Science - USA (2020-09-25)

SCIENCE sciencemag.org 25 SEPTEMBER 2020 • VOL 369 ISSUE 6511 1555

PHOTO: KATE CLARK/GNS SCIENCE

M

any of the world’s most danger-

ous earthquake faults are a silent

menace: They have not ruptured

in more than a century. To gauge

the hazard they pose to buildings

and people, geologists cannot rely

on the record of recent strikes, captured by

seismometers. Instead, they must figure out

how the faults behaved in the past by look-

ing for clues in the rocks themselves, in-

cluding slickenlines, scour marks along the

exposed rock face of a fault that can indicate

how much it slipped in past earthquakes.

Now, researchers in New Zealand say

slickenlines, when curved, can also reveal

which end of a fault slipped first. “This is

important information to know,” says Jean

Paul Ampuero, a seismologist at the Cali-

fornia Institute of Technology who was not

involved in the work. Knowing how an

earthquake ruptured in the past could help

seismologists refine hazard assessments for

cities, such as Los Angeles and Istanbul,

that sit at the end of known faults. That’s

because earthquakes beam their energy in

the direction of rupture, Ampuero says. “If

an earthquake is coming toward you, it’s

coming to kick you in the face.”

Earthquakes don’t happen all at once.

Rather, the slip between rocks begins

at one spot on the face of the fault—the

hypocenter—and travels along it, like a

zipper being unzipped. As the rupture ad-

vances, the earthquake waves it generates

pile up and intensify, like the siren of an

approaching ambulance. Los Angeles lies at

the northern terminus of the southern San

Andreas fault, Ampuero notes. “If it breaks

north, toward LA, that would be pretty bad.”

The researchers first noticed the curved

slickenlines after the 7.8-magnitude Kaiko-ura

earthquake, which struck New Zealand’s

South Island in 2016. It was a chaotic event:

The quake propagated from the southeast

to the northeast and its energy jumped from

fault to fault, causing dozens of them to slip

and shake. One, the Kekerengu, crosses

a series of canyons, creating more than a

dozen exposures that revealed the marks

of rock scraping against rock. Soon enough,

the team noticed a consistently curved pat-

tern in these striations on one side of the

fault, says Jesse Kearse, a doctoral student

in earthquake geology at Victoria University

of Wellington and co-author of the work. “It

was like a rainbow.”

More than 2 decades ago, Paul Spudich,

a seismologist who died last year, had seen

the same pattern in slickenlines from a Jap-

anese earthquake. But no one had shown

that such curves indicated anything about

the rupture’s direction. Kearse showed his

findings to Yoshihiro Kaneko, an earth-

quake dynamicist at GNS Science, New Zea-

land’s national geoscience research center.

Kaneko thought his models could reproduce

the motion that caused such an arch. When

they fed his model with the Kekerengu data,

the same shapes appeared. As in the real

world, they formed on the side of the fault

that had slipped toward the northeast.

The model also suggested how the arches

form. The fault slip was mostly horizontal,

but once the rupture reached the surface,

no overlying rock constrained its slight up-

ward motion. That allowed the side of the

fault traveling with the rupture to bend

slightly upward before leveling off. “It gets

dragged off course by the seismic waves and

makes an arch,” Kearse says.

That work, which Kearse and Kaneko

published last year in Geology, covered just

one earthquake. Kearse then plunged him-

self into the literature, unearthing 60 large

historical earthquakes that had reached

the surface and been documented by a geo-

logist. Of those, one-third had curved slick-

enlines. Only eight of those 20 had the other

constraints Kearse and Kaneko needed to

test their model, like the earthquake’s hypo-

center. But for all eight, the location of the

curving slickenlines corresponded to the

rupture direction predicted by their model,

regardless of the earthquake’s magnitude

or fault type, they reported in a paper pub-

lished last month in the Journal of Geophysi-

cal Research. “It’s really compelling,” says

Katherine Scharer, a paleoseismologist at the

U.S. Geological Survey. “I’d love to see people

go out after every rupture and see if they can

document this.”

If similar slickenlines are discovered for

older faults, that won’t immediately trans-

late to better risk assessments, cautions

Laura Wallace, a geodetic scientist also at

GNS Science. Just because a fault ruptured

in one direction in the past does not neces-

sarily mean it will break in the same direc-

tion again. There are physical reasons to

suggest that might be true, but the modern

record is simply too short to say for sure.

“It’s a huge question,” she says.

Slickenlines may hold the answer to that

question, too, Scharer says. Not every fault

has the right soft mudstones to preserve

these lines, and even those that do likely only

retain the last rupture. But under just the

right conditions, a fault might capture mul-

tiple ruptures, she says, giving researchers

a chance to look for sets of curved slicken-

lines that indicate the directions of multiple

earthquakes. “It’s a precious moment of time

being recorded.” j

Curved scour marks trace the

directions of ancient quakes

“Slickenlines” etched in rocks could help refine shaking

hazard for cities near the ends of faults

PALEOSEISMOLOGY

Researchers found curved slickenlines at nine

exposures of the Kekerengu fault in New Zealand.

By Pa u l Vo ose n