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MOZART: A Seismological Investigation of the
East African Rift in Central Mozambique
by J. F. B. D. Fonseca, J. Chamussa, A. Domingues, G. Helffrich, E.
Antunes, G. van Aswegen, L. V. Pinto, S. Custódio, and V. J. Manhiça
Online Material: Table of station coordinates; preliminary
seismicity catalog.
INTRODUCTION
From Afar to the Malawi–Mozambique border, the East African Rift System (EARS; Fig. 1) is well described on the basis of
geological and geophysical evidence (Ebinger et al., 1987; Ring
et al., 1992; Simiyu and Keller, 1997; Chorowicz, 2005; Roberts et al., 2012), seismicity (Nyblade and Langston, 1995; Yang
and Chen, 2008, 2010; Kim et al., 2009; Delvaux and Barth,
2010), and Global Positioning System (GPS) data (Fernandes
et al., 2004, 2013; Calais et al., 2006; Stamps et al., 2008; Saria
et al., 2013). However, its extent south of the Malawi rift is
unclear. Various trajectories have been proposed: a split near
Lake Tanganyika continuing southwest through the Okavango
delta in Botswana (Scholz et al., 1975); offshore through the
Mozambique Channel (Mougenot et al., 1986); through the
Urema Rift in Central Mozambique (Ebinger et al., 1987;
Hartnady, 1990; Steinbruch, 2010); distributed between the
Okavango rift and the offshore Davie Ridge (Grimison and
Chen, 1988). The M w 7 Machaze earthquake (Fenton and
Bommer, 2006; Yang and Chen, 2008; Raucoules et al., 2010;
Copley et al., 2012) highlighted the importance of the Central
Mozambique sector of the EARS.
Project MOZAmbique Rift Tomography (MOZART) set
out to investigate the seismicity and the crustal structure of
central Mozambique with four main goals:
1. to delineate the active structures of the EARS in this
region;
2. to characterize the imprint left in the crust and upper
mantle by the tectonic history of the study area;
3. to identify any crustal and upper mantle signature of
continental rifting at an incipient stage, thus shedding
light on the still obscure processes of rift initiation;
4. to understand the role of inherited structures in the development of continental rifting.
This report describes the MOZART seismic network operated in the study area, characterizes the quality of the data
through power spectral analysis of the ambient noise, and
presents seismicity results derived from a preliminary analysis
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of the dataset. We focus the discussion on the delineation of
the active structures and the factors controlling their location,
and discuss also the implications of the new data for seismichazard assessment.
TECTONIC SETTING
Proterozoic Inheritance
The study area has a complex tectonic inheritance (Fig. 2). It is
flanked on the west by two blocks of early Archaean crust (i.e.,
the Kaapvaal craton to the southwest and the Zimbabwe craton
to the northwest) which were brought together ∼2:6 Ga ago
along the Limpopo collision belt (Mason, 1973). In the northeast, the structures of the Lurio belt belong to the Mesoproterozoic Kibaran Orogeny (∼1:4 Ga), with a strong imprint of the
Pan-African Orogeny, 800–500 Ma ago (Grantham et al., 2003).
The Zimbabwe craton is bounded in the north by the east–west
Zambezi belt and in the east by the north–south Mozambique
belt, parts of the Pan-African Orogen (Grantham et al., 2003).
These two belts, formed during the assembly of Gondwana, join
in the northernmost sector of our study area.
Mesozoic Rifting
Karoo basalts, erupted at the early stages of Mesozoic rifting,
were drilled at depths of 3100–3300 m (Flores, 1973) at different sites on the Mozambique coastal plains (MCP, see Fig. 2 for
locations). According to Frankel (1972), these basalts were
probably extruded through parallel sets of faults east of the
Lebombo monocline. Thick Upper Cretaceous and Tertiary
sediment layers overlie this basaltic basement (Fig. 2 shows the
extent of the sediment cover). The nature of the crust in the
MCP is debated, hinging on the location of the ocean-continent boundary. Cox (1970) thought the western limit of the
MCP, the Lebombo monocline, to be “a line of complete crustal
disruption,” and Watts (2001) expected the MCP to be underlain by thinned continental, oceanic, or mixed-type crust.
