|A singularity of destruction in a sea of normalcy: The collapsed Weiguan Jinlong residential building in Tainan after the 6.4 earthquake. (Photo: Taipei Times)|
Pictures showing the destruction after a devastating temblor are never pretty. But the latest aerial photos coming out of the earthquake-struck city of Tainan in southern Taiwan are outright surreal. As far as the wide angle lens of the camera can see, the pictures show buildings which look undamaged and pristine. Only in the photo's center do two colossal grey apartment towers lie on their sides - keeled over as if they were felled like trees cut with a chainsaw. Rescue teams are still frantically searching for survivors. As of this writing more than one hundred people have been pulled out alive but more than 150 others are still missing, presumably trapped deep inside the fallen residential towers. The bodies of 13 residents who perished in the collapse have also been recovered.
To a seismologist's eye, however, this picture shows more than a surreal scene after one of nature's most devastating forces has struck. Why is it, one has to ask, that hundreds of buildings in Tainan's Yongkang District are intact while the twin 17-story towers of the Weiguan Jinlong residential building were left lying on their sides on Yongda Road when the shaking stopped? The earthquake itself, which occurred on Saturday shortly before 4:00 am (local time), had a magnitude of 6.4 and hence was only slightly larger than M=6.0 the South Napa earthquake of August 24, 2014.
According to the Central Weather Bureau, Taiwan's government agency tasked with seismic monitoring, the peak ground acceleration caused by the quake's seismic waves did not exceed more than 30 percent of a "g", the standard gravitational pull of the Earth. Seismic engineers call this amount of shaking "moderate". It will certainly be felt but should not lead to a catastrophic building failure, particular in a country like Taiwan, which has one of the most stringent earthquake resilient building codes in the world.
|Map: Complex plate tectonics under Taiwan. The red star depicts the epicenter of Saturday's quake. (Source: National Central University, Taiwan)|
Even though the blogger does not know the history of the Weiguan Jinlong residential building with its 96 apartments, something seems fishy: Damage in cities hit by moderate or strong earthquakes is never concentrated just in a single building. If only one building fails catastrophically and everything else is left standing with only cosmetic damage, one can only assume that the structure had a fundamental flaw in its design, or that its construction was very shoddy.
The reason that Taiwan has such stringent and mostly well enforced building codes is the island's location. The country sits on one of the tectonically most complex regions in the world, at the boundary between the Philippine Sea and Eurasia plates. There the two plates converge with a velocity of about 3 inches per year. However, the collision of the two plates is not straight forward subduction like under South America. North and east of the island, the Philippine Sea plate subducts beneath Eurasia along the Ryukyu Trench. South of the island, the South China Sea (on the the Eurasia plate) subducts beneath the Philippine Sea plate in the Manila Trench.
Besides the earthquakes associated with this complex subduction, Taiwan is often pummeled by temblors originating within the island's crust itself. Taiwan is dissected by a major north-south trending fault, which leads to devastating thrust earthquakes. The strongest of these occurred in April of 1935 and cost 3200 people their lives. The location of the so called Chi-Chi Earthquake on September 21, 1999 was also on this thrust fault, its epicenter a mere 60 miles from Saturday's quake. It had a magnitude of 7.6 and more than 2500 people were killed. The Taipei Times, an English language newspaper in Taiwan, reported on Sunday that the company which built the now-collapsed towers in 1994 was also involved in the construction of some of the structures which collapsed during the Chi-Chi earthquake. (hra115)
It does not happen very often, that scientists are invited to the White House. Of course, once you've received a Nobel Prize, every president is happy to pose with you for a photo opportunity. Or you may have achieved something special in your research, and be honored with the Medal of Science. Or perhaps you have even discovered something very special, for which you deserve the Presidential Medal of Freedom - but don't count on it. Since the inception of the Freedom Medal in 1960, only 19 scientists have been bestowed with this high honor. So when a whole group of West Coast seismologists, among them several from our Berkeley Seismological Laboratory (BSL), gather at 1600 Pennsylvania Avenue in Washington on Tuesday, something unusual must be going on.
John Holdren, President Obama's Science Adviser, has invited experts to the first ever "White House Earthquake Resilience Summit". Politically, this is an unusual move indeed. When we look at the history of earthquakes in this country, we find that politicians and administrations mostly act retroactively: Only after a big temblor has caused major damage, are funds made available to strengthen buildings and structures, educate the public about earthquake risks, improve seismic networks or upgrade earthquake research facilities. In contrast, Tuesday's gathering is a proactive step. The folks in the White House Office of Science and Technology Policy want to strengthen earthquake resilience now, before the next Big One happens.
Rumor is that this initiative was triggered by an article in the New Yorker in the summer about the long trong earthquake in the Pacific Northwest. But this great story alone wouldn't have done the trick. Seismologists also have something important to offer to the public, which may indeed save lives during the next big quake. Their efforts in developing an Earthquake Early Warning (EEW) system for the West Coast are slowly paying off. The demonstration system, which has been operational for several years is ready to be upgraded to a "Prototype Production System". Instead of having a couple of computers scanning the seismic records continuously for signs of an earthquake, the upgrade uses a robust network of linked computers with a lot of redundancy to automatically perform the scans and run the warning algorithms. This new system is a collaborative effort between the BSL, Caltech in Pasadena and the USGS offices in Menlo Park and Pasadena.
