Geotechnical challenges to geological ha

Geotechnical challenges to geologtcalhazard and protection of environment
D6fis geotechniques contre les risques g6ologiques et protection de I'environnement
Geotech n ische H erausforderungen zu geologische Risiken und U mweltschutz
HITOSHI KOIDE, Geological Survey of Japan, AIST/MlTl, Tsukuba, Japan

ABSTRACT: Tectonic instability of the Japanese islands, due to the interaction of four major plates, causes frequent geological disasters
such as the 1.995 Kobe earthquake and the 1991-1995 eruption of Unzen volcano, Kyushu. Short-range prediction of earthquakes and
volcaniceruptions has not been sufficiently successful to prevent casualties, although long-range and middle-range prediction ofearthquakes
and volcanic eruptions has made much progress in recent years.
The Kobe earthquake shockingly showed the risks to society from active faults in urban basement rocks. Extensive studies for
earthquake prediction and prevention of geological disasters are in progress, inctuding the assessment of fault activity by direct excavation,
drilting, geophysical and geochemical surveys of active faults, in situ stress measuroments, groundwater monitoring, continuous
measurement of crustal displacement by GPS and VLBI and mapping of crustal deformation by SAR interferometry.
Waste is the Achilles' heel of modem human society. The burning of fossil fuels, which is today the main energy source, emits a
huge volume of COz and causes global warming. Nuclear power, the most promising alternate energy source, produces small amounts of
high-level radioactive waste that are extremely toxic for more than several tens of thousands of years.
Subterranean disposal of COz is the safest effective method to reduce the emission of COzwhile buming fossil fuels. Formation of
CO2 hydrate in cool, deep aquifers virtually completely blocks the leakage of CO2 in submarine sediments and land sedimentary basins at

high latitudes.
I-ong-range stability of the geological environment is necessary for 0re safe geological disposal of high-level mdioactive waste. Rapid

tectonic deformation occurs near plate boundaries, volcanic fronts and major active faults. There exist few active faults and no volcanoes in
stable regions between the subduction zone and volcanic fronts in the island arc region,
Geological disposal ("mines for environment") is the most natural solution for the waste problems that arise from the use of mineral
resources extracted from mines. Although many mining technologies are available for "mines for environment", safe geological disposal
requires the accurate geological prediction of the nonexistence of future abnormal phenomena, instead of prospecting for past products of
abnormal mineral concentration. Only the geological evidence from the earth's history of nearly five billion years can verify long-range
predictions for subterranean containment of waste that are currently most often based on short-range experiments and numerical modeling.

RESUME: L'instabilit6 tectonique de I'archipel japonais due

d I'interaction de quatre grandes plaques lithosph6riques provoque de fr6quents

d6sastres g6ologiques, tels que [e tremblement de terre de Kob6 en ].995 et l'6ruption du volcan Ut:zer, l99l-95, Kyushu. I-a pr6diction d
court terme des tremblements de terre et 6ruptions volcaniques n'a pas 6t6 assez juste pour 6viter les d6g6ts, bien que de grands progrds aient

i moyen terme et i Iong terme au cours des demidres ann6es.
tremblement de terre de Kob6 a montr6 d'une manidre choquante les risques des failles actives dans les roches du socle des zones
urbaines. I-es 6tudes approfondies pour la pr6diction des tremblements de terre et la pr6vention des d6sastres gdologiques sont en cours,
incluant l'dvaluation de l'activit6 des failles par excavation directe, sondage, 6tude g6ophysique et g6ochimique des failles actives, mesures de
contrainte in situ, suweillance des eaux souterraines, mesure continue du d6placement de la cro0te par GPS, VLBI et cartographie de la

d6formation de la crotte par interf6rom6trie SAR.
Irs d6chets constituent le talon dAchitle de la soci6t6 humaine modeme. [a combustion des 6nergies fossiles, qui sont Ies principales
sources d'6nergie actuelles, produit d'6normes volumes de CO2 qui provoquent le r6chauffement de la plandte. l,lrcrgie nucl6aire, qui
6t6 faits dans [a prediction

Ir

constitue la source d'6nergie de remplacement la plus prometteuse, produit peu de d6chets d forte radioactivit6 demeurant trEs toxiques plus
d'une dizaine de milliers d'ann6es.
stockage souterrain du CO2 est la m6thode la plus efficace pour r6duire les 6missions de COz pendant la combustion des 6nergies

Ir

fossiles.

