Search USGS⌕ Search

SEARCH · Search USGS

Results for “Journal of Geophysical Research - Planets”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,027 records · Page 57Linked to original sources
USGS2012

Tektites in Cretaceous-Tertiary boundary rocks on Haiti and their bearing on the Alvarez impact extinction hypothesis

Relic tektites are associated with a Pt-group metal abundance anomaly and shocked minerals in a thin marl bed that marks the K-T boundary on Haiti. The presence of these three impact-produced materials at the precise K-T boundary enormously strengthens the Alvarez impact extinction hypothesis. The tektites occur in smectite spherules that have external shapes typical of tektites. Their chemical and physical properties are broadly similar to those of other tektite groups, except that the Haitian tektites have lower Si and higher Fe, Ca, and Na. On average, they contain more Sc, V, Cu, Zn, Ga, Sr, Sn, and Ba and less Cr, Ni, Co, B, Mn, and Hf than Other tektite groups. Amounts of rare earth elements (REE) in the tektites indicate that their progenitor materials were not melted mafic or ultramafic oceanic crust; rather they were sedimentary deposits having a bulk composition of andesite. Rare tektites contain unusually high amounts of CaO (∼20%) and S (0.4%), and these data suggest that some target materials consisted of CaSO 4 . Anhydrite beds occur in the subsurface at two candidate impact sites (Chicxulub and Manson). Sm-Nd isotopic data for the tektites indicate that the melted precursor sediments were most likely deposited less than −400 m.y. between the Late Cretaceous and Silurian periods. Major chemical changes accompanied the diagenetic change of glass to smectite. The Haitian tektites are the first datable impact products in K-T boundary rocks, and 40 Ar- 39 Ar ages of the glass show that the K-T boundary and impact event are coeval at 64.5±0.1 Ma.

Journal of Geophysical Research↗
USGS2012

Limitations of quantitative analysis of deep crustal seismic reflection data: Examples from GLIMPCE

Amplitude preservation in seismic reflection data can be obtained by a relative true amplitude (RTA) processing technique in which the relative strength of reflection amplitudes is preserved vertically as well as horizontally, after compensating for amplitude distortion by near-surface effects and propagation effects. Quantitative analysis of relative true amplitudes of the Great Lakes International Multidisciplinary Program on Crustal Evolution seismic data is hampered by large uncertainties in estimates of the water bottom reflection coefficient and the vertical amplitude correction and by inadequate noise suppression. Processing techniques such as deconvolution, F-K filtering, and migration significantly change the overall shape of amplitude curves and hence calculation of reflection coefficients and average reflectance. Thus lithological interpretation of deep crustal seismic data based on the absolute value of estimated reflection strength alone is meaningless. The relative strength of individual events, however, is preserved on curves generated at different stages in the processing. We suggest that qualitative comparisons of relative strength, if used carefully, provide a meaningful measure of variations in reflectivity. Simple theoretical models indicate that peg-leg multiples rather than water bottom multiples are the most severe source of noise contamination. These multiples are extremely difficult to remove when the water bottom reflection coefficient is large (>0.6), a condition that exists beneath parts of Lake Superior and most of Lake Huron.

Great Lakes↗
USGS2012

Interseismic deformation at the Nankai Trough, Japan, subduction zone

Uplift along the coast of southwestern Japan following the 1944 Tonankai ( M s = 8.0) and 1946 Nankaido ( M s = 8.2) earthquakes has been inferred from the 1950–1985 tide gage records. Although uplift rates vary with geographic position, the temporal dependence at each site can be described as the superposition of an exponentially decaying (time constant ∼5 years) transient, significant only in the first decade postseismic, and a steady interseismic rate. The steady interseismic uplift rate is defined only over the 1956–1985 interval, but the linearity in time ofthat uplift record is such that uplift over the remainder of the interseismic cycle (∼80 years) might reasonably be expected to occur at the same rate. Moreover, the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction. However, the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle. Thus, either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Seismic constraints on the nature of lower crustal reflectors beneath the extending Southern Transition Zone of the Colorado Plateau, Arizona

