David K. Yamaguchi

Summary of Coastal Geologic Evidence for Past Great Earthquakes at the Cascadia Subduction Zone

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

Mutans Streptococci Dose Response to Xylitol Chewing Gum

Xylitol is promoted in caries-preventive strategies, yet its effective dose range is unclear. This study determined the dose-response of mutans streptococci in plaque and unstimulated saliva to xylitol gum. Participants (n = 132) were randomized: controls (G1) (sorbitol/maltitol), or combinations giving xylitol 3.44 g/day (G2), 6.88 g/day (G3), or 10.32 g/day (G4). Groups chewed 3 pellets/4 times/d. Samples were taken at baseline, 5 wks, and 6 mos, and were cultured on modified Mitis Salivarius agar for mutans streptococci and on blood agar for total culturable flora. At 5 wks, mutans streptococci levels in plaque were 10x lower than baseline in G3 and G4 (P = 0.007/0.003). There were no differences in saliva. At 6 mos, mutans streptococci in plaque for G3 and G4 remained 10x lower than baseline (P = 0.007/0.04). Saliva for G3 and G4 was lower than baseline by 8 to 9x (P = 0.011/0.038). Xylitol at 6.44 g/day and 10.32 g/day reduces mutans streptococci in plaque at 5 wks, and in plaque and unstimulated saliva at 6 mos. A plateau effect is suggested between 6.44 g and 10.32 g xylitol/day.

Sudden, probably coseismic submergence of Holocene trees and grass in coastal Washington State

Growth-position plant fossils in coastal Washington State imply a suddenness of Holocene submergence that is better explained coseismic lowering of the land than be decade- or century-long rise of the sea. These fossils include western red cedar and Sitka spruce whose death probably resulted from estuarine submergence close to 300 years ago. Rings in eroded, bark-free trunks of the red cedar show that growth remained normal within decades of death. Rings in buried, bark-bearing stumps of the spruce further show normal growth continuing until the year of death. Other growth-position fossils implying sudden submergence include the stems and leaves of salt-marsh grass entombed in tide-flat mud close to 300 years ago and roughly 1,700 and 3,100 years ago. The preservation of these stems and leaves shows that submergence and initial burial outpaced decomposition, which appears to take just a few years in modern salt marshes. In some places the stems and leaves close to 300 year old are surrounded by sand left by an extraordinary, landward-directed surge-probably a tsunami from a great thrust earthquake on the Cascadia subduction zone.

Earthquake recurrence inferred from paleoseismology

Publisher Summary This chapter describes three North American examples of earthquake history inferred from Quaternary geology and discusses earthquakes in the interior of the North America plate––in the New Madrid seismic zone of Missouri, Arkansas, and Tennessee. The study of prehistoric earthquakes––paleoseismology––provides long-term rates of earthquake occurrence to improve confidence in such forecasts. These earthquakes suggest the rates and patterns of recurrence that help define earthquake hazards. The eastern California shear zone, centered about 150 km northeast of Los Angeles, exhibits geologic evidence for prehistoric surface ruptures during episodes thousands of years apart. Typical intervals between the earthquakes span hundreds of years in the New Madrid and Cascadia examples and thousands of years in the eastern California example. Apart from enabling such estimates of recurrence intervals, paleoseismology can provide evidence for the regional clustering of earthquakes in seismic zones and for aperiodic rupture along the same part of a fault. Such findings have made paleoseismology an essential part of earthquake-hazard assessment in the United States.

New tree-ring dates for recent eruptions of Mount St. Helens

Abstract Distinctive patterns of growth rings in increment cores from old-growth Douglas-fir (Pseudotsuga menziesii) stands identify A.D. 1800 as a more precise date for the eruption of tephra layer T by Mount St. Helens, Washington. Layer T was previously inferred to date to about A.D. 1802. Growth patterns also establish A.D. 1480 as the date of eruption of the earlier layer Wn, previously estimated as dating to about A.D. 1500. The timing of radial tree growth places a small limitation on the seasonal resolution of these new tree-ring dates.

Tree-ring evidence for a two-year interval between recent prehistoric explosive eruptions of Mount St. Helens

Distinctive annual-ring patterns in trees in a forest stand 10 km east of Mount St. Helens indicate that the volcano9s voluminous layer We tephra was erupted after the A.D. 1481 growth season, late in 1481, or early during 1482. Combined with earlier research that established A.D. 1480 as the date of eruption of the underlying, similar layer Wn tephra, the new tree-ring data provide evidence that these two major explosive tephra eruptions about 500 yr ago were separated by about 2 yr. During that eruptive sequence, a voluminous explosive eruption was therefore followed by more than 1 yr of minor explosive activity before another voluminous eruption occurred. This record suggests that Mount St. Helens might produce a second large explosive eruption during its current period of activity. Examinations of the tephra deposits that correlate with narrowring patterns in trees at Mount St. Helens also demonstrate that distinctive ring patterns can form in trees across a large range of tephra-layer thicknesses. The examinations also suggest that tephra coarseness interacts with tephra-layer thickness in determining the effects that tephra fallout will have upon forest trees.