[a

formation d'hydrate de COz dans des aquifdres frais et profonds bloque pratiquement totalement les fuites de COz dans les

s6diments sous-marins et les bassins s6dimentaires terestres d hautes latifudes.

I-a stabilit6 i long terme de I'environnement g6ologique est n6cessaire pour le stockage g6ologique str des d6chets i forte radioactivit6.
Des d6formations tectoniques rapides suwiennent aux limites des plaques, aux ilcs volcaniques et aux principales failles actives, II y a peu
de failles actives et pas de volcans dans les r6gions stables entre la zone de subduction et les fronts volcaniques des arcs insulaire.
stockage g6ologique ("mine pour I'environnement") est la solution la plus naturelle aux problEmes de d6chets d6coulant de
I'utilisation des ressources min6rales extraites des mines. Bien que beaucoup de technologies miniEres soient disponibles pour les "mines
pour I'environnement", le stockage g6ologique exige Ia prddiction g6ologique de la non-existence de ph6nomdnes anormaux futurs, au lieu
de la prospection des produits pass6s des concentrations min6rales anormales. Seule une preuve g6ologique de I'histoire de Ia Terre sur
environ cinq milliards d'ann6es peut v6rifier les pr6dictions long terme pour Ie confinement souterrain des d6chets g6n6ralement bas6 sur

Ir

i

des exp6riences

i

court terme et une mod6lisation num6rique

.

1

005

ZUSAMMENFASSUNG: Die tektonische Instabilitdt der japanischen Inselgruppe aufgrund der Wechselwirkung von vier gro0en Platten
verursacht hiiufige geologische Katastrophen wie das Erdbeben 1995 in Kobe und der Ausbruch des Vulkans Unzen 1991-1995 in
Kyushu. Die kurzfristigen Vorhersagen von Erdbeben und Vulkanausbriichen waren bisher nicht erfolgreich genug, um Menschenopfer zu
verhindem. Allerdings hat sich die Zuverliissigkeit von lang- und mittelfristigen Voraussagen in den letzten Jahren deutlich verbessert.
Das Erdbeben von Kobe zeig[e mit schockierender Deutlichkeit das Risiko aktiver Verwerfungen in Gesteinen unter besiedelten
Gebieten. Es sind umfassende Untersuchungen iiber die Vorhersage von Erdbeben und die Verhindenrng geologischer Katastrophen im
Gange, einschlieBlich der Beurteilung der Verwerfungsaktivitiit durch direkte Ausschachtungen, Bohrungen, geophysikalische und
geochemische Messungen aktiver Verwerfungen, In-situ-Spannungsmessungen, Grundwassermessungen, Dauermessungen der
Krustenverschiebung durch GPS und VLBI sowie Kartierung von Krustenverformungen durch SAR-lnterferenzmeBverfahren.
Abfallprodukte sind die Achillesferse unserer modemen Industriegesellschaft. Die Verbrennung von fossilen Brennstoffen, unserer
gegenwiirtigen Hauptenergiequelle, erzeugt groBe Mengen von CO2-Gas und ftihrt zu einer globalen Erwiirmung. Kemkraftenergie, die
vielversprechendste altemative Energiequelle, evotgl geinge Mengen hoch radioaktiver Abfallprodukte, die ihre Giftigkeit iiber
zehntausende von Jahren beibehalten.

