Spencer H. Wood

Distribution, correlation, and radiocarbon dating of late Holocene tephra, Mono and Inyo craters, eastern California

Two pumiceous tephra layers, widespread in meadow topsoils of the southern Sierra Nevada, are correlated on the basis of radiocarbon dates and trace-element analyses with two eruptive centers at the northern and southern ends of the Mono Craters—Inyo craters volcanic chain in eastern California. Pumice and obsidian that were erupted in the northern part of the chain are uniform in trace-element content, whereas those erupted from the southern part are nonuniform and distinctly different, particularly in Sr content. Similar differences are recognized in the two most recent and widespread tephra layers originating from these sites. These tephra layers are the deposits of the most recent explosive eruptions of magma from the Mono Craters and the Inyo craters. Tephra 1, characterized by sanidine microphenocrysts and a Sr content of about 215 ppm, was erupted 720 ± 60 yr B.P. Its distribution defines a south-trending lobe extending over the Sierra Nevada from the upper San Joaquin drainage area to the Little Kern drainage area. Sr, Rb, and Zr contents of the ash are similar to those of a tephra-ringed obsidian dome at the south end of the Inyo craters. Tephra 2, characterized by a lack of microphenocrysts and a Sr content of less than 20 ppm, was erupted 1190 ± 80 yr B.P. It is encountered as a fine ash layer in the Sierra Nevada from northernmost Yosemite to Kings Canyon. Its low Sr content indicates geochemical affinity with the Mono Craters. Panum Crater, a tephra-ringed dome at the north end of the chain, appears to be its most likely source vent.

The Borah Peak, Idaho Earthquake of October 28, 1983—Hydrologic Effects

Spectacular groundwater effects accompanied the October 28, 1983 Idaho earthquake (M s = 7.3). Groundwater discharge increased from many springs and base flow of major rivers significantly increased over an 18,000 km2 region. In the epicentral area a 35-m surge in artesian pressure produced spectacular bursts of muddy water from carbonate bedrock hills at Chilly Buttes. This pressure surge was also manifested as a 1.4-km long zone of large sand boils erupted through the valley alluvium. At Smiths Fish Hatchery, sediment eroded by the surge of water clogged the fresh spring-water supply inlet pipes and led to suffocation of the entire crop of trout. North of the epicentral area a major warm springs dried up for 8 days and then recommenced flowing at an ever increasing rate until it stabilized at about 9 times the pre-earthquake flow. This flow of about 1400 l/s caused severe bank erosion of pastureland until diverted into an artificially excavated channel. At Clayton Silver Mine, 50 km north of the epicenter, increased seepage into the underground mine workings overwhelmed the mine-pump capacity and flooded the 330-meter level within 14 hrs. Thirty kms south of the epicenter a 200,000 m3 mudflow was apparently caused by increased spring flows. The valley fill failed two days after the earthquake and devastated a 3.5-km reach of Lupine Creek valley. Similar coseismic and post-seismic hydrologic effects were reported in studies of the 1952 Kern County, California earthquake, and the 1959 Hebgen Lake, Montana earthquake. Several mechanisms for these hydrologic phenomena are considered, but the most plausible cause is the sudden release of elastic strain in aquifer rocks. Sudden strain release of elastic rebound increases pore pressure in some confined aquifers thus leading to increase in discharge from springs that contribute to the base flow of major rivers.

Fluoride: A naturally-occurring health hazard in drinking-water resources of Northern Thailand.

In Northern Thailand, incidences of fluorosis resulting from the consumption of high-fluoride drinking-water have been documented. In this study, we mapped the high-fluoride endemic areas and described the relevant transport processes of fluoride in enriched waters in the provinces of Chiang Mai and Lamphun. Over one thousand surface and sub-surface water samples including a total of 995 collected from shallow (depth: ≤ 30 m) and deep (> 30 m) wells were analysed from two unconnected high-fluoride endemic areas. At the Chiang Mai site, 31% of the shallow wells contained hazardous levels (≥ 1.5 mg/L) of fluoride, compared with the 18% observed in the deep wells. However, at the Lamphun site, more deep wells (35%) contained water with at least 1.5mg/L fluoride compared with the shallow wells (7%). At the Chiang Mai site, the high-fluoride waters originate from a nearby geothermal field. Fluoride-rich geothermal waters are distributed across the area following natural hydrological pathways of surface and sub-surface water flow. At the Lamphun site, a well-defined, curvilinear high-fluoride anomalous zone, resembling that of the nearby conspicuous Mae Tha Fault, was identified. This similarity provides evidence of the existence of an unmapped, blind fault as well as its likely association to a geogenic source (biotite-granite) of fluoride related to the faulted zone. Excessive abstraction of ground water resources may also have affected the distribution and concentration of fluoride at both sites. The distribution of these high-fluoride waters is influenced by a myriad of complex natural and anthropogenic processes which thus created a challenge for the management of water resources for safe consumption in affected areas. The notion of clean and safe drinking water can be found in deeper aquifers is not necessarily true. Groundwater at any depth should always be tested before the construction of wells.

Early 20th-century uplift of the northern Peninsular ranges province of southern California

Repeated leveling in the northern Peninsular Ranges province identifies an early 20thcentury episode of crustal upwarping in southern California. The episodic vertical movement is broadly bracketed between 1897 and 1934, and the main deformation is bracketed within 1906–1914 and involved regional up-to-the-northeast tilting of the Santa Ana block of as much as 4 · 10−6 rad and elevation changes exceeding 0.4 m in the Perris block and parts of the San Jacinto block, Transverse Ranges, and the Mohave block. Primary tide station records containing occasional entries since 1853 at San Pedro and San Diego show no evidence of episodic crustal movement, suggesting that the uplifted area hinged along coastal fault zones forming the west boundary of the Santa Ana block. Physiographic features and recent studies of Quaternary marine terraces by others show that this episode of regional tilting and uplift is a part of the continuing tectonic process in southern California. A crude, questionable coincidence exists between the uplift episode and a period of increased seismicity (1890–1923) in the northern Peninsular Ranges characterized by a number of moderate-size (M > 6) earthquakes on NW-trending strike-slip faults. However, releveling data are too sparse to associate the uplift development clearly with any one event.

