RELOCATION OF THE MACHAZE AND LACERDA
EARTHQUAKES IN MOZAMBIQUE AND THE
RUPTURE PROCESS OF THE 2006 Mw7.0 MACHAZE
EARTHQUAKEPaulino C. FEITIO*
MEE07165

Supervisors:

Nobuo HURUKAWA**
Toshiaki YOKOI**ABSTRACT
In the present study, the 2006 Machaze earthquake (Mw7.0) and the 2006 Lacerda earthquake (Mw5.6) inMozambique are relocated. The main purpose of the relocation is to determine fault planes related tothose earthquakes. The result of the relocation of the Machaze earthquake indicates that the strike and dipof the fault plane is about 172° and 65° westward, respectively, and it represents a normal faulting.
Analysis of the Lacerda cluster suggests that this is an earthquake swarm. The fault plane corresponding
to the biggest event presents a strike and dip of about 350° and 40°, respectively, dipping to the east. The
rupture process of the Machaze earthquake is also determined in the present study. The result of the slip
inversion shows that two asperities characterize the Machaze earthquake. The maximum slip is 3.4 m
located in the south asperity near the initial break point and the asperity in the north of the initial breakpoint has a slip of about 2.5 m. The last asperity is located at surface and generated the most prominent
offset observed at the surface. The aftershocks are located near the two asperities. Both analyses show asize of the fault plane corresponding to the mainshock about 50 km in length.
Key words: Mozambique earthquake, relocation, joint hypocenter determination, slip inversion.
INTRODUCTION
African continent is divided in two major tectonic blocks, Nubia and Somalia. The boundary betweenthese blocks is along the East African Rift System (EARS). Mozambique is located in the boundary zone
between Nubian plate and Somalia plate, at the southern end of the EARS. The seismicity in the eastern
Africa is related to the tectonics of the region.
The majority of events are located along themajor tectonic lines, mainly along the EARS(Figure 1). The number of earthquakes occurringin the region, according to the historical
recordings including the last strongest (Mw7.0)
Nubianon February 22, 2006 at 22:19 UTC, withPlateepicenter in Machaze, in the western province ofSomalia
Manica, denotes that earthquakes must bePlateconsidered as one of the geohazards in thecountry. In the same year, on September 24, one
event with magnitude Mw5.6 occurred in theoffshore of Mozambique Channel. BothLacerda cluster
Machaze and Lacerda events are located in thesouth end of western and eastern branches of theMachaze cluster
EARS, respectively (circles in Figure 1). Precise
earthquake locations play an important role inunderstanding earthquake source process andone of the most important parameters among theseismological ones on that strong motion
estimation. The strong motion prediction is one
Figure 1. The seismicity map of eastern Africaof the key for earthquake disaster mitigation.
region. The data used is from from Global CMT
within the range from 1976 to 2007
*National Directorate of Geology (DNG), Mozambique.
**International Institute of Seismology and Earthquake Engineering, JapanIn the present study the Machaze and Lacerda clusters are simultaneously relocated to determine thefault plane and seismic plane, respectively. Moment tensor inversion and Rupture process of theMachaze event are also determined in this study using the inversion technique developed by Yagi andFukahata (2008).
METHODS
Three methods are used in the present study. The modified joint hypocenter determination (MJHD)
method (Hurukawa and Imoto 1990, 1992) is used to determine the fault plane for the Machaze andthe Lacerda earthquakes and two inversion methods are used to determine the moment tensor
components and the rupture model for the Machaze earthquake, respectively. The MJHD technique isdesigned to minimize all travel-time residuals simultaneously and to find a common set of station
corrections. This method removes the effect of horizontal heterogeneity of the earth through theintroduction of station corrections. Due to the trade-off between station corrections and focal depth ofearthquakes, the original JHD method becomes unstable and unreliable. The MJHD method
introduced by Hurukawa and Imoto (1990, 1992) and Hurukawa (1995, 1998) overcome this problem
by introducing the following priors. The station correction is independent of the distance and theazimuth from the center of the region to the station. This is very effective, especially in the case inwhich no earthquakes are observed clearly at all stations.
In order to understand the fault rupture process, waveform modeling is one of the mostpowerful tools available. The method developed by Yagi and Fukahata (2008) is used, in whichcovariance components in analysis of densely sampled observed data are considered. The source depth
and source time function are determined using a grid search method. Assuming that those parameters
are known, the moment tensor components were determined by inversion technique. For the slip
inversion, the seismic fault is gridded into many sub-faults. Then the whole rupture process is
considered to be the summation of all these sub-events, which are considered point source,respectively. The unknown parameters include the amount, direction and temporal change of slip ateach grid point. The result of the inversion which gives the amount of slip at each grid point isobtained using least square fitting. From this inversion the initial depth can be corrected, hypocentercan be relatively accurate located, the initial rupture time, and the moment tensor can be obtained.
DATA AND RESULTS OF RELOCATION
The data used for relocation of both Machaze cluster and Lacerda swarm consist of P-arrival times
from International Seismological Centre (ISC) and National Earthquake Information Center (NEIC)
operated by United States Geological Survey (USGS). From ISC, the data was retrieved within theperiod between January 1, 1964 and December 31, 2004 and from NEIC the data ranges between
January 1, 2005 and December 31, 2007.
Machaze cluster
The criterion used for station and events selection is MSTN (minimum number of stations which
record the event) is 15 and for the MEVN (minimum number of events recorded by each station) isalso 15. By using this criterion initially 69 stations were selected worldwide to locate 103 events.
Figure 2 shows the hypocenters distribution determined by ISC and NEIC-USGS. The fault
mechanism used is obtained from Global CMT catalog. Since depths of hypocenters were fixed at 10km, it is difficult to identify the fault plane of the mainshock. The MJHD method is used to relocate
the Machaze event. In these analyses only hypocenters with travel time residuals less than 2.0 seconds
are relocated.

