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NATURE|Vol 437|15 September 2005 NEWS & VIEWS
329
the associated tsunamis and seen in layers of
intercalated sand and soil extending horizon-
tally for nearly a kilometre close to the mouth
of the Río Maullín. This region subsided
approximately 2 m as a consequence of the
1960 event, but the stratigraphy has been
preserved because of long-term net tectonic
uplift. Overall, the authors were able to identify
a sequence of eight large earthquakes that
have occurred over the past 2,000 years,
with an average recurrence interval of about
300 years.
But what about the apparent recurrence
sequence of 1575, 1737, 1837 and 1960, which
does not fit this pattern? The consequences of
the 1575 event are evident both from carbon
dating and in documents written by the Span-
ish conquistadors, which tell of effects that
were similar to those seen in 1960. Documen-
tation of the earthquakes of 1737 and 1837 is
less clear, but they are frequently cited in the
seismological histories of the region as being
large (estimated magnitudes of 7.5 and 8.0,
respectively)
8
.
Cisternas et al., however, find that although
the 1575 earthquake appears clearly in their
tsunami stratigraphic record, those of 1737
and 1837 do not. A tsunami was associated
with the event of 1837, reaching Hawaii with
an amplitude of 6 m. But Cisternas et al. sug-
gest that it originated outside their study area,
to the south, and they conclude that the earth-
quake of 1737 was too small to generate a
sizeable tsunami. All in all, they propose that
much of the fault dislocation that occurred in
1960 stemmed from a release of energy that
had remained ‘locked in’ since 1575 — that is,
the earthquakes that occurred between those
times had expended comparatively little of the
accumulated stress.
The relevance of these studies is underlined
by the more recent occurrence of a huge earth-
quake elsewhere in the world. In December
2004, and again in March 2005, the eastern
Indian Ocean was seriously affected by tec-
tonic movement at a subduction zone. The
giant December event stemmed from a dis-
placement of up to 30 m along a rupture 1,100
km in length
9
, which produced the devastating
tsunami that swept across the Indian Ocean —
a startling reminder of the need to learn more
about the behaviour of giant earthquakes.
■
Sergio Barrientos is at the Preparatory
Commission for the Comprehensive
Nuclear-Test-Ban Treaty Organization,
PO Box 1200, A-1400 Vienna, Austria.
e-mail: sergio.barrientos@ctbto.org
1. Cisternas, M. et al. Nature 437, 404–407 (2005).
2. Kanamori, H. J. Geophys. Res. 82, 2981–2987 (1977).
3. Aki, K. Bull. Earthq. Res. Inst. Tokyo Univ. 44, 23–88 (1966).
4. Benioff, H., Press, F. & Smith, S. J. Geophys. Res. 66, 605–619
(1961).
5. Plafker, G. & Savage, J. C. Geol. Soc. Am. Bull. 81, 1001–1030
(1970).
6. Barrientos, S. & Ward, S. N. Geophys. J. Int. 103, 589–598
(1990).
7. Klotz, J. et al. Earth Planet. Sci. Lett. 193, 437–446 (2001).
8. Lomnitz, C. Seism. Res. Lett. 75, 368–378 (2004).
9. Vigny, C. et al. Nature 436, 201–206 (2005).
EARTHQUAKES
Giant returns in time
Sergio Barrientos
The behaviour of a seismic fault in Chile seemed to confound predictions
of how often giant earthquakes should recur. Examination of a 2,000-year
record of tsunami deposits in the region clarifies matters.
In May 1960, south-central Chile experienced
a huge earthquake, the largest since instru-
ment records began. The consequences were
felt not only in Chile, but also in Hawaii, the
Philippines, Japan and other locations across
the Pacific, which were all hit by the ensuing
tsunami. But this giant seismic event has long
puzzled seismologists, because the energy
released by the earthquake should have taken
several centuries to build up, through the
accumulation of stress on the fault concerned,
as the Nazca and South American tectonic
plates converge at a rate of 8–9 centimetres per
year. Yet the immediately preceding events are
historically documented as occurring not only
in 1575, which would fit expectations, but also
in 1737 and 1837.
As they describe on page 404 of this issue
1
,
Cisternas et al. have revisited this puzzle by
examining records of land movement and
tsunamis left in buried layers of sand and soil in
an estuary that runs across the central part of
the fault zone (see Fig. 1 of the paper, page 404).
