Geophysical Journal International
Geophys. J. Int. (2010) 183, 375–380
doi: 10.1111/j.1365-246X.2010.04732.x
Earthquake source parameters for the 2010 January Haiti main
shock and aftershock sequence
Meredith Nettles1,2 and Vala Hjörleifsdóttir1 ∗
1 Lamont-Doherty
2 Department
Earth Observatory of Columbia University, New York, NY, USA. E-mail: [email protected]
of Earth and Environmental Sciences, Columbia University, New York, NY, USA
SUMMARY
Previous analyses of geological and geodetic data suggest that the obliquely compressive
relative motion across the Caribbean–North America plate boundary in Hispaniola is accommodated through strain partitioning between near-vertical transcurrent faults on land and
low-angle thrust faults offshore. In the Dominican Republic, earthquake focal-mechanism
geometries generally support this interpretation. Little information has been available about
patterns of seismic strain release in Haiti, however, due to the small numbers of moderateto-large earthquakes occurring in western Hispaniola during the modern instrumental era.
Here, we analyse the damaging MW = 7.0 earthquake that occurred near Port au Prince on
2010 January 12 and aftershocks occurring in the four months following this event, to obtain
centroid–moment-tensor (CMT) solutions for 50 earthquakes with magnitudes as small as
MW = 4.0. While the 2010 January main shock exhibited primarily strike-slip motion on a
steeply dipping nodal plane (strike=250◦ , dip=71◦ and rake=22◦ ), we find that nearly all of
the aftershocks show reverse-faulting motion, typically on high-angle (30◦ –45◦ ) nodal planes.
Two small aftershocks (MW 4.5 and 4.6), located very close to the main shock epicentre, show
strike-slip faulting with geometries similar to the main shock. One aftershock located off the
south coast of Haiti shows low-angle thrust faulting. We also examine earthquakes occurring
in this region from 1977–2009; successful analysis of four such events provides evidence for
both strike-slip and reverse faulting. The pattern of seismic strain release in southern Haiti
thus indicates that partitioning of plate motion between transcurrent and reverse structures
extends far west within Hispaniola. While we see limited evidence for low-angle underthrusting offshore, most reverse motion appears to occur on high-angle fault structures adjacent
to the Enriquillo fault. Our results highlight the need to incorporate seismogenic slip on
compressional structures into hazard assessments for southern Haiti.
Key words: Earthquake source observations; Seismicity and tectonics.
1 I N T RO D U C T I O N
A large and destructive earthquake of MW 7.0 occurred on 2010
January 12 near the densely populated capital city of Haiti, Port
au Prince. The earthquake occurred in a region of known, but
poorly characterized, historical seismicity (e.g. Scherer 1912), on
the Enriquillo–Plaintain Garden fault system. This system accommodates primarily transform motion between the Caribbean and
North American plates (e.g. Mann et al. 1995). The overall relative
plate motion is transpressional (e.g. Sykes et al. 1982; Deng & Sykes
1995; Dixon et al. 1998; DeMets et al. 2000; Manaker et al. 2008),
and many compressional geological features have been mapped in
the region (Mann et al. 1995). However, the only mapped through∗ Now at: Universidad Nacional Autónoma de México.
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the Enriquillo fault, exhibits primarily transcurrent motion, and is
believed to dip nearly vertically (Mann et al. 1995, 2002).
No earthquakes larger than M ∼ 5 have been recorded in southern Haiti during the last 34 years, approximately the era of modern
instrumental recording and the timespan (1976–2009) covered by
the Global Centroid–Moment Tensor (GCMT) catalogue (Ekström
et al. 2005; Dziewonski et al. 1981). One earthquake in northern
Haiti, on 1994 March 2 of MW 5.4, exhibits left-lateral strike-slip
motion on a nodal plane striking east–southeast, consistent with
the mapped geometry of the Septentrional fault associated with
the northern part of the plate-boundary zone (Prentice et al. 1993;
Mann et al. 1998). The most recent large earthquake on the island
of Hispaniola, prior to the 2010 January 12 event, occurred on 2003
September 22 in the Dominican Republic, with moment magnitude MW 6.4. The focal mechanism for that event, as for all other
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Accepted 2010 July 10. Received 2010 July 7; in original form 2010 May 27
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M. Nettles and V. Hjörjleifsdóttir
earthquakes of MW 6.0 or larger occurring in the Dominican Republic and included in the GCMT catalogue, shows thrust faulting.
