This RAPID response is focused on post-seismic surface-deformation, coastal tsunami hazards and probable stress adjustment leading to the forecast occurrence of a second Mw¡Ã7.0 earthquake in the Port au Prince region of Haiti in the next decade or so.
The PI and his group will monitor post seismic deformation of the Leogane area of Haiti where a 2-4 m slip deficit currently exists in the near surface following the 12 January Mw=7 quake using two tide gauges with 1 mm precision and remote telemetry for this work. They will measure the tidal harmonics and seiche amplitudes of the Bay of Logane for use in coral uplift and subsidence modeling currently being undertaken by USGS. They will also install a supplemental tsunami gauge to monitor drawdown and surge should a large aftershock or an anticipated future mainshock, occur and install a GPS unit near the region of maximum uplift to monitor vertical and horizontal components of the deformation field.
The results will be communicated by web pages and articles to the public as they become known. The availability of telemetered data is a considerable bonus and the following web page will be enlarged http://cires.colorado.edu/~bilham/Haiti/index.html to make this data available. The PI is also translating a public outreach poster into Creole and placed in a Caribbean context.
The Mw=7.0 Haiti earthquake in 2010 was a moderate earthquake that occurred close to a densely populated capital city. At around 80,000, the resulting death toll was huge, but not as large as the inflated number claimed a year later by Haitian officials (more than 300,000- see figure). Port au Prince was previously destroyed in the 18th century by an earthquake and a second earthquake that followed within several years. There remains some concern that a second shock may follow the 2010 earthquake with equal or greater intensity. One way to investigate the possible future sequence of earthquakes is to examine previous earthquakes in Haiti's history. However, this is rendered difficult because its history starts only in 1492. A disruption in the growth of shoreline corals was observed to accompany the 2010 Haiti earthquake because parts of the coast were raised by the tectonic motions accompanying the earthqake. Accordingly various groups from the US and France initiated coral studies in the area to see whether the corals might record a long history of previous earthquakes. They discovered, however, that they could not easily interpret the coral growth activity because Haiti lacked a tide gauge. The prime objective in the present project was to provide this missing sea level variation data. Accordingly five temporary tide gauges were distributed around the epicenter and their data analyzed both for the time variation in sea level, and for possible differential heights changes symptomatic of continuing tectonic deformation. No tectonic signature was evident. The instruments were removed after a year, but not before we had discovered that unusually large sea level changes are the norm in Haiti. A comparison of our realtime data with a satellite derived measure of sea level from the past decade show that these long term fluctuations are caused by currents to the north and south of the island. Sea level prior to the Haiti earthquake was at its lowest level in a decade - so low that it had resulted in the death of corals close to mean sea level in the nine months prior to the earthquake. We found no evidence for differential vertical motions above approximately 1 cm noise level of the five sites. We examined a number of other effects of the earthquake on apparent mean sea level. One was the realization that sea level prior to the earthquake had been recorded as solution notches on a number of limestone blocks tumbled into the sea by different earthquakes. By making certain assumptions the current height of the sea level notch above mean sea level can be used to indicate the precise amount of uplift at locations where these blocks had been tumbled.
