Archives for: April 2012
This seismogram, recorded at Berkeley's seismic station CMB in the Sierra foothills shows the ground movement associated with the sonic boom generated by the meteor. When the sonic boom reached the station, the ground was slightly pushed down by the pressure wave, hence the strong downward movement in the record. After a few seconds, the ground recovered elastically.
The loud boom, which was heard on Sunday morning around 8 am PDT over large areas of eastern California and Nevada, not only rattled the nerves of many who were rudely awakened by the bang. It also set off house and car alarms, and many frightened people called the police. Astronomers quickly determined that a meteor the size of a washing machine had entered the Earth's atmosphere traveling eastward over the West Coast. It may have been part of the annual Lyrid meteor shower, which peaked over the weekend. Similar to a jet flying at supersonic speed, the bolide's travel generated a sonic boom, which was heard over hundreds of square miles on both sides of the Sierra Nevada. However, the meteor was at least ten times faster than the speediest fighter jets.
The loud bang was also registered by many seismometers in the region, among them the sensitive instruments of the Berkeley Digital Seismic Network and its sister networks operated by the University of Nevada in Reno and by the United States Geological Survey.
How can these seismometers, which are designed to notice the slightest movement of the ground, also pick-up sound? In order to find out why, we need to venture a little bit into the physics of waves. Sound waves and seismic waves are essentially the same. Both belong to a group called elastic waves. A sound wave causes a back-and-forth motion of air molecules. When this vibration reaches our ear drums - or the diaphragm of a microphone - it is sensed or recorded as noise. In a similar fashion, an earthquake wave causes particles in the ground to vibrate, albeit with much lower frequencies than audible sound waves.
While most humans can hear sound waves with frequencies between a few hundred and about 15,000 Hertz (vibrations per second), earthquakes cause vibrations in the infrasound range and below. In general, these frequencies are too low to be heard by the human ear. However, seismometers are designed to operate best in the infrasound range down to extremely low frequencies of a few milli-Hertz. Hence, when the sound's frequency is low enough, it can be registered by a seismometer.
There are two reasons why a loud sonic boom like the one heard on Sunday morning leaves distinctive traces on the seismic networks. Firstly, a sonic boom consists of sound waves of a myriad of frequencies, among them some in the infrasound range. These low frequency vibrations can directly trigger a seismometer.
The other reason is the sonic boom's intensity. As an aircraft - or a meteor - jets through the air traveling faster than the speed of sound, it creates a series of sound waves in front of it and behind it. These waves are slower than the jet itself, because they cannot travel faster than the speed of sound in air - or in simple terms: They cannot get away from the aircraft fast enough. As the jet keeps producing more and more of these sound waves, they get stuck, are forced together and eventually, they merge into a single shock wave - the sonic boom. When it hits the ground, it not only dazzles our eardrums. It also generates a pressure pulse on the Earth's surface which leads to a good sized vibration. That is what gets picked-up by the seismometer
Seismic signatures of sonic booms were studied extensively when NASA's Space Shuttle fleet was still flying. Every time a Shuttle landed on the dry lake bed of Edwards Air Force Base east of LA, its sonic boom was registered by the seismometer network in southern California. More on these unique seismograms can be found here. (hra078)
The magnitude 8.6 earthquake, which struck off the coast of Sumatra last week, was one of those rare megaquakes. It belongs to the group of the ten or so strongest quakes ever measured. As explained in the previous blog entry, scientists were surprised - and certainly breathed a deep sigh of relief - that this quake did not cause a significant tsunami. In fact, hardly anybody was even seriously hurt when the extremely strong seismic waves caused by this quake hit the Indonesian islands closest to the epicenter. But once the immediate danger of a tidal wave was over, seismologists began to realize that this quake and its aftershock of 8.2 two hours later pose an immensely interesting, multifaceted scientific puzzle. Here is why:
These two quakes occurred in the immediate vicinity of the Sumatra subduction zone, where the Indian Ocean plate dives under fragments of the Eurasian plate. The Great Sumatra Quake of December 26th, 2004 had its origin in the same general area. But while this quake, which produced the devastating tsunami seven and a half years ago, had a focal mechanism expected for subduction zones, last week's quakes did not. They were earthquakes of the California type, that is the two flanks of the rupture slid past each other horizontally instead of slipping vertically against each other as in most subduction temblors. Such horizontal strike slip quakes have almost never been observed on the seaward side of a subduction zone. How and why they occurred off Sumatra is puzzle number 1.
