NSF MARGINS Program, Seismogenic Zone Experiment (SEIZE) Workshop, Mt. Hood, Oregon, 2008.

Tectonic geomorphology and paleoseismology along the Nicoya Peninsula seismogenic zone, Costa Rica

Jeffrey S. Marshall ¹, Eli J. LaFromboise ², John D. Utick ¹, Fookgiin Khaw ¹, Shawn C. Morrish ¹, Peter Piestrzeniewicz ¹, Thomas W. Gardner ³, J. Marino Protti ⁴, James A. Spotila ⁵

1. Geological Sciences Department, Cal Poly Pomona University, Pomona, CA, 91768, USA

2. Geological Sciences Department, California State University, Northridge, CA, 91330, USA

3. Geosciences Department, Trinity University, San Antonio, TX, 78212, USA

4. OVSICORI, Universidad Nacional, Heredia, Costa Rica

5. Department of Geosciences, Virginia Tech University, Blacksburg, VA 24061, USA

The Nicoya Peninsula, Costa Rica deforms in response to rapid NE subduction of the Cocos plate at the Middle America Trench (9 cm/yr). This emergent outer forearc peninsula lies only 60-80 km inboard of the trench axis and coincides with a locked segment of the seismogenic zone. The Nicoya segment is a high-potential seismic gap, with an apparent slip deficit of nearly 5 m since the last major earthquake (M7.7, 1950). That event produced widespread damage and up to 1.0 m of coseismic coastal uplift. During ensuing decades, the Nicoya coast has experienced gradual interseismic subsidence, reflecting strain accumulation leading toward the next earthquake. While elastic seismic-cycle strain produces decadal-scale shoreline fluctuations on the Nicoya Peninsula, net tectonic uplift has resulted in coastal emergence throughout the late Quaternary. This longer-term uplift is recorded by emergent marine terraces at the coast, and by incised alluvial fill within interior valleys. This investigation examines both short-term seismic cycle deformation and the longer-term geomorphic imprint of fore arc uplift associated with the Nicoya Peninsula seismogenic zone.

Ongoing field mapping, surveying, and isotopic dating have provided new constraints on net Quaternary deformation patterns and upper-plate faulting. Observed differences in coastal uplift along the Nicoya Peninsula coincide with three contrasting domains of subducting seafloor offshore (EPR, CNS-1, CNS-2). Uplift rates vary from 0.1-0.2 m/k.y. inboard of EPR crust (north of Punta Guiones), 0.2-0.3 m/k.y. inboard of CNS-1 crust (south of Punta Guiones), and 1.0-2.0 m/k.y. inboard of CNS-2 seamounts impacting Cabo Blanco (the peninsula’s southern tip). Tectonic segmentation of the upper-plate may reflect along-strike variations in subducting plate roughness, thermal structure, fluid flow, plate coupling, and seismogenic zone structure (e.g., dip angle, length and width, limits of up-dip and down-dip locking). In addition, local geomorphic anomalies reveal shallow faults that may accommodate a significant fraction of net fore arc deformation (crustal shortening and/or lateral sliver transport).

Seismic cycle elastic strain accumulation and release produce significant changes in local tidal levels, wave erosion, and littoral sediment dynamics. As a result, coastal wetlands and beaches on the Nicoya Peninsula may preserve stratigraphic records of vertical shoreline fluctuations and tsunami associated with prior earthquake cycles. Sediment coring at strategic sites may produce useful paleo-seismic records. In addition, detailed pre- and post-earthquake topographic surveying of the Nicoya coast (using LIDAR and/or differential GPS) may also establish important constraints on seismic cycle deformation patterns. These approaches represent promising new avenues for our ongoing studies of upper plate deformation along the Nicoya seismogenic zone.

Based on the rapid convergence rate (9 cm/yr) and the frequency of historic seismicity, the recurrence interval for large Nicoya earthquakes has been estimated at 50-60 years. While these events may produce meter-scale coseismic uplift along the Nicoya coast, a large fraction is recovered during interseismic subsidence. The net result is gradual uplift and the emergence of Quaternary marine terraces at the observed rates. Our ongoing investigation is aimed at developing further constraints on both short- and long-term deformation patterns within the upper plate. The results may have implications for understanding the rupture behavior, paleoseismology, and earthquake hazards of the Nicoya Peninsula seismogenic zone.