VOLCANIC HAZARDS ASSESSMENT OF SAVAI'I, SAMOA - NET
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VOLCANIC HAZARDS ASSESSMENT
OF SAVAI’I, SAMOA
Paul Taylor’ and Lameko Talia
August 1999 SOPAC Technical Report 295
’Australian Volcanological Investigations
Apia Observatory, Samoa
Funding for this Project was provided by the
United Nations Development Programme through the
South Pacific Disaster Reduction Programme[3]
CONTENTS
SUMMARY.................................................................................................................................... 5
ACKNOWLEDGEMENTS............................................................................................................. 6
INTRODUCTION.......................................................................................................................... 7
THE GEOLOGY OF SAVAI’I......................................................................................................... 7
AGE RELATIONSHIPS OF PREHISTORIC VOLCANIC ACTIVITY.............................................8
RECENTVOLCANICACTIVITY......................................_...........................................................10
FEATURES OF VOLCANIC ACTIVITY ON SAVAI’I................................................................... 10
STRUCTURAL INFLUENCES ON RECENT VOLCANIC ACTIVITY......................................... 14
FREQUENCY AND DISTRIBUTION OF VOLCANISM THROUGH GEOLOGICAL TIME......... 14
VOLCANIC HAZARDS ON SAVAI’I........................................................................................................ 16
VULNERABLE ELEMENTS ON SAVAI’I..................................................................................... 19
CONCLUSIONS AND THE RISK OF FUTURE VOLCANIC ACTIVITY ON SAVAI’I.................. 22
RECOMMENDATIONS.................................................................................................................. 23
REFERENCES......................................................................................................................... 24
APPENDIX
Volcanic Hazards Map of Savai’i, Samoa
Introduction...........................................................................................................................
25
ExplanatoryNotes.................................................................................................................. 25
Precautionary Notes...............................................................................................................26
[SOPAC Technical Report 295 -Taylor &Talia][4]
LIST OF FIGURES
Figure
1 Locality Maps.......................................................................................................................8
2 Simplified Geological Map...................................................................................................8
. Topography.............................................................................................................. 9
3 Savai'i 1.
4 1760 Lava Flows................................................................................................................ 11
5 1902 Flows and Vents....................................................................................................... 11
6 1905 -11 Flows........................................................ 11
7 Features of the 1905-11 Eruption................................................................................ 12-13
8 Features of Savai'i Volcanism............................................................................................ 13
9 The Ologogo Fault System................................................................................................ 14
10 Temporal Distribution of Volcanic Cones and Lavas on Savai'......................................... 15
11 Vulnerable Elements..........................................................................................................20
LIST OF TABLES
Table
1 Volcanic Stratigraphy of .,. Savai'i........................................................................... 9
.,.
2 Historic Eruptions onSavai'i.............................................................................................. 12
3 Frequency and Distribution of Volcanism on Savai'i.......................................................... 16
4 Summary of Volcanic Hazards on Savai'i.......................................................................... 17
[SOPAC Technical Report 295-Taylor &Talia][5]
SUMMARY
The island of Savai’i is the largest and westernmost of the Samoan archipelago and is the home
for 50,000 Samoans living in villages located around the coast. Savai’i is a mature volcanic
environment, but exhibits the features of recent “monogenetic” volcanism. Geological studies
conducted on Savai’i suggest that volcanism has been occurring, more or less continuously,
since at least Pliocene times. It was suggested that two volcanic episodes have occurred during
the development of Savai’i. The initial episode, the ”shield-building phase”, produced the basal
lava shield during Pliocene to early-Pleistocene times. Following a period of relative quiescence
during the early to mid-Pleistocene the “post-erosional phase” has produced a sequence of
lavas and pyroclastics that overly the marked erosional boundary. This phase of volcanism has
continued during recent times. The most recent eruption occurred during 1905-11 and resulted
in the destruction of large areas of arable land and a number of villages located along the
northeast coast.
A volcanic hazard assessment conducted by the authors has concluded that activity is likely to
continue. Future eruptions will be effusive (lava-forming), explosive (tephra-forming) or a combi-
nation of both. Due to the lack of comprehensive chronological data for eruptions prior to the
mid-1700's it is difficult to forecast when future eruptions will occur. Furthermore, because of
the “monogenetic” nature of the volcanism it is also difficult to forecast where future vents will
form.
A preliminary volcanic hazards map has been constructed using the spatial density of prehis-
toric and recent vents. The hazard zones shown thus define the relative probability of a vent
erupting in that zone. However, due to the possibility that a vent may erupt anywhere on Savai’i,
the entire island must therefore be considered vulnerable to the hazards associated with future
activity. The likely hazards include: lava flows, pyroclastic flows/surges, lahars, tephra falls,
volcanic gases, ballistic ejecta, lightning and volcanic edifice collapse.
The recommendations put forward in the report provide a firm basis for planning for the future.
The priority activities include the development of a comprehensive chronological database of
prehistoric activity, the development of mitigation programs aimed at reducing the impact of
future activity and public awareness programs to increase the general level of knowledge of all
levels within the community.
