As of this writing, the death toll of Wednesday's earthquake in Central Italy has risen to 280. However, rescue workers expect to find more bodies in the rubble in the three mountain communities most affected by the shaking of the magnitude 6.2 quake: Amatrice, Accumoli and Arquata del Tronto. Of particular worry is the situation in Amatrice, where a hotel has completely collapsed. Many tourists had spent the week there to attend the town's annual Pasta Festival. The celebrated specialty dish „Spaghetti all’amatriciana“ is well known all over Italy.
While rescue and recovery work is still continuing in the epicentral region, experts have begun to ask why relatively weak earthquakes always seem to generate enormous amounts of death and destruction in Italy. Compare, for instance, the Napa earthquake that occurred in Northern California almost exactly two years ago with Wednesday's temblor in the Italian Appenine mountains. With a magnitude of 6.0 the Napa quake was just a tad smaller than the magnitude 6.2 quake in Italy. But only one person was killed in California's wine country and the damage to buildings and structures was far less than the severe destruction in Italy. Elsewhere in the industrialized world even much stronger earthquakes with magnitudes of 7.5 or more do not generate as much damage as it seems to be the norm for smaller quakes in Italy.
|Rescue and recovery teams are still working in the town of Amatrice.|
Take the L'Aquilla event from 2009 with a magnitude of 6.3, only 25 miles from Wednesday's temblor: 300 people died and more than 10,000 buildings were destoyed. In 2002 more than 30 people, among them 27 pupils, died in a magnitude 5.9 quake south of the current epicenter. Other devastating Italian quakes also do not belong into the class of strong or severe earthquakes: One near Assisi in 1997 with a magnitude of 6.4 caused 11 fatalities, in a magnitude 6.4 quake south of Naples in 1997 more than 2700 people died and one thousand were killed in the Friuli quake of 1976 (M=6.5).
One reason for the enormous destruction caused by relatively small quakes in Italy can be traced to the age of many buildings, particularly in small, rural towns and villages. Many structures there date back centuries or even to the Middle Ages. When they were built, nobody cared or had a clue about earthquake resistant construction methods. However, earthquake engineers have also encountered modern buildings which did not comply with those aspects in the Italian building code, which relate to earthquake safety and resiliance. The 27 pupils mentioned earlier died in a modern school building in the town of San Giuliano di Puglia. It turned out that during its construction the earthquake safety measures required by law were bypassed or ignored.
|Built by modern standards: The red building in this picture is the only structure in the historic center of Armatrice left undamaged by Wednesday's quake. Photos: AP|
Another reason is that - in contrast to other industrialized countries located in seismic zones - Italy does not seem to have a culture of preparing for natural disasters. This sentiment was echoed after Wednesday's quake by Francesco Peduto, the president of Italy's National Council of Geologists. He said in a newspaper interview, that Italy was "lacking a culture of disaster prevention". He added that 40 percent of the 60 million Italians live in zones with a high natural hazard, mostly due to seismic activity but also in volcanic zones, like those around Vesuvius near Naples and Etna in Sicily. Even in areas with high seismic risk, there is little education of the general public on how to behave during earthquakes. While in Japanese and California schools kids are regularly taught to "Drop, Cover and Hold on" when they feel seismic shaking, very few Italian schools have any disaster drills. Peduto estimated that between 20 and 50 percent of all deaths in earthquakes could be avoided, if people knew how to react properly when shaking starts.
Another critic is Massimo Cocco, one of the research directors of the Italian National Institute for Geophysics and Volcanology (INGV). He estimates that almost 70 percent of Italy's buldings are built with little or no earthquake resistance. The Italian government, he claims, does not even have a "Earthquake Safe" plan for schools and hospitals. Peduto concurs and demands nationwide legislation to enforce earthquake retrofitting. Not only would such a measure reduce the loss of life, it would also save money in the long run. The national Association of Builders, an industry group, estimates that since 1968 Italy has spent about 200 billion Dollars in reconstruction and recovery efforts after earthquakes. Applying well tested retrofitting and building techniques to increase earthquake resistance would have cost a fraction of that amount, Peduto notes. (hra126)
The strong magnitude 6.2 earthquake that shook Central Italy around 3:30 this morning (local time) must have revived horrible memories for many people in the Rieti and Abbruzo regions about 100 miles north-northeast of Rome. Seven years ago, a temblor of similar size shook the same area and left the medieval town of L'Aquila in ruins. More than 300 people died in that quake, which has become one of the most infamous temblors in recent Italian history. First reports about today's quake in Italian media put the death toll at about 120. The reports speak of strong shaking and widespread damage.