Unterirdische Entsorgung von CO2 ist das sicherste und effizienteste Verfahren zur Reduzierung von CO2-Abgasen bei der

Verbrennung von fossilen Brennstoffen. Die Bildung von CO2-Hydrat in kiihlen, tiefen wasserfiihrenden Schichten verhindert nahezu
vollstiindig die Versickerung von CO2 in Unterwassersedimenten und landsedimentiiren Talkesseln in hohen geografischen Breiten.
Die I-angzeitstabilitdt der geologischen Umgebung ist notwendig fiir die sichere geologische Beseitigung von radiaktiven Abfiillen.
Pltitzliche tektonische Verformung geschieht in der Niihe von Plattengrenzen, wlkanischen Fronten und groBen aktiven Verwerfungen. Es
gibt nur wenige aktive Verwerfungen und keine Vulkane in den stabilen Regionen zwischen der Subduktionszone und vulkanischen
Fronten in den gebogenen Inselregionen.
Geologische Beseitigung ("Umweltgruben") ist die nattrlichste Iiisung fiir das Abfallproblem, die aus der Verwendung von
Mineralressourcen herriihrt, die Untertage abgebaut wurden. Zwar gibt es eine Reihe von Bergbautechnologien fiir "Umweltgruben", aber
die geologische Beseitigung erfordert geologische Vorhersage des Nichtvorhandenseins von kiinftigen anomalen Phiinomenen anstelle von
Prospektierung von alten Produkten anomaler Mineralkonzentration. Nur der geologische Beweis aus der nahezu fiinf Milliarden Jahre alten
Geschichte der Erde kann zu einer lantgzeitvoraussage fiir die unterirdische Sicherung von Abfall, die bisher lediglich auf
Kurzzeitversuchen und numerischen Modellen basiert, verifizieren.

1 1995

KOBE EARTHQUAKE

westem Japan, the Shinkansen(bullet hain) and Tokaido lines, were
disconnected for almost 80 days. The earthquake destroyed all 35
container berths at Kobe harbor, which is the country's largest container

port and the sixthlargest cargo port inworld. The economic loss caused
by the earthquake exceeded 10 trillion yen, but the indirect losses are

The January 77,1995 Kobe earthquake(also known officially as the
"Hyogo-Ken Nanbu earthquake" and popularly as the "Gteat Hanshin
earthquake") with a Japan Meteorological Agency magnitude of 7.2
(Fig.1) was the most disastrous earthquake in Japan since the Great
uncountable.
Kwanto earthquake that killed 143,000 people in and around the Tokyo
The Kobe earthquakeoriginatedata depthofabout 14 lon below
metropolitan arca m 7923. The toll from the Kobe earthquake has
Akashi charmel at the northem tip of Awaji island(Fig, 1).
reached 5,502 dead, 2 missing and 41,501 injured. Approximately
Kikuchi(1995) estimated from seismograph records that ttuee major
320,000 people were evacuated, mostly in Kobe and adjacent cities in
successive fault ruptures gensrated the Kobe earthquake(Fig.2). The first
Hyogo Prefecture as of January 23. A total
of 186,613 buildings were destroyed and
Active faults
531 fires bumt 7,120 houses. About 897o

Earthquakefaults.
of the fatalities were crushed or choked to
wtln sunace DreaKs
death and anolhet 70Vo died from the fires.
Earthquake faults
A maximum horizontal acceleration of 833
-,
lntensity 10
gal wd a maximum horizontal velocity of
138 cmlsec of ground motion were
Epicenter
*'
recorded in Kobe.
Akashi-Kaikyo
.
'
Bridge
Kobe, with its population of 1.5
million, is traditionally thegateway to the
+

Displacement
Vestem world' thanks to its role as the
leading intemational trade port in Japan.
The main part of Kobe is located along a
narrow corridor that connects westem and
cental Japan between the Rokko
mountains and Osaka bay. The ollapse of
elevated expressways and the destruction
railways in Kobe and adjacent cities
disrupted the traffic system of westem

Kishiwada

P

lP

2oh

Japan.

