Hebgen Lake Research Articles

Shallow Lake, Strong Shake: Record of Seismically Triggered Lacustrine Sedimentation From the 1959 M7.3 Hebgen Lake Earthquake Within Henrys Lake, Idaho

AbstractWe investigate a shallow lake basin for evidence of a large historic intraplate earthquake in western North America. Henrys Lake, Idaho is an atypical candidate for lacustrine paleoseismic study given its shallow depth (∼7 m) and low relief (≤2° slopes). Here, we test the earthquake‐recording capacity of this basin type by showing sedimentological evidence of the 1959 M7.3 Hebgen Lake earthquake within sediment cores, using anthropogenically produced 137Cs activity to constrain timing. In addition to expanding the morphologic range of basins targeted for lacustrine paleoseismic studies, this work has implications for sediment response in dam‐enhanced basins. Lack of sedimentological evidence for other earthquakes coupled with radiocarbon chronology reveals that the 1959 event is the only clearly recorded earthquake within Henrys Lake since the mid‐Holocene. Henrys Lake offers a proxy for paleo‐earthquake signatures within similar lacustrine environments and underscores the importance of further paleoseismic studies in the region.

Revisiting the 1959 Hebgen Lake Earthquake Using Optical Image Correlation; New Constraints on Near‐Field 3D Ground Displacement

AbstractNear‐field surface displacement measurements allow us to quantify the on‐ and off‐fault proportion of earthquake‐related deformation. The Hebgen Lake earthquake was a large normal event with a complex surface rupture, which broke across mountainous terrain. This study takes advantage of high‐resolution historical aerial stereo‐imagery to measure three‐dimensional displacement from correlation of the orthorectified pre‐ and post‐earthquake image mosaics. The results reveal new strike‐slip ruptures which are possibly associated with the aftershocks from 18th August 1959. These structures likely reflect internal block deformation induced by the complex geometry of the mainshock. Additionally, comparison of our results with the existing displacement data shows that the optical image correlation‐derived offsets often exceed the field measurements by >50%. We attribute this difference to inelastic off‐fault deformation.

3-D displacement profiles along the Grayling Arm (Hebgen Lake)

3-D displacement profiles along the Grayling Arm (Hebgen Lake)

3-D displacement profiles obtained from optical image correlation of the Hebgen Lake area

3-D displacement profiles obtained from optical image correlation of the Hebgen Lake area

Microseismic Evidence for Bookshelf Faulting in Western Montana

AbstractOne of the most seismically active regions in the United States, located hundreds of kilometers inland from the nearest plate boundary, is the Intermountain Seismic Belt (ISB). The 6 July 2017 M 5.8 earthquake occurred 11 km southeast of Lincoln, Montana, within the ISB. This was the largest earthquake to rupture in the state of Montana since the 1959 M 7.3 Hebgen Lake earthquake. We use continuous seismic data from the University of Montana Seismic Network, the Montana Regional Seismic Network, and the U.S. Geological Survey to investigate the Lincoln aftershock sequence and to evaluate crustal stress conditions. We manually picked P- and S-wave arrival times, computed 4110 hypocenter locations and 2336 double-difference relocations, and generated focal mechanisms for 414 aftershocks (12+ polarities) in the 2 yr following the mainshock. Based on the alignment of aftershocks, we infer that the mainshock occurred on a north–northeast-trending left-lateral strike-slip fault. The orientation of the fault is unexpected, given that it strikes nearly perpendicular to the prominent Lewis and Clark line (LCL) faults in the area. Although most aftershocks concentrate near the mainshock, several distinct clusters of microseismic activity emerge along subparallel faults located primarily to the west of the mainshock. The subparallel faults also exhibit left-lateral strike-slip motion oblique to the LCL. We postulate that the aftershocks reveal the clockwise rotation of local-scale crustal blocks about vertical axes within a larger, right-lateral shear zone. The inferred block rotations are consistent with a bookshelf-faulting mechanism, which likely accommodates differential crustal motion to the north and south of the LCL region. The tension axes of well-constrained focal mechanisms indicate local northeast–southwest extension with a mean direction of N60°E.