Watkeys (2002) considered the MCP a pull-apart basin floored
by continental crust, formed during the stage of dextral strike
slip between east and west Gondwana. Figure 3 shows the freeair gravity anomaly of the region (Sandwell and Smith, 2009;
Pavlis et al., 2012). The strong positive anomaly associated
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doi: 10.1785/0220130082
▴
Figure 1. Project MOZAmbique Rift Tomography (MOZART)
study area (white rectangle) in relation to the main surface
features of the East African Rift System (EARS; dark gray; after
Chorowicz, 2005 and Kinabo et al., 2007).
with the Lebombo and Nuanetsi–Sabi monoclines was interpreted by Leinweber and Jokat (2011) as a possible indicator of
the transition between continental and oceanic crust.
Tertiary Deformation
According to Flores (1973), Central Mozambique deformed
along two fault systems since the Cretaceous: the Zambezi Tectonic System (ZTS) and the Inhaminga Tectonic System (ITS).
The former consists mainly of the northwest–southeast border
faults of the Lower Zambezi graben (Fig. 4), well exposed in the
northeast of the study area; the latter comprises, from north to
south, the north–south Shire graben and the north-northeast–
south-southwest Urema graben (Fig. 4). Hartnady (2006) proposed an additional seismically active structure further south,
the Mazenga graben. In the north, the structures of the Shire
graben connect with the Malawi rift (Ebinger et al., 1987). The
crosscutting relationships divide the Tertiary deformation into
two epochs, the ZTS being older than the ITS. The M w 7 Machaze 2006 earthquake appears to be associated with the
southern sector of the younger set. In the field, the clearest
expression of these tectonic systems is the Inhaminga fault
(Fig. 4), striking at ∼30° and bordering the Urema rift on
the southeast, with an estimated throw of 700 m (Flores, 1973).
▴ Figure 2. Tectonic provinces of the study area. The dotted line
is the limit of the sediment cover. See text for details.
Regional Seismicity and Microplate Tectonics
Figure 5 shows the seismicity and published focal mechanisms
in the study area and its vicinity. The seismicity traces a northnortheast–south-southwest trend across central Mozambique
between latitudes 18° S and 21° S. There is a cluster of activity
at the epicentral area of the 2006 earthquake (black star in
Fig. 5; no aftershocks were included for clarity). In view of
the regional scarcity of permanent seismographic stations
(Fig. 6), large location errors can be expected in general, impeding clear association of the seismicity with active structures.
In particular, the M s 6.2 earthquake of southwest Mozambique
in 1940, included in the compilation of Gutenberg and Richter
(1949), should be regarded with caution because Trepa
(1970b) reported that the Mozambican press did not contain
any echo of such event.
Stamps et al. (2008) used sparse Global Positioning System (GPS) and earthquake slip data to model the relative motion between the Nubia and Somalia plates, and proposed the
clockwise rotation of the Rovuma microplate, christened by
Hartnady (2002). According to this model, rifting in central
Mozambique between the diverging Nubia plate and Rovuma
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▴ Figure 4. Tertiary tectonic structures of central Mozambique,
after Flores (1973) and Hartnady (2006).
▴ Figure 3. Free-air gravity anomaly of the study area (Sandwell
and Smith, 2009; Pavlis et al., 2012). The map shows the strong
signature of the transition from the Kaapvaal (southwest) and
Zimbabwe cratons (northwest) to the Mozambique coastal plains
(southeast).
microplate progresses at rates between 3.8 in the north and
1:5 mm=year in the south. More recently, Saria et al.’s
(2013) new model revised the position of the pole of rotation,
predicting spreading rates of ∼4 mm=year in south Tanzania
and, possibly, convergence in south Mozambique. The focal
mechanism of the M w 7 Machaze earthquake of 2006 (Yang
and Chen, 2008) is a strong indication that crustal extension is
underway in that region (Delvaux and Barth, 2010).
THE MOZART PROJECT
Political instability in Mozambique prevented adequate seismic
monitoring until recently. In 2011, project MOZART deployed
30 temporary broadband seismographic stations in central and
southern Mozambique and across the South African border
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(Fig. 6). Typical interstation distances are ∼100 km. The
equipment was provided by the SEIS-UK Equipment Pool,
and consists of CMG-3T (120s) seismometers and Nanometrics Taurus or Güralp CMG-DCM dataloggers. The deployment started in March 2011 and ended in August 2013.