The blockage of highways in the
Kobe area halted the operation of extremely
efficient automobile assembly plants some
200 lon away by intenupting the supply of
auto parts. The main railway links to

Figurel-. Epicenter of the 1995 Kobe earthquake, associated faults and
severely damaged areas ( after Tsukuda,L995).

1006

Awaji island, where an 11
km long series of surface fault ruptures with right-lateral horizontal

fault rupture propagated southwestward in

deformation along a hidden fault zone. The strong shock of the sudden
displacement along thedeep partof the fault propagatedupward directly
along the fault, causing severe damage in a narrow belt above the fault

zone, which forms a seismic waveguide(Li et a1.,L994).

of, at most, 1.7 m and reverse vertical displacement of 1.3
appeared along the Nojima fault. This fault had previously been
recognized as an active fault due to about 20 m of dextral and 9.5 m
verticaldisplacement of azl or 22 thousand yearold tenace (Mizuno
et al., 1990). As active faults in Japan only moves during earthquakes,
the Nojima fault possibly had about ten episodes of similar seismic
activityover the past 22 thousand years.
The fault break that generated the Hyogo-ken Nanbu earthquake
displacement

m

2

EARTHQUAKE PREDICTION AND ACTIVE FAULTS

ofland along the converging
of four major plates @urasia, Pacific, Philippine Sea

and North America) formed the ftamework of the Japanese islands. The
northwestward subduction of ttre Philippine Sea plate along the Nankai
trough, where major earthquakes have occurred at an intewal of some
100 years, induces right-lateral slip along the nearly east-west Median
Tectonic Line (MTL). The MTL is very active over a length of 300 km
in the Shikoku district and in the western Kinki dishict, with a
displacement rate of 5-10 m/1000 years. The right-lateral slip of the
east-west trending MTL induces intensive east-west compression(Fig.3),
forming a basin-and-range structure bounded by north-south reverse
faults in the central part of the Kinki districl, where the old civilization of
Japan flourished in the basins of Nara,
and Osaka.
The accumulation of sediments and blocks
plate boundaries

J\

Epicenter
10lan

Awaji
Figtre 2. Fault break of the Kobe earthquake estimated from
seismograph records (Kikuchi, 1995).
also propagated north-eastward intodeep basement rocks directly below

downtown Kobe(Fig.2). However, none of the many active faults in
Kobe showed clear surface evidence of fault displacement, although
numerous surface fissures appeared due to slope instability triggered by
the earthquake(Kamai et al., L995).
The Akashi-Kaikyo bridge, linking Honshu(the largest island of
Japan) and Awaji island at the Akashi channel, is under constmction and
will be completed in 1998. The total length of the suspension bridge is
3,910 m and its centerspan of 1990 m will be the world's longest. The

tru,r1...Stress

concentration r

Earthquake

+

south tower( on the Awaji island side) of the main span of the
suspension bridge moved 1.3 m westward due to the earthquake relative
to the north tower, elongating the main span to 1.990.8 m(Fig.1). The
south end of the bridge(on the Awaji island side) moved 1.4 m
westward relative to the Kobe side, elongating the total length of the
bridge to 3,911.1 m. It is likely that the ftac€ of the earthquake fault
(Nojima fault) on Awaji island turns eastward in the Akashi channel
going under the main span of the Akashi-Kaikyo bridge.
The most impressive feature was the narrow "belt of severe quake
damage" (Shimamoto et al., 1995) that extended about 30 km with a
width of only 1 to 2 km(Fig.1). Damage was much milder to thenorth
ofthe belt in spite of tIrc rugged topography in and around the Rokko
mountains and milder to the south of the belt, too, even on wide,
reclaimed lands along the coast of Kobe and adjacent cities, although
liquefaction occurred extensively in the filled lands around Osaka bay.
A serious question is the cause of severe quake damage in a long
narrow belt. Soft, unconsolidated sediments and artificially buried soils
caused several spots of extremely severe damage. However, generally
thin unconsolidated sediments in the damaged belt are insufficient to
induce the entire belt of severe quake damage. Faults are the main cause
of severe quake damage in a long narrow belt. Seismic refleclion
surveys and gravity measurements revealed, hidden, very active faults
under some survey lines across the belt of severe quake damage(Endo et
al., 1995), although any displacements along the hidden faults dueto the
1995 earthquake are not apparent on the ground surface. Synthetic
aperture radar(SAR) interferometry images by the Japanese Earth
Resources Satellite(JERS-1)(Murakami, 1995) also revealed crustal
deformation along the belt of severe earthquake damage, suggesting

