Source Parameters for Nicaragua and Costa Rica Earthquakes
Subduction megathrust faults provide a large number of earthquakes with which to explore several key seismological questions about how fault conditions can affect earthquake behavior. The proposed study leverages past NSF and internationally funded experiments in Central America, using large datasets of earthquakes recorded locally with temporary land and ocean bottom seismometers in order to address a key question: How do fault conditions impact earthquake rupture? We will use a rich earthquake dataset from the NSF-funded Costa Rica Seismogenic Zone Experiment and subsequent networks and German-funded SFB 574 project to characterize earthquake source characteristics over a range of magnitudes and compare these parameters to specific tectonic and geologic variations. Additionally, we will also calculate near earthquake source compressional-to-shear velocity ratios (Vp/Vs) to complement already existing coarser scale velocity tomography to provide more detailed information on fault conditions. We target the following hypotheses:
1) There will be distinct differences in source spectra for interplate events along the Nicaragua and Costa Rica margins that relate to variable fault conditions along each portion of the subduction seismogenic zone.
2) New estimates of Vp/Vs throughout the region will reflect differences in fluid content and/or composition in localized regions that may mimic variations in earthquake source spectra and source parameters.
While more details are provided in the presentations and manuscripts listed below, here we outline the significant results:
1) We find significantly different earthquake source spectra for earthquakes that occurred within the southern Nicaragua previous tsunami earthquake rupture zone relative to northern Costa Rica. Earthquakes in this area occurred between 7-15 years after the 1992 tsunami event. Corner frequency estimates for the Nicaragua events are ~45% lower than events outside of the 1992 rupture zone, indicating lower stress drops for these events. This suggests a fundamental difference in how earthquakes behave within a few hundred km region and suggests some temporal stability in the features that lead to the slow or low stress drop events.
2) Mean stress drop for the events along Costa Rica is 2.2 MPa, ~70% of the global subduction zone average. Stress drop is highest in the locked region of central Nicoya Peninsula near the 5 September 2012 hypocenter, and lowest north of the Osa Peninsula, where significant topography is being subducted. This is somewhat at odds with our hypotheses, as we expected to see more heterogeneity in the area of most heterogeneity in incoming plate topography. It is possible that the significant heterogeneity leads to large amounts of fault zone deformation, thus not allowing the fault zone to develop patches of high and low strength, and “homogenizing” the stress drops.
3) Stress drop is not significantly different for earthquakes located in slow slip and tremor zones along the Nicoya Peninsula, which suggests those phenomena may involve transient changes to the plate interface.
4) Aftershocks of the 2012 Mw 7.6 Nicoya earthquake show spatial variations in stress drop that can be linked to spatial variations in slip during the mainshock. We find higher median stress drop within the zone of strong coupling and high slip relative to the adjacent regions without significant mainshock slip.
Rotman, H., Bilek, S.L., and Phillips, W.S. (2014), Corner frequency variation in the southeastern region of the 1992 Nicaragua tsunami earthquake, AGU meeting.
Bilek, S.L., Phillips, W.S., Walter, J., S.Y. Schwartz, and Peng, Z. (2014), Temporal and spatial variations in earthquake source parameters within the 2012 Nicoya, Costa Rica Mw=7.6 earthquake rupture zone, AGU meeting.
Rotman, H.M.M., Bilek, S.L., and Phillips, W.S. (2014), Examining microseismicity along the Costa Rica and southern Nicaragua segments of the Middle America trench, SSA Annual meeting, 2014.
Bilek, S.L., Rotman, H., and Phillips, W.S. (2014), Examining large and small earthquakes in the Costa Rica subduction zone to probe temporal and spatial megathrust heterogeneity, SSA Annual Meeting.
Bilek, S.L, H.R. DeShon, E.R. Engdah, and M. El Hariri (2013), What can earthquake rupture heterogeneity tell us about subduction zone fault heterogeneity, SSA Annual Meeting (invited).
Bilek, S.L., J. Walter, A.V. Newman, S.Y. Schwartz, and Z. Peng (2013), Analysis of small earthquake source parameters along the Nicoya Peninsula: Probing changes following the 2012 Mw=7.6 earthquake and within slow slip and tremor zones, AGU Annual Meeting, 2013.
DeShon, H.R. and M. Moore-Driskell (2013), Reduced compressional wavespeeds and Poisson’s ratios along the Nicoya Peninsula Seismogenic Zone, Costa Rica, AGU Meeting of the Americas, Cancun, Mexico, 14-17 May.
Moore-Driskell, M. and H.R. DeShon (2012), Variability of High Resolution Vp/Vs and Seismic Velocity Structure Along the Nicaragua/Costa Rica Segment of the Middle America Subduction Zone, T13F-2685 presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec.
Bilek, S.L. Rotman, H.M., and Phillips, W.S., Low Stress Drop Earthquakes in the Rupture Zone of the 1992 Nicaragua Tsunami Earthquake, Geophys. Res. Lett., 43, doi:10.1002/2016GL070409, 2016.
Rotman, H.M.M., Bilek, S.L., and Phillips, W.S., Heterogeneous earthquake stress drop with non-self-similar scaling in the Middle America subduction zone along Costa Rica, J. Geophysical Research, in prep.
Susan Bilek, New Mexico Tech, Susan.Bilek@nmt.edu, 575-835-6510
Heather DeShon, Southern Methodist University, email@example.com, 214-768-2916
This study is supported by NSF-EAR (EAR-1141898 to SLB, EAR-1141900 to HRD). Graduate students have also been supported through NMT/SMU funds. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the Universities.