Source Parameter Scaling and the Cascadia Paleoseismic Record

Several approaches to interpreting the Cascadia paleoseismic record are used to derive relationships between fault area, slip, and moment and to compare the results with the scaling relationships determined by Somerville et al. (2015) for recent subduction-zone events. In two models (CA12a and CA12b), taken from Goldfinger et al. (2012), paleoevents are classified into five characteristic areas (CA), with the slip during each event estimated based on the time between the event and either the following or the previous event. In model CA14, taken from Scholz (2014), slip on four characteristic segments is determined from the plate tectonic convergence rate, assuming a constant stress drop. In model CL, introduced in this article, the fault length for paleoevents is defined by the along-strike length over which the observations have been correlated; width and slip are interpolated from model CA14. CA12a and CA12b show large scatter compared with the global compilation because of large variations in slip for a given area. Models CA14 and CL reproduce the relationship derived for asperities (defined as patches in finite-fault models with slip >1:5 times the average slip). These models can be reconciled with the total area and average slip from Somerville et al. (2015) by increasing the fault area and decreasing the slip using scaling factors derived from the analysis of recent earthquakes (CLmod1) or by reducing the slip by a factor of ∼8 (CLmod2). CLmod1 implies that the paleoearthquake observations are controlled by high-slip patches, whereas CLmod2 implies that much of the plate tectonic convergence is accommodated aseismically. A scenario intermediate between CLmod1 and CLmod2 is considered most likely. This study demonstrates the value of using scaling relationships based on modern earthquakes as a tool for evaluating earthquake histories derived from paleoseismic data

Subsurface temperatures beneath Southern Hydrate Ridge

During Ocean Drilling Program Leg 204, 80 in situ measurements of subseafloor temperature were made; 68 of these showed good frictional pulses on insertion and extraction from the seafloor and were used to constrain the subsurface temperature. Considering uncertainties from various sources, uncertainties in the in situ temperatures are estimated to generally be less than ±0.3°C. The data are consistent with a purely conductive temperature regime at all sites, and there is no resolvable difference in heat flow between sites on the flanks of southern Hydrate Ridge and sites near the summit, where other data indicate that free gas is venting into the ocean, gas hydrate is forming rapidly, and free gas and gas hydrate coexist in the sediments. We interpret this apparent paradox to indicate that the aqueous fluid flow is decoupled from free gas flow and that advection of free gas does not have a significant effect on the temperature field. The temperature data, which include several measurements within a few meters of the predicted base of the methane hydrate stability field (calculated for the measured pore water salinity at each site) also indicate that the bottom-simulating reflection (BSR) corresponds to the base of gas hydrate stability within measurement uncertainties, although a systematic shallowing of the BSR by as much as 10 m is possible. The heat flow indicated by the Leg 204 measurements and the regional BSR depth is significantly lower than the heat flow predicted based on the age of the subducting plate and the thickness of the accretionary complex. Several measurements made near the summit at depths shallower than 60 meters below seafloor show anomalous behavior consistent with low in situ thermal conductivity, possibly because of the presence of free gas and/or massive gas hydrate in these sediments

Seismicity and structure of the Orozco transform fault

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982.Microfiche copy available in Archives and ScienceVita.Bibliography: leaves 312-321.by Anne Martine Tréhu.Ph.D

Relationship between subduction erosion and the up‐dip limit of the 2014 Mw 8.1 Iquique earthquake

The aftershock distribution of the 2014 Mw 8.1 Iquique earthquake offshore northern Chile, identified from a long‐term deployment of ocean bottom seismometers installed eight months after the mainshock, in conjunction with seismic reflection imaging, provides insights into the processes regulating the up‐dip limit of coseismic rupture propagation. Aftershocks up‐dip of the mainshock hypocenter frequently occur in the upper plate and are associated with normal faults identified from seismic reflection data. We propose that aftershock seismicity near the plate boundary documents subduction erosion that removes mass from the base of the wedge and results in normal faulting in the upper plate. The combination of very little or no sediment accretion and subduction erosion over millions of years has resulted in a very weak and aseismic frontal wedge. Our observations thus link the shallow subduction zone seismicity to subduction erosion processes that control the evolution of the overriding plate. Key Points: - We investigate structure and seismicity at the up-dip end of the 2014 Iquique earthquake rupture using amphibious seismic data. - Seismicity up-dip of the 2014 Iquique earthquake occurs over a broad range likely interpreted to be related to the basal erosion processes. - Coseismic stress changes and aftershocks activate extensional faulting of the upper plate and subduction erosion

