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Brian F. Atwater

Publications and source records attributed to Brian F. Atwater.

At least 37 records · Page 2Linked to original sources
USGS2010

Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands

Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. The overwash, after crossing a fringing coral reef and 1.5 km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. Most of the breaches extend tens to hundreds of meters perpendicular to a 2-km stretch of Anegada’s windward shore. Remnants of the breached ridges stand 3 m above modern sea level, and ridges seaward of the breaches rise 2.2–3.0 m high. The overwash probably exceeded those heights when cutting the breaches by overtopping and incision of the beach ridges. Much of the sand-and-shell sheet contains pink bioclastic sand that resembles, in grain size and composition, the sand of the breached ridges. This sand extends as much as 1.5 km to the south of the breached ridges. It tapers southward from a maximum thickness of 40 cm, decreases in estimated mean grain size from medium sand to very fine sand, and contains mud laminae in the south. The sand-and-shell sheet also contains mollusks—cerithid gastropods and the bivalve Anomalocardia—and angular limestone granules and pebbles. The mollusk shells and the lime-mud cap were probably derived from a marine pond that occupied much of Anegada’s interior at the time of overwash. The boulders and cobbles, nearly all composed of limestone, form fields that extend many tens of meters generally southward from limestone outcrops as much as 0.8 km from the nearest shore. Soon after the inferred overwash, the marine pond was replaced by hypersaline ponds that produce microbial mats and evaporite crusts. This environmental change, which has yet to be reversed, required restriction of a former inlet or inlets, the location of which was probably on the island’s south (lee) side. The inferred overwash may have caused restriction directly by washing sand into former inlets, or indirectly by reducing the tidal prism or supplying sand to post-overwash currents and waves. The overwash happened after A.D. 1650 if coeval with radiocarbon-dated leaves in the mud cap, and it probably happened before human settlement in the last decades of the 1700s. A prior overwash event is implied by an inland set of breaches. Hypothetically, the overwash in 1650–1800 resulted from the Antilles tsunami of 1690, the transatlantic Lisbon tsunami of 1755, a local tsunami not previously documented, or a storm whose effects exceeded those of Hurricane Donna, which was probably at category 3 as its eye passed 15 km to Anegada’s south in 1960.

Natural Hazards↗
USGS2005

The orphan tsunami of 1700—Japanese clues to a parent earthquake in North America

Summary A puzzling tsunami entered Japanese history in January 1700. Samurai, merchants, and villagers wrote of minor flooding and damage. Some noted that no parent earthquake had been felt; they were wondering what had set off the waves. They had no way knowing that the tsunami had been spawned during an earthquake along the coast of northwestern North America. This orphan tsunami would not be linked to its parent earthquake until the middle 1990s, through an extraordinary series of discoveries in both North America and Japan. The Orphan Tsunami of 1700 , now in its second edition, tells this scientific detective story through its North American and Japanese clues. The discoveries underpin many of today’s precautions against earthquakes and tsunamis in the Cascadia region of northwestern North America. The Japanese tsunami of March 2011 called attention to those hazards as a mirror image of the transpacific waves of January 1700.

Professional Paper↗
USGS2004

Paleomagnetic correlation of late Holocene earthquakes among estuaries in Washington and Oregon

Paleomagnetic directions of estuarine mud provide additional evidence that individual earthquakes, or rapid series of earthquakes, caused widespread coseismic land-level changes during the past 2000 years in western Washington and Oregon. Most of the paleomagnetic measurements were made on mud dating from the first decades after coseismic subsidence from plate-boundary earthquakes at the Cascadia subduction zone. Mud deposited soon after the A.D. 1700 Cascadia earthquake has similar remanent directions among all five sites ( k = 171) sampled along 80 km of Pacific coast between Grays Harbor and the mouth of the Columbia River. Likewise, internally consistent directions were obtained along this stretch of coast from mud deposited soon after a plate-boundary earthquake (or earthquake series) in A.D. 340–410 and soon after another such event in A.D. 680–720. Also analyzed were remanent magnetizations of mud deposited shortly before (or shortly after) land-level changes from seismicity in the North America plate beneath Puget Sound. A mean direction for sites on the Snohomish River delta, near Everett, from the time of an earthquake on the Seattle fault in A.D. 900–930 is statistically identical (95% confidence level) to a mean direction in mud that was uplifted in A.D. 800–1000 at potentially correlative sites near Tacoma and Olympia. The paleomagnetic direction from Everett for the upper-plate earthquake of A.D. 900–930 differs substantially from that for a plate-boundary earthquake in A.D. 810–1190. This difference implies that the upper-plate earthquake preceded the plate-boundary earthquake by a century or two on the basis of comparisons of their paleomagnetic poles with a previously reconstructed path of geomagnetic paleosecular variation in western North America.

Oregon, Washington↗
USGS1997

Recurrence intervals for great earthquakes of the past 3,500 years at northeastern Willapa Bay, Washington

Seven great earthquakes, or earthquake series, probably ruptured the southern Washington part of the Cascadia subduction zone in the past 3,500 years. Each earthquake was probably of magnitude 8 or larger. The earthquakes define six recurrence intervals that average about 500 years. The longest interval, about 700-1300 years, was followed by two of the shortest, which together lasted less than 800 years. Another long interval, 600-1000 years, ended with an earthquake 300 years ago.

