Concepción is the natural laboratory to understand the spatial difference in seismic demand

The magnitude of 8.8 on the MW scale made the February 27, 2010 (27F) a megaquake; the second strongest in the history of Chile after that of Valdivia in 1960 and the eighth most powerful of which humanity has registered.

It affected several regions, but the most devastated were Maule and Biobío. Its inhabitants know that they survived a cataclysm, a devastating tsunami and a major collapse of structures that claimed 525 lives and left 23 people missing.

It is also known that “there are areas of the city where there was much more damage than in others,” highlighted Dr. Gonzalo Montalva, an academic from the Department of Civil Engineering of the Faculty of Engineering of the University of Concepción (UdeC) and a researcher at the Nucleus Milenio Cyclo (Seismic Cycle Along Subduction Zones).

But, he warned, it is not known precisely or with objective evidence what was the difference in movement between one area and another. “By 2010 we had no more instrumentation installed than old equipment that starts measuring when the earthquake starts. Therefore, the entire previous phase, which is very important, is not recorded and is lost. Now we do have instrumentation and information, ”he said.

Less magnitude, more data

Now the story is different and with data from an earthquake of lesser magnitude and impacts: the 6.2 MW with its epicenter in Lebu on November 12, which caused minor damage to infrastructure in nearby communes and was felt in Greater Concepción.

Measuring the event served to better estimate the behavior of the city and its structures in the face of earthquakes, distinguishing significant spatial differences. “The differences are up to 6 times between one area and another. Therefore, it is not surprising that after a large earthquake there are areas with much more damage than others,” Montalva said about the information obtained from the Alluvial Basin Site Response Observatory (Orsca) that was implemented through the Fondequip 160015 project. who leads

Initiative that stood out as a pioneer in South America and uses Concepción as a natural laboratory, with an alluvial basin with highly variable basal rock, to investigate the effects of surface or site geology on earthquakes; topic of global interest. The geology of the site is the characteristics of the rocks and soils in the upper 500 meters, she said.

Specifically, Orsca showed that the intensity that affects deeper areas and especially tall buildings is up to 6 times higher, versus an area with a rocky basement of about 60 meters. Montalva clarified that said condition was observed as a result of 27F, but without having information about the previous process.

Due to the new local evidence, he asserted that the expected structural response will depend on the location and structural typology of the civil works. Reason why the amplification for tall structures could be recurrent in the central Concepción area, which with 180 meters is the deepest in the basin.

Observatory

“Orsca has the main objective of understanding the differences in seismic demand and structural behavior in different areas of alluvial basins. We installed it in Concepción, but the interest is to generate scientific knowledge that goes beyond the city”, highlighted Montalva about the laboratory made up of seismometers that cover various conditions of the site, soil and rock, as well as different depths to the rocky basement.

There are 6 surface teams that are distributed in places with different geological and geotechnical conditions: the Freire-UdeC Home, the UdeC Civil Engineering Geotechnics Laboratory, the UdeC Earth Sciences Department, the Sagrados Corazones College, the Fourth Fire Company of Concepción and the Faculty of Law of the Catholic University of the Santísima Concepción.

In addition, two instruments are part of an observation well down to the rocky basement that was built at Hogar Freire-UdeC. One of the seismic stations is at a depth of 40 meters and another is in the bedrock at a depth of 80 meters.

And to reveal its potential impact, the academic said that the observatory imitates a European project with more time and resources, but also emphasized that “we have fewer instruments, but much more seismicity than the European ones and we can get much more information.”

Orsca: data that can reduce uncertainty regarding the seismic hazard from Concepción

The Orsca obtains information from waves that propagate through the earth’s crust, explained Gonzalo Montalva. It even measures without tectonic activity and records environmental sound effects such as waves or city noise.

“When a wave of any type propagates through a medium, its shape, amplitude and frequency change. And when it reaches the surface, where we have our structures built, in some cases they are going to be very different,” he said.

Hence the significance of the laboratory in an area as seismic as Concepción in particular and Chile in general, whose territory is on the subduction zone between the Nazca and South American plates.

Advance regulation

As a result of the data being processed to transform it into knowledge, the academic stated that it can improve the prediction of the specific seismic response of the site under analysis. It is that it seeks to reduce the uncertainty associated with the seismic hazard of Concepción with extrapolation to other alluvial basins in Chile and the world.

Because he asserted that “a better understanding of the seismic demand and the structural behavior of different areas has important practical implications: it allows progress towards a regulation of specific constructions for alluvial basins such as Concepción or Valdivia and Viña del Mar that have a similar situation.”

Progress that is critical in light of the evidence: “for the current standard, all sites are identical and all structures are designed as if the seismic demand were the same. It is current in Chile, Japan (also in the subduction zone) and everywhere, ”he warned.

The reason that Montalva gave for a dangerous paradigm is that “the most superficial meters are evaluated for their engineering properties. But the response we see is more linked to deeper phenomena related to how the basin is structured, which are at least 200, maybe 500 or 1,000 meters deep.”

Of course, he highlighted the project to update the seismic design regulations for buildings in Chile that is in public consultation and introduces improvements in this aspect. And he revealed that evolution has been influenced by the information provided by Orsca.

The Seismic Threat Studies standard is also being studied at the request of the Ministry of Housing and Urban Planning, under coordination of the Construction Institute and the leadership of Montalva.

This is intended to standardize seismic hazard studies for specific sites; analysis that allows improving what a design standard determines for a structure in seismic terms and, even, determining design values ​​for special situations that are outside the scope of existing design standards. Another context in which the Orsca data is of special relevance.

impact that transcends

The Orsca provides data to feed different investigations. One is the Fondecyt project led by Montalva on soil liquefaction, which he defined as the second cause of structural damage and deaths in earthquakes. The first is tsunamis and they don’t always happen.

Liquefaction is determined by the existence of a soft and saturated soil (full of water) and the seismic demand. “The more saturated the soil is, the more likely it is to liquefy. And more likely to liquefy if the intensity of the earthquake at that point is greater, ”he said.

The work proposes new methodologies to calculate the liquefaction in subduction zones, particularly to determine where it will occur before an earthquake. “For this, instrumentation and understanding the seismic demand is very important,” he assured.

And, of course, Orsca provides key information for the Cyclo, where they seek to address the complete seismic cycle in subduction zones.

The results of these studies, he affirmed, have a tremendous social impact in the country, for example, determining the seismic threat in pursuit of better decisions.

In this regard, he clarified that, due to the characteristics of the territory, the population of all of Chile is threatened by all types of earthquakes. But, he maintained that “from rationality, mixing all seismogenic sources and understanding how seismic energy accumulates and what signs could be indicators of a next earthquake is very important.”

In this sense, he stressed that “we can calculate the probability of an event of greater intensity that is capable of significantly damaging a structure in an area. And if we can do it, we can take steps to design better structures in a high-hazard area or cheaper ones in areas where you don’t need as much security.”

And the potential impact of the Orsca operation is not limited to the works or entities in which Montalva is involved: the data it records is available in http://www2.udec.cl/~geotecnia/. “They can access from all over Chile and the world to download and use the data for academic, teaching, professional or industrial purposes. They are technical data, but in some cases very useful ”, he closed.