swarm

\L

lvlajor fault
Displacement

Kobeearthquake

Figure 3. I-arge compressive stress at the central Kinki
block induced by the rightJateral slip along MTL and ATL.
Regional tectonic stress is northwest-southeast compression.
Mechanical analysis was made on a simplified block model
which allows sliding along major active faults(Hamajima,
199s).

E
6

t
3r'
L

I
I

I

I

Figure 4. Temporal variation ofcrustal stress in
western Japan and great earthquakes along the Nankai,
Suruga and Sagami troughs (after Mogi, L981)
The Arima-Takatsuki Tectonic Line (ATL), which is an en
of the Median Tectonic Line, is a 50 km long,
east-west right-lateral active fault system(Fig.3). The Rokko-Awaji
echelon subsidiary fault

1007

westem cental Japan from the evidence of earthquake archeology,
history and direct excavation of active faults. He suggested a generally
eastward migration of earthquakes in the Kinki district, but potential
source faults of major earthquakes remain in both the east and west
sides of westem cenfial Japan.
Koide et al.(1979) classified the stages of development of a typical
fault system(Fig.5). The Median Tectonic Une is a fault in the old stage
(the 6th of 7 stages) but its segments in westem cental Japan were
rejuvenated into ttre vigorous stage(the 5th of 7 stages) in which
intermittent major earthquakes occur along a series of en echelon and
conjugate faults. lrft-stepping offset and branching of the Median
Tectonic Line(Fig.3) are similar to the bending of the right-lateral San

active fault system is mainly composed of northeaslsouthwest revsrse
faults, which accomodate rightJateral slip. Most of the reverse faults dip

northwest, but the Nojima fault, along which the Kobe earthquake
occurred, dips southeast in most of its segments. The northeastsouthwest active faults are arranged in a right-stepping, en echelon
arrangement forming a wide fault belt with right-lateral shear in the
Rokko-Awaji area. The Kobe earthquake and its aftershocks clearly
revealed the existeDce of a narower, nearly vertical, right-lateral fault
zone below ttre broader zone of surface en echelon active faults in the
Rokko-Awaji area(Fig.l). The deep earthquake source fault is straighter
than its surface extension, which branches into numerous en echelon
faults, but it still has several bends and jogs. The initial rupture ofthe
Kobe earthquake started at a jog in the Akashi channel(Fig.1). The
second and third ruptures started at other jogs under downtown
Kobe(Fig.2). Strong shocks are produced by the rupture of asperities

@lled seismologically "earthquake bnght spots" (Umeda

Andreas fault near Los Angeles, Califomia, U.S.A. Mogi (1974,1981)
noted a characteristic space-time distribution, i.e. a "doughnut pattem" of
seismic activity where precursory seismic quiescence precedes a major
earthquake in the future source region while seismic activity is high in its
surroundingregion(Fig.6). Mogi's "doughnut" is the most reliable clue

and

Yamashita, 1994).

for

Mogi (1981) noted that seismicity is high in western Japan about
50years before great earthquakes along the Nankai trough due to stress
accumulation and predicted that westem Japan should become
seismically active near the end of the 20th century(Fig.4). It is feared that

middle-r ange

earthquake

prediction.
c)

