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Abstract:
We analyzed data from the EarthScope Transportable Array in Alaska in order to understand the nature of near-surface melting in the permafrost region. Data consisted of (1) temperature, (2) vertical seismic data, (3) horizontal seismic data, and (4) wind data. Observations at some stations indicate a rapid melting phenomena in the summer that causes a thousandfold increase in horizontal power spectral density. 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 December. However, some stations 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. This suggests that the melt layer remains at depth as surface 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 occurs in February and November that appears to be correlated with winds. We summarize the temporal characteristics of individual stations, their seasonal geographic patterns throughout Alaska and a potential mechanism to explain a sudden increase of horizontal noise.
