Dynamically Triggered Seismic Activity in Alaska

Abstract:

Alaska is the most seismically active state of the USA. In the past 100 years, over nine thousand earthquakes occurred in Alaska with magnitudes greater than 4.5. The number of stations increased about fourfold after USArray (TA) deployed in Alaska. These new stations provide the opportunity to detect smaller earthquakes, including dynamically triggered earthquakes and tremor. Peterson and Christensen observed tectonic tremor in south-central Alaska between 1998 to 2001. Between 2006 and 2012, Gomberg and Prejean have found tremor spots in central mainland Alaska and in the islands of Unalaska and Akutan. Several studies also have reported dynamically triggered earthquakes in central Alaska. Because the current amount of seismic stations provides an overwhelming number of waveforms to analyze, we enlisted the help of over five thousand volunteer scientists through an online project, called “Earthquake Detective”. In our project, we ask volunteer scientists to classify weak signals from small, potentially triggered seismic events by listening to relevant sections of seismograms that are converted to audible frequencies. By comparing the volunteer scientists’ classifications with our own expert classifications for data from 5 of the 30 teleseismic earthquakes, we determined that volunteer scientists achieve over 90% accuracy in detecting earthquakes or tremor when at least 7 of 10 volunteer scientists agreed. Volunteer scientists detected earthquakes and tremor in central and southcentral Alaska within thousands of records of 30 teleseismic earthquakes with Mw > 7.5 from 2013 to 2018. These local earthquakes and tremor occurred during the first 30 minutes for surface wave propagation. However, some earthquakes were not associated with triggering. About 150 thousand earthquakes occurred in central and south-central Alaska from 2013 to 2018. Therefore, it is likely that some detections of earthquakes by volunteer scientists are part of this average rate of ~3 such earthquakes every hour. Nevertheless, the possibly triggered earthquake and tremor detections also emerged from volunteer scientists’ classifications. These local earthquake and tremor signals were recorded by stations clustered in central and southcentral Alaska. The signals may originate about 100 km to 250 km away from NE Anchorage, which is close to the place where Peterson and Christensen reported their tremor observations.