FOCUS | LETTERSPUBLISHED ONLINE: 10 OCTOBER 2010 | DOI: 10.1038/NGEO975

High tsunami frequency as a result of combinedstrike-slip faulting and coastal landslidesMatthew J. Hornbach1*, Nicole Braudy2, RichardW. Briggs3, Marie-Helene Cormier4,5,Marcy B. Davis1, John B. Diebold5, Nicole Dieudonne6, Roby Douilly7, Cliff Frohlich1,Sean P. S. Gulick1, Harold E. Johnson III4, Paul Mann1, Cecilia McHugh2,5, Katherine Ryan-Mishkin2,Carol S. Prentice8, Leonardo Seeber5, Christopher C. Sorlien9, Michael S. Steckler5,Steeve Julien Symithe7, FrederickW. Taylor1 and John Templeton5

Earthquakes on strike-slip faults can produce devastatingnatural hazards. However, because they consist predominantlyof lateral motion, these faults are rarely associated withsignificant uplift or tsunami generation1–4. And althoughsubmarine slides can generate tsunami, only a few per centof all tsunami are believed to be triggered in this way4–6.The 12 January Mw 7.0 Haiti earthquake exhibited primarilystrike-slip motion but nevertheless generated a tsunami. Herewe present data from a comprehensive field survey thatcovered the onshore and offshore area around the epicentre todocument that modest uplift together with slope failure causedtsunamigenesis. Submarine landslides caused the most severetsunami locally. Our analysis suggests that slide-generatedtsunami occur an order-of-magnitude more frequently alongthe Gonave microplate than global estimates5,7–12 predict.Uplift was generated because of the earthquake’s location,where the Caribbean and Gonave microplates collide obliquely.The earthquake also caused liquefaction at several riverdeltas that prograde rapidly and are prone to failure. Weconclude that coastal strike-slip fault systems such as theEnriquillo–Plantain Garden fault produce relief conduciveto rapid sedimentation, erosion and slope failure, so thateven modest predominantly strike-slip earthquakes can causepotentially catastrophic slide-generated tsunami—a risk that isunderestimated at present.

The 12 January 2010 Haiti earthquake was a predominantlystrike-slip event that generated vertical deformation and severallocal tsunami; however, both the US Geological Survey and GlobalQuick Centroid Moment Tensor solutions indicate ∼65% left-lateral motion accompanied by 30–33% thrusting. The epicentre ofthe earthquake was within a few kilometres of the surface trace ofthe Enriquillo–Plantain Garden fault, a plate-bounding strike-slipfault separating the Caribbean and Gonave microplates13,14 (Fig. 1inset). Although the 12 January earthquake was only a moderatelylarge event with strike-slip motion running approximately parallelto shore15, it generated tsunami: waves were reported west, northand south of the epicentre, with the most significant wave observedto the west (Fig. 1).

To assess the location and deformation of faults and slidesand the cause of the 12 January tsunami, in February–March

1The University of Texas Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, Austin, Texas 78758-4445, USA, 2QueensCollege, City University of New York, Flushing, New York 11367, USA, 3United States Geological Survey, Golden Colorado 80225, USA, 4University ofMissouri, Columbia, Missouri 65211, USA, 5Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA, 6Bureau desMines et de l’Energie (BME), Delmas 19, Rue Nina no. 14 PO Box 2174, Port-au-Prince, Haíti (WI), 7Universite d’Etat de Haiti, 21 Rue Riviere, Port-au-Prince,Haiti (WI), 8United States Geological Survey, Menlo Park, California 94025, USA, 9University of California, Santa Barbara, Santa Barbara, California 93106,USA. *e-mail: matth@ig.utexas.edu.

2010 we conducted a comprehensive onshore/offshore field surveynear the epicentre. Our offshore survey imaged the sea floor andshallow subsurface using a suite of geophysical tools includingmultibeam bathymetric sonar, seismic profilers (chirp) and side-scan sonar. The onshore survey measured shoreline subsidence anduplift, found tsunami markers, conducted interviews with tsunamiwitnesses and searched for surface rupture16,17.