[SOPAC Technical Report 295 -Taylor & Talia][6]
ACKNOWLEDGEMENTS
This report would not have been possible without the assistance of the Assistant Director of the
Apia Oservatory, Ausetalia Titimaea, and his staff who provided support during the project, and
allowed the junior author of this report to take part in fieldwork on Savai’i.
Warren Jopling, of Safua Tours in Savai’i provided much information during our many discus-
sions about the geology of Savai’i. Many of the localities could not have been visited without his
advice.
David Kear of Ohope, New Zealand is thanked for his assistance on the available 14C dates for
the Puapua Volcanics. Ian all of the Australian National University in Canberra is also
thanked for his valuable insights on K/Ar dating of the older rocks of Savai’i.
Thoughtful reviews by Shane Cronin, of Massey University and Russell Blong of the Natural
Hazards Research Centre at Macquarie University helped to improve this report.
Russell Howorth, Program Manager with SOPAC is also thanked for co-ordinating the project
and making many arrangements with the Government of Samoa.
Funding for this project was provided by the United Nations Development Program through the
South Pacific Disaster Reduction Program.
[SOPAC Technical Report 295 -Taylor &Talia][7]
INTRODUCTION
This report on the volcanic hazard assessment of Savai’i was initiated as a recommendation put
forward at the ”Workshop on Volcanic Hazards and Emergency Management in the South
Pacific”, held in Port Vila, Vanuatu from 24-28 February 1997 (Howorth and Elaise, 1997). The
workshop identified eight volcanic areas within the South Pacific region that require immediate
attention. One of these was Savai’i, Samoa, specifically the recent vent of Matavanu' located
on the northeast slopes of Savai’i. Matavanu erupted almost continuously from August 1905 to
November 1911 destroying several villages, large areas of arable land as well as extensive
areas of the lagoon and fringing/barrier reef along the northeast coast. The activity at Matavanu
was not the only activity to occur on Savai’i in recent times, Mauga Afi(1760 est.) and Mauga
Mu (October - November 1902) have also erupted.
Savai’i is the westernmost and largest island of Samoa. It supports a population of approxi-
mately 50,000, living mainly in villages along the coastal regions. The economy of Savai’i, as
part of Samoa, is based on subsistence agriculture, fishing and tourism. Hence, future destruc-
tion of arable land or reef systems by volcanic activity, could have a serious effect on the
economy of Savai’i and Samoa.
This report will refer to both published information on the geology of Savai’i and observations
made during fieldwork conducted between 2 - 17 September 1998 in an attempt to assess the
volcanic hazards on Savai’i. The resulting volcanic hazards map (Appendix 1), can be used to
develop operational support plans for the vulnerable regions of Savai’i.
THE GEOLOGY OF SAVAI'I
The island of Savai’i is the westernmost and largest island of the Samoan group (Figure 1).
Covering an area of some 1700 km, Savai’i is composed primarily of the products that suggest
a long period of volcanism, dating back to at least the Pliocene. The majority of volcanism that
produced the many lava flows cropping out on Savai’i was concentrated along the Tuasivi
Ridge. The Tuasivi Ridge runs in an E-W direction through the centre of the island.
Few detailed geological studies of Savai’i have been undertaken. The most recent geological
mapping of Samoa (including Savai’i mapped at 1:100,000 and 1:400,000 scales) was con-
ducted by Kear and Wood (1959). Although their report was comprehensive in terms of the
general geological features, little information was provided on the volcanic processes and
associated hazards that have occurred during the recent past.
The major geological units on Savai’i are shown in Figure 2 and the volcanic stratigraphy is
summarised in Table 1. The oldest unit that outcrops on Savai’i is the Fagaloa Volcanics (the
“shield-building’’ phase) exposed in the Vaipouli River (on the north coast), the valleys that form
the headwaters for the Vanu River and the Alia-ole Gao'a (southern Tuasivi Ridge). This unit is
separated from the younger volcanics (the “post-erosional’’ phase) by a marked erosional
boundary.
Although Kear and Wood conducted extensive fieldwork on Savai’i, many of the geological
boundaries were determined by aerial photograph interpretation or based on the development
of vegetation or the degree of erosion. Hence, due to the lack of exposure, many of the bounda-
ries are only approximate.
Keating (1992) in a summary of previous work suggested that the concentration of volcanic
cones along the Tuasivi Ridge infers the presence of five rift zones. However, we consider an
alternate orientation of rift zones (Figure 3). It should be noted that many of the vents on the
lower slopes may not be directly associated with one of the rift zones. This interpretation differs
All village names and names referring to geographical and geological features will be given in italics.
[SOPAC Technical Report 295 -Taylor & Talia][8]
from Keating, in that the inferred rift zones are curved and merge around a central point that
coincides with the highest point on Savai’i, the cone of Mt Silisili. This interpretation is consistent
with the orientation of rift zones and summit caldera complexes on other oceanic volcanoes, eg.
Kilauea and Mauna Loa on Hawaii.
AGE RELATIONSHIPS OF PREHISTORIC VOLCANIC ACTIVITY
Few reliable dates are available for rocks that comprise the volcanic units of Savai’i.