1995 Kobe

o
C

may

.9

(tr

o

ndicate the
iruring of a

t

ically active
ln westem

Sangawa
found
evidence

of

Great earthquake

rthquakes
Japan

survey of old
ruins ruptured by

Figure 6. The space-time distribution of seismic activity between
two successive great earthquakes - "Mogi's doughnut" (after
Mogi, 198L). The number 1 denotes foreshocks in a strict
sense and 2,3,4 and 5 are different kinds of foreshocks in a

liquefaction and

broad sense. The number

man-made tomb
hills deformed by

second kind.

nan
archeological

archeological

jogs

Seismicity has been low

Kinki district
suggest that the
Arim a-Takatsuki
Line and

active

fault system were
the source of the

1596 Fushimi
earthquake
(M7.5), which
severelydamaged

the main
metropolitan
areas

denotes the seismic gap of the

generate, ftequent small earthquakes.
the westem central Kinki district in the
compressional jog of the MTL, whereas numerous small earthquakes
occurred north of the ATL and south of the MTL, where extensional
teclonic deformation occurs due to rightJateral slip of the faults(Fig.3).
The seismicity formed a Mogi's "doughnut." However, many people
and organizationswere unprepared for the 1995 Kobe earthquake. Many
people believed that the cites Osaka, Kyoto and Kobe in the westem
cental Kinki district are safe from earthquakes as they had experienced
only a few remote tremors. Japan has constructed a systematic
observation network for short-term prediction targeted only to the Tokai
earthquake, which is a presumed great earthquake along ttre Suruga
trough east of the Nankai trough(Fig.4). Active faults 4long the
subduction zone have been dormant for 140 years, but form a typical
Mogi's "doughnut" as the Nankai trough is seismically quiet now and is
surrounded by seismically active areas.
Groundwater level changes due to the Kobe earthquake were
detected in monitoring wells about 300 km away, which were installed
in the Tokai district to monitor stress changes in confined aquifers

whereas extensional

sites in the central

Rokko

6

As rocks are very weak under tensile stress but much stronger
under compressive stress, compressional jogs are more resistant than
other parts of the fault system (Koide & Bhattacharji, 1977). Ruptures of
compressional jogs are not frequent, but trigger big earthquakes,

active faults.
Numerous traces
of liquefaction in

of Japan in

tlrat

age-Kyoto,
Osaka, Sakai and
Kobe ( Sangawa,
Lee4).

in

related to the coming Tokai earthquake (Iakahashi, 1993). Many
coseismic and a few possible precursory groundwater level changes in

Tsukuda
Figure 5. Stages of development of a typical
fault system ( Koide et a1.,1979 ).
1.primary, 2.infant, 3 juvenile, 4.young,
5.vigorous, 6.old and T.creeping stages.

Great earthquake

(1987) estimated
source faults of

some monitoring wells were detected

for several earthquakes

(fakahashi,L993, Sato et al., 1995).
It is expected that precursory crustal deformation can be detected
by the extensive continuous earthquake observation network in the Tokai

historical
earthquakes in

1

008

Temperature('C)

40

uplift

60

80

100

0

120
.

I 40

.

0.1

sea

subsidence

/t, /

f

loor
s00

1

..-

gl
10

000

pressure

Depth(m)
1

'I

500

5
(MPa)

20

25
Figure 7. Graben subsidence in the center of domal uplift caused
by underground dike intrusion in the lzu-Oshima volcano. The
theoretical curve closely fit the measured values of vertical

3o

displacement.

Figure8. Density(kg/1)

district for prediction of the presumed great Tokai earthquake of
magnitude 8 class(Mogi,1974,1981). However, reliable short-range
prediction of other possible earthquakes is still virtually impossible for
several reasons: systematic monitoring networks for earthquake
prediction have not been constructed other than the Tokai earthquake
observation network, smaller earthquakes induce na[ower and smaller
precursory changes than major earthquakes, and a reliable theory for
short-range earthquake prediction is still needed.
Recent developments in laboratory exPeriments have made it
possible to map precisely the generation process of acoustic emissions,
which are miniature earthquakes in rock specimens (Satoh et al., 1994).