Earthquakes in central Italy are a direct consequence of the complex collision of two tectonic plates. Although geographically a European country, Italy is actually a nail shaped protrusion of the African plate which rams into Europe, creating the Alps far north of this morning's epicenter. This collision has also created the Apennine Mountains. They run along the shaft of the "boot" which Italy resembles when viewed on a map. The Apennines are dominated by a series of north-south trending earthquake faults, which run parallel to the crest of the mountain range through most of central Italy. The epicenters of both the 2009 earthquake and today’s quake lie less than 25 miles apart on those fault lines. Today's quake occurred near the town of Amatrice to the north-northwest of L'Aquila.
|Map of the seismic risk in the Italian Apennines: areas in green and blue are considered very low risk. The pink in the center of the map has the highest risk. The white star denotes the location of today's quake. Source: INGV, Rome|
Besides the difference in the extent of damage and in the number of lives lost, there are several reasons why the two quakes are hard to compare. While today's quake was followed by several significant aftershocks - one of them reached a magnitude 5.5 - it was not preceded by any noticeable foreshocks. This was different in 2009. In the months before the April 6th quake, the area around L'Aquila was pummeled by hundreds of quakes. Although most of them were small, they were felt by the local population and rattled their nerves. As a consequence, Italy's government established a commission of experts, who were tasked with evaluating the seismic hazard in relation to this earthquake swarm, which seemed to go on and on.
While these experts were discussing various scenarios, an engineer without any prior knowledge of seismology stole the show. He had observed that the concentration of radon in some groundwater wells had changed during the swarm. Because he had read somewhere that such a change may be an indicator for an upcoming strong earthquake, he predicted in public that a strong quake was imminent. He did not discuss his findings with any seismologist. The commissioners reacted to this surprise announcement by trying to assure the population that such predictions were nonsense - only to be superseded by reality. A few days after the engineer went public, the devastating quake did indeed happen. Because their reaction was deemed completely inadequate, several commissioners were later found guilty of manslaughter.
Before today's quake, however, nobody had attempted a prediction and no commission was in session to gloss over the seismic risk along the fault lines in the Apennines. In fact, this region of Central Italy is considered to have one of the highest seismic risks in the nation.
One factor that certainly contributed to the widespread damage is the focal depth of today's quake. The seismologists at the Italian national seismic network computed a focal depth of only 2.5 miles. The closer a quake's origin is to the Earth's surface, the stronger the shaking and, hence, the more damage can be expected. (hra125)
California is full of faults. Although this sentence is a cheap pun amongst seismologists, it was the first thing that came into the blogger's mind when he heard about the surprising temblor in Lake County on Tuesday night (August 9th) around 8 pm. With a magnitude of 5.1, the quake was felt from Point Arena on the coast all the way east to Chico and Oroville in the Sierra foothills. It was located in a remote area of the southern Mendocino National Forest about 25 miles northeast of Ukiah and 10 miles south of Lake Pillsbury at a depth of 8 miles.
The epicenter of tuesday evening's quake (yellow star) was located on the southern section of the Bartlett Springs Fault. This fault is the eastern limit of the plate boundary. To the west are the Maacama and the San Andres Fault. (Modified after USGS)
It occurred along a fault that most Californians will have never heard of: the Bartlett Springs Fault. This faultline runs for about 100 miles through Lake County, from the Round Valley Indian Reservation in the North through the waters of Lake Pillsbury until it ends in the south, when it crosses Bartlett Springs Road east of Clear Lake. This road and the nearby spring gave the fault its name. Much of the fault runs through the new "Berryessa Snow Mountain National Monument," which President Obama created by proclamation about a year ago.
Even though the Bartlett Springs Fault was identified by geologists decades ago, it has hardly been investigated in detail. One reason for the lack of research activity is that rather few quakes happen along this fault when compared to the seismicity along other faults in Northern California like the San Andreas or Calaveras Faults. Tuesday's quake was the strongest temblor along this fault for more than 50 years. The other reason is that the population density along the Bartlett Springs fault is very low - keep in mind that all of Lake County has a population of less than 70,000 people. Both points together make the seismic risk, which is the potential for damage and losses due to an earthquake, along the Bartlett Springs Fault considerably lower than here in the Bay Area.