The 2017–2018 Maple Creek Earthquake Sequence in Yellowstone National Park, USA

AbstractWe explore the detailed spatiotemporal evolution of 3,345 earthquakes that occurred near Maple Creek, Yellowstone, for the time period of 12 June 2017 to 13 March 2018. We generate high‐accuracy relocations and near source VP/VS ratios using 4.4 million P wave and S wave differential travel times derived from waveform cross correlation. The hypocenters can be subdivided geographically into two major subpopulations: a northern cluster with planar structures striking mainly NW‐SE and a southern cluster with planar structures striking mainly E‐W. We observe VP/VS ratios of 1.39–1.66 in the northern cluster and a steady ratio of 1.50 in the southern cluster, suggesting the presence of CO2‐filled cracks. We interpret the northern earthquake cluster primarily as long‐lived aftershocks of the 1959 Mw 7.2 Hebgen Lake earthquake but with some influence of magmatic fluids. We interpret the southern earthquake cluster as a more classic, swarm‐like sequence induced primarily by the migration of magmatic fluids.

Surface Rupture Morphology and Vertical Slip Distribution of the 1959Mw7.2 Hebgen Lake (Montana) Earthquake From Airborne Lidar Topography

AbstractThe 1959Mw∼7.2 Hebgen Lake earthquake is among the largest continental normal faulting events recorded, as well as one of the earliest associated with a multifault rupture. Multimeter vertical slip was observed on three main, morphologically distinct strands: the Hebgen fault and southeastern section of the Red Canyon fault, which both follow sharp topographic rangefronts, and the Red Canyon fault Kirkwood Ridge section, which cuts steep topography in the footwall of the Hebgen fault. We augment early field, seismological, and geodetic studies by investigating the modern surface rupture using newly acquired airborne lidar topography. By estimating throw from scarp profiling of the ∼36.5 km primary surface rupture, we show both that peak 1959 slip occurred at a structurally mature part of the fault and that many 1959 slip minima are associated with clear structural complexities. Vertical slip often substantially exceeds throw measured at the fault free face immediately after the earthquake; the scarps do not conclusively express beveled forms characteristic of repeated slip and degredation, yet must in places capture both the 1959 earthquake (for which we estimate an average throw of 2.64 m) and one or two preceding latest Pleistocene–Holocene events known from trenching. This has wider, cautionary implications for interpreting paleo‐earthquake chronologies and deriving magnitudes from morphologically simple scarps. By comparing 1959‐only and multievent vertical displacement populations, and considering preliminary paleoseismic data, we suggest that large surface‐rupturing earthquakes on the Hebgen and Red Canyon faults involve highly variable slip distributions.

Improving TRANS4D’s model for vertical crustal velocities in Western CONUS

Abstract The “Transformations in Four Dimensions” (TRANS4D) software was developed to enable geospatial professionals and others to transform 3-D positional coordinates referred to one date to corresponding positional coordinates referred to another date. For this purpose, TRANS4D incorporates 3-D crustal velocity models for most of the United States and Canada. In this report, an improved model for the vertical velocity field of that part of the conterminous United States that resides west of longitude 107°W is introduced. A new estimation process was employed so that this newer velocity field would have a spatial resolution of 0.0625° × 0.0625° in latitude and longitude, whereas the spatial resolution of TRANS4D’s previous model for this area was 0.25° × 0.25°. The realized improvements benefited from the inclusion of repeated geodetic data at approximately 1300 new locations and from the longer time spans of repeated geodetic data at other locations. After removing that part of the current vertical velocity field due to the glacial isostatic adjustment associated with the Last Glacial Maximum, features of the remaining vertical velocity field are discussed in terms of ongoing geophysical processes. These processes include subduction in the Pacific Northwest, uplift along the San Andreas Fault System, and subsidence due to groundwater extraction in California’s Central Valley. They also include uplift within both the Yellowstone Caldera and the Long Valley Caldera, uplift near Hebgen Lake in Montana, and subsidence near Lassen Peak Volcano in California.