Analysis of gridded seafloor images from multibeam, chirp andside-scan sonar data delineates the offshore extension of two tracesof the Enriquillo–Plantain Garden fault mapped on land16 (Figs 2and 3). This fault system skirts the coast, entering the southeastcorner of Baie de Grand Goâve as a series of sharp en echeloneast–northeast striking lineaments sub-parallel to shore (Figs 2 and3b). Chirp data collected over these lineaments showvertically offsetshallow beds indicative of faulting (Fig. 3d). Vertical displacementat the sea floor on the northernmost fault is nomore than 30 cm (forexample Fig. 3d). It is unclear whether this fault ruptured duringthe 12 January event; however, vertical offsets in shallow reflectors(<5m below the sea floor) indicate recent activity. Along thewestern half of Baie de Grand Goâve northwest–southeast strikingridges align closely with onshore folds (Fig. 2). The submarineslopes along many of these faulted shorelines are steep, someexceeding 30◦ (Fig. 4). We interpret Baie de Grand Goâve as atransition zone where predominantly strike-slip motion in thesoutheast corner of the bay evolves into transpressive thrustingfurther west.

Analysis of seafloor images in Baie de Petit Goâve indicates thatan equally significant tectonic transition exists there. Chirp andmultibeam data show a northeast–southwest striking 40-m-deeparcuate depression cutting through Baie de Petit Goâve (Fig. 3a).Sediments are thick within this depression, but steadily thin tothe east, consistent with an east-dipping normal fault (Fig. 3c).The transition from observed shoreline uplift to subsidence atBaie de Petit Goâve during the 12 January event also correlateswith the location of this fault17. Furthermore, the shape, locationand orientation of this fault closely mimics four-dimensionalanalogue transtensional pull-apart basin models18. We thereforesuggest Baie de Petit Goâve represents a pull-apart basin andthat the Enriquillo–Plantain Garden fault system converts froma transpressive regime in western Baie de Grand Goâve to a

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Baie de Grand GoaveBaie de Petit Goave

18° 25’ N

18° 35’ N

18° 30’ N

2010 Tsunami

1692

1907

1842

18601852

1932

1887

1770

Tsunamicoincident with slide

s

s

s

s

s

s

s

Caribbean plate

Gonave microplate

North American plate

Hispanola

Cuba

JamaicaEPGF

SF

8 mm yr¬1

20° N

18° N

76° W 74° W 72° W

sss

Releasing bend

Restraining bendReleasing bend

s s

Southern Peninsula

1.41.4

s1.1

s3.0 0.2

s2.5?

s~1.0

s 0.1

0.3

2.0

0.5

2.5s

s

Tsunami with centre value indicating estimated max flow depth (m)

Tsunami where only draw-down observed

72° 45 ’ W 72° 35 ’ W72° 55 ’ W

Figure 1 | Tsunami reports across Haiti and Gonâve microplate. The 12 January epicentre is denoted by the yellow US Geological Survey moment tensorsolution. Circles indicate locations of reported tsunami. Inset: The location of the Gonâve microplate with respect to the North American and Caribbeanplates. SF is the Septentrional fault and EPGF is the Enriquillo–Plantain Garden fault. Different coloured dots show the locations where tsunami werereported for specific earthquakes7–12. ‘S’ denote tsunami adjacent to shoreline slides. The yellow box shows the location of the main part.

~8 mm yr¬1

Transpression

Mapped faults

Possible faults (?)

Transtension

18° 28’ N

Fig. 3bFig. 4a

Fig. 4b

Fig. 3a (Baie de Petit Goave)D

epth

(m

)

0

200

Baie de Grand Goave

72° 52’ W

18° 24’ N

72° 46’ W

North

2 km

Figure 2 | Bathymetry from gridded multibeam and chirp surveys of Baie de Petit Goave and Baie de Grand Goave. Multibeam data used in water depthsgreater than 50 m are gridded at 8 m resolution; chirp data were gridded at 50 m resolution; deeper bathymetry from the General Bathymetric Chart of theOceans (GEBCO) has a resolution of 900 m. Blue lines are primary river drainages in this region. Land topography is from the Shuttle Radar TopographyMission data, gridded at 30 m resolution. Black lines show interpreted faults. Eastern land-based faults show the continuation of the Enriquillo–PlantainGarden fault towards Port-au-Prince16.

transtensional regime in Baie de Petit Goâve over a distance ofless than 10 km. Observed shoreline uplift combined with ourtectonic interpretation suggests that the 12 January event hadenhanced vertical motion because it ruptured across a pairedrestraining–releasing fault segment extending through Baie deGrand Goâve and Baie de Petit Goâve.