With these records, and earlier ones (Fig. 1), the
history of activity on this fault can be rewritten.
Since the inception of instrumental record-
ing in the late 1800s, the largest earthquakes
have all occurred in subduction zones
2
, where
one tectonic plate is being driven beneath
another. These earthquakes involve ruptures
on the order of a thousand kilometres long by
a couple of hundred kilometres wide, with
fault displacements of tens of metres. Hun-
dreds of years are necessary to accumulate the
stresses released in these giant events, which
leave traces of their consequences that can
later be identified.
Thanks to the work of Cisternas et al.
1
, we
now have more insight into the seismic cycle in
south-central Chile. The earthquake of May
1960 resulted from a rupture, about 1,000 km
long and 150 km wide, along the north–south-
trending fault where the Nazca plate dives
beneath South America. Earthquakes of this
size saturate the recorders of the seismic waves
that are usually used to estimate earthquake
magnitude and which are reported in terms of
the familiar Richter scale. Instead, a measure
known as seismic moment is used
3
. This is now
more commonly applied to seismic events in
general, and is calculated by multiplying the
area of the rupture zone by the fault displace-
ment and a quantity reflecting the rigidity of
the volume in which the rupture takes place.
The moment magnitude, derived from this
quantity, reflects the real size in terms of the
elastic energy release of an earthquake.
The 1960 earthquake measured 9.5 on
the moment-magnitude scale. It has a special
place in seismological history, because it pro-
vided experimental confirmation of the idea
that earthquakes can cause free oscillations of
the Earth
4
— that is, set the Earth ringing. The
observed changes in land elevation ranged
from 6 m of uplift to 2 m of subsidence; these
displacements have been modelled
5,6
as the
elastic response of the Earth to an average dis-
location of 20 m along the fault, with localized
peaks of more than 30 m. Even now, in 2005,
post-seismic readjustments continue to be
observed in the area
7
.
Cisternas et al.
1
undertook a detailed exam-
ination of a local stratigraphic record of earth-
quakes in south-central Chile, as produced by
Figure 1 | Spot the
surveyor. In this
print, produced in
1874, the figure of
Francisco Vidal
Gormaz can be
seen at work in the
coastal village of
Carelmapu in
southern Chile.
Here he surveys part
of the region that
will be hit by the
1960 earthquake; his
records have helped
in interpreting
the earthquake.
15.9 News & Views 325 NEW 12/9/05 2:40 PM Page 329
Nature
Publishing
Group
© 2005
Predecessors of the giant 1960 Chile earthquake
Marco Cisternas
1
, Brian F. Atwater
2
, Fernando Torrejo
´
n
3
, Yuki Sawai
5
, Gonzalo Machuca
4
, Marcelo Lagos
6
,
Annaliese Eipert
7
, Cristia
´
n Youlton
1
, Ignacio Salgado
1
, Takanobu Kamataki
5
, Masanobu Shishikura
5
,
C. P. Rajendran
8
, Javed K. Malik
9
, Yan Rizal
10
& Muhammad Husni
11
It is commonly thought that the longer the time since last earth-
quake, the larger the next earthquake’s slip will be. But this logical
predictor of earthquake size
1
, unsuccessful for large earthquakes
on a strike-slip fault
2
, fails also with the giant 1960 Chile earth-
quake of magnitude 9.5 (ref. 3). Although the time since the
preceding earthquake spanned 123 years (refs 4, 5), the estimated
slip in 1960, which occurred on a fault between the Nazca and
South American tectonic plates, equalled 250–350 years’ worth of
the plate motion
3,6–10
. Thus the average interval between such
giant earthquakes on this fault should span several centuries
3,9,10
.
Here we present evidence that such long intervals were indeed
typical of the last two millennia. We use buried soils and sand
layers as records of tectonic subsidence and tsunami inundation at
an estuary midway along the 1960 rupture. In these records, the
1960 earthquake ended a recurrence interval that had begun
almost four centuries before, with an earthquake documented
by Spanish conquistadors in 1575. Two later earthquakes, in 1737
and 1837, produced little if any subsidence or tsunami at the
estuary and they therefore probably left the fault partly loaded
with accumulated plate motion that the 1960 earthquake then
expended.
