The Alaska Amphibious Community Seismic Experiment (AACSE) is a shoreline-crossing passive- and active-source seismic experiment that took place from May 2018 through August 2019 along an similar to 700 km long section of the Aleutian subduction zone spanning Kodiak Island and the Alaska Peninsula. The experiment featured 105 broadband seismometers; 30 were deployed onshore, and 75 were deployed offshore in Ocean Bottom Seismometer (OBS) packages. Additional strong-motion instruments were also deployed at six onshore seismic sites. Offshore OBS stretched from the outer rise across the trench to the shelf. OBSs in shallow water (<262 m depth) were deployed with a trawl-resistant shield, and deeper OBSs were unshielded. Additionally, a number of OBS-mounted strong-motion instruments, differential and absolute pressure gauges, hydrophones, and temperature and salinity sensors were deployed. OBSs were deployed on two cruises of the RN Sikuliaq in May and July 2018 and retrieved on two cruises aboard the RN Sikuliaq and RN Langseth in August-September 2019. A complementary 398-instrument nodal seismometer array was deployed on Kodiak Island for four weeks in May-June 2019, and an active-source seismic survey on the RN Langseth was arranged in June 2019 to shoot into the AACSE broadband network and the nodes. Additional underway data from cruises include seafloor bathymetry and sub-bottom profiles, with extra data collected near the rupture zone of the 2018 M-w 7.9 offshore-Kodiak earthquake. The AACSE network was deployed simultaneously with the EarthScope Transportable Array (TA) in Alaska, effectively densifying and extending the TA offshore in the region of the Alaska Peninsula. AACSE is a community experiment, and all data were made available publicly as soon as feasible in appropriate repositories.

Few constraints exist on the major element chemistry of ancient oceans. Turritellid marine snails precipitate aragonitic shells and are abundant in the Cenozoic fossil record, and therefore may be ideal for reconstructing past seawater chemistry. Here we report strontium to calcium (Sr/Ca) ratios of modern and early Cenozoic turritellid shells that still retain an aragonitic mineralogy. By applying partition coefficients determined from modern specimens to data for Paleocene and Eocene shells, we calculate paleo-seawater Sr/Ca values. The high Sr/Ca values recorded in fossil shells may imply that seawater Sr/Ca ratios varied from at least 9.5-15.2 mmol/mol, much higher than modern average seawater values (8.6 mmol/mol). High paleo-seawater Sr/Ca ratios may indicate that substantial changes in the biogeochemical cycling of Sr have occurred over the past similar to 37 million years. A decrease in seawater strontium concentrations could have arisen from an increase in the proportion of weathered radiogenic silicate rocks relative to carbonates, reducing the riverine flux of strontium to the oceans, or changes in the exposure and erosion of aragonitic carbonates on tropical shelves due to variations in sea level. (C) 2009 Elsevier B.V. All rights reserved.

The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (Mw > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (VP and VP/VS) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center-derived oceanic lithosphere is ∼5 km, revising earlier estimates suggesting ∼10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low VP along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge P-wave velocities suggest 20–30 per cent serpentinization across the Nicoya Peninsula region while calculated VP/VS values suggest 0–10 per cent serpentinization. Interpretation of VP/VS resolution at depth is complicated however due to ray path distribution. We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes.

New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (∼1 year) observation period. We also estimate northwest motion of a coastal “sliver block” at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos‐Caribbean relative motion is partitioned here, with ∼8 cm/yr on the Cocos‐Panama block boundary (including a component of permanent shortening across the Fila Costeña fold and thrust belt) and ∼1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from ∼10–50 km depth. This large locked zone, as well as associated forearc and back‐arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.