Field estimate of paleoseismic slip on a normal fault using the Schmidt hammer and terrestrial LiDAR: Methods and application to the Hebgen fault (Montana, USA)

AbstractThe weathering characteristics of bedrock fault scarps provide relative age constraints that can be used to determine fault displacements. Here, we report Schmidt hammer rebound values (R‐values) for a limestone fault scarp that was last exposed in the 1959 Mw 7.3 Hebgen Lake, Montana earthquake. Results show that some R‐value indices, related to the difference between minimum and maximum R‐values in repeated impacts at a point, increase upward along the scarp, which we propose is due to progressive exposure of the scarp in earthquakes. An objective method is developed for fitting slip histories to the Schmidt hammer data and produces the best model fit (using the Bayesian Information Criterion) of three earthquakes with single event displacements of ≥ 1.20 m, 3.75 m, and c. 4.80 m. The same fitting method is also applied to new terrestrial LiDAR data of the scarp, though the LiDAR results may be more influenced by macro‐scale structure of the outcrop than by differential weathering. We suggest the use of this fitting procedure to define single event displacements on other bedrock fault scarps using other dating techniques. Our preliminary findings demonstrate that the Schmidt hammer, combined with other methods, may provide useful constraints on single event displacements on exposed bedrock fault scarps. Copyright © 2018 John Wiley & Sons, Ltd.

Evaluation of the evolving stress field of the Yellowstone volcanic plateau, 1988 to 2010, from earthquake first-motion inversions

Although the last rhyolite eruption occurred around 70ka ago, the silicic Yellowstone volcanic field is still considered active due to high hydrothermal and seismic activity and possible recent magma intrusions. Geodetic measurements document complex deformation patterns in crustal strain and seismic activity likewise reveal spatial and temporal variations in the stress field. We use earthquake data recorded between 1988 and 2010 to investigate these variations and their possible causes in more detail. Earthquake relocations and a set of 369 well-constrained, double-couple, focal mechanism solutions were computed. Events were grouped according to location and time to investigate trends in faulting. The majority of the events have normal-faulting solutions, subordinate strike-slip kinematics, and very rarely, reverse motions. The dominant direction of extension throughout the 0.64Ma Yellowstone caldera is nearly ENE, consistent with the perpendicular direction of alignments of volcanic vents within the caldera, but our study also reveals spatial and temporal variations. Stress-field solutions for different areas and time periods were calculated from earthquake focal mechanism inversion. A well-resolved rotation of σ3 was found, from NNE-SSW near the Hebgen Lake fault zone, to ENE-WSW near Norris Junction. In particular, the σ3 direction changed throughout the years around Norris Geyser Basin, from being ENE-WSW, as calculated in the study by Waite and Smith (2004), to NNE-SSW, while the other σ3 directions are mostly unchanged over time. The presence of “chocolate tablet” structures, with two sets of nearly perpendicular normal faults, was identified in many stages of the deformation history both in the Norris Geyser Basin area and inside the caldera.

Effects of lithospheric viscoelastic relaxation on the contemporary deformation following the 1959 Mw 7.3 Hebgen Lake, Montana, earthquake and other areas of the intermountain seismic belt

The 1959 Mw 7.3 Hebgen Lake, MT, normal‐faulting earthquake occurred in an extensional stress regime near the Yellowstone volcanic field. Time‐dependent crustal deformation data following this major earthquake were acquired by precise trilateration and GPS surveys from 1973 to 2000 around the Hebgen Lake fault zone. Modeling the changes of baseline lengths across and near the fault reveals a lateral variation of transient rheology, in which the lithosphere is stronger near the Hebgen Lake fault zone than in the vicinity of the Yellowstone volcano system. The models also imply that the lower crust is stronger than the upper mantle, in agreement with results from studies of postseismic and post‐lake‐filling relaxations (<~100 years). In addition, evaluations of the postseismic motion produced by the Hebgen Lake and the 1983 Mw 6.9 Borah Peak, ID, earthquakes indicate that horizontal transient motion of up to ~1 mm/yr contribute significantly to the contemporary regional crustal deformation near the epicentral areas. For the eastern Basin and Range, ~500 km south of the Hebgen Lake fault, similar rheologic models were derived from the observed uplift associated with the Lake Bonneville rebound and were used to evaluate the postseismic deformation associated with six most recent paleoearthquakes of the Wasatch fault zone and three M ≥ 5.6 historic earthquakes of northern Utah. The results show ≤0.1 mm/yr of horizontal postseismic motion at present time that are within the horizontal uncertainties of continuous GPS velocity from the Basin and Range and significantly smaller than the contemporary extension of 1–3 mm/yr in the Wasatch Front.

