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At least 1,063 records · Page 59Linked to original sources
USGS2008

Ultrahigh resolution topographic mapping of Mars with MRO HiRISE stereo images: Meter-scale slopes of candidate Phoenix landing sites

The objectives of this paper are twofold: first, to report our estimates of the meter‐to‐decameter‐scale topography and slopes of candidate landing sites for the Phoenix mission, based on analysis of Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images with a typical pixel scale of 3 m and Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) images at 0.3 m pixel −1 and, second, to document in detail the geometric calibration, software, and procedures on which the photogrammetric analysis of HiRISE data is based. A combination of optical design modeling, laboratory observations, star images, and Mars images form the basis for software in the U.S. Geological Survey Integrated Software for Imagers and Spectrometers (ISIS) 3 system that corrects the images for a variety of distortions with single‐pixel or subpixel accuracy. Corrected images are analyzed in the commercial photogrammetric software SOCET SET (® BAE Systems), yielding digital topographic models (DTMs) with a grid spacing of 1 m (3–4 pixels) that require minimal interactive editing. Photoclinometry yields DTMs with single‐pixel grid spacing. Slopes from MOC and HiRISE are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully; only the Mars Exploration Rover (MER) B site in Meridiani Planum is smoother. MOC results at multiple locations have root‐mean‐square (RMS) bidirectional slopes of 0.8–4.5° at baselines of 3–10 m. HiRISE stereopairs (one per final candidate site and one in the former site) yield 1.8–2.8° slopes at 1‐m baseline. Slopes at 1 m from photoclinometry are also in the range 2–3° after correction for image blur. Slopes exceeding the 16° Phoenix safety limit are extremely rare.

Journal of Geophysical Research E: Planets↗
USGS2008

Mars Exploration Program 2007 Phoenix landing site selection and characteristics

To ensure a successful touchdown and subsequent surface operations, the Mars Exploration Program 2007 Phoenix Lander must land within 65° to 72° north latitude, at an elevation less than −3.5 km. The landing site must have relatively low wind velocities and rock and slope distributions similar to or more benign than those found at the Viking Lander 2 site. Also, the site must have a soil cover of at least several centimeters over ice or icy soil to meet science objectives of evaluating the environmental and habitability implications of past and current near‐polar environments. The most challenging aspects of site selection were the extensive rock fields associated with crater rims and ejecta deposits and the centers of polygons associated with patterned ground. An extensive acquisition campaign of Odyssey Thermal Emission Imaging Spectrometer predawn thermal IR images, together with ∼0.31 m/pixel Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment images was implemented to find regions with acceptable rock populations and to support Monte Carlo landing simulations. The chosen site is located at 68.16° north latitude, 233.35° east longitude (areocentric), within a ∼50 km wide (N‐S) by ∼300 km long (E‐W) valley of relatively rock‐free plains. Surfaces within the eastern portion of the valley are differentially eroded ejecta deposits from the relatively recent ∼10‐km‐wide Heimdall crater and have fewer rocks than plains on the western portion of the valley. All surfaces exhibit polygonal ground, which is associated with fracture of icy soils, and are predicted to have only several centimeters of poorly sorted basaltic sand and dust over icy soil deposits.

