Post-eruptive seismic activity of Mount Cameroon (Cameroon), West Africa: a statistical analysis

JOUlTl&JfVOkaIlOlogv

andgeothermalreseamh

ELSEVIER

Journal of Volcanology and Geothermal

Research 79 (1997) 25-45

Post-eruptive seismic activity of Mount Cameroon ( Cameroon) West Africa: a statistical analysis Bekoa Ateba avb3 *, Nfomou Ntepe a,b a The Institutefor Geological and Mining Research (IRGM), P.O. Box 4110, Yaounde, Cameroon b Branch for Geophysical and Volcanological Research, Ekona, P.O. Box 370, Buea, Cameroon

Abstract Recent seismological studies of the Cameroon Volcanic Line show that Mt. Cameroon is the most active centre, so a permanent seismic network of six seismographs was set up in its region between 1984 and 1986. The network was reinforced with temporary stations up till 1987, and the local seismicity was studied. Here we emphasise a statistical analysis of seismic events recorded by the permanent seismic stations. Four swarms lasting 9 to 14 months are identified at intervals of 2-3 years. Most earthquakes are felt (intensity and magnitude, respectively, less than VI MM and 5) during the first three swarms and a few during repose periods. The main focal regions are the northwest and southeast flanks, the Bimbia and Bioko regions in the South of the volcano. Hypocentres are distributed from the surface to 60 km depth indicating crustal and subcrustal activities. The subcrustal events are observed only in the southeast flank, they are the most regular earthquakes with a monthly frequency of 9 to 15 events. They are characteristic earthquakes with magnitude 2.8 + 0.1. Between 1984 and 1992, their yearly mean time interval between successive events range from 50 to 86 hours. For that period their occurrence can be modelled as a stationary renewal process with a 3-day period. But the analysis of variance shows possible significant differences among yearly means. A Weibull’s distribution confirms that the time intervals between successive deep events are not independent, and in 1993 a swarm of deep earthquakes is recorded, hence a non-loglinear magnitude/frequency relation. The deep seismicity is thought to be associated with a zone of weakness (perhaps a magmatic conduit) and may have some close relationship with the magmatic activity. 0 1997 Elsevier Science B.V. Keywords: seismic activity; crust; Mount Cameroon;

statistical

analysis

1. Introduction Mt. Cameroon is located in the Gulf of Guinea, West Africa, it is a member of an alignment of volcanoes stretching from islands of the Atlantic ocean (Pagalu, Sao Tome, Principe and Bioko) to the main land (Mounts Cameroon, Manenguba, Bamboutos and Oku), this alignment known as the

* Corresponding

author.

0377-0273/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO377-0273(97)00022-X

Cameroon Line (CL) is on its northern extension subdivided into two parts (Fig. l), one trending northward to Biu Plateau (northeastern Nigeria), the other westwards to Chad through the Foumban shear zone and the Adamawa Plateau. The origin of the CL is not yet well known. The hypotheses can be classified into two main categories: The CL is thought to be underlined by an important Precambian fracture, the Foumban shear

26

B. Ateba, N. Ntepe/Journal

Soloma ’

/ PO alu 3 9

,8’ 6

Fig. I. Main volcanic Benue trough.

centres

of Volcanology

B 0”

’

0

of the Cameroon

Line, and the

zone which extension to Brazil seems to be the Pemambuco fault. The reactivation of this fault during Cretaceous to Tertiary times could be the origin of the CL (Le Mar&ha1 and Vincent, 1971; Browne and Fairhead, 1983). It is also thought to be the result of the movement of the African plate over a hot spot (Morgan, 1983). Fitton (1980) suggests that the similarity of the shape between the Benue trough and the CL could be related to a common hot zone over which the African plate rotated. Halliday et al. (1990) established that a 206Pb/ 204Pb ratio of less than 10 Ma lava exhibits an anomaly at the continent-ocean boundary, with values decreasing northward in the continent and southward in the ocean. The anomaly is attributed to a migration following emplacement of a fossil upper mantle plume at 125 Ma. Lee et al. (1994) showed from 40Ar-39Ar datation of Pagalu basalts that the age of the earliest exposed volcanic rocks decreases oceanward from Principe (3 1 Ma) to Sao Tome (13 Ma) and to Pagalu (4.8 Ma). All these recent observations suggest a time variation of the volcanic activity of the CL.