We determine the reflection polarity and exploit variations in P and S wave reflectivity and P wave amplitude versus offset (AVO) to constrain the origin of lower crustal reflectivity observed on new three-component seismic data recorded across the structural transition of the Colorado Plateau. The near vertical incidence reflection data were collected by Stanford University in 1989 as part of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that traversed the Arizona Transition Zone of the Colorado Plateau. The results of independent waveform modeling methods are consistent with much of the lower crustal reflectivity resulting from thin, high-impedance layers. The reflection polarity of the cleanest lower crustal events is positive, which implies that these reflections result from high-velocity contrasts, and the waveform character indicates that the reflectors are probably layers less than or approximately equal to 200 m thick. The lower crustal events are generally less reflective to incident S waves than to P waves, which agrees with the predicted behavior of high-velocity mafic layering. Analysis of the P wave AVO character of lower crustal reflections demonstrates that the events maintain a constant amplitude with offset, which is most consistent with a mafic-layering model. One exception is a high-amplitude (10 dB above background) event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets. The event has a pronounced S wave response, which along with its negative AVO trend is a possible indication of the presence of fluids in the lower crust. The Arizona Transition Zone is an active but weakly extended province, which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers. Instead, we favor horizontal basaltic intrusions as the primary origin of high-impedance reflectors based on (1) The fact that most xenoliths in eruptive basalts of the Transition Zone are of mafic igneous composition, (2) indications that a pulse of magmatic activity crossed the Transition Zone in the late Tertiary period, and (3) the high regional heat flow observed in the Transition Zone. The apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion. We present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust.

Arizona↗
USGS2012

Paleomagnetism and geochronology of 23 Ma gabbroic intrusions in the Keku Strait, Alaska, and implications for the Alexander Terrane

Samples of Tertiary gabbro from 24 sites in the Keku Strait, Alaska, help constrain the displacement history of the Alexander terrane. Step heating experiments on a plagioclase separate from these previously undated intrusions indicate a discordant 40 Ar/ 39 Ar age of 23.1 ± 1.7 Ma. The characteristic magnetization resides in magnetite, is easily isolated by thermal and alternating field demagnetization, and has both normal and reversed polarities. The mean paleomagnetic pole, with no structural correction, is latitude 87.1°N, longitude 141.6°E, A 95 = 10.1°, with N = 20 sites. This pole indicates insignificant tectonic displacement (0.5° ± 8.2° southward) and rotation (0.6° ± 15.2° counterclockwise). We therefore conclude that any northward displacement or vertical axis rotation of the Alexander terrane, and/or tilting in the vicinity of the Keku Strait must have occurred before 23 Ma.

Alaska↗
USGS2012

Paleomagnetism of the Late Triassic Hound Island Volcanics: Revisited

The collision and accretion of the Alexander terrane profoundly influenced the geologic history of Alaska and western Canada; however, the terrane's displacement history is only poorly constrained by sparse paleomagnetic studies. We studied the paleomagnetism of the Hound Island Volcanics in order to evaluate the location of the Alexander terrane in Late Triassic time. We collected 618 samples at 102 sites in and near the Keku Strait, Alaska, from the Late Triassic Hound Island Volcanics, the Permian Pybus Formation, and 23-Ma gabbroic intrusions. We found three components of magnetization in the Hound Island Volcanics. The high-temperature component (component A) resides in hematite and magnetite and was found only in highly oxidized lava flows in a geographically restricted area. We think it is primary, or acquired soon after eruption of the lavas, principally because the directions pass a fold test. The paleolatitude indicated by this component (19.2° ± 10.3°) is similar to those determined for various portions of Wrangellia, consistent with the geologic interpretation that the Alexander terrane was with the Wrangellia terrane in Late Triassic time. We found two overprint directions in the Hound Island Volcanics. Component B was acquired 23 m.y. ago due to intrusion of gabbroic dikes and sills. This interpretation is indicated by the similarity of upper-hemisphere directions in the Hound Island Volcanics to those in the gabbro. Component C, found in both the Hound Island Volcanics and the Permian Pybus Formation, is oriented northeast and down, fails a regional fold test, and was acquired after regional deformation around 90 to 100 Ma. This overprint direction yields a paleolatitude similar to, but slightly higher than, slightly older rocks from the Coast Plutonic Complex, suggesting that the Alexander terrane was displaced 17° in early Late Cretaceous time. The occurrence of these two separate overprinting events provides a satisfying explanation of the earlier puzzling results from the Hound Island Volcanics (Hillhouse and Grommé, 1980). Finally, great-circle analysis of the paleomagnetic data from the Pybus Formation suggests the Alexander terrane may have been in the northern hemisphere in Permian time.

