Jessie Kathleen Pearl

Determining the timing and cause for ancient hillslope failures is challenging in the western United States, yet it is critical because it can inform ground-motion estimates for large-to-great earthquakes on plate-boundary faults. We identify two preinstumental debris avalanches (3×106 and 4.28×106 m3) in the Coast Ranges of northern California. These debris avalanches are well suited for studying the timing (to the exact year) and trigger, as the densely forested landscape enables effective use of dendrochronology and high-resolution radiocarbon to pinpoint the time of failure, and the legacy of landslide studies in the region provides context for evaluating a seismic trigger as the most likely triggering mechanism. Neither of the debris avalanches was triggered by the largest precipitation event of the instrumental record in 1964 C.E., and the two debris avalanche sites are physiographically suited to experience topographic amplification of seismic shaking. Through a suite of geologic and dendrochronologic evidence, we establish the likely time of failure of the two debris avalanches as 1906 C.E. and 933 C.E. The failure of the younger landslide coincides with the 1906 C.E. San Francisco earthquake on the Pacific/North American plate boundary, whereas the older landslide dates to 933 C.E., within the broad age range (850–966 C.E.) of a likely megathrust earthquake on the Cascadia subduction zone as recorded in coastal marshes in Humboldt Bay, California. If the older landslide was triggered by a subduction earthquake, then the precise age of the 933 C.E. debris avalanche would trim the uncertainty on a Cascadia subduction zone event from 116 yr to 1 yr. Utilizing debris avalanche records from sites suitable to record seismic shaking improves understanding of plate-boundary earthquake timing and extent of shaking.

The Seattle Fault Zone is an approximately 70-km-long, east-west-trending zone of south-dipping blind reverse faults within the Puget lowland region in Washington. Because of the proximity, the Seattle Fault Zone poses a significant earthquake hazard to the Puget sound and Seattle metropolitan regions. We present preliminary mapping and trench-site information from a paleoseismic investigation across a newly identified active fault scarp located within the hanging wall of the Seattle Fault Zone on southern Bainbridge Island, Washington. The trench exposed monoclinally folded Miocene bedrock, fractured and faulted glacial-related deposits, and laminated lacustrine deposits capped by slope-derived colluvium. The observations from this investigation record late Pleistocene to Holocene north-vergent folding and faulting along this new fault scarp.

How forest structure regulates the amount and timing of snowmelt across a climatic gradient of snow stability

Compound earthquakes involving simultaneous ruptures along multiple faults often define a region’s upper threshold of maximum magnitude. Yet, the potential for linked faulting remains poorly understood given the infrequency of these events in the historic era. Geological records provide longer perspectives, although temporal uncertainties are too broad to clearly pinpoint single multifault events. Here, we use dendrochronological dating and a cosmogenic radiation pulse to constrain the death dates of earthquake-killed trees along two adjacent fault zones near Seattle, Washington to within a 6-month period between the 923 and 924 CE growing seasons. Our narrow constraints conclusively show linked rupturing that occurred either as a single composite earthquake of estimated magnitude 7.8 or as a closely spaced double earthquake sequence with estimated magnitudes of 7.5 and 7.3. These scenarios, which are not recognized in current hazard models, increase the maximum earthquake size needed for seismic preparedness and engineering design within the Puget Sound region of >4 million residents.

We describe the utility of false rings in Taxodium distichum (i.e. baldcypress) as a proxy for hydroclimatic extreme events in three different river basins (Pascagoula, Mobile, and Choctawhatchee) that discharge into the northern Gulf of Mexico. False rings occur as a result of a change in the environmental limiting resource for tree stem growth, and in T. distichum, false ring production is usually a result of increases in mid-growing season water availability. Our results show that false ring occurrence (from 1931 to 2018) is similar across sites but occur in different years, suggesting that false ring production is indicative of tree response to its local environment. False ring production in T. distichum has previously been correlated with summer streamflow, the season when tropical cyclone precipitation (TCP) is highest. To assess a stand-wide response, we define high false ring (HFR) years as all years when ⩾ 20% of trees produced a false ring. We show total TCP in July is the best predictor for HFR years in T. distichum, and false ring production in smaller river basins captures local TCP better than larger river basins. Additionally, HFR years coincide with summers of anomalously high precipitation, anomalously low temperatures, and a positive phase of the North Atlantic Oscillation. 77% of HFR years occur in seasons when there is heavy tropical cyclone activity near sample sites, building a foundation to use false ring records as robust TCP proxies with hydroclimate reconstruction potential.

From its inception as a scientific discipline, tree-ring research has been used as a trans-disciplinary tool for dating and environmental reconstruction. Tree-ring chronologies in some regions extend back many thousands of years, opening up new potential for the study of climate, people, and ecology at annual and sub-annual resolution. As such, they are a frequently used resource for a diverse range of studies spanning the Holocene. They are also the focus of a constantly evolving array of analytical techniques and multidisciplinary approaches to research questions. This literature review discusses case studies at the cutting-edge of interdisciplinary tree-ring research, notes recent breakthroughs and limitations, and identifies key frontiers for the future of tree-ring research.

High‐resolution paleoclimate records are essential for improving our understanding of internal variability and the detection and attribution of forced climate system responses. The densely populated northeastern United States is at risk from increasing temperatures, severe droughts, and extreme precipitation, but the region has limited annual and seasonal‐resolution paleoclimate records beyond the instrumental record. Chamaecyparis thyoides, L. (B.S.P.), Atlantic white cedar, a wetland conifer found within 200 km of the Atlantic coastline of the United States, is a promising tree‐ring proxy that can fill in these data gaps. Here, we develop and analyze a new network of Atlantic white cedar tree‐ring chronologies across the northeastern United States and demonstrate that site selection is important for regional paleoclimate reconstructions. Ring width variability reflects winter through summer temperatures at inland and hydrologically stable sites in the northernmost section of the species' range. Ombrotrophic sites along the coast record hydrological signals and correlate with growing season precipitation. We demonstrate skillful regional climate field reconstructions for the last several centuries and show the increased skill from incorporating our moisture sensitive sites into broad‐scale products like the North American Drought Atlas. This comprehensive understanding of the species' climate responses leads to a tree‐ring network that provides the long‐term multivariate climate context at multidecadal and centennial time scales for the large‐scale ocean‐atmospheric processes that influence the climate of the region. We use this network to examine the covariance of temperature and drought across the New England area over the past two centuries.

Hydroclimate extremes in North America, Europe, and the Mediterranean are linked to ocean and atmospheric circulation anomalies in the Atlantic, but the limited length of the instrumental record prevents complete identification and characterization of these patterns of covariability especially at decadal to centennial time scales. Here we analyze the coupled patterns of drought variability on either sides of the North Atlantic Ocean basin using independent climate field reconstructions spanning the last millennium in order to detect and attribute epochs of coherent basin‐wide moisture anomalies to ocean and atmosphere processes. A leading mode of broad‐scale moisture variability is characterized by distinct patterns of North Atlantic atmosphere circulation and sea surface temperatures. We infer a negative phase of the North Atlantic Oscillation and colder Atlantic sea surface temperatures in the middle of the fifteenth century, coincident with weaker solar irradiance and prior to strong volcanic forcing associated with the early Little Ice Age.