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The last decade has provided unexpected lessons in the enormous risks from great subduction earthquakes: Sumatra 2004, Chile 2010, and Japan 2011 were each devastating, resulting in surprising impacts distinct from shallow seismic events. Similar large-magnitude earthquakes are known to occur on the Cascadia subduction zone (CSZ), with the potential of rupturing the entire 1100 km length of the Pacific Northwest plate boundary. Coseismic landslides represent one of the greatest risks to the millions of people living along the Cascadia Subduction Zone, from northern California to southern British Columbia. Empirically derived relationships between earthquake magnitude and landsliding are well studied, and suggest a magnitude 9 earthquake is likely to trigger thousands of landslides. Because a magnitude 9 subduction earthquake is well known to have occurred just over 300 years ago, evidence of coseismic landslides triggered by this event should still be present in the landscapes of the Washington and Oregon Coasts. We are systematically hunting for these landslides through field and LiDAR mapping and are using a combination of radiocarbon dating and surface roughness analysis, a method first developed to study landslides near to the Oso 2014 disaster site, to develop more robust regional landslide chronologies. In addition, we compare our results to new probabilistic quantification of ground motions from a M9 earthquake, including uncertainties, which is a novel approach to delivering synthetic seismograms for engineering and other purposes. With these new data, we hope to better characterize how the landscape will respond to the next large subduction zone earthquake in the Pacific Northwest.
The ~150 km wide dextral Marlborough Fault System (MFS) and adjacent Kaikōura Mountains accommodate oblique convergence between the Pacific and Australian plates at the southern end of the Hikurangi subduction zone, New Zealand. In this presentation, I will present results from recent investigations of both the longer-term and shorter-term deformation and landscape evolution in the MFS. New low-temperature thermochronology data from this region places limits on the timing and style of mountain building and the relationship between the mountains and adjacent faults. We sampled rocks for apatite and zircon (U-Th/He) and apatite fission track dating from a range of elevations spanning ~2 km within the Kaikōura Mountains, which stand high above the active Marlborough dextral faults. The data reveal Miocene cooling localized to hanging wall rocks, first along the Clarence Fault in the Inland Kaikōura Range, then along the Jordan Thrust in the Seward Kaikōura Range, followed by widespread, rapid cooling reflected in all samples across the study area starting at ~5 Ma. Pliocene to present rapid exhumation across the field site, including at low-elevation sample sites fault footwalls, may relate to an increase in relative plate convergence rates and new fault development in the eastern MFS at this time. Our results suggest that topographic relief in this region predates the onset of dextral faulting and that portions of the Marlborough Faults were once thrust faults that coincided with the early development of the transpressive plate boundary. I will also discuss results from studies of the near-fault geomorphology in response to dextral strike-slip fault motion over the last several thousands to 1 million years and comparison with complementary landscape evolution models. In particular, we examine the role of shutter ridges, as well as fault slip rate, in producing characteristic elements of strike-slip landscapes: long stream channel offsets that make the landscape “signature” of the fault more dramatic, and the process of stream capture, which reduces these offset lengths and therefore the signature of strike-slip faulting by re-directing stream flow paths.