Ohnaka(1995) suggested, based

on his

underground

AB,

(1975) noted that a rock mass could be depressed in the central area by
the increase of magma pressure above a vertically elongated, needleshaped or dike-like magma body. Fissure eruption after the summit
eruption and trough-like subsidence surrounded by wide domal uplift in
the 1986 eruplion of Izu-Oshima volcano indicate underground dike
intrusion of magma(Fig. 7, Koide et al., 1987). Central trough
subsidence is observed in the eruption of Unzen volcano, too.
Subsurface dike intrusion ofmagma caused lateral expansion ofthe host
geologic body in the 1986 eruption oflzu-Oshima volcano, in the 1989
submarine eruption off Ito city(lada, 1990, Tsuneishi, 1990) and in the

experimentally-derived

1991 eruption of Unzen volcano in Japan(Yasuda et al. 1993).

Takada(1994) studied ascent and interaction of magma-filled
oacks with gelatin model experiments and theory. He suggested that the
stress balance betweenthe local stress induced by magma accumulation
and the existing regional stress field, and stress relaxation govern the
structure of a volcano.

of groundwater wells, electomagnetic
measurements, dense and precise seismographic measurement and
continuous geodetic measurement by a dense, nation-wide global
stress measurement, monitoring

3 PREDICTION OF

at the

D indicate typical geothermal gradients.
T denotes the critical point 31.1"C,7.39MPa.

earthquake, the Japanese govemment expanded the earthquake prediaion
program to include direct excavation and drilling of active faults, in situ

very long baseline interferometry

COz

C and

nucleation model for shear rupture, that earthquakes of magnitude 7
class have precursory zones of rupture nucleation of the scale order of
several km. The nucleation process ofearthquake rupture may provide a
key to short-range earthquake prediction. After the 1995 Kobe

positioning system (GPS) network, satellite laser ranging (SLR)

of

temperature and hydrostatic pressure. Lines

It is expected that the prediction of volcanic eruptions will be
improved by the funher development of GPS and SAR interferometry
techniques for measurement of crustal deformation.

and

(VLBI)'

4 SUBTERRANEAN DISPOSAL OF COz AS A

VOLCANIC ERUMIONS

COUNTERMEASURE FOR GLOBAL WARMING

On June 3, 1991, a pyroclastic flow killed 43 people near Unzen

The probable principal cause ofglobal warming is the hugeamount of
CO2 emitted by buming fossil fuels extacted from underground
deposits. Although the carbon dioxide is not very poisonousto animals
and is useful for plants, he enormous volume of manmade CO2
emissions makes the development of effective mitigation technology
extremely difficult.
Abottt 6OVo of COz emissions in Japan are from fixed massive
sources such as electric power plants, mills, etc. While intensive effots
are being made for the development of CO2 recovery technology from
flue gas of power plants and mills, we should find suitable spaces for the
disposal of separated CO2. Oceanic disposal of CO2is still questionable

volcano, Kyushu, Japan. Continued inflation of the lava dome near the
top ofUnzen volcano, accompanied by frequent collapses ofportions of
lhe dome, produced pyroclastic flows that threatened people around the
volcano until 1995. Upwelling of magma under the volcano was
revealed by the migration of earthquake swarms.Lateral inflation of the
volcano before magma outflow was clearly detected by measuring the
distance to the mountainside from an adjacent hill by an electro-optical
distance measurement instrument(EDM) (Saito et al. 1993).
On November 15, 1986, the first volcanic eruption since 1974
started in the summit crater of Izu-Oshima volcano, a volcanic island in
Sagami bay, southem Kanto district, Japan. Earthquake swarms and
volcanic tremors signaled the earlier subsurface magmatic activity, but
the subsidence of the central surface of the volcano prevented short-term

due to the risk to the marine environment.

