The Geomorphic Footprint of Megathrust Earthquakes: A Field Investigation of Convergent Margin Morphotectonics, Nicoya Peninsula, Costa Rica
What: Students will investigate the morphotectonic footprint of earthquake-generated uplift on the Nicoya Peninsula, Costa Rica. This project will expand upon preliminary geomorphic, geodetic, and seismologic data showing patterns of coseismic rupture and coastal uplift generated by the recent Mw7.6 Nicoya Earthquake of 5 September 2012. Project students will build upon several decades of prior research on subduction generated coastal uplift on the Nicoya Peninsula [e.g., Hare and Gardner, 1985; Marshall and Anderson, 1995; Marshall et al., 2001-2012], including a highly successful 1998 Keck project [Gardner et al., 2001]. The participating students will conduct fieldwork along the Nicoya Peninsula coastline, learning research techniques of tectonic geomorphology, paleoseismology, and GPS geodesy.
When: June 24-July 20
Where: Nicoya Peninsula, Costa Rica, Central America (passport is required).
Who: Nine undergraduate students and three project leaders: Dr. Jeff Marshall (Cal Poly Pomona), Dr. Tom Gardner (Trinity University), and Dr. Marino Protti (Observatorio Volcanológico y Sismológico de Costa Rica, OVSICORI-UNA).
Project Overview and Goals
Megathrust earthquakes along subduction zones are among Earth’s most powerful and deadly natural hazards. During the past decade, more than a quarter-million people lost their lives to megathrust earthquakes and tsunami in Sumatra (M9.3, 2004), Chile (M8.8, 2010), and Japan (M9.0, 2011). Such catastrophic events are also notable for sudden geomorphic changes along coastlines caused by coseismic uplift or subsidence [e.g., Plafker, 1972; Atwater, 1987]. Earthquake-induced changes in land level result in either emergence or submergence of the coast, shifting the relative position of the shoreline, and all subsequent tides. Evidence of past events is preserved in the sedimentary record of beaches and coastal wetlands, and by such features as emerged tidal platforms and coral reefs [e.g., Taylor et al., 1987; Nelson et al., 1996; Natawidjaja et al., 2006]. Geomorphic and stratigraphic analysis of these features allows geoscientists to unravel the paleoseismic history of convergent margin coastlines [e.g., Sieh, 2006; Satake and Atwater, 2006], and to investigate how earthquake induced crustal displacements affect the long-term growth and decay of coastal topography [e.g., Bull, 1985; LaJoie, 1986; Marshall and Anderson, 1995; Sato and Matsuura, 2007; Rehak et al., 2008].
An excellent place to study these processes is the Nicoya Peninsula on the Pacific coast of Costa Rica, Central America (Fig. 1) [Marshall, 2008]. The Nicoya Peninsula is unique because it is one of the few landmasses along the Pacific Rim located directly above the seismogenic zone of a subduction megathrust fault. Due to its proximity to the subduction zone, the peninsula is particularly sensitive to vertical movements related to the earthquake cycle [Marshall and Anderson, 1995; Feng et al., 2012]. Costa Rica is part of the Central American convergent plate margin, where the Cocos oceanic plate subducts beneath the Caribbean plate at the Middle America Trench [von Huene et al., 2000]. The two plates converge at a rapid rate (~8 cm/yr) along the Nicoya Peninsula [DeMets et al., 2010], resulting in a high seismic potential, as demonstrated by repeated large magnitude (>M 7.5) earthquakes over the past few centuries, including events in 1853 (M≥7.5), 1900 (M≥7.5), 1950 (Ms=7.7), and 2012 (Mw=7.6).
On 5 September 2012, a major earthquake (Mw=7.6) ruptured the megathrust plate interface beneath the Nicoya Peninsula [Dixon et al., 2012]. This large event was centered 12 km offshore of the central Nicoya coast, at a depth of 18 km (Fig. 2a). Near the hypocenter, the maximum slip exceeded 2 m, and the rupture spread outward along the plate interface to encompass >3000 km2 of the Nicoya seismogenic zone. More than 1700 aftershocks were recorded within the first 5 days (Fig. 2b), outlining two distinct rupture patches, one centered on the central coast, and the other beneath the southern tip of the peninsula.
