Hi Allan,

I read your message, and I try to access some of the aspects you mention. I believe you main question is to evaluate if there is a possibility of not having a rupture to the surface, in the circumstances you describe.

- you say that the fault trace crosses a flat area where floodings are common. In this situation, it would be very difficult to expect that a paleo - surface rupture would be preserved unless 1) it is very recent or 2) it would have a significant vertical component, which seems not to be the case since a strike-slip main component is inferred. In this case, I'm guessing that it would also be tricky to limit the real fault trace length.

- In unconsolidated sediments, it is very difficult to preserve a recent fault plane near the surface. Depending on the way the rupture propagated to the surface and the type of sediments and geometry of the sedimentary bodies, you may never see the surface rupture. There are cases of trenches opened just a couples of years after the earthquake, where the rupture was indeed recognized and it was not possible to recognized ant deformation in the trench. In my opinion, holocene ruptures could in the situation you describe be more difficult to identify, which is not the same that consider than they didn't happen.

- Considering that this fault is capable to generate an maximum earthquake of circa 6.9 Mw, based in fault length (maybe not fully constrained? is the length based upon the foothills?) I would expect that a fault rupture would propagate to the surface, even if generating a minor geomorphic expression. I am not sure of how much was estimated for the slip per event, but some lateral slip is expected and again this could not be visible in the surface if frequently covered by floods. Was there any 3D trenching conducted in the paleoseismological excavations?

- You have not mention if there is any liquefaction. In this type of sediments, liquefaction could be possible which would also cause deformation at the topographic surface. 

So, basically, I consider that the topographic surface can be deformed due to a local earthquake, meaning don't consider this an adeuqate place to build a Hospital. How about long term slip rates? you have only mention the last three events and you have not mention the slip rate for this fault. Is there any late Pleistocene references faulted? 

Hope this helps

best regards
Paula

-----------------------------------------------
Paula Marques Figueiredo
PhD Geology
Institute Dom Luiz
Science Faculty
Lisbon University
C6, 2.78

phone: +351 217500000 (ext.26278)
cell phone: +351 919969298

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De: Tectonics & structural geology discussion list <[log in to unmask]> em nome de Dirk Nieuwland <[log in to unmask]>
Enviado: 16 de julho de 2015 20:31
Para: [log in to unmask]
Assunto: Re: Your opinion on Neo-tectonic situation in Costa Rica

Hallo Allan

Please find below my view on this interesting and very practical question that you ask.

In terms of stress, 50 m of overburden will add very little to the vertical stress acting on the fault zone. If the main movement is indeed strike-slip, then the vertical stress is not the driving stress, this will be one of the horizontal stress components, the vertical stress is the intermediate stress Sigma-2 (S2). The depth of the main fault zone is, with respect to the vertical stress of some interest, but probably the 50 m overburden is insignificant.

However, groundwater flow through the overburden may play a role, if it can enter the fault zone and then increase the fluid pressure in the fault zone. Water pressure can easily build up, simply because of the weight of a water column. Every vertical 10 m is approximately 1 bar (0.1 MPa).
For a depth of 1000 m, de water pressure is approximately 100 bar or 10 MPa, acting as pore pressure in the fault zone. The fluid pressure will reduce the normal stress on the fault zone and will make it easier to slip (the slip tendency will increase with increasing fluid pressure).
It is likely that the strike of the fault zone varies and that consequently the angle between the maximum horizontal compression and the fault zone will also vary. Where this angle is at its minimum, the risk of water entering the fault zone is probably the greatest. Similarly, a steeper fault zone dip will also make it easier for water to enter the fault zone. For the water pressure the fault dip is of no consequence, only the vertical depth is of importance (I ignore friction of flowing water along the fault wall, because the flow rate is probably not so high to make this a significant parameter).

The flat morphology is good in so far that the risk of landslides due to increase water pressure is small, but the water will need to go somewhere and a relatively open fault zone (a pull-apart geometry) will form a potential risk.
If the fault zone is a Riedel shear in sediments above a main basement strike-slip zone, then you can expect an angle of about 30 degrees between the fault zone and the maximum horizontal stress Sigma-1 (S1). With advancing deformation this ideal angle will vary locally along the fault zone as it (the faults zone) will change shape locally.
A small angle between the fault zone and S1 indicates a relatively low normal stress on the fault zone, however, this does not mean that the normal stress on the fault is tensile and that the fault is open. The really open fault situation can be expected at pull-apart geometries.
In locations where the normal stress on the fault is less than the weight of a (vertical) watercolumn, the water pressure is in theory high enough to wedge the fault open, provided that the fault has no cohesion. Could there be a permeable pathway into the deeper fault zone? a fracture zone maybe? Do you have any adea of the nature (permeability) of the fault rock? This can vary considerably laterally along a strike-slip zone.

I hope that this helps, should you have questions or comments please do not hesitate to contact me at: <[log in to unmask]>.

Regards,   Dirk

Dirk Nieuwland
4e Binnenvestgracht 13
2311NT Leiden
URL: www.newtec.nl
E: [log in to unmask]
M: +31 (0)621547949

> On 16 Jul 2015, at 01:37, Allan Lopez <[log in to unmask]> wrote:
>
> 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