Fate of geothermal mercury from Yellowstone National Park in the Madison and Missouri Rivers, USA

Mercury is a worldwide contaminant derived from natural and anthropogenic sources. River systems play a key role in the transport and fate of Hg because they drain widespread areas affected by aerial Hg deposition, transport Hg away from point sources, and are sites of Hg biogeochemical cycling and bioaccumulation. The Madison and Missouri Rivers provide a natural laboratory for studying the fate and transport of Hg contributed by geothermal discharge in Yellowstone National Park and from the atmosphere for a large drainage basin in Montana and Wyoming, United States of America (USA). Assessing Hg in these rivers also is important because they support fishery-based recreation and irrigated agriculture. During 2002 to 2006, Hg concentrations were measured in water, sediment, and fish from the main stem, 7 tributaries, and 6 lakes. Using these data, the geothermal Hg load to the Madison River and overall fate of Hg along 378km of the Missouri River system were assessed. Geothermal Hg was the primary source of elevated total Hg concentrations in unfiltered water (6.2–31.2ng/L), sediment (148–1100ng/g), and brown and rainbow trout (0.12–1.23μg total Hg/g wet weight skinless filet) upstream from Hebgen Lake (the uppermost impoundment). Approximately 7.0kg/y of geothermal Hg was discharged from the park via the Madison River, and an estimated 87% of that load was lost to sedimentation in and volatilization from Hebgen Lake. Consequently, Hg concentrations in water, sediment, and fish from main-stem sites downstream from Hebgen Lake were not elevated and were comparable to concentrations reported for other areas affected solely by atmospheric Hg deposition. Some Hg was sequestered in sediment in the downstream lakes. Bioaccumulation of Hg in fish along the river system was strongly correlated (r2=0.76–0.86) with unfiltered total and methyl Hg concentrations in water and total Hg in sediment.

When Landslides Are Misinterpreted as Faults: Case Studies from the Western United States

We present several case studies from the western United States where faults are mapped on the basis of geomorphic and structural evidence that is equally likely to indicate landsliding. In some examples, faults have obscured evidence of landslides that utilized fault planes as rupture surfaces. In the Southern California examples, late Pleistocene or Holocene faults are mapped solely based on linear scarps. Such faults are often better explained by landsliding. Similarly, both landslides and faults have been proposed to explain prominent scarps and grabens in the Saddle Mountains of Washington. We note that both faulting and landsliding have been invoked by consultants and reviewers to explain offset Quaternary colluvium in observation pits and linear scarps in a subdivision in central Utah. Several subparallel linear scarps in granitic rock on a ridge top in the Southern California desert have also been mapped as faults. Recent studies, however, show that the features more likely indicate incipient landsliding that grades laterally into fully developed landslides. The Hebgen Lake, Montana, earthquake of 1959 produced landsliding as well as tectonic ground rupture. We suggest that an arcuate scarp that formed north of the primary ground rupture zone, previously interpreted as a fault, was likely produced by reactivation of a 6-mi-wide (9.7 km) landslide. We include a final case study where a combination of normal and thrust faulting mimics landsliding near St. George, Utah. Failure to correctly differentiate between landslides and faults leads to incorrect evaluation of a site's stability as well as incorrect evaluation of seismic hazard and ultimately impacts public health and safety.

Spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 from InSAR and GPS observations

In this study, the spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 is monitored using interferometric synthetic aperture radar (InSAR) data acquired by the European Remote-Sensing Satellites (ERS-1 and ERS-2) and the Environmental Satellite (ENVISAT). These data are combined with continuous global positioning system (GPS) measurements to identify four discrete episodes of caldera subsidence and uplift, these episodes are: 1992–1995 (subsidence of 2.7 cm/year), 1996–2000 (subsidence of 0.5 cm/year, with local uplift of 1.7 cm/year at Norris), 2000–2004 (subsidence of 0.7 cm/year, with local uplift of 0.6 cm/year at Norris), and 2004–2009 (uplift of 3–8 cm/year, with local subsidence of 1–4 cm/year at Norris). We construct the full three-dimensional velocity field of Yellowstone deformation for 2005–2006 from ascending and descending ENVISAT orbits. The InSAR three-dimensional velocity field and three-component GPS measurements indicate that the majority of the observed deformation (3–8 cm/year) across the Yellowstone caldera and near Norris Geyser Basin (NGB) occurred in the vertical direction between the summers of 2005 and 2006. During this time, significant lateral displacements of 3–7 cm/year also occurred in the east–west direction at the southeastern and western rims of the Yellowstone caldera and in the area between Hebgen Lake and NGB. Minor north–south displacements of about 0.2 cm/year also occurred, however, in the southwestern section of the caldera and near Yellowstone Lake during the same period. The calculated three-dimensional velocity field for 2005–2006 implies the existence of two pressure-point sources, beneath the two structural resurgent domes in the Yellowstone caldera, connected by a planar conduit, rather than a single, large sill as proposed in previous studies. Furthermore, no measurable displacements occurred along any fault zone across the caldera during the entire period of observation (1992–2009). Therefore, we infer that magmatic and hydrothermal processes beneath the Yellowstone caldera and NGB were the main sources of deformation.

A Mass Failure Model for the Initial Degradation of Fault Scarps, with Application to the 1959 Scarps at Hebgen Lake, Montana

Abstract Calculation of earthquake scarp ages from scarp morphology usually assumes that scarp materials reach their angle of repose immediately after a rupture. However, observations of the 1959 Hebgen Lake, Montana, earthquake scarp and similar features worldwide confirm that scarps require a finite period of mass failure to reach the initial conditions for hillslope diffusion, so the age of features less than 1000 yr old cannot be accurately estimated with methods based only on the linear diffusion equation. We apply a numerical model of this interval of mass failure degradation to vertical initial-angle scarps from the 1959 rupture at Hebgen Lake, Montana. The mass failure rate coefficient, R M , ranges from 1.0×10 -2 to 1.2×10 -1 m·yr -1 for young scarps at Hebgen Lake and nine other locations, and has little or no dependence on climate conditions such as annual temperature range or average rainfall. Including an interval of mass failure gives more accurate age estimates where a scarp age is of the same order as the characteristic mass failure relaxation time of 10–1000 yr.

Seismicity and earthquake hazard analysis of the Teton–Yellowstone region, Wyoming

