En echelon cracks where Highway 25 crosses the San Andreas Fault (Photo by Horst Rademacher)

There is a whole lot of shaking going on in Tres Pinos in these days. Well, it is not exactly the earth shattering rumbling associated with big earthqakes. But the 500 or so residents of this tiny hamlet south of Hollister have certainly been taking note. Joe Lucido tells the blogger: "We felt quite a bit of shaking last night...The last couple of weeks have been like nothing we can remember here in San Benito County. The talk is all over town." So what is going on in this section of the state, about one hour's drive south from Silicon Valley?

Tres Pinos is located between Hollister and the Pinnacles. In the immediate vicinity of this town, the Calaveras Fault branches off from the San Andreas Fault. South of the split, the section of the San Andreas is "creeping" all the way to Parkfield. Creeping means that the mechanical stress associated with the movement of the plates is not stored in the rock until it breaks in a big temblor. Instead the energy is more or less continuously released. You can see evidence of this creep in the pavement of Highway 25 south of Tres Pinos. The fault crosses the road in many places. There, the asphalt is broken in a typical, staggered pattern that seismologists call "en echelon cracks" (see picture).

However, not all of the energy is released in fault creep. Sometimes the friction between the two flanks of the fault is too high and some energy accumulates. When the rocks crack under this load, earthquakes happen. These quakes with magnitudes of 3 or 4 are small by worldwide standards, but strong enough to be felt. Even though the movement of the plates - and thereby the transfer of the mechanical stress into the fault - is very steady, friction along the fault is not nearly as constant. This is the reason that small temblors sometimes occur in clusters. If there are enough of them, they begin to rattle the nerves of the folk in Tres Pinos and nearby.

That is exactly what's happening now. Since January 13, there were nine small quakes with magnitudes above 3 and dozens more between 2 and 3. Such clusters are by no means unusual. Since 1970, a total of 328 quakes with magnitudes above 3 happened on this stretch of the fault. As the graph below shows, most of them come in clusters. If you want to stay abreast of the rumbling under Tres Pinos and beyond, look at the realtime earthquake map provided by the USGS. (hra029)

Is your water heater properly strapped to the wall? Do you have additional insurance on your house with the California Earthquake Agency (CEA)? If so, then you can thank a horrible event, which happened 15 years ago today in Southern California.

	Damage from the Northridge earthquake (Photo courtesy of M. Celebi, USGS)

It was about 4:30 am when Clarence Dean was rudely awakened in his home in Lancaster in northern Los Angeles County. A major earthquake had shaken his house badly. He got up, dressed quickly and then jumped onto his Kawasaki Police 1000 motorcycle. Dean was an LA motorcycle cop, and this Martin Luther King Day would have been his day off. But he knew he would be needed at his Van Nuys police station because of the earthquake. From his home, it took him only a couple of minutes until he reached Highway 14. There he took the westbound direction and sped towards LA, the blue emergency strobe flashing on his bike. But he never made it to his assigned post. The swooping viaduct where Highway 14 meets Interstate 5 had collapsed in the quake. Where two lanes of concrete had been built in a gentle arc high above the ground, there was nothing left. In the darkness, Officer Dean spotted the gap too late and flew 75 feet through the air before crashing, 30 feet below, in a cascade of sparks and screaming metal.

The policeman was one of 72 people who died on that morning in a 6.7 quake, with a hypocenter 10 miles beneath the town of Reseda in the San Fernando Valley. Had it not been a federal holiday, many more people might have died during the early morning commute. Besides the 5/14 interchange, the Santa Monica Freeway (I 10) had collapsed near Cienega Boulevard, making this major artery unusable for weeks. Most of the damage, however, occurred in the town of Northridge, hence the name for this costliest temblor in US history. Homes, apartment buildings and even hospitals collapsed all over the San Fernando Valley. It was later estimated that the quake had caused more than $20 billion worth of damage. As a consequence, many insurers of private homes stopped offering protection against earthquakes in our state.

The same region had been the victim of a similarly damaging temblor 23 years earlier, the San Fernando earthquake of 1971. Seismologists later found out that these two quakes occurred on two different, previously unknown faults. After the Northridge quake, the California Legislature passed several laws in a rare form of bipartisan unity. One gave life to CEA, which now underwrites earthquake insurance statewide. The building code was also amended, requiring each newly installed gas-powered water heater to be strapped to the wall. Too many heaters had fallen on that fateful morning 15 years ago, breaking the gaslines and causing devastating fires. (hra028)

	Picture 1: The first X-ray photograph

It is often said that seismologists use earthquake waves to illuminate the interior of the Earth like a radiologist uses X-rays to study the insides of the human body. There is indeed some similarity, as both of these branches of physics are of the same age. The German researcher Wilhelm Conrad Röntgen took his first X-ray photograph in 1896 (see picture 1). Roughly at the same time, the first seismographs were installed and earthquake researchers learned to read the "trembling of the rock" (see blog January 9, 2009).

In a strict physical sense, however, major differences exist between X-ray pictures and seismograms. Through the different shades of grey in the X-ray picture one can clearly distinguish between the wedding ring, the bones, and the soft tissue in the hand. This is due to the fact that metal, the calcium structure of the bone, and the soft tissue absorb electromagnetic waves differently. Metals swallow X-rays almost completely, but the calcium in the bones is not quite as opaque as metal, while the soft tissue lets most of the rays pass.