We use elemental ratio (Mg/Ca, Sr/Ca, U/Ca and Ba/Ca) and oxygen isotope data from a Porites lutea coral head collected from offshore Amédée Island, New Caledonia (22°29′S, 166°28′E) to assess the degree to which changes in these geochemical variables reflect variations in sea surface conditions. We have assessed the robustness of each geochemical proxy by comparing 25 years (1992–1968) of monthly geochemical variations with a local record of sea surface temperature (SST) and with the appropriate 1° by 1° grid box from a global gridded SST data set. We conclude from our comparison that the most consistent proxies of monthly SST variations at this site are δ 18 O (r 2 = −0.84; p = <.0001) and Sr/Ca (r 2 = −0.84; p = <.0001). The fidelity of the coral‐based Sr/Ca‐SST proxy was assessed via calibration‐verification exercises at New Caledonia and Rarotonga. The “paleoclimate” accuracy of the coral Sr/Ca‐SST technique is judged to be robust, and the precision of the technique is estimated to be ∼0.3° ± 0.5 (2σ) based on the calibration‐verification exercise. In contrast, monthly variations in Mg/Ca and U/Ca ratios are only moderately correlated with monthly SST variations (r 2 = 0.55; p = <.0001 for Mg/Ca and r 2 = 0.47; p = <.0001). The strength of the correlation between SST and Mg/Ca and U/Ca varies as a function of time, suggesting that variations in these ratios are not a simple function of SST variations. This variability in goodness of fit between U/Ca‐SST and Mg/Ca‐SST may ultimately limit the use of these ratios as coral paleothermometers. Averaging or stacking individual proxy SST records (Sr/Ca, Mg/Ca and U/Ca) to generate a composite proxy SST record does not necessarily improve the proxy record if the signal‐to‐noise ratio in each proxy is highly variable. For example, stacking of Sr/Ca‐Mg/Ca‐U/Ca records at New Caledonia produces a proxy SST time series that has greater standard error than the individual Sr/Ca‐SST time series (cf. ordinary least squares standard error of 1.16°C versus 0.86°C). Ba/Ca variations have little correlation with SST, Sr/Ca, Mg/Ca, or U/Ca variations at New Caledonia. In conclusion, variations in oxygen isotopic composition of seawater, calculated by removing the Sr/Ca‐based temperature component of the δ 18 O signal, agree reasonably well with observed variations in sea surface salinity, especially at the interannual timescale.

Although many arid badland channels erode rapidly, channel flows appear to be rare, raising the issue of the character and frequency of flows responsible for channel erosion, Using an acoustic stream gauge, we recorded nine flash flood hydrographs over a 3 yr period from the Upper Blue Hills badlands, Utah, with maximum discharges up to similar to 0.9 m(3)/s. Flow hydrographs reveal bores and rapid depth changes that are similar to flash floods observed elsewhere. Bore and hydrograph peak translation velocities are greatest in narrow channel segments, Rapid runoff generation during short-duration thunderstorms produced complex hydrographs whose shapes appear to reflect channel network geometry, Storm runoff response is highly sensitive to antecedent moisture, which greatly reduces the regolith infiltration capacity, High antecedent moisture coupled with a relatively low intensity, long-duration rainstorm produced the largest flow event. Estimating flow frequency in this landscape therefore requires knowledge of the distributions of both storm sizes and temporal spacing relative to the short time required for the regolith infiltration capacity to recover following wetting, here roughly 24 hr, Landscape changes can be produced not only by rare, large rainfall events, but by a broad range of storm size and frequency under optimal antecedent moisture conditions.

The Nd and Sr isotopic compositions of Late Pleistocene to Recent mafic to silicic volcanic rocks from the Inyo Domes and Mono Craters were analyzed to constrain the origin and evolution of magmas erupted during the most recent cycle of silicic volcanism in the Long Valley region. Whole rock samples of biot-, opx-, and ol-bearing rhyolites from the Mono Craters have similar epsilon/sub Nd/(0) and /sup 87/Sr//sup 86/Sr ratios, ranging from 0.9 to -1.4, and .70633 to .70680, respectively. Intermediate composition inclusions in the Mono Crater rhyolites have slightly higher epsilon/sub Nd/(0)=2.90 and lower /sup 87/Sr//sup 86/Sr=.70526, which suggests that the rhyolites evolved via crustal assimilation from more mafic, higher epsilon/sub Nd/(0), parental magmas. However, young basaltic rocks at June Lake and Black Point have epsilon/sub Nd/(0)=-1.5 and 1.6, while the Mesozoic batholithic crust in the region has epsilon/sub Nd/(0)=-2.0, suggesting that the mantle-derived magmas parental to the rhyolites, and the assimilated crust, may have differed only by 2-4 epsilon/sub Nd/ units. Therefore, the uniform isotopic compositions of the Mono Crater rhyolites, Inyo Domes rhyolites, and Bishop Tuff may only reflect the similar isotopic compositions of the mantle and crustal source regions, and do not require that the rhyolites weremore » all derived from the same magma body. More precise isotopic measurements will be required to establish the proportions of mantle and crustally-derived components in the rhyolites, and to determine if this proportion varies with time or rhyolite composition. The apparent similarity in the isotopic compositions of the upper mantle and continental crust in this region is enigmatic, but could be an artifact of extensive mass transfer between the upper mantle and lower continental crust during the subduction-related Mesozoic magmatism.« less