and Geothermal

Research 79 (1997) 25-45

Six eruptions have been reported this century on Mt. Cameroon (1909, two in 1922, 1954, 1959, 1982) and one in Bioko (DCruelle et al., 1987). Then, Mt. Cameroon is the most active volcanic centre of the CL (Ruxton, 1922; De Swardt, 1956; Jennings, 1959; DCruelle et al., 1987). Mt. Cameroon is a strato-volcano with its summit at 4095 m. It has the shape of an ellipse, with a long axis of 50 km bearing 30”N, and a short axis of 25 km. It is made of three series of basaltic lavas: the black, the medium and the white series (Geze, 1953). Mt. Cameroon has more than a hundred cones; the dynamism of eruption being mostly strombolian or hawaiian and vulcanian. Many faults are recognised around Mt. Cameroon (Zogning, 1989). The most important are in the east flank, the Tiko fault, and in the west flank, the Boa fault. Many smaller faults are observed in the summit region. They strike in the direction of the line (Fig. 2) or 70”N. Volcanic hazards are associated with volcanic explosions, lava flows, landslides and to some extent with lahars (Deruelle, 1982; DCruelle et al., 1987; Wouatong, 1990) in 1994 a flash flood was observed in Buea. The 1986 lake Nyos gas explosion which killed 1700 people is the biggest disaster ever recorded in Cameroon (Sigurdsson et al., 1987; Le Guem and Sigvaldasson, 1989). To prevent or to mitigate volcanic disasters, the Cameroon government through IRGM (The Institute for Geological and Mining Research) decided to build an observatory at the foot of the volcano in 1990. The seismicity project started late in 1984 and a permanent seismic network was installed on and around Mt. Cameroon. The results obtained so far have been reported in many internal reports, thesis and scientific papers. In this study we focus our attention on more recent and important events like swarms, felt earthquakes, and an explosion that occurred on the Mt. Cameroon area, to better assess future seismic or volcanic activity of the massif.

2. Seismic network and data acquisition The initial network of the Mt. Cameroon region (1984-1990) was composed of six seismic stations (Fig. 3a). Each of them had a 1Hz vertical component seismometer (Willmore MK III A). At the

27

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 (1997) 25-45 I

9'

KUMBA

n

A

Summit (4100rn)

0 Volcanic cone w Town /Fault ~‘Fossible

~~Z7eruptive l-

fault

vents

1909

2 - 1922 3 - 1922 4 - 1954 MUYUKA

5-

n

1982

6 - 1959

MISSELELE ~

n

Fig. 2. Cones, recent eruptive vents and tectonics on Mt. Cameroon.

station, signals from the seismometer are amplified, modulated and telemetered toward the recording centres at KBC and BTE. Because of the transfer of the central recording sites from KBC to EKC and BTE to DBC, the network was slightly modified in 1990. Fig. 3b illustrates the actual geographical distribution of the seismic stations. In both cases a common and absolute time base is achieved using electronic clock synchronised to VLF radio signals from the Omega navigation transmitter in Liberia or Argentina. Signals are recorded on 24 hour ink paper drum recorders. On daily basis, the staff picks arrival times and measures the amplitudes and periods of identified waves of local earthquakes. The values are reported in a monthly catalogue. We deal only with earthquakes having their epicentre within 100 km radius from Mt. Cameroon.

From November 1985 to March 1986, and from December 1986 to April 1987, the network was reinforced with eight temporary seismic stations of the University of Leeds, of which three had three components. The equipment was similar and data from both the temporary and permanent networks were recorded on multichannel FM tape recorder (Ambeh et al., 1989). The corresponding analogic paper seismograms were produced in Leeds, and copies sent to Cameroon are available.

3. General seismicity Data collected with the permanent and temporary seismic stations were analysed by Ambeh (1989) and Ambeh et al. (1989). However, in our study, only data from the permanent seismic network stations are

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 (1997) 25-45

28

m--e_

Contour line

A Seismic station

0

Recording Station

I

5 PI

6-

Statfon

1OKm

KBC

A

relay station

0

recording

To%

StOtiOn

Fig. 3. Mt. Cameroon permanent seismic network: (a) recording centre in Kumba (formal situation 1985-1989); (b) recording centre in Ekona (1990 to present)

processed. Events with at least three P and one S phases with picking errors smaller than 0.2 and 0.5 second, respectively, are located using HYPOCENTER (Lienert et al., 1986) with the velocity model found in The Adamawa Plateau by Stuart et al. (1985). The depth of the Moho for the model is 33 km. Hypocentres with root mean square travel time residual greater than 0.5 second, or horizontal or vertical error greater than 5 km are disregarded. These events as well as those recorded by fewer than three stations are, however, used for events counting. It should be noted that errors given by HYPOCENTER are formal errors at 95% confidence level. The accuracy depends on the velocity model, the number and the quality of phases and the relative position of the hypocentre with respect to the seismic stations. The tests show that the locations within the network can be associated with a 5-km error. Outside, the error increases with the distance to the network. The main focal regions identified around Mt. Cameroon region are (Fig. 4a): the summit region (above 1000 contour line), the northwest flank (near BOK), the southeast flank and Bimbia region. Earthquakes also occurred in the Bioko area. The vertical profiles (Fig. 4b and c) show hypocentres from the surface to 60 km, indicating both: crustal and subcrustal seismic activities. The subcrustal or ‘deep’ activity is located in the southeast flank. The seismicity was also characterised by swarms and felt earthquakes with maximum intensities less than VI (Modified Mercalli scale). When all events are considered in the general histogram from 1985 to 1994 (Fig. 5a), the approximate monthly mean frequency of local earthquakes is 30. During swarms the level of seismicity is higher and can reach 180 events per month. We consider that a swarm starts if the monthly number of earthquakes is at least 60 otherwise, it is a repose period which varies from one to 28 months. The mean repose period is 9 months. A swarm episode is ended if the monthly number of earthquakes falls under 60 during at least 9 months. With these criteria, four main swarms are identified between 1984 and 1994 in the Mt. Cameroon region: 1st period: January 1986 to February 1987 - 14 months

