'Inchworm' pattern of Indonesian earthquake rupture powered seismic
'boom'

Date:
July 27, 2020
Source:
University of Tsukuba
Summary:
A sonic boom-like seismic phenomenon of supershear rupture
occurred during the 2018 Palu earthquake in Indonesia. Researchers
investigated the relationship between this phenomenon and the
complex geometry of the Palu-Koro fault. An 'inchworm-like' pattern
of repeated rupture deceleration and acceleration along the fault
was detected, associated with bends in the fault trace. This slip
evolution may have enhanced the propagation of supershear rupture
and contributed to the generation of the 2018 Palu tsunami.



FULL STORY ========================================================================== Earthquakes are often imagined as originating from a single point where
the seismic waves are strongest, the hypocenter underground or the
epicenter at the Earth's surface, with seismic energy radiating outward
in a circular pattern.

But this simplified model fails to account for the complex geometry of
the actual fault systems where earthquakes occur. The real situation may
be much more complex -- and more interesting. In some remarkable cases,
a phenomenon called "supershear" rupture can occur, where the earthquake rupture propagates along the fault at a speed faster than the seismic
waves themselves can travel -- a process analogous to a sonic boom.


==========================================================================
In a new study published in Earth and Planetary Science Letters,
researchers at the University of Tsukuba investigated a case of supershear rupture, the 2018 Palu earthquake (moment magnitude: 7.6) in Sulawesi, Indonesia, and its relationship with the complex geometry of the fault
system.

Study co-author Professor Yuji Yagi explains, "We used globally
observed teleseismic wave data and performed finite-fault inversion
to simultaneously resolve the spatiotemporal evolution of slip and the
complex fault geometry." The results of this analysis showed that the propagation of supershear rupture of the Palu-Koro fault southward from
the earthquake's epicenter was sustained by a pattern of repeated delay
and advancement of slip along the fault, associated with the fault
system's complex geometry. Areas with particularly high slip rates,
referred to as "slipping patches," were identified near the epicenter
as well as 60, 100, and 135 km south of the epicenter. In addition,
three distinct episodes of rupture after the process initiated were distinguished, with delays in the advancement of the slipping patches
between them.

Tracing the surface rupture of the earthquake showed two major bends
in the earthquake fault, 10-25 km south of the epicenter and 100-110
km south of the epicenter. Supershear rupture persisted along this geometrically complex fault.

As lead author Professor Ryo Okuwaki describes, "Our study shows that the geometric complexity of a fault can significantly influence the velocity
of rupture propagation. Our model of the 2018 Palu earthquake shows a
zigzag pattern of slip deceleration and acceleration associated with bends
in the fault, which we have named inchworm-like slip evolution. We propose
that the geometric complexity of a fault system can promote persistent supershear rupture, enhanced by repeated inchworm-like slip evolution."
These findings may have significant implications regarding assessment
of future earthquake impacts and related disasters. For example, the
authors suggest that the slipping patch they detected beneath Palu Bay
may have contributed to generation of the 2018 Palu tsunami, which added
to the devastation of the earthquake.


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


========================================================================== Journal Reference:
1. Ryo Okuwaki, Shiro Hirano, Yuji Yagi, Kousuke Shimizu. Inchworm-like
source evolution through a geometrically complex fault fueled
persistent supershear rupture during the 2018 Palu Indonesia
earthquake. Earth and Planetary Science Letters, 2020; 547: 116449
DOI: 10.1016/ j.epsl.2020.116449 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200727114716.htm

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