Both onshore and offshore data in Baie de Petit Goâveand Baie de Grand Goâve show evidence for recent slumpingand liquefaction, especially at river deltas. Several river deltasexperienced metre-scale deformation during the earthquake(Fig. 4). Analysis of aerial photographs, and interviews conductedwith local residents, reveal many river delta shorelines are

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y

dept

h (m

)

0

200

Slide debris

GEBCOMB

Chirp

Uplift

Subsidence

500 m

y'

50 m

Fig. 3c

b

x

Northern fault

500 m

Southern fault

0

50

Dep

th (

m)

x'

a

cc

Sediments thin --> Sediments thin -->

VE = 6:120 m

y'

Northern fault

Relative uplift

Relativedown-drop

x

20 m

VE = 5:1

c

d

x'

y

Figure 3 |Detailed view of offshore faults. a, Bathymetry across Baie de Petit Goâve shows a depression extending southwest to northeast across the baythat aligns with subsidence and uplift that occurred during the earthquake17. Inset: A slide imaged with side-scan sonar. b, Faults along the southeastcorner of Baie de Grand Goâve indicated with the General Bathymetric Chart of the Oceans (GEBCO), multibeam (MB) and chirp data sets. c, Chirp lineextending across Baie de Petit Goâve that shows westward sediment thickening. d, Chirp seismic line in the southeast corner of Baie de Grand Goâveshowing sediment offsets indicative of faulting.

prograding at high rates, some in excess of 3m yr−1 (Fig. 4).Rivers near the Enriquillo–Plantain Garden fault system aresteep, littered with slide debris and commonly end at truncateddeltas prograding onto steep submarine slopes19,20. Steep slopescombined with severe rainy and dry seasons and deforestationprobably contribute to high near-shore sedimentation rateshere. High sedimentation produces under-consolidation andoverpressure in offshore sediments, substantially increasingthe susceptibility of slope failure21. Accordingly, during the12 January earthquake, numerous shoreline slope failures andliquefaction occurred where sedimentation rates are highest(for example Fig. 4).

Analysis of witness accounts, tsunami flow-depth estimates andoffshore data indicate that the most significant tsunami occurredimmediately adjacent to slides, with many of these areas experienc-ing local flow depths in excess of 1m (Fig. 1; see SupplementaryInformation). At shorelines adjacent to slides, witnesses generallyreported a tsunami that beganwith a rapid sea-level retreat followedby a rapid wave inundation (see Supplementary Information).Away from shoreline slides, the tsunami was generally less severe.Slow draw-down over several minutes, followed by little if anyrun-up, was observed across much of Baie de Grand Goâve inmost areas away from slides (Figs 1 and 5). Detailed shorelinemeasurements of corals indicate all of Baie de Grand Goâve

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500 m

Extent ofshoreline slide

Slope = ~30°

25 January, 2010

June 2005

100 m

100 m

~3 m yr¬1 shoreline progradation

near river mouth

Shoreline locations

Jan. 2010

May 2005

June 2002

1986

197850 m

Lateral spreadsupto ~1 m deep

~1 m

Dep

th (

m) 0

200

a

b

Offshore tracksof slide

Tsunami withflow-depth >1 m

Only minortsunamireported

Figure 4 | Sediment deformation in Baie de Grand Goâve. a, Combined Google Earth and bathymetry image near the town of Grand Goâve (see Fig. 2 forlocation). Depths greater than 50 m are gridded at 8 m, with shallower chirp data gridded at 50 m. Insets show shoreline slope failure caused by theearthquake that extend offshore. b, Google Earth image of a delta near L’Acul reef that underwent liquefaction (see Fig. 2 for location). Delta progradationis based on two Google Earth photos, interviews and a 1978 air photo acquired from the Haiti Government (flight 78-HAI-01/400 UAG412).

tectonically uplifted by tens of centimetres during the event17, andthis might explain the regional observation of steady draw-downwith little if any run-up following the earthquake at several sitesaway from slides (Fig. 5a,b). This pattern of shoreline uplift andthe general absence of tsunami run-up in regions where no slidesoccurred indicate that the centroid of the 12 January rupture wasprobably offshore of Baie de Grand Goâve.