The 1960 Chile mainshock resulted from a rupture nearly
1,000 km long on a north–south trending fault that conveys the
subducting Nazca plate beneath South America at rates averaging 8 m
per century
3
. Lurching westward above the rupture, the South
America plate rose in a mostly offshore area while subsiding 1–2 m
in a coastal downwarp
6
(Fig. 1b). The ensuing tsunami, with crests
10–15 m high in Chile
11
, reached maximum heights of 10 m in
Hawaii
12
and 6 m in Japan
13
.
The 1960 earthquake was preceded historically by earthquakes in
1575, 1737 and 1837 (Fig. 1b; Supplementary Table S1). The reported
effects from 1575 most nearly resemble those from 1960 (ref. 4).
Conquistadors, at forts limited to the northern half of the 1960
rupture area, wrote of persistent marine inundation near Imperial,
Valdivia and Castro that implies widespread tectonic subsidence.
They also described a devastating tsunami near Valdivia (Sup-
plementary Table S1, record 1). The 1737 earthquake, known only
from secondary sources, damaged the few Spanish settlements then
remaining south of Concepcio
´
n. It lacks a reported tsunami, even
though tsunamis from central Chile in 1730 and 1751 were noted
locally
14
and in Japan
13,15
. The 1837 earthquake damaged towns along
the central third of the 1960 rupture area and changed land levels
along the southern half of that area. Its associated tsunami, by
reportedly cresting 6 m high in Hawaii
12
, provides evidence that
the 1837 earthquake released almost half the seismic moment of the
LETTERS
Figure 1 | Index maps. a, Plate-tectonic setting of south-central Chile.
Paired arrows indicate plate convergence at 8.4 cm yr
21
. b, Documented
effects of the 1960 earthquake and its historical predecessors. Compiled
from refs 4, 5, 13 and 14, and from Supplementary Table S1. c, Study area
along the Rı
´
o Maullı
´
n. Barbed lines in a and b show seaward edges of
subduction zones; teeth point down the plate boundary.
1
Facultad de Agronomı
´
a, Pontificia Universidad Cato
´
lica de Valparaı
´
so, Casilla 4-D, Quillota, Chile.
2
US Geological Survey at University of Washington, Seattle, Washington
98195-1310 USA.
3
Centro EULA-Chile,
4
Departamento de Ciencias Histo
´
ricas y Sociales, Universidad de Concepcio
´
n, Casilla 160-C, Concepcio
´
n, Chile.
5
Active Fault Research
Center, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8567 Japan.
6
Instituto de Geografı
´
a, Pontificia Universidad
Cato
´
lica de Chile, Casilla 306, Santiago, Chile.
7
National Oceanic and Atmospheric Administration, Seattle, Washington 98115-6349, USA.
8
Centre for Earth Science Studies,
Thiruvananthapuram 69603, India.
9
Civil Engineering, Indian Institute of Technology, Kanpur 208016, India.
10
Geological Department, Institute of Technology, Bandung 40132,
Indonesia.
11
Potential Geophysics Division, Meteorological and Geophysical Agency of Indonesia, Jakarta 10720, Indonesia.
Vol 437|15 September 2005|doi:10.1038/nature03943
404
© 2005 Nature Publishing Group
1960 mainshock
16
. However, according to primary sources in Sup-
plementary Table S1, the same tsunami caused little if any flooding at
Valdivia and no reported damage anywhere in Chile or Ancud.
To further compare the 1960 earthquake with these historical
earthquakes, and to gain perspective from earlier earthquakes as well,
we reconstructed a 2,000-yr history of repeated subsidence and
tsunamis at the Rı
´
o Maullı
´
n estuary (Fig. 1b, c). Because of the
estuary’s central location, this history probably includes earthquakes
from full-length breaks of the 1960 rupture area, while perhaps
excluding earthquakes from partial ruptures to the north or south.