Figure 2. The epicentral distribution and two
vertical cross sections along A-B and C-D lines,
which are perpendicular to the strikes of the two
nodal planes of the Glonbal CMT, are illustrated.
The size of symbols represents the magnitude ofthe event and the shapes and the colors of thesymbols denote the depth.

Figure 3. Relocated hypocenters. The bars
in the circles represent the standard
deviation. The sym,bols are the same as in
Figure 2

Observing the cross sections in Figure
3, it seems to be clear that the nodal plane
shown in the cross section C-D is the fault
plane of the Machaze earthquake. Figure 4
shows only immediate aftershoks.
Although there is some scatter in
hypocenter distribution in horizontal direction,
it still clear that the nodal plane shown in thecross section C-D in Figure 4 is the fault planeof the Machaze event. The strike and dip of
this fault are 172° and 65°, respectively. The
length of fault plane is estimated to beapproximately 50 km and the epicenter islocated near the center of the fault. This event
has only one immediate foreshock observed in
the relocated results and the epicenter islocated less than 1 km from the mainshock.
The time difference between the foreshock andmainshock is approximately 27 hours and 8minutes.

Figure 4. Immediate aftershocks within a week
after the mainshock of the Machazeearthquake. The symbols are the same as in
Figure 3.

Lacerda cluster
For the case of the Lacerda cluster the criteria used is as follows: MSTN is 12 and MEVN is also 12.
In this relocation 21 stations were used to locate 15 events. Figure 5 shows the epicentral distribution
determined using data from NEIC-USGS. Analyses of the relocated events shows that this cluster iscomposed by events with magnitude ranging between Mw4.0 and Mw5.6 and the biggest event(Mw5.6) is one of the last in the cluster, which suggest that the cluster can be considered anearthquake swarm. The fault mecanism used is obtained from Global CMT catalog, considering thebiggest event.

The focal depth is fixed at 10 km, it is impossible to identify the fault plane. Figure 6 shows
the results of relocation. It is clear that all events follow the N-S lineament, which is consistent withthe tectonic lineament of the Lacerda graben. Although the scatter is large, the hypocenter distributionof these events suggests that the nodal plane shown in cross section A-B could be the fault plane
related to the major event. The strike and dip of the fault plane determined by the Global CMT is 350°
and 40°, respectively. This fault plane dips to the east.

Figure 5. Epicentral distribution map using data fromNEIC-USGS. The mechanism of the event
considered is from Global CMT. The symbols are asin Figure 2.

Figure 6. Relocated events from Lacerda cluster.
The symbols are as in Figure 3.

DATA AND RESULT OF SLIP INVERSION
The rupture model of Machaze earthquake is determined in the present study using the inversion
method by Yagi and Fukahata (2008). The data used was retrieved from Data Management Center(DMC) of Incorporated Research Institute of Seismology (IRIS). Fifteen vertical components frombroadband seismograph stations are used to pick P-arrivals. To retrieve the source information thestations were selected within the epicentral distance between 30° and 90°.
For slip distribution waveforms are inverted for both nodal planes. For the nodal plane I,
13x10 grid points with spacing of 4 km was adopted. For the nodal plane II, grid points with 13x13
and spacing of 4 km was adopted. The fault models are determined for both nodal planes. The
hypocenter is determined by grid search method varying the depth for every 2.0 km for both faultmodels. According to Figure 7, the differences between the variances of the strike, dip and rake ofboth fault models are small, but the fault model I present slightly smaller variances. Using the result
from aftershock distribution it is assumed that fault model I is the actual fault plane. The spatiotemporal slip distribution is presented in the Figure 8d.