Alaska↗
USGS2012

Inferred depth of creep on the Hayward Fault, central California

A relation between creep rate at the surface trace of a fault, the depth to the bottom of the creeping zone, and the rate of stress accumulation on the fault is derived from Weertman's 1964 friction model of slip on a fault. A 5 ± 1 km depth for the creeping zone on the Hayward fault is estimated from the measured creep rate (5 mm/yr) at the fault trace and the rate of stress increase on the upper segment of the fault trace inferred from geodetic measurements across the San Francisco Bay area. Although fault creep partially accommodates the secular slip rate on the Hayward fault, a slip deficit is accumulating equivalent to a magnitude 6.6 earthquake on each 40 km segment of the fault each century. Thus, the current behavior of the fault is consistent with its seismic history, which includes two moderate earthquakes in the mid-1800's.

Journal of Geophysical Research↗
USGS2012

Paleoseismology along the 1980 surface rupture of the Irpinia Fault: Implications for earthquake recurrence in the southern Apennines, Italy

The Irpinia fault was the source of the M s 6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. Trenching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre-1980 paleoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average 61 cm. The net vertical displacement of 2.12–2.36 m since 8600 cal year B.P. observed in the trenches gives a vertical slip rate of 0.25–0.35 mm/yr, a dip slip rate of 0.29–0.40 mm/yr, and an extension rate of 0.14–0.20 mm/yr. Although fault behavior data are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude ( M 7) earthquakes and (2) the Holocene rate of extension across the Apennines is ≤1 mm/yr. The 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation.

Apennine Mountains↗
USGS2012

Deformation from 1973 to 1987 in the epicentral area of the 1959 Hebgen Lake, Montana, earthquake (Ms = 7.5)

A 40‐km aperture trilateration network centered on the 1959 Hebgen Lake earthquake epicenter has been surveyed in 1973, 1974, 1976, 1978, 1981, 1984, and 1987. The deformation inferred from those surveys is described roughly by a uniaxial, 0.266 ± 0.014 μstrain/yr, N15°E ± 1°extension that is uniform in both time and space. That extension is orthogonal to the strike (N78°W ± 5°) of the 1959 rupture plane inferred from focal mechanism solutions. The absence of strain accumulation in the N75°W direction suggests that the source of deformation must be greatly elongated in that direction. Looked at in greater detail, the deformation is found to be concentrated in the northern part of the network. The zone of concentrated deformation coincides with a part of a well‐defined trend in seismicity that extends at least 100 km N75°W from the Sour Creek resurgent dome in the Yellowstone caldera. Extension perpendicular to that trend is indicated by focal mechanism solutions. Thus, the zone of rifting identified within the Hebgen Lake network apparently extends east southeast to the Yellowstone caldera. About 8 mm/yr extension is observed across a 20‐km width within the zone.

Montana, Wyoming↗
USGS2012

Edge-driven microplate kinematics

It is known from plate tectonic reconstructions that oceanic microplates undergo rapid rotation about a vertical axis and that the instantaneous rotation axes describing the microplate's motion relative to the bounding major plates are frequently located close to its margins with those plates, close to the tips of propagating rifts. We propose a class of edge-driven block models to illustrate how slip across the microplate margins, block rotation, and propagation of rifting may be related to the relative motion of the plates on either side. An important feature of these edge-driven models is that the instantaneous rotation axes are always located on the margins between block and two bounding plates. According to those models the pseudofaults or traces of disrupted seafloor resulting from the propagation of rifting between microplate and major plates may be used independently to approximately trace the continuous kinematic evolution of the microplate back in time. Pseudofault geometries and matching rotations of the Easter microplate show that for most of its 5 m.y. history, block rotation could be driven by the drag of the Nazca and Pacific plates on the microplate's edges rather than by a shear flow of mantle underneath.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Seasonal fluctuations in sea level on the South Carolina shelf and their relationship to the Gulf Stream