Journal of Geophysical Research E: Planets↗
USGS2008

Hematite spherules at Meridiani: results from MI, Mini-TES, and Pancam

We report on observations of hematite‐bearing spherules at Meridiani Planum made using the Microscopic Imager (MI), Mini‐Thermal Emission Spectrometer (Mini‐TES), and Panoramic Camera (Pancam) instruments on the Mars Exploration Rover Opportunity. Spherules were observed on soil surfaces and in outcrop rocks, both on undisturbed surfaces and in abraded surfaces ground using the Rock Abrasion Tool (RAT). Spherule size and shape change little along the 850 m eastward traverse from Eagle Crater to Endurance Crater, but spherules decrease and then slightly increase in size along the 6 km traverse from Endurance south to Victoria Crater. Local populations range from submillimeters to several millimeters in diameter. An additional small diameter (100 μ m) size population is possible. An increase in irregular shapes is found near Victoria Crater. This, combined with the size decrease south of Endurance, suggests either a changing depositional environment, or variation in the duration and timing of diagenetic events. The dominant smaller size population observed early in the mission in aeolian areas and ripple crests is observed as the primary size population in abraded outcrop farther south. This suggests that successively younger beds are exposed at the surface along the southward traverse. Stratigraphically higher units removed by erosion could be recorded by the present surface lag deposit. Coordinated systematic observations are used to determine optical and infrared hematite indices of the surface soils in Pancam and Mini‐TES. In spite of the systematic variation seen in MI, both Pancam and Mini‐TES indices are highly variable based on the local surface, and neither show systematic trends south of Endurance. The lack of a 390 cm −1 feature in Mini‐TES spectra suggests concentric or radial interior structure within the spherules at scales too fine for MI to observe. Mini‐TES does not detect any silicate component in the spherules. A bound water component in soils or in exchange with the atmosphere is observed. These spherules have been previously interpreted as concretions formed within what were once water‐saturated, diagenetically altered “dirty evaporate” sandstone sediments. Our observations support this interpretation; however, no single terrestrial analog provides a model that can account for all attributes of the spherules on Mars.

Journal of Geophysical Research↗
USGS2008

A prelanding assessment of the ice table depth and ground ice characteristics in Martian permafrost at the Phoenix landing site

We review multiple estimates of the ice table depth at potential Phoenix landing sites and consider the possible state and distribution of subsurface ice. A two-layer model of ice-rich material overlain by ice-free material is consistent with both the observational and theoretical lines of evidence. Results indicate ground ice to be shallow and ubiquitous, 2-6 cm below the surface. Undulations in the ice table depth are expected because of the thermodynamic effects of rocks, slopes, and soil variations on the scale of the Phoenix Lander and within the digging area, which can be advantageous for analysis of both dry surficial soils and buried ice-rich materials. The ground ice at the ice table to be sampled by the Phoenix Lander is expected to be geologically young because of recent climate oscillations. However, estimates of the ratio of soil to ice in the ice-rich subsurface layer suggest that that the ice content exceeds the available pore space, which is difficult to reconcile with existing ground ice stability and dynamics models. These high concentrations of ice may be the result of either the burial of surface snow during times of higher obliquity, initially high-porosity soils, or the migration of water along thin films. Measurement of the D/H ratio within the ice at the ice table and of the soil-to-ice ratio, as well as imaging ice-soil textures, will help determine if the ice is indeed young and if the models of the effects of climate change on the ground ice are reasonable.

Journal of Geophysical Research E: Planets↗
USGS2009

Transport slopes, sediment cover, and bedrock channel incision in the Henry Mountains, Utah

[1] Field data from channels in the Henry Mountains of Utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds (the cover effect). We compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit (Navajo Sandstone) but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources. Bedrock channels that contain abundant clasts (diorite-rich) have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment (diorite-poor). The diorite-poor tributaries have incised more deeply to lower average slopes and have more reach-scale slope variability, which may reflect bedrock properties, longitudinal sediment sorting, and incision at lower sediment supply. Diorite-rich channels have less bedrock exposed and smoother longitudinal profiles than diorite-poor channels. We find that (1) coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision, validating the hypothesized cover effect in fluvial incision models; (2) the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock, suggesting that the slope of incising bedrock channels can become adjusted to the sediment load; (3) when abundant sediment is available, transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope; and (4) cover effects can be important even when moderate amounts of bedrock are exposed in channel beds.

Utah↗
USGS2007

Influence of disturbance on carbon exchange in a permafrost collapse and adjacent burned forest

We measured CO 2 and CH 4 exchange from the center of a Sphagnum ‐dominated permafrost collapse, through an aquatic moat, and into a recently burned black spruce forest on the Tanana River floodplain in interior Alaska. In the anomalously dry growing season of 2004, both the collapse and the surrounding burned area were net sinks for CO 2 , with a mean daytime net ecosystem exchange of −1.4 μ mol CO 2 m −2 s −1 , while the moat was a CH 4 source with a mean flux of 0.013 μ mol CH 4 m −2 s −1 . Regression analyses identified temperature as the dominant factor affecting intragrowing season variation in CO 2 exchange and soil moisture as the primary control influencing CH 4 emissions. CH 4 emissions during the wettest portion of the growing season were four times higher than during the driest periods. If temperatures continue to warm, peatland vegetation will likely expand with permafrost degradation, resulting in greater carbon accumulation and methane emissions for the landscape as a whole.