(a)(b)(c)(d)

Figure 7. Comparison of the variances obtainedfor fault model I and fault model II.

Figure 8. (a) Source parameters, (b) fault mode I
mechanism of the event, (c) source time function,(d) amount of slip in each grid point. The arrows
indicate the slip direction of the hangingwall.

The final source parameters of the Machaze earthquake determined in the present study can besummarized as follows: Strike, dip and rake, 176, 74 and -78.3, respectively; the initial break point islocated at 12.0 km depth; the source duration is 13 seconds (Figure 8c); there are two asperities, themaximum slip is 3.4 m located in the south asperity near the initial break point and the asperity in thenorth of initial break point has a slip of about 2.5 m. The length of the fault plane is approximately 50km.
DISCUSSION
The Machaze fault plane determined using the MJHD method dips to the west. It is normal fault andthe strike, dip and rake (172°, 65° and -78°) of the determined fault plane are adopted from Global
CMT. The result based on relocated aftershock distribution is consistent with the field observations byFenton and Bommer (2006). Those authors observed that the Machaze fault strikes N10-20°W, and thelinear trace of the fault indicates that the fault plane is relatively steep, which displacement is normal,
down-to-the-west on a west-dipping fault plane. In the rupture process determination, there are twoasperities observed. One of them is located at the surface, 15 to 25 km on the north of the initial breakpoint. Source time function (Figure 8c) shows two peaks corresponding to two observed asperities.
The second peak occurs five seconds after the first peak which means that the second major slip wasobserved at the surface five seconds after the major slip. The slip inversion showed also that other thanthe dominant dip slip offset, there are a small left-lateral offset (Figure 8). This result is consistent
with the observations presented by Fenton and Bommer (2006). Aftershock distribution as observed inFigure 9 points that events in the northern side of the mainshock are shallower and in the southern side
are deeper. That aftershocks distribution is consistent with the observed asperities from slip inversion.
From slip inversion it is observed that the asperity located in the north of the initial break point isshallower and the southern one is deeper. In Figure 9 the fault plane is projected assuming the result offault plane model determined from the slip inversion. This Figure shows a steeper fault plane (dipping74°) and it fits well the aftershock distribution reducing the scatter observed when the orientation offault plane determined by Global CMT is adopted.
Hashimoto at el., (2007) observed from the analyses of ENVISAT images that the source fault
is elongated in both north and south directions from the surface rupture, but they couldn’t observe
clearly the extent in the north of rupture area. In the fault slip distribution presented in this study, it’sclear that there is no extension of slip distribution in the north of surface rupture area, related tocoseismic rupture.

Figure 9. Aftershock distribution of Machaze event
adopting the mechanism determined from slip
inversion. The symbols are as in Figure 4.

The surface offset determined from theslip inversion in the present study is about 2.5
meters and that obtained by Hashimoto at el.,(2007) is about 2.0 ~ 2.7 meters. Those results
are consistent each other. The fault lengthinterpreted by Hashimoto et al., (2007) isabout 50 km.
The result from aftershock distribution
suggests a fault length of approximately 50 km
which is the same obtained from slip inversion.
Furthermore, all analyses indicate bilateral
rupture of the earthquake. The fault plane of
the biggest event of Lacerda dip to the east andthe strike is approximately N-S, which isconsistent with the general strike of theLacerda graben. Hypocenter distribution ofthis swarm shows that all of events lie along
the same line suggesting that they belong to
the same seismic plane.

CONCLUSIONS
Using the MJHD technique, a foreshock, the mainshock and aftershocks of the 2006 Machazeearthquake are accurately relocated in the present study. The fault plane dips to the west with a steep
angle. The length of the aftershocks area is approximately 50 km. This study revealed also that theLacerda cluster is an earthquake swarm. Although it should be an earthquake swarm, all events lie
along the same line suggesting that they belong to the same seismic plane which dips to the east.
Analyzing the orientation of both Machaze fault plane and Lacerda fault plane determined in thisstudy and relating them with the tectonics of the area where they are located, it can be concluded thatthey are related to the progressive continental break-up, and intra-continental tectonism related toEARS. From the slip inversion analyze of the Machaze fault, it can be concluded that the Machazeearthquake had two main asperities, one of them located on the surface, which originated an offset of
2.5 meters. The initial break point was not the point of major slip of the fault. The fault slip confirmedthat the fault was mainly normal faulting, but there is a small left-lateral slip along the fault plane. Thelength of the rupture zone is about 50 km and is consistent with the length of aftershock area.

ACKNOWLEDGEMENTS
Expression of gratitude is addressed to Professor Yagi, from University of Tsukuba for the program,and to Prof. Manabu Hashimoto and Prof. Julian Bommer for the valuable manuscripts provided.

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