The 2012 Nicoya earthquake was felt throughout much of Central America and resulted in widespread damage to homes, businesses, schools, and health centers across Costa Rica (>$45 million). Thanks to extensive prior public outreach by geoscientists and government officials, Costa Rican citizens were acutely aware of the seismic hazard posed by the Nicoya seismogenic zone. For this reason, the population was well prepared and emergency personnel reacted swiftly, minimizing earthquake casualties (<200 injured, 0 deaths). Although a major disaster was averted, this powerful earthquake was a stark reminder to local residents that they live in a region of substantial seismic hazard.
For geoscientists, the 2012 Nicoya earthquake was a watershed event. The last major earthquake in this area (MS=7.7) occurred in 1950 [Protti et al., 2001], causing widespread damage and casualties, and producing landslides, liquefaction, and pronounced coseismic uplift along the Nicoya coast [Marshall and Anderson, 1995]. Since then, seismologic, geodetic, and geomorphic studies had recognized the Nicoya Peninsula as a mature seismic gap, with a high probability of rupturing in the near future [e.g., Protti et al., 1995 and 2001; Marshall and Anderson, 1995; Marshall et al., 2003-2012; Norabuena et al., 2004; Feng et al., 2012]. In 1989, the USGS gave a 93% probability of a large earthquake occurring here before 2009, listing Nicoya as fourth among the top seismic gaps of the Pacific Rim [Nishenko, 1989]. To monitor precursory seismicity and the build-up of crustal strain, the Observatorio Volcanológico y Sismológico de Costa Rica (OVSICORI-UNA), working with international collaborators, developed a dense network of seismometers and GPS stations across the Nicoya Peninsula (Fig. 3). On September 5, 2012, after 62 years of tectonic strain accumulation, the forecast Nicoya Earthquake finally occurred, generating a wealth of geophysical data [Dixon et al., 2012], and providing an unprecedented opportunity for geologists to capture the near-field pattern of coseismic deformation produced by a major megathrust earthquake.
In the wake of the 2012 Nicoya earthquake, an NSF Rapid Response Team was organized to collect preliminary geomorphic and geodetic field data to constrain patterns of coseismic deformation across the peninsula [Newman et al., 2012, unpublished]. Geomorphic spot measurements at more than a dozen field sites (Fig. 3) indicate that the earthquake produced up to 0.8 m of coseismic uplift along the central Nicoya coast (Fig. 4) [Marshall and Morrish, 2012, unpublished]. Inversion modeling of preliminary GPS data from the OVSICORI geodetic network yielded consistent results (Fig. 5a), showing maximum uplift adjacent to the earthquake epicenter and decaying outward with distance. Preliminary models based on seismic wave inversion (Fig. 5b) show a bull’s eye of maximum slip (>2m) adjacent to the hypocenter, surrounded by a broader area of decreasing slip across the seismogenic zone beneath the central coast. This rupture pattern is roughly similar to the area of pre-earthquake locking suggested by GPS modeling (Fig. 5a) [Feng et al., 2012].
Earthquake cycle deformation has been observed along convergent margin coastlines worldwide. Famous examples in the geomorphic literature include Chile, Alaska, Japan, Cascadia, Vanuatu, and Indonesia [Plafker, 1972; Matsuda et al., 1978; Bull, 1985; LaJoie, 1986; Atwater, 1987; Taylor et al., 1987; Sieh, 2006]. As the locked interface between two converging tectonic plates snaps free, the upper plate springs forward releasing stored elastic energy in the form of seismic waves (the earthquake). The seaward edge of the plate nearest the subduction trench rebounds upward, resulting in sudden coseismic uplift (and often a tsunami). In contrast, the landward region further from the trench subsides as strain is released. As the plates become locked again and elastic strain begins to build, gradual interseismic movements generally occur in the opposite direction (subsidence in the cosesimic uplift zone and vice versa).
This cycle of vertical motion in response to elastic strain accumulation and release is an integral part of the way subduction zones work, and is a dramatic manifestation of the forces that generate deadly megathrust earthquakes and tsunami.