Two other locations besides Baie de Petit Goâve and Baie deGrand Goâve—the southeast tip of Grand Goâve Island and thetown of Luly on the northern edge of Port-au-Prince Bay—alsoreported a tsunami on the north side of the peninsula (Fig. 5a).Our analysis reveals no clear evidence for slope failure near theselocations and no shoreline uplift. According to witness accounts,the waves that arrived at these two locations were less severe andstruck several minutes after the earthquake, with the wave at Lulyarriving more than 30min after the event.

To determine possible tsunami sources, we modelled tsunamigenerated by both coseismic uplift and slides (Fig. 5 andSupplementary Information). Computer models indicate thatcoseismic uplift originating near Baie de Grand Goâve explainsthe Gonâve Island and Luly tsunami and the slow and steadydraw-down observed at several locations in the Baie de GrandGoâve (Fig. 5a,b). This suggests that modest (∼33%) thrustingcontributed to tsunamigenesis .

The most significant waves, however, were generally reportedin isolated locations adjacent to submarine slides (Figs 1, 4 and 5,Supplementary Information). The phase, relative amplitude andarrival times of waves predicted by the earthquake-generatedtsunami model are inconsistent with witness observations at siteswhere slides occurred (Fig. 5a,b and Supplementary Informa-tion). However, when we use a slide-generated tsunami model,predicted wave phase, relative amplitude and arrival times atslide locations closely match witness accounts (Fig. 5b,c and Sup-plementary Information), indicating slides caused these waves.Slides also probably generated tsunami on the south coast of thepeninsula (Fig. 1), as there is no evidence for significant coseis-mic deformation here15.

Slide-generated tsunami during the 12 January earthquake areconsistent with historic tsunami reports along the Enriquillo–Plantain Garden fault system. Previous studies suggest sliding andshoreline liquefaction are the probable causes of the 1907 Kingston,Jamaica, and 1692 Port Royal, Jamaica, tsunami7,8,11,12, implying atleast three tsunami have been generated in-part by sediment failurein the past four centuries along this strike-slip fault. Historicalaccounts indicate that motion along the northern (Septentrional)and southern (Enriquillo–Plantain Garden) strike-slip faultsbounding the Gonâve microplate frequently causes large tsunami(Fig. 1), and the 2010 Haiti tsunami may pale in comparison

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18° 30’ N

72° 20’ W72° 40’ W73° 00’ W

18° 50’ N

Port-au-Prince

Gonave Island

Luly

Grand Goave

Peak

am

plitu

de

(m)

0

0.75

Shorelines where tsunami reported10 km

Beloc reef

L’Acul reef

1

¬10

1

¬10

1

¬10

Petit Goave

Petit Goave

10 20 10 20 10 20 10 20

Grand Goave L’Acul reef Beloc reef

(m)

(min)

Maximum amplitude of earthquake-generated tsunami

A r e-Generated T

Grand GoaveL’Acul reefPetit Goave

Beloc reef

Peak

am

plitu

de

(m)

0

0.75

72° 50’ W 72° 40’ W

18° 30’ N

18° 25’ N

Maximum amplitude of slide-generated tsunami

Approximate slidelocation and direction

Baie de Grand Goave

Earth-quake

Slides

Sum

a

b

c

Figure 5 | Tsunami wave models. a, Maximum wave amplitude generatedby coseismic deformation during the earthquake. Much of the coastlinealso uplifted during the earthquake, thereby reducing the net waveamplitude along parts of the coast. b, Tsunami at four synthetic tide gaugesgenerated by the earthquake (red), slides (blue), and both (black). Theblack lines generally produce the best fit with observations (seeSupplementary Information). Red lines show sea permanently retreating inBaie de Grand Goâve as a result of uplift. c, Maximum tsunami waveamplitude generated by four slides. Constructive interference causescomplex amplitudes near Beloc reef.

with the 1770 event where waves reportedly washed severalkilometres inland22. At least nine documented tsunami havebeen linked to earthquakes along these faults during the last 318years, or once every ∼35 years, although intervals are highlysporadic7–12 (Fig. 1 inset).