Palaeoseismic investigations and historical reports make it
clear that large earthquakes have occurred in Haiti previously
(e.g. Scherer 1912; Prentice et al. 1993). Recent studies using geological and geodetic data have also highlighted the potential for
earthquakes of M ∼ 7–7.5 on the Enriquillo and Septentrional
faults (Calais et al. 2002; Manaker et al. 2008). Little is known,
however, about the character of smaller, recent earthquakes in the
Enriquillo fault region, or about background seismicity rates, because of the absence of a local seismic network in Haiti. Following
the great 2004 Sumatra–Andaman earthquake and the large tsunami
it generated, the United States Geological Survey (USGS) invested
in a significant improvement to broad-band seismological observing capabilities in the Caribbean, installing nine broad-band seismometers in the region (network code CU), each with near-realtime
telemetry capabilities. The combined network coverage provided
by the CU and global-network stations, along with improvements
to moment-tensor determination techniques (Arvidsson & Ekström
1998; Ekström et al. 1998) now make analysis of smaller earthquakes possible.
Knowledge of seismic strain-release patterns prior to and following the 2010 January main shock is important for understanding
how strain is accommodated throughout the region, and for understanding possible effects of static stress changes related to the main
shock and larger aftershocks. The minimum magnitude threshold
for global CMT analysis is normally set at M ∼ 5, which would
lead to GCMT analyses of fewer than a dozen events for the current
Haiti sequence. Here, we present CMT solutions for 50 earthquakes
of the 2010 Haiti main shock–aftershock sequence, including events
with magnitudes as small as MW = 4.0. We also present analyses of
four earthquakes of magnitude 4.3–4.9 occurring prior to the main
shock, during the years 1990–2008.
2 D ATA A N D M E T H O D S
We apply the CMT approach (Dziewonski et al. 1981; Dziewonski
& Woodhouse 1983; Ekström et al. 2005) to obtain moment tensors,
centroid locations and centroid times for the 2010 January 12 Haiti
main shock and aftershock sequence. We attempt to analyse all
earthquakes in the map region shown in Fig. 1 reported by the
USGS National Earthquake Information Center (NEIC) as of 2010
May 20 with preliminary magnitudes of 4.0 and larger for the four
months following the main shock (2010 January 12–2010 May
12). In addition, we attempt to analyse all events reported in the
USGS monthly listing with magnitude 4.0 or larger during the period
1977–2009.
For larger events (approximately MW ≥ 5.0), we follow the standard procedures used for global CMT analysis (Ekström et al.
2005). These analyses incorporate long-period body waves in
the period range 50–150 s; mantle waves in the period range
125–350 s and intermediate-period surface waves in the period
range 50–150 s. The determination of source parameters for smaller
Figure 1. Top panel: focal mechanisms for 50 earthquakes of the 2010 January 12 Haiti main shock–aftershock sequence. Bottom panel: focal mechanisms
for four earthquakes occurring in 1990–2008, prior to the 2010 main shock. All events are plotted at the NEIC epicentral locations. The mechanisms shown in
grey are less well constrained than those shown in red. Solutions could not be obtained for 56 events attempted, owing to high noise levels.
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events relies primarily on the intermediate-period surface waves; the
surface-wave analysis is implemented as described by Arvidsson &
Ekström (1998) and Ekström et al. (1998). For some events, we adjust the filter towards shorter periods, which typically provide better
signal-to-noise ratios for shallow earthquakes. Most such events are
analysed in the 40–100 s band. For the smallest events, typically
those of MW ≤ 4.5, the closest stations ( ≤ 15◦ ) are analysed
in the period range 35–75 s. The filter is selected on a stationby-station basis in this case. We use data recorded at the stations
of the GSN, Geoscope, Geofon, the Lamont-Doherty Cooperative
Seismographic Network (LCSN) and the USGS Caribbean network
(CU) for our analyses, as well as stations of the Abbreviated Seismic
Research Observatory (ASRO) network for early events.
