The Oaxaca subduction zone is an ideal area for detailed studies of plate boundary deformation as rapid convergent rates, shallow subduction, and short trench‐to‐coast distances bring the thermally defined seismogenic and transition zones of the plate interface over 100 km inland. Previous analysis of slow slip events in southern Mexico suggests that they may represent motion in the transition zone, defining the downdip edge of future megathrust earthquakes. A new deployment consisting of broadband seismometers distributed inland along the Oaxaca segment provide the means to examine whether nonvolcanic tremor (NVT) signals can also be used to characterize the boundary between the seismogenic and transition zones. In this study, we established that NVT exists in the Oaxaca region based on waxing and waning of seismic energy on filtered day‐long seismograms that were correlated across neighboring stations and were further supported by appropriate relative time moveouts in record sections and spectrograms with narrow frequency bands. Eighteen prominent NVT episodes that lasted upwards of a week were identified during the 15 months analyzed (June 2006 to September 2007), recurring as frequently as every 2–3 months in a given region. We analyze NVT envelope waveforms with a semiautomated process for identifying prominent energy bursts, and analyst‐refined relative arrival times are inverted for source locations. NVT burst epicenters primarily occur between the 40–50 km contours for depth of the plate interface, except in eastern Oaxaca where they shift toward the 30 km contour as the slab steepens. NVT hypocenters correlate well with a high conductivity zone that is interpreted to be due to slab fluids. NVT is more frequent, shorter in duration, and located further inland than GPS‐detected slow slip, while the latter is associated with a zone of ultra‐slow velocity interpreted to represent high pore fluid pressure. This zone of slow slip corresponds to approximately 350°C–450°C, with megathrust earthquakes, microseismicity, and strong long‐term coupling occurring immediately updip from it. This leaves NVT primarily in a region further inland from the thermally defined transition zone, suggesting that transition from locking to free slip may occur in more than one phase.

The southwestern coast of Mexico is marked by active subduction of the Rivera and Cocos plates, producing megathrust earthquakes that tend to recur every 50-100 years. Herein, we use seismic and GPS data from this region to investigate the potential relationship between earthquakes, tectonic (non-volcanic) tremor, and transient slip along the westernmost 200 km of the Mexico subduction zone. Visual examination of seismograms and spectrograms throughout the 18-month-long MARS seismic experiment reveals clear evidence for frequent small episodes of tremor along the Rivera and Cocos subduction zones beneath the states of Jalisco, Colima, and Michoacán. Using a semi-automated process that identifies prominent energy bursts in envelope waveforms of this new data, analyst-refined relative arrival times are inverted for source locations using a 1-D velocity model. The resulting northwest-southeast trending linear band of tremor is located downdip from the rupture zones of the 1995 Mw 8.0 Colima-Jalisco and 2003 Mw 7.2 Tecoman subduction-thrust earthquakes and just below the regions of afterslip triggered

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We report estimated stable isotope compositions of depositional waters and paleoprecipitation from the Cretaceous Arctic to further elucidate the role of the global hydrologic cycle in sustaining polar warmth during that period. Estimates are based on new hydrogen isotopic analyses of n-alkane biomarkers extracted from Late Cretaceous and mid-Cretaceous terrestrial deposits in northern Alaska and the Canadian High Arctic. We integrate these new results with earlier published work on oxygen isotopic analyses of pedogenic siderites, dinosaurian tooth enamel phosphates, and pedogenic clay minerals from the same field areas. Average Late Cretaceous δD values of −143‰ VSMOW corresponded with average δ18O values of −24.1‰ VSMOW, and average mid-Cretaceous δD values of −106‰ VSMOW corresponded with average δ18O values of −22.1‰ VSMOW. The distributions of water isotope δD and δ18O values from Cretaceous Arctic deposits do not intersect with the Global Meteoric Water Line, suggesting an apparent deuterium excess ranging from about 40 to 60 per mil. We considered several possible explanations for these Cretaceous results including (1) mass-balance changes in zonal patterns of evaporation and precipitation at lower latitudes, (2) concentration of 2H in leaf tissue waters from continuous transpiration by coniferous paleofloras during the Arctic growing season, and (3) concentration of 2H in the groundwaters of methane-emitting Arctic soils.