Palaeomagnetic and radiometric dates were summarised by Keating (1985 and 1987). The
oldest rocks exposed on Savai’i (the Fagaloa Volcanics) are likely to be up to 2.5 Ma.[9]
Fagaloa Volcanics Interbedded pahoehoe and aa basaltic lava flows Pliocene to Early
interbedded with scoria and tephra beds. Most flows Pleistocene
are porphyritic (olivine) and include a variety of
xenoliths including peridotite. Flows are deeply weath-
ered with thick soils developed on the surface of the
units.[10] In a series of K/Ar dates on Savaii rocks, McDougall (1986) suggested that none were older than 0.4 Ma. McDougall (pers. comm. 1998) further suggested that ages of those samples from the units mapped as the Mulifanua Volcanics were generally from 0.2 to
[12]
Table 2.. Historic eruptions on Savai'i.
episode of collapse the vents coalesced to form a single elongate
crater 250 x 300 m. Lava fountains to a height of 200-300 m were
reported. Voluminous lava flows (10' m3)moved toward the NE,
reaching to coast on a 8 km wide front. The flows were observed to
be in excess of 8 m thick at several localities. Many of the flows,
particularly those produced during the later stages of the eruption,
flowed from the vent to the coast in lava tubes. Several villages,
(10' m') covered an estimated area of 190 km on the NW flanks of
the island with flows reaching the NW coast on a 10 km wide front.
The village of Aopo was reportedly surrounded and partially de-
stroyed by the lava flows. A number of other villages and extensive
areas of arable land along the NW coast were destroyed. No fatalities
are known.[14]
STRUCTURAL INFLUENCES ON RECENT VOLCANIC ACTIVITY
Kear and Wood (1959) mapped numerous normal faults along the Tuasivi Ridge in Western
Savai'i. They suggested that many of the faults were coincident with, and formed fissures, along
which were erupted many cones and lava flows of the Middle to Late-Holocene Puapua and
MulifanuaVolcanics.
Apart from the Tuasivi ridge faults, the remaining faults on Savai'i are not well defined. The
prominent fault system present on the northern side of the Tuasivi Ridge (here known as the
Ologogo Fault system) extends approximately 50 kms from near Safaua village to near Sasina
village on the north coast (Figure 9). The fault scarp varies from 10-20 metres high near its
western end to in excess of 150 metres at altitudes of 1000 metres. Several other major longitu-
dinal topographic features occur east of the village of Manase on the north coast and east of the
village of Paia (Figure. 9). Vertical relief of these features exceeds 20-30 metres. These fea-
tures are interpreted as the eastern end of the Ologogo fault system. It is suggested that this
fault zone is analogous to the Pali fault systems that are characteristic to the flanks of Kilauea
volcano in Hawaii.
Historic eruption vents (i.e. the Aopo Volcanics) formed in close proximity to the southern and
eastern part of Ologogo fault system. In addition, many lava flows of the Puapua and Aopo
Volcanics flow northward of this fault system. It is also important to note that, few recent vents
occur between the Ologogo fault system and the north coast.
No recent movements along the Ologogo fault system have been reported. Since past move-
ments have formed extensive scarps that dissect both the Mulifanua and the older Salani
Volcanics, we suggest that the majority of movement occurred during the early-Holocene.
FREQUENCY AND DISTRIBUTION OF VOLCANISM THROUGH GEOLOGICAL TIME
The vents and volcanic cones active during each of the major geologic units (after Kear and
Wood, 1959) are shown in a series of maps (Figure. 10) and described in Table 3.
Several apparent trends in the frequency and distribution of volcanism through geological time
are noted:
i) Volcanic activity during the Middle to Late-Pleistocene (the Salani Volcanics) involved both
explosive and effusive activity in the eastern half of Savai'i, along the East and North Rift
zones.
ii) Volcanic activity during the Late-Pleistocene (the Mulifanua Volcanics) also involved both
explosive and effusive activity, but was mostly centred in the western half of Savai'i along the
West and South rift zones, apart from minor activity at the eastern end of the east rift zone.iii) Both explosive and effusive activity continued during the Middle to Late-Holocene (the
Puapua Volcanics, but was spread more randomly along the East-West Rift zone.
iv) Activity during historic times (the Aopo Volcanics) occurred along the West and North Rift
zones and may have been associated with the Ologogo fault system.
v) The frequency of eruptive events appears to have increased rapidly through time. The calcu-
lations of eruption frequencies, however, are limited by the poor age constraints on the
volcanic periods (except the historical period). In addition, for the older units, evidence of
many events is probably buried by younger product thus reducing the apparent eruptive
frequency. However, despite these issues - there remains a very strong trend of increasing
eruptive frequency that may continue to future activity on the island.
[SOPAC Technical Report 295 -Taylor &Talia][16]
Table 3: Frequency and distribution*of volcanism through geological time on Savai'i.
