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The May 31, 1970, Peru earthquake; the disastrous consequences and
mitigation of inevitable future events
-John Prince
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Abstract:
On May 31st
1970 an earthquake occurred off the western coast of Peru. The magnitude 7.7 event was
responsible for the deaths of 70000 people including some 18000 that were buried after catastrophic debris
flow was triggered high atop Mount Huascaran. The avalanche of rock, dirt, snow, ice and water reached
speeds greater than 300km/hr as it descended the slopes of the enormous mountain to wreak havoc on thetowns and people at the base of the slopes. The death toll was greatly increased by construction techniques
which are prone to failure during seismic events. Large magnitude earthquakes are inevitable in the
tectonically active western coast of South America. In order to minimize both human casualties and economic
losses actions need to be taken to raise awareness and to find an alternative to adobe brick constructions.
Introduction:
On the last day of May 1970 an earthquake
occurred off the coast of Peru, which resulted in the
deaths of over 70,000 people in the surrounding
area. About 18000 of the deaths were associated
with a catastrophic failure of an over-steepened
precipice high on the slopes of Mount Huascaran
and the subsequent debris flow which completely
buried the town of Yungay and parts of Ranrahirca.
The earthquake which was a magnitude 7.7 lasted
for 30-90 seconds, according to eye witnesses and
was said to have started gently but to have quickly
become more violent (Plafker et al, 1971). The
earthquake and a series of aftershocks which
ranged in scale from magnitude 4 to magnitude
6.25 on the Richter scale triggered hundreds of
landslides and rock falls in a 7500km2
area in the
two mountain ranges, the Cordillera Negra to the
west and the Cordillera Blanca in the east (Ericksen
et al, 1970). Damage to the infrastructure was
extensive and was worsened by the fact that many
of the buildings in the area were constructed from
adobe mud bricks, a construction style that is
exceptionally susceptible to failure during an
earthquake. Damage associated with landslides and
debris flows occurred where towns had been
constructed on top of previous debris flow deposits.
Although this study focuses on the area effected by
the May 31, 1970 Peru earthquake these
considerations apply all areas where earthquake
hazards are high and particularly in countries and
regions where the infrastructure is poorly
developed and the population is less aware of the
intrinsic risks. The purpose of this case study is to
detail the effects of the 1970 earthquake disaster in
Peru, to see what changes can and have been done
in order to minimize the death and destruction due
to this type of event in the future.
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Figure 2: Above, Cross-sectional profile through central Peru
showing the shallow hypocenters of earthquakes,
characteristic of flat subduction. Abscissa values are
distance from coast in km, ordinate values are depth in km.
Figure from Rhea et al., 2010
Figure 3: Above, Cross section profile through Ecuador and
Northern Peru showing seismic data associated with normal
subduction. Yellow triangles are active volcanoes. Abscissa
values are distance from coast in km, ordinate values are
depth in km Figure from Rhea et al., 2010
Figure 1: Table of largest ever recorded earthquakes
compiled by the USGS
http://earthquake.usgs.gov/earthquakes/world/10_largest_
world.php
Location Date UTC Magnitude Lat. Long. Reference
1. Chile 1960 05 22 9.5 -38.29 -73.05 Kanamori, 1977
2. Prince William Sound, Alaska 1964 03 28 9.2 61.02 -147.65 Kanamori, 1977
3. Off the West Coast of Northern Sumatra 2004 12 26 9.1 3.30 95.78 Park et al., 2005
4. Near the East Coast of Honshu, Japan 2011 03 11 9.0 38.322 142.369 PDE
5. Kamchatka 1952 11 04 9.0 52.76 160.06 Kanamori, 1977
Figure 4: Left Map of South America showing location and relative
earthquake foci. Red dots are shallow, green are intermediate and
deep focus earthquakes. Figure fromRhea et al., 2010
