HISTORIC AND PALEOSEISMIC EVIDENCE FOR NON-CHARACTERISTIC EARTHQUAKES AND THE SEISMIC CYCLE AT THE DELTA RIVER CROSSING OF THE DENALI FAULT, ALASKA

The Delta River valley segment of the Denali fault ruptured on exactly the same surface trace during the M 7.9 earthquake of November 3, 2002, during an M 7.2 earthquake on July 6, 1912, and during two paleoseismic events. In all events, displacements are known or inferred to be dextral with a subordinate north-side-up dip slip component.

The Delta River valley, 90 km east of the 2002 epicenter, lies within a transition zone 10 km wide in which surface dextral slip diminishes from 6 m or more east of the valley to 4.5 m west of the valley. Geodetic data for the 2002 rupture along the TAPS oil pipeline indicate a total of 5.8 m dextral slip and 1.3 m dip slip distributed over a zone 1,000 m wide. Surface trace of the fault is poorly developed in the thick unconsolidated deposits on the valley floor because of distributive deformation. Offsets on individual surface fissures are less than 1.3 m dextral and about 0.6 m vertical.

Tree ring counts from pre-2002 damaged trees on the fault trace closely date the age of the penultimate event as a widely-felt M 7.2 earthquake on July 6, 1912 (Carver et al, 2004). Empirical data for the 1912 earthquake suggest a surface rupture length of 60—84 km, average horizontal displacement of 1.4—2.1 m, and maximum horizontal displacement of 2.4—3.9 m.

Paleoseismologic data from test pits at the west bank of the Delta River indicate two earlier faulting events on the Denali fault probably comparable in magnitude to the 1912 earthquake based on their vertical component of fault displacement (0.5 and 0.54 m). Radiocarbon dates bracket these paleo-events at 310—460 ybp, and 650—780 ybp and recurrence intervals are ~290 and 330 years.

Our data indicate the following displacement history: 1. The last seismic cycle at the Delta River consisted of three events of ~M 7.2 followed by the major M 7.9 event; 2. Cumulative fault slip for the cycle is 10.4—11.2 m dextral and ~2.3 m vertical; 3. Average long term dextral slip rate is ~10.7—10.9 mm/yr; and 4. Cycle duration and return time for 2002-type events, is 950—1,030 years.

The derived slip rate is compatible with the higher end of average long-term rates between 8.7 and 11.6 mm/yr that were estimated from offset late Pleistocene moraines along the Denali fault east and west of the Delta River (Plafker et al, 1994, Table 3), and with recent cosmogenic isotope studies (Matmon et al, in press).