3 A N A LY S I S A N D R E S U LT S
We are able to obtain CMT solutions for 50 earthquakes of the
2010 January 12 sequence, and four events occurring prior to 2010.
Focal mechanisms for these events are shown in Fig. 1 and source
parameters are presented in Tables S1 and S2 (see the Supporting
Information section). The source parameters can also be downloaded in electronic format from our website (www.globalcmt.org).
In the figures and table, we identify 14 of the events as having less
well-constrained focal mechanisms. These are events with fewer
usable data records, either due to small event size or the presence
of large amplitude waveforms from earlier events. As can be seen
in Fig. 1, the results for these events are consistent with those for
the best-constrained events.
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separation, fixed to 7, 10 or 15 s, but we find that a reverse-faulting
subsource with a strike of ∼250◦ –270◦ , dip of ∼65◦ and scalar
moment 30–40 per cent that of the total moment, combined with a
subsource of near-vertical dip and left-lateral slip, explains the data
well. Similar geometry for the reverse mechanism is obtained when
the strike-slip fault plane is allowed to dip steeply to the south.
Second, we conduct an analysis of how well we are able to resolve
the dip of the main shock fault plane under the assumption that the
fault slipped obliquely on a single planar fault. We perform a series
of inversions in a grid search over the source geometry. The strike
is fixed to either 252◦ (for a north-dipping nodal plane) or 72◦ (for
a south-dipping nodal plane), the rake is varied between −45◦ and
+45◦ and the dip is varied between 55◦ and 90◦ , in 5◦ increments,
in an approach similar to that taken by Henry et al. (2000) for the
Antarctic plate earthquake. The residual misfit between observed
and synthetic waveforms is shown in Fig. 2(a). We find two minima
in the misfit function, with the deeper minimum corresponding to
the best-fit double-couple solution described earlier, with a northdipping fault plane. The shallower minimum corresponds to a southdipping fault plane, with strike 72◦ , dip 65◦ and rake 25◦ . The
residual variance is dominated by the contribution from the longperiod mantle waves, and the difference in misfit for the northand south-dipping solutions is relatively small. However, the body
waves for the south-dipping fault plane are fit poorly, especially at
several stations in nodal directions. An example of the difference in
waveform fits to the body waves for the north- and south-dipping
fault planes is shown in Fig. 2(b). We conclude that the fault plane
must indeed dip to the north, under the assumption that all of the
slip occurred on a single planar fault.
3.1 The 2010 January 12 main shock
The source parameters obtained for the main shock are similar to
those for the quick CMT distributed within hours of the event.
The refined solution has scalar moment M0 = 4.4 × 1026 dynecm (MW = 7.0). The presumed fault plane strikes 250◦ and dips
71◦ north–northwest. The slip angle of 22◦ indicates primarily leftlateral motion, with a moderate thrust component. The best-fitting
moment tensor includes a non-double-couple component that is
relatively large, with the intermediate eigenvalue ∼20 per cent the
size of the absolutely largest eigenvalue. The size of the non-doublecouple component is not well constrained, but is also consistent with
a thrust contribution to the main shock. The geometry of the focal
mechanism obtained when the solution is required to represent a
perfect double couple is very similar to the double-couple part of
the unconstrained solution, with strike 252◦ , dip 66◦ and rake 28◦ .
The main shock focal mechanism, which corresponds to the best
point-source representation of the earthquake geometry, is consistent with oblique slip on a single fault plane or, if the non-doublecouple component represents a source feature, nearly simultaneous
slip on two separate fault planes experiencing left-lateral and reverse slip. To assess seismological constraints on the geometry of
the main shock fault or faults, we perform two analyses.
