2012 Nicoya Earthquake (Mw7.6), Costa Rica

Dr. Jeff Marshall’s Research

Coseismic Coastal Uplift from the 2012 M_w7.6 Nicoya Earthquake, Costa Rica

Overview:

On 5 September 2012, a major megathrust earthquake (M_w=7.6) ruptured the plate interface beneath the Nicoya Peninsula, Costa Rica. This large event was centered 12 km offshore of the central Nicoya coast, at a depth of 18 km (Fig. 1). 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 (Fig. 2). More than 1700 aftershocks were recorded within the first 5 days (OVSICORI-UNA), outlining two distinct rupture patches, one centered on the central coast, and the other beneath the southern tip of the peninsula (Fig. 1).

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 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 (M_S=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., 2001; Marshall et al., 2003-2012; Norabuena et al., 2004; Feng et al., 2012). In 1989, USGS experts gave a 93% probability of a large earthquake occurring here before 2009, listing Nicoya as fourth among the top seismic gaps of the circum-Pacific region (Nishenko, 1989). To monitor precursory seismicity and the build-up of crustal strain, the Observatorio Volcanológico y Sismológico de Costa Rica (OVSICORI) established a dense network of seismometers and GPS stations across the Nicoya Peninsula. On September 5, 2012, after 62 years of tectonic strain accumulation, the forecast earthquake finally occurred, generating a wealth of geophysical data, and providing an unprecedented opportunity for geologists to capture the near-field pattern of coseismic deformation produced by a major megathrust earthquake.

Figure 1. a) Earthquake epicenter map and seismogenic zone profile for Costa Rica (by LIS-UCR) showing location of 2012 M_w=7.6 Nicoya earthquake (red circle) with respect to two years of prior seismicity (2010-2012). b) Map of Nicoya Peninsula earthquakes for September 2012 (recorded by OVSICORI-UNA) showing distribution of aftershocks and triggered events (red dots) associated with the 5 September 2012 mainshock (blue star). Note two distinct rupture patches outlined by aftershocks beneath the central and southern portions of the peninsula.

Figure 2. Map of the Nicoya Peninsula with preliminary dislocation model (based on seismic wave inversion) showing slip distribution for the 5 September 2012 M_w7.6 Nicoya earthquake (by Laboratorio de Ingenieria Sismica, Universidad de Costa Rica [UCR-LIS]). Colored contours (key at right) show variable slip decaying from a maximum of >2m near the hypocenter. Area of maximum slip corresponds with area of greatest observed coseismic uplift along the coastline (both geomorphic and GPS data).

NSF Rapid Response Team

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 (Fig. 3). Geomorphic spot measurements at a dozen field sites indicate that the earthquake produced 0.1 to 0.8 m of coseismic uplift along the central Nicoya coast (Fig. 4). Inversion modeling of preliminary GPS data from the OVSICORI geodetic network yielded consistent results (Fig. 5), showing maximum uplift adjacent to the earthquake epicenter and decaying outward with both coast parallel and coast perpendicular distance. Preliminary models based on seismic wave inversion (Fig. 2) 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. 5; Feng et al., 2012).

Click here to view roster: NSF Nicoya Earthquake Rapid Response Team

Figure 3. NSF Nicoya Earthquake Rapid Response Team collecting geodetic and geomorphic field data to characterize the coseismic deformation pattern produced by the September 5, 2012 Nicoya Peninsula earthquake, Costa Rica. a) Geodesy field team at the SAMA campaign GPS site at Playa Sámara, just onshore of the earthquake epicenter, left to right: Sarah Polster (Grad Student, Georgia Tech), Dr. Andy Newman (Professor, Georgia Tech), and Jacob Richardson (Grad Student, University of South Florida). b) Geomorphology field team surveying pre- and post-earthquake tidal debris lines, Playa Samara, left to right: Dr. Jeff Marshall (Professor, Cal Poly Pomona) and Shawn Morrish (Grad Student, Cal Poly Pomona).