An interesting question for geomorphologists is how this short-term cycle of elastic motion translates into longer-term permanent deformation that generates topographic relief. How much of the seismic cycle deformation is non-recoverable and permanent? Does coastal topography mirror earthquake cycle deformation patterns? Are earthquake rupture zones long-lived features or are they transient, changing location and shape through time? What other processes contribute to the creation of permanent coastal topography along convergent margins?
Along the Nicoya Peninsula’s seaward-facing coastline, net Quaternary uplift is recorded by emergent marine terraces (ancient shorelines) and uplifted alluvial fill (ancient river deposits) [Hare and Gardner, 1985; Marshall and Anderson, 1995; Gardner et al., 2001; Marshall et al, 2001-2012; Sak et al., 2009]. Along the peninsula’s landward-facing gulf coast, net subsidence results in drowned rivers and broad mangrove estuaries. Ongoing geomorphic, paleo-geodetic, and paleoseismic studies [e.g., Marshall et al., 2010; Spotila et al., 2010; Marshall and Spotila, 2011] are revealing upper plate deformation patterns that provide important clues about seismogenic zone segmentation and the periodicity of megathrust earthquakes beneath the Nicoya Peninsula.
Field mapping, surveying, and isotopic dating of uplifted paleo-shorelines, river deposits, and wetland sediments allows for calculation of Holocene and Pleistocene uplift rates [e.g., Marshall et al., 2012]. Results indicate that sharp variations in uplift patterns on the Nicoya Peninsula coincide with three distinct domains of subducting seafloor identified through offshore geophysical studies. These seafloor segments (Fig. 1), designated EPR, CNS-1, and CNS-2 [Barckhausen et al., 2001], each originated at distinct oceanic spreading ridges and exhibit contrasts in crustal thickness, surface roughness, and heat flow [e.g., von Huene et al., 2000; Fisher et al., 2003]. Such contrasts may exert important controls on seismogenic zone geometry, seismic coupling, and earthquake rupture behavior [e.g., Newman et al., 2002; Norabuena et al., 2004; DeShon et al., 2006; Schwartz and DeShon, 2007].
This Keck research project will build upon preliminary post-earthquake field studies by filling critical data gaps, addressing key questions about how earthquake-generated uplift impacts coastal geomorphology, and investigating how seismic cycle motions contribute to net deformation and topographic growth. Project students will gain valuable professional experience applying modern field techniques of coastal geomorphology, paleo-seismology, and GPS geodesy. Their efforts and research results will contribute to the growing body of scientific knowledge on convergent margin morphotectonics.
Potential Student Projects
- Coseismic Coastal Uplift and Impacts on Beach Morphology. Preliminary geomorphic field measurements and GPS data indicate that the 2012 Nicoya earthquake generated up to 0.8 m of coseismic uplift along the central Nicoya coastline. Students working on this project will investigate this coastal uplift through detailed site investigations, collecting additional geomorphic field data, and surveying coastal landforms to further constrain the magnitude and pattern of uplift. Students will use laser range finders, barometric altimeters, hand held GPS, and stadia rods to survey topographic profiles and measure uplift. Students will also investigate the impacts of recent coseismic uplift on the morphology of beaches and rocky shorelines by measuring uplift-related geomorphic changes, including modification of beach profiles, stream incision, shifts in tidal levels, changes in wave erosion, and displacement of tidal ecozones (e.g. mortality of sessile organisms).
- Coastal Wetland Stratigraphy and Paleoseismic Records. To investigate the history of paleo-earthquakes on the Nicoya Peninsula, students will use hand-gouge augers to extract sediment cores from coastal wetlands. These cores will be examined for evidence of previous coseismic land level changes (e.g. abrupt changes in grain size, soil color, etc.). A pilot sediment coring study [Spotila et al., 2010] revealed compelling evidence of abrupt stratigraphic breaks, consistent with coseismic uplift in several Nicoya wetlands. In this project, we will target additional wetland sites for paleoseismic records in areas of known vertical motion during the 2012 earthquake. The core stratigraphy will be measured, photographed, and described in detail. Organic samples will be collected from cores for radiocarbon dating. In addition, students will document the impact of the recent coseismic uplift on wetland morphology and sedimentation. This will provide important reference data for interpreting stratigraphic evidence of past events.
