Ocean Storms

Ocean storms are known to be the primary producers of secondary microseisms. These signals are formed from pressure fluctuations at the ocean floor due to large standing waves caused by ocean waves of similar frequency traveling in opposite directions. Ocean storms, hence, have a strong influence in producing secondary microseisms. Seismic data thus becomes an important dataset that predates the satellite era in monitoring ocean storms and studying their decadal patterns. Learning the changes in patterns and intensity of storm systems is becoming extremely important, especially during times of dramatic climate change.

The movie on the right shows the ocean wave heights in conjunction with the power of secondary microseisms. The arrow mark in the movie points towards the maximum gradient direction of Secondary microseisms. The length of the arrow is plotted in proportion to the magnitude of the gradient. This movie illustrates the clear influence of storms in secondary microseismic noise. To learn more, please refer to my publication (expected to be out soon).

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Sea ice

Sea ice coverage in the Arctic Ocean fluctuates with the seasons, peaking around March and reaching its lowest point near September. The seasonal growth of sea ice is known to reduce the generation of microseisms because sea ice coverage prevents the formation of ocean waves. The effect of sea ice on microseism is strongest in the short-period secondary microseismic band (SPSM).

The movie on the right shows the evolution of short-period secondary microseismic power at station A21K, sea ice, and wave height in northern Alaska. We can see that the SPSM amplitude follows the ocean wave amplitude in coastal waters. It is also evident that the onset of sea ice significantly dampens SPSM. This means that SPSM can be used to study sea ice in coastal oceans. Monitoring coastal sea ice is important because arctic communities depend on it for transport and subsistence. Other implications of the increased melting of sea ice are accelerated coastal erosion and increasing shipping traffic in the region. To learn more, please refer to my publication (expected to be out soon).

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Seasonal and spatial patterns of microseisms

Microseisms exhibit very different trends both seasonally and spatially. The Secondary microseism (SM) noise floor is generally consistent across Alaska but rises when moving from north to south. Short period secondary microseism (SPSM), however, are stronger near the coast and attenuates significantly toward the inland. Seasonal changes of both of these signals are overlaid on these spatial trends.

These differences in seasonal and spatial patterns arise due to the influence of various environmental factors and the signal's attenuation properties. Carefully studying these trends provides new insights into the storm systems, sea ice, and coastal erosion in the region. We find the Gulf of Alaska as the dominant source of SM in Alaska, while SPSM is produced in the coastal ocean. We also find that sea ice in the northern ocean significantly attenuates SPSM, but SM is unaffected by sea ice. By comparing sea ice and shorefast ice datasets, we also find that SPSM clearly marks the open ocean (ice-free) period when the coast is susceptible to erosion. We also find that the amplitude of SPSM is proportional to the amplitude of coastal ocean waves, which causes erosion, a growing concern in the region.

Resonance of microseisms due to Bathymetry

The bathymetry of the ocean plays a vital role in resonation secondary microseisms. The animation on the right shows the evolution of a noticeable storm in the Gulf of Alaska and the corresponding secondary microseisms recorded in the northeast region. We can clearly see that the microseismic amplitude jumps up when the arm of the storm strikes the eastern coast of Alaska. Kedar et al., 2008 shows that the bathymetry of the Gulf of Alaska, especially the eastern Gulf, is conducive for resonating secondary microseisms.

Also, the coastline of Alaska is roughly perpendicular to the incoming storms here. This could result in a reflection of ocean waves traveling in the opposite direction to the incoming storm. The waves that travel in opposite directions with similar frequencies can form standing waves, and secondary microseisms form in this kind of setting. This could be another reason for the increased secondary microseism generation in the region. Whatever the cause, it is clear that the Gulf of Alaska resonates secondary microseism efficiently.

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Glacial Quakes

Alaska's glaciers are thinning at an alarming rate, contributing substantially to global sea level rise. This is partly due to the large volume of Glaciers in Alaska (Millan et al., 2022) but also due to the warming climate in the region. The large glaciers of Alaska contain a huge volume of glacier ice, enough to raise global sea levels by a total of 46.4 mm if it all melted (Millan et al., 2022). Around 25 % of global ice loss is from Alaska, and that amounts to 66.7 billion tons each year (Hugonnet et al., 2021). At this rate, all Alaskan glaciers could disappear in 250 years. Unfortunately, studies suggest that the rate of glacier melting in Alaska is accelerating (Berthier et al., 2018). Columbia glacier in Alaska is a world reference glacier, and its recession has been monitored for several decades.

Large calving events at the glacier terminus of Columbia Glacier cause cavity formation and collapse in the water. Such collapse generates strong energy and produces glacial quakes (Bartholomaus et al., 2012). The figure on the right shows the relocated yearly glacial quakes in Columbia Glacier using the hypoDD algorithm. hypoDD is based on Double-differencing, a technique used in earthquake location to improve the precision of hypocenter determinations by comparing the travel-time differences between two closely spaced events recorded at the same station (Waldhauser, 2000). Here, we see that the glacial quakes migrate along with the glacier terminus, suggesting an origin from glacial calving.

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