Geology topics

W. Thatcher

Publications and source records attributed to W. Thatcher.

At least 19 recordsLinked to original sources

USGS2012

Lithospheric loading by the 1896 Riku-u earthquake, northern Japan: Implications for plate flexure and asthenospheric rheology

Under favorable circumstances the time-dependent aseismic deformation resulting from the loading of the lithosphere by the stress drop of large dip slip earthquakes can be used to determine both the effective elastic plate thickness and the asthenospheric viscosity. The deformation has several similarities with the deflection of the lithosphere by surface loads and with movements due to postglacial rebound. Level changes obtained in the 80 years since the M = 7.5, 1896 Riku-u earthquake, an intraplate thrust event in northern Honshu, provide convincing evidence that asthenospheric readjustments are responsible for the observed movements. Leveling surveys crossing the zone of surface faulting have been repeated five times since 1900 and delineate a localized depression that has subsided at a continually decreasing rate. The depression is centered close to the 1896 faulting, and its shape and width, about 75 km, are matched by our model using a plate thickness of 30 km. The decaying subsidence rate constrains the viscosity of the uppermost asthenosphere to be 1×10 20 P. A linear viscous rheology matches the observed decay quite well, although measurements are sparse during the several decades following the earthquake.

Journal of Geophysical Research Solid Earth↗

USGS2012

Quaternary and geodetically measured crustal movements in the Tokai District, Central Honshu, Japan

Geodetic measurements made in this century demonstrate that modern movements correlate well with deformation that has occurred during the past 2 m.y. Both show effects of subduction and of the Quaternary collision of Izu Peninsula with Central Honshu. Westward from Suruga Bay, deformation is greatest near the coast, decreases markedly towards the uplifting Akaishi Mountains, and then increases again in the conjugate block-faulted terranes of Chubu district. Quaternary faults, historic earthquakes, and the distribution of horizontal shear straining since ∼1880 all reflect this deformation pattern. Since 1900 the west coast of Suruga Bay has undergone secular subsidence, horizontal contraction and tilting towards Suruga Trough, all typical of the strain buildup process that occurs landward of subduction zones. However, futher westward, active mountain building is occuring, and the Akaishi Ranges are rising 3 mm/a. Unlike the episodic aseismic uplift of the Transverse Ranges of southern California, movements here have been relatively uniform. It seems likely that an increase in the tilt rate of the Suruga coast recognized since ∼1973 is part of a long-term readjustment to the great 1944 Tonankai earthquake. Data show that a monotonic increase has occurred since at least 1949, and similar effects are seen adjacent to the 1944 rupture zone over 200 km southwest along the Nankai Trough.

Journal of Geophysical Research Solid Earth↗

USGS2012

Nonlinear strain buildup and the earthquake cycle on the San Andreas fault

Two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault, accompanied by a progressive broadening of the zone of deformation adjacent to it. In the thin lithosphere model, transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere. For an earth model with a thick elastic lithosphere (plate thickness » depth of seismic slip), transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane. Data from the San Andreas fault indicate a long-term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle. Observations support a cycle-long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar. If so, the usually lower strain rates and broader deformation zone currently observed on the southern San Andreas represent a later evolutionary stage of the northern locked section, where a great earthquake is a more recent occurrence. Although the data allow some extreme models to be discarded, no sufficiently strong constraints exist to decide between the thin and thick lithosphere models. Regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time-predictable model.

Journal of Geophysical Research Solid Earth↗

USGS2012

Order and diversity in the modes of circum-Pacific earthquake recurrence

Recurrence characteristics of great circum-Pacific earthquakes and determinations of spatial distribution of seismic moment release are surveyed in order to delineate their general features and relate earthquake slip distribution to models of recurrent rupture. As noted by others, the pattern of moment release is typically very irregular, with strong concentrations in a few isolated regions of a much larger aftershock zone. Despite this complexity, rupture nucleation is notably systematic, with mainshock epicenter showing a strong tendency to locate in or immediately adjacent to identified regions of high moment release. This generalization suggests that earthquake recurrence is more likely to be controlled by maximum rather than average fault slip. Well-documented case histories from twelve plate boundary regions demonstrate that seismic strain release tends to be temporally well-ordered, while source dimensions, slip, and cumulative moment release vary considerably from cycle to cycle. On none of the segments studied do earthquakes consistently recur in nearly identical events. Instead, individual great earthquakes differ significantly from cycle to cycle or rerupture takes place in a sequence of two or more smaller events. Despite these differences the duration of the seismic cycle is approximately uniform. Furthermore, when strain release occurs in a sequence of large earthquakes, these events take place towards the end of the cycle and occur with increasing rupture length and magnitude. These ordered and irregular features of earthquake recurrence argue for the existence of corresponding elements on plate boundary faults. The ordered characteristics are identified with zones of concentrated moment release that slip comparable amounts in each cycle and have high shear strength. The irregular features are associated with intervening weaker regions that move in response to the stress concentration of dynamic rupture and slip by differing amounts in each event. Although these mechanistic associations are indirect and tentative, the features of recurrent behavior documented here have implications for long-term earthquake hazard assessment that are not dependent on the models proposed to explain them. Chief among these are the cycle-to-cycle differences among gap-filling events and the absence of shocks that fill major slip-deficient regions early in the seismic cycle.