First, we invert the waveforms used to determine the main shock
moment tensor for a CMT solution composed of two subsources
to determine the geometry of the reverse mechanism that, paired
with a near-vertical strike-slip fault, best explains the observed
waveforms. One subsource is constrained to have a nodal plane
oriented 250◦ with pure left-lateral slip and a dip of either 71◦ or
90◦ , while the geometry of the second subsource is constrained to
be a double couple, but otherwise left free. The retrieved geometry
of the second subsource depends somewhat on the subsource time
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The most surprising result of our CMT analysis is that nearly all
of the aftershocks for which we are able to obtain solutions show
reverse faulting (Fig. 1). Indeed, we find only two aftershocks, an
MW 4.5 event on January 16 and an MW 4.6 event on February 23,
both located very close to the main shock hypocentre, that show
dominantly strike-slip motion (Fig. 1). Most of the reverse-faulting
events exhibit steeply dipping (30◦ –45◦ ) nodal planes, though a few
have one nodal plane of shallower dip. One event, which occurred
off the south coast of Haiti (2010 January 15; MW 4.6), shows
underthrusting on a plane dipping 13◦ to the north. All of the events
are found to be shallow (≤22 km), with most solutions returning
depths of 12 km, the minimum depth allowed for standard CMT
inversions in the absence of additional depth constraints.
Aftershocks occurring immediately after the main shock can be
difficult to analyse because of interference from the large main
shock surface waves at all stations. We have paid particular attention to these early aftershocks to assess whether they might exhibit
transcurrent, rather than reverse, motion. In several cases, we have
performed joint inversions for earthquakes occurring close in time
to confirm that interference between wave packets does not bias
our results. We find that even those events occurring within the
first hours after the main shock are dominated by reverse motion,
including a MW 6.0 event 7 min after the main shock.
The directions of maximum compression for the reverse-faulting
events are oriented NE–SW in all but three cases, with P-axis azimuths ranging from 0◦ to 50◦ , and most falling in the range 5◦ –35◦ .
We find some suggestion for two preferred P-axis orientations, one
near 10◦ and one near 30◦ , but the clustering is not strong. We
believe the variability in retrieved CMT parameters reflects true
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Figure 2. (a) Residual misfit for pure double-couple solutions, with strike 252◦ (left-hand side of image) and 72◦ (right-hand side of image) and varying rake
and dip; note non-linear colour bar. There are two minima, one for the north-dipping fault plane (left-hand side) and one for the south-dipping fault plane
(right-hand side). The residual misfit for the north-dipping plane is slightly smaller than that for the south-dipping plane. (b) An example of waveform fits to
the body waves for the solutions corresponding to the two minima in (a). Shown is the vertical component of displacement (blue: data; red: model), bandpass
filtered between 50 and 150 s, from station KDAK (Alaska), at a distance of 69◦ and azimuth of 326◦ . Shaded regions lie outside the body-wave window and
are excluded from the inversion.
variability in the geometry of the earthquake sources: the relative
orientations of the individual sources are well constrained by our
analysis, which uses a nearly identical subset of stations for earthquakes of similar magnitude. The sensitivity of the waveforms to
variations in source geometry is illustrated in Fig. 3.
3.3 Earthquakes prior to 2010
We are able to obtain CMT solutions for four earthquakes, of MW =
4.3–4.9, occurring prior to the 2010 January main shock. The focal
mechanisms we retrieve for the pre-main shock events are consistent
with those observed for the aftershock sequence: three of the four
moment tensors show reverse faulting, with the fourth showing
strike-slip faulting on an approximately E–W trending nodal plane.
The reverse-faulting events are located to the west of the 2010
January main shock, and the geometry of the events occurring in
the aftershock zone is similar to that observed for the 2010 sequence.
The geometry of the strike-slip event is similar to that of the 2010
main shock, but is located to the east of the main shock, in a region of
relatively weak aftershock activity. One event (1990 December 31)
for which we are unable to obtain a robust CMT solution nonetheless
appears to have both a location and source geometry similar to the
underthrusting event of 2010 January 15 off the south coast. As for
the 2010 aftershocks, the P-axis orientations of these earlier events
lie in the range 0◦ –50◦ .
4 D I S C U S S I O N A N D C O N C LU S I O N S
The dominance of reverse faulting in the aftershock sequence for
this large strike-slip earthquake is unusual, and we are not aware
of previous examples of aftershock sequences which are both so
different from the main shock geometry and so internally consistent.
While the 1989 Loma Prieta earthquake generated an aftershock
sequence in which many events differed in geometry from the main
shock, the aftershocks also exhibited great internal variability, with
examples of left- and right-lateral strike-slip, normal, and reverse
faulting (Beroza & Zoback 1993).