http://earthquake.usgs.gov/earthquakes/world/10_largest_world.phphttp://earthquake.usgs.gov/earthquakes/world/10_largest_world.phphttp://earthquake.usgs.gov/earthquakes/world/events/1960_05_22.phphttp://earthquake.usgs.gov/earthquakes/world/events/1960_05_22.phphttp://earthquake.usgs.gov/earthquakes/states/events/1964_03_28.phphttp://earthquake.usgs.gov/earthquakes/states/events/1964_03_28.phphttp://earthquake.usgs.gov/earthquakes/eqinthenews/2004/us2004slav/http://earthquake.usgs.gov/earthquakes/eqinthenews/2004/us2004slav/http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/http://earthquake.usgs.gov/earthquakes/world/events/1952_11_04.phphttp://earthquake.usgs.gov/earthquakes/world/events/1952_11_04.phphttp://earthquake.usgs.gov/earthquakes/world/events/1952_11_04.phphttp://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usc0001xgp/http://earthquake.usgs.gov/earthquakes/eqinthenews/2004/us2004slav/http://earthquake.usgs.gov/earthquakes/states/events/1964_03_28.phphttp://earthquake.usgs.gov/earthquakes/world/events/1960_05_22.phphttp://earthquake.usgs.gov/earthquakes/world/10_largest_world.phphttp://earthquake.usgs.gov/earthquakes/world/10_largest_world.php8/2/2019 Natural Disasters Term Paper Final
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Geologic Setting:
The earthquake of May 31st
1970 occurred
off the western coast of South America near the
Peruvian town of Chimbote. The western coast of
South America is part of the Pacific ring of fire,
which refers to the tectonically active border
surrounding the Pacific Ocean. The exact mechanics
of earthquakes are still poorly understood, but what
is clear from data gathered in the past is that themost powerful earthquakes tend to be associated
with subduction zones (Rhea et al, 2010). Since
subduction is taking place at nearly all of the plate
boundaries surrounding the Pacific it follows that
most of the largest magnitude earthquakes ever
recorded have occurred in subduction zones
surrounding the Pacific Ocean (Figure 1).
The coast of central Peru is a unique area
tectonically, since it is the only location on earth
which displays flat subduction, where an oceanic
(Nazka) plate underthrusts a continental (South
American) plate (Norabuena, 1992) (Figure 2). In all
other convergent boundaries involving an oceanic
plate and a continental plate the oceanic plate is
pushed into the mantle partially melting the
subducting slab. In these cases strato-volcanoes
form at the surface above the subducting slab due
to the rising and eruption of the melt created at
depth (Figure 3). The type of subduction can also be
deduced from the seismic record, since most of the
hypocenters of earthquakes in subduction zones are
within the subducting slab. When the foci of
earthquakes in a subduction zone are plotted there
is a general relationship between distance inland
and depth of the hypocenter (Figure 2, 3, 4). The
foci of the earthquakes get deeper as their
epicenters move inland, this is due to the fact that
most of the earthquakes are focused within the
subducting slab. However in central Peru none of
this conventional subduction zone evidence is
present, indeed normal subduction of the NazkaPlate does take place in southern Peru and in
Ecuador but northern and central Peru do not
display any of the normal subduction zone
volcanism nor does the seismic data agree with
normal subduction of an oceanic plate (Hasegawa
and Sacks, 1981). In the case of central Peru the
evidence from hypocenters of earthquakes suggests
that up until about 100km depth the Nazka plate
subducts at a normal angle of about 300
but then it
bends back to horizontal and continues eastward
for approximately another 300km (Norabuena,
1992)(Figure 2). This flat subduction of the Nazka
plate generates shallow hypocenters of earthquakes
even relatively far inland. Shallow earthquakes are
generally more hazardous than deep ones because
of the proximity of the hypocenter to the surface.
Earthquake:
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On the afternoon of May 31st
1970 an
earthquake occurred off the coast of Peru. The
Epicenter of the magnitude 7.7 earthquake was
located about 25km west of the coast of central
Peru, South America (Plafker et al, 1971). The
Hypocenter of the earthquake was at a depth of
56km according to the geodetic survey. It is
estimated that the earthquake caused 70,000
deaths in the surrounding areas, destroyed 200,000
homes and left 800,000 people homeless (Cluff,
1971). The astonishing amount of death and
damage would have been severely worsened ifmovement had been close enough to the surface to
have caused a tsunami.
Since there was no associated tsunami
Plafker et al (1971) concluded that fault plane
movement must have only occurred at depth with
no associated underwater landslides or thrusting.