Journal of Geophysical Research Solid Earth↗

USGS2008

Temporal evolution of continental lithospheric strength in actively deforming regions

It has been agreed for nearly a century that a strong, load-bearing outer layer of earth is required to support mountain ranges, transmit stresses to deform active regions and store elastic strain to generate earthquakes. However the dept and extent of this strong layer remain controversial. Here we use a variety of observations to infer the distribution of lithospheric strength in the active western United States from seismic to steady-state time scales. We use evidence from post-seismic transient and earthquake cycle deformation reservoir loading glacio-isostatic adjustment, and lithosphere isostatic adjustment to large surface and subsurface loads. The nearly perfectly elastic behavior of Earth's crust and mantle at the time scale of seismic wave propagation evolves to that of a strong, elastic crust and weak, ductile upper mantle lithosphere at both earthquake cycle (EC, ???10?? to 103 yr) and glacio-isostatic adjustment (GIA, ???103 to 104 yr) time scales. Topography and gravity field correlations indicate that lithosphere isostatic adjustment (LIA) on ???106-107 yr time scales occurs with most lithospheric stress supported by an upper crust overlying a much weaker ductile subtrate. These comparisons suggest that the upper mantle lithosphere is weaker than the crust at all time scales longer than seismic. In contrast, the lower crust has a chameleon-like behavior, strong at EC and GIA time scales and weak for LIA and steady-state deformation processes. The lower crust might even take on a third identity in regions of rapid crustal extension or continental collision, where anomalously high temperatures may lead to large-scale ductile flow in a lower crustal layer that is locally weaker than the upper mantle. Modeling of lithospheric processes in active regions thus cannot use a one-size-fits-all prescription of rheological layering (relation between applied stress and deformation as a function of depth) but must be tailored to the time scale and tectonic setting of the process being investigated.

GSA Today↗

USGS2006

Inference of postseismic deformation mechanisms of the 1923 Kanto earthquake

Coseismic slip associated with the M7.9, 1923 Kanto earthquake is fairly well understood, involving slip of up to 8 m along the Philippine Sea-Honshu interplate boundary under Sagami Bay and its onland extension. Postseismic deformation after the 1923 earthquake, however, is relatively poorly understood. We revisit the available deformation data in order to constrain possible mechanisms of postseismic deformation and to examine the consequences for associated stress changes in the surrounding crust. Data from two leveling lines and one tide gage station over the first 7-8 years postseismic period are of much greater amplitude than the corresponding expected interseismic deformation during the same period, making these data suitable for isolating the signal from postseismic deformation. We consider both viscoelastic models of asthenosphere relaxation and afterslip models. A distributed coseismic slip model presented by Pollitz et al. (2005), combined with prescribed parameters of a viscoelastic Earth model, yields predicted postseismic deformation that agrees with observed deformation on mainland Honshu from Tokyo to the Izu peninsula. Elsewhere (southern Miura peninsula; Boso peninsula), the considered viscoelastic models fail to predict observed deformation, and a model of ???1 in shallow afterslip in the offshore region south of the Boso peninsula, with equivalent moment magnitude Mw = 7.0, adequately accounts for the observed deformation. Using the distributed coseismic slip model, layered viscoelastic structure, and a model of interseismic strain accumulation, we evaluate the post-1923 stress evolution, including both the coseismic and accumulated postseismic stress changes and those stresses contributed by interseismic loading. We find that if account is made for the varying tectonic regime in the region, the occurrence of both immediate (first month) post-1923 crustal aftershocks as well as recent regional crustal seismicity is consistent with the predicted stress pattern. This suggests that the influence of the 1923 earthquake on regional seismicity is fairly predictable and has persisted for at least seven decades following the earthquake.

