No surface rupture?

As of 2 February no evidence for faulting has been obtained.

Magnitude 7 plus, strike-slip earthquakes sometimes to do not result in surface ruptures. An example is the Mw=7.1 Loma Prieta earthquake of 1989 on the San Andreas fault system.

It is possible that a surface rupture will eventually be manifest, delayed by afterslip. An example is the Mw=6.6 Superstition Hills Fault rupture (1987) in which surface rupture was less than 1 cm on the day of the earthquake, but amounted to 1 m after a year.

It is also possible that our initial reconnaisance was insufficiently thorough.

The causal fault of the Port-au-Prince earthquake follows a deep east-west valley in which surface slip was anticipated, but not found, during an areal traverse 25 January. Nor are fissures apparent on satellite imagery. Thus if a surface rupture occurred it has yet to be found. Several potential cracks and linear features will be examined on the ground in the coming few days.

Why search for rupture?

1. A surface rupture is of value in pin-pointing the fault that slipped in this earthquake, permitting future detailed paleoseismic trenching that can determine the recurrence interval of historical earthquakes on the fault. A bi-product of these trench excavations is the long term slip rate of the fault, and an estimate of the future probability of major earthquakes.

2. The location of the surface rupture provides an important constraint on the geometry of the rupture. Current models for slip are constrained by teleseismic inferences, and by the inversion of Insar data.

3. The monitoring of continued slip on the fault can provide clues about the loading of contiguous ruptures to the east and west. These earthquakes (also M7) are anticipated in the next several decades, but may occur sooner than later.

Why is there urgency in the search for ruptures?

Immediately after an earthquake, subtle cracks are driven over, smoothed over by villagers, and repaired by road crews. Heavy rain can fill, erode or obliterate surface cracks. Because of the importance of (1) above, an immediate search for ruptures in the aftermath of a catastrophic earthquake can be justified as a vital investment in mitigating future disasters. The loss of such information is indefensible.

Imagery

Two dozen hi-res (25 megapixel) oblique views of the fault were facilitated by the Pioneer Production Film crew (Simon Ludgate and Mike Hutchinson). These were analyzed 25 Jan. by (from left to right) Roger Bilham,CO, Richard Koehler, AK, and Paul Mann (TX).

30 Jan: This in from Richard Koehler-
"I am typing on an iPhone so will make this short. We walked along the fault in the fault valley at two different locations. We observed a well-traveled trail perpendicular to the fault that showed no signs of deformation. We documented a few slumps with 6 cm vertical headscarps that were on the fault. Shadowed scarps that were observed on Roger's photographs and google earth appear to be man made based on an irigation canal at the base and their discontinuous expression (the people may have modified an existing Paleo scarp). There is also vegetation rooted in the face of the scarps indicating that they are not fresh. Thus our recon verifies satelite image analyses that support no surface rupture in the areas we saw. More later............................Rich Koehler"

Coastal indications of uplift and subsidence are also shown on this page. A submarine slump caused a local tsunami at the location below (18,433,-72.762). We are looking east along the head of the slump which has slid still standing trees 20 m offshore. Note the absence of a beach which to the east and west of this location consists of a sandy slope more than 10 m wide.

Link to Assemblage of hi-res photos

Helicopter traverse 25 Jan

A prominent east-west fault-controlled valley that offsets north-flowing drainages up to 20 km to the west, is located near the epicenter. The valley marks the path of the transform plate boundary. It is floored by anastamosing streams with (at this season) very modest flow. Numerous fresh landslips and rock falls scar the south-facing valley side and these are likely to be re-activated during heavy rain. The 20-km-offset of the streams in this valley has been caused by the equivalent of ten thousand earthquakes each slipping 2 m, every two centuries or so, for the past 3 million years.

View east along transform boundary (Port au Prince 20 km to NE behind limestone hills on left). The tiny village of Henry (18.468,-72.452) in foreground was largely undamaged by the earthquake

View NW along transform boundary showing landslips on limestone cliffs, and alluvial plain and Gulf of La Gonave in the distance (Photo taken near 18.459, 72.493). These landslips will be reactivatd during heavy rain.

River Terrace 18.4721N, 72.5375W viewing south (Roger1 from Carol Prentice). Click on image for 600k jpg. Click here for 20 Mb jpg. Rich Koehler and Paul Mann have checked the apparent scarp here - no fractures. See note below left.

The above image views north at 18.4740,-72.4460. Two very large images can be accessed: One Two

Rock fall on south-facing valley slope near stream bed at western end of transform valley (18.4796,-72.557). The largest boulder (left) measures 3.5m x 2.5m x 2m and weighs an estimated 30 tonnes.

(Above) Uplifted corals were identified at 18.525 -72.651 (Click here) and at 18.446,-72.690 (view above). The largest of the donut-shaped corals are about 1 m in diameter. These corals have been raised about 50 cm relative to in-situ corals located about 2 km to the south of the above photo. i.e the above relative uplift estimate is independent of the tidal stage. Sections of these corals will be used to determine uplift and subsidence history of this shoreline for the past several hundred years.

Slump fissures can be found parallel to the shoreline along most of the coastal sediments. In this location (view west at 18.431,-72.760) the slump has removed more than 30 m of beach plus an unknown volume of submarine sediments, taking with it local vegetation and the foreshore for a distance of approximately 1 km to the west. Several dead trees offshore stand in more 2-4 m of sea water. The submarine collapse caused a local tsunami, which is reported by CNN to have drowned 7 Haitians. Its amplitude is purely speculative but most probably exceeded 1 m at the shore. The inland penetration of the tsunami (run-up) was not obvious from areal views but its passage may have been recorded by staining of the grass roughly 50 m inland.