Alaska↗
USGS2008

Short-term response of methane fluxes and methanogen activity to water table and soil warming manipulations in an Alaskan peatland

Growing season CH 4 fluxes were monitored over a two year period following the start of ecosystem-scale manipulations of water table position and surface soil temperatures in a moderate rich fen in interior Alaska. The largest CH 4 fluxes occurred in plots that received both flooding (raised water table position) and soil warming, while the lowest fluxes occurred in unwarmed plots in the lowered water table treatment. A combination of treatment and soil hydroclimate variables explained more than 70% of the variation in ln-transformed CH 4 fluxes, with mean daily water table position representing the strongest predictor. We used quantitative PCR of the α -subunit of mcr operon to explore the influence of soil climate manipulations on methanogen abundances. Methanogen abundances were greatest in warmed plots, and showed a positive relationship with mean daily CH 4 fluxes. Our results show that water table manipulations that led to soil inundation (flooding) had a stronger effect on CH 4 fluxes than water table drawdown. Seasonal CH 4 fluxes increased by 80–300% under the combined wetter and warmer soil climate treatments. Thus, while warming is expected to increase CH 4 emissions from Alaskan wetlands, higher water table positions caused by increases in precipitation or disturbances such as permafrost thaw that lead to thermokarst and flooding in wetlands will stimulate CH 4 emissions beyond the effects of soil warming alone. Consequently, we argue that modeling the effects of climate change on Alaskan wetland CH 4 emissions needs to consider the interactive effects of soil warming and water table position on CH 4 production and transport.

Alaska↗
USGS2008

Lightning‐driven electric fields measured in the lower ionosphere: Implications for transient luminous events

Transient luminous events above thunderstorms such as sprites, halos, and elves require large electric fields in the lower ionosphere. Yet very few in situ measurements in this region have been successfully accomplished, since it is typically too low in altitude for rockets and satellites and too high for balloons. In this article, we present some rare examples of lightning‐driven electric field changes obtained at 75–130 km altitude during a sounding rocket flight from Wallops Island, Virginia, in 1995. We summarize these electric field changes and present a few detailed case studies. Our measurements are compared directly to a 2D numerical model of lightning‐driven electromagnetic fields in the middle and upper atmosphere. We find that the in situ electric field changes are smaller than predicted by the model, and the amplitudes of these fields are insufficient for elve production when extrapolated to a 100 kA peak current stroke. This disagreement could be due to lightning‐induced ionospheric conductivity enhancement, or it might be evidence of flaws in the electromagnetic pulse mechanism for elves.

Journal of Geophysical Research A: Space Physics↗
USGS2009

Three‐dimensional model of Hellenic Arc deformation and origin of the Cretan uplift

[1] The Hellenic Arc of Greece is the most seismically active part of Europe, but little is know about its mechanics. We modeled deformation along the arc using a finite element model. The model was intended to capture large‐scale 3‐D structure of Nubian plate subduction beneath the Aegean block and its deformational consequences. The shape of the interface was developed using mapped traces at the surface and earthquake hypocenters at depth. Model block motions were constrained by recent compilations of GPS velocity vectors. We simulated a 10 ka period of convergence between Nubia and the Aegean and calculated the strain field in the overriding plate as well as the spatial distribution and orientation of differential stress (∣ σ 1 − σ 3 ∣). From these calculations we derived testable quantities such as the expected seismic moment rate on the interplate contact, uplift pattern, and distribution of strain modes. Our relatively simple model broadly reproduced observed uplift patterns, earthquake activity, and loci of extension and contraction. The model showed a localization of uplift near the island of Crete, where the fastest Aegean uplift rates are well documented. Comparison of calculated expected seismic moment and observed earthquake catalogs implies a nearly fully coupled interplate contact. On the basis of our modeling results, we suggest that south Aegean deformation is driven primarily by the fast moving (∼33 mm a −1 ) Aegean upper plate overriding a nearly stalled (∼5 mm a −1 ) Nubian lower plate. This tectonic setting thus more closely resembles a continental thrust than it does a typical oceanic subduction zone.

