Slow and regular earthquakes interact near Istanbul

Date:
August 12, 2021
Source:
GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre
Summary:
Earthquakes typically last only a few seconds, although sometimes
the shifts in the subsurface occur in slow motion. Understanding
these 'slow quakes', known as 'slow slip events', and their
interplay with the short - sometimes violent - tremors is critically
important to define the seismic hazard and subsequent risk.



FULL STORY ========================================================================== Earthquakes typically last only a few seconds, although sometimes the
shifts in the subsurface occur in slow motion. Understanding these
'slow quakes', known as 'slow slip events', and their interplay with
the short -- sometimes violent -- tremors is critically important to
define the seismic hazard and subsequent risk. An international group
led by Patricia Marti'nez-Garzo'n, Junior Group Leader at the GFZ German Research Center for Geosciences, has now published a study in the journal Seismological Research Letters in which they investigate this interplay
of different seismic events near the metropolis of Istanbul with its
millions of inhabitants.


==========================================================================
A suspicious calm in the 'seismic gap' The Armutlu Peninsula is currently
the most seismically active area directly south of the densely populated Istanbul megacity. The region is part of the North Anatolian Fault,
separating Eurasia from the Anatolian plate. This fault is an active
tectonic plate boundary known to generate destructive earthquakes causing
large numbers of casualties. The last such major earthquake occurred in
1999 near Izmit resulting in almost 20,000 fatalities. A portion of the
fault, running between Istanbul and Armutlu, is currently identified as a "seismic gap" because there is a suspicious calm there, so to speak. The
region is therefore considered overdue to produce a major earthquake.

Observation of slow quakes In this region, slow earthquakes could
be first identified in 2019 thanks to special borehole strainmeter
instruments deployed by researchers from the GFZ German Research Centre
for Geosciences, in collaboration with the Turkish Disaster and Emergency Management Presidency (AFAD) and the UNAVCO institute from US. To further
study the interaction between slow slip events and "regular" earthquakes
in the region, a dense temporary seismic network "SMARTnet" was deployed
in the Armutlu Peninsula, as part of the GONAF Plate Boundary Observatory.

Framing a regular MW 4.6 earthquake in December 2018, another 30-day
lasting slow slip event was now recorded, activating the shallower
portion of the same small fault. After that, the fault continued to be seismically active during an entire year, with more than 1000 earthquakes located on the same fault section.

Higher seismicity rates after larger earthquake The recent study in "Seismological Research Letters" reports on these observations and
discusses that the larger seismicity rates during the year after the
MW 4.6 event are promoted by the occurrence of the slow slip event, as
well as the re-distribution of stresses after the main rupture. "The strainmeters near the active fault allowed to identify the slow slip
signal that presumably occurred at shallow depth level and that released
an amount of energy equivalent to that of a magnitude 5.5 earthquake,
but distributed over weeks, not seconds," says Dr. Marti'nez-Garzo'n, lead-author of the study.

Exact role of slow quakes still to be clarified How the slow slip event
could have affected the state of stress along nearby faults remains yet
to be investigated in detail. Prof. Marco Bohnhoff, head of the GONAF observatory and a co-author of the study states: "How the slow and fast earthquakes interact remains to be understood in detail. In any case,
our results will allow to better quantify the regional seismic risk, in particular for the 15-million population center of Istanbul in the light
of the pending big one." Dr. Marti'nez-Garzo'n is funded by the Helmholtz Young Investigators Group SAIDAN: Seismic and aseismic deformation in
the brittle crust. GONAF is part of the GFZ Plate Boundary Observatory initiative and was co-funded by German, Turkish and US agencies as well
as from the International Continental scientific Drilling Programme ICDP
that is based at GFZ Potsdam. The temporary seismic network SMARTnet
was also funded by GFZ Travel Expedition Funds.

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


========================================================================== Journal Reference:
1. Patricia Marti'nez-Garzo'n, Virginie Durand, Stephan Bentz, Grzegorz
Kwiatek, Georg Dresen, Taylan Turkmen, Murat Nurlu, Marco
Bohnhoff. Near- Fault Monitoring Reveals Combined Seismic and Slow
Activation of a Fault Branch within the Istanbul-Marmara Seismic
Gap in Northwest Turkey.

Seismological Research Letters, 2021; DOI: 10.1785/0220210047 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210812103422.htm

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