Figure 7 depicts the probabilistic power spectral density of
the ambient noise (McNamara and Buland, 2004) recorded at
MOZART stations. Noise levels are low, well below the highnoise model of Peterson (1993). Sites showing lower noise levels in the northwest (squares) and in the southwest (triangles)
are on the Zimbabwe and Kaapvaal cratons, respectively. The
highest noise sites (circles) are on the sedimentary cover of the
MCP (Fig. 2).
ANALYSIS OF THE LOCAL SEISMICITY
Seismicity detected by the network averages one or two small
earthquakes daily. We estimated preliminary locations with a
velocity model derived from James et al. (2003), using the program HYPOCENTER (Lienert and Havskov, 1995) as distributed in the software package SEISAN 9.1 (Ottemoller et al.,
2011). Once the full dataset is recovered, we anticipate better
locations with an improved local velocity model.
Figure 8a shows the epicenters of 307 earthquakes with
M L magnitudes ranging from 0.9 to 3.9, recorded between
April 2011 and July 2012. We adopted the South African
M L magnitude calibration of Saunders et al. (2013), adjusting
the coefficients to minimize the residuals in order to account
for attenuation differences. We retained all the events with
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▴ Figure 5. (a) Seismicity of Mozambique and surrounding regions, 1912–2006. Machaze 2006 earthquake (star) included; aftershocks
excluded for clarity. Source: International Seismological Centre. (b) Zoom in of the study region highlighting the higher-magnitude historical earthquakes and available focal mechanisms (Global CMT catalog; Ekström et al., 2012).
horizontal errors below 10 km. This large allowance reflects the
unsuitability of a single uniform preliminary velocity model,
clearly inadequate for this tectonic setting. Figure 8b and c
shows a map view of a subset of events with depth uncertainty
lower than 5 km, and the projection of the hypocenters on a
vertical section along the strike of the Inhaminga fault, respectively. A selection of four earthquakes with M L > 2:4 in the
Urema graben yields a joint focal mechanism solution that indicates pure normal faulting on a plane striking N31°E
(Fig. 8a), in good agreement with the location and orientation
of the Inhaminga fault.
DISCUSSION
The northeast sector of the observed seismicity pattern correlates well with the topography, tracing the Urema rift valley
(Fig. 8a). The linearity of the seismicity distribution supports
its association with the Inhaminga fault. The linear northnortheast–south-southwest seismicity pattern extends ∼300 km,
reaching the epicentral area of the Machaze earthquake, and sug-
gests that the Inhaminga fault continues to the southwest across
the Lower Zambezi graben.
Focal depths range from the surface to 30 km (Fig. 8c)
indicating a brittle lower crust, possibly due to Karroo mafic
intrusions (Manninen et al., 2008). Yang and Chen (2010) located a Machaze earthquake aftershock at 27 4 km depth
and reported that similar lower crustal depths are common
for earthquakes in amagmatic sectors of the EARS. The concentration of shallow seismicity near the center of the section
may result from the intersection of the Inhaminga and
Zambezi tectonic systems, leading to a locally weaker upper
crust. A cluster of seismicity in the northeastern end, ranging
in depth from 10 to 30 km, corresponds to the location where
the Inhaminga fault connects with the northwest–southeast
Zambezi fault (Chorowicz, 2005).
The joint focal mechanism obtained in the Urema graben
(Fig. 8a) is consistent with west-northwest–east-southeast extension, at right angles to the strike of the Inhaminga fault.
This deviates significantly from the east-northeast–westsouthwest extension revealed by the focal mechanism of the
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▴ Figure 6. Location of the MOZART stations (white triangles), and
other deployments in the region. Crossed circles are permanent
broadband stations, open circles are Africa Array stations (Nyblade et al., 2011), and black dots show previous temporary networks. The white squares show the ongoing SAFARI temporary
deployment (Gao et al., 2013).