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 (1997) 25-45

1986-1990

k

Seismic station

+

Epicentre, depth s 33km

W

Epicentre, depth > 33 km

29

.'.. Contour line -

Costa1 line

+

BOK 'c

+

.

.

DBC

-

4”N

9"30

9'E

BOK

DBC

(W

MOK

EKG

KBC

MIS

+

+

+

tt

0

ti++’ 1

tt tr +t

+A+ t++ tt

t

+

+

++

3

*

t

+$

+I

t t +

t

4

++ t

+r,

+

+

t

t

‘+

++ + * ++

+ t

t

+ + ++

+a

++ 4 +

t +

++ ++t 9 +*+i + t

; +

t

A T

t

t

+44+ t

t

t

l

+ t+ +*

60 J Fig. 4. Seismicity map for the period 198551992. (a) Dotted lines are 1000 m, 2000 III and 3000 m contour lines; A = seismic station; + = depth of hypocentre less or equal to 33 km; n = depth greater than 33 km. (b) E-W profile. Cc) N-S profile.

30

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 ( I997) 25-45

2nd period: February 1988 to February 1989 13 months 3rd period: April 1990 to December 1990 - 9 months 4th period: March 1993 to November 1993 - 9 months Starting from the last eruption (October-November 19821, an approximate 2-3 year time interval is established between the beginning of the main swarms. The seismicity of the southeast flank is

BIOKO

100

-1,

-2, Mt

+.3 ._

O -4

$j s &

SE

FLANK

8

(0)

*

150

2

I

200

150 i

-24

-1,

’

ALL

4. I. The south flank (Bimbia region)

4.2. The summit and northwest flank region

CAMEROON

i

c

quakes)

,4...,

50

(b)

4. Crustal seismic activity (swarms and felt earth-

In February 1986, a swarm took place in the Bimbia region. It lasted 6 days for a total of 60 earthquakes. The highest magnitude was 2.7. The epicentres fall in the Atlantic ocean (Fig. 6) with an E-W trend. Their mean focal depth was 15 to 25 km. From time to time small series of 5 to 10 events from that area are recorded. They are characterised by clear impulsive P onset and well identified S waves.

loo-

(Cl

quite steady from 1985 to 1992, swarms in the Bioko area occur before, at the same time, or just after those on Mt. Cameroon (Fig. 5).

-2

i

*

EVENTS

Fig. 5. (a) Monthly histogram of Mt. Cameroon local seismicity, period 1985-1994. (b) Contribution of the southeast flank. (c) Contribution of the rest of Mt. Cameroon, (d) Contribution of the Bioko area.

In terms of swarms and felt earthquakes, the summit region and particularly the north and northwest flank have been the most active during the period of observation. The first Bokosso swarm started on February 13, 1986 and lasted 10 days. A total of 120 earthquakes were recorded during this period. Five shocks with magnitude 3.7 to 4.2 were felt and frightened local population, with intensity V (Modified Mercalli scale: MM). In August 1986, another swarm of 8 earthquakes was observed, 3 of them were felt (Ambeh, 1989). The epicentres are distributed on the northwest flank, some of them are clustered near BOK (Fig. 6). The focal depth ranges from 5 to 20 km. Discrete events and minor swarms are intermittently observed in this region and they are also characterised by impulsive P onsets and clear S phases. Two earthquakes of magnitudes 3.0 and 3.5, respectively, were felt in Buea, intensity IV (MM) on March 19 and 26, 1989 (Ateba and Ntepe, 19891. They are poorly located and correlated to the Limbe fault. On May 29, 1989 an explosion earthquake was recorded and heard 100 km north of the mountain. It was associated with a volcanic explosion that took

31

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 (1997) 25-45

1) O-

10 km

A

1986-1987

seismic station Epicentre

+

'30

hb BOK

\

EKC

I

(b) o

I 9OE

I

I

DBC c

1

BOK ,

I

I

I

MOK +

‘i””

““t

I go30

I

MIS t

9 b;;

+ 2 20 -

g b 4 30 -

tt

Jt

t

I-

r

+ + 10 .

I

l

t

-

t

40 4

t t

t

;

+L

Fig. 6. a)

Epicentres of the 1986/1987 crustal seismic swarm. Dotted lines are 1000 m, 2000 m and 3000 m contour lines; A = seismic station: + = epicentre. (b) E-W profile. place in the northeast flank of the central rift zone of the volcano. A black cloud (or plume) rose from the northern flank, a few seconds after the detonation and an on-the-spot assessment revealed a new crater at 2860 m a.s.1. at the foot of an old volcanic cone. No lava flow was observed.