The coastal city of Chimbote located just 25km from
the epicenter of the 1970 earthquake was built on a
delta plane and much of the city is within 20m of
normal sea-level, making this city especially
vulnerable to tsunamis. The earthquake lasted an
estimated 45 seconds and was followed by several
aftershocks which were as large as magnitude 6 on
the Richter scale (Plafker et al, 1971). According to
Erickson et al (1970) shaking had a pronounced side
to side motion that making it hard to move around,
however the shaking was not strong enough to
throw people to the ground. Most of the damage to
infrastructure was concentrated in a 300km long
stretch within 165km of coast (Ericksen et al 1970).
Fault plane movement as indicated by the
hypocenters of the initial earthquake and its
aftershocks was along a fault surface approximately
140 km long parallel to the coast and 65 km wide
(Plafker et al, 1971).
Debris Flow:
The largest debris flow generated by the
May 31st
earthquake occurred in the valley between
the Cordillera Blanca and the Cordillera Negra
where the towns of Yungay and Ranrahirca lay
(Figure 5). The debris flow was generated from a
collapse atop Mount Huascaran which is located on
the eastern fringe of the Cordillera Blanca; it is the
tallest peak in the mountain range. The Cordillera
Blanca is composed primarily of Tertiary
granodiorites and Mesozoic marine sediments
(Bodenlos and Ericksen, 1955). Granodiorite rocks
are composed primarily of felsic minerals such as
quartz and plagioclase giving them a large
proportion of covalent bonds with Si making them
very resistant to both chemical and physical
erosion. For this reason granites and granodiorites
can often form sheer precipices 1000s of meters
tall. The Cordillera Negra to the west are composed
primarily of mafic dark minerals. Many landslides
were also generated in this mountian chain but
none with the same impact associated with that
from Mount Huascaran. The rocks of the Cordillera
Negra generally fail more easily than those of the
Cordillera Blanca because of theyre chemical
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bonding which makes them less resistant to
chemical weathering and physical erosion. Mafic
rocks tend to have more metals in theyre structure
and thus more ionic bonding. The mountains of the
Cordillera Negra do not form the same type of
oversteepened peaks that can be observed in the
Cordillera Blanca, since ionic bonds are much more
easily broken due to chemical weathering.
Figure 5: View of Mount Huascaran with Yungay, Ranrahirca
and the debris flow deposits from Plafker et al., 1971
The debris flow started near the peak of
Mount Huascaran and gained a considerable
amount of momentum as the snow ice and rock
virtually free fell for a full kilometer to the base of
the cliff at the peak. The flow was able to reach
astounding speeds on the order of 200 miles per
hour or about 320 kilometers per hour (Cluff, 1970).
It was estimated by eyewitnesses that the flow
started immediately after or during the earthquake
and had reached the town of Yungay within 3
minutes after it had commenced (Plafker et al,
1971). The extreme speeds attained by this debris
flow are likely attributed to: the initial free fall of
material and the fact that the upper portion of theground that needed to be covered was a steep
glacier offering very little friction to slow the flow,
the snow and ice incorporated into the flow likely
helped it maintain high speeds by reducing its
internal friction (Plafker et al, 1971). The energy of
the flow by the time it had reached the towns of
Yungay and Ranrahirca was still sufficient to carry
several boulders weighing up to 7000 metric tons
(Figure 6). The town of Yungay was completely
buried under an estimated 5m of debris there are
only a few relects of the old town. Since the event a
new town has been constructed further to the
north off of the debris flow deposits from the 1970
event. The town of Ranrahirca was also mostly
buried in the 1970 event, it has also been
reconstructed however it still lies on old debris flow
deposits (Plafker et al, 1971).
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Construction:
Many of the buildings in the area that was
most severely affected by the 1970 Earthquake
were constructed with adobe bricks, commonly
made of a combination of clay, straw, sand and
water. Adobe constructions usually consist of large
mud or clay bricks held in place by comparatively
weak mortar (usually mud) (Glass et al, 1977).
Although adobe constructions do have many
advantages as a building material; inexpensive easyto construct and good insulating properties, these
constructions are far from ideal during an
earthquake. The heavy bricks are easily shaken free
of the weak mortar which holds them in place. Glass
et al (1977) found while studying the effects in
Guatemala of an earthquake of magnitude 7.5 on
the Richter scale, that all deaths associated with
building collapse in the study area occurred in
adobe constructions. While houses that were built
in other styles either remained intact or collapsed
without causing death. A different choice of
building materials may not have helped those that
were overcome by the debris flow from Mount
Huascaran, but it certainly would have made a
significant difference in the death toll of the
earthquake as a whole.