Distribution of Vents
* Many vents that may have been active during the older phases of activity (eg. those formed during the Salani phase)
may have been covered by the products of the more recent phases,
VOLCANIC HAZARDS ON SAVAI’I
The most common volcanic hazards likely to occur during future eruptions on Savai’i are lava
flows, volcanic gases and aerosols, tephra falls and volcanic earthquakes. Although not re-
ported during recent activity, other hazards that are likely include; ballistic ejecta, lightning,
pyroclastic surges, lahars and volcanic edifice collapse. Table 4 summarises the volcanic haz-
ards that may occur during future eruptive activity on Savai’i, probable warning periods, likeli-
hood to cause damage, injury or death, areas affected and probable locations. Each of the
hazards are outlined in the following section.
Lava flows are by far the most common volcanic hazard on Savai’i. Both pahoehoe and aa
flows are likely to occur and they may flow long distances from vents along the Tuasivi Ridge
toward the coast. Recent eruptions indicate the likely magnitude and effects of lava flows. The
1760 eruption aa lavas destroyed several villages on the northwest coast. Flows travelled 10-15
km from the vent and crossed the coast on an 8 km front. The 1905-11 predominantly
pahoehoe flows travelled 15 km, destroying five villages on the northeast coast. Both the 1760
and 1905-11 flows destroyed large tracts of plantation land and large expanses of the fringing
reef. Prehistoric lava flows on Savai’i have also travelled more than 10 km from their vents and
often reached the sea.
Lava flow hazards include inundation and burning of everything in their paths, generation of
widespread fires, volcanic gas emission (where they enter the sea), and local explosions (where
flows encounter standing water).
The hazard for lava flows is greatest along stream valleys and canyons and in coastal regions
of Savai’i where streams valleys cross the coast. On the hazard map (Appendix 1), likely flow
paths of lava flows are shown by the red arrows.
Volcanic gases (including in general order of abundance; H2O CO2,, SO2, H,2 S, HCI, NH,, HF
and other toxic components) are commonly emitted during eruptive activity. Volcanic gases
interact with moist atmospheres to almost immediately produce aerosols (very fine droplets) of
[SOPAC Technical Report 295 -Taylor &Talia][17]
Table
Probable Location
Edifice Minutes to High to High to Very large Northwest section of the island;
collapse hours extreme extreme tsunamis may be formed depend-
ing on scale of collapse that may
affect coastal areas of Savai’i and
surrounding islands[18]
easterly and north-easterly. .....in this direction the destruction of the trees was complete,
and to nearly as far as Olonono, a distance of2 1/2 miles, their they were bleached skel-
etons were all that remained...... I heard from credible sources that the cocoa-nut palms
had been killed as far as 5 miles to the west of the crater."(p634)
Anderson (1910) has also described the effects of the Ua Sami (acid rain) formed by the inter-
action of the lava and seawater in coastal localities. Anderson states:
"At Matautu, 2 miles west of the lava, the damage was due to the Ua Sami formed by the
action of the lava on the sea-water, which was considered even more poisonous than that
discharged from the crater. Many cocoa-nut palms had been damaged and some had
died, but most had recovered; while several breadfruit trees were killed entirely, and some
recovered only with the loss of their upper branches."(p635)
This damage caused at coastal areas was a result of lava haze (Laze), aerosols of sulphuric
acid and hydrochloric acid produced by the reaction of seawater and hot lava.
The hazard from volcanic gases and aerosols is greatest near the vent and in downwind areas,
as well as an additional hazard from lava flows where they enter the sea, Due to the prevailing
east to southeast winds, the greatest volcanic gas and aerosol hazards are likely to be to the
west or northwest of the vent that forms.
Volcanic Ash (Tephra) falls: Tephra or volcanic ash falls probably accompany many Savai’i
eruptions. Tephra is ejected and carried high above the vent, before being dispersed by the
wind and deposited in downwind areas. Significant tephra falls were not recorded during the
historic eruptions.
Tephra falls of >10cm thickness commonly causes collapse of building roofs, destruction of
crops and occasionally injury to people and animals that are not sheltered from the heaviest of
falls (e.g Blong, 1984). Fine tephra may contaminate food supplies and water, as well as caus-
ing damage to electrical and mechanical equipment.
The greatest hazard from tephra falls is similar to that for volcanic gases, ie in downwind and
near-vent areas. The risk from tephra falls is greatest in Tephra Zone 1 on the hazard map
(Appendix 1), while a moderate degree of risk from tephra falls occurs within Tephra Zone 2.
Ballistic ejecta - are hot, usually large fragments of rock debris ejected explosively from an
erupting vent. This process probably occurs in most future eruptions on Savai’i. The ejecta may
either be solid (scoria or lava blocks), or partially fluid fragments of lava (bombs). Fragments
are normally blasted within a few hundred metres radially around the cone. Impact of ballistic
ejecta will destroy most structures and will probably be fatal to any life within their range.
The greatest hazard from ballistic ejecta is confined to within a few hundred metres of vent
locations.
Lightning is a secondary hazard that may occur during explosive eruptions. Lightning is caused
by the electrostatic effects within an column. These do not strike the ground as frequently as
normal meteorologic lightning, but still may cause fires and damage buildings and crops.
The greatest hazard from volcanic-induced lightning is likely to be in the areas that are also
subject to the greatest volcanic ash fall hazard.
Pyroclastic surges are generated by the explosive interaction of rising magma and water.
