Fault-Plane Solution of the Earthquake of 19 March 2005 in Monatele ( Cameroon )

An earthquake of magnitude Ml = 3.04 was detected on the 19 of March 2005 at 11:49:18.31 (local time) by seven broadband seismometers located around Cameroon (Central Africa). Its epicenter has been relocated and found to be in Monatele (Cameroon) with latitude 4 ̊26.34' and longitude 10 ̊59.62'. The fault-plane solution calculated using double couple fault-plane solutions based on P-wave polarity readings, also taking into consideration the dextral nature of the Sanaga Fault (SF) underlying this region of the epicenter, is a strike-slip fault with a normal faulting component. The beach ball representation of this fault-plane solution has strike, dip and rake values of 289, 70, −169 for the principal fault plane and 195, 80, −20 for the auxiliary fault plane.


Introduction
On the 19 th of March 2005, an earthquake occurred in the Center Province of Cameroon and the preliminary location of this earthquake situated it at a latitude of 4˚10.86', a longitude of 11˚1.38' with a depth of 10 km by the US National Earthquake Information Center (NEIC); and at a latitude of 4˚12.74', a longitude of 11˚6.28' with a depth of 10 km by the International Seismological Center (ISC) [1].The effect of this earthquake was felt some about 81 km in the vicinity of Yaounde, the capital city of Cameroon.At the moment of the occurrence of the earthquake, a temporary deployment of 8 broadband seismic stations was operating across Cameroon [2] in order to study the Cameroon Volcanic Line (CVL) which is one of the most intriguing geological features in West Africa.In this current study, records from 7 stations of this deployment were used to relocate the epicenter of this earthquake in Monatele, the divisional headquarters of the Lekie Division of the Center Region of Cameroon.When the different alignments of fault lines in Cameroon are considered, Monatele is located along the SW section of the Sanaga Fault which is the source of this earthquake (Figure 1).

Locating the Earthquake
Data from the different stations in miniseed format were converted to the Seismic Analysis Code (SAC) format.The SAC program was used to filter the events (using a butterworth filter with two poles and corner frequencies of 0.5 and 2 Hz), pick first arrivals, determine their polarities and obtain the maximum amplitude of the event.This information was then used to build the input file for the earthquake location program HYPOINVERSE [5].The results from this location program stored in an archive output file showed that this earthquake has its epicenter at Monatele (Figure 3) with latitude 4˚26.34' and longitude 10˚59.62';and its focus at a depth of about 7 km.The program also determined the azimuths and takeoff angles of the events (Table 1).

Determination of Fault Plane Solution
The archive output data of HYPOINVERSE was used as an input file into the computer program FPFIT by Reasenberger and Oppenheimer [6] which calculates double couple fault-plane solutions based on P-wave polarity readings.The program was used to generate possible fault-plane solutions allowing room for multiple solutions to be presented.The resulting solution was then plotted using FPFIT program to yield the fault-plane solution in a beach-ball representation (Figure 4).

Discussion of Results
The beach ball brought out two fault planes with strike, dip and rake values of 289, 70, −169 and 195, 80, −20 respectively.Previous studies in the region suggested that the Sanaga Fault is a dextral fault [7]- [9].Therefore, considering the orientation of the two fault planes in the beach ball representation of the fault-plane solution generated with FPFIT (see Figure 4), the fault plane with strike, dip and rake values of 289, 70, −169 is oriented such that the solution is dextral.Consequently, the principal fault plane is that with strike, dip and rake values of 289, 70, −169; while the other fault plane constitute the auxiliary plane.When beach ball representation of fault plane solutions are taken into consideration, the beach ball solution generated from this earthquake also suggests that this fault is a strike-slip fault with a normal faulting component.

Conclusion
The local earthquake of 19 th March 2005 of magnitude Ml = 3.04 was located by the computer-aided program Table 1.Output archive data from HYPOINVERSE showing earthquake location information (retyped and not to scale by author for the purpose of clarity and presentation).Line 1 gives a summary of the location information (time/date: 19/03/2005 at 11:49:18:31, latitude: 4˚26.34',longitude: 10˚59.62',….).From lines 3 to 9: column 1 gives station name (as first four characters), nature of onset phase: I = impulsive or E = emergent (fifth character), type of first arrival phase used: either P or S (sixth character), direction of first arrival: either U = up or D = down or -= not clear (seventh character), quality of phase: ranging from 0 = highest quality to 4 = lowest quality (eighth character); column two gives event date and time of event; column eleven gives the takeoff angles of the events for the stations in the first two characters; column fourteen contains the azimuth to station of the event and the last column gives the component of the SAC signal used (first three characters being the Z component of the broadband seismometer) and the seismic network used (last two characters).

Figure 1 .
Figure 1.Position of the Sanaga Fault (SF) and Monatele on the map of Cameroon.The star indicates the position of Monatele (the epicenter of the earthquake) and the arrow points to the Geological limit of the Congo Craton (adapted from[4]).

Figure 2 .
Figure 2. First arrivals of earthquake (indicated by vertical black lines) registered at stations CM09, CM06, CM18, CM24, CM26, CM29 and CM32.(First arrivals are labeled with four characters which represent; nature of onset phase: I = impulsive or E = emergent (first character), type of first arrival phase used: either P or S (second character), direction of first arrival: either U = up or D = down or-= not clear (third character), quality of phase: ranging from 0 = highest quality to 4 = lowest quality (fourth character).

Figure 3 .
Figure 3. Plot of great circle path (GCP) between epicenter of event (star) and stations (squares).

Figure 4 .
Figure 4. Beach-ball representation of the fault-plane solution of the Monatele earthquake.HYPOINVERSE to have epicenter at Monatele (in Cameroon) situated at latitude 4˚26.34' and longitude 10˚59.62'.This earthquake is seen to have a source the underlying Sanaga Fault.The fault-plane solution obtained by the help of the computer program FPFIT suggests that the section of the Sanaga Fault (SF) around this region is a strike-slip fault with a normal faulting component.Nevertheless, more accuracy would have been added to these results if more stations had recorded this event and their signals processed.