Conclusions:
The west coast of South America is a very
tectonically active boundary. Large scale
earthquakes will continue to occur in this area as
long as subduction continues along this coast. In
order to mitigate loss of life and livelihood a few
steps need to be taken, some of which are under
way already. First, towns and villages should never
Figure 6: Boulder transported by debris flow estimated to weigh over 7000 metric tons. Note meter stick in central photo is 4m tallFigure from Plafker et al., 1971
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be constructed atop debris flow deposits. These
deposits are situated in areas where debris flows
have already occurred and will occur again at some
point in the future. Furthermore these types of
deposits tend to be poorly compacted and un-
cemented, making them vulnerable to liquefaction
amplifying damage during earthquakes. If towns are
constructed away from previous debris flow
deposits out of the way of future landslides, 10s of
thousands of lives can be saved. Second, a more
earthquake friendly construction style must be
adopted by the people living in high risk areas. Thereason why adobe construction is so popular in this
part of the world is because it is a very cost
effective way of building structures with good
insulating properties. Blondet et al. (2003)
published a report detailing how to improve adobe
brick constructions performance during seismic
events. If cost effective alternatives and
improvements of this kind can be made readily
available to the general populations of these
developing countries then it might be possible to
prevent deaths associated with building collapse as
well as to minimize damage to infrastructure and
prevent hundreds of thousands of people from
becoming homeless every time an earthquake
occurs. Third, general awareness of the hazards
associated with living in a tectonically active area
should be a priority. If the population knew what
were the safest steps to take immediately after an
earthquake then loss of life could be minimized.
Since the recurrence rate for events of this
magnitude is not very high for one particular area
the population may be lulled into a sense ofsecurity. Ericksen et al. (1970) found that an event
of the magnitude of the May 31, 1970, earthquake
had not occurred in that area before for at least
three generations according to locals. If the villagers
in towns such as Yungay or Ranrahirca knew that
the safest thing to do in the moments after an
earthquake was to move to a specific rally point at
high ground then potentially 10s of thousands of
lives could have been saved on May 31st
1970.
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Sources
E., Bodenlos A. J. and Ericksen G. "Lead Zinc deposits of Cordillera Blanca and northern Cordillera Huayhuash,
Peru." U.S. Geological Survey Bulletin, 1955: 1-166.
Ericksen G. E., Plafker G., Fernandez Concha J. "Preliminary Report on the geologic events associated with the
May 31st, 1970, Peru earthquake." Geological Survey Circular, 1970: 1-25.
Glass R. I., Urrutia J. J., Sibony S., Smith H., Garcia B., Rizzo L. "Earthquake Injuries Related to Housing in a
Guatamalan Village." The American Association for the Advancement of Sciences, 1977: 38-43.
Marcial Blondet, Gladys Villa Garcia M. and Svetlana Brzev. Earthquake-Resistant Construction of Adobe
Buildings: A Tutorial. Tutorial, Oakland, California: Earthquake Engineering Research Institute, 2003.
O., Norabuena. "Velocity Structure of the Subducting Nazca Plate beneath central Peru as inferred from Travel
Time Anomalies." MSc Thesis, Virginia, 1992.
Plafker G, Erickson G. E. and Fernandez Concha J. "Geological Aspects of the May 31, 1970, Peru Earthquake."
Bulletin of the Siesmological Society of America, 1971: 543-578.
Rhea, S., Tarr, A.C., Hayes, G., Villaseor, A., Furlong, K.P., and Benz, H.M. Seismicity of the Earth 1900-2007,
Nazca plate and South America. Open File Report, U.S. Geological Survey , 2010.
S., Cluff L. "Peru earthquake of May 31, 1970; engineering geology observations." Bulletin of the Siesmological
Society of America, 1971: 511-533.
S., Hasegawa A. and Sacks. "Subduction of the Nazka Plate beneath Peru as Determined from Siesmic
Observations."Journal of Geophysical Research, 1981: 4971-4980.