Three days after the explosion, on June 1, 1989, another earthquake was widely felt as far as Kumba and in all villages and towns around Mt. Cameroon with an intensity V (MM), magnitude 4.5. The perplexity and the fear were great among the population, and it was rumoured that big damages and

32

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 I1 997) 25-45

-

1967 I

1968

I

1989

YEAR E : explosion

S : earthquake

Fig. 7. Number of local earthquakes

per 10 daya. period May 1987 to December

many injuries were recorded. But field investigations did not establish any serious casualties. This earthquake was also recorded by SRO station in Central African Republic. According to the general histogram, these earthquakes were felt during repose period. But, plotting the number of events recorded per 10 days from 1987 to 1989 (Fig. 7) the seismicity of the region increased 11 months before the first felt earthquake on March 19, 1989. During this period of swarm, the epicentres of the events are scattered on the summit area, the northwest flank and the east part of the volcano (Fig. 8). In September 1990, earthquakes were felt mostly in the northwest flank. The biggest (magnitude 4.5) was recorded on September 20 and also detected by the Ivory Coast network (Bert& 1991). The second occurred on September 25, with magnitude 3.9. The maximum intensity was V (MM). These events occurred during the third swarm period previously identified. The epicentres of this swarm period are distributed in the North, the west and northwest flanks of the massif, in the Bokosso area (Fig. 9). A light earthquake (magnitude 3.7) was also felt in Buea with an intensity III (MM) on August 6, 1992. 4.3. The Bioko island Many earthquakes originate from this region as discrete or swarm events. Their foci are out of the network, so their locations are poor. Swarms of these events remarkably occur simultaneously (or a little bit before or after) with the Mt. Cameroon seismic swarms (Ateba and Ntepe, 1989; Ubangoh, 1994). The maximum frequencies for the two regions are

1989.

observed during the first three swarms (Fig. 5c and d). The most significant periods of seismic activity are observed in February-March 1985, 1988-1989 and 1990. Their magnitude is less than 3 and their epicentres are scattered on and around the Bioko island in the Atlantic ocean (Ambeh et al., 1989).

5. Subcrustal

activity

An interesting aspect of the seismicity of Mt. Cameroon is the regular appearance of similar southeast flank earthquakes with constant magnitude 2.8 k 0.1. Their depth range from 35 to 60 km. They are considered as ‘deep earthquakes’ (Fig. 10). Due to their regularity, one might expect that they are triggered by earth tides. But Ambeh and Fairhead (1991) showed that there is no correlation with them. The deep activity can be divided into two periods: the steady state between 1985-1992, the monthly frequency is 9 to 15 events, and the swarm period in 1993 when more than 30 events per month were recorded during 7 months (Fig. 5b). 5.1. The 1984-I 992 period The first step is to look for a general trend. The yearly number of these southeast flank events is calculated and their cumulative number is a linear function of the duration of the period 1985-1992 (Fig. 11). The cumulative function H(t) can then be written as: H(t) = t/7. T is the mean time interval between two earthquakes. When H(t) is a linear function of time, the appearance of earthquakes is similar to a stationary renewal process (Lomnitz,

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 (1997) 25-45

the average recurrence interval, and its uncertainty (Schwartz and Coppersmith, 1986). The next step is to test the hypothesis of constant rate of appearance of deep southeast flank events at a

1974). The experimental value T is about 3 days for the period 19851992. Renewal models consider that the probability of the occurrence of earthquakes depends on the elapsed time since the last event and

(a)

10 km

9

33

A

1988-1989

+

1

seismic station Epicentre

**....contour line -Costa1

line

ABOK

I

DBC

I

I

I

1

I

I

9"E

(b)

DBC o

~-T-----T9'30

BOK J

4

MOK

EKC 4

4

KBC -1

MIS J+

+

+ ++ +

+

+

+

+

t

Fig. 8. (a) Epicentres associated with the 1988/1989 Mt Cameroon 3000 m contour lines; A = seismic station; + = epicentre.

t

++ ++ + + + t*

+ ++ + t

crustal seismic swarm activity. Dotted lines are 1000 m, 2000 m and

B. Ateba, N. Ntepe/ Journal of Volcanology and Geothermal Research

34

smaller time scale (one year). Time differences in hours between successive events are calculated and their values reported in 12-hour intervals. This leads to annual files of inter-occurrence time between two events. Their normalised yearly histogram can then be plotted, year after year (Fig. 12). We observed that for most of the years, as a first approximation,

(a)

1

0

10

km

’

79 11997) 25-45

the distribution was normal. Each year, there is a central value around which are distributed the most probable values. Away from this value, the frequency decreases towards zero but, more slowly for times greater than the central value. These characteristics are observed in the general histogram 19851992. It should be noted that when an event is not

A

1990

station

Seismic

+

EpiCentR

Contour line cost.31line

.,...

