Haiti Mw 7.0 Strike-Slip Earthquake
John Elliott, Zhenhong Li, Barry Parsons
Figure 1: ALOS ascending interferogram covering the portion of the fault rupture west of Port-au-Prince overlain on digital elevation topography (Click figure to enlarge). Coloured fringes indicate surface deformation in the satellite line-of-sight. Cycles of colour from blue to red indicate the ground has moved away from the satellite. SAR data from the Japanese Space Agency (JAXA). Detailed modelling remains to be done, but the location of the fringes indicate the fault rupture occurred somewhat to the west of Port-au-Prince. A kml file to display this figure in GoogleEarth is available here.
Figure 2: LANDSAT image of the region of Haiti struck by the earthquake to the west of Port-au-Prince. The Enriquillo fault is a clear topographic feature running ENE-WSW through the landscape and the causative fault is likely to be located on this.
InSAR Observations of Ground Displacements and Implications for Future Seismic Hazard
COMET researchers are investigating the ground deformation associated with the devastating recent Haiti earthquake using satellite radar measurements. They hope to more accurately locate the fault that ruptured in this event in order to improve our understanding of faulting and seismic hazard.
Interferometric Synthetic Aperture Radar (InSAR) is a satellite based technique which can provide measurements of ground motion in an earthquake. These observations are represented in an interferogram as fringes which mark contours of ground motion in the line-of-sight of the satellite (40 degrees off vertical). This enables a much more precise identification of the fault location and extents of slip for the earthquake rupture when compared to the seismological solutions. In the case of the Haiti earthquake, the InSAR observations indicate that the rupture was somewhat to the west of Port-au-Prince, with a significant amount of the rupture occurring offshore (Figures 1 & 3). This observation has very important implications regarding the future earthquake hazard for this region. It indicates that the portion of the Enriquillo fault immediately to the south of Port-au-Prince has not ruptured in this event, but has been brought closer to failure from the stresses imparted from the recent earthquake. Therefore, rebuilding and restructuring efforts focused on the capital must take account of this continuing seismic hazard.
Figure 3: ALOS descending interferogram covering the fault rupture overlain on digital elevation topography. Coloured fringes indicate surface deformation in the satellite line-of-sight. Cycles of colour from blue to red indicate the ground has moved away from the satellite. SAR data from the Japanese Space Agency (JAXA). A kml file to display this figure in GoogleEarth is available here.
Figure 4: ALOS ascending interferogram covering the most westerly portion of the fault rupture overlain on digital elevation topography. Coloured fringes indicate surface deformation in the satellite line-of-sight. Cycles of colour from blue to red indicate the ground has moved away from the satellite. SAR data from the Japanese Space Agency (JAXA). A kml file to display this figure in GoogleEarth is available here.
Background
Figure 5: Map of recent earthquakes and GPS vectors for Hispaniola
with the mainshock and largest aftershock indicated to the west of
Port-au-Prince. Earthquakes are magnitude 5 and above from the Global CMT catalogue for the period 1973-2008. GPS vectors
are motion of Hispaniola relative to a fixed North American
Plate from Manaker et al. (2008). Slip rate estimates for the Enriquillo fault and Septentrional fault are 8 ± 5 and 7 ± 2 mm/yr respectively as calculated by Manaker et al. (2008).
On the 12th January the USGS reported a magnitude 7.0 earthquake had occurred in Haiti just before 5 pm, west of the capital Port-au-Prince. The initial seismological solutions indicated a predominately strike-slip fault that ruptured at a shallow depth of about 10 km. The ENE-WSW striking nodal plane is the most consistent with the tectonics of the region. The causative fault is most likely the East-West striking Enriquillo left-lateral strike-slip fault. An earthquake of this size is likely to have ruptured over a fault 40 km long and slipped 1-2 metres.
The largest aftershock to date was a magnitude 5.9 which occurred eight days after the mainshock on the 20th January (USGS), and most likely is located at the western end of the initial fault rupture. However the mechanism of this earthquake was mostly thrustal rather than predominately strike-slip like mainshock.
Similar previous large earthquakes to have occurred along this fault in 1751 and 1770 and were thought to have been of a slightly larger magnitude (7.5 – see Manaker et al. (2008)). Their estimates of fault slip rate from GPS observations have the Enriquillo fault accumulating interseismic strain at a rate of about 7 mm/yr. Over 250 years, a fault locked at this rate has a slip deficit of almost 2 metres.
Figure 6: Map of the tectonic plate boundaries and names for Central America. The arrows indicate the motion of parts of the Caribbean plate relative to a fixed North American plate as recorded by GPS on Jamaica and Barbados. Plate boundary data is from Bird (2003) and GPS velocities from Manaker et al. (2008).
Figure 7: Map of the population distribution for Hispaniola with the location of the mainshock and largest aftershock. The fault rupture occurred to the south-west of the Haitian capital Port-au-Prince with a population of about 700,000. Population data from SEDAC.
Figure 8: Map of the mainshock epicentres from various sources (indicated by the focal mechanisms/beach balls) and for the aftershocks (red circles). The mainshock solutions indicate that the earthquake is dominantly strike-slip with a component of thrust. The largest aftershock on the 20th January is labelled Mw 5.9 and is west of the mainshock. The aftershocks are distributed in an east-west direction over about 100 km and are aligned with the Enriquillo fault. USBW – USGS Body wave solution. GCMT – Global Centroid Moment Tensor solution. USWPhase – USGS W phase solution. USCMT – USGS Centroid Moment Tensor Solution. Earthquake data from USGS.
Links to Web pages and articles discussing the earthquake