However, even though it has the word "Production" in its name, the new system is still not ready for a big time roll-out to the public. Currently only a few beta-testers like BART and emergency management agencies in the Bay Area and in LA receive the alerts. The reader may rightly ask, why this system is not available to everybody, like similar warning systems in Japan, Taiwan and Mexico. There are at least three important reasons, which are holding us back. To make the warnings faster and more reliable, many more additional seismic stations are needed, particularly in Northern California, Oregon and Washington. In addition, we need to develop multiple ways to deliver the warnings to the public within fractions of a second. There are many means of distributing such warnings, from cell phone apps, emails, and sirens, to automatic announcements on TV screens and through the radio. But each of them has to be fast, reliable and also robust. The last thing one wants is that a hacker gets into the system and spreads false messages about impending shaking. And thirdly, a huge educational effort is needed to teach members of the public what to do, when an EEW message is received.
While seismologists have laid the foundations for Earthquake Early Warning, it is now up to all levels of government and the private sector to finish the job and bring this system to life. Tuesday's meeting at the White House seems like a good start. (hra114)
The second significant seismic event of the still very young year 2016 was not an earthquake at all. The first event last Sunday (Pacific time), a tectonic earthquake with a magnitude of 6.7, caused major damage in the far northeastern corner of India near the border with Burma. Several hundred people were either killed or injured. The next event, on Wednesday, had a much lower magnitude and did not cause any direct harm, but it was politically far more significant: The Democratic Peoples Republic of Korea, commonly known as North Korea, detonated its fourth nuclear weapon since the beginning of its atomic weapons test program in 2006. Seismometers all over the globe registered the seismic waves from this underground explosion. Politicians of all major countries, even North Korea's allies in China, expressed outrage. In the meantime, the propaganda machine in North Korea's capital Pyongyang jubilated, that for this latest weapons test, the country successfully detonated its first hydrogen bomb.
North Korea is currently the only country in the world which conducts nuclear weapons tests. Its test site Punggye Ri in the province of North Hamgyong is located about 235 miles northeast of Pyongyang and about the same distance to the southwest from the Russian Pacific port city of Vladivostok. North Korea began testing atomic weapons there in 2006, with further detonations in 2009 and almost three years ago on February 12, 2013. These tests have gotten progressively bigger. This can be inferred from the equivalent earthquake magnitudes computed from the strength of the explosions' seismic waves. These values increased from 4.3 in 2006, to 4.7 in 2009 to 5.1 in 2013.
According the the calculations of the scientists at the USGS National Earthquake Information Center (NEIC) in Golden, Colo., Wednesday's event also had a magnitude of 5.1. Germany's Geoforschungszentrum in Potsdam estimated a slightly higher magnitude of 5.2. Based on more than 2100 nuclear weapons test conducted by the five official nuclear powers (US, USSR, Britain, France and China) during the Cold War, these magnitude values can be converted into the strength of the explosion, the so called "detonation yield". Usually, an underground nuclear explosion which generates seismic waves with a magnitude of 5.1 has roughly the same yield as the detonation of about 7000 tons of the conventional chemical explosive TNT. As a comparison: the atomic bomb which destroyed Hiroshima close to the end of World War was about twice as strong as this explosion.
While seismological research has made it possible to determine the yield of an underground weapons test with reasonable accuracy, we seismologists can't tell which type of weapon was detonated. Therefore it remains to be seen, if North Korea's claim of having successfully tested a hydrogen bomb is true. However, even if Wednesday's test was used to perfect a "regular" atomic bomb, it is scary enough. (hra113)
When two earthquakes with magnitudes of 7.6 struck the Latin American country of Peru on Tuesday evening (local time) within five minutes of each other, one had to fear for the worst. After all, in contrast to its southern neighbor Chile, Peru is not known for its earthquake resistant structures and strictly enforced building codes. But, although the shaking of both quakes was widely felt across western South America, it caused hardly any damage. Even in the epicentral region in the sparsely populated Peruvian jungle near the Brazilian border to the east of the Andes, no buildings collapsed. The head of Peru's emergency services, Alfredo Murgueytio, was quoted by Reuters as saying that "there are no damages reported." Several residents of the Brazilian city of Brasileia, 150 miles east of the epicenter, told the same news agency, that they felt the ground shake and that chairs and tables rattled during the quake, but that there was no visible damage.
At first glance, the absence of any significant damage was very surprising. After all, the single quake which occured earlier this year in Nepal wreaked havoc in Kathmandu and other areas of the Himalayas . It had a magnitude of 7.8 and hence was only slightly stronger than Tuesday's double temblor.