Our stratigraphic records are tied to modern analogues from 1960
along a nearly marine reach of the Rı
´
o Maullı
´
n. There, 8 km inland
from the sea (Fig. 1c, purple dot), markers of the 1960 earthquake
extend across faint terraces and beach ridges stranded by net late
Holocene emergence
17
. Eyewitnesses recall that the 1960 tsunami
coated upper terraces with sand
18
. We traced the sand, up to 15 cm
thick, more than 1 km inland across the buried 1960 soil in areas
covered only by the highest post-earthquake tides (Figs 2a, b). In this
same area, the sandy record of post-1960 storms extends just a few
metres inland from the shore. On lower terraces, now covered
routinely by tides, a 1960 pasture soil has been eroded and biotur-
bated on post-earthquake tidal flats. Waves and currents are now
Figure 2 | Stratigraphic evidence for 1960 earthquake and its ancestors in
area shown by purple dot in Fig. 1c.
Supporting data in Supplementar y
Figs S1–S4 and Supplementary Tables S2–S4. a, Records of the 1960
earthquake that serve as modern analogues for inferring past occurrence of a
tsunami and of coseismic subsidence. b, Sequences of such records
correlated among trenches. Tides measured 1989, 2003, and 2004.
c, Chronology of the inferred events compared with the historical sequence
in Fig. 1b. Field ev idence for subsidence and tsunami (solid blue circles and
triangles at left) comes from all transects (Supplementary Figs S2 and S3).
The average of the historical recurrence intervals, 128 yr, contrasts with the
longer average intervals between the events recorded stratigraphically.
NATURE|Vol 437|15 September 2005 LETTERS
405
© 2005 Nature Publishing Group
burying the remains of this soil with sand as much as 1 m thick
(Fig. 2a), and with mud in sheltered areas.
Additional sand sheets mantle buried marsh and meadow soils
beneath the 1960 soil. Using criteria from the 1960 examples, we
interpret some of these sand sheets as tsunami deposits (blue dots in
Fig. 2) and others as indicators of subsided, post-earthquake tidal
flats (blue triangles). We traced these event records, which probably
represent eight earthquakes in all (events A–H), among 60 trenches
scattered along 2 km of transects (example, Fig. 2b). Like the 1960
earthquake (event A), four earlier events (B–D, G) produced tsunami
deposits on meadows that post-earthquake tides rarely reached and
correlative tidal-flat deposits on lower ground. Such evidence,
assembled from all transects, is summarized by solid blue symbols
in Fig. 2c. Some events are recorded less widely than the 1960
earthquake. The D sand sheet tapers landward without crossing a
former beach ridge. The E sand sheet, found entirely inland from that
ridge, may have been removed by erosion on the seaward side.
Diatom assemblages from soils that shortly predate and postdate
tsunami deposition provide further evidence for subsidence during
events A, B and D. In all three cases the assemblages above the
tsunami sand are more nearly marine than those in the soil below
(summary, Fig. 2c). The difference is clearest for the 1960 event. An
attempted comparison for event C failed because the upper part of
the buried soil is probably missing from erosion on a post-C tidal flat,
and because the remnant soil is contaminated with burrow-filling
tidal-flat sand.
In sum, our stratigraphy and paleoecology provide evidence for
seven inferred pre-1960 earthquakes from the past 2,000 years
(Fig. 2c). The youngest three (B–D), each marked by evidence for
both subsidence and tsunami, occurred within the past 1,000 years.
Event D dates to the two-sigma range
AD 1020–1180
—
the age of
growth-position stem bases of a rush (Juncus procerus) that tsunami
sand surrounded. The event C tsunami similarly left sand around
Juncus balticus and Scirpus americanus culms in a swale along a spur
transect (Supplementary Fig. S3b); below-ground stems (rhizomes)
that probably belonged to such plants yielded three statistically
indistinguishable ages pooled as
AD 1280–1390.
The tsunami deposit from event B probably exceeds the one from
1960 in thickness and landward extent. Because the 1837 tsunami was
large in Hawaii
12,16
, we expected this penultimate sand sheet to date
from the early nineteenth century. Instead, a burned horizon mostly
2 cm below the sand dates to
AD 1450–1510 or 1590–1620, as judged
from four statistically equivalent ages on charred twigs. Because it
followed the fire, probably by a century at most, we correlate event B
with the extensive subsidence and devastating tsunami of 1575
(Fig. 1b).