M w 7 Machaze earthquake and its aftershocks (Yang and
Chen, 2008, 2010; Delvaux and Barth, 2010), 300 km further
south. The latter strike is in better agreement with the orientation of the Mazenga graben (Fig. 4), proposed as an active
structure by Hartnady (2006). Such change in orientation of
the active structures is also noticeable in the seismotectonic
map of Trepa (1970a) that was based on the seismicity for
the period 1905–1967. By changing from the trend of the
Urema graben (subparallel to the Nuanetsi–Sabi monocline)
to the trend of the Mazenga graben (subparallel to the Lebombo monocline) the southward-propagating rift could avoid
the stronger crust of the Kaapvaal craton. In the north of the
study area, the eastward shift from the Malawi rift to the Shire
and Urema grabens through the Zambezi fault may also reflect
the avoidance of the protrusion of the Zimbabwe craton into
Mozambique (Fig. 2).
Gwavava et al. (1996) from gravity data estimated an effective elastic thickness of 56 km for the Kaapvaal craton, and
from 21 to 39 km for the Mesozoic basin east of the Lebombo
monocline. Petit and Ebinger (2000) pointed out that rifting
tends to follow cratonic margins, where strong rheological contrasts concentrate stress. The 300 km long north-northeast–
south-southwest active segment presented in this paper runs
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along the border of the Zimbabwe craton, in agreement with
that observation. Why it should be truncated near the Machaze
epicentral area is not clear, because the transition to the Kaapvaal craton is still 250 km away. The 1940 M s 6.2 earthquake,
with epicenter ∼200 km south-southwest of the Machaze epicentral area (Fig. 3), may indicate that incipient rifting continues with the same strike until it reaches the Lebombo
monocline, with only a lateral offset in the Machaze epicentral
area (however, as mentioned above, we prefer to take this epicenter with caution). If a change in extension direction is confirmed, it may indicate that a complex second-order stress
pattern exists in this region resulting from the juxtaposition of
cratonic and extended crust at the Lebombo and Nuanetsi–
Sabi monoclines and the sharp difference in the orientation
of the latter structures. A satisfactory seismotectonic framework for the Machaze earthquake should also explain its
dip of 76° 4°, very unusual for normal faulting (Yang and
Chen, 2008).
The scale of the ∼300 km long active fault segment now
proposed, to be confirmed once the complete MOZART dataset is gathered, compares well with the seismotectonic map of
Mozambique proposed by Hartnady (2006), and exceeds the
dominant lengths of the tectonic map of Flores (1973), in
which the structures of the ITS are truncated by those of the
ZTS. This observation is relevant for hazard assessment, because a straight active fault segment of ∼300 km indicates that
magnitudes higher than 7 (currently the historical maximum)
may occur in the region, albeit with long return periods in view
of the low-strain rates predicted by plate tectonic models. Hartnady (2006) compared seismic moment release with the predicted strain rate, and concluded that M w 8 earthquakes
should be expected with a return period of ∼2300 years. However, the predicted strain rates are strongly dependent on the
uncertain location of the Rovuma–Nubia pole of rotation and
on the assumed breadth of the deforming zones. Paleoseismological investigations are therefore required to assess maximum
magnitudes and return periods before significant advances can
be made regarding seismic-hazard assessment.
Near the north-northeast end of the seismicity trend, at
which the Inhaminga and the Zambezi faults connect, a swarm
was recorded in July 2011 (25 earthquakes with 0:9 < M L <
3:8 in 12 days). Trepa (1970b) reported also a concentration of
seismicity at the same location. Like the Machaze epicentral
area, this is another point of inflection of the orientation of
the active structures, which can also be regarded as a candidate
for stress accumulation and possibly higher seismic hazard.
All the hypocentral locations presented in this preliminary
report are conditional on the crustal velocity model. It presently pertains to the Kaapvaal craton (James et al., 2003)
and is inappropriate to the MCP. However, the key observation
of a linear trend of seismicity connecting the Zambezi fault in
the north to the Machaze epicentral area in the south is
unlikely to be affected in a significant way. Maximum earthquake depths may change with incorporation of the likely
lower MCP velocities into the model.
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▴
Figure 7. (a) Nearest-neighbor plot of probabilistic power spectral density (PPSD) of the background noise recorded at the MOZART
stations (vertical component) averaged over the period range 4–6 s (double-frequency microseisms). (b) Median PPSD (vertical component). Squares, stations located on the Zimbabwe craton; triangles, stations located on the Kaapvaal craton; circles, stations located on
the MCP. The gray lines are the new high-noise model (NHNM) and the new low noise model (NLNM) of Peterson (1993).
