Earthquake-swarms, slow-slip and fault-interactions at the western-end
of the Hellenic Subduction System precede the Mw 6.9 Zakynthos
Earthquake, Greece
Abstract
The month-to-year-long deformation of the Earth’s crust where active
subduction zones terminate is poorly explored. Here we report on a
multidisciplinary dataset that captures the synergy of slow-slip events,
earthquake swarms and fault-interactions during the ~5
years leading up to the 2018 M 6.9 Zakynthos Earthquake at the western
termination of the Hellenic Subduction System (HSS). It appears that
this long-lasting preparatory phase initiated due to a slow-slip event
that lasted ~4 months and released strain equivalent to
a ~M 6.3 earthquake. We propose that the slow-slip
event, which is the first to be reported in the HSS, tectonically
destabilised the upper 20-40 km of the crust, producing alternating
phases of seismic and aseismic deformation, including intense
microseismicity (M<4) on neighbouring faults, earthquake
swarms in the epicentral area of the M 6.9 earthquake
~1.5 years before the main event, another episode of
slow-slip immediately preceding the mainshock and, eventually, the large
(M6.9) Zakynthos Earthquake. Tectonic instability in the area is
evidenced by a prolonged (~4 years) period of overall
suppressed b-values (<1) and strong earthquake interactions on
discrete strike-slip, thrust and normal faults. We propose that
composite faulting patterns accompanied by alternating
(seismic/aseismic) deformation styles may characterise multi-fault
subduction-termination zones and may operate over a range of timescales
(from individual earthquakes to millions of years).