Aegean Sea, Hellenic Arc↗
USGS2008

Postearthquake relaxation after the 2004 M6 Parkfield, California, earthquake and rate-and-state friction

An unusually complete set of measurements (including rapid rate GPS over the first 10 days) of postseismic deformation is available at 12 continuous GPS stations located close to the epicenter of the 2004 M6.0 Parkfield earthquake. The principal component modes for the relaxation of the ensemble of those 12 GPS stations were determined. The first mode alone furnishes an adequate approximation to the data. Thus, the relaxation at all stations can be represented by the product of a common temporal function and distinct amplitudes for each component (north or east) of relaxation at each station. The distribution in space of the amplitudes indicates that the relaxation is dominantly strike slip. The temporal function, which spans times from about 5 min to 900 days postearthquake, can be fit by a superposition of three creep terms, each of the form ??l loge(1 + t/??l), with characteristic times ??, = 4.06, 0.11, and 0.0001 days. It seems likely that what is actually involved is a broad spectrum of characteristic times, the individual components of which arise from afterslip on different fault patches. Perfettini and Avouac (2004) have shown that an individual creep term can be explained by the spring-slider model with rate-dependent (no state variable) friction. The observed temporal function can also be explained using a single spring-slider model (i.e., single fault patch) that includes rate-and-state-dependent friction, a single-state variable, and either of the two commonly used (aging and slip) state evolution laws. In the latter fits, the rate-and-state friction parameter b is negative.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

Premonitory acoustic emissions and stick-slip in natural and smooth-faulted Westerly granite

A stick-slip event was induced in a cylindrical sample of Westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 MPa. Continuously recorded acoustic emission (AE) data and computer tomography (CT)-generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault. The dynamic stick-slip event, considered to be a laboratory analog of an earthquake, generated an ultrasonic signal that was recorded as a large-amplitude AE event. First arrivals of this event were inverted to determine the nucleation site of slip, which is associated with a geometric asperity on the fault surface. CT images and AE locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault. This experiment is compared with a stick-slip experiment on a sample prepared with a smooth, artificial saw-cut fault surface. Nearly a thousand times more AE were observed for the natural fault, which has a higher friction coefficient (0.78 compared to 0.53) and larger shear stress drop (140 compared to 68 MPa). However at the measured resolution, the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments, suggesting a similar dynamic rupture process. We propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field-scale fault system in terms of the presence of asperities, fault surface heterogeneity, and interaction of branching faults. ?? 2009.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

Hydrometeor-enhanced tephra sedimentation: Constraints from the 18 May 1980 eruption of Mount St. Helens

Uncertainty remains on the origin of distal mass deposition maxima observed in many recent tephra fall deposits. In this study the link between ash aggregation and the formation of distal mass deposition maxima is investigated through reanalysis of tephra fallout from the Mount St. Helens 18 May 1980 (MSH80) eruption. In addition, we collate all the data needed to model distal ash sedimentation from the MSH80 eruption cloud. Four particle size subpopulations were present in distal fallout with modes at 2.2 Φ, 4.2 Φ, 5.9 Φ, and 8.3 Φ. Settling rates of the coarsest subpopulation closely matched predicted single‐particle terminal fall velocities. Sedimentation of particles <100 μ m was greatly enhanced, predominantly through aggregation of a particle subpopulation with modal diameter 5.9 ± 0.2 Φ (19 ± 3 μ m). Mammatus on the MSH80 cloud provided a mechanism to transport very fine ash particles, with predicted atmospheric lifetimes of days to weeks, from the upper troposphere to the surface in a matter of hours. In this mechanism, ash particles initiate ice hydrometeor formation high in the troposphere. Subsequently, the volcanic cloud rapidly subsides as mammatus develop from increased particle loading and cloud base sublimation. Rapid fallout occurs as the cloud passes through the melting level in a process analogous to snowflake aggregation. Aggregates sediment en masse and form the distal mass deposition maxima observed in many recent volcanic ash fall deposits. This work provides a data resource that will facilitate tephra sedimentation modeling and allow model intercomparisons.