Professional Paper↗
USGS1996

Earthquake-induced burial of archaeological sites along the southern Washington coast about A.D. 1700

Although inhabited by thousands of people when first reached by Europeans, the Pacific coast of southern Washington has little recognized evidence of prehistoric human occupation. This apparent contradiction may be explained partly by geologic evidence for coastal submergence during prehistoric earthquakes on the Cascadia subduction zone. Recently discovered archaeological sites, exposed in the banks of two tidal streams, show evidence for earthquake-induced submergence and consequent burial by intertidal mud about A.D. 1700. We surmise that, because of prehistoric earthquakes, other archaeological sites may now lie hidden beneath the surfaces of modern tidelands. Such burial of archaeological sites raises questions about the estimation of prehistoric human population densities along coasts subject to earthquake-induced submergence.

Washington↗
USGS1992

A tsunami about 1000 years ago in Puget Sound, Washington

Water surged from Puget Sound sometime between 1000 and 1100 years ago, overrunning tidal marshes and mantling them with centimeters of sand. One overrun site is 10 kilometers northwest of downtown Seattle; another is on Whidbey Island, some 30 kilometers farther north. Neither site has been widely mantled with sand at any other time in the past 2000 years. Deposition of the sand coincided—to the year or less—with abrupt, probably tectonic subsidence at the Seattle site and with landsliding into nearby Lake Washington. These findings show that a tsunami was generated in Puget Sound, and they tend to confirm that a large shallow earthquake occurred in the Seattle area about 1000 years ago.

Washington↗
USGS1987

Evidence for great Holocene earthquakes along the outer coast of Washington state

Intertidal mud has buried extensive, well-vegetated lowlands in westernmost Washington at least six times in the past 7000 years. Each burial was probably occasioned by rapid tectonic subsidence in the range of 0.5 to 2.0 meters. Anomalous sheets of sand atop at least three ofthe buried lowlands suggest that tsunamis resulted from the same events that caused the subsidence. These events may have been great earthquakes from the subduction zone between the Juan de Fuca and North America plates.

Washington↗
USGS1982

Geologic maps of the Sacramento-San Joaquin Delta, California

The Sacramento-San Joaquin Delta, the arm of the San Francisco Bay estuary that reaches into the Central Valley of California, differs from typical coastal-plain deltas in three important respects. First, rather than meeting the ocean individually and directly, all major waterways of this delta discharge via a single constricted outlet into a chain of estuarine bays and straits. Second, in the most common vertical sequence of deposits, peat and mud deposited in tidal marshes and swamps (tidal wetlands) directly overlie alluvium or eolian sand, a sequence recording a landward spread of tidal environments rather than the seaward migration of fluvial environments that is typical of coastal-plain deltas (Cosby, 1941, p. 43; Thompson, 1957, p. 12; Shlemon and Begg, 1975, p. 259; Atwater and Belknap, 1980). Finally, intensive human use has led to a peculiar set of conflicts involving rights to water and responsibilities for flood-control levees (Kockelman and other, 1982).

California↗
USGS1977

Late Quaternary depositional history, Holocene sea-level changes, and vertical crustal movement, southern San Francisco Bay, California

Sediments collected for bridge foundation studies at southern San Francisco Bay, Calif., record estuaries that formed during Sangamon (100,000 years ago) and post-Wisconsin (less than 10,000 years ago) high stands of sea level. The estuarine deposits of Sangamon and post-Wisconsin ages are separated by alluvial and eolian deposits and by erosional unconformities and surfaces of nondeposition, features that indicate lowered base levels and oceanward migrations of the shoreline accompanying low stands of the sea. Estuarine deposits of mid-Wisconsin age appear to be absent, suggesting that sea level was not near its present height 30,000–40,000 years ago in central California. Holocene sea-level changes are measured from the elevations and apparent 14 C ages of plant remains from 13 core samples. Uncertainties of ±2 to ±4 m in the elevations of the dated sea levels represent the sum of errors in determination of (1) sample elevation relative to present sea level, (2) sample elevation relative to sea level at the time of accumulation of the dated material, and (3) postdepositional subsidence of the sample due to compaction of underlying sediments. Sea level in the vicinity of southern San Francisco Bay rose about 2 cm/yr from 9,500 to 8,000 years ago. The rate of relative sea-level rise then declined about tenfold from 8,000 to 6,000 years ago, and it has averaged 0.1–0.2 cm/yr from 6,000 years ago to the present. This submergence history indicates that the rising sea entered the Golden Gate 10,000–11,000 years ago and spread across land areas as rapidly as 30 m/yr until 8,000 years ago. Subsequent shoreline changes were more gradual because of the decrease in rate of sea-level rise. Some of the sediments under southern San Francisco Bay appear to be below the level at which they initially accumulated. The vertical crustal movement suggested by these sediments may be summarized as follows: (1) Some Quaternary(?) sediments have sustained at least 100 m of tectonic subsidence in less than 1.5 million years (<0.07 mm/yr) relative to the likely elevation of the lowest Pleistocene land surface; (2) the deepest Sangamon estuarine deposits subsided tectonically about 20–40 m in about 0.1 million years (0.2±0.1–0.4±0.1 mm/yr) relative to the assumed initial elevations of the thalwegs buried by these sediments; and (3) Holocene salt-marsh deposits have undergone about 5 m of tectonic and possibly isostatic subsidence in about 6,000 years (0.8±.0.7 mm/yr) relative to elevations which might be expected from eustatic sea-level changes alone.

California↗