Enigmatic seismic excitation of the 1957 Andreanof, Aleutian Is., earthquake

SUMMARY The 1957 Andreanof, Aleutian Is., earthquake (1957 March 9, 51.53°N, 175.63°W, d = 25 km) is among the most enigmatic great earthquakes instrumentally recorded. The length of the aftershock area is very long (about 1200 km), and tsunami excitation has been recently confirmed to be very extensive, yet its instrumental seismic magnitude Ms is only about 8.1 to 8.3. Detailed analyses of long-period surface waves in the past gave an Mw = 8.4, and the seismic-tsunami disparity remains unresolved. The main difficulty in seismic studies is the absence of high-quality seismic data. Here we investigate the cause of this disparity by carefully analysing some historical seismograms with modern digitization methods. We also take advantage of the 1996 Aleutian Is. earthquake (Mw = 7.87) that occurred very close to the 1957 event. For the 1996 event, high-quality modern broad-band seismograms are available which can be effectively used as empirical Green’s functions for the analysis of the 1957 event. Using the Wiechert (Strasbourg, Uppsala), Milne–Shaw (Wellington) and Benioff (Uppsala, Pasadena) seismograms of the 1957 event, we could determine that the 1957 event had significant secondary excitation of long-period (150 s) waves during about 1000 s following the first event. The Mw of the combined source is approximately 8.4. Because of the limited bandwidth of the old instruments, we cannot detect long-period energy beyond 150 s. However, the unusually long-lasting excitation over nearly 1000 s suggests that the event had significant excitation at periods longer than 150 s with a much larger Mw for the total event. Although we cannot address this question quantitatively because of our band-limited data, our numerical experiment using a source with a slow component shows that if the time scale of the slow source is longer than 500 s, our data can be made compatible with an Mw = 8.8 to 8.9 event, thereby reconciling the results from seismic and tsunami data.