Intense foreshocks and a slow slip event preceded the 2014 Iquique Mw 8.1 earthquake

Abstract

The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. The moment magnitude (Mw) 8.1 Iquique earthquake of 1 April 2014 broke a highly coupled portion of this gap. To understand the seismicity preceding
this event, we studied the location and mechanisms of the foreshocks and computed Global Positioning System (GPS) time series
at stations located on shore. Seismicity off the coast of Iquique started to increase in January 2014. After 16 March, several
Mw > 6 events occurred near the low-coupled zone. These events migrated northward for ~50 kilometers until the 1 April earthquake
occurred. On 16 March, on-shore continuous GPS stations detected a westward motion that we model as a slow slip event situated
in the same area where the mainshock occurred.

Full text

DOI: 10.1126/science.1256074
, 1165 (2014);345 Science
et al.S. Ruiz
8.1 earthquake
w
M
Intense foreshocks and a slow slip event preceded the 2014 Iquique
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excluded for SN2014J (34). But in this case, the
accreted material is mostly He, and the accre-
tion rate can be very high, up to 10
−4
solar masses
per year (44). If the white dwarf is rapidly ro-
tating or if mass is accreted faster than it loses
angular momentum and thus spreads over the
white dwarf, a He belt will be accumulated.
An equatorial ring as inferred here might not
be that uncommon. Recently, Hubble Space Tele-
scope imaging of the light echo from the re-
current nova T Pyx revealed a clumpy ring (45).
Once this belt becomes dense enough, explosive
He burning may be ignited, leaving an ejecta con-
figuration as shown in Fig. 3. This may be con-
sistent with the observed gamma-ray and optical
signals. Our radiation transfer simulations in UV/
optical/near-IR (fig. S10) show that the Ni belt
would not produce easily distinguishable features
but would result in a normal SN Ia appearance,
not only for a pole-on observer but also for an
equatorial observer. In view of this, the inter-
pretation of having this type of explosion as a
common scenario is not rejected by statistical
arguments (see the supplementary materials for
more details).
The evolution of the
56
Co gamma-ray signal
should reveal further aspects of the
56
Ni distri-
bution in SN2014J. These lines with associated
continua have been recognized to emerge in data
from both INTEGRAL instruments (46),asmore
of the total
56
Ni produced in the SN becomes
visible when the gamma-ray photosphere recedes
into the SN interior.
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ACKNOW LEDGM ENTS
This research was supported by the Deutsche Forschungsgemeinschaft
(DFG) cluster of excellence “Origin and Structure of the Universe”
and from DFG Transregio Project No. 33 “Dark Universe.”
S.A.G. acknowledges support from the Russian Academy of
Sciences, program RAS P-21. F.K.R. was supported by the DFG
(Emmy Noether Programm RO3676/1-1) and the ARCHES
prize of the German Ministry for Education and Research. The
work by K.M. is partly supported by a Japan Society for the
Promotion of Science Grant-in-Aid for Scientific Research (grant
no. 23740141 and 26800100) and WPI, at the Ministry of
Education, Culture, Sports. Science & Technology. We are grateful
to E. Kuulkers for handling the observations and to X. Zhang for
preparing our SPI data for the INTEGRAL SN2014J campaign.
The INTEGRAL/SPI project has been completed under the
responsibility and leadership of CNES Toulouse. We are grateful to
the agencies and institutions of ASI, CEA, CNES, DLR, ESA, INTA,
NASA, and OSTC for support of this ESA space science mission.