The other unexpected feature of this quake should concern everybody in California. Most quakes along the San Andreas and its sister faults also shift in a strike slip fashion. Seismologists have always assumed that such strike slip events would be able to generate strong temblors, which are likely to cause a lot of damage. However, all the scientific evidence collected here in California and in other strike slip regimes around the world suggests that strike slip quakes cannot grow into really devastating megaquakes with magnitudes of 8 and greater. It seems that the horizontal slip fault zones are too weak to accommodate enough tectonic stress to delay rupturing until the breaking results in a megaquake. Well, since last Wednesday researchers have such an unexpected strike slip megaquake in their collection. And now they are scratching their heads as to how to fit these two unexpected aspects of the puzzle into current earthquake theory.
One possible solution was pointed out at an earthquake conference in Memphis last week by Lori Dengler, a Professor of Geology at Humboldt State University and California's foremost expert on tsunamis. She noted that the Great Sumatra Earthquake released a large amount of tectonic stress along its extremely large rupture area. As a consequence the mechanical stress along the fringes of the rupture area grew and put the surrounding rock under an immense strain, with two forces acting in opposite directions. This pattern is similar to the forces acting on the San Andreas Fault. When the rocks finally gave way last week, the result was a strike slip earthquake. And why was it so large? Dengler thinks that the rocks in the hypocentral area were virgin. They had never been broken in an earthquake before - in contrast to the rocks along the San Andreas fault, which have ruptured many times during the past eons. As a consequence, the oceanic crust off Sumatra was able to accumulate an enormous amount of stress. When the rocks finally gave way, the result was last week's megaquake. (hra077)
|Epicenters of last night's earthquakes and the great Sumatra quake of 2004. (Click to view larger image.)|
When their alarm pagers went off around 2 am PDT last night, most seismologists on the West Coast feared the worst. A magnitude 8.6 earthquake had struck the very area off the coast of the Indonesian island of Sumatra that had suffered extreme damage by the now infamous Boxing Day earthquake on December 26, 2004. Then, hundreds of thousands of people were swept away by one of the biggest tsunamis in recent centuries. This monster wave was caused by a giant magnitude 9.2 earthquake. Although somewhat smaller, last night's magnitude 8.6 quake could have set off a similarly strong tsunami. Adding to the anxiety was the fact that about two hours after the first temblor, a second very strong earthquake shook almost the same area. It had a magnitude of 8.2. But after a few hours of anxious waiting, seismologists and emergency responders could relax. Only a small wave less than 3 feet high hit the west coast of Sumatra. It caused no significant damage. Along the shores of the rest of Indian Ocean just a few sensitive tidal stations registered a small disturbance.
Why did these two very strong earthquakes last night spare us a natural disaster similar to those that we witnessed after the 2004 Indonesia earthquake, or after the Tohoku quake, which struck in March of last year along the east coast of Japan, with a magnitude of 9.0? The differences in magnitude can not have been the only reason, because any offshore quake with a magnitude of more than 7.5 is potentially able to cause a tsunami. With their magnitudes of 8.6 and 8.2, last night's quakes were certainly within the tsunamogenic range and each one by itself could have generated a devastating tsunami.
One possible explanation for the tameness of the most recent quakes is their location. Both temblors occurred under the sea floor more than 60 miles to the southwest of the epicenter of the 2004 quake. This epicenter lay very close to the deep ocean trench, which represents the boundary line between the Indian Ocean and the Eurasian plates. Last night's quakes were much further from the trench and thereby further from Sumatra's coastline. But as we have seen again and again, distance is no protector from the devastating effects of a tsunami. These waves can cross an ocean basin with the speed of a jetliner and are able to wreak havoc thousands of miles away from their source.
The actual reason why the last two quakes did not generate any tsunami is their focal mechanism. Seismologists use this term to describe the relative direction in which the two rock masses actually move when they spring past each other in an earthquake. Take the temblors on our doorstep for example: Their mechanism is called strike slip, because our two home plates, the North American and the Pacific plates, scrape past each other horizontally. This results in quakes in which the two sides of the fault slip past each other without much vertical movement.
In contrast, the Great Sumatra Quake of 2004 had a completely different focal mechanism. It was a thrust event, in which almost all movement was vertical: One flank of the fault sprang upwards. This upwards thrust of the sea floor hit the water column above very hard, generating the monstrous killer wave. Last night's earthquakes were not of this dangerous thrust type, because their mechanisms were California style: The two flanks of the undersea fault slipped past each other horizontally. (hra076)