Scientific American 201905

ADVANCES

18 Scientific American, May 2019

Illustration by Thomas Fuchs

TURJOY CHOWDHURY

Getty Images

SEISMOLOGY

Missing

Earthquakes

Found

Geologists fill in mysterious gap

in Nepal’s quake record

Like a slow-motion car crash, the Indian
subcontinent is colliding with Eurasia. This
impact, along a fault known as the Main
Himalayan Thrust, is the force driving the
rise of the Himalayas. In April 2015 it trig-
gered Nepal’s magnitude 7.8 Gorkha
earthquake, which destroyed villages and
parts of Kathmandu, killing thousands.
Nepal is no stranger to such temblors—
but in the western part of the country, no
significant earthquakes have been recorded
since 1505. Such a “seismic gap” could be
bad news: if the region’s faults are not rel­
easing their pent­up stress every so often,
one or more very large and potentially cata-
strophic earthquakes could result. “With
more than 500 years of waiting, the stored
energy due to the convergence of India
[and Eurasia] could be considerable,” says

Zakaria Ghazoui, a geologist at the Institute

of Earth Sciences in Grenoble, France. The

sudden release of this energy could devas-

tate nearby places such as Pokhara, one of

Nepal’s largest cities.

To determine whether this seismic

gap actually exists, Ghazoui and a team

of researchers ventured onto Rara Lake in

western Nepal’s Himalayas and retrieved

cores of sedimentary layers from the bot-

tom. They suspected the cores might con-

tain records of when any past earthquakes

occurred because quakes can cause under-

water avalanches that leave behind layers of

what Ghazoui calls “disorganized” sediment.

The team found evidence of at least

eight avalanches since 1505, correspond-

ing to moderate to large quakes. “We were

hoping to find the trace of the 1505 earth-

quake, but the discovery of the other

earthquake­triggered avalanches was a

real surprise,” he says. This means that the

area’s faults may not be storing as much

stress as has been thought. But it also

NEURAL ENGINEERING

Decoding

Speech

New approach is a step

toward translating thoughts

into machine­spoken words

Neurological conditions that can cause
paralysis, such as amyotrophic lateral scle-
rosis (ALS) and strokes in the brain stem,
also rob many patients of their ability to
speak. Assistive technologies enable key-
board control for some of these individuals
(like the famed late physicist Stephen
Hawking), and brain­computer interfaces
make it possible for others to control
machines directly with their thoughts. But
both types of devices are slow and imprac-
tical for people with locked­in syndrome
and other communication impairments.
Now researchers are developing tools to

eavesdrop on speech­related brain activity,

decode it and convert it into words spoken

by a machine. A recent study used state­of ­

the ­art machine learning and speech­syn-

thesis technology to yield some of the most

impressive results to date.

Electrical engineer Nima Mesgarani of

Columbia University’s Zuckerman Institute

and his colleagues studied five epilepsy

patients who had electrodes implanted in

or on their brain as part of their treatment.

The electrodes covered regions involved in

processing speech sounds. The patients

listened to stories being read aloud as

their brain activity was recorded. The team

trained a “deep learning” neural network to

match this activity with the corresponding

audio. The test was then whether, given

neural data it had not seen before, the sys-

tem could reproduce the original speech.

When the patients heard the digits zero

through nine spoken four times each, the

system transformed the neural data into

values needed to drive a vocoder, a special

kind of speech synthesizer. A separate

group of participants heard the synthe-

sized words and identified them correctly

75 percent of the time, according to the

study, published in January in Scientific

Reports. Most previous efforts have not

measured how well such reconstructed

speech can be understood. “We show that

it’s intelligible,” Mesgarani says.

Researchers already knew it was possi-

ble to reconstruct speech from brain activ-

Aftermath of the 2015 Nepal earthquake in Sankhu.

© 2019 Scientific American © 2019 Scientific American