These radial blasts of rock debris, steam and gas travel up to 3 km from the vent at velocities of
up to 300 km/hr. These magma-water eruptions (hydrovolcanic) are most likely io occur within a
lagoon or shallow near-coastal area, but may also occur from lakes and swamps. Deposits
produced in this manner are exposed along the coast at the base of Tafua cone.
[SOPAC Technical Report 295 -Taylor&Talia][19]
The hazards of pyroclastic surges are probably confined to within 1 km of the vent under most
circumstances, but larger blasts may affect areas of up to 3 km radially. Surges damage all
crops and structures in their paths, and are highly hazardous to people and animals within their
range.
Coastal villages in the immediate area of an active cone would be at greatest hazard from
pyroclastic surges.
Volcanic earthquakes commonly precede and accompany eruptions regardless of the charac-
ter of the activity. Although violent earthquakes were reported to precede and accompany the
1902 eruption (Jensen, 1907; Thomson, 1921), no damage that could be directly related to the
events was reported. Jensen (1907) reported that the initial activity at Matavanu that occurred
between 9-10.00 pm on 4 August 1905 was accompanied by an earthquake that:
".....was strongly felt only near the point of eruption, but nevertheless registeredby the seismo-
graphat Apia. "(p656-657)
These two reports indicate that earthquakes accompanying activity on Savai’i, probably small in
magnitude and not likely to cause substantial damage.
Hazards associated with earthquakes are probably greatest in near vent areas.
Lahars or volcanic mudflows have not been reported during historical activity of Savai’i. How-
ever, intense tropical rainfall coinciding or soon following future activity, could potentially gener-
ate lahars. Lahars travel rapidly (>30km/hr), contain large amounts of sediment, and sweep
away everything in their paths.
The greatest hazards from lahars are within stream valleys radiating from areas of eruptive
activity (particularly areas of volcanic ash fall) and sweep away everything in their paths.
The greatest hazards from lahars are within stream valleys radiating from areas of eruptive
activity (represented by red arrows on the hazard map in Appendix 1). It should be noted that a
number of villages are located on or within a short distance of streams that are present on
Savai’i, and as such would be at risk from the effects of lahars.
Volcanic edifice collapse: The products that comprise volcanic islands are inherently unstable,
with alternate layers of lava and porous breccias dominating. A number of volcanoes in the
Southwest Pacific may have experienced large-scale collapses (e.g. Johnson, 1987; Taylor
1998). Edifice collapse may or may not relate to eruptive activity and commonly occurs along
major zones of structural weakness in the cone (eg. major fault zones). Events such as earth-
quakes or intrusion of magma into the edifice can act as triggers to collapse. Large magnitude
collapses into the sea are capable of generating regionally destructive tsunamis.
On Savai’i, the Ologogo fault system is a major zone of potential weakness on the northwest
slopes (see the hazard map in Appendix 1), where movement on a large scale could occur.
Although movements along the system have not been reported during historic times, it should
still be treated as a site of potential slope failure. Such an event will have impacts that reach far
from Savai'i itself.
VULNERABLE ELEMENTS ON SAVAI’I
It is important to consider land-use patterns when considering the vulnerability of an area to any
hazard/s On Savai’i, forest cover, population patterns, land tenure, social infrastructure, agricul-
tural activities and fishing activities are summarised by Ward and Ashcroft (1998). It is there-
fore, pertinent to briefly summarise the land-use patterns on Savai’i. For the purpose of this
report the land-use categories (vulnerable elements) are Village and commercial activities,
[SOPAC Technical Report 295 -Taylor &Talia][20]
Agricultural activities, Bushland, and Fishing activities. All of the functions within the outlined
land-use categories are vulnerable to the affects of volcanic activity to a varying degree depend-
ing on the location of the vent. Figure 10 summarises the location of many of the vulnerable
elements on Savai’i.
i) Village and Commercial Activities: include all regions of the island devoted to primarily to
residential, commercial, administration, tourist and transport functions. In general terms the
majority of these activities are located within several kilometres of the coast.
a) Villages have been developed along most of the coast of Savai’i. The major population
centres are located along the southeast and east coast between Satupaitea and Puapua
villages, along the north coast between Saleaula and Sasina villages and in the northwest
including villages in and around Asau, Viasala and Falealupo-uta. In general terms, village
and commercial activities are concentrated in an area within about 1 to 1.5 km of the coast-
line. Several other village areas are also located along the south and southwest coast.
Several other villages including Aopo, Letui, Patamea, Japueleele and Vaolia are located up
to 8 km inland of the coastline.
All the population centres on Savai’i are susceptible to the effects of volcanic activity, high-
lighting that at least 50,000 Savai'ians are at risk. During activity the flow hazards (lava,
pyroclastic or mud flows) may destroy dwellings and other buildings in their path. Heavy
tephra falls in downwind localities (western and northwestern parts of the island) may cause
the collapse of buildings, particularly those with low pitch roofs. Tephra and volcanic gases
may have a direct effect on individuals by causing respiratory problems or in extreme cases,
death may result. Volcanic earthquakes more also result in damage to buildings, although
the areas would be localised. The overall impacts on the population will depend on the type
of activity and the location of the vents.
b) The administrative centre of Salelologa, is located on the southeastern coast, and is
where most government and other services are located. Other government services such
as the police office and the immigration office are located at Tuasivi. The only well-equipped
hospital on Savai’i is also located at Tuasivi
If Salelologa or Tuasivi are affected by an eruption, most of the government services would
be disrupted. Should an eruption destroy or severely damage the hospital it would effec-
tively leave Savai’i without sufficient medical facilities to cope with expected casualties.
