Variable Slip Modes in Postseismic Deformation North of the April 16,
2016 Mw 7.8 Pedernales, Ecuador Megathrust Earthquake
Abstract
The north Ecuador subduction zone has a history of experiencing a range
of slip modes including megathrust and other fast slip, slow, and
aseismic slip. In 1906, a Mw 8.8 megathrust ruptured 500 km along the
north Ecuador/Colombia margin. Parts of this region re-ruptured in
events (south to north): ‘42 (Mw 7.8), ‘58 (Mw 7.7), and ‘79 (Mw 8.2).
The April 16, 2016 Pedernales megathrust rupture overlapped the ‘42
rupture. Postseismic deformation following the 2016 event exhibited a
range of slip behaviors and associated seismicity. A dense temporary
land and offshore deployment augmented permanent stations of the
national network (RENSIG) to record postseismic deformation for one
year. Aftershocks concentrate spatially in bands or clusters mirroring
patterns in background seismicity marking persistent asperities which
cause variations in plate coupling. Bands of aftershocks outline the
2016 rupture and two patches of larger slip within the rupture;
additional bands are observed to the south and to the north. North of
the rupture, bands and clusters are observed near Punta Galera,
Atacames, and Esmeraldas. Seismicity near Punta Galera outlines the
north edge of a patch of aseismic slip that occurred in the month
following the mainshock. One month after the mainshock, Mw 6.7 and 6.9
aftershocks occurred. Calibrated relocations show these are interface
events north of the 2016 rupture, downdip of the aseismic slip. On 7/11/
2016, Mw 5.9 and 6.3 interface events occurred, causing an increase in
local seismicity. In June intermittent seismicity began in Esmeraldas,
near the 1958 rupture. An earthquake swarm and a transient in GPS data
in July 2016 suggests possible slow slip in the region. Relocations of
earthquakes in the swarm outline a splay fault in the upper plate. An
increase in seismicity near Atacames in December suggests fast slip.
Calibrated relocations of the 5 largest events (M 4.7-5.2) and automatic
locations of the remaining 246 events show they are upper plate events.
In the months following the Pedernales event, fast, aseismic, and slow
slip occur north of the rupture. Near Atacames and Esmeraldas upper
plate seismicity is predominant.