These observations raise a fundamental question: what causesso many tsunami along strike-slip faults, especially when eventssuch as the 12 January earthquake may have onshore epicentreswith horizontal displacements running approximately parallel tothe coast? Although large tsunami along strike-slip faults are rare,tsunami sometimes accompany strike-slip earthquakes, and whythis happens is often unclear2,23. In special instanceswhen strike-slipruptures are perpendicular to the shore and extend along steep un-

derwater coasts, their horizontal displacement can trigger tsunami1.Regional coseismic vertical deformation during strike-slip earth-quakes is often less than 1m, with MW 7.0 strike-slip earthquakesgenerally producing no more than 2m of total displacement, only afraction of which is vertical3,24. In some instances, however, metre-scale vertical displacements can occur along strike-slip faults25.

One possible explanation for the anomalous frequency oftsunami in this region is that motion along these faults, althoughgenerally strike-slip, may commonly include enough verticalmotion to generate a wave, as our analysis suggests (Fig. 5a,b). Theyoung 2,000–3,000-m-high mountains associated with restrainingbends across Haiti and Jamaica, relative plate motions andearthquake focal mechanisms all indicate a significant thrustcomponent exists in earthquakes across this region13,26–28.

Along the Gonâve microplate, a second factor contributingto frequent tsunamigenesis is that conditions are conducive toslope failure. Slide-generated tsunami are considered anomalous,accounting for only a small per cent of all tsunami reportedglobally5. This is because slides require specific conditions togenerate tsunami, including relatively shallow water, significantvolumetric displacement and high acceleration rates5,6.

Global compilations by the National Geophysical Data Centerattribute only 3% of tsunami to slides; our study, combined withprevious work, implies at least three of the nine reported tsunamiacross the Gonâve microplate were in-part generated by slopefailure near strike-slip faults. This suggests slide-generated tsunamimay occur an order-of-magnitude more frequently along this faultsystem than global estimates suggest.

Tectonics, climate and deforestation enhance the chances ofslide-generated tsunami in this region. Oblique strike-slip faultscan produce steep vertical offsets29,30. Steep slopes combined withextreme dry andwet seasons result in high upland erosion and near-shore sedimentation. Both steep slopes and high sedimentationreduce slope stability. Deforestation may further reduce stability bypromoting additional erosion and sedimentation. At least two slide-generated tsunami (1907 and 2010) along the Enriquillo–PlantainGarden fault were activated by earthquakes of reportedly modestmagnitude31,32 (MW 6.5 and 7.0, respectively) in which coseismicdeformation often causes no significant wave. We therefore aresubstantially underestimating the potential for slide-generatedtsunami along this and similar fault systems.

Received 17 May 2010; accepted 8 September 2010;published online 10 October 2010

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AcknowledgementsThis study was made possible by an NSF RAPID Response grant nos OCE-1028045, NSFEAR1024990, and a Jackson School of Geosciences Rapid Response Research Grant. Wethank the crew of the R/V Endeavor, and S. De Bow for a successful cruise. This work isdedicated to J. B. Diebold.

Author contributionsS.P.S.G., M-H.C., C.M. and M.S.S. planned and directed the offshore study. J.T., R.D.,S.J.S., L.S., N.D., H.E.J., J.B.D., K.R-M., N.B., C.C.S. and M.B.D. carried out marinechirp, multibeam and side-scan data acquisition, processing and interpretation. R.W.B.,P.M., C.S.P. and F.W.T. planned, acquired and analysed all of the land-based study. C.F.developed the earthquake–tsunami rupture model. M.J.H. wrote the manuscript, withall co-authors commenting and discussing results and implications.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturegeoscience. Reprints and permissionsinformation is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests for materials should be addressed to M.J.H.

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