-

i BOK

9

+

A

t

-4'N

9”30

9’E

(b)

DBC

BOK t

MOK t

EKC

KBC

MIS

t

t

t

t t*

t

t

t t

a

c

I

t

F %

t +

t t+ t

Fig. 9. 1990 seismic swarm activity. Dotted lines are 1000 m, 2000 m and 3000 m contour lines; A = seismic station: + = epicentre.

B. Ateba, N. Ntepe /Journal

recorded by the seismographs (due to break down), the time interval between two earthquakes will be at least twice the mean value. Only 2% of the data are more than 144 hours, then during the studied period, the network performed well and the data base is quite representative. Choosing the yearly mean (7”) as the central value, the results obtained are listed in Table 1. The means range from 50 to 86 hours or a monthly frequency of 9 to 15 events. For the first five years, there is a regular increase of the yearly mean of inter-occurrence time between southeast flank events, but that pattern did not continue. To estimate the variations and to decide whether or not they are significant, we applied to the data set

0 90

80 YEAR

Fig. 11. Regression line of the cumulative east flank earthquakes. MagnitudeFrequency deep SE flank events period (1995-1992) 1000

-I-

100

22

5

&

1c) --

1

7

1

I,5

2

2.5

3

3.5

Magnitude

Fig. 10. Size distribution 1985-1992.

of deep southeast

35

of Volcanology and Geothermal Research 79 (1997) 25-45

flank events, period

number of deep south-

a classical method of analysis of variance (ANOVA). The use of the technique is to compare the means of normal populations supposed to have the same variance (Davis, 1973). The sources of variations have to be identified: here they are between the yearly means and the monthly ones of inter-occurrence time (basic data). Then the mean of the sum of squares of each source is calculated and the value of their ratio is compared to the theoretical value given by Fisher distribution tables. The results are summarised in Table 2. The calculated ratio is 31.85, while the theoretical value given for a 95% probability is 1.99. This means that the yearly mean of time interval between successive events can be significantly different from one year to another. The calculated least significant difference for 88 degrees of freedom and 95% confidence interval is 6 hours. Two years are considered to have the same mean if the difference in their yearly mean is less than 6 hours. Plotting the results in Fig. 13 shows that from 1985 to 1992, the yearly mean was increasing with almost two stationary periods: one of two years 1985-1986, and the other of 4 years 1987 to 1990, separated by an important variation between 1986 and 1987. From 1991 to 1992 a decrease is observed which means increased activity confirmed in 1993 by a seismic swarm. To test the independence of the events, a Weibull distribution is used to model the time interval be-

B. Ateba, N. iVtepe/Journal of Volcanology and Geothermal Research 79 (1997) 25-45

36

1985

12

36

60

81.

10.3

1989

132

12

36

60

6L

101

132

r-5-

taa

1985 1992

0.8 P6 '1

i

0.6

Lx.

0.2.

12

36

60

86

106

132

12

36

60 12

‘I

12

60

inter-occurrence (hr)

1987

I

36

36

60

84

IO8

132

12

36

36

60

SL

108

132

12

36

inter-occurrence (hr)

time

60

60

64

108

132

dL

IO6

132

6L

106

time

132

interval

interval

Fig. 12. Yearly and general histogram of the annual inter-occurrence to 1992. Each histogram is normalised to its maximum frequency.

time between successive

deep southeast flank earthquakes,

period 1985

B. Ateba, N. Ntepe/Journal

Table 1 Mean time interval CT.,) between

two successive

southeast

flank

earthquakes Year TY (h) Std.error

31

of Volcanology and Geothermal Research 79 (1997) 25-45

n

1985 1986 1987 1988 1989 1990 1991 1992 49.9 53.4 61.7 12.3 78.8 75.1 85.8 66.5 a (h) 6 3 7 6 11 8 11 4

a Std = standard.

BY5I9861987 198819891990199119921993199L tween successive earthquakes. If T, is the minimum repose period, T, the mean repose period, the survivor function which is the probability for the repose period to be greater or equal to T is defined by: F(T)

=P(t=T)

=exp-

[@(T-T,)]’

where @ = 1/CT, - T,,), j3 is a positive constant. G(T) = log{ - log[ F(T)]} is a linear function of log(T) with a slope p. If p = 1, we have a Poisson’s distribution (Moutou, 1984). The method applied to one year period (1991) gives j3 = 1.73 (Fig. 14). On a yearly basis, the appearance of southeast flank events does not follow a Poisson’s law. Then two successive time intervals of southeast flank earthquakes are not independent. This is confirmed by a swarm in 1993. The quasi-steady state between 1985 and 1992 is apparent; the yearly mean time interval between successive events is not constant and a significant increase is observed between 1986 and 1987. 5.2. The 1993 seismic swarm

