Better prediction of megathrust earthquakes: Illuminating slow slip
plate tectonics in south-western Japan

Date:
February 14, 2022
Source:
Kobe University
Summary:
Within the next 30 years, a highly destructive Nankai
Trough megathrust earthquake is predicted to hit southwest
Japan. Understanding long-term slow slip events that occur along
the plate interface between the subducting Philippine Sea plate
and overriding Amurian plate under the Bungo Channel is essential
for pinpointing when such an earthquake will happen.



FULL STORY ========================================================================== Within the next 30 years, a highly destructive Nankai Trough megathrust earthquake is predicted to hit southwest Japan. Understanding long-term
slow slip events (L-SSE (*1)) that occur along the plate interface between
the subducting Philippine Sea plate and overriding Amurian plate under
the Bungo Channel is essential for pinpointing when such an earthquake
will happen. To this end, Kobe University's Professor YOSHIOKA Shoichi (Research Center for Urban Safety and Security) and SESHIMO Yukinari
(1st year Masters student, Department of Planetology, Graduate School of Science) analysed the 2018-2019 Bungo Channel L-SSE using longitudinal
GNSS data (*2) provided by the Geospatial Information Authority of Japan.


==========================================================================
They revealed that even though the 2018-2019 event was shorter than past
L-SSEs in this region, it was also bigger in terms of slippage amount
and slip velocity, as well as seismic moment and moment magnitude. This
and their other findings provide further insight into the behaviour of
L-SSEs in the Bungo Channel.

These results were publicized in the online journal Scientific Reports
on January 10, 2022 at 10 am GMT. This journal is published by the Nature Publishing Group (UK).

Main points
* Illuminating slow slip distribution in the Bungo Channel is
important for
understanding the causal mechanism behind a Nankai Trough megathrust
earthquake, which has been predicted to occur in the near future.

* This research group illuminated the spatiotemporal distribution
of the
slow slip event that occurred in 2018-2019
* Despite taking place over the shortest time period, the 2018-2019
event
was the highest recorded so far in terms of slippage amount and
slip velocity, as well as seismic moment (*3) and moment magnitude
(*4) compared to past slow slip events in the Bungo Channel.

*

=========================================================================
The 2018-2019 event resembled the 2002-2004 slow slip in terms
of starting location, slip distribution and the order in which
sub-events occurred.

Research Background In the Bungo Channel, which is located between Shikoku
and Kyushu in southwest Japan, the Philippine Sea plate is subducting
in a north-westerly direction beneath the Amurian plate. Every 6 or so
years, long-term slow slip events (L- SSE) repeatedly occur along the
plate interface beneath this region. Each of these L-SSEs last between
several months to a couple of years.

Studies conducted up until now have analyzed the slip distributions of
L-SSEs that occurred in the following periods: 1996-1998, 2002-2004, and 2009-2011. It has been indicated that there is a possible relationship
between these events and the occurrence of a Nankai Trough megathrust earthquake.

Illuminating the slip distributions of L-SSE in the Bungo Channel is
considered vital for understanding how such a megathrust earthquake
occurs. Therefore, Kobe University researchers decided to conduct a new analysis of the L-SSE that happened during the 2018-2019 period.



========================================================================== Research Methodology and Findings Using longitudinal GNSS data,
the researchers calculated the spatiotemporal distribution of the
Bungo Channel L-SSE that occurred in 2018-2019. From the results, they understood that this event could be divided into 2 sub-events; the 1st sub-event spanning March to July 2018 (0.1 of a year = 36.5 days) and
occurring under the southwest side of the Bungo Channel, and the 2nd
sub-event that occurred from August 2018 to April 2019 directly beneath
the central part of the Channel.

The 2018-2019 L-SSE, at one year in duration, lasted for the shortest
length of time compared to past Bungo Channel L-SSEs that have been
analyzed. Despite this, the 2018-2019 L-SSE was the biggest event in
terms of slippage amount and slip velocity, as well as seismic moment
and moment magnitude. Furthermore, the researchers also discovered that
the 2018-2019 L-SSE was similar to the 2002- 2004 L-SSE in terms of
starting location, slip distribution and the order in which sub-events occurred. In the past, there have been intervals of approx. 6 years
between the occurrence of each L-SSE, however the 2018-2019 L-SSE took
place around 8 years after the previous event, which was in 2009-2011.

Further Research The Bungo Channel L-SSEs have taken place at the
downdip of the plate interface adjacent to the hypocenter of the Nankai
Trough megathrust earthquake, which is estimated to occur with a 70~80% probability in the next 30 years. Monitoring the spatiotemporal changes in
the behaviour of these kinds of slow earthquake (*5) is vital for enabling seismologists to detect the early signs of a megathrust earthquake. These observations have revealed spatiotemporal changes in the slip distribution
of the 2018-2019 Bungo Channel L-SSE, highlighting the great importance
of comparing recent L-SSE with past events. Next, the researchers will
monitor the slippage and adhesion where the plates intersect beneath the
Bungo Channel. By furthering their analysis, they hope to illuminate the mechanisms by which earthquakes occur and to enable the early detection
of the coming Nankai Trough megathrust earthquake.

Glossary 1. Long-term slow slip event (L-SSE):This phenomenon occurs at
a subduction interface (where one plate is thrust beneath another). Over
a period ranging from a few months to a few years, part of the plate
interface slowly slips- this is an L-SSE. 2. Longitudinal GNSS data:The Geospatial Information Authority of Japan has around 1300 GNSS (Global Navigation Satellite System) stations located throughout Japan that
collect temporal data on seismic shifts.

3. Seismic moment:This is a measure of an earthquake's size based on the
area of fault rupture, the average amount of slip, and the force that was required to overcome the friction sticking the rocks together 4. Moment magnitude: Magnitude that is calculated based on long seismic waves (that
have a duration of over 10 seconds) and the shape of these waves. It can
also be determined from seismic moments. 5. Slow earthquake:An earthquake phenomenon in which a fault rupture gradually occurs. Slow earthquake
is a general term for a variety of time constant phenomena where a fault
slips; including low frequency earthquakes, ultra-low frequency tremors
and earthquakes, and short-term and long-term slow slip events.

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


========================================================================== Journal Reference:
1. Yukinari Seshimo, Shoichi Yoshioka. Spatiotemporal slip
distributions
associated with the 2018-2019 Bungo Channel long-term slow slip
event inverted from GNSS data. Scientific Reports, 2022; 12 (1)
DOI: 10.1038/ s41598-021-03982-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220214095551.htm

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