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Hi Allan, I hope there are experienced engineering geologists and geotechnical engineers associated with this project to incorporate detailed seismic parameters into the design (see below). As for your question as to whether comparatively unconsolidated sediments over a fault trace inhibit or stop ground deformation during an (otherwise) surface rupturing event, I know of no such cases. The converse is more common. Several types of deformation in loosely consolidated sediments are common during earthquakes, many of which are actually the result of strong ground motion (ie shaking from the earthquake) rather than the fault displacement itself. Such types of deformation include lateral spreading and liquefaction. Ground motion is commonly amplified in soft sediments (this is the phenomenon known as “soft sediment amplification”): this has been recorded by instrumentation during a number of earthquakes. Accordingly you are likely to have the dual threat of ground deformation owing to fault displacement as well as ground deformation owing to strong ground motion from the earthquake. In addition to the threat posed by the particular fault in question, it isn’t clear to me where your site is relative to the off shore subduction zone. Depending on the proximity to the subduction zone megathrust rupture, a M8+ (or whatever the closest subduction zone segment is capable of) may produce more damaging ground motion at the site (and resultant soft sediment deformation) than the upper plate M6.5-6.9, and the recurrence interval for the megathrust quake may actually be shorter. Engineering analyses can be run to determine whether the ground motion on the nearest subduction megathrust segment is more of a problem than the proximal fault. I guess what I am advising is the tidbits of advice you get here are just a starting point. Specific information on the active fault segment locations, their estimated maximum magnitudes, and recurrence intervals, as well as the site geology should all be input for detailed engineering analyses that should be done for your site. I hope this helpful, Cheers John ----- Original Message ----- From: "Allan Lopez" <[log in to unmask]> To: [log in to unmask] Sent: Wednesday, July 15, 2015 4:37:12 PM Subject: Your opinion on Neo-tectonic situation in Costa Rica I hope you all are fine. I would appreciate your opinion on this tectonic situation in Costa Rica: An active regional strike-slip fault dipping 70 degrees and responsible for several destructive historical earthquakes, the last in 1910 (Mw 6.4) depict at least three surface ruptures during the last 1000 years according to absolute ages determined in materials recovered at paleo-seismological trenches sampled in the foothills very close to its main trace. The structure is covered by unconsolidated Quaternary fillings (silty sediments, lahars and alluvium) with an estimated thickness around 50 meters. Electrical resistivity is in the range of 15 and 180 Ohm / m and seismic velocities of 0.1 to 2.5 km / sec. The local basement is igneous with resistivity between 200 and 2500 Ohm / m and Vp speed > 7.50 km / sec. According to the length trace it is assigned a potential of 6.5 to 6.9 Mw, and a recurrence period of 500 years (could be somewhat higher). The rheological contrast between igneous and fill cover is strong. The site is fairly flat and there is no nearby morphological evidence of surface trace since the erosion rate seems higher than that of deformation and local floodings are common. The specific question, according to your experience and knowledge, is: Do you know cases where under similar or equivalent scenarios it has been determined that unconsolidated covers have mechanically stoped-inhibited the overall displacement and deformation, and no propagation have developed and consequently without surface rupture ?? The concern is that there are plans to build a Regional Hospital (correct) just above the said trace or between active blocks within the damaged area. Thank you very much for your valuable opinions and suggestions Best regards, Allan López Colegio de Geólogos de Costa Rica