natural disasters term paper final

8/2/2019 Natural Disasters Term Paper Final

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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.php


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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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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.

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