A sequence of ML 5+ earthquakes in 2000 at the southern end of the Minto Flats seismic zone.
On November 29, 2000 people living in interior Alaska were awakened around 2 a.m. by the strong shaking caused by two magnitude 5+ earthquakes. The shocks occurred within 40 sec of each other, which made it impossible to obtain a reliable location for the second event. Analysis of the seismograms recorded at stations MCK and COLA indicates, however, that these shocks originated at the same location, given that the S-P times are identical for both of them. The magnitude of the first event was estimated between 5.6 (ML AEIC) and 5.8 (MW CMT). The second event was somewhat smaller at about magnitude 5.3. The AEIC located the main shock at 63.88N and 150.15W at 22.47 km depth. Another significant aftershock (ML 5.0) occurred on December 6, 2000 and was located at 63.91N and 150.28W at a depth of 18.71 km.
Over 60 aftershocks of the ML 5.6 earthquake were located through the end of December 2000 by the AEIC. These earthquakes together with the regional seismicity were relocated using the Joint Hypocenter Determination (JHD) method. Only 32 aftershocks have been successfully relocated (see figure below). The main shock has moved to a shallower depth of 19.6 km and is positioned at 63.85N and 150.26W. The JHD location of the ML 5.0 aftershock is 63.86N and 150.32W, 13.77 km deep. Aftershocks align along a SSW-NNE striking plane dipping to the east and are located between 5 and 45 km depth. There were no earthquakes below 20 km depth in the ML 5.6 source region prior to this sequence. The main shock is located at the edge of the deepest crustal seismicity that have been previously recorded.
The moment tensor solutions for the main shock and two of its largest aftershocks were obtained using regional broadband data (see Figure below). One fault plane of the main shock solution is striking at 12o and dipping at 79o, and the second plane's strike and dip are 106o and 70o, respectively. The calculated MW magnitude is 5.6.
In summary, the sequence of magnitude 5+ earthquakes in the interior that occurred in November and December 2000 provides additional information for understanding of interior Alaska tectonics. These earthquakes are interesting in several ways. First of all, the main shock occurred at a depth of 20 km where no earthquakes were previously observed, while some of the aftershocks are located even deeper. Secondly, the north-south striking fault plane is rotated by about 20o from the general trend of seismicity that composes the Minto Flats seismic zone. Closer examination of the fault plane solutions available for the earthquakes within the MFSZ shows, that the general trend of fault plane orientations changes from SW-NE to S-N north and south of 64oN latitude, respectively. It confirms the previous suggestion, that different faults have been developing as composite parts of the MFSZ (Page et al., 1995).
Future work on this earthquake sequence will include relocation of the aftershocks using BEAAR (Broad-Band Experiment Across Alaska Range) data, when these data become available in summer 2001.
Relocated seismicity and M 5+ earthquake sequence.
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