001

Southern Peru (8.4)

Kunlun, China (7.8)

2002

Denali fault quake in Alaska (7.9)

2003

Hokkaido, Japan (8.3)

2004

Sumatra-Andaman Island (9.2)

Macquarie Ridge, near New Zealand (8.1)

2005

Sumatra, Indonesia (8.7)

2006

Java, Indonesia (7.7)

Major quakes trigger smaller jolts worldwide

The 2004 Sumatra quake caused mini-quakes in far-off Ecuador and Alaska. How? Scientists aren't sure, but they know surface waves from major quakes travel the globe. One theory is that they can affect the water table, causing water to spill into faults, which are cracks in the Earth's crust. This could trigger a tremor.

Seismic waves and a possible theory

PRESSURE OR P WAVE: A body wave that moves at high speed with an up-and-down motion.

SHEAR OR S WAVE: A body wave that moves from side to side, causing much damage from an earthquake.

LOVE WAVE: A surface wave that moves in a shearing fashion.

RAYLEIGH WAVE: A surface wave that has a rolling motion.

A THEORY APPLIED: Water in a fault can trigger an earthquake.

CARRIE COCKBURN/ THE GLOBE AND MAIL

SOURCE: ENCYCLOPEDIA BRITANNICA, INC.