INTEGRAL’s data archive (http://www.isdc.unige.ch/integral/
archive#DataRelease) is at the INTEGRAL Science Data Center
in Versoix, Switzerland, and includes the SN2014J data used
in this paper.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/345/6201/1162/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S10
Tables S1 and S2
References (46–58)
14 April 2014; accepted 21 July 2014
Published online 31 July 2014;
10.1126/science.1254738
EARTHQUAKE DYNAMICS
Intense foreshocks and a slow
slip event preceded the 2014
Iquique M
w
8.1 earthquake
S. Ruiz,
1
*M. Metois,
2
A. Fuenzalida,
3
J. Ruiz,
1
F. Leyton,
4
R. Grandin,
5
C. Vigny,
6
R. Madariaga,
6
J. Campos
1
The subduction zone in northern Chile is a well-identified seismic gap that last ruptured
in 1877. The moment magnitude (M
w
) 8.1 Iquique earthquake of 1 April 2014 broke a
highly coupled portion of this gap. To understand the seismicity preceding this event,
we studied the location and mechanisms of the foreshocks and computed Global
Positioning System (GPS) time series at stations located on shore. Seismicity off the coast
of Iquique started to increase in January 2014. After 16 March, several M
w
> 6 events
occurred near the low-coupled zone. These events migrated northward for ~50 kilometers
until the 1 April earthquake occurred. On 16 March, on-shore continuous GPS stations
detected a westward motion that we model as a slow slip event situated in the same
area where the mainshock occurred.
Since the giant moment magnitude (M
w
)
8.8 megathrust earthquake of 1877 (1–4),
the ≥500-km-long region stretching from
Arica (18°S) to the Mejillones Peninsula of
Chile (24.5°S) (Fig. 1) has had relatively few
major seismic events, with only moderate M
w
<8
events in 1933, 1967, and 2007 (4–9). On the basis
of recent geodetic data (5,10,11), the degree of
interseismic coupling (that is, the ratio between
theinterseismicsliprateandtheplate-convergence
velocity) in the area shows two distinct highly
coupled segments (Loa and Camarones) separated
by a low-coupling zone (LCZ) off the coast of Iquique
(5). On 1 April 2014, a ~150-km-long portion of
thegapbrokeinaM
w
8.1 earthquake after a strong
precursory activity that started on 16 March (12,13).
Understanding the complex nucleation phase
SCIENCE sciencemag.org 5 SEPTEMBER 2014 •VOL 345 ISSUE 6201 1165
1
Departamento de Geofísica, Facultad de Ciencias Físicas y
Matemáticas, Universidad de Chile, Santiago, Chile.
2
Istituto
Nazionale di Geofisica e Vulcanologia, Centro Nazionale
Terremoti, Rome, Italy.
3
School of Environmental Sciences,
University of Liverpool, Liverpool, UK.
4
Centro Sismológico
Nacional, Facultad de Ciencias Físicas y Matemáticas,
Universidad de Chile, Santiago, Chile.
5
Institut de Physique
du Globe de Paris, Sorbonne Paris Cité, Université Paris
Diderot, UMR 7154 CNRS, Paris, France.
6
Laboratoire de
Geologie, UMR 8538 CNRS Ecole Normale Superieure, Paris,
France.
*Corresponding author. E-mail: sruiz@dgf.uchile.cl
RESEARCH |REPORTS

preceding the M
w
8.1 mainshock and its coseismic
rupture should provide insights into the seismic
gap history, present-day seismic hazard, and nu-
cleation process of megathrust earthquakes.
Seismic activity preceding the Iquique earth-
quake initiated in a region between 19.5°S and
21°S, where coupling ranges from 0.2 to 0.5 (5);
that is, the plates are slowly creeping past each
other at a fraction of the plate rate (when the
plates are fully locked, coupling is 1.0). This
region was actively monitored because its seis-
mic activity had steadily increased since 2008,
1166 5SEPTEMBER2014•VOL 345 ISSUE 6201 sciencemag.org SCIENCE
Fig. 1. Northern Chile seismic gap. Interseismic coupling was calculated from GPS measurements acquired in the zone since 2008. Slip contours are shown
with continuous lines for the 2007 M
w
7.7 Tocopilla, 1995 M
w
8.0 Antofagasta, and 2014 M
w
8.1 and 7.6 Iquique earthquakes. Precursory seismicity from January to
March 2014 is shown with light blue circles. Foreshocks from March 2014 are shown with open squares.The dashed lines denote the surface projection of the
subduction interface isodepths.(Inset) Normalized probability that two or more M
w
< 7 earthquakes occurred since 2008 (CSN catalog) as a function of the value
of interseismic coupling. The upper right corner indicates the mean and standard deviation of the best-fit normal distribution.
