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Surface Rupture on the Denali Fault Interpreted from Tree Damage During Bulletin of the Seismological Society of America, Vol. 94, No. 6B, pp. S58–S71, December 2004 Surface Rupture on the Denali Fault Interpreted from Tree Damage during the 1912 Delta River Mw 7.2–7.4 Earthquake: Implications for the 2002 Denali Fault Earthquake Slip Distribution by Gary Carver, George Plafker, Mike Metz, Lloyd Cluff, Burt Slemmons, Elden Johnson, Jim Roddick, and Steve Sorensen Abstract During the 3 November 2002 Denali fault earthquake, surface rupture propagated through a small, old-growth forest in the Delta River valley and damaged many trees growing on the fault. Damage was principally the result of fault offset of tree roots and tilting of trees. Some trees were split by surface faults that intersected the base of their trunks or large taproots. A few trees appear to have been damaged by strong shaking. Many of the older trees damaged in 2002 were deformed and scarred. Some of these scarred trees exhibit past damage indicative of surface faulting and have abrupt changes in their annual ring patterns that coincide with the past damage. Annual ring counts from several of these older scarred trees indicate the damage was caused by surface rupture on the Denali fault in 1912. The only earth- quake of sufficient magnitude that fits the requirements for timing and general lo- cation as recorded by the damaged trees is a widely felt Ms 7.2–7.4 earthquake on 6 July 1912 informally referred to as the 1912 Delta River earthquake. Seismologic data and intensity distribution for the 1912 Delta River earthquake indicate that its epicenter was within 60–90 km of the Delta River and that rupture probably propa- gated toward the west. Inferred fault length, displacement, and rupture direction suggest the 1912 rupture was probably largely coincident with the western, lower slip section of the 2002 rupture. Introduction The 3 November 2002 Denali fault earthquake (Mw 7.9) Most of the surface rupture along the Denali fault was produced about 340 km of surface faulting in the central in glacially scoured rock or thin, unconsolidated glacial and Alaska Range (Fig. 1) (Eberhart-Phillips et al., 2003). The colluvial sediments overlying bedrock, and was confined to earthquake initiated with rupture of a 45-km-long segment a single, linear trace or a narrow zone of closely spaced, en of the previously unrecognized Susitna Glacier thrust fault. echelon faults. However, in the Delta River valley the fault Slip was immediately triggered on the adjacent Denali fault displaced thick glaciofluvial sediments. There, much of the and propagated eastward 240 km, to the intersection with lateral slip was accommodated as a broad zone of defor- the Totschunda fault system. Rupture continued southeast mation with numerous small displacements across multiple along the Totschunda system an additional 65 km. Most of surface traces. Post-earthquake surveys of the Trans-Alaska the surface rupture on the Denali fault coincided with well Pipeline at the fault crossing revealed 5.8 m of right-lateral preserved late Holocene stream offsets and scarps. Displace- and 1.2 m of vertical (up to the north) slip distributed ment was right-lateral with a north-side-up vertical compo- across a 1000-meter-wide zone (Metz, 2004). About 3.5 m nent of as much as 1 meter along most of the Denali fault of the lateral component and most of the vertical component surface break. Lateral displacement at the surface was about was concentrated within a ϳ25-meter-wide zone marked 1–2 m on the low-slip, western 68 km section of the Denali by multiple short discontinuous surface faults, en echelon fault rupture; increased to 4–6 m on the central, 106-km- scarps, mole tracks, fault-line graben, fissures, and fault-line long section between the Delta and Slana River valleys; as folds. The maximum lateral displacement measured on a sin- much as 8.8 m on the 54-km-long high-slip part of the fault gle fault trace in the thick unconsolidated sediments was near Gillett Pass; and 1–2 m on a 24-km-long transfer zone 1.2 m. at the Totschunda fault intersection (Haeussler et al., 2004). S58 Surface Rupture on the Denali Fault Interpreted from Tree Damage during the 1912 Delta River Mw 7.2–7.4 Earthquake S59 Figure 1. Neotectonic map of south-central Alaska showing epicenters of large earthquakes (yellow circles), volcanoes (yellow triangles), the 3 November 2002 rup- ture segment of the Denali fault system (pink shading), and major Quaternary faults and earthquake focal regions. Inset shows the seismologically determined 6 July 1912 Delta River earthquake epicenter and circle of error (gray) from Boyd and Lerner-Lam (1988); red arrow indicates location of damaged trees along the fault and the preferred 1912 epicenter location. Base