'Beach ball' representations calculated for US underground nuclear tests
can aid monitoring

Date:
September 21, 2021
Source:
Seismological Society of America
Summary:
Researchers have calculated moment tensors for 130 underground
nuclear and 10 chemical test explosions that took place at the
Nevada National Security test site.



FULL STORY ========================================================================== Researchers at Lawrence Livermore National Laboratory have calculated
moment tensors for 130 underground nuclear and 10 chemical test explosions
that took place at the Nevada National Security test site.


========================================================================== Often represented graphically to resemble a striped "beach ball," moment tensors "are essentially a mathematical concept that's used to describe
forces that drive earthquakes," said Andrea Chiang, co-author of the new
study published in the Bulletin of the Seismological Society of America.

Instead of the forces along an earthquake fault, the moment tensor
solutions calculated by Chiang and Michael Pasyanos represent the
forces of implosion, explosion and collapse that occurred during the underground tests. These calculations, the researchers show, can be
used to distinguish explosions from earthquakes, and estimate yield for
the explosions.

Pasyanos and Chiang said compiling an extensive database of moment tensor solutions for this unique set of carefully documented explosions will be
useful for researchers tasked with explosion monitoring. Their data set reflects nuclear explosions at the site from 1970 to 1992, and chemical explosions from 1993 to 2019.

"Because of the moratorium on nuclear testing, it's kind of more important
than ever to do analysis on legacy data," said Pasyanos. "We're hoping
that this data set can be used by others in the community to test and
validate and improve on methods." In the past, moment tensor solutions
were less useful for explosion monitoring than earthquake analysis due
to assumptions made in the equations -- such as ignoring any volume
changes during the event -- that sidestepped the important physics of explosions. More recently, work by Chiang and others has refined these equations to include this type of variable.

In some preliminary tests, the researchers were able to use aspects
of the Nevada moment tensor solutions to categorize seismic events as
either earthquakes or explosions. They also tested whether moment -- a combination of force, slip and area of fault rupture that seismologists
use to determine earthquake size -- could be used to determine yield
for an explosion.

"We feel as a community that moment and moment magnitude is kind of a
better quantifying parameter for an earthquake, for instance, so the
natural question is, is it also better for yield estimation?" Pasyanos
said.

The study suggests that moment doesn't always translate directly to
yield, but is complicated by the type of rock surrounding the explosion,
as earlier studies have also indicated.

"If the explosion happens in strong rock, that will couple well and
produce a high seismic moment," Pasyanos explained. "If it's in weak rock,
the same size explosion would couple poorly, and you'd get a smaller
seismic moment for the same yield." The Nevada explosion recordings are
a mix of analog and digital data, and the study highlights the importance
of preserving seismic data sets for future analyses, the researchers
note. In many cases, it can be a race against the clock to preserve data
on media that is physically deteriorating or can no longer be read with
current technologies, or a time-consuming effort to read and digitize
paper records.

Chiang also stressed the importance of the people who worked at seismic networks, and the details they can contribute about the stations and instruments, in parsing past seismic records. "It's important to preserve
their knowledge as well," she said.

========================================================================== Story Source: Materials provided by
Seismological_Society_of_America. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Michael E. Pasyanos, Andrea Chiang. Full Moment Tensor Solutions
of U.S.

Underground Nuclear Tests for Event Screening and Yield Estimation.

Bulletin of the Seismological Society of America, 2021; DOI:
10.1785/ 0120210167 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210921134342.htm

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