South Coast Geological Society and Association of Environmental and Engineering Geologists invite you to join our joint meeting on Wednesday, August 22nd at 6PM to be held at Dave & Buster’s in Orange, CA. We are thrilled to co-host Dr. Jeff Marshall, Professor for the Geological Sciences Department at Cal Poly Pomona. Dr. Marshall will share his presentation titled “Sizing up the subduction beast: A tale of earthquakes and ancient shorelines in Costa Rica and New Zealand”
Sizing up the subduction beast: A tale of earthquakes and ancient shorelines in Costa Rica and New Zealand
The seismic hazards of subduction zones are substantial, as exemplified by the 2004 Sumatra (Mw 9.2) and 2010 Japan (Mw 9.0) megathrust earthquakes and tsunami, which killed more than a quarter-million people. Convergent plate margins account for more than 80% of global seismic moment release, and recent research has revealed a broad spectrum of slip behavior on megathrust faults, ranging from stick-slip earthquakes to tremor and slow-slip. This talk will explore ongoing geomorphic and paleoseismic investigations along the coastlines of Costa Rica and New Zealand that provide new insights into megathrust fault segmentation and rupture behavior over multiple seismic cycles.
The Nicoya Peninsula, Costa Rica forms a prominent morphologic high along the Middle America forearc, where the Cocos plate subducts beneath the Caribbean plate at 90 mm/yr. This emergent coastal landmass overlies the megathrust along a seismogenic zone that produces frequent major earthquakes, as well as periodic slow slip events. Quaternary marine and fluvial terraces record a net uplift pattern consistent with the peninsula’s overall topographic form. Terrace mapping, surveying, and geochronology (14C, OSL, TCN) reveal uplift variations that coincide with three domains of subducting seafloor (EPR, CNS-1, CNS-2), with uplift rates of 0.1-0.2 m/ky inboard of older EPR crust in the north, 0.2-0.5 m/ky inboard of younger CNS-1 crust along the central coast, and 1.5-2.5 m/ky inboard of CNS-2 seamounts impacting the peninsula’s southern tip. The two most recent large Nicoya earthquakes (1950 Mw7.8; 2012 Mw7.6) generated decimeter-scale coseismic uplift along the central coast. The 2012 uplift pattern coincides with the area of mainshock slip, pre-event locking, and prior 1950 coseismic uplift. Most of the 1950 uplift was recovered by interseismic subsidence during six decades of strain accumulation leading to the 2012 rupture. While elastic strain accumulation and release produce short-term cycles of uplift and subsidence, long-term net uplift results in gradual coastal emergence and the growth of topographic relief. Net uplift along the central Nicoya segment may be the product of irrecoverable seismic-cycle strain (shortening), coupled with tectonic erosion near the trench and subsequent underplating of eroded material at depth beneath the peninsula. Our results are consistent with geophysical observations that indicate along-strike segmentation of the Nicoya seismogenic zone and the presence of three principal earthquake source areas: 1) Papagayo (1916, M>7.0), 2) Nicoya (1950, Mw7.8; 2012 Mw7.6), and 3) Cobano (1990 Mw7.3). Historic ruptures within all three segments have produced damaging and deadly earthquakes in the past and should be expected to do so again in the future.
The Hikurangi subduction margin along North Island, New Zealand accommodates oblique convergence of the Pacific Plate beneath the Australian plate at 45 mm/yr. Along the southern margin, pronounced frontal accretion and forearc uplift occur inboard of the subducting Hikurangi plateau, in an area of strong interface locking and deep slow slip. In the north, subduction erosion and serrated coastal uplift occur inboard of subducting seamounts on the plateau flank, in an area of weaker locking and shallow slow slip. Emergent marine terraces and paleo-shorelines along the Hikurangi margin record tectonic uplift over two time scales: 1) short-term episodic uplift events during the Holocene (0-10 ky), and 2) long-term net deformation during the Pleistocene (10-500 ky). Along the Wairarapa segment in the south, a narrow coastal lowland preserves uplifted Holocene terraces and beach ridges that record up to seven prehistoric earthquakes. LiDAR imagery allows for correlation of paleo-shorelines and field sites where 14C ages constrain paleo-earthquake timing. Up to four higher elevation Pleistocene marine terraces in this area characterize long-term uplift and margin-parallel folding, with OSL ages constraining deformation rates. Along the southern Hawke’s Bay segment (central Hikurangi margin), up to three Holocene terraces record paleo-earthquakes near Cape Kidnappers. LiDAR mapping, field surveying, and 14C dating constrain the timing and magnitude of coseismic uplift events. Several higher Pleistocene terraces characterize long-term uplift and deformation along the Kidnappers anticline. Along the northern Hawke’s Bay segment, a continuous flight of Holocene and Pleistocene marine terraces at Mahia Peninsula record outer forearc uplift and tilting above the upper-plate Lachlan thrust. Five steps within the Holocene terrace are interpreted as discrete coseismic uplift events. Along the Raukumara segment in the north, localized Pleistocene terraces and Holocene platforms occur at varying elevations, recording differential coastal uplift and paleo-earthquake events. Along the Hikurangi margin as a whole, tectonic uplift is a product of complex interactions between deep megathrust slip and shallow upper-plate faulting. Uplifted Holocene paleo-shorelines record single coseismic uplift events, while Pleistocene terraces characterize net deformation patterns. Continued mapping, field surveying, and age dating along strike of the Hikurangi margin will help differentiate between very large margin-wide megathrust ruptures (M8.0-9.0+) and smaller, more localized, upper-plate thrust events (M7.0-8.0). Both earthquake types pose a significant seismic and tsunami hazard for New Zealand residents.