Earthquakes of the Teton–Yellowstone region represent a high level of seismicity in the Intermountain west (U.S.A.) that is associated with intraplate extension associated with the Yellowstone hotspot including the nearby Teton and Hebgen Lake faults. The seismicity and the occurrence of high slip-rate late Quaternary faults in this region leads to a high level of seismic hazard that was evaluated using new earthquake catalogues determined from three-dimensional (3-D) seismic velocity models, followed by the estimation of the probabilistic seismic hazard incorporating fault slip and background earthquake occurrence rates. The 3-D P-wave velocity structure of the Teton region was determined using local earthquake data from the Jackson Lake seismic network that operated from 1986–2002. An earthquake catalog was then developed for 1986–2002 for the Teton region using relocated hypocenters. The resulting data revealed a seismically quiescent Teton fault, at M L, local magnitude > 3, with diffuse seismicity in the southern Jackson Hole Valley area but notable seismicity eastward into the Gros Ventre Range. Relocated Yellowstone earthquakes determined by the same methods highlight a dominant E–W zone of seismicity that extends from the aftershock area of the 1959 (M S surface wave magnitude) 7.5 Hebgen Lake, Montana, earthquake along the north side of the 0.64 Ma Yellowstone caldera. Earthquakes are less frequent and shallow beneath the Yellowstone caldera and notably occur along northward trending zones of activity sub-parallel to the post-caldera volcanic vents. Stress-field orientations derived from inversion of focal mechanism data reveal dominant E–W extension across the Teton fault with a NE–SW extension along the northern Teton fault area and southern Yellowstone. The minimum stress axes directions then rotate to E–W extension across the Yellowstone caldera to N–S extension northwest of the caldera and along the Hebgen Lake fault zone. The combination of accurate hypocenters, unified magnitudes, and seismotectonic analysis helped refine the characterization of the background seismicity that was used as input into a probabilistic seismic hazards analysis. Our results reveals the highest seismic hazard is associated with the Teton fault because of its high slip-rate of approximately 1.3 mm/yr compared to the highest rate of 1.4 mm/yr in southern Yellowstone on the Mt. Sheridan fault. This study demonstrates that the Teton–Yellowstone area is among the regions highest seismic hazard in the western U.S.

Earthquake swarm and b-value characterization of the Yellowstone volcano-tectonic system

The Yellowstone volcanic field, Yellowstone National Park, is one of the most seismically active areas of the western U.S., experiencing the deadly 1959 M7.5 Hebgen Lake, MT, earthquake adjacent to the 0.64-Ma caldera, as well as more than 30,000 earthquakes from 1973 to 2007. This well-recorded seismic activity offers the opportunity to study the temporal and spatial occurrence of earthquakes and extensive earthquake swarms and how they relate to active volcanic and tectonic processes. We characterize the distribution of earthquakes by analyzing the rate of occurrence characterized by the b-value. To accurately determine b-values, the earthquake catalog was filtered to identify statistically time- and spatially-dependent related events, defined as swarms, from independent single main and aftershocks. An algorithm was employed that identified 69 swarms for 1984–2006 based on inter-event times and spatial clustering. The swarms varied in duration from 1 to 46 days with the number of events varying from 30 to 722 with magnitudes of − 1.2 to 4.8. All of the swarm events as well as the 597 events triggered by the 2002 Denali fault, AK, earthquake were removed from the catalog for analysis. The catalog data were then filtered for a magnitude of completeness ( M COMP) of 1.5 and the b-value distribution for the Yellowstone region was determined with the de-swarmed data. b-values ranged from 0.6 ± 0.1 to 1.5 ± 0.05 with the highest values associated with the youthful 150,000-year old Mallard Lake resurgent dome. These variations are interpreted to be related to variations in stresses accompanying the migration of magmatic and hydrothermal fluids. An area of high b-values (up to 1.3 ± 0.1) associated with the Hebgen Lake fault zone west of the Yellowstone caldera could be related to the transport of magmatic fluids out of the Yellowstone volcanic system or could be indicative of a relative low stress regime resulting from the stress release by the Hebgen Lake earthquake. An area of low b-values (0.6 ± 0.1) south of the Yellowstone caldera is interpreted as evidence of a relatively higher stress regime associated with an area of dominantly extensional stress. This seismicity was associated with a nearly 90° change in the principal stress axes direction to northeast-southwest, compared to east-west within the Yellowstone caldera, and may be influenced by buoyancy loading by the Yellowstone hotspot.

