Figure 1

When you read the headline and notice the words "Love Waves", please don't think the Seismo Blogger is diverging into the X-rated territory of the web. Instead, he is delivering on the promise made two weeks ago, to explain more about surface waves (see blog July 15, 2009). These waves, which in contrast to P- and S-Waves do not travel through the interior of the Earth, race along its surface instead. They also come in two flavors which differ in at least two aspects: the particle motion they generate and the speed with which they circle the globe.

One type of surface wave was first mathematically described by John William Strutt, a young British physicist. He later not only followed his father into nobility as the third Baron Rayleigh, but also became one of the most prominent researchers of his time (1842-1919) and was honored for his work with the Nobel prize in 1904. At first glance, the Rayleigh waves look like the surface waves in the water (see blog July 15, 2009), but when observing carefully, one will notice that their respective particle motions are different. In a water wave, each particle makes a circular motion in the direction of the propagation of the wave. In a Rayleigh wave, the particles make an elliptical movement against the propagation direction. Hence, their motion is retrograde (see Figure 1). A very nice animation of the difference in particle motion between a water and a Rayleigh wave can be found here.

	Figure 2

The second type of surface wave was discovered in 1911 by another Englishman, Augustus Edward Hough Love. Although not quite as famous as Lord Rayleigh, Love nevertheless held the position of Professor for Natural Philosophy at Oxford University for 41 years. Love found that the particles in the waves named after him do not move in a rotating fashion at all. Instead, they jerk back and forth perpendicular to the direction of wave propagation (see Figure 2). They are therefore similar to an S-wave (see blog September 10, 2008).

	Lord Rayleigh		A.E.H. Love

The speed with which both types of waves circle the globe is truly mind boggling. Love waves race around the Earth at almost 10,000 miles per hour. Their relatives, the Rayleigh waves, lag behind slightly, but still speed at about 7800 miles an hour. It seems that only the International Space Station is faster. As of this writing, the 13 astronauts aboard ISS plough through their orbit at 16,218 miles per hour (hra042).

	Buzz Aldrin next to the first seismometer on the moon, which he installed 40 years ago today. In the background is Eagle, Apollo's lunar lander. Photo: NASA (Click to view larger image.)

 
When the first human set foot on the moon 40 years ago today, it was not only a "small step for a man and one giant leap for mankind"; it was also a red-letter day for seismology. During their 21-hour long stay in Mare Tranquillitatis, Neil Amstrong and Buzz Aldrin set up a seismometer (see Figure 1). And minutes after the sensor had settled down, it recorded the first moonquake and transmitted its data back to Earth. Never before had seismologists seen the interior rumblings of another heavenly body. The seismometer, which the astronauts of Apollo 11 installed, returned data for just three weeks. More advanced instruments were deployed later by the crews of Apollo 12, 14, 15, and 16 near their respective landing sites. These sensors transmitted their recordings down to Earth for years, until the broadcasts ceased in September 1977.

The lunar seismometers registered more than 12,000 seismic events. Over the years, researchers discovered that these moonquakes fall into four distinct categories. Most exciting was that about half of the moonquakes originate much deeper than any quake on Earth. The foci of these deep moonquakes are located in the lower mantle of the moon at a depth of between 435 and 750 miles. It turned out that these quakes do not occur at random time intervals. Instead, they repeat in a 27-day pattern. As this is the time it takes the moon to circle the Earth once, researchers concluded that these quakes are caused by the tidal forces that the Earth exerts on the moon. All of these deep moonquakes had magnitudes smaller than 3 (see left panel of Figure 2).

Three types of moonquakes are shown here (explanation: see text). The top three rows are the recordings of a three-component long-period seismometer; the bottom row are the registriations of a short period instrument.Graphics by Yosio Nakamura, UT Austin.

The average magnitude of the second largest group of lunar temblors was even smaller. These moonquakes have a very shallow origin of only a few miles at most and are generated by the thermal stresses along fracture planes in the uppermost lunar crust. Their cause is the enormous temperature difference of more than 400 degrees F between day and night on the lunar surface

At least 1700 other moonquakes were the rumblings of the lunar crust as a result of meteorite impacts on the moon's surface (see right panel of Figure 2). Because the moon does not have an atmosphere of any significance, most meteorites do not burn up due to the frictional heat as they do on their way through the Earth's atmosphere, but instead make it all the way to the lunar surface with a bang.

Most worrisome, however, is the smallest group yet discovered in the lunar dataset. Researchers found 28 sizeable shallow moonquakes with magnitudes of up to 5 (see central panel of Figure 2). They occur at depths of less than 100 miles and would certainly be felt by astronauts working on the moon. Because of these quakes, any future lunar base envisioned by NASA would have to be built with seismic safety codes in mind. (hra041)

Fig 1. Photo: Horst Rademacher

When you throw a stone in a quiet pond or dip your finger in a puddle, a nice circular wave emerges on the surface of the water. It gently moves across the pond in a concentric pattern (see photo). Physicists call these ripples "surface waves", because only the molecules in the top few inches of the water are being moved by such waves. The deeper you probe the pond, the less effect such waves have. Already a foot or so beneath the surface, the water stays completely calm as these waves pass overhead.

A very similar pattern of waves follows every earthquake. When the Earth moves in a temblor, it generates more than just the P- and S-waves we described a while ago (see blog September 10, 2008). Surface waves also emerge, but unlike their relatives on the water surface of a pond, they can be anything but subtle. Take the strong earthquake which happened last night (Pacific Daylight Time) off the southern tip of the South Island of New Zealand. Although it had a magnitude of 7.6 and was located just 20 miles beneath the surface, it caused surprisingly little damage in New Zealand's coastal towns. But even though the quake's hypocenter lay all the way across the Pacific, it made the ground in Berkeley move by 1/10 of an inch. The blogger doubts that anybody would have felt this swaying of the ground, because the period of such surface waves from far away earthquakes is 20 seconds or more. As you can see in the seismogram, it took more than an hour for all the surface waves generated by the New Zealand quake to pass California.

In contrast to their relatives in the pond, these seismic surface waves did not travel across the surface of the Pacific Ocean. They traveled along top layers of the Earth's crust, and during their journey they don't care if the rocks are hidden under water or exposed on a continent. The seismic surface waves spread as elastic waves through the crust in the same pattern as the water waves on a pond.

Even though the spreading pattern of such surface waves looks nice and simple and the movement is - mostly - gentle, the physics behind them is rather complex. More about that in the next blog. (hra040)