- Geomorphology and Petrology of Uplifted Holocene Beachrock Horizons. To study coastal uplift, students will examine Holocene-age carbonate beachrock deposits, a common feature of the Nicoya Peninsula coastline [Marshall et al., 2012]. These tabular horizons of lithified beach sediment extend laterally along the shoreline, creating a natural pavement similar to a concrete sidewalk. Beachrock forms by precipitation of carbonate cement (calcite and aragonite) within intergranular pore spaces of beach sediments in the groundwater excursion zone between high and low tide. These horizons form preferentially where groundwater is abundant near streams and wetlands. As earthquakes elevate the coastline, beachrock horizons are moved upward on the beach face and eventually into the landscape beyond. By surveying their current elevation above sea level and collecting samples for age dating, we will use beachrock horizons as timelines to track the history of net Holocene uplift. Students will survey beachrock outcrops using laser range finders, hand held GPS, and stadia rods. Sites will be photographed and described in detail, and samples will be collected for radiocarbon dating and thin section analysis.
- Geodetic Evaluation of Co-Seismic and Post-Seismic Deformation. The Costa Rican Volcanologic and Seismologic Observatory (OVSICORI-UNA) operates a dense network of 12 continuous GPS receivers on the Nicoya Peninsula (in collaboration with University of South Florida and Georgia Tech University). In addition, there are more than 15 campaign GPS monuments located throughout the peninsula. Students will work under the direction of Dr. Marino Protti (Senior Research Geophysicist, OVSICORI) to install campaign GPS receivers, service continuous GPS stations, download data, and process/evaluate horizontal and vertical GPS motions to interpret seismotectonic deformation related to the 2012 Nicoya earthquake.
Fieldwork will take place in relatively remote locations. While tourism has brought rapid development in some areas, most of the Nicoya Peninsula is still rural and rugged. Field sites will be accessed by four-wheel drive vehicle and by hiking along rocky coastlines and steep trails with dense vegetation. Weather conditions will be hot and humid, with intense tropical sun and occasional heavy thunderstorms. Hats, sunscreen, water, and proper footwear are essential. Insect repellent is recommended. Safety is a critical aspect of this project. Students will work in teams under the supervision of project faculty. Participants are required to be vigilant and aware of potential hazards, including dehydration, heat exhaustion, sunburn, rugged terrain (cliffs, rocky shore platforms, streams/rivers), unfriendly plants (thorny, sharp, or stinging shrubs/trees), and potentially aggressive insects and animals (fire ants, scorpions, mosquitoes, wasps/bees, snakes, livestock). Participants may be exposed to new and significant allergies (mold, mildew, insect bites, poison plants). Those with asthma or other allergy sensitive conditions should take appropriate caution. Emergency medical care is available, but located at substantial distance from field sites. We will stay in small eco-tourist lodges with modest facilities. These hotels provide a relatively safe and comfortable home base and easy access to field areas. We will eat meals prepared by the hotel staff or at local restaurants. Dietary options are limited to what is available on site. While tropical fruits and vegetables are common, most meals consist of rice, beans, meat, poultry, or seafood. We will have limited Internet access, but cell phone coverage and public phones are generally not available. Use of personal electronic devices is prohibited anywhere outside of hotel rooms. To prevent theft, students must be vigilant of personal belongings at all times. Participants should be comfortable living and working in a remote rural setting, within a developing country, immersed in the Spanish language. A valid passport is required for travel to Costa Rica.
This project is designed for advanced undergraduate students with an interest in geomorphology and tectonics. Students should be prepared for working in challenging field conditions, in a foreign country. Fieldwork in a tropical setting requires astute observational skills, creative thinking, and an ability to work with limited geologic exposure. Demonstrated experience in field methods is strongly preferred. Students should have taken core courses in geomorphology, mineralogy/petrology, and structural geology/tectonics. Additional coursework in sedimentary geology, stratigraphy, geophysics, optical mineralogy, or GIS is desirable. Acceptance on this project requires academic standing as a rising senior, and completion of a supervised senior thesis (or equivalent) during the following academic year (2013-14), as an integral part of this research effort. The application must include a supporting letter from the student’s home campus thesis advisor. This letter must indicate how the thesis requirement is included in the senior year course load. The advisor should also describe the student’s prior research background and field experience, and how well the applicant will function in the working conditions described above.
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