Although seismic waves experience some absorption in the rocks of the Earth, it is by no means the dominant feature in a seismogram. Look at picture 2. It is a recording of the magnitude 7.6 earthquake in eastern Indonesia on January 4, 2009 (see blog January 5, 2009). The blue line represents 40 minutes of ground movement at our seismic station "CMB" in Columbia in Tuolumne County. Starting from the left, it begins as an almost straight line. Then, roughly at 19:57 UTC, the ground starts to wiggle. There is another shaking at minute 20:02 UTC, a bigger one at 20:12, and an even larger one at 20:17, with the biggest one at 20:31. Each of the arrows marks one of these "onsets" or "phases," as seismologists call them.

What are these phases? As the seismic waves travel from the earthquake focus through the earth, they hit major boundary layers in its interior. At each of such "discontinuities" the waves are reflected, refracted or diffracted. Each of these processes alters the path of the waves, which in turn makes them travel longer until they reach CMB. In addition, different kinds of waves travel through the Earth with different velocities. The P-waves (red arrows) are faster than the S-Waves (green arrows) (see also blog September 10, 2008). In one of the next blogs, we will explore, how these phases can be used to "X-ray" the Earth's interior. (hra027)

Picture 2: Seismogram showing M 7.6 earthquake in West Irian Jaya.

	Map illustrating the distribution of BDSN stations in Northern and Central California.
	Inscription on the entrance to the Göttingen University earthquake station (Photo by Horst Rademacher)

Recording earthquakes is one of the main jobs of the folks at the Berkeley Seismological Laboratory (BSL), the host of this blog. But measuring those tiny wiggles of the ground caused by temblors near and far is by no means trivial. Compared to reading an electrical current off an ampere meter or the temperature from a thermometer, running a network of seismometers is a rather complex undertaking. BSL operates a network of earthquake stations all over Northern California, from Fresno County in the south to the Oregon border (see map).

Undoubtedly the most important items in this network are the seismometers. Most of these sensors are handmade masterpieces of mechanical engineering. Their function is to pick up even the slightest movement of the ground. Modern feedback sensors are easily capable of registering a millionth of an inch or less. Even the tiny footsteps of a mouse on the concrete floor inside a station can be noticed. These seismometers convert the ground vibrations into electrical voltages. They in turn are tranformed into computer readable bits by a digitizer, the second most important item in a seismic station. The bits are then electronically packaged into groups, which in turn are transmitted to BSL's data center on UCB's campus. Sometimes, the data packets follow circuitous paths before they reach Berkeley. Some travel by satellite; others by phone lines, by radio or simply over the internet.

Once they reach the BSL, the data are checked, analyzed and stored forever on specially secured computer disks. BSL's archive of earthquake recordings is one of the most complete collections in the world, dating back almost 100 years. All of these data are accesible to all researchers free of charge.

And what do scientists do with Berkeley's data? There is indeed a wealth of information stored in those seismograms. Some seismologists use them to study the actual physical processes during an earthquake. Others compute how strongly the ground shook in response to the seismic waves, thereby laying the ground work for the planning by civil engineers. But one of the most important aspects of seismology is the unravelling of the Earth's interior. One of the first professors of seismology in the world, Göttingen's Emil Wiechert, once described this scientific endeavour in very poetic terms: "The trembling rocks bear tidings from afar. Learn to read their meaning."

This sentence, in its original German, is cut into the stone above the entrance to the earthquake station at Göttingen University (see picture). In one of the next blogs, we will see what scientists do to read the meaning from the "trembling rocks." (hra026)

Map showing Sunday's two large earthquakes on the Bird's Head Penninsula in West Irian Jaya.

Earthquakes, of course, do not recognize religious or other holidays. Neither do they take notice of the - from a geophysical perspective - completely arbitrary choice of the date of the beginning of the new year. Nevertheless, during the last two weeks, the world's seismicity was exceptionally low, which allowed the blogger to take a few days off. The relative quiescence, however, ended with a bang over the weekend, when Indonesia was struck by two major earthquakes within less than three hours of each other. According to local news reports, at least four people were killed and dozens injured; several buildings collapsed.

The first quake happened shortly before 5 am on Sunday morning (local time) and had a magnitude of 7.6. It was followed by dozens of aftershocks, the strongest of which occured nearly three hours after the main shock with a magnitude of 7.4. On the long term average, only about ten earthquakes of such strength happen worldwide every year r 14, 2008). The latest earthquake sequence originated under Indonesia's eastern province, West Irian Jaya. Geologically this province belongs to the island of New Guinea. The epicenters were located beneath the island's westernmost "Bird's Head Peninsula", which derives its name from its unmistakable shape. The casualties and building collapses were reported from the provincial capital Manokwari, more than 1800 miles east of Indonesia's federal capital Jakarta.

The tectonic situation in this part of the world is complex. Three major lithospheric plates converge in this region and break up into half a dozen microplates, which tend to wiggle under the onslaught of tectonic forces. Under the Bird's Head Peninsula, the Pacific plate is moving southwest with respect to the Australia plate. Compared to its speed in California of about 2 inches per year, the Pacific plate races under New Guinea, moving with a velocity of almost 5 inches per year. According to the USGS, the focal-mechanism of Sunday's earthquakes are "broadly consistent with Pacific plate lithosphere being subducted beneath Australia plate lithosphere." This subduction zone along the northwest coast of New Guinea is characterized by an offshore oceanic trench, the New Guinea trench.

The Tsunami Warning Center in Jakarta (see blog November 14, 2008) issued a tsunami alert immediately after the main shock, but it was revoked within an hour after scientists determined that the quake's epicenter was on land. (hra025)