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Lake Crescent, a 180-m-deep, glacially carved lake located on the Olympic Peninsula in western Washington, USA, overlies the Lake Creek-Boundary Creek fault zone, a system of structures with at least 56 km of late Pleistocene to Holocene surface rupture. Investigation of the lake’s sediment, including a reflection seismic survey and analysis of piston cores, reveals evidence that the fault beneath the lake has ruptured four times in the past ∼7200 years, producing unusually thick deposits termed megaturbidites. The earthquakes triggered rockslides that entered the lake and caused displacement waves (lake tsunamis) and seiches, most recently ca. 3.1 ka. Seismic reflection results from beneath the depth of core penetration reveal at least two older post-glacial ruptures that are likely to have similarly affected the lake. The stratigraphy of Lake Crescent provides insight into the behavior of a fault system that partially accommodates regional clockwise rotation and contraction of the northern Cascadia forearc through oblique dextral shear, and highlights the potential for disruption to critical infrastructure, transportation corridors, and industry on the North Olympic Peninsula during future surface-rupturing earthquakes. Our results illustrate the potential synergism between lacustrine paleoseismology and fault-scarp trench investigations. More precise dating of strong earthquake shaking afforded by continuous accumulation of lake sediment improves earthquake histories based on trenched fault scarp exposures, which are commonly poorly dated.

The creation of ocean crust by rapid injection of magma at mid-ocean ridges can lead to eruptions of lava onto the seafloor and release of “event plumes,” which are huge volumes of anomalously warm water enriched in reduced chemicals that rise up to 1 km above the seafloor. Here, we use seismic data to show that seafloor eruptions and the release of hydrothermal event plumes correspond to diking episodes with high injection velocities and rapid onset of magma emplacement within the rift zone. These attributes result from high excess magma pressure at the dike source, likely due to a new influx of melt from the mantle. These dynamic magmatic conditions can be detected remotely and may predict the likelihood of event plume release during future seafloor spreading events.

Never before has a volcanic eruption on a slow- or ultraslow- spreading mid-ocean ridge been both observed seismically and confirmed on the seafloor. During the first half of 1999, a long-lived volcanic-spreading event occurred on the ultraslow-spreading Gakkel Ridge in the Arctic Ocean. The seismicity associated with this event was unprecedented in duration and magnitude for a seafloor eruption. Sonar images from the U.S.S. Hawkbill, which passed over the area within four months of the start of activity, are consistent with the presence of a large, recently erupted flow and a volcanic peak directly in the area of seismic activity. Seismic activity began in mid-January and continued vigorously for three months; a reduced rate of activity persisted for an additional four months or more. In total, 252 events were large enough to be recorded on global seismic networks. Although a limited number of volcanic-spreading events have been observed globally, the duration and magnitude of the Gakkel Ridge swarm, when compared with volcanic seismicity at ridges spreading at intermediate and fast spreading rates, suggest that a negative power-law relationship may exist between these parameters and spreading rate. Fault activation, in response to magmatic emplacement, appears to have occurred over a broad region, suggesting that magma may have been tapped from mantle depths. The slow migration of the largest magnitude events along the axis of the rift valley suggests multiple magmatic pulses at depth. In combination with bathymetric setting and sidescan sonar confirmation, the seismic data for this event have provided a unique look at the scale and character of eruption processes at ultraslow-spreading rates.

New data from the eastern flank of the Mid-Atlantic Ridge (~25‐27°N) show that slow-spreading abyssal hill faults maintain maximum displacement-length ratios that are systematically greater than those reported on the fast-spreading East Pacific Rise. Lower displacement-length ratios in the fast-spreading environment may reflect the importance of fault linkage (rather than lateral propagation) in determining the lengths of abyssal hill faults and the limited ability of fault systems that evolve within an extremely thin lithosphere to acquire additional displacement during or following linkage.