RESEARCH |REPORTS

with repeated interplate thrust events of magni-
tude <4.0. Several seismic clusters were located
in this region by the National Seismological Cen-
terofChile(CSN)(Fig.1andfig.S1);someof
these clusters were associated with persistent mi-
croseismicity observed by the nearest seismolog-
ical stations (fig. S2). Global catalogs reveal an
increase in seismicity near Iquique since 2005,
compared with the previous 10 years (fig. S3).
To understand the seismicity that preceded the
1 April mainshock, we used the events listed in
the CSN catalog to relocalize and estimate the
focal mechanism of several foreshocks of this
sequence. Simultaneously, we calculated the Glob-
al Positioning System (GPS) time series of the
closest permanent stations up to the date of
the mainshock and inverted for the slip rate on
the plate interface (14).
The most recent seismic activity in northern
Chile started on 4 January 2014, when a M
w
5.7
interplate thrust event took place at 20.69°S,
70.80°W on the southern edge of the Iquique
LCZ (5). On 8 January, another M
w
5.7 event
occurred in the same area. The automatic loca-
tion system of CSN identified 30 events in a
smaller region of 15 km by 15 km, in the period
from 4 to 24 January 2014. Then on 12 January,
a small cluster of six identified events occurred
north of the previous ones at (19.7°S, 71.0°W).
During February 2014, another small cluster oc-
curred near 19.4°S, 71.0°W, where 16 events with
local magnitude (ML) 2.4 to 4.0 were identified
(Fig. 1, fig. S4, and table S1). On 15 March, the
area was reactivated with 11 events of ML 2.6 to
4.6, followed on 16 March by the first big fore-
shock of M
w
6.7, ~50 km south and with a slightly
deeper centroid (fig. S4). This event triggered a
persistent precursory seismicity, including a
M
w
6.3 earthquake on 22 March, which slowly
moved northward and lasted until the occur-
rence of the 1 April 2014 M
w
8.1 event (Fig. 2 and
fig. S4).
Overall, these foreshocks delineate a region
that spans ~150 km along the strike of the sub-
duction zone. We relocated this precursory seis-
micity and computed regional seismic moment
tensor using a linear time-domain, broadband
waveform inverse method (15)(Fig.2).Thecen-
troid of the M
w
6.7 precursor of 16 March was
only 10 km deep, in an area where the seismo-
genic interface is at a depth of ~20 km (16)(Fig.2
and fig. S5). This event had a reverse focal mecha-
nism with a strike of 277° that is at a sharp angle
with respect to the trench (Fig. 2A). Over the
course of 1 week, a persistent seismicity occurred
in a zone of 10-km radius (Fig. 2C); most of these
events were located inside the shallow South
American plate and had very diverse focal mecha-
nisms. On 22 March, a new M
w
6.3 foreshock
occurred ~30 km north of the 16 March foreshock.
After this event, precursory seismicity moved to
the vicinity of the 22 March foreshock with depths
and mechanisms indicatingthatitoccurredat
the plate interface (Fig. 2B).
From 16 March until the 1 April mainshock
(i.e., for 17 days), all of the continuous GPS (cGPS)
stations located along the coast between Iquique
and Pisagua started to move trenchward (Fig. 3).