Ancient Floods, Modern Hazards: The Ping River, Paleofloods and the ’Lost City’ of Wiang Kum Kam

This paper demonstrates that the importance of rivers in northern Thailand was anchored upon society’s dependence on them for sustenance and defense. Concurrently, rivers were also of deep religious and cultural significance. Hence, many northern Thai settlements were located near rivers. This resulted in their susceptibility to flood hazards. Our study investigates the interactions between the Ping River and the population of Wiang Kum Kam. Wiang Kum Kam was one of the former capitals of the Lanna Kingdom, a thirteenth- to sixteenth-century polity in northern Thailand. Described as the ‘Atlantis’ of the Lanna kingdom, the city was buried under flood sediments several centuries ago. Based on the floodplain sediments excavated, we argue that the city was abandoned after a large flood. Radiocarbon dating of charcoal found in the coarse sand layer deposited by the flood suggests that the deposition occurred sometime after ca. 1477 AD–1512 AD. Prior to this large flood, persisting floods in the city were noted in the Chiang Mai Chronicle and were also recorded in the floodplain stratigraphy. We show that an elongated mound on the floodplain in Wiang Kum Kam was a dyke constructed after ca. 1411 AD to alleviate the effects of persisting floods. From this story of paleofloods and Wiang Kum Kam, we conclude with two lessons for the management of modern floods in urban Thailand.

Tectonic tilt rates derived from lake-level measurements, Salton Sea, California

Tectonic tilt at the Salton Sea was calculated by differencing lake-level measurements from two points on the sea. During the past 26 years, tilting was down toward the southeast. By 1970 differential vertical movement amounted to 110 millimeters between two gages situated 38 kilometers apart on the southwest shore. A reversal in tilt direction in late 1972 has diminished the net differential vertical movement to 60 millimeters.

Student-based archaeological geophysics in northern Thailand

Summary As part of the 2010 near-surface geophysics workshop in Chiang Mai, Thailand, local archaeological targets were used as a basis for teaching geophysical data collection, processing, and interpretation techniques. By addressing local issues and interests, the workshop was able to demonstrate to participants and the local community how near-surface geophysics can be applied using simple survey methods and low-cost processing techniques.

Seismic reflection imaging of hydrostratigraphic facies in Boise: A tale of three scales

Summary We have acquired, processed and interpreted seismic reflection data from the Boise Valley at three scales to help build a hydrostratigraphic model for regional groundwater studies and to better understand the hydrostratigraphic significance of seismic boundaries. We use existing industry seismic reflection data to identify the structural and stratigraphic framework of the western Snake River Plain, a normal-fault bounded basin that contains more than 2 km of Neogene and younger sediments. To directly tie structure and stratigraphy to water well lithology and geophysical logs, we have acquired seismic reflection data throughout Boise to image near-surface sediments, where prograding delta and fluvial sediments control groundwater flow. To correlate seismic boundaries to hydrologic properties in Boise, we have also acquired seismic reflection data from a highly characterized wellfield. We find the seismic boundaries directly correspond to bulk changes in porosity at this site. Seismic reflection data from these three scales better define basin morphology and help map discrete hydrostratigraphic units necessary to understand hydraulic connectivity, groundwater flow directions, and aquifer capacity in the Boise Valley.

Natural degradation of earthworks, trenches, walls and moats, Northern Thailand

We here investigate the geometry, age, and history of several enigmatic northern Thailand earthwork entrenchments that are mostly located on hills and could not have held water to form moats. The earthworks are either oval or rectangular in plan view and typically encircle 0.3 to 1 sq km areas with no potsherd debris that would indicate former towns. Most trenches are 3–5 m deep with inner walls 4.5–8 m high. Some encircling earthworks are concentric double trenches spaced approximately 10 m apart. Historians have suggested these earthworks enclosed defensible areas where people in outlying villages sought refuge when under attack by neighboring rulers, the Chinese Ho, or the Burmese. We argue that some encircling entrenchments may have been for the capture or containment of elephants. Nearly all of the once near-vertical original walls have degraded to slopes of 32–47°. Fitting calculated curves of the diffusion-based scarp-degradation model to our height-slope data, and assuming most scarps have degraded since the end of La Na Kingdom time a.d. 1558, we derive a diffusion coefficient of 0.002 m2 y−1. Slopes of the rectangular earthwork at Souvannkhomkham, Laos, across the Mekong River from Chiang Saen Noi, are significantly more degraded (approximately 32°), indicating an age of 800–1200 years. Locations of these earthworks are established in hope that they will be preserved as part of the Thai and Lao archaeological legacy.

The establishment of a geophysics field camp in northern Thailand

As a participant in SEGs Geoscientists Without Borders program, we have developed a geophysics field camp in northern Thailand to train students and professionals from throughout Southeast Asia in field-based geophysical methods. Over the past two years, faculty, technicians, professionals, and students from 18 institutions and 11 countries have acquired, processed, and interpreted geophysical data at field sites in and around Chiang Mai, Thailand. Participation from undergraduate students, graduate students, and private and public sector geoscience professionals provides a broad base of experience, background, and insight. Our training has provided opportunities for cross-cultural collaboration and education, and a greater use of field-based geophysical methods for academic, private sector, and government agencies throughout Southeast Asia.