Near-bottom pressure measurements obtained over a 9-month period in 1978 on the outer continental shelf off South Carolina document that sea level rose 35 to 50 cm between July and October. Records of coastal sea level showed a similar rise. When the pressure and coastal sea level records were corrected for the effects of wind stress and temperature, the largest portion of the rise in the two variables remained. The amplitude of the change in residual sea level was largest at the shelf break, 28 cm, and decayed to 22 cm at the coast. The relatively small magnitude of the decay suggests that alongshelf pressure gradients imposed at the shelf break have minimum alongshelf wavelengths of 2000 km. The changes in residual sea level were seasonal and associated with seasonal changes in the transport of the Gulf Stream. The 3 years of residual sea level records examined here indicate that the amplitude of a typical decrease in transport of the Gulf Stream off South Carolina between July and October has a strong interannual variability. The change in transport can be 50% lower to over 100% higher than the historical average. The residual sea level records also suggest that a change in transport of the Gulf Stream is not strongly related to the onshore/offshore position of the Gulf Stream.

South Carolina↗
USGS2012

Modeling tidal exchange and dispersion in Boston Harbor

Tidal dispersion and the horizontal exchange of water between Boston Harbor and the surrounding ocean are examined with a high-resolution (200 m) depth-averaged numerical model. The strongly varying bathymetry and coastline geometry of the harbor generate complex spatial patterns in the modeled tidal currents which are verified by shipboard acoustic Doppler surveys. Lagrangian exchange experiments demonstrate that tidal currents rapidly exchange and mix material near the inlets of the harbor due to asymmetry in the ebb/flood response. This tidal mixing zone extends roughly a tidal excursion from the inlets and plays an important role in the overall flushing of the harbor. Because the tides can only efficiently mix material in this limited region, however, harbor flushing must be considered a two step process: rapid exchange in the tidal mixing zone, followed by flushing of the tidal mixing zone by nontidal residual currents. Estimates of embayment flushing based on tidal calculations alone therefore can significantly overestimate the flushing time that would be expected under typical environmental conditions. Particle-release simulations from point sources also demonstrate that while the tides efficiently exchange material in the vicinity of the inlets, the exact nature of dispersion from point sources is extremely sensitive to the timing and location of the release, and the distribution of particles is streaky and patchlike. This suggests that high-resolution modeling of dispersion from point sources in these regions must be performed explicitly and cannot be parameterized as a plume with Gaussian-spreading in a larger scale flow field.

Massachusetts↗
USGS2012

Effects of hydrothermal alteration on the magnetization of the Oligocene Carpenter Ridge Tuff, Bachelor Caldera, San Juan Mountains, Colorado

Intracaldera Oligocene Carpenter Ridge Tuff fills the Bachelor caldera in the central San Juan caldera complex and hosts mineral deposits of the Creede mineral district. The Carpenter Ridge Tuff and unaltered portions of its intracaldera Bachelor Mountain Member, have strong, high‐coercivity, reverse magnetizations with average magnetic susceptibility (MS) and natural remanent magnetization (NRM) of 6 × 10 −3 volume SI and 8 A/m, respectively. Oxide phenocrysts in these rocks are titanomagnetite and ilmenite; however, magnetization appears to be controlled by microcrystic titanomaghemite based on thermal demagnetization unblocking temperatures and Curie temperatures between 580°C and 620°C. Much of the intracaldera tuff was affected by potassic metasomatism, a type of hydrothermal alteration characterized by addition of K and loss of Ca and Na, between 27.3 Ma and 25.1 Ma. Potassic metasomatism resulted in the replacement of original feldspars by potassium feldspar and quartz, oxidation of the original oxide phenocrysts to hematite and rutile, and consequent suppression of MS and NRM by a factor of 5. Also present, however, are metasomatized rocks that have high magnetizations even though their original oxide phenocrysts were destroyed; values of MS and NRM are similar to those of outflow tuff. Such rocks are suspected of containing secondary magnetite on the basis of (1) Curie and thermal unblocking temperatures at and below 580°C, indicating that magnetization is carried by magnetite but that primary microcrysts of maghemite were destroyed, and (2) observation in one sample of magnetite rimming cores of hematite+rutile. The presence of secondary magnetite in metasomatized rocks requires a local shift to more reducing fluid chemistry, although we are unable to determine whether this shift occurred during potassic metasomatism or during later alteration or ore deposition. These altered rocks may record a change from a pervasive alteration by alkaline, oxidizing fluids to a vein‐controlled alteration dominated by more reduced fluids in which magnetite was stable.