CONCLUSIONS
Preliminary epicentral locations of microearthquakes in Central Mozambique delineate a straight north-northeast–southsouthwest linear pattern extending ∼300 km, to connect
the Zambezi fault in the north to the epicentral region of
the M w 7 Machaze earthquake of February 2006. Preliminary
earthquake depths extend to ∼30 km, revealing the presence of
an active fault potentially of crustal scale. The fault probably
corresponds to the Inhaminga fault of Flores (1973), which
appears to continue toward the south-southeast after the
Urema graben cuts the older Lower Zambezi graben. A joint
first-motion focal mechanism for four earthquakes in the
Urema rift shows normal faulting with strike N31°E, in good
agreement with the Inhaminga fault’s orientation. These new
results clarify the southward continuation of the EARS, and
contribute to a preliminary fault source model for seismic-hazard assessment in Central Mozambique. It is expected that the
full MOZART dataset will allow a better identification of the
factors controlling the southward propagation of the rift, the
delineation of the second-order stress field in Central Mozambique, and the test of alternative models to explain those
results.
DATA AND RESOURCES
Under the loan conditions for the equipment (Brisbourne,
2012), the full MOZART dataset will be freely available
through the Incorporated Research Institutions for Seismology
Data Management Center (IRIS-DMC) and the Observatories
and Research Facilities for European Seismology (ORFEUS)
Data Centre after an embargo period. Ⓔ Station coordinates
and preliminary hypocentral locations are available as an electronic supplement to this paper.
ACKNOWLEDGMENTS
Project MOZAmbique Rift Tomography (MOZART) was
funded by the Portuguese research foundation FCT, Lisbon,
under contract PTDC/CTE-GIX/103249/2008. The equipment was loaned from Natural Environment Research Council
(NERC)’s SEIS-UK Equipment Pool, United Kingdom (UK).
The authors acknowledge great logistic support from Alex Brisbourne, Victoria Lane, and David Hawthorn (SEIS-UK). Logistic support from the Council of Geosciences (Michelle
Grobbelaar) for the South African sites is also appreciated.
The following staff of the Provincial Geology Directorates
were key to the success of the deployment: Ussene Elias (Sofala), António Riama and Célio Anil (Manica), Carlos Cumbane and Ângelo Mutolo (Inhambane), and Bernardo Cossa
and Francisco Palalane (Gaza). Thanks to Hélder Ferreira,
João Narciso, and Patrícia Pinheiro, from Instituto Superior
Tecnico (IST), for their contributions to the MOZART project.
Cynthia Ebinger and an anonymous reviewer helped us to improve the manuscript. The following software were used to
produce this report: SEISAN (Ottemoller et al., 2011), ObsPy
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▴
Figure 8. Preliminary locations of the earthquakes recorded during the first half of the MOZART deployment (April 2011 to July 2012).
(a) The map view of all the epicenters with horizontal uncertainty below 10 km (307 events). A joint focal mechanism solution is shown for
four earthquakes with M L > 2:4 located in the Urema graben, inside the outlined rectangle (P-wave first motions, 21 polarities, 3 violations). (b) The map view of all earthquakes (error ellipses) with depth uncertainty below 5 km (111 events). (c) Cross section with error
ellipses for the events shown in (b).
(Beyreuther et al., 2010), Generic Mapping Tools (GMT;
Wessel and Smith, 1998), and Seismic Analysis Code (SAC;
Goldstein et al., 1998).
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P.O. Box 217
Maputo, Mozambique
G. Helffrich
University of Bristol
School of Earth Sciences
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Bristol BS8 1RJ, United Kingdom
G. van Aswegen
Council for Geosciences
280 Pretoria Street
Silverton, Private Bag X112
Pretoria, Republic of South Africa
J. F. B. D. Fonseca
A. Domingues1
E. Antunes
L. V. Pinto
Instituto Superior Técnico
Avenida Rovisca Pais, 1
Lisbon 1049-001 Portugal
S. Custódio
Instituto Dom Luiz
Universidade de Lisboa
Lisbon 1749-016 Portugal
[email protected]
J. Chamussa
V. J. Manhiça
National Directorate for Geology
Av. Samora Machel 380
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Also at University of East Anglia, United Kingdom.
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