Educational facilities, both government and non-government sponsored, are located in most
villages and a college is located in Tuasivi. Depending on the location and type of activity
schools and the pupils could be affected but because of the numerous schools around
Savai’i, students could be relocated until the eruption ceases.[21]
c) A telecommunications centre and an electric power plant are located along the main coast
road west of Saleleloga.
Telecommunications and power supply functions are highly susceptible to volcanic activity.
Although lava flows may disrupt both phone and power transmission by destroying the lines,
tephra may cause considerable disruption due to the shorting-out of the electrical systems. It
is also known that tephra can interfere with radio signals, which may cause disruption to
radio-telephones.
d) Water is an important commodity for an island like Savai’i. Water is drawn from permanent
streams or from springs that are located around the coast. To ensure that water is supplied to
all localities, an extensive water reticulation system has been developed. water is pumped
gravity fed to villages by a network of pipes that commonly parallel the main coast road.
Water supply systems are particularly susceptible to the effects of volcanic activity both
through the destruction of the supply lines and contamination of storage facilities. As much of
the water is distributed by a system of pipelines, their destruction by lava flows and earth-
quakes would have a considerable impact on village life. Both chemical and physical con-
tamination may result from only small tephra falls on storage facilities. An increase in the
acidity of the water or contamination by elevated levels of elements including flourine, may
render the entire supply unsuitable for use. Tephra may also cause blockages in filters and
pipes resulting in disruptions. Supplies drawn from bores and springs may be less suscepti-
ble and these may provide alternate supplies, but it should be noted that changes in the
chemistry of the waters may occur during or following the activity.
e) A wharf and other anchorage facilities that service most of the island have been estab-
lished at Salelologa. Regular daily ferry services link Savai’i with Upolu. Other less developed
anchorage facilities are also present at Asau, on the northwest coast. Port facilities are
critical to the economy of Savai’i as all the produce is exported through Salelologa and Asau.
Closure or disruption to either port during an eruption will thus have long-lasting effects on
the economy of Savai’i.
f) Airports are located near Salelologa and Asau. Regular services from and to Upolu are
scheduled once or twice daily.
Although both airports are at risk from lava flows, a far more significant threat is from tephra
and volcanic gases that may accompany an eruption. Due to the dominant E to SE winds the
airport at Salelologa is exposed to a low degree of risk from tephra and gas. The domestic
air route to and the airport at Asau, however, are subject to a relatively high degree of risk.
Significant damage and loss of power may occur when planes fly through tephra. Falls of
tephra on the runways may also cause disruption to services. It should also be noted that a
number of international air routes traverse airspace in the region of Savai’i that may be
affected by tephra plumes that reach altitudes of 8-10 km. Costly diversions may be neces-
sary to avoid the tephra hazards.
g) An extensive system of roads has been developed to serve as the major lifeline on Savai’i.
A sealed road connects all populated areas around the coast. An extensive network of un-
sealed roads and tracks connect inland villages and plantations to the main coast road. The
majority of villages are serviced by a bus service on at least a daily basis. Outlets for fuel and
other automotive services are located in Salelologa, Avao and Asau.
The road around the coast of Savai’i is vulnerable to the effects of future activity, particularly
lava flows. Should an eruption result in the closure of a section of the main coast road, only
minor disruption would be experienced. Closure of the inland roads would have a consider-
able impact on access to the isolated villages and plantation lands.
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h) Tourism is an important industry for Savai’i. A number of hotels, guest houses and ac-
commodation facilities have been established around the island. Larger hotels are concen-
trated along the southeast coast near Salelologa and along the north coast near Manase.
Other guesthouses have been established in a number of other villages along the north-
western and southwestern coast.
The occurrence of an eruption could have a significant impact on tourism on Savai’i by
reducing the number of visitors because of the possible risk to life. However, due to the
spectacular nature of the phenomena, an eruption could also have a positive effect on
tourism due to man’s curiosity.
ii) Agricultural Activities: are the regions, devoted primarily to the production foodcrops,
coconuts, cocoa, bananas, the raising of cattle and other commercial activities such as
timber. Agricultural activities dominate the majority of the landscape of Savai’i. Plantation
activities are undertaken on the lower slopes of the Tuasivi Ridge, below altitudes of about
500 metres above sea level.
Most crops are vulnerable to damage by volcanic activity and since the economy of Savai’i
is based largely on agriculture there would be wide ranging and long lasting effects. Be-
sides destruction by lava flows, foliage is susceptible to both tephra and volcanic gases.
Falls of tephra may either destroy or severely damage the foliage causing no or poor yields.
Volcanic gases, in particular aerosols, that adhere to the grasses and the foliage of other
crops may lead to retarded growth or poisoning of animals or humans that consume them.
iii) Fishing activities: for both domestic and commercial purposes are conducted within the
lagoons, on the reefs and at offshore locations adjacent to the reef faces.