Colorado↗
USGS2012

Broad trends in geomagnetic paleointensity on Hawaii during Holocene time

Paleointensity determinations have been obtained from 22 basaltic lava flows on the island of Hawaii using the Thelliers' method. Radiocarbon dating indicates that these flows erupted at intervals ranging from about 200 to 1000 years, and results of the experiments provide an estimate of broad trends in geomagnetic paleointensity during Holocene time in the vicinity of Hawaii. Most of the samples were obtained from quickly cooled flow margins and, as a consequence, typically contain two titanomagnetite populations. Only two of the 79 samples analyzed failed to yield an estimate of the paleofield, demonstrating that paleointensities can be obtained from such samples if carefully selected. Virtual dipole moments calculated for the flows are compared with a published curve of dipole field intensity that was constructed using worldwide archeomagnetic data. The large nondipole fields that were previously postulated for the vicinity of Hawaii are confirmed, and the present data indicate that they were present from about 5000 years B.P. to perhaps as recently as the past 200 years. The data indicate, however, that these nondipole sources must have been virtually absent between about 12,000 and 5000 years B.P. as they are at the present time.

Hawaii↗
USGS2012

Magnetic grain-size variations through an ash flow sheet: influence on magnetic properties and implications for cooling history

Rock magnetic studies of tuffs are essential to the interpretation of paleomagnetic data derived from such rocks, provide a basis for interpretation of aeromagnetic data over volcanic terranes, and yield insights into the depositional and cooling histories of ash flow sheets. A rhyolitic ash flow sheet, the Miocene-aged Tiva Canyon Member of the Paintbrush Tuff, contains both titanomagnetite phenocrysts, present in the magma prior to eruption, and cubic Fe-oxide microcrystals that grew after emplacement. Systematic variations in the quantity and magnetic grain size of the microcrystals produce large variations in magnetic properties through a section of the ash flow sheet penetrated in a borehole on the Nevada Test Site. Natural remanent magnetization varies from less than 1 × 10 −4 to more than 8 × 10 −4 A m 3 kg −1 , and in-phase magnetic susceptibility varies from less than 1 × 10 −6 to more than 10 × 10 −6 m 3 kg −1 . The microcrystals, which include both magnetite and maghemite, have Curie points and maximum unblocking temperatures between 580°C and 640°C. Rock magnetic data, including in-phase and quadrature magnetic susceptibilities as well as hysteresis parameters, demonstrate that these microcrystals are of superparamagnetic and single-domain sizes. Titanomagnetite phenocrysts are the dominant remanence carriers in the central 50 m of the section, whereas microcrystals are important contributors to remanent magnetization and magnetic susceptibility in two 15-m-thick zones at the top and bottom. Within these zones the size of microcrystals decreases both toward the quenched margins and toward the interior of the sheet. The decrease in microcrystal size toward the interior of the sheet is interpreted to indicate the presence of a cooling break; possibly represented by a concentration of pumice. Laboratory heating produces changes in magnetic properties which vary complexly from sample to sample. Such changes include (1) decrease in saturation magnetization, (2) both increase and decrease in in-phase and quadrature magnetic susceptibilities, and (3) shifts in coercivity spectra. The complex sample-to-sample variation in response to heating is attributed to differences in initial grain-size distributions.