Washington↗
USGS2009

Inducing in situ, nonlinear soil response applying an active source

[1] It is well known that soil sites have a profound effect on ground motion during large earthquakes. The complex structure of soil deposits and the highly nonlinear constitutive behavior of soils largely control nonlinear site response at soil sites. Measurements of nonlinear soil response under natural conditions are critical to advancing our understanding of soil behavior during earthquakes. Many factors limit the use of earthquake observations to estimate nonlinear site response such that quantitative characterization of nonlinear behavior relies almost exclusively on laboratory experiments and modeling of wave propagation. Here we introduce a new method for in situ characterization of the nonlinear behavior of a natural soil formation using measurements obtained immediately adjacent to a large vibrator source. To our knowledge, we are the first group to propose and test such an approach. Employing a large, surface vibrator as a source, we measure the nonlinear behavior of the soil by incrementally increasing the source amplitude over a range of frequencies and monitoring changes in the output spectra. We apply a homodyne algorithm for measuring spectral amplitudes, which provides robust signal-to-noise ratios at the frequencies of interest. Spectral ratios are computed between the receivers and the source as well as receiver pairs located in an array adjacent to the source, providing the means to separate source and near-source nonlinearity from pervasive nonlinearity in the soil column. We find clear evidence of nonlinearity in significant decreases in the frequency of peak spectral ratios, corresponding to material softening with amplitude, observed across the array as the source amplitude is increased. The observed peak shifts are consistent with laboratory measurements of soil nonlinearity. Our results provide constraints for future numerical modeling studies of strong ground motion during earthquakes.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

Seismic wave triggering of nonvolcanic tremor, episodic tremor and slip, and earthquakes on Vancouver Island

[1] We explore the physical conditions that enable triggering of nonvolcanic tremor and earthquakes by considering local seismic activity on Vancouver Island, British Columbia during and immediately after the arrival of large-amplitude seismic waves from 30 teleseismic and 17 regional or local earthquakes. We identify tremor triggered by four of the teleseismic earthquakes. The close temporal and spatial proximity of triggered tremor to ambient tremor and aseismic slip indicates that when a fault is close to or undergoing failure, it is particularly susceptible to triggering of further events. The amplitude of the triggering waves also influences the likelihood of triggering both tremor and earthquakes such that large amplitude waves triggered tremor in the absence of detectable aseismic slip or ambient tremor. Tremor and energy radiated from regional/local earthquakes share the same frequency passband so that tremor cannot be identified during these smaller, more frequent events. We confidently identify triggered local earthquakes following only one teleseism, that with the largest amplitude, and four regional or local events that generated vigorous aftershock sequences in their immediate vicinity. Earthquakes tend to be triggered in regions different from tremor and with high ambient seismicity rates. We also note an interesting possible correlation between large teleseismic events and episodic tremor and slip (ETS) episodes, whereby ETS events that are “late” and have built up more stress than normal are susceptible to triggering by the slight nudge of the shaking from a large, distant event, while ETS events that are “early” or “on time” are not.

Vancouver Island↗
USGS2009

Constraints on the stress state of the San Andreas fault with analysis based on core and cuttings from SAFOD drilling phases 1 and 2

Analysis of field data has led different investigators to conclude that the San Andreas Fault (SAF) has either anomalously low frictional sliding strength ( μ < 0.2) or strength consistent with standard laboratory tests ( μ > 0.6). Arguments for the apparent weakness of the SAF generally hinge on conceptual models involving intrinsically weak gouge or elevated pore pressure within the fault zone. Some models assert that weak gouge and/or high pore pressure exist under static conditions while others consider strength loss or fluid pressure increase due to rapid coseismic fault slip. The present paper is composed of three parts. First, we develop generalized equations, based on and consistent with the Rice (1992) fault zone model to relate stress orientation and magnitude to depth‐dependent coefficient of friction and pore pressure. Second, we present temperature‐ and pressure‐dependent friction measurements from wet illite‐rich fault gouge extracted from San Andreas Fault Observatory at Depth (SAFOD) phase 1 core samples and from weak minerals associated with the San Andreas Fault. Third, we reevaluate the state of stress on the San Andreas Fault in light of new constraints imposed by SAFOD borehole data. Pure talc ( μ ≈0.1) had the lowest strength considered and was sufficiently weak to satisfy weak fault heat flow and stress orientation constraints with hydrostatic pore pressure. Other fault gouges showed a systematic increase in strength with increasing temperature and pressure. In this case, heat flow and stress orientation constraints would require elevated pore pressure and, in some cases, fault zone pore pressure in excess of vertical stress.