Depleted natural gas and oil fields have porous underground
reservoir rocks and fiap structures that can contain gas and liquids safely
over several million years. Some natural gas deposits contain a large
fraction of CO2. It is possible to use depleted underground natural gas

prediction of the eruption.

of a volcano, which Mogi(1957)
with a spherical magma body model, is the

Usually, the inflation
investigated theoretically

most reliable precursor of volcanic eruptions. Koide and Bhattacharji

and oil reservoirs as storage sites

1

009

for

COz because impermeable

Te m pe ratu re (" C)

Den sity (kgll)

o o.1 0.20.3 0.40.5().60.70.a o.9

5 '1o 15

1

o

500

\

20

25 30 35

Om
R

t

.x

c

000
Depth
1 5()0
(m)
2000
1

1
I

Depth

Pressure

D

(MPa)

t

A

\

B

a

I

\

2soo

:

I

30()0
Figure.9 Density

of

I

underground simple CO2

100

A: geothermal gradient 4'Cl100m,
surface temperafure 20"C,

Figure10. Depths where CO2 hydrate

B:geothermal gradient 3'Cl100m,

will

be

formed in sediments(dotted area).
Curve RPQS is the criterion for
decomposition of CO2 hydrate. Line OD shows a
simplified possible temperature change with depth
in very cool sedimentary basins at high latitudes.

surfacetemperafure 15'C,

C:geothermal gradient 2" ClL00m,
surface temperature 10'C,
D:geothermal gradient 1'C/100m,
surface temperafure 0'C.

JBK represents

a typical temperature Fofile in the
Japan sea. Line BA indicates temperature increase
with depth in marine sediments, where B is the sea

caprocks prevent the leakage of CO2.

The underground injection of CO2 should be operated so as to
minimize the change of reservoir fluid pressure from the primary
pressue, which is usually almost equivalent to the hydrostatic
pressure(Fig.8). The critical point for COz is at 31.1"C and 7.39MPa.
Simple carbon dioxide is stored as a supercritical fluid in reservoirs
deeper than about 800m. Evidently, cooler reservoirs having a low
geothermal gradient can store more carbon dioxide than warmer
reservoirs having a high geothermal gradient(Fig.9).
Prediction of the fate of underground CO2 is necessary for the
safe subterranean disposal of COz. About 3 MPa of pressure at about

floor.
reservoir capacity becomes very large and operating costs are reduced.
Three dimensional geological mapping of caprocks and careful leakage
tests aenecessary to store freeCO2 in subterranean reservoirs, whereas
good caprocks are available in natural gas reservoirs.
In and around the Japanese islands, the Engineering Advancement
Association of Japan (ENAA) estimated (fanaka er a1.,1944) that as
much as 2 billion tons of CO 2 can be stored as a supercritical fluid in oil
and gas reservoirs, as much as 1..5 billion tons in aquifers with rap
structures, as much as L6 billion tons as solution in groundwater in other
aquifers on land and as much as 72 billion tons in offshore aquifers.
Koide et al.(1992) estimated that 320 billion tons of COz, at least, can be

5"C is suffrcient to form CO2hydrate in aquifers(Fig.10). Formation of
CO2hydrate in the pores of rocks virtually completely prevents leakage
ofcarbon dioxide(Koide et al., 1994). Carbon dioxide injecled in deep
reservoirs may migrate upward in aquifer systems and forms a CO2
hydrate cap at the depth range of 200m-400m below the water surface in
cool sedimentary basins in high latitudes and in wide areas of cool
marine sedimentary basins deeper than two or three hundred
meters(Fig.11). Precipitation of calcium carbonate may also seal rock
fractures, thereby preventing leakage of CO2.
Underground reservoirs can store about 10 times more carbon

stored in aquifers in sedimentary basins in theworld.

Virtually complete isolation of huge amounts of CO2 is possible
in cool sedimentary basins at high
latitudes by CO2 hydrate self-trapping. Sub-seabed disposal and
subterranean disposal in cool districts an reali,n CO2 emission ftee
in deep cool marine sediments and

fossil fuel power generation.

dioxide as a supercritical fluid ttran as a gas dissolved in groundwater. If
caprocks are tight and sfiong enough to contain the superoitical fluid,

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