Crustal deformation of the Yellowstone–Snake River Plain volcano‐tectonic system: Campaign and continuous GPS observations, 1987–2004

The Yellowstone–Snake River Plain tectonomagmatic province resulted from Late Tertiary volcanism in western North America, producing three large, caldera‐forming eruptions at the Yellowstone Plateau in the last 2 Myr. To understand the kinematics and geodynamics of this volcanic system, the University of Utah conducted seven GPS campaigns at 140 sites between 1987 and 2003 and installed a network of 15 permanent stations. GPS deployments focused on the Yellowstone caldera, the Hebgen Lake and Teton faults, and the eastern Snake River Plain. The GPS data revealed periods of uplift and subsidence of the Yellowstone caldera at rates up to 15 mm/yr. From 1987 to 1995, the caldera subsided and contracted, implying volume loss. From 1995 to 2000, deformation shifted to inflation and extension northwest of the caldera. From 2000 to 2003, uplift continued to the northwest while caldera subsidence was renewed. The GPS observations also revealed extension across the Hebgen Lake fault and fault‐normal contraction across the Teton fault. Deformation rates of the Yellowstone caldera and Hebgen Lake fault were converted to equivalent total moment rates, which exceeded historic seismic moment release and late Quaternary fault slip‐derived moment release by an order of magnitude. The Yellowstone caldera deformation trends were superimposed on regional southwest extension of the Yellowstone Plateau at up to 4.3 ± 0.2 mm/yr, while the eastern Snake River Plain moved southwest as a slower rate at 2.1 ± 0.2 mm/yr. This southwest extension of the Yellowstone–Snake River Plain system merged into east‐west extension of the Basin‐Range province.

Spatial variation in the response of tree rings to normal faulting during the Hebgen Lake Earthquake, Southwestern Montana, USA

Tree rings have frequently been used to identify the effects of earthquakes on forests, but little is known about spatial variation in the response of trees to intraplate normal faulting. This paper documents and describes the effects of tree location (distance from and position above or below the fault scarp), size and age on the response of tree rings to the 1959 magnitude 7.5 Hebgen Lake earthquake, which occurred along a normal fault in the Gallatin National Forest in southwestern Montana. Core samples from 88 trees were collected along nine 100-m transects straddling the Hebgen scarp, and from 28 additional large-diameter trees near the scarp. The most common tree-ring response to the earthquake was a suppression in growth, usually lasting for several years. Among samples from the transects, suppressions were significantly more common below vs. above the scarp, but this pattern was not found among the large tree samples. Distance of trees (within 58 m) from the fault scarp had little effect on tree-ring responses. These results illustrate the importance of interactions between tree location and tree size/age in identifying tree-ring responses to earthquakes. Smaller, younger trees required the direct movement of the downthrown block below the scarp to incur sufficient damage to record a suppression, whereas larger, older trees were damaged even on the stationary slope above the scarp. The small effect of distance from the scarp on suppressions suggests that event-response trees may be found further from a fault than previously thought.

Probabilistic Earthquake Relocation in Three-Dimensional Velocity Models for the Yellowstone National Park Region, Wyoming

Recorded seismicity for the Yellowstone National Park region, comprising 25,267 earthquakes from November 1972 to December 2002, has been relocated using three-dimensional velocity models and probabilistic earthquake location. In addition, new coda magnitudes for earthquakes between 1984 and 2002 were computed by using an improved coda magnitude equation. Three-dimensional velocity models for earthquake location were computed by inverting subsets of high-quality data of three different periods, 1973–1981, 1984–1994, and 1995–2002, for hypocenter locations and seismic velocities. Earthquakes were relocated by using a nonlinear, probabilistic solution to the earthquake location problem. Fully nonlinear location uncertainties included in the probabilistic solution allow a better and more reliable classification of earthquake locations into four quality classes. Earthquake locations show an improvement in location accuracy with time, which we attribute to improved network geometry and more precise timing of arrival times. No large systematic shifts of the relocated earthquake locations are observed, except a systematic shift of ∼2 km to greater depth. The new relocated earthquake locations show tighter clustering of epicenters and focal depths when compared with original earthquake locations. The most intense seismicity in terms of number of earthquakes and cumulative seismic moment release in the Yellowstone National Park region occurs northwest of the Yellowstone caldera between Hebgen Lake and the northern rim of the caldera. Seismicity within the Yellowstone caldera is diffuse, and shallow individual clusters of earthquakes can be associated with major hydrothermal areas.