CSU Student Research Competition Summary Submission

Transcription

1 Name: CSU Student Research Competition Summary Submission Shawn Morrish Title: Coseismic Uplift and Geomorphic Response to the September 5, 2012 Mw7.6 Nicoya Earthquake, Costa Rica Abstract: The Nicoya Earthquake of September 5 th, 2012 occurred in the central region of the Nicoya Peninsula, Costa Rica and had a moment magnitude of 7.6.Historically, the peninsula coast has experienced measurable coseismic uplift (maximum of ~1m) and uplift was again observed during this event. Fifteen study sites along the 150km Pacific coast were visited one week after the event where field evidence of the geomorphic response to uplift was observed and characterized. Five field techniques were utilized to measure coastal uplift. Methods varied at each location dependent on local site characteristics, geomorphic features, and prior reference data sets. 1) Prior survey lines were reoccupied using eye level, stadia rod, and tape measure; 2) this technique was also used to measure differences in pre/post earthquake tidal levels; 3) A barometric altimeter was used to survey tidal levels relative to known coastal survey monuments, and a stadia rod and tape measure were used for direct measurements of displaced 4) intertidal zone staining and 5) desiccation of displaced ecosystems. Maximum uplift of ~1m was observed closest to the epicenter at Playa Carrillo with a gradual decrease in observed uplift to the North terminating South of Tamarindo and a sharper decrease in uplift to the South terminating North of Playa Carmen. The combined field measurements reveal an uplift signature that corresponds with the earthquake rupture geometry, GPS-based geodetic measurements, as well as previously recognized patterns of morphotectonic segmentation along the Nicoya Peninsula. Coseismic uplift is an active process resulting from the subduction earthquake cycle. Narrative: On September 5 th, 2012, a large megathrust earthquake ruptured beneath the Nicoya Peninsula, Costa Rica. The moment magnitude was 7.6 and the hypocenter was 12km offshore at a depth of 18km. The estimated rupture area of the earthquake was extent across the Pacific coast of the peninsula and the event released 62 years of strain along the recognized Nicoya

2 seismic gap (Figure 1). The Nicoya Peninsula undergoes a recognized seismic cycle of interseismic subsidence and coseismic uplift. Previous years of research have focused on the long term net effect of uplift on the peninsula, but the September 5 th event provided a chance to observe and quantify the coseismic aspect of research in the field. One week after the September 5 th event, a multi- university research group was sent to the Nicoya Peninsula as an NSF Figure 1 4 : Digital elevation model of Nicoya Peninsula with relationship to the Middle America Trench. Outlined is the estimated rupture area of the September 5, 2012 event. (National Science Foundation) Rapid Response team. Both geodetic GPS and geomorphic data were obtained to characterize the coseismic deformation of the event. This study focuses on the geomorphic data obtained in the field as part of this Rapid Response team (Figure 2), the processes utilized, the data processing, and the end preliminary results. Fifteen sites of focus were visited along the 150km Pacific coast and measurements of varying styles were conducted while in the field. Field days were conducted from a central field base in the town of Nicoya with few problems as local infrastructure and services were not severely affected by the earthquake. Standard days during the Rapid Response rip involved an early morning start, driving to a predetermined known locality, scouting said locality for evidence of deformation,

3 surveying/ characterizing the evidence, continuing to another site to do the same or returning to base to work up the data and plan for the next day. Field data was recorded each day in a field notebook and digitized for processing into a graphing program. The multiple forms of data require varying types of Figure 2 2 : Digital elevation model of Nicoya Peninsula with offshore bathymetry. Icons represent NSF Rapid Response team sites with the coastal blue dots representing locations from this particular study. analysis and calculations to be carried out. Of note are calculations related to the way in which data was obtained with a hand eye level survey. A simple calculation to extract the surveyor s eye height must be carried out to retrieve true elevation difference between points. A key factor (and source of potential error) is a recognition and calculation that must be carried out to determine the elevation of the tide at the time of the survey beginning (at the present tide). This correction allows the survey to be correlated to MSL (mean sea level) and thus allows reoccupied surveys and tide line surveys to be properly correlated (utilizing MSL as a constant variable). Provided for your inspection is a data table (Table 1) from a particular site that shows the data corralling process and multiple data columns that are recorded and calculated for a survey. Upon calculating corrections and applying them to the obtained raw data, the data can then be plotted as a simple X-Y plot as Elevation vs. Horizontal Distance (Figure 3). This is helpful in visualizing the deformation as well as correlating known points to one another in