Figure 4. Pre & post-earthquake photographs of high tide at Playa Carrillo estuary, showing the magnitude of 5 September 2012 coseismic uplift directly inland of earthquake epicenter: a) July 5, 2012, 3:50 pm, +3.0m tide, b) Sept 13, 2012, 12:30 pm, +2.4m high tide. The tide pictured at left was the highest tide for the 2 months preceding the earthquake. Note the coconut debris line left by this tide still visible in the post-earthquake photo at right. While the pre-earthquake high tide at left is 0.6m higher than the post-earthquake high tide at right, the surveyed difference in these tidal levels is ~1.4m, indicating uplift of ~0.8m (Marshall et al., unpublished data).

Figure 5. a) Map of Nicoya GPS network (from Feng et al., 2012) showing continuous stations (yellow circles) and campaign sites (red diamonds). Blue vectors show horizontal velocities relative to stable Caribbean plate between 1996 and 2010. 2-D 2s error ellipses represent 86.5% confidence. b) Preliminary rapid GPS solution for continuous stations showing horizontal (black) and vertical (blue) displacement vectors for the 5 September 2012 M_w7.6 Nicoya earthquake (solution by JPL, based on data from OVSICORI-UNA). Red beach ball shows preliminary focal mechanism for mainshock. Contoured colors show modeled distribution of pre-earthquake locking on megathrust fault.

Coseismic Coastal Uplift Results

The Geomorphology Team, consisting of Dr. Jeff Marshall and graduate student Shawn Morrish (Cal Poly Pomona), measured coastal uplift at over 22 sites along the Nicoya Peninsula coastline. Measurements were made using six different techniques as described below.

1. Rapid Altimeter Surveying of Prior Coastal Monuments – Barometric altimeter used to determine comparative pre & post-earthquake elevations of known survey monuments (e.g. coast geodetic survey "mojones", telephone poles, sign posts, etc.). These sites were measured rapidly to determine comparative elevations above present tide, previous high tide line, and highest high tide debris lines (along beach berm and/or inner edge of pocket beaches).

2. Reoccupation of Pre-Earthquake Beach Profile Survey Lines – Hand level, stadia rod, and tape measure used to survey coast-perpendicular beach profiles at sites that had been surveyed prior to the earthquake (over the past several years). These survey data provide comparative pre & post-earthquake elevation profiles relative to mean sea level (based on tidal levels at time of survey).

3. Surveying of Pre & Post-Earthquake High Tide Debris Lines - Hand level, stadia rod, and tape measure used to survey coast perpendicular beach profiles at new sites to determine elevation difference between the post-earthquake recent high tide debris lines (at time of survey) and pre-earthquake highest high tide debris line (along former beach berm or sea cliff inner edge).

4. Spot Measurements of Pre-Earthquake High-Tide Staining – Hand level and stadia rod used to measure difference between post-earthquake high tide levels and pre-earthquake high-tide staining on rocky cliffs (e.g., colored zones of former high-high tides, salt spray, etc).

5. Spot Measurements of Post-Earthquake Low-Tide Desiccation Bands – Measuring tape and stadia rod used to measure horizontal width and vertical thickness of mortality zones of sub-tidal organisms (e.g. algae, plants, coral, shellfish, etc) now exposed in post-earthquake low tide zone by coseismic uplift.

6. Spot Measurements of Post-Earthquake Coastal Stream Incision and Mangrove Root Exposure – Stadia rod used to measure depth of post-earthquake coastal stream incision into beach sediments (i.e., measuring change in local base level), and height of exposed mangrove roots at high tide.

Figure 6. Digital elevation model of the Nicoya Peninsula showing preliminary geomorphic field measurements of coseismic coastal uplift made during NSF rapid response fieldwork 13-18 September 2012 (Marshall and Morrish, unpublished data). Note spatial similarity of uplift pattern shown here, and the modeled patterns of fault slip (Fig.2) and pre-event geodetic locking (Fig. 5).