Source of variations

Sum of squares

flank

peaked in March, when more than 170 events were recorded. Through June the monthly number of earthquakes remained greater than 60. Thereafter, it dropped to less than 50 and in 1994 the monthly mean returned to 9-12 southeast flank events. Most epicentres are scattered on the southeast flank of Mt. Cameroon (Fig. 15a). A small number also occurred on the west flank. Fig. 15b displays an E-W profile where a trend is not clearly recognised. As in the N-S profile (Fig. 15~1, the depth extend from the surface to 60 km; the lateral extension of hypocentres is smaller than in the E-W profile, and the hypocentres show a subvertical distribution between 20 and 60 km. Due to the similarity of waveforms and epicentral determination, we distinguished

G(T)

After the apparent quasi-steady state of appearance of deep southeast flank events between 1985 to 1992 slight increase is observed from 11 events in December 1992 to 18 in February 1993. The swarm

Table 2 ANOVA table of the inter-occurrence east flank earthquakes

Fig. 13. Yearly mean time interval between deep southeast events period 1985-1994.

.

3 \

time between deep south-

Degrees of

Squares

F

means

freedom Between years Between monthly means Total variation

12480.50 4928.57 17409.07

7 88 95

1783.64 56.01

31.85

(hC=d Fig. 14. Weibull distribution of inter-occurrence time between successive deep southeast flank events. The mean repose period is obtained for G(T) = 0, here T,, = 86 hours.

B. Ateba, N. Ntepe / Journal of Volcanology and Geothermal Research 79 (1997) 25-4.5

38

five main groups of earthquakes originating from the southeast flank during that period: A family of deep southeast flank events in Tiko area. Their depth is 30 to 60 km, the range of their magnitude 2.5 to 3, is larger than during the 1985-1992 period (Fig. 10). Examples of seismograms of these events are shown on Fig. 16a. A family of events with no clear P nor S phases (Fig. 16b). They are difficult to locate and their magnitude range from 1.6 to 2.9 were the most

regular during the period of crisis from March to June. - The other earthquakes were crustal, of tectonic origin and belong to three focal regions: the Wouri estuary, seven events recorded in March; the Mungo-Misselele, nine events in September; the Bimbia bay, nineteen events recorded in June. The 1993 seismic crisis which affected all these focal regions in a short time can be seen as a sudden, large perturbation of the stress field which changed

(a) 4’ 30

4’ N

DBC

03

BOK

MOK EKC

MIS

MIS

!

E

W O-

BOK

MOK !

1

N

r

. lo-

.

20ii

30-

l

Y 7

40-

k x

506070-

60.

l

. .

.

l

70-

Fig. 15. 1993 seismic swarm. (a) Epicentres map. Dotted lines are 1000, 2000 and 3000 contour lines; A = seismic station: hypocentre less or equal to 33 km; n = depth greater than 33 km. (b) E-W profile. cc> N-S profile.

fB = depth of

39

B. Ateba, N. Ntepe/ Journal of Volcanology and Geothermal Research 79 (1997125-45

I

__-.-. ._ ___ _.__ ..-_-:I~.~._ _ --.--_.__.. r.-*_

I I I i

_

-.

..-

-;

_-_

___;_ .._- ---- ..- _--- i -... _ ‘._‘.~_ ._.. _. . _. --. ---__ 7‘--:-::. _ .__--r:z--.r~~~~ T.-x. 11 I-1 r,__-_r r---. ._ _ _..... .___... ____3‘ -’ “. ,-_.----. ,_ ,.,- .‘,. .__.-..-._,.... - __... ,-’ _.._, __ ..__ -.,_ _. .._.x+:~r ..‘.(-. .--.~__ _..___._ .__.._ -.. -.-

i

i

-: --. __ -----. ._-...

: _

I

.._.-

.-.

I

---

: ._.

_

I,._..__

.._..

-

-..._._.._.

--.

-...--

--

-

t

I

1 mn Fig. 16. Seismograms of some southeast Mungo-Misselele region.

flank events,

1993 seismic swarm. (a) Constant

magnitude

2.8 events. (b) Emergent

events. (c)

B. Ateba, N. Ntepe/Joumal

40

of Volcanology and Geothermal Research 79 11997) 25-45

I

I 1 mn

Fig. 16 (continued).

the rate of deep earthquakes frequency. The energy was globally released from zones of weakness under the southeast flank and Douala basin. Usually the seismicity of intraplate volcanoes is concentrated in the crust. In Hawaii, subcrustal seismic activity has been observed on Kilauea and, Klein et al. (1987) suggested that the events are associated with magma conduits. So the 1993 seismic swarm may have been due to magma injection from depth like in Mount St Helens in 1982 (Weaver et al., 1983). Observed swarms and felt earthquakes are summarised in Table 3. They are located on the summit, the northwest and southeast flanks, and the South of the volcano. Moreover, the swarms in the Mt. Cameroon region correlate with the increase of the

seismicity around Bioko area. After the 1982 eruption, the level of deep seismicity remained quite high though decreasing with time, whereas the activity in the crust was characterised by swarms and felt earthquakes, from 1986 to 1992. When deep activity resumed with the 1993 swarm, the level of seismicity in the crust (central part of the volcano) became very low. This behaviour may indicate a close relation to the state of stress under the volcano.