California↗
USGS2008

Seismicity and deformation induced by magma accumulation at three basaltic volcanoes

We analyzed the evolution of volcano‐tectonic (VT) seismicity and deformation at three basaltic volcanoes (Kilauea, Mauna Loa, Piton de la Fournaise) during phases of magma accumulation. We observed that the VT earthquake activity displays an accelerating evolution at the three studied volcanoes during the time of magma accumulation. At the same times, deformation rates recorded at the summit of Kilauea and Mauna Loa volcanoes were not accelerating but rather tend to decay. To interpret these observations, we propose a physical model describing the evolution of pressure produced by the accumulation of magma into a reservoir. This variation of pressure is then used to force a simple model of damage, where damage episodes are equivalent to earthquakes. This model leads to an exponential increase of the VT activity and to an exponential decay of the deformation rate during accumulation phases. Seismicity and deformation data are well fitted by such an exponential model. The time constant, deduced from the exponential increase of the seismicity, is in agreement with the time constant predicted by the model of magma accumulation. This VT activity can thus be a direct indication of the accumulation of magma at depth, and therefore can be seen as a long‐term precursory phenomenon, at least for the three studied basaltic volcanoes. Unfortunately, it does not allow the prediction of the onset of future eruptions, as no diverging point (i.e., critical time) is present in the model.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

Postseismic relaxation following the 1992 M7.3 Landers and 1999 M7.1 Hector Mine earthquakes, southern California

Postseismic relaxation (measured postseismic deformation less the deformation that would have occurred at the preseismic rate) has been monitored at the same 10 monuments over ???6 years following both the 1992 Landers and the 1999 Hector Mine earthquakes. For both earthquakes the displacement components of the observed relaxation are well described by ?? i + ?? i f 1 (t), where ?? i and ?? i are constants peculiar to each component at each monument, t is the time after the earthquake, and f 1 (t) is a temporal function common to all components and all monuments for that earthquake. The temporal fanction f 1 (t) can be approximated by bt + c log e (1 + t /??), where ?? = 38.7 ?? 15.2 days and 25.6 ?? 7.7 days for the Landers and Hector Mine relaxations, respectively. Because the estimated values of ?? do not differ significantly, the transient term log e (1 + t/??) in the temporal function may be the same for both earthquakes. The asymptotic (t ??? ???) relaxation rates ?? i b are only a few mm/a and do not appear to be significantly different following the two earthquakes. The asymptotic deformation rates appear to be slightly greater than the preseismic deformation rates, but the preseismic rates were not measured directly. Thus, the deformations of the Landers array measured over the first 5.6 years following the Landers earthquake and over the first 6.4 years following the Hector Mine earthquake are generally consistent with a simple model in which the transient postearthquake relaxation depends upon time as log e (1 + t/??) with ?? = 28 ?? 5 days and the asymptotic postseismic speeds of the monuments exceed the preseismic speeds by at most only a few millimeters per annum.

Journal of Geophysical Research B: Solid Earth↗
USGS2009

A search in strainmeter data for slow slip associated with triggered and ambient tremor near Parkfield, California

We test the hypothesis that, as in subduction zones, slow slip facilitates triggered and ambient tremor in the transform boundary setting of California. Our study builds on the study of Peng et al. (2009) of triggered and ambient tremor near Parkfield, California during time intervals surrounding 31, potentially triggering, M ≥ 7.5 teleseismic earthquakes; waves from 10 of these triggered tremor and 29 occurred in periods of ambient tremor activity. We look for transient slow slip during 3-month windows that include 11 of these triggering and nontriggering teleseisms, using continuous strain data recorded on two borehole Gladwin tensor strainmeters (GTSM) located within the distribution of tremor epicenters. We model the GTSM data assuming only tidal and “drift” signals are present and find no detectable slow slip, either ongoing when the teleseismic waves passed or triggered by them. We infer a conservative detection threshold of about 5 nanostrain for abrupt changes and about twice this for slowly evolving signals. This could be lowered slightly by adding analyses of other data types, modeled slow slip signals, and GTSM data calibration. Detection of slow slip also depends on the slipping fault's location and size, which we describe in terms of equivalent earthquake moment magnitude, M. In the best case of the GTSM above a very shallow slipping fault, detectable slip events must exceed M~2, and if the slow slip is beneath the seismogenic zone (below ~15 km depth), even M~5 events are likely to remain hidden.

Journal of Geophysical Research B: Solid Earth↗