Friday, 24 April 2026, 7:08 am
Article: The Conversation
Jesse
Kearse, Te
Herenga Waka — Victoria University of
Wellington
The magnitude of an
earthquake depends on how far a rupture travels along a
fault line before it stops. For the first time, we have now
directly observed how a large earthquake comes to a
halt.
By analysing seismic recordings taken within
a few kilometres of faults, we have identified a ground
motion signal we call a “stopping phase”. It records the
moment the earthquake stops.
This discovery, published in
Science today, provides direct seismic evidence that
large ruptures stop suddenly, rather than slowing down
gradually.
It also helps identify where shaking
may be strongest along strike-slip faults, which could help
with disaster planning and preparation.
Earthquakes
grow as the rupture spreads along a fault. The further it
goes, the larger the earthquake magnitude becomes. The vast
majority of earthquakes stop before they grow large enough
to be felt.
For example, the earthquake monitoring
network GeoNet has recorded
more than 20,000 earthquakes in Aotearoa New Zealand
over the past year, but all stopped soon after they
began.
A very small percentage of earthquakes keep
on going, and can travel for many hundreds of kilometres
before coming to a halt, reaching up to magnitude 9 and
often causing widespread damage.
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Although rupture
stopping is clearly important for determining earthquake
magnitude, the process itself has been extremely difficult
to observe directly.
Our findings now provide
planners with a new way of identifying where the most
damaging ground motions are likely to happen along a fault
line.
A hidden signal of reversed ground
motion
Using seismic, GPS and satellite data from
12 large strike-slip earthquakes worldwide, we found
consistent patterns of ground motion at the ends of the
faults which was not observed at their centres.
We
observed that during the final moments of the earthquake,
the ground suddenly moves in the opposite direction to the
fault’s motion, creating a “whiplash” effect.
It’s like riding in a fast car that suddenly
slams on the breaks – your body continues to go forward,
but then sharply whips back the other way.
The
stopping phase of an earthquake is similar, with the
reversal in ground movement signalling a rupture’s abrupt
halt.
As the stopping phase arrives at the
Earth’s surface from depth, the ground can change
direction in a fraction of a second, and move up to a metre
or more backwards.
Because it occurs close to the
fault and over such a short time, this process has not been
recognised before. These signals are only becoming clear
with increasingly dense modern seismic monitoring
networks.
We simulated earthquakes using numerical
models and were able to reproduce the stopping phase
observations only when the rupture stopped suddenly. When
the model forced a rupture to slow down gradually, the
signal disappeared.
This animation simulates
an earthquake, showing the abrupt arrest and stopping phase
ground motion. Video by Jesse
Kearse.
Do
all earthquakes stop like this?
This research
focuses on large strike-slip earthquakes – events where
the ground moves sideways along near-vertical faults, rather
than up and down.
Because near-fault observations
are so rare, the science is severely data limited.
Strike-slip faults provide the best opportunity to study
these processes because they often rupture all the way to
the Earth’s surface, where instruments can capture ground
motion close to the source.
Abrupt arrest may also
occur on other types of faults, such as subduction zones
where one tectonic plate slides beneath
another.
But we need further research to confirm
whether this process is common to all earthquakes or only
strike-slip fault systems such as the San Andreas Fault
running along the coast of California in the US or New
Zealand’s Alpine Fault.
The domino effect of
cascading rupture
Large strike-slip faults are
made up of a series of segments, linked together end to
end.
To become large events, earthquakes that ride
these fault networks must break multiple segments in one go.
In the seismic data from one such event in
Turkey-Syria in 2023, we found stopping phases were recorded
both at the end of the earthquake and at the edges of
internal segments.
This stop-start behaviour during
a single event means an earthquake rupture comes to an
abrupt halt at the end of one fault segment, before
triggering a slip on its neighbouring segment, like a
cascade of falling dominoes.
How this helps
people living near fault lines
Stopping phases
help pinpoint where earthquake shaking may be most
intense.
Our research shows these signals occur
near the ends of ruptures and at boundaries between fault
segments – places where earthquakes are most likely to
stop.
This means the strongest and most complex
shaking is likely to occur at the edges of fault segments,
rather than their centres.
For strike-slip fault
systems, this is particularly useful because fault segments
and ends can be mapped before an earthquake happens.
This allows scientists and planners to identify
locations where damaging ground motions are expected to
occur.
Incorporating stopping phases into hazard
models will improve how we anticipate shaking for cities
near faults, such as New Zealand’s capital Wellington.
Jesse
Kearse, Postdoctoral Research Fellow, Te
Herenga Waka — Victoria University of
Wellington
This article is
republished from The
Conversation under a Creative Commons license. Read the
original
article.