6. Magnitude /frequency

If N is the number of earthquakes having a magnitude M, the Gutenberg-Richter frequencymagnitude relation is given by: log,,N=

Table 3 Main swarms and felt earthquakes on Mt. Cameroon region 1985 to 1994. Intensities are in Modified Mercalli scale (MM) Period

I I II II II II 111 III IV

Date

Type

Location

Magnitude

13 02 1986 25 02 1986 19 03 1989 26 03 1989 29 05 1989 01 06 1989 2009 1990 25 09 1990 03 1993

swarm swarm felt felt explosion felt felt felt SW_

NW flank S flank S flank S flank summit summit summit summit SE flank

4.2 2.7 3.0 3.5 3.2 4.5 4.5 3.7 3.0

Intensity

V I III III-IV III-IV V IV III 1

relation

a - bM

u and b are constants. Usually N, the cumulative number of earthquakes whose magnitude is equal or greater to M is preferred. This formula is valid if the earthquakes appearance follows a Poisson statistical law. The events are then supposed to be independent. Experimental values obtained for b lie from 0.6 to 1.2 and can be linked to the heterogeneity of the structure (Magi, 1963), or to the state of stress of the medium (Scholz, 1968). But, in volcanic area, greater b values have been observed. Ambeh (1989) and Ambeh et al. (1992) have determined for the year 1985/1986 a b value 0.69

41

B. Ateba, N. Ntepe/ Journal of Volcanology and Geothermal Research 79 (I 997) 25-45 Magnitude Frequency curues (1993) (Mt. Cameroon region) 100

4

It 0

03

13

1

2

2,5

3

3,5

Magnitude Fig. 17. Magnitude/frequency

relation for the 1993 seismic swarm period

and 0.18 f 0.01 for the field season 1986/1987. These values seem to be weak in volcanic zone, and that result was due to the presence of a great number of southeast flank constant magnitude events compared to smaller magnitude events (Ambeh et al., 1992). Tabod et al. (1992) also investigated on b value for the whole volcanic province of Cameroon; for the period 1984 to 1992, they + 0.07

determined a b value equal to 1.38 _t 0.11 which is more representative in a volcanic area. We were interested by the relation for the 1993 year. The results plotted on Fig. 17 show a nonloglinear relationship, but a maximum distribution between magnitude 2.2 to 2.8, This is due to the strong dependence on swarm type earthquakes observed in 1993. The main contributions are given by

42

B. Ateba, N. Ntepe/ Journal of Volcanology and Geothermal Research 79 (1997) 25-45

the family of emergent type signals and the deep southeast flank events which have maximum frequency, respectively, for magnitude 2.4 and 2.8. The influence of the rest of the events is low. Okada (1983) also revealed inconsistencies of the Gutenberg-Richter relation for earthquake swarms in Japan. The highest magnitude for Mt. Cameroon earthquakes is less than 5 according to available data and historical reports.

7. Discussion Although all recent eruptions of Mt. Cameroon are associated with felt earthquakes or seismic swarms (Vogt, 1982; Fairhead, 1985; Ambraseys and Adams, 1986), the permanent seismic network recorded many swarms and/or felt earthquakes not necessarily associated to eruptions. This is particularly true in lowland flank zone: Bokosso (northwest), Bimbia (S). The spatial distribution of epicentres plotted for one year does not follow a simple pattern. But the seismic activity is concentrated at a few distinct localities. Some swarms or earthquakes may be related to existing faults suggesting that the seismic activity seems to be controlled by the tectonic setting of the volcano, which may have played an important role in the distribution of volcanic vents. But the small number of phases used to locate the hypocentres in a region where the velocity model is not well known, do not give accurate locations to confirm this relation. Intense crustal seismic activity has been observed in 1986 on Mt. Cameroon, Bioko and North of Mt. Cameroon, in the Foumban Shear Zone where the highest 4.8 magnitude earthquake has been recorded (Tabod et al., 1992). Again in 1990, seismic activity increased around Mt. Cameroon and Bioko, we also recorded during that period, earthquakes from Sudan. This may be the result of regional stress concentration, perhaps at the level of the African Plate. Ambeh (1989) also proposed that subcrustal seismic activity on Mt. Cameroon is driven by the forces which cause plate motions. The analysis of the number of deep southeast flank events shows that two main periods of activity can be considered: first the quasi-stationary 1984 to