Journal of Geophysical Research↗
USGS2012

Predictive modeling of the seismic cycle of the greater San Francisco Bay region

The seismic cycle for the San Francisco Bay region is synthesized by a model combining the pre- and post-1906 seismic histories. The long-term acceleration of seismic release (seismic moment, Benioff strain release, or event count) in the seismic cycle and the shorter-term accelerations preceding the larger earthquakes within that cycle are modeled using an empirical predictive technique, called a time-to-failure analysis, in which rate of seismic release is proportional to an inverse power of the remainng time to failure. The exponent of time to failure in the accelerating sequences appears to be scale invariant, and the length of the full cycle is estimated at 269±50 yr. The 1989 Loma Prieta earthquake should have been predictable with an uncertainty of two years in time and 0.5 in magnitude, although the specific location (at Loma Prieta) was not predictable by this technique. If the model is correct, the San Francisco Bay region should be entering a relatively long (20-50 yr) period of seismic quiescence above magnitude 6.

California↗
USGS2012

An episode of reinflation of the Long Valley Caldera, eastern California: 1989-1991

Following the episodes of inflation of the resurgent dome associated with the May 1980 earthquake sequence (four M 6 earthquakes) and the January 1983 earthquake swarm (two M 5.2 events), 7 years of frequently repeated two-color geodimeter measurements spanning the Long Valley caldera document gradually decreasing extensional strain rates from 5 ppm/yr in mid-1983, when the measurements began, to near zero in mid-1989. The corresponding seismic activity within the caldera persisted at a low rate of fewer than 10 M ≥ 1.2 earthquakes per week from 1985 through November 1989 with no events exceeding M 3.0. Early October 1989 marked a change in activity when measurements of the two-color geodimeter network showed a significant increase in extensional strain rate (9 ppm/yr) across the caldera. The seismic activity began exceeding 10 M ≥ 1.2 per week in early December 1989 and rapidly increased to a sustained level of tens of M ≥ 1.2 per week with bursts having hundreds of events per day. Many events exceeded M 3.0 and the largest event was M ≈ 4. The 1989–1991 inflation episode is the first time that we have sufficient geodetic measurements in Long Valley to define the temporal relation between onset of an inflation episode and onset of brittle failure (earthquake swarm within the caldera). Here, the onset of deformation preceded the onset of increased earthquake activity by more than 2 months. The seismicity rate began to decrease in mid-July 1990, consistent with a gradually slowing of extension across the caldera as measured by the two-color geodimeter. The recent episode of inflation can be modeled by a single Mogi point source located about 7 km beneath the center of the resurgent dome. In contrast, the deformation pattern observed between mid-1983 and mid-1989 is best reproduced by fault slip in the south moat, inflation at 6.5 km depth near Casa Diablo Hot Springs and inflation beneath the resurgent dome. It appears that the 7-km source beneath the resurgent dome that was active for the earlier episodes is the primary source for the more recent episode. The model used to satisfy the line length observations predicts 7.5 cm of uplift along leveling route along highway 395 from mid-1983 to mid-1989 and an additional 11 cm through the end of 1991. To comp are with the energy release from seismicity, the modeled inflation from late 1989 through the end of 1991 has a moment that is a factor of 40 more than the cumulative seismic moment from earthquakes located within the caldera from the same period. Thus the recent inflation episode represents a significant portion of the observed geodetic deformation with only little seismic energy release.

Journal of Geophysical Research↗
USGS2012

Helium isotope and gas discharge variations associated with crustal unrest in Long Valley Caldera, California, 1989-1992

The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within ∼4 months by a large increase in 3 He/ 4 He in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 RA in July 1990 and subsequently declined to values near 5 R A . Potential sources of the 3 He‐rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a 3 He‐rich gas chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7 R A ), the persistence of relatively high values (>5 R A ) over a period of 3 years, the increase in the flux of total He relative to gases in air‐saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased 3 He/ 4 He ratios in the fumarolic gas, and that the intrusive process may have persisted for ∼1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome‐south moat region during the 1989–1991 period resulted in at most relatively small changes in 3 He/ 4 He in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of 3 He‐rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with Mammoth Mountain.

California↗