Jeff
Marshall’s Research
TECTONICS & EARTHQUAKES
OTHER RESEARH TOPICS:
Watersheds
& Rivers | Coastlines &
Beaches | Mountains
& Deserts
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Tectonic setting of the
Nicoya Peninsula, Costa Rica. Lower image: SRTM-DEM linked to
offshore bathymetry (courtesy C.J. Petersen, IFM-GEOMAR). Upper image: SRTM-DEM of the Nicoya
Peninsula. Uplifted marine and fluvial terraces occur within the late
Pleistocene Iguanazul, Carrillo-Camaronal, Cobano, and La Mansión geomorphic
surfaces (outlined in red). Differences in Quaternary uplift rates at three
coastal study sites (black boxes) correspond with variations in subducting
seafloor bathymetry and seismogenic zone structure across the EPR, CNS-1, and
CNS-2 segment boundaries on the Cocos Plate. |
View of Punta Pochotes
and Playa Junquillal, northern Nicoya Peninsula, showing the active Holocene
wavecut platform and an uplifted Pleistocene marine terrace cut across
Cretaceous Nicoya Complex seafloor basalt. The uplifted terrace (Qt3) is the
lowest of three discrete treads within the Iguanazul geomorphic surface
(Marshall et al., 2003). At this site, the terrace inner edge elevation =12 m
above mean sea level. Correlation with OSL dated terraces at the peninsula’s
southern tip (Cabo Blanco) indicates this terrace formed at 80 ka (OIS 5a)
and has been uplifted at 0.1-0.2 m/k.y. Emergent marine terraces along the
Nicoya coast record the net pattern of upper plate deformation resulting from
the subduction earthquake cycle. Differences in uplift rates coincide with
three contrasting domains of subducting seafloor offshore (EPR, CNS-1,
CNS-2). Fore arc segmentation may reflect along-strike variations in
subducting plate roughness, coupling, and seismogenic zone geometry. |
Geologic map of the
Nicoya Peninsula showing boundaries of the Nicoya seismic gap (orange dashed
lines), epicenters of large subduction earthquakes (red circles), and
aftershock zones of the 1978, 1990, and 1992 events (red dashed lines). The
limits of the 1990 and 1992 rupture zones coincide with the edges of the
Nicoya seismic gap. The last major rupture of the Nicoya segment (1950;
M=7.7) produced up to 1.0 m of coseismic coastal uplift (Marshall and
Anderson, 1995). |