What is so different about these two earthquakes in Peru and the one in Nepal? The Nepal quake caused massive destruction with over 9000 casualties while the doublet on Tuesday hardly damaged anything at all. The main reason for the mild effects of the two Peruvian earthquakes was their focal depth. While the Nepal earthquake had its origin less than 10 miles below the surface, seismologists at several seismological centers around the world calculated the depth of the Peru double quakes to be between 375 and 400 miles below the surface. By the time seismic waves from such deep earthquakes reach the surface, they have lost a lot of their initial energy.
Think about what would happen when an earthquake with a magnitude of 7.6 were to occur in the Los Angeles area. While we are sure that it will cause quite a bit of damage in the epicentral region down south, we do not expect this earthquake to cause significant structural damage in the Bay Area. The seismic waves may be felt in our area around the San Francisco Bay, but they will have lost much of their punch on their almost 400 mile long journey way northward. It so happens that the distance between San Francisco and LA is similar to the depths of Tuesday's earthquakes in Peru.
While deep earthquakes like the two on Tuesday under Peru are usually rather gentle on buildings and structures on the Earth's surface right above their focus, they can be felt over a surprisingly large area. A magnitude 8.2 quake, which had its source almost 450 miles below Bolivia in 1994, made skycrapers in Toronto sway. Similarly, the deepest earthquake ever measured, the magnitude 7.8 quake in the Izu-Bonin-Trench in late May this year, caused a slight swinging movement of a 50 story building in Pasadena. The distances between Bolivia and Toronto and the Bonin Trench and Southern California are 4500 and 5500 miles respectively.
What is the reason for such ultra-long distance effects? In contrast to shallow focus quakes, the seismic waves of deep temblors do not have to travel through the Earth's crust on their way to distant locations. They start their journey in the Earth's mantle. Compared to the Earth's crust, this section of the Earth's interior is much more homogeneous, and therefore attenuates seismic waves much less strongly than the more heterogeneous rocks of the crust.
In summary: deep earthquakes usually cause less damage than shallower ones, but they can be felt over much, much longer distances. If you happen to be in a skyscraper which gently rocks from side to side, the motion is not necessarily due to the wind. The swaying of a tall structure can be caused by seismic waves from an earthquake continents away. (hra112)
|More than 400 small earthquakes have occured in the seismic swarm under San Ramon during the last two weeks (Map: USGS)|
There was an era in earth science more than thirty years ago, when earthquake prediction was at the top of the seismological research scale. After some apparent successes in China and other places, seismologists thought they were close to being able to tell the world exactly where and when devastating temblors would occur. Parkfield, a tiny hamlet in central California northeast of Paso Robles, was the epicenter of this research. There, where the San Andreas Fault is so strikingly exposed, dozens of scientists from the US and abroad placed hundreds of sensors into the ground, hoping to catch the latest precursor. Unfortunately, after more than a decade of enormous effort in Parkfield, earthquake prediction proved as elusive as the search for the Midas' touch to turn everything into gold.
One of the questions seismologists tried to answer was, if earthquake swarms could be the prelude to a Big One. This hypothesis got some notoriety in the summer of 1975. Six years before, the construction of the Oroville Dam north of Sacramento had been finished and this tallest earthen dam in the US had impounded the Feather River without any problems. But when suddenly in June 1975 earthquakes started to happen under the reservoir, authorities became worried. Would the quakes shake to dam to pieces? However, within a month the swarm had subsided and people breathed a little easier - until August 1, when a quake of magnitude 4.7 shook the dam. Bruce Bolt, then the director of the Berkeley Seismological Laboratory, was asked if there was more to come. He said it was possible and indeed, a few hours later, a magnitude 5.7 quake happened in the area of the swarm.
Not only scientists began to ask whether Bolt's statement was a true prediction or if the occurence of the strong quake under the dam was an untimely coincidence. Even though we seismologists have clearly failed to find any reliable precursor so far, the same question is being asked again today, as it was after the Oroville incident: Is the current earthquake swarm under San Ramon and Danville in the East Bay a prelude to something bigger? Starting with a 0.8 magnitude microquake on October 13, more than 400 temblors have since occured along the Calaveras Fault under the Crow Canyon Country Club. At least eight of these quakes had magnitudes greater than 3 and were clearly felt all the way to Concord in the North and San Jose to the South. As of this writing, the swarm goes on and on.
To say it very clearly: Earthquake swarms are not at all an indicator that something bigger is about to come. Too many times such swarms have happened without culminating in a big, destructive quake. This is especially true for the I-680 corridor. In May a swarm under Concord came and went. And since 1970 the northern segment of the Calaveras Fault in the region of Alamo, Danville and San Ramon has seen at least four swarms, most recently in 2003. Each lasted for weeks or months, but so far none was closely followed by a significant quake.
Does that mean we can exclude that a bigger, destructive quake will happen soon on this section of the Calaveras Fault? No, we can't! This segment of the fault has not had a significant quake since it last ruptured in the 1860's. Given the stresses continuously exerted on it by the movements of the tectonic plates here in the Bay Area, the likelihood of a quake of magnitude 6.7 or greater under San Ramon and Alamo in the next 30 years is about eight percent. At first glance this number seems very low - but it is not zero. Such a quake can happen today, next week, in thirty years or even later than that. And this is independent of whether the area is experiencing an earthquake swarm or not. (hra111)