Search

A monitoring well must be investigated before it is destroyed or abandoned to determine both its condition and the details of its construction. The well will be sounded (its depth [...]

The material properties and distribution of faults above the seismogenic zone promote or inhibit earthquake rupture propagation. We document the depths and mechanics of fault slip along the seismically active Hurricane fault, UT, with scanning and transmission electron microscopy and hematite (U‐Th)/He thermochronometry. Hematite occurs as mm‐scale, striated patches on a >10 m2thin, mirror‐like silica fault surface. Hematite textures include bulbous aggregates and cataclasite, overlain by crystalline Fe‐oxide nanorods and an amorphous silica layer at the slip interface. Textures reflect mechanical, fluid, and heat‐assisted amorphization of hematite and silica‐rich host rock that weaken the fault and promote rupture propagation. Hematite (U‐Th)/He dates document episodes of mineralization and fault slip between 0.65 and 0.36 Ma at ∼300 m depth. Data illustrate that some earthquake ruptures repeatedly propagate along localized slip surfaces in the shallow crust and provide structural and material property constraints for in models of fault slip. Earthquake ruptures can travel to the Earth's surface along discrete, large faults, or earthquake energy may be consumed in the shallow crust by the creation of small fault networks and fractured rock, which may reduce ground shaking intensity. Estimating earthquake hazards requires knowledge of subsurface material properties and how they change to promote or inhibit localized faulting. We investigate the Hurricane fault, UT, part of the Intermountain Seismic Belt or a north–south trending zone of recorded seismicity in the western US, which has the potential for large earthquakes (up to magnitude 7). We target hematite, an iron‐oxide mineral, on a mirror‐like, silica fault surface with microscopy and radiometric dating to document textural changes and the timing and depth of past fault slip. Nanoscale textures indicate the physical breakdown of hematite and surrounding rock, followed by the growth of new hematite and solidification of a silica surface layer, during an earthquake. Radioisotopic analyses capture hematite mineralization and fault slip 0.65–0.36 million years ago at shallow depths (∼300 m). In this example, the combination of mechanical and hydrothermal processes weaken fault materials, leading to repeated propagation of earthquake ruptures toward the surface along a discrete fault. Hematite textures and (U‐Th)/He dates record mineralization and slip at ∼0.65–0.36 Ma and ∼300 m depth on the seismogenic Hurricane faultComminution and hydrothermal fluids cause amorphization of hematite and adjacent host rock that weaken the fault during seismic slipData demonstrate earthquake ruptures repeatedly propagate along localized slip surfaces in the shallow crust Hematite textures and (U‐Th)/He dates record mineralization and slip at ∼0.65–0.36 Ma and ∼300 m depth on the seismogenic Hurricane fault Comminution and hydrothermal fluids cause amorphization of hematite and adjacent host rock that weaken the fault during seismic slip Data demonstrate earthquake ruptures repeatedly propagate along localized slip surfaces in the shallow crust

Analysis of 201 crayfish burrows (Camborygma eumekenomos) from three fluvial beds of the Chinle Formation (Upper Triassic, Utah, U.S.A.) provides results consistent with knowledge of Holocene crayfish ecology. Thus, many aspects of their population ecology may have remained unchanged since the early Mesozoic. A significant increase in crayfish size away from the fluvial channel reflects size (age) segregation along an environmental gradient. The high lateral (within-bed) variation in burrow density may have been caused by spatial heterogeneity in water table and soil moisture levels. In each of the three analyzed beds, the burrows record a single ecologic generation of a monospecific crayfish population. The three beds differ in terms of the mean burrow diameter (this may reflect differences either in the average size/age of the resident crayfish or in the lateral extent of sampling). However, the overall shape of the burrow-size distribution is similar for all three beds (unimodal and close-to-normal). This reflects similar paleohydrologic and paleoecologic conditions through time, as substantiated by independent ichnologic and sedimentologic evidence.

Several letters to the editor are presented in response to articles and topics that appeared in previous issues of "Prevention," including "Field of Dreams," "The Joy of Yes" by Jeanne Marie Laskas," and an article about how to make energy bars.