XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
S2_002
The 1868 (Southern Peru) and 1877 (Northern Chile) tsunamis
recorded at Fort Point, California.
Barrientos, S.1, Ward, S.N.2
(1) Departamento de Geofísica, Facultad de Ciencias Físicas y Matemáticas, Universidad
de Chile.
(2) Earth Sciences Board, University of California, Santa Cruz.
[email protected]
Summary
Southern Peru and northern Chile have been exposed to several large earthquakes
and catastrophic tsunamis. Two of the largest events that have affected the coasts of these
two countries took place in the later part of the XIX century; these are the August 13,
1868, and the May 9, 1877 (local time), events. Their estimated magnitudes are of the
order of 9 rupturing contiguous segments of approximately 400-500 km each. Both of
them accommodated the convergence between Nazca and South American plates
producing large seafloor and coastal elevation changes generating significant tsunamis
that affected most of the coastlines of the Pacific basin. Reported local run-ups reached
20 m. Both trans-pacific tsunamis were recorded at one tide gage in Fort Point, in the
Presidio area of San Francisco Bay, California. Records at Sausalito (approximately 6 km
away from Fort Point) for the 1877 event mimic the signal recorded at Fort Point. Here
we report on the characteristics of these tide gage records and compare them to recent
records of tsunamis produced by the largest earthquakes in the region. Numerical
simulations of the tsunamis have been constructed.
Background and Discussion
Seismogenic zones in Chile are basically well established: large shallow (0-50 km)
thrust earthquakes along the coast, large deeper (70-100 km) tensional as well as
compressional events within the subducting Nazca plate, and very shallow seismicity (020 km) in a few places, such as the cordilleran region of central Chile and the southern
extremity of the continent by the Magellan Strait. Deeper seismicity (150 to 650 km)
occurs farther to the east, beneath Bolivia and north-western Argentina.
The large thrust earthquakes, responsible for most of the damage recorded in
history, are located along the coast from Arica (18°S, the northernmost extreme of coastal
Chile) to the triple junction at Taitao Peninsula (46°S). With magnitudes that can reach
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XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
values well over eight, these events are usually accompanied by noticeable coastal
elevation changes and, depending on the amount of seafloor vertical displacement, by
catastrophic tsunamis. Their rupture zones are limited to the coupled region between the
Nazca and South American plates which extends down to 45-53 km depth [1] and their
lengths could reach well over one thousand kilometers long. Their spatial and time
characterizations have been studied ([2, 3, 4]), so that the hazard due to these large events
is well recognized. More than ten events with magnitudes equal or greater than
magnitude 8 have taken place in Chilean coasts during the XX century alone. Among
these earthquakes is the 1960 event, the largest earthquake ever recorded since the
beginning of instrumental seismology. Return periods for magnitude ~8 events are of the
order of 80 to 130 years for any given region in Chile, but about a dozen years when the
country is considered as a whole. Mega-thrust earthquakes seem to have much longer
return periods, of the order of a few centuries for any given region ([5, 6, 7]).
The two very large shocks that took place in the XIX century severely damaged
Arica and other important coastal villages by the ensuing tsunami which reportedly
reached runups of up to 20 and 15 m respectively (Fig. 1; [8]). Analysis of the arrival
times of the tsunamis at different locations along the coast, along with the characteristics
of the runups for the 1868 and 1877 events, allowed Díaz [9] to estimate maximum
rupture lengths of over 600 km and over 500 km with corresponding displacements of
nearly 14 and 10 m generating magnitudes (MW) of 9.0 and 8.9 respectively. The higher
values correspond very well with those estimated by Abe [10]. The southern extreme of
the 1868 rupture reached beyond the Arica bend, farther to the south than the dislocation
associated with the recent 2001 event. In 2007, a Mw=7.7 earthquake took place from
Mejillones Peninsula to Antofagasta, thus, a stretch of nearly 500 km along the coupled
region of southern Peru - northern Chile (Ilo-Arica-Tocopilla) has not been subjected to
significant earthquakes since 1868-1877. Comparison of tsunami amplitudes recorded in
Japan, at Hakodate, indicate maximum values reaching 2 m and 3 m for the 1868 event
[9] compared to maximum values of 1 m for the 2001 event.
The estimations obtained by Diaz [9] are then incorporated as tsunamigenic sources
to model the observed time series at Fort Point, California (Fig. 2), where a tide gage was
installed in 1854. This tide gage has provided the longest continuous tidal observations
in the Americas [11].
Conclusions
Tide gauge records at Fort Point and Sausalito, California, recorded the tsunami
waves originated by both the 1868 and 1877 earthquakes in southern Peru and northern
Chile. Even though the 1868 earthquake was larger in magnitude, the records in
California show a smaller amplitude perturbation; almost 0.2 m for the 1877 and 0.1 m
for the 1868 events respectively. This is mainly due to the different tsunami radiation
patterns and, to a lesser degree, that a part of the energy is blocked by the sub-continent
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Santiago, 22-26 Noviembre, 2009
itself for the 1868 one. We are in the process of modeling these records and a first
attempt is shown in Figs. 3 and 4.
Acknowledgements
The authors would like to thank the Núcleo Científico Milenio “International
Earthquake Research Center Montessus de Ballore” P06-064-F
References
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Escuela de Ciencias del Mar, Facultad de Recursos naturales, Universidad Católica de
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[11] Theberge, A.E., 2005. 150 years of tides on the Western coast: The longest series of
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Santiago, 22-26 Noviembre, 2009
Fig 3. Planar dislocation models used to
numerically simulate the tsunami
waveforms shown in Fig. 4.
Fig. 1. Maximum amplitudes reported
for the 1868 and 1877 tsunamis along
the coasts of southern Peru and northern
Chile
Fig. 4 Simulated tsunami waveforms
generated by the 1877 earthquake as
seen from the four red sites located at the
latitude of San Francisco.
Fig. 2. Comparison of tsunamigrams
recorded at Fort Point, California, of
three of the largest earthquakes that have
taken place in the southwestern coast of
South America in the last 150 years.
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