1992 period which can be modelled as a stationary renewal process; and the swarm period in 1993; characteristic of volcanic area. When the hypothesis of a constant yearly time interval between successive deep seismic events is tested, the ANOVA method reveals significant differences. The quasi-steady state from 1985 to 1992 is apparent. For the first period (1985-1992), three years of observation have been sufficient for Ambeh (1989) and Ambeh et al. (1992) to interpret the deep seismic events as characteristic earthquakes. According to Schwartz and Coppersmith (1984), characteristic earthquakes have similar waveforms and narrow range of magnitude. Paleoseismicity suggests that they can be generated by some specific fault segment (Okada et al., 1981). Aki (1984) proposed that characteristic earthquakes can be generated by breaking an asperity, when waveforms and the magnitude are the same, or by breaking a barrier if waveforms are similar but magnitudes are different. So the tectonic origin of southeast flank events is quite well established. But, the spectral analysis performed by Ambeh (1989) showed that they have predominant low frequencies, 2 to 3 Hz, and the 1993 seismic swarm with emergent P waves from southeast flank confirmed the double nature of the mechanism of deep earthquakes: (1) clear tectonic like events with at times small foreshock and aftershocks for the period 1984-1992, constant magnitude and strike slip focal mechanism (Ambeh et al., 1992); (2) swarm-type events with emergent P phases, no detectable S phase, and non loglinear magnitude-frequency relationship. They may have some direct link with the magmatic activity of the volcano. The percentage of success in predicting an eruption is not 100%, although the study of seismic swarms has been effective in other volcanoes (Zobin. 1979; Yokoyama et al., 1981; Tokarev, 1985). The accuracy depends on a good characterisation of the behaviour of the edifice based on its history. In this respect, the north and northwest flanks where most of the swarms are observed, may be seen, as the site of the most probable future eruption, if all the stress was not released there during the 1986, 1989 and 1990 seismic crisis. The mean repose period of Mt. Cameroon is around 20 years, which is longer than our period of instrumental observation. It is then quite early to draw a conclusion. But, according to

B. Ateba, N. Ntepe/ Journal of Volcanology and Geothermal Research 79 (1997) 25-45

Fitton et al. (1983), who extrapolated a plot of the cumulative volume of magma erupted with respect to time since 1909, the next major eruption will be around 2003. Archambault et al. (1980) studied the subsurface temperature variations at three stations around the volcano. They determined an increase of temperature at Ekona station prior to 1959 eruption. Then, this method, if implemented can be a good complement to seismological studies on Mt. Cameroon. Due to the dimensions of the volcano and the problems of accessibility, the permanent seismic network is not well distributed and the number of stations is also below average. Even if the main results obtained when the network was reinforced by temporary seismic stations of The University of Leeds are confirmed, individual localisations of hypocentres should be less reliable. The analogue paper seismogram is another limiting factor in advancing analysis of seismic signals. Nevertheless, we have been able to monitor all the Mt. Cameroon seismic crisis since 10 years.

8. Conclusion The post-eruptive seismicity is characterised by seismic swarms and felt earthquakes in the northwest and southeast flanks, Bimbia (south of Mt. Cameroon) and Bioko. Four main episodes of seismic swarms with more than 60 events per month, lasting 9 to 14 months were indentified at intervals of 2-3 years. Earthquakes were felt mostly during these swarms with an intensity less than VI (MM), and a local magnitude less than 5. Negligible damage to houses have been observed but, local populations are always terrified by their effects and the suddenness. The depth of hypocentres range from the surface to 60 km indicating crustal and subcrustal activity. All felt earthquakes are located on the crust. The subcrustal activity on the southeast flank is due to characteristic earthquakes of magnitude 2.8 + 0.1. The statistical analysis shows that their monthly frequency was 9- 15 events (198% 1992) period during which, their appearance is modelled as a stationary renewal process with a 3-day period. When this hypothesis is tested using ANOVA method, the re-

43

sults reveal that significant differences can exist among annual mean time interval between successive deep southeast flank events. This is confirmed by a swarm in 1993. That deep seismicity is thought to be associated with an important zone of weakness (magmatic conduit) and may have some direct relationship with the volcanic activity on Mt. Cameroon.

Acknowledgements The ‘seismicity of the Cameroon Volcanic Line’, a joint project between IRGM (Cameroon) and The University of Leeds (UK) enabled preliminary data collection and interpretation. Many researchers have been involved: J.D. Fairhead, W.B. Ambeh, C. Tabod and J.M. Nnange. The IRGM managers, the late D. Soba and G.E. Ekodeck, have always been attentive to the seismic and volcanic activity on Mt. Cameroon. Ubangoh R., F. Aka and J. Ngninpeye also took an important part in data collection while many other workers also gave their best to keep the network running: they are congratulated. Diagrams were drawn by B.A. Yem and F. Nkoa. We received encouragements from Professor P. Bormann and Professor J. Zschau. Special thanks to Dr. R. Schick who made his comments before submitting the paper. Dr. R. Gaulon and Prof. R. Madariaga offered a PC which helped in processing the data and typing the manuscript. We are also grateful to anonymous reviewers whose suggestions improved the manuscript.

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