Nevertheless, as Tuesday's temblor showed, this fault is active. In addition, Jim Lienkaemper, the geologist with the US Geological Survey in Menlo Park who probably did more seismic investigations along the Bartlett Springs Fault than anybody else, found creep along some sections of the fault. He found that the two sides of the fault move past each other at a rate of about a quarter of an inch per year.
The Bartlett Springs Fault is part of the wide zone of earthquake faults generated by the boundary between the Pacific and the North American plates, which governs the geology of most of California. Although such boundaries are drawn on maps as single, narrow lines, they are in fact broad zones, which can be dozens of miles wide. Look at the Bay Area itself. Here the plate boundary is split into several faults, which run parallel to each other. The westernmost extension of this zone is the San Gregorio Fault, which lies mostly offshore. It is followed to the east by the San Andreas, the Hayward, the Calaveras, and finally the Greenville Fault, which is located east of Livermore. The distance between the San Gregorio Fault and the Greenville Fault is more than 50 miles - which is in fact the width of the plate boundary here in our area.
The situation is similar in Lake County up north. Coming from the east, the Bartlett Springs Fault is the first stepping stone from the North American to the Pacific Plate. It is followed in the west by the Maacama Fault, which essentially runs along Highway 101 between an area northeast of Santa Rosa and Laytonville. Finally, the western limit of the boundary zone is defined by the San Andreas Fault itself. The San Andreas Fault picks up about half of the movement between the plates, which is estimated to be about two inches per year. The Maacama Fault creeps by at about half an inch per year, and the Bartlett Fault moves, as mentioned above, by about a quarter inch. (hra124)
Earthquake faults come in very different flavors. No, we are not talking about the classic distinction based on the direction of movement of the two flanks of a fault, like a normal, a thrust or a stike slip fault. When he is talking about different flavors, the blogger means more subtle differences. Think for instance about how many earthquakes happen on a given fault segment during a certain interval of time. First and foremost, of course, the number of quakes depends on the strength of the tectonic activity along the fault line. If there is no plate movement, well, there won't be any quakes, because there is no tectonic stress to build up. But even with significant tectonic activity, some faults may not show any signs of seismicity. Such faults are "locked" - and they are the most dangerous beasts in the seismological family.
Earthquake events in the past 7 days south of Hollister, CA. Events plotted are M > 2.0. [Source USGS]
The reason they are so hazardous: Locked faults act like an absorber of tectonic energy. For years, decades and sometimes even centuries such faults pick up the movement of the plates and store the resulting tectonic stress. Unfortunately, their storage capacity is not infinitely large. At some point, exactly when the stored tectonic stress overcomes the frictional and mechanical resistance within the rocks, the fault breaks suddenly and explosively - usually resulting in a large, catastrophic earthquake. One example of a locked fault is the long segment of the San Andreas Fault stretching from south of Parkfield across Tejon Pass almost to San Bernardino. This section last generated a big, destructive quake in 1857, called the Fort Tejon Earthquake. Since then, it has been locked and is collecting tectonic stress.
The other extreme is a "creeping" fault. Creeping faults do not accrue any tectonic stress at all. Instead the underlying movement of the plates shows up directly along the fault line at the Earth's surface. The two sides of the fault glide past each other - as if they had been lubricated and lost all resistance. The famous shifting curbs in Hayward, which were recently removed, are one example showing fault creep. Another impressively creeping fault is the very southern end of the Calaveras fault in the town of Hollister. We can also see fault creep wherever the San Andreas Fault crosses California Highway 25 south of Hollister. At these crossings the asphalt is cracked in a distinctive pattern.
Unfortunately for us seismologists, nature is rarely black or white. Most earthquake faults are neither fully locked nor are they freely creeping. Instead, they exhibit more complex behavior. They can be mostly locked, but may also release some of their stored energy in small earthquakes and they may even show a small amount of shallow creep along the surface. Such faults are called "hybrid", like a hybrid vehicle, which sometimes runs on electricity and at other times is propelled by a gasoline-powered combustion engine. The earthquakes on such hybrid faults usually come in bunches, a sequence of temblors which we call a cluster.
During the last fifty years, the earthquakes near Tres Pinos did not occur randomly, but in clusters instead - like the current uptick in seismicity.
One area in northern California where the San Andreas Fault shows a hybrid behavior and typically releases its seismic energy in clusters is the section south of Hollister near the hamlet of Tres Pinos. Every few years, the town is rattled for a few weeks by a dozen or more earthquakes with magnitudes of less than 4.5. Then everything calms down again for some months or years. The current seismic activity near Tres Pinos is such an earthquake cluster.