4 Location: UTM- Start GPS: , Date: 9/13/12 End GPS: , Surveyors: Jeff Marshall, Shawn Morrish Eye height (m): 1.63 Station # Time Measured Distance (ft) Measured Cumulative Measured Distance (m) Distance (m) Height (m) Eye Height Cumulative Corrected (m) Height (m) 0 9:44a current tide 1 9: wave runup 2 9: Wave runup 3 9: wave runup 4 9: wave runup 5 9: wave runup 6 9: wave runup 7 9: wave runup 8 9: wave runup 9 9: up beach 10 9: up beach 11 9: beach slope (flat) wet sand 12 9: beach slope (flat) wet sand 13 9: beach slope (flat) wet sand 14 10: up beach 15 10: lower edge of recent HT (previous tide) 16 10: upper edge of recent HT 17 10: up slope 18 10: up slope 19 10: Up beach berm 20 10: Up beach berm 21 10: edge of damp sand 22 10: up sand 23 10: edge of gravel berm 24 10: up berm 25 10: up gravel flat 26 10: Middle upper (crser) debris line (0.8ft wide) 27 10: along flat of gravel 28 10: along flat of gravel 29 10: Lower edge of highest (coarse) debris line 30 10: upper edge of highest debris line 31 10: along gravel 32 10: along gravel 33 10: into vegetation (dormilona) 34 10: edge of road 35 10: middle of road 36 10: edge of road Table 1 4 : Example MS Excel spreadsheet data table of a pre/post tidal debris line survey at East Samara Beach. Note pre earthquake tide level in blue and post level in red (lower due to coseismic uplift of beach face). (Data is unpublished) reoccupied surveys. Figure creation is necessary for providing visual representation of the data as well as providing character to the research itself. Upon calculating all the data sets and determining the various parameters at each site, uplift values can be calculated for each site and applied to an overarching model of the peninsula. Uplift was greatest nearest the event epicenter with an estimated value of m. Notes

5 Elevation (m) Playa Carrillo Este Comparative VE Horizontal Distance (m) Figure 3 4 : Example MS Excel plotted graph of a reoccupied hand level survey at East Carrillo beach across the beach face and two roads. The pre earthquake elevation is in blue and the post elevation in red. Coseismic uplift at this particular site is estimated to be 0.97m. ±0.2 (Data is unpublished) The obtained data reveals a sharp drop off in uplift values to the South and a steady drop off in values to the North (Figure 4). This pattern coincides with the geodetic data and the rupture area of the event. The measured uplift pattern and coseismic deformation of the peninsula during this most recent step in the Nicoya Peninsula seismic cycle provide tangible and quantitative data for the previously recognized seismic cycle pattern. This project has been a great experience to obtain unique and important data for the scientific community and it will have impact on the understanding of fthe regional tectonics as well as influence ongoing research within the area. While research had been previously guided by historic accounts and second hand information, we now have actual data to help guide the characterizing seismic cycle on the peninsula and thus future work that will be carried out in the area. Future work could certainly involve measuring and tracking interseismic subsidence to coincide with the new measured coseismic data from this study so as to potentially determine net seismic deformation over long duration multiple seismic cycles (which is a driving force of ongoing geomorphic research within the region).

6 Figure 4 4 : Digital Elevation Model of Nicoya Peninsula with estimated coastal uplift pattern outlined along the coast. (Figure is unpublished) References: 1) Marshall, J. S., and R. S. Anderson (1995), Quaternary uplift and seismic cycle deformation, Península de Nicoya, Costa Rica, Geol. Soc. Am. Bull., 107, ) Dixon, T. H., Schwartz, S., Protti, M., Gonzalez, V., Newman, A., Marshall, J., Spotila, J. (2013), Detailed Data Available for Recent Costa Rica Earthquake, EOS, Vol. 94, No. 2, 8 January ) Feng, L., A. V. Newman, M. Protti, V. Gonzalez, Y. Jiang, and T. H. Dixon (2012), Active deformation near the Nicoya Peninsula, northwestern Costa Rica, between 1996 and 2010: Interseismic megathrust coupling, J. Geophys. Res., 117, B06407, doi: /2012JB ) Marshall, J.S. and Morrish, S. C (unpublished)