Journal of Geophysical Research B: Solid Earth↗

USGS2005

Coseismic slip distribution of the 1923 Kanto earthquake, Japan

The slip distribution associated with the 1923 M = 7.9 Kanto, Japan, earthquake is reexamined in light of new data and modeling. We utilize a combination of first-order triangulation, second-order triangulation, and leveling data in order to constrain the coseismic deformation. The second-order triangulation data, which have not been utilized in previous studies of 1923 coseismic deformation, are associated with only slightly smaller errors than the first-order triangulation data and expand the available triangulation data set by about a factor of 10. Interpretation of these data in terms of uniform-slip models in a companion study by Nyst et al. shows that a model involving uniform coseismic slip on two distinct rupture planes explains the data very well and matches or exceeds the fit obtained by previous studies, even one which involved distributed slip. Using the geometry of the Nyst et al. two-plane slip model, we perform inversions of the same geodetic data set for distributed slip. Our preferred model of distributed slip on the Philippine Sea plate interface has a moment magnitude of 7.86. We find slip maxima of ???8-9 m beneath Odawara and ???7-8 m beneath the Miura peninsula, with a roughly 2:1 ratio of strike-slip to dip-slip motion, in agreement with a previous study. However, the Miura slip maximum is imaged as a more broadly extended feature in our study, with the high-slip region continuing from the Miura peninsula to the southern Boso peninsula region. The second-order triangulation data provide good evidence for ???3 m right-lateral strike slip on a 35-km-long splay structure occupying the volume between the upper surface of the descending Philippine Sea plate and the southern Boso peninsula. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2005

Northwest Basin and Range tectonic deformation observed with the Global Positioning System, 1999-2003

We use geodetic velocities obtained with the Global Positioning System (GPS) to quantify tectonic deformation of the northwest Basin and Range province of the western United States. The results are based on GPS data collected in 1999 and 2003 across five new quasi-linear networks in northern Nevada, northeast California, and southeast Oregon. The velocities show ???3 mm/yr westward movement of northern Nevada with respect to stable North America. West of longitude 119??W the velocities increase and turn northwest, parallel to Sierra Nevada/Great Valley microplate motion, and similar to velocities previously obtained to the south. The observations are explained by a kinematic model with three domains that rotate around Euler poles in eastern Oregon and western Idaho. Northeast California experiences internal dextral shear deformation (11.2 ?? 3.6 nstrain/yr) subparallel to Pacific/North America motion. Relative motions of the domains imply 2-5 mm/yr approximately east-west extension in northwest Nevada and 1-4 mm/yr approximately north-south contraction near the California/Oregon border. The northward decreasing approximately east-west extension in northwest Nevada is consistent with the northern termination of Basin and Range deformation, faulting and characteristic topography. No significant extension is detected in the Oregon Basin and Range. The Oregon Cascade arc moves north at ???3.5 mm/yr and is possibly influenced by the approximately eastward motion of the Juan de Fuca plate. These results disagree with secular northwest trenchward motion of the Oregon forearc inferred from paleomagnetic rotations. South of latitude 43??, however, trenchward motion exists and is consistent with block rotations, approximately east-west Basin and Range extension, and northwest Sierra Nevada translation. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

USGS2004

Contemporary tectonic deformation of the Basin and Range province, western United States: 10 years of observation with the Global Positioning System

[1] We have estimated patterns and rates of crustal movement across 800 km of the Basin and Range at ∼39° north latitude with Global Positioning System surveys in 1992, 1996, 1998, and 2002. The total rate of motion tangent to the small circle around the Pacific‐North America pole of rotation is 10.4 ± 1.0 mm/yr, and motion normal to this small circle is 3.9 ± 0.9 mm/yr compared to the east end of our network. On the Colorado Plateau the east end of our network moves by ∼1–2 mm/yr westerly with respect to North America. Transitions in strain rates delimit six major tectonic domains within the province. These deformation zones coincide with areas of modern seismicity and are, from east to west, (1) east‐west extension in the Wasatch Fault zone, (2) low rate east‐west extension centered near the Nevada‐Utah border, (3) low rate east‐west contraction between 114.7°W and 117.9°W, (4) extension normal to and strike‐slip motion across the N10°E striking Central Nevada Seismic Zone, (5) right lateral simple shear oriented N13°W inside the Walker Lane Belt, and (6) shear plus extension near the Sierra Nevada frontal faults. Concentration of shear and dilatational deformation across the three westernmost zones suggests that the Walker Lane Belt lithosphere is rheologically weak. However, we show that linear gradients in viscosity and gravitational potential energy can also effectively concentrate deformation. In the Basin and Range, gradients in gravitational potential are spatially anticorrelated with dilatational strain rates, consistent with the presence of horizontal variations in viscosity of the lithosphere.