The locations of the Haiti aftershocks are distributed over a wide
area, mostly to the west of the main shock hypocentre. The centroid
locations we determine are systematically shifted to the north with
respect to the hypocentre locations reported by the NEIC, perhaps
due to ray paths through the northern Caribbean and Gulf of Mexico slower than modelled and station coverage dominated by North
American stations. However, the centroid locations remain as widely
distributed as the reported hypocentre locations, and the combination of geographical distribution of the earthquake locations and the
variation in focal geometry suggest that the aftershocks occur on
multiple, distributed fault structures, rather than on a single reverse
fault.
Nearly all of the aftershocks occur on planes striking WNW–ESE.
If the reverse component of the main shock occurred on a separate
reverse fault, rather than as oblique slip on the Enriquillo fault, our
analysis suggests that this reverse fault strikes ENE–WSW to E–W,
like the Enriquillo fault itself, and that it must be large enough to
accommodate an earthquake of approximately MW 6.8. However,
further knowledge of the accommodation of reverse motion during
the main shock will require detailed analysis of higher frequency
body waves and satellite remote-sensing data.
The reverse motion we observe in the Haiti aftershocks is consistent with the presence of multiple thrust structures mapped in
the region (Mann et al. 1995) and with a measured component
of compression across the island of Hispaniola (Manaker et al.
2008). However, previous studies of strain partitioning in the northern Caribbean have concluded that the overall east–northeastward
motion is divided between the Enriquillo and Septentrional strikeslip faults on land and low-angle underthrusting structures off
the north and southeast coasts of Hispaniola (Mann et al. 2002;
Manaker et al. 2008). Our results, from the aftershock sequence
and earlier earthquakes, indicate that strain partitioning in southwest Hispaniola occurs through left-lateral slip on the Enriquillo
fault and reverse slip on adjacent high-angle faults. We infer that
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Figure 3. Observed (blue) and synthetic (red) surface wave seismograms (vertical component of displacement) for events 201001130132A (black arrows) and
201001130136A (green arrows) at several stations located at epicentral distance and azimuth α. All records are bandpass filtered between 50 and 150 s. The
relative amplitudes of the arrivals for the two events vary with azimuth, indicating different focal geometries for the two events, which have similar depths and
source locations. The filter used in the analysis is acausal, leading to the apparent arrival of surface wave energy prior to the event onset (0 s).
many anticlinal structures observed on land (Mann et al. 1995) are
fault cored.
The prevalence of reverse faulting for the earthquakes analysed,
apart from the 2010 January 12 main shock, emphasizes the need
to incorporate seismogenic slip on compressional structures into
hazard assessments for southern Haiti. While the reverse events appear to occur on multiple fault structures, rather than a single, large
thrust fault, probably limiting the maximum size of such events,
even earthquakes of MW 4–5 are large enough to cause damage at
local distances. Further, we see some evidence for thrust faulting
off the south coast of Haiti with geometry similar to that of larger
underthrusting events that have occurred to the east in the Dominican Republic. The seismic deformation occurring on reverse faults
should also be taken into account in stress-transfer models like
those used to assess probable loading on the Enriquillo fault and its
branches (e.g. Lin et al. 2010).
AC K N OW L E D G M E N T S
We thank R. S. Stein, F. Waldhauser, T. Diehl, N. Seeber and G.
Ekström for suggestions and discussions, and two anonymous re
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Journal compilation viewers for helpful comments. The GSN data were collected and
distributed by IRIS and the USGS. We thank the operators of the
GSN, LCSN, Geoscope and Geofon for collecting and archiving the
seismic data used here. This work was supported by NSF awards
EAR-0824694 and EAR-0710842.
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S U P P O RT I N G I N F O R M AT I O N
Additional Supporting Information may be found in the online version of this article:
Table S1. Centroid—moment-tensor solutions for 54 earthquakes
occurring in southern Haiti, 1990–5/2010. Headings are as in standard Global CMT reports (Ekström et al. 2005).
Table S2. Principal axes and best-double-couple parameters.
Please note: Wiley-Blackwell are not responsible for the content or
functionality of any supporting materials supplied by the authors.
Any queries (other than missing material) should be directed to the
corresponding author for the article.
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