Dispatch from AGU-Temblor.net

Friday’s coverage of the Fall 2020 meeting of the American Geophysical Union (AGU) focuses on preparing for cascading hazards, looking for unusual earthquakes in the subduction zone and helping shed light on earthquake hazards in Haiti.

Written by Megan Siefer, science writer and editor (@ megansever4)

Citation: Sever, M, 2020, Dispatches from AGU, Temblor, http://doi.org/10.32858/temblor.145

The fall meeting of the American Geophysical Union (AGU) may not sound much normal this year – especially since I am writing this from my home office in Portland, Oregon, rather than the media room at the Moscone Center. But that doesn’t mean that not many exceptional search results are presented. Today, Friday, December 11th, I found dozens of posters and chats that I wanted to explore. Below, you’ll find a little light, in case you missed it.

Cascading risks

Several sessions today focused on the idea of ​​cascading risks, whether predicting them or building community resilience. Cascading hazards, also known as compound hazards, concurrent or cascading hazards, are natural hazards that depend on each other – think floods caused by hurricanes or fires caused by earthquakes that break a gas line. The terminology also sometimes includes the built environment, such as water pipelines, fiber-optic cables for communications, or power plants, which are often built along coasts and in populated areas that are already exposed to natural hazards. When the built environment is contained, cascading hazards are often called “natech” events: technological disasters caused by natural hazards. The organizers of the natech session wrote today that these events “have a ripple effect on risks to society, infrastructure and the environment”. But they write that these risks are largely overlooked in disaster risk science and management.

One poster showed that more than half of all structures built in the neighboring United States over the past 70 years are in hazard hotspots. The research team reports that more than 1.5 million of these structures are located in hotspots of two or more hazards.

Virginia Iglesias, an environmental scientist at the Earth Laboratory at the University of Colorado Boulder (CU) and a team from CU, the University of Florida and Boise State, first identified “hotspots to emerge and scale the most damaging risks” from 1945 to 2015.These hazards include floods, earthquakes, hurricanes, hurricanes and wildfires. Next, the team used a dataset showing the more than 350 million buildings where people live, work, and recreate to estimate development trends over the same period. Analyzed together, these two datasets reveal the extent, pattern and trend of exposure to risk in the United States [the contiguous U.S.], “They wrote on their poster.

These hotspots cover 31% of the land in the lower 48 states but account for 57% of the structures built in hot risk areas. In addition, the team found, “the number of structures in typhoon and earthquake hotspots has increased faster than the national average.”

These findings confirm what disaster scientists have been saying for some time: “For decades the United States has been on a precarious and unsustainable development path, preferentially placing more assets at risk of more frequent and severe natural hazards.” This situation, they write, is exacerbated by Americans’ perceptions of “building back larger” after disasters, which put more property value at risk and outweighs any gains in mitigation. Other factors compounded by the fact that more and more people are moving to coasts, where many of the most devastating risks are occurring, and that weather and climate risks are getting worse due to climate change.

I want to shout about the results of this rooftop team. everybody get up! The risks will not disappear. We need to be smarter about how and where to evolve!

Discovering Haiti’s Hidden Flaws

On January 12, 2010, Haiti was hit by a magnitude 7.0 earthquake on an unidentified fault. Not only was it devastating, killing more than 200,000 people, but it was also surprising to scientists, although it was not the first devastating earthquake to strike an unplanned mistake in Haiti. So researchers have spent the past decade trying to ensure that this doesn’t happen again. “It is imperative to verify if there are more unknown energetic structures that could increase the seismic threat this region actually faces,” wrote Hsin-Yu Lee, a graduate student at the University of California, Riverside, and an international team in a city. a poster.

Haiti has many active bugs, some of them are yet to be set. Credit: Lee et al., 2020

In their poster, Lee and her team presented preliminary results from an ongoing study to identify previously unknown faults in this geologically complex state. They started by perusing a yearlong earthquake catalog produced from a new network of 27 seismometers published in 2013. However, this catalog lacked a lot of seismic activity: all the small, frequent earthquakes that are often overlooked, told me to Temblor: Earthquake background noise. To identify faults that were previously unknown, the team developed a technology to spot those missing earthquakes and thus expand the catalog. “By detecting and transmitting those earthquakes, we can highlight active, planned and unassigned faults,” Lee said.

Daily earthquakes occur between January 2014 and June 2014. The red bars are published and the earthquakes and blue bars are those included in the new catalog. Credit: Lee et al., 2020

In their poster, the team wrote: “In this study, we use events carried from the recently published catalog as template waveforms and apply an identical filtering technique linking these blocks to continuous data in order to discover missing events.” The team tripled their previously published earthquake catalog and located more than 1,000 earthquakes that were not in the previous catalog between January and June 2014, Lee Templore told me. “With the addition of these new discoveries, we have been able to better define structures in southern Haiti,” she said. Ultimately, she said, this could lead to “an improvement in the seismic risk map for this area, which could be used by engineers in designing buildings.”

He told me this was just the beginning of the team’s work. The team noted in the poster that much more needs to be done to add to the earthquake catalog at longer timescales and to use additional seismic data from the Dominican Republic.

Search for unusual earthquakes in subduction zones

Subduction zone earthquakes are complex monsters. Earthquakes can occur in the inclined plate mantle, on the crust of the slope, on the subduction interface or in the mantle wedge, and it is really difficult to locate earthquakes in the system. But it is important because the location of the earthquake helps elucidate the generation mechanisms, according to Felix Ballab, a postdoctoral researcher at the University of Bergen in Germany, and his team, who presented their poster today.

Although it is generally believed that medium-depth earthquakes occur in the plate mantle and in the plate crust, the team wrote in their summary, “Recent observations indicate that some medium-depth earthquakes may occur in other unusual locations, such as over the deep section (> 50 km). ) From the subduction front, and at the top of the mantle wedge. However, current techniques for determining the location of these earthquakes cannot determine these locations.

So Palab and his team tried to determine the location of these deep earthquakes from a waveform, or seismic waveform. They started by looking at the shapes of small, local earthquake waves. They then modeled and analyzed how the waves exit from the general simulated subduction zones using a new three-step approach. They were able to characterize the features of waveforms emerging from each site – the inclined plate mantle, fused shell, subduction interface, and mantle wedge. They then applied this approach to the West Hellenic subduction zone, where previously clusters of earthquakes had been observed in the mantle wedge, crust plate and façade.

“Our results highlight the powerful potential of full waveform analysis of local earthquakes to precisely locate hypocenters in relation to the subterranean structure, and by doing so, search for earthquakes that may occur in unusual environments,” the team wrote.

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