As our figure of seismic activity near Tres Pinos for the last 50 years shows, there were lots of clusters in the early 70's, then again in 1990 and 2002. The last significant earthquake cluster in this area happened in early 2009. Fortunately, none of these quakes had magnitudes above 4.6. This is big enough to rattle the nerves of the few people who live there, but not strong enough to cause significant damage. (hra123)
It is not for naught that we call the Hayward Fault a "tectonic timebomb in our backyard". Almost 150 years have passed since the last major earthquake rattled the fault itself and, in fact, the entire San Francisco Bay Area. It was in October 1868 when the Hayward Fault last shook off its tectonic load in what is now estimated to have been a magnitude 7 earthquake. Since then plate tectonics has continued its relentless job: Mechanical stress generated by the eternal the movement of the Pacific Plate relative to North America has again accumulated along the Hayward Fault. In fact, so much potential seismic energy has been stored since 1868, that this tectonic demarcation line along the foothills of the East Bay now has the greatest probability for a big shaker in all of Northern California.
Although the Hayward Fault currently poses the largest seismic hazard of all Bay Area earthquake faults, to the layman it remains something of an enigma. Despite numerous published maps and even virtual tours on Google Earth, its exact location seems hidden deep beneath the urban jungle of the East Bay. There are however a few places, where the fault reveals itself at the Earth's surface. The cause: In some of its sections the fault responds to the ever increasing tectonic load by subtle creep. Instead of waiting for the tectonic stress to be released in a sudden explosive quake, in a few places the ground at the fault gives way in a slow crawl along the fault line.
For the uninitiated, it takes a little training to find the subtle marks with which the creeping fault expresses itself. On the campus of Contra Costa College in San Pablo, the fault is visible through pavement cracks and offset curbs. There, a patio area next to Campus Circle bears an especially interesting mark of the fault's creep. Its bricks, originally laid in a straight and rectangular design, have been moved by the fault and now exhibit a clearly curvy pattern. While most of the fault line is covered by urban infrastructure, it also cuts through the Oakland Zoo, where it runs past the otter tank and a bear pen.
|Over the decades the steady creep of the Hayward Fault has moved apart the two curbs at the intersection of Rose and Prospect streets in Hayward - making it a world famous icon of the restless Earth.|
The upper section of the UC Berkeley campus straddles the fault as well. Some of the residence halls near the Greek Theater and the Memorial Stadium at the foot of Strawberry Canyon are built either directly on or just a few yards off the surface trace of the fault. The effects of fault creep in the stadium are clearly visible in the concrete wall above section KK in the south curve, where two sections of the wall have separated from each other by more than a dozen inches.
Some of the most impressive displays of the fault's creep can be seen in downtown Hayward. Less than half a block east of Mission Boulevard, parking lots and roads are full of characteristic cracks, which align themselves along the fault in a staggered fashion called "en echelon pattern". The curbs of nearly every residential street which crosses the fault are offset by a few inches. Nowhere was this clearer than at the intersection of Rose and Propect streets. Since 1971, and perhaps earlier, geologists and laymen alike have documented how the fault has been separating two segments of the curb, inch by inch (see sequence of pictures). Hundreds of students from colleges all over the world have marvelled at this otherwise unremarkable intersection, where plate tectonics could be seen in action. Over the years, the offset curbs at Rose and Prospect had moved more than 7 1/2 inches away from each other - a crawl which made them an icon among geologists. They were one of the few places in the world, where the restless Earth could be easily observed in an urban environment.
No more curb: a contractor has erased decades of fault creep. Photo: Andrew Arden
But this icon is no more. During the last few weeks a contractor has straightened out the curb and replaced it with a wheelchair-accessible ramp. While making the sidewalks safe for people with disabilities in itself is a good thing, nobody in Hayward's city government understood that in doing so they destroyed a world famous marker pointing to the earthquake fault that bears the city's name. "We just weren't aware of it", assistant city manager Kelly McAdoo told the Los Angeles Times. Adding that it is the administration's mandate from the City Council to have safe sidewalks, accessible for all members of the community.
To Earth scientists, the erasure of the iconic curb is a loss indeed. But thinking in the longer time frame of geology, the loss is only temporary: Because we know that even the City Council of Hayward cannot stop the lithospheric plates from moving. As soon as the concrete under the new ramp has cured, the creeping fault will begin to tag and tear and the curbs will be on the move again. In a decade or so, the Hayward Fault will again have visibly left its mark. (hra122)