This slowly increasing westward motion is in
contrast to the usual inland-directed interseis-
mic motion. The displacements measured dur-
ing this period were quite large (>5 mm for the
stations located between Iquique and Pisagua
and ~1 cm at the PSGA station). Only a fraction of
this cumulative displacement (up to 20%) can be
attributed to the largest foreshock of 16 March
(M
w
6.7), suggesting that slow aseismic slip was
taking place offshore, concurrently with the de-
velopment of the precursory seismicity. Whether
this motion started slightly before or coincided
with the 16 March foreshock is beyond the cur-
rent GPS resolution. We inverted for the slip
distributions on the subduction interface that
best reproduce the observed displacements using
Okada’s formulas for an elastic half-space (14)
(Fig. 4). We found a slip of ~0.8 m for the M
w
6.7 events of 16 March, located in a narrow area
SCIENCE sciencemag.org 5 SEPTEMBER 2014 •VOL 345 ISSUE 6201 1167
Fig. 2. Seismicity preceding the Iquique earthquake. (A) Gray dots show the foreshocks from 16 to 31 March; the intensity of the gray color indicates the
depths of the events.The slip distribution of the M
w
8.1 and M
w
7.6 earthquakes inverted from far-field broadband records of the International Federation of Digital
Seismograph Networks is shown with the color. (B) A 15-km-wide cross section along the A-A′line shown in (A). (C) A 15-km-wide cross section along the B-B′line
shown in (A). In the vertical cross sections, we plot the focal mechanisms of events with M
w
> 4.6. Mechanisms were computed by broadband moment tensor
analysis. The gray curve shows the seismogenic contact according to (16).
RESEARCH |REPORTS

in the vicinity of the CSN epicenter. We then com-
puted the aseismic slip for the period from 10
March to the mainshock that extends over an
area of 70 km by 20 km located between the fore-
shocks and the coast (Fig. 4).
The 1 April 2014 Iquique earthquake started
with a small shock at the northern end of the
region activated by the precursors that occurred
in March (19.57°S, 70.91°W). The peak seismic
moment release rate during the M
w
8.1 earth-
quake took place ~30 s after the initial nucleation
(fig. S6). We used standard teleseismic methods
(17) to invert for the coseismic slip of the main
event and its large M
w
7.6aftershockthatoc-
curred 2 days later (Fig. 2 and figs. S6 and S7).
The maximum slip associated with these events
was deeper than that of the March precursors
and ~20 km inland, affecting areas of higher
coupling (>0.6). Coseismic slip appears to over-
lap (at least partially) with the slow slip event
(SSE) (Fig. 4), but no substantial coseismic slip
occurred in the LCZ (Fig. 1). Whereas precursory
seismicity migrated northward, the mainshock
and its aftershock propagated toward the south,
similar to what was observed for the 2007 M
w
7.8
Tocopilla earthquake (7,8).
TheLCZoffthecoastofIquiquecanbeas-
sumed to play a key role in the events that
occurred in northern Chile during, and in the
20 years preceding, this precursory sequence.
The seismic swarms detected since 2008 oc-
curred on the edges of the LCZ, and most mod-
erate seismicity took place preferentially in zones
of intermediate coupling (see inset in Fig. 1 and
fig. S8), suggesting that aseismic creep occur-
ring on the LCZ triggered seismic activity in its
vicinity. Up to now, SSEs have remained unde-
tected by the cGPS network operating in north-
ern Chile for more than a decade. Nevertheless,
the very inference of a LCZ off the coast of Iquique
implies some degree of accommodation of plate
convergence by aseismic slip, which might op-
erate by repeated SSEs. We postulate that until
now, either the magnitude of these SSEs was too
small, or they occurred too far from the coast
to be detected by GPS measurements. The only
major change we detected in plate convergence
in the area before 2014 was a long-term velocity
change at the cGPS station operating in Iquique
since 1995 (UAPE):its eastward velocity decreased
after 2005 by ~20%, from 19.5 to 15.2 mm/year
(fig. S9). This suggests that interseismic loading
has been decreasing in the Iquique area during the
past decade, probably reflecting a very SSE oc-
curring on the decadal scale. This change could
have been triggered by the deep intraslab 2005
M
w
7.7 Tarapacá earthquake that generated
little postseismic relaxation (fig. S9).