Transpression between two warm mafic plates: The Queen Charlotte Fault revisited

The Queen Charlotte Fault is a transpressive transform plate boundary between the Pacific and North American plates offshore western Canada. Previous models for the accommodation of transpression include internal deformation of both plates adjacent to the plate boundary or oblique subduction of the oceanic plate; the latter has been the preferred model. Both plates are warm and mafic and have similar mechanical structures. New multichannel seismic reflection data show a near-vertical Queen Charlotte Fault down to the first water bottom multiple, significant subsidence east of the Queen Charlotte Fault, a large melange where the fault is in a compressive left step, and faulting which involves oceanic basement. Gravity modeling of profiles indicates that the Pacific plate is flexed downward adjacent to the Queen Charlotte Fault. Upward flexure of North America along with crust thickened relative to crust in the adjacent basin creates topography known as the Queen Charlotte Islands. Combined with other regional studies, these observations suggest that the plate boundary is a vertical strike-slip fault and that transpression is taken up within each plate

Seismic attribute inversion for velocity and attenuation structure using data from the GLIMPCE Lake Superior experiment

A simultaneous inversion for velocity and attenuation structure using multiple seismic attributes has been applied to refraction data from the 1986 GLIMPCE Lake Superior experiment. The seismic attributes considered include envelope amplitude, instantaneous frequency, and travel time of first arrival data. Instantaneous frequency is converted to t* using a matching procedure which approximately removes the effects of the source spectra. The derived seismic attributes are then used in an iterative inversion procedure referred to as AFT inversion for amplitude, (instantaneous) frequency, and time. Uncertainties and resolution of the velocity and attenuation models are estimated using covariance calculations and checkerboard resolution maps. A simultaneous inversion of seismic attributes from the GLIMPCE data results in a velocity model similar to that of previous studies across Lake Superior. A central rift basin and a northern basin are the most prominent features with an increase in velocity near the Isle Royale fault. Although there is an indication of the central and northern basins in the attenuation model for depths greater than 4 km, the separation is not evident for shallower depths. This may result from microstructures masking compositional variations in the attenuation model for shallower depths. Attenuation Q values range from approximately 60 near the surface to ear 500 at 10 km depth. A relationship between inverse Q and velocity of Q¯¹=0.0210-0.0028*v was found between Q¯¹ and velocity beneath Lake Superior which supports previous laboratory results. The invereted velocity and attenuation models provide important constraints on the lithology and physical properties of the Midcontinent rift beneath Lake Superior.Copyrighted by American Geophysical Union

Seismogenic up-dip limit of the 2014 Mw 8.1 Iquique earthquake links subduction erosion and upper plate deformation

The 2014 Mw 8.1 Iquique earthquake ruptured the boundary between the subducting Nazca Plate and the overriding South American Plate in the North Chilean subduction zone. The broken segment of the South American subduction zone had likely accumulated elastic strain since an M~9 earthquake in 1877 and what therefore considered a mature seismic gap. The moderate magnitude of the 2014 earthquake and its compact rupture area, which only broke the central part of the seismic gap, did not result in a significant tsunami in the Pacific Ocean. To investigate the seismo-tectonic segmentation of the North Chilean subduction zone in the region of the 2014 Iquique earthquake at the shallow seismic/aseismic transition, we combine two years of local aftershock seismicity observations from ocean bottom seismometers and long- offset seismic reflection data from the rupture area. Our study links short term deformation associated with a single seismic cycle to the permanent deformation history of an erosive convergent margin over millions of years. A high density of aftershocks following the 2014 Iquique earthquake occurred in the up-dip region of the coseismic rupture, where they form a trench parallel band. The events spread from the subducting oceanic plate across the plate boundary and into the overriding continental crust. The band of aftershock seismicity separates a pervasively fractured and likely fluid-filled marine forearc farther seaward from a less deformed section of the forearc farther landward. At the transition, active subduction erosion during the postseismic and possibly coseismic phases of the 2014 Iquique earthquake leads to basal abrasion of the upper plate and associated extensional faulting of the overlying marine forearc. Landward migration of the seismogenic up-dip limit, possibly at similar rates compared to the trench and the volcanic arc, leaves behind a heavily fractured and fluid-filled outermost forearc. This most seaward part of the subduction zone might be too weak to store sufficient elastic strain to nucleate a large megathrust earthquake