Search for: All records

Due to limited observational coverage, monitoring the warming of the global ocean, especially the deep ocean, remains a challenging sampling problem. Seismic ocean thermometry (SOT) complements existing point measurements by inferring large‐scale averaged ocean temperature changes using the sound waves generated by submarine earthquakes, calledTwaves. We demonstrate here that Comprehensive Nuclear‐Test‐Ban Treaty Organization (CTBTO) hydrophones can recordTwaves with a higher signal‐to‐noise ratio compared to a previously used land‐basedT‐wave station. This allows us to use small earthquakes (magnitude <4.0), which occur much more frequently than large events, dramatically improving the resulting temporal resolution of SOT. We also find that the travel time changes ofTwaves at the land‐basedT‐wave station and the CTBTO hydrophone show small but systematic differences, although the two stations are only about 20 km apart. We attribute this feature to their different acoustic mode components sampling different parts of the ocean. Applying SOT to two CTBTO hydrophones in the East Indian Ocean reveals signals from decadal warming, seasonal variations, and mesoscale eddies, some of which are missing or underestimated in previously available temperature reconstructions. This application demonstrates the great advantage of hydrophone stations for global SOT, especially in regions with a low seismicity level.

 

Seismically generated sound waves that propagate through the ocean are used to infer temperature anomalies and their vertical structure in the deep East Indian Ocean. TheseTwaves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, aTwave's travel time changes in response to temperature anomalies along the wave's path. What part of the water column the travel time is sensitive to depends on the frequency of the wave, so measuring travel time changes at a few low frequencies constrains the vertical structure of the inferred temperature anomalies. These measurements reveal anomalies due to equatorial waves, mesoscale eddies, and decadal warming trends. By providing direct constraints on basin‐scale averages with dense sampling in time, these data complement previous point measurements that alias local and transient temperature anomalies.

 

Strong small‐scale seismic scatters (<10 km) have been recently observed at 660 km depth, but their origin remains uncertain. We systematically conduct both high‐resolution 2‐D geodynamic computations that include realistic thermodynamic properties, synthetic seismic waveforms, and insight from shallow seismic observations to explore their origin. We demonstrate that neither short‐term subduction, nor long‐term mechanical mantle mixing processes can produce sufficiently strong heterogeneities to explain the origin of such small‐scale seismic scatters. Instead, the intrinsic heterogeneities inside the oceanic lithosphere which subducts into the mantle transition zone and the uppermost lower mantle can explain the observed short‐wavelength scatter waves.

 

Topography, or depth variation, of certain interfaces in the solid Earth can provide important insights into the dynamics of our planet interior. Although the intermediate- and long-range topographic variation of the 660-kilometer boundary between Earth’s upper and lower mantle is well studied, small-scale measurements are far more challenging. We found a surprising amount of topography at short length scale along the 660-kilometer boundary in certain regions using scattered P'P' seismic waves. Our observations required chemical layering in regions with high short-scale roughness. By contrast, we did not see such small-scale topography along the 410-kilometer boundary in the upper mantle. Our findings support the concept of partially blocked or imperfect circulation between the upper and lower mantle. 

Vigorous convection in Earth's outer core led to the suggestion that it is chemically homogeneous. However, there is increasing seismic evidence for structural complexities close to the outer core's upper and lower boundaries. Both body waves and normal mode data have been used to estimate awave velocity,, at the top of the outer core (thelayer), which is lower than that in the Preliminary Reference Earth Model. However, these lowmodels do not agree on the form of this velocity anomaly. One reason for this is the difficulty in retrieving and measuringarrival times. To address this issue, we propose a novel approach using data from seismic arrays to iteratively measure‐differential travel times. This approach extracts individualsignal from mixed waveforms of theseries, allowing us to reliably measure differential travel times. We successfully use this method to measuretime delays from earthquakes in the Fiji‐Tonga and Vanuatu subduction zones.time delays are measured by waveform cross correlation betweenand, and the cross‐correlation coefficient allows us to access measurement quality. We also apply this iterative scheme to syntheticseismograms to investigate the 3‐D mantle structure's effects. The mantle structure corrections are not negligible for our data, and neglecting them could bias theestimation of uppermost outer core. After mantle structure corrections, we can still see substantial time delays of,, and, supporting a lowat the top of Earth's outer core.