Together with the fact that the M
w
8.1 main-
shock and M
w
7.6aftershockrupturesdidnot
penetrate into the LCZ, the foreshock sequence
and slow slip preceding the M
w
8.1 event argue
for a creeping Iquique LCZ. Because SSEs are
often associated with seismic swarms (18), we
propose that the seismicity observed in northern
Chile since 2008 was triggered by a SSE, devel-
oping for several years and accelerating during
the final foreshock sequence as in the preslip
model of nucleation (19). As suggested by many
laboratory experiments (20), the SSE might have
occurred in the nucleation zone of the impend-
ing megathrust rupture (Fig. 4). This precursory
sequence included several shallow crustal events
that took place near the 16 March foreshock;
these events may be associated with the activation
of a listric fault in the outermost fore-arc. This
area is poorly known due to the lack of marine
seismic profiles, but it may be similar to the
eroded wedge enhanced by fracturing imaged
at 22°S (21).
Several other subduction earthquakes were
preceded by precursory seismic activity (12); in
particular, the 1985 Valparaiso M
w
8.0 (22)and
2010 Maule M
w
8.8 (23) Chilean events and the
1168 5SEPTEMBER2014•VOL 345 ISSUE 6201 sciencemag.org SCIENCE
Fig. 3. Motion of coastal GPS stations preced-
ing the Iquique earthquake. (A)Northand(B)
east components relative to a linear evolution
model with seasonal variations estimated since
2012 (14).Thethickredlinedenotestheorigintime
of the mainshock, whereas the black dotted lines
show the occurrence times of the M
w
>6fore-
shocks. Error bars indicate 1sformal uncertainty.
RESEARCH |REPORTS

2011 Tohoku-Oki earthquake (24). However, the
size and duration of the precursory events of
the 1 April mainshock are distinct. The occurrence
of a SSE before the 2011 M
w
9.0 Tohoku-Oki
earthquake and possibly the 2001 M
w
8.4 Arequipa
earthquake (24,25) implies that SSE may be a
common precursory feature and a potential trig-
gering mechanism for large megathrust ruptures.
On the other hand, whether the shallowest part
of the Camarones segment is still coupled and
capableofproducingalargeearthquakeorcreep-
ing aseismically is beyond the current resolution
of the GPS network. The large aftershock of 3
April left the highly coupled Loa segment south
of Iquique largely untouched. Several earthquakes
of equivalent or larger magnitude may still rup-
ture the deep intermediate-coupling areas of
this segment.
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ACKNO WLED GME NTS
We thank J. C. Ruegg for his initiative to study northern Chile
with GPS, followed by J. B. De Chabalier, A. Socquet, D. Carrizo,
and others after him. S.R., J.R., R.M., and J.C. acknowledge
the support of the Chilean National Science Foundation
project FONDECYT no.1130636 and S.R. of project FONDECYT
no.11130230. This work received partial support from
ANR-2011-BS56-017 and ANR-2012-BS06-004 of the French
Agence Nationale de la Recherche. This is Institut de Physique
du Globe de Paris contribution no. 3551. We thank Incorporated
Research Institutions for Seismology Data Management Center
(www.iris.edu/data/), International Plate Boundary Observatory
Chile (http://geofon.gfz-potsdam.de/waveform/), Centro
Sismológico Nacional (www.sismologia.cl), and International
Associated Laboratory Montessus de Ballore for making raw
data available to us.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/345/6201/1165/suppl/DC1
Materials and Methods
Figs. S1 to S13
Tables S1 and S2
References (26–32)
14 May 2014; accepted 15 July 2014
Published online 24 July 2014;
10.1126/science.1256074
SCIENCE sciencemag.org 5SEPTEMBER2014•VOL 345 ISSUE 6201 1169
Fig. 4. Slip distribution preceding
the Iquique earthquake. The slip
distributions were inverted using
Okada’s equations for an elastic
half-space from the surface
displacements observed during
the preseismic phase (14) (Fig. 3).
(A) Coseismic slip due to the 16
March M
w
6.7 earthquake.
(B) Aseismic slip for the period
ranging from 10 to 31 March 2014.
(C) Cumulative slip model for
displacements observed from
10 to 31 March 2014. Black and
green contours are the slip
distributions for the mainshock
and the main aftershock,
respectively, already presented in
Fig. 2. Mo, seismic moment.
RESEARCH |REPORTS