map after Plafker et al. (1994). Evidence in the Delta River Valley for a 1912 younger of these moraines as less than about 735 years based Earthquake on the Denali Fault on two 14C analyses of a log buried in the moraine. They report the age of the older moraine, based on lichenometry, During the 3 November 2002 Denali fault earthquake, to be at least 3300 years. No zonal soil has formed in the surface rupture propagated through a small old-growth forest outwash, but a silty peat composed of wind-deposited silt, in the Delta River valley. This forest is composed predom- decayed moss, and tree litter forms an organic horizon up to inately of old-growth white spruce (Picea glauca) and scat- 30 cm thick that covers the gravelly outwash. The organic tered cottonwood (Populus balsamifera), birch (Betula mat contains a widespread and distinctive 1–2-cm-thick eo- spp.), alder (Alnus spp.), and willow (Salix spp.). Reger and lian (loess) layer near its base. A twig recovered from this Pewe (1991) conducted dendrochronology studies of trees silt layer yielded a 14C age of less than 305 years. in the forest growing on late-Holocene moraines as part of The trees are rooted almost entirely in the organic ho- a study of recent advances of the nearby Canwell glacier. rizon and few roots penetrate into the underlying gravel. They found the forest includes first-generation trees, some Thus each tree is supported by a dense, shallow root mat on of which are at least 250 years old. Stands of trees are dense top of the outwash. Eyewitnesses near the fault report that and, except along the fault, the spruce trees are straight and during the earthquake trees “waved like windshield wipers” symmetrical with living branches that extend to within 1– with their “tops nearly reaching the ground.” During our 2 m of the ground. The largest trees are about 1 m in di- study, we found concentric cracks in the organic soil bor- ameter and more than 25 m in height. dering the edge of the root mats of some trees near the fault. The forest occupies a late-Holocene outwash plain of The cracks appeared to reflect shaking-induced movement the Canwell glacier and adjacent late-Holocene terraces of the roots over the underlying outwash and may have been along the lower part of Miller Creek and the Delta River caused by whipping of the trees during the earthquake. Such (Fig. 2). Upvalley the Canwell outwash plain grades to two soil cracking was not observed more than ϳ100 m away prominent late-Holocene end moraines of the Canwell gla- from the fault. Very few trees were uprooted or toppled by cier. Reger and Pewe (1991) estimated the age of the the shaking. S60 G. Carver, G. Plafker, M. Metz, L. Cluff, B. Slemmons, E. Johnson, J. Roddick, and S. Sorensen shattered and fell during the earthquake (Fig. 3b). The trunks of two of the shattered trees had been previously split and healed. Other trees rooted across surface traces of the fault were split but remained standing (Fig. 3c). The splitting of trees appeared to be the result of either direct propagation of a surface fault trace through the base of the tree or from leverage generated by offset taproots. In 2002 trees rooted directly across faults and split by fault propagation through the base of the trunk were offset right-laterally up to 80 cm. A few trees growing near surface faults were split when their root mats were offset and the large taproots acted as levers, splitting the trunk where the taproots joined. Trees split by taproot leverage commonly were not offset. The earthquake- generated splits in the trees are widest at the base of the trunk and narrow rapidly as they extend up the tree. Most splits end 1–2 m above the ground. Figure 2. Oblique air photo viewed to south up the Delta River valley. The Richardson highway (left) and Trans-Alaska Pipeline intersect the Denali fault in an old-growth white spruce forest. Tree damage Evidence of Pre-2002 Tree Damage surveys discussed in this report were conducted be- Many of the older trees damaged in the 2002 Denali tween the highway and the pipeline along the fault (yellow line “A”) and about 300 m south of the fault fault earthquake have scars and deformities that record simi- (yellow line “B”). lar damage from the past. The old damage includes bent and deformed trunks, healed splits in the base of the trunks, trees lacking living lower branches, broken trees with multiple Effects of the 2002 Surface Faulting on the Trees stump sprouts, dead trees, and trees with abrupt changes in annual ring width coincident with past damage. The 2002 Denali fault earthquake rupture damaged most Many older spruce trees that are rooted on the 2002 fault trees growing on surface traces of the fault in the Delta River scarps have bent or deformed trunks and few living branches valley.
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