COSEISMIC LANDSLIDE INVENTORY FOR THE CINCHONA EARTHQUAKE (01-08-2009, Mw 6.2), COSTA RICA. A CASE OF HIGH LANDSLIDE DENSITY AT POÁS VOLCANO

In the last 250 yr, Poás volcano has had six M> 5 shallow earthquakes linked to local faults that caused coseismic landslides. We present a landslide inventory and mass wasting calculations for the 2009 (M_w 6.2) Cinchona earthquake, caused by the Ángel fault and whose epicenter was located 6.5 km E from the active crater of Poás. The landslides occurred all around Poás volcano, the W flank of Barva and at SE flank of Platanar volcano. Our catalog covers an area of 519 km², and was obtained through analysis of high resolution LiDAR images taken 2 months after the event. The total number of landslides (N_Lt) produced by this event is 4781. From the N_Lt, 53% are translational slides, 37% slumps, 10% debris flows, and < 1% rock falls. Most of the landslides (82%) occurred in areas of moderate to high slopes (30^o-60^o), which were covered by primary, secondary and gallery forests. The total area exposed by the landslides in the 3 volcanoes is 21.82 km² (4.2 % of the study area) for an average density of 9.2 landslides/km². We estimate that coseismic landsliding produced ~0.10-0.39 km³ of erodible material, most of which (95 %) is located in the NE part of the volcanic range, near the epicenter. Thus most of the wasting material will eventually travel to the Caribbean side and only about 5% will be evacuated to the Pacific side. Of the landslides that occurred on Poás volcano, 54% are located on the La Paz Andesites Unit, the remainder occurred in 9 other volcanic units. We attribute the high landslide density in the La Paz Andesites Unit to a site effect produced by: proximity to the epicenter, steep slopes, geology (~0.5 Ma old porphyritic lavas intensively weathered) and a considerable coverage ~25m thick of pyroclastic fall material. Using a previously estimated 50 yr return period for earthquakes (M ~5.5-6) in the study area, we calculate a mass flux from coseismic mass wasting of ~130-487 km³/km/Myr, a rate comparable to estimates of magma flux at arc volcanic systems. However estimations of the amount of material that stay in the edifice are still needed. The same recurrence interval yields an average erosion rate due to landslides of (0.38-1.50mm/yr). We speculate that large >150 km³ and mature ~1Ma massifs like Poás are susceptible to erosion processes like this one, due to their mass and the active faults within the edifice that produce coseismic landslides.