We checked additional estuarine records in a further, futile search
for signs of the 1837 earthquake. These records include trees that the
1960 earthquake lowered into tidal freshwater farther up the Rı
´
o
Maullı
´
n (red triangle, Fig. 1c). Residents on hand for the 1960
earthquake testify that a forest, green and emergent before the
earthquake, lost its foliage from routine tidal submergence in the
first few years thereafter. Several decades later, defoliated trunks
dominated an area of 10 km
2
. But several decades after the 1837
earthquake, a nautical chart
19
depicted all trees in this area as leafy
(Fig. 3a). In an accompanying report
20
, the expedition botanist does
not mention dead or dying trees among the forest’s riparian plants
and animals, which he studied for four days. We cut slabs of 15 dead
standing trees in 2003 to estimate their lifespans by counting annual
rings. We assume these trees died in 1960. In that case, ten of them
were alive in 1837 and two in 1737 (Fig. 3b). This finding suggests
that the forest failed to subside in 1837 as much as it did in 1960, in
agreement with the nautical survey and the botanist’s report.
Shoreline changes provide additional evidence that the 1837
earthquake did not produce 1960-size subsidence along the Rı
´
o
Maullı
´
n. Some of the islands and pastures that subsided in 1960
into the middle or lower part of the intertidal zone are barren
intertidal or subtidal flats (Fig. 1c, green triangles). At a similar
time after the 1837 earthquake, these areas were charted
19
as
emergent and vegetated (Supplementary Fig. S1b).
Earthquakes evident in these various estuarine records thus
recurred less often than did earthquakes in the historical sequence:
1575, 1737, 1837, 1960 (Fig. 2c). The best-defined of the earthquake
intervals recorded geologically, which together span most of the past
millenium, average nearly 300 yr
—
more than double the historical
average of 128 years. The 1960 earthquake ended a 385-year interval
that includes the years 1737 and 1837. The poorly understood
earthquakes of 1737 and 1837 probably released too little seismic
moment midway along the 1960 rupture to leave tsunami deposits or
subsidence stratigraphy at the Rı
´
o Maullı
´
n.
Where size varies markedly among successive earthquakes on the
same part of a fault, much of the fault slip during the largest
earthquakes may have thus accumulated before earlier earthquakes
of smaller size. Such storage through multiple recurrence intervals
probably helps to explain the enormity of the 2004 Sumatra–Anda-
man earthquake. The fault slip in 2004 near the Nicobar Islands
amounted to 10 m (ref. 21) in an area where the fault had last
ruptured in 1881 during an earthquake of estimated magnitude 7.9
(ref. 22). By contrast, the fault loading between 1881 and 2004
amounted to less than 4 m at plate-convergence rates recently
estimated from satellite geodesy
22
and less than 7 m at rates inferred
from long-term plate motions
3
. As in the 1960 Chilean case, the 2004
earthquake may thus have used accumulated plate motion that a
previous earthquake left unspent.
Received 9 March; accepted 15 June 2005.
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(See Supplementary
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Supplementary Information is linked to the online version of the paper at
www.nature.com/nature.
Acknowledgements This work was supported by Chile’s Fondo Nacional de
Desarrollo Cientı
´
fico y Tecnolo
´
gico (Fondecyt) and by the US Geological Survey.
Logistical help came from the Municipality of Maullı
´
n and its people (M. I. Silva,
P. Soto, R. Vergara, J. Gallardo, G. Andrade, J. Soarzo and C. Ruiz), and from the
Servicio Hidrolo
´
gico y Oceanogra
´
fico of the Armada de Chile. The manuscript
incorporates suggestions from S. Barrientos, S. Bondevik, C. Lomnitz, A. Nelson,
K. Wang, J. Clague and E. Geist.
Author Contributions M.C. and B.A. led the fieldwork and writing. F.T. studied
documents; Y.S. studied diatoms; G.M. studied tree slabs. M.L. and I.S.
contributed to three seasons of fieldwork, G.M. and C.Y. to two, and A.E., M.H.,
T.K., J.K.M., C.P.R., Y.R. and M.S. to one.
Author Information Reprints and permissions information is available at
npg.nature.com/reprintsandpermissions. The authors declare no competing
financial interests. Correspondence and requests for materials should be
addressed to M.C. (marco.cisternas@ucv.cl).
NATURE|Vol 437|15 September 2005 LETTERS
407
© 2005 Nature Publishing Group
