ALBERT

Description: We summarize the importance of great earthquakes (Mw ≳ 8) for hazards, stratigraphy of basin floors, and turbidite lithology along the active tectonic continental margins of the Cascadia subduction zone and the northern San Andreas Transform Fault by utilizing studies of swath bathymetry visual core descriptions, grain size analysis, X-ray radiographs and physical properties. Recurrence times of Holocene turbidites as proxies for earthquakes on the Cascadia and northern California margins are analyzed using two methods: (1) radiometric dating (14C method), and (2) relative dating, using hemipelagic sediment thickness and sedimentation rates (H method). The H method provides (1) the best estimate of minimum recurrence times, which are the most important for seismic hazards risk analysis, and (2) the most complete dataset of recurrence times, which shows a normal distribution pattern for paleoseismic turbidite frequencies. We observe that, on these tectonically active continental margins, during the sea-level highstand of Holocene time, triggering of turbidity currents is controlled dominantly by earthquakes, and paleoseismic turbidites have an average recurrence time of ~550 yr in northern Cascadia Basin and ~200 yr along northern California margin. The minimum recurrence times for great earthquakes are approximately 300 yr for the Cascadia subduction zone and 130 yr for the northern San Andreas Fault, which indicates both fault systems are in (Cascadia) or very close (San Andreas) to the early window for another great earthquake. On active tectonic margins with great earthquakes, the volumes of mass transport deposits (MTDs) are limited on basin floors along the margins. The maximum run-out distances of MTD sheets across abyssal-basin floors along active margins are an order of magnitude less (~100 km) than on passive margins (~1000 km). The great earthquakes along the Cascadia and northern California margins cause seismic strengthening of the sediment, which results in a margin stratigraphy of minor MTDs compared to the turbidite-system deposits. In contrast, the MTDs and turbidites are equally intermixed on basin floors along passive margins with a mud-rich continental slope, such as the northern Gulf of Mexico. Great earthquakes also result in characteristic seismo-turbidite lithology. Along the Cascadia margin, the number and character of multiple coarse pulses for correlative individual turbidites generally remain constant both upstream and downstream in different channel systems for 600 km along the margin. This suggests that the earthquake shaking or aftershock signature is normally preserved, for the stronger (Mw ≥ 9) Cascadia earthquakes. In contrast, the generally weaker (Mw = or

Description: We relate the late Holocene northern San Andreas fault (NSAF) paleoseismic history developed using marine sediment cores along the northern California continental margin to a similar dataset of cores collected along the Cascadia margin, including channels from Barclay Canyon off Vancouver Island to just north of Monterey Bay. Stratigraphic correlation and evidence of synchronous triggering imply earthquake origin, and both temporal records are compatible with onshore paleoseismic data. In order to make comparisons between the temporal earthquake records from the NSAF and Cascadia, we refine correlations of southern Cascadia great earthquakes, including the land paleoseismic record. Along the NSAF during the last approximately 2800 yr, 15 turbidites, including one likely from the great 1906 earthquake, establish an average repeat time of approximately 200 yr, similar to the onshore value of approximately 240 yr. The combined land and marine paleoseismic record from the southern Cascadia subduction zone includes a similar number of events during the same period. While the average recurrence interval for full-margin Cascadia events is approximately 520 yr, the southern Cascadia margin has a repeat time of approximately 220 yr, similar to that of the NSAF. Thirteen of the 15 NSAF events were preceded by Cascadia events by approximately 0-80 yr, averaging 25-45 yr (as compared to approximately 80-400 yr by which Cascadia events follow the NSAF). Based on the temporal association, we model the coseismic and cumulative postseismic deformation from great Cascadia megathrust events and compute related stress changes along the NSAF in order to test the possibility that Cascadia earthquakes triggered the penultimate, and perhaps other, NSAF events. The Coulomb failure stress (CFS) resulting from viscous deformation related to a Cascadia earthquake over approximately 60 yr does not contribute significantly to the total CFS on the NSAF. However, the coseismic deformation increases CFS on the northern San Andreas fault (NSAF) by up to about 9 bars offshore of Point Delgada, most likely enough to trigger that fault to fail in north-to-south propagating ruptures.

Description: Integrated Ocean Drilling Programme (IODP) Expedition 308 studied overpressure and fluid flow on the Gulf of Mexico continental slope. The scientific program examined how sedimentation, overpressure, fluid flow, and deformation are coupled in a passive continental margin setting. The expedition investigated the model of how extremely rapid deposition of finegrained mud leads to rapid build-up of pore pressure in excess of hydrostatic (overpressure), underconsolidation and continental slope instability. Expedition 308 tested this model by examining how physical properties, pressure, temperature, and pore fluid compositions vary within low-permeability mudstones that overlie a permeable, overpressured aquifer. Three sites were drilled in the Ursa Basin off the Mississippi Delta, using the research drillship R/V JOIDES RESOLUTION (Fig. 1). In the Ursa Basin rapid, late Pleistocene sedimentation was known to be present. Drilling documented severe overpressure in the mudstones overlying the aquifer. The most important achievement of IODP Expedition 308 is to have successfully recorded in situ formation pressure and temperature in an overpressured basin. This is the first time that a coherent data set of such measurements has been obtained.