Past eruptions that have produced lava flows, have destroyed extensive areas in the coastal
zone, particularly the valuable fishing grounds associated with the reefs. Future eruptions,
particularly those producing lava, could be expected to have a similar effect both on the
coastal environment and the livelihood of the Savai'ians.
iv) Bushland: are those regions not generally used for any of the above activities, and includes
the remainder of the interior of Savai’i, that in the most part is inaccessible. Several large
areas of this bushland have already been designated as areas having significant conserva-
tion value but because of the probable location of future vents may be affected by activity.
CONCLUSIONS AND THE RISK FROM FUTURE VOLCANIC ACTIVITY ON SAVAI’I
Although the island of Savai’i is a mature volcanic environment, it exhibits the features of recent
monogenetic volcanism and as such it is reasonable to conclude that activity will continue in the
future. Activity may be effusive, explosive or a combination of both.
From the apparent random location of the vents formed during past periods of activity, it is
difficult to forecast the location of future activity. However, considering the location of historic
eruptions and the youngest prehistoric events may occur along either the West or North Rift
zones.
Due to the lack of comprehensive chronological data for eruptions prior to the mid-1700s it is
difficult to forecast when future activity may occur. The periodicity of historic activity suggests
[SOPAC Technical Report 295 -Taylor&Talia][23]
that eruptions have an approximate recurrence period of about 150-200 years. For Savai’i that
would suggest a moderate-high possibility of an eruption within the next 100 years.
Due to the unknown location of vents during future periods of activity, the entire island is consid-
ered vulnerable to the effects of future activity. If future activity is effusive, the movement of lava
flows will be controlled by the local topography, with the flows taking a route of least resistance
to the lower flanks, i.e. along river channels and canyons. It is thus suggested that coastal areas
along river channels and downslope of highland canyons are more vulnerable than other locali-
ties. Should future activity be explosive and produce tephra, localities downwind of the vents, i.e.
to west and northwest, would be vulnerable to the effects of tephra fail.
The Ologogo fault system may be a possible zone of weakness to engender large scale col-
lapses along the northwest part of Savai’i. A further investigation of this structure is required to
evaluate the risk of future activity of this type.
RECOMMENDATIONS
Following the completion of this volcanic hazards assessment of Savai’i, it is recommended
that:
1. A program of volcano monitoring should be established. Since recent activity was accompa-
nied by periods of seismic activity, a seismic station would be an appropriate technique.
This station, once established, should have the capability of continuous transmission of data
to a recording station located at the Apia Observatory.
2. Officers of the Apia Observatory should be provided with appropriate training in volcanologi-
cal techniques to ensure that data collected during the monitoring program can be effec-
tively analysed. This should enable trends to be monitored and realistic forecasts of activity
to be made.
3. A detailed study of the volcanic geology of Savai’i should be undertaken to develop an
understanding of the volcanic processes that have occurred, and to develop a comprehen-
sive chronology of the eruptive activity. The study should include the use of both absolute
and relative dating techniques.
4. An operational support plan should be developed for Savai’i. This plan should include:
a. the likely hazards that will occur,
b. preventive and protective measures that may be appropriate,
c. the organisational infrastructure necessary to implement and maintain the plan,
d. hazard zones, alerts and warnings,
e. operational support plans to be followed during a future volcanic emergency, and
f. a series of eruption scenarios to indicate the likely effects Of future activity.
5. In-country training and workshop programs should be developed to inform and educate
government and business agencies involved in the development and planning process for
Savai’i.
6. Public awareness programs should be developed to promote an understanding of volcanic
activity on Savai’i and what should be done during an emergency.
[SOPAC Technical Report 295 -Taylor &Talia][24]
REFERENCES
Anderson, T. (1910) The Volcano of Matavanu in Savai'i. QuarterlyJournalof the Geological society of
London. vol.66, pp621-639.
Blong, R.J. (1984) Volcanic Hazards, a Sourcebookof the Effects of Eruptions. Academic Press, Sydney.
Grant-Taylor, T.L. Rafter, T.A. (1962) New Zealand Radiocarbon Age Measurements - 5. Newzealand
JournalofGeology and Geophysics.vol.5,p331-359.
Howorth, R, Elaise, A. ( I 997) Workshop on Volcanic Hazards and Emergency Management in the South
Pacific. SOPAC MiscellaneousReport, No.245.
Jensen, H.I. (1 907) The Geology of Samoa, and the eruptions in Savai'i. Proceedings of the Linnean
Society of NSW. vol1.31, pt.4, pp641-672.
Johnson, R.W. (1987) Large-scale Volcanic Cone Collapse: The 1888 Slope Failure of Ritter Volcano and
other Examples from Papua New Guinea. Bulletin of Volcanology. vol.49, p669-679.
Kear, D., Wood, B.L. (1959) The Geology and Hydrology of Western Samoa. Newzealand Geological
SurveyBulletin, No.63.