Rapid resetting of an estuarine recorder of the 1964 Alaska earthquake

Tides and plants have already restored much of a landscape that the 1964 Alaska earthquake destroyed. At the head of a macrotidal estuary near Anchorage, in the vicinity of Portage, subsidence during the earthquake changed meadows, thickets, and spruce groves into barren tidal flats. Tidal-flat silt and sand soon buried the pre- earthquake landscape while filling intertidal space that the subsidence had made. The flats supported new meadows and thickets by 1973 and new spruce by 1980. Three new findings confirm that the flats aggraded rapidly and that their vegetation is maturing. (1) Most of the postearthquake deposits at Portage date from the first decade after the 1964 earthquake. Their thickness of 23 sites in a 0.5 km 2 area was 1.4 ± 0.2 m in 1973, 1.6 ± 0.2 m in 1991, and 1.6 ± 0.3 m in 1998. (2) Many of the deposits probably date from the first months after the earthquake. The deposits contain sedimentary couplets in which coarse silt or very fine sand is capped by fine or medium silt. About 100 such couplets make up the lowest 0.5 m or more of the postearthquake deposits in two outcrops. These couplets thicken and thin rhythmically, both as groups of 5–20 couplets and as pairs of successive couplets. Probably, the groups of thick couplets represent the highest tides, the groups of thin couplets represent some of the lesser high tides, and the pairs record inequality between twice-daily high tides. (3) In the 1980s and 1990s, thickets expanded and spruce multiplied. The vegetation now resembles the fossil assemblage rooted in the buried landscape from 1964. Had the 1964 Alaska earthquake been repeated a decade later, the two earthquakes would now be recorded by two superposed, buried landscapes near Portage. Much more than a decade is probably needed to reset similar recorders at mesotidal estuaries of the Cascadia subduction zone.

Linear response of mutans streptococci to increasing frequency of xylitol chewing gum use: a randomized controlled trial [ISRCTN43479664]

BackgroundXylitol is a naturally occurring sugar substitute that has been shown to reduce the level of mutans streptococci in plaque and saliva and to reduce tooth decay. It has been suggested that the degree of reduction is dependent on both the amount and the frequency of xylitol consumption. For xylitol to be successfully and cost-effectively used in public health prevention strategies dosing and frequency guidelines should be established. This study determined the reduction in mutans streptococci levels in plaque and unstimulated saliva to increasing frequency of xylitol gum use at a fixed total daily dose of 10.32 g over five weeks.MethodsParticipants (n = 132) were randomized to either active groups (10.32 g xylitol/day) or a placebo control (9.828 g sorbitol and 0.7 g maltitol/day). All groups chewed 12 pieces of gum per day. The control group chewed 4 times/day and active groups chewed xylitol gum at a frequency of 2 times/day, 3 times/day, or 4 times/day. The 12 gum pieces were evenly divided into the frequency assigned to each group. Plaque and unstimulated saliva samples were taken at baseline and five-weeks and were cultured on modified Mitis Salivarius agar for mutans streptococci enumeration.ResultsThere were no significant differences in mutans streptococci level among the groups at baseline. At five-weeks, mutans streptococci levels in plaque and unstimulated saliva showed a linear reduction with increasing frequency of xylitol chewing gum use at the constant daily dose. Although the difference observed for the group that chewed xylitol 2 times/day was consistent with the linear model, the difference was not significant.ConclusionThere was a linear reduction in mutans streptococci levels in plaque and saliva with increasing frequency of xylitol gum use at a constant daily dose. Reduction at a consumption frequency of 2 times per day was small and consistent with the linear-response line but was not statistically significant.

Tree-ring dating of pre-1980 volcanic flowage deposits at Mount St. Helens, Washington

Tree-ring dating (ring-width pattern matching) was used to determine emplacement dates for eight subfossil-tree–bearing, pre-1980 pyroclastic-flow and lahar deposits in stream valleys draining Mount St. Helens, Washington. Limiting dates were also established for nine other pre-1980 flowage deposits from the ages of trees rooted on or near them, or from exhumed trees. The 17 new dates improve understanding of the chronology of events during and after Mount St. Helens9s Kalama eruptive period, which extended from a.d. 1479 to the mid-1700s. We assign bracketing dates to the three phases of the Kalama eruptive period: 1479–1510 to the early Kalama, during which Mount St. Helens erupted explosive dacitic tephras, pyroclastic flows, and one or more domes; 1489–1566 to the middle Kalama, during which Mount St. Helens produced andesitic tephras, pyroclastic flows, and lava flows; and 1489–1750 to the late Kalama, during which Mount St. Helens again erupted dacite. Overlap of the brackets reflects the limits of our dating resolution. The revised chronology confirms that little time (perhaps 50 yr) elapsed between the end of Mount St. Helens9s Kalama eruptive period and the start of the succeeding Goat Rocks eruptive period (a.d. 1800–1857). Eruptions apparently occurred sporadically during the entire 1479–1857 interval. The record also indicates that a large pyroclastic flow occurred 7–10 yr after the explosive eruptions that began the Kalama eruptive period. The dates further imply that extrusion of the pre-1980 summit dome continued intermittently for ≈ 100 yr and limit the ages of two tephras. These findings demonstrate the usefulness of relict tree-bearing deposits to volcanic history studies.

Corrigendum to “Renewal of Tidal Forests in Washington State after a Subduction Earthquake in A.D. 1700” [Quaternary Research, 56, 2, 139–147 (2001)]

The authorswish to point out that there is an error in the footnote on page 1 of this article. The corrected text should read: Supplementary data for this article (Tables A1–A3 and Figs. A1–A8) are available on IDEAL (http://www.idealibrary.com). The Publisherwould like tomake readers aware that these supplementary data are now available via the HTML version of this article, at http://dx. doi.org/10.1006/qres.2001.2251.