09:15 〜 09:30
[SSS07-02] Seasonal Changes in Shallow Earth Structure in the Permafrost Region of Alaska from Seismic Noise
キーワード：Seasonal melting in permafrost region, Extreme changes in horizontal noise, Rayleigh-wave eigenfunctions
Using data from the EarthScope Transportable Array in Alaska, we report a surprising increase in horizontal seismic noise in summer. Our analysis is based on a simultaneous examination of (1) temperature, (2) vertical seismic data, (3) horizontal seismic data, and (4) wind data and can provide a unique perspective to the seasonally changing shallow process in the permafrost region.
Observations at some stations indicate a rapid, massive melting phenomena in the summer that causes a thousandfold increase in horizontal power spectral density (PSD). This rapid rise in horizontal seismic noise typically occurs in July, lasting about 30 days. The initiation of this major melting period does not happen immediately after the surface temperature exceeds 0oC; instead, there is a delay of about a month. After the peak horizontal amplitude is reached, it gradually returns to the pre-melting level. Many stations show that this return occurs by the end of the year, or by the end of December; some stations, however, require until March or April in the following year to arrive back to their pre-melting level. For all stations, this return occurs well after the surface temperature becomes negative in September or October. These observations suggest that the melt layer remains at depth as temperatures drop below freezing, perhaps sandwiched between the developing ice from the surface and the underlying permafrost ice. However, we noted some caution is required at a few stations because a transient surge in horizontal amplitudes seems to occur in February and November that appears to be correlated with winds. But that seems to be limited to a few stations near the west coast of Alaska. We summarize the temporal characteristics of individual stations, their seasonal geographic patterns throughout Alaska and a potential mechanism that can explain a sudden increase of horizontal noise with little effects on vertical noise.
Observations at some stations indicate a rapid, massive melting phenomena in the summer that causes a thousandfold increase in horizontal power spectral density (PSD). This rapid rise in horizontal seismic noise typically occurs in July, lasting about 30 days. The initiation of this major melting period does not happen immediately after the surface temperature exceeds 0oC; instead, there is a delay of about a month. After the peak horizontal amplitude is reached, it gradually returns to the pre-melting level. Many stations show that this return occurs by the end of the year, or by the end of December; some stations, however, require until March or April in the following year to arrive back to their pre-melting level. For all stations, this return occurs well after the surface temperature becomes negative in September or October. These observations suggest that the melt layer remains at depth as temperatures drop below freezing, perhaps sandwiched between the developing ice from the surface and the underlying permafrost ice. However, we noted some caution is required at a few stations because a transient surge in horizontal amplitudes seems to occur in February and November that appears to be correlated with winds. But that seems to be limited to a few stations near the west coast of Alaska. We summarize the temporal characteristics of individual stations, their seasonal geographic patterns throughout Alaska and a potential mechanism that can explain a sudden increase of horizontal noise with little effects on vertical noise.