Kear, D., Wood, B.L. (1962) Structure, Landforms and Hydrology. In J.W. Fox (Ed), Western Samoa; Land
Life andAgriculture in TropicalPolynesia. Whitecombe and Tombs, Christchurch.
Kear, D. (1967) A Summary of the Geology of Western Samoa. Newzealand Journalof Geology and
Geophysics,vol.10,pp1200-1201.
Keating, B.H. (1985) Paleomagnetic Studies of the Samoa Islands: Results from the Islands of Tutuila and
Savaii. In Brocher, T. (Ed), Geological Investigations of the Melanesian Borderland, Circum-Pacific
Council for Energy and Mineral Resources - Earth Science Series, Vol.3, ppl87-199.
Keating, B.H. (1987) Summary of the Radiometric Ages from the Pacific. Inter-governmental Oceano-
graphic Commission, Technical Series Report, No.32.
Keating, B.H. (1992) The Geology of the Samoan Islands. In Keating, B.H., Bolton, B.R. (Eds), Geology
and Offshore Mineral Resources of the Central Pacific Basin, Circum-Pacific Council for Energy and
Mineral Resources -Earth Science Series, Vol.14,ppl27-178.
McDougall, I. (1986) Samoan Island Chain - A Hot Spot Trace. International Volcanological Congress,
Auckland, Hamilton, Rotorua New Zealand, Abstracts volume, p333.
Richard, J.J. (1962) Kermadec Tonga and Samoa. Catalogue of Active Volcanoes of the World. pt.13,
pp30-38
Taylor, P.W. (1998) A Review of Some Processes Occurring Along the Tofua Volcanic Arc, Kingdom of
Tonga. In K.A.Crook, P.Rodda and R.Howorth (Eds), Abstracts of Papers Presented at the STAR
Session 1998, SOPAC Miscellaneous Report, No.245, p37.
Thomson, J.A. (1 921) The Geology of Western Samoa. The New zealand Journal of Science and Tech-
nologyV01.4, No.2,p49-66.
Ward, R.G., Ashcroft, P. (1998) Samoa: Mapping the Diversify Institute of Pacific Studies, Suva.
[SOPAC Technical Report 295-Taylor & Talia][25]
APPENDIX 1
Volcanic Hazards Map of Savai’i, Samoa
Introduction
Savai’i is technically a “monogenetic” volcanic field and as such each new eruption site may occur at any
location on the island. Future eruptions may occur at or near old vents or at an entirely new site. Due to
the monogenetic character of volcanism it is difficult to predict the location of future eruptions.
The volcanic hazards map has been constructed using the spatial density of past eruptive vents, with the
hazard zones representing the relative probability for the outbreak of a new vent.
Explanatory Notes
Three zones delimit increasing risk from the flow and near vent hazards i.e. lava flows, lahars, pyroclastic
surges/flows, explosions, ballistic ejecta and hydrovolcanic activity.
Zone A (Red zone): includes the summit and the upper parts of the North and West Rift zones and is
where there is the greatest density of recent and prehistoric vents. This zone has the highest probability of
future vents erupting and is at greatest risk from near-vent hazards. Only a small portion of this zone will
be affected by a single eruption.
Zone B (Orange zone): includes the remainder on the North and West Rift zones and the East and South
Rift zones. This zone has a moderate probability of future vents erupting and is at moderate risk from
near-vent hazards. Only a small portion of this zone will be affected by a single eruption,
The red arrows are the most likely routes of flow hazards from vents that may erupt in Zones A and B.
Coastal regions of the island have a moderate to high risk of the effects of the flow hazards.
Zone C (White zone): includes the remainder of the island not part of Zones A and B. This zone is subject
to the lowest risk of near-vent hazards but is still subject to a moderate to high risk from flow hazards.
Hydrovolcanic activity may occur in the near-shore and off-shore areas that form part of this zone.
Two zones delimit increasing risk from tephra fall originating from vents that may erupt in Zones A and B.
Tephra Zone 1: is the area bounded by the black dashed line. This zone is subject to a high probability of
tephra falls of > 20 cm from vents that may erupt within Zone A. Only a small portion of this zone will be
affected by a single eruption.
Tephra Zone 2 is the area bounded by the grey dotted/dashed line. This zone is subject to a high prob-
ability of tephra falls of > 20 cm from vents that erupt within Zone B and tephra falls of > 10 cm from vents
that erupt within Zone A. Only a small portion of this zone will be affected by a single eruption.
The vent location and the prevailing wind direction will control the size and location of the area affected.
Volcanic edifice collapse: The part of Savai’i that is likely to be affected by this hazard is the area to the
northwest of the Ologogo fault system (represented by the heavy broken line). Should this type of hazard
occur, the likely direction of movement is shown by the heavy blue arrows.
Regional destructive tsunamis may be generated during edifice collapse events.
[SOPAC Technical Report 295 -Taylor &Talia][26]
Precautionary Notes
Due to the lack of reliable age data for the volcanic units cropping out on Savai'i, this map should be only
considered as preliminary.
Due to the scale of the map the boundaries of the hazard/probability zones are only approximate.
The map is suitable for general disaster management, response and public awareness. However, for
detailed land-use planning, building site investigationsand engineering projects, further detailed site
investigations are required.
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