EARTHQUAKES IN THE PAMPLONA ZONE, YAKUTAT BLOCK, SOUTH CENTRAL ALASKA

The Pamplona zone is a region of complex deformation and moderate seis micity located within the Yakutat block, a region that has been relati vely aseismic since a series of large (M>7.8) earthquakes in 1899. In 1970 a sequence of moderate to large sized earthquakes occurred within the Pamplona zone (largest event of M-w=6.7). Together with a M-w=6.1 event in 1958, these events are the only M greater than or equal to 5 .5 events known to have occurred in the Pamplona region since 1900. Th us these events give important information on internal deformational p rocesses within the Yakutat block. Waveform modeling of three earthqua kes in April 1970, showed rupture complexity along low angle, thrust f aults. Focal depths indicate that two of the events occurred above the Wrangell-Aleutian megathrust, while the largest event may have occurr ed on the megathrust. Events in 1958 and February 1970 indicate that d eformation within the western Pamplona zone is occurring along high an gle (>60 degrees) faults with reverse-oblique motion. We believe the P amplona spur, the easternmost part of the Pamplona zone, may have beha ved as an asperity during the 1899 sequence. The location of the spur may be influenced by a north-south trending fault zone in the subducti ng Pacific plate that appears to be responsible for the 1987-1992 Gulf of Alaska sequence, occurring 50 to 200 km south of the Pamplona zone .