Reports: DNI849877-DNI8: Active Fold Growth Studies with InSAR

Rowena Lohman , Cornell University

This project involved the use of satellite-based interferometric synthetic aperture radar (InSAR) observations of ground deformation and how they can help us understand the contribution of earthquakes vs. aseismic professes to the growth of individual folds in southeastern Iran – a region of active hydrocarbon extraction.  In the previous year’s report, we described our studies of two M6 earthquakes that occurred in close spatial and temporal proximity to eachother.  We modeled the stress triggering aspects of the sequence and found that they were consistent with the 2nd earthquake being brought closer to failure by the first.  In other words, no additional mechanical constraints or variable pre-stress conditions are required by the sequence.  As a side note, this proved to be a useful exercise in preparation for the larger sequence of earthquake now occurring in New Zealand.

In the past year, we have moved towards a more synoptic view of the entire Zagros mountain range.  Our success in looking at one sequence of earthquake led us to consider whether time series of the entire mountain belt might reveal similar triggered behavior.  We have now completed deformation time series using all available InSAR data for the mountain belt, which has also required a side effort on assessing the quality of these rate maps and how they are affected by anthropogenic activity, climate, topographic relief, etc.  Over the next year, we expect to now interpret the observed deformation signals as we complete this project.

Our progress on the tasks we set for ourselves for this project is as follows:

Task 1: Predict Coulomb stress changes associated with the two Qeshm Island earthquake using earthquake locations and slip distributions previously studied by the PI, under the assumption of an elastic half space:  Done, and published.

Task 2: Generate model of folding associated with these earthquakes, based on maps of surface geology (which are publicly available) and published cross sections through other folds in the region:  We are still examining the depth range of seismicity (aftershocks) associated with these earthquakes, which have the very curious feature of being disjoint from where we infer fault slip to occur.  This implies that the observed ground deformation may be do to shallow, aseismic creep rather than from coseismic slip.  Our next effort will involve seeing whether we can “hide” the main shock deeper, in the realm of where aftershocks are observed, or if the InSAR precludes the sort of large, spatially broad signal that would be caused by an earthquake deeper than where we infer slip.

Task 3: Calculate shear stress changes along bedding planes in fold model, as well as regions where randomly oriented faults would be stressed to the point where they would be likely to slip. Identify the zones with the highest stress changes (positive or negative) and compare with the general geometrical characteristics of the Zagros folds: We have yet to make progress on this task, as we are still verifying whether the fault model we are using is actually correct, or if it represents later deformation in response to the main earthquake (see Task 2).

Task 4: Set up finite element  (using combination of Pylith and CUBIT).  Beginning with a model containing no contrasts within the crust, benchmark stress change results with those generated in Task 1:  We have performed the initial benchmarking exercise, comparing our results to those in a more simple elastic half space, for which there is an analytical solution.

Task 5: Using the actual distribution of salt domes in the Qeshm Island region, rerun the finite element stress change calculation with columns of salt that are more compliant than the surrounding rock.  Explore how this addition changes the predicted stress field and it’s relationship to structures within the fold: We have not begun work on this task yet.

Task 6: Big picture- Revisit the problem of whether currently observed earthquakes could actually build the geological structures found in nature.  Given the range of stress changes predicted by our modeling, would we expect deformation that agrees with the shape of folding updip and downdip of the earthquakes?   Within how large of a volume would we expect stress changes that have the potential to fracture the rock?:  Our observed difference between the deformation and seismicity indicates that there is a very complicated relationship between the faulting that is commonly observed in basement rocks within the region and the processes controlling fold growth.

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