Arizona, California, Nevada, Utah↗

USGS2003

GPS constraints on the kinematics of continental deformation

Recent GPS observations from the western United States, New Zealand, central Greece, and Japan indicate that present-day continental deformation is typically focused in narrow deforming zones whose extent is much smaller than the intervening largely inactive regions. However, these narrow zones are heterogeneously distributed, reflecting the inherent heterogeneity of continental lithospheric strength and internal buoyancy. Plate driving and resisting forces stress plate boundary zones and plate interiors and drive deformation. These forces change continuously and discontinuously, leading to continental deformation that typically evolves and migrates with time. Magmatic and tectonic processes alter lithospheric rheology and internal buoyancy and also contribute to the time-varying character of continental deformation.

International Geology Review↗

USGS2002

Interferometric synthetic aperture radar studies of Alaska volcanoes

Interferometric synthetic aperture radar (InSAR) imaging is a recently developed geodetic technique capable of measuring ground-surface deformation with centimeter to subcentimeter vertical precision and spatial resolution of tens-of-meter over a relatively large region (/spl sim/10/sup 4/ km/sup 2/). The spatial distribution of surface deformation data, derived from InSAR images, enables the construction of detailed mechanical models to enhance the study of magmatic and tectonic processes associated with volcanoes. This paper summarizes our recent InSAR studies of several Alaska volcanoes, which include Okmok, Akutan, Kiska, Augustine, Westdahl, and Peulik volcanoes.

Alaska↗

USGS2001

Mantle flow beneath a continental strike-slip fault: Postseismic deformation after the 1999 Hector Mine earthquake

Two recent large earthquakes in the Mojave Desert, California - the magnitude 7.3 1992 Landers and magnitude 7.1 1999 Hector Mine earthquakes - have each been followed by elevated crustal strain rates over periods of months and years. Geodetic data collected after the Hector Mine earthquake exhibit a temporally decaying horizontal velocity field and a quadrant uplift pattern opposite to that expected for localized shear beneath the earthquake rupture. We interpret the origin of this accelerated crustal deformation to be vigorous flow in the upper mantle in response to the stress changes generated by the earthquake. Our results suggest that transient flow in the upper mantle is a fundamental component of the earthquake cycle and that the lower crust is a coherent stress guide coupling the upper crust with the upper mantle.

Science↗

USGS2000

Aseismic inflation of Westdahl volcano, Alaska, revealed by satellite radar interferometry

Westdahl volcano, located at the west end of Unimak Island in the central Aleutian volcanic arc, Alaska, is a broad shield that produced moderate-sized eruptions in 1964, 1978-79, and 1991-92. Satellite radar interferometry detected about 17 cm of volcano-wide inflation from September 1993 to October 1998. Multiple independent interferograms reveal that the deformation rate has not been steady; more inflation occurred from 1993 to 1995 than from 1995 to 1998. Numerical modeling indicates that a source located about 9 km beneath the center of the volcano inflated by about 0.05 km3 from 1993 to 1998. On the basis of the timing and volume of recent eruptions at Westdahl and the fact that it has been inflating for more than 5 years, the next eruption can be expected within the next several years.

Geophysical Research Letters↗

USGS2000

Studies of volcanoes of Alaska by satellite radar interferometry

Interferometric synthetic aperture radar (InSAR) has provided a new imaging geodesy technique to measure the deformation of volcanoes at tens-of-meter horizontal resolution with centimeter to subcentimeter vertical precision. The two-dimensional surface deformation data enables the construction of detailed numerical models allowing the study of magmatic and tectonic processes beneath volcanoes. This paper summarizes our recent: InSAR studies over the Alaska-Aleutian volcanoes, which include New Trident, Okmok, Akutan, Augustine, Shishaldin, and Westdahl volcanoes. The first InSAR surface deformation over the Alaska volcanoes was applied to New Trident. Preliminary InSAR study suggested that New Trident volcano experienced several centimeters inflation from 1993 to 1995. Using the InSAR technique, we studied the 1997 eruption of Okmok. We have measured ???1.4 m deflation during the eruption, ???20 cm pre-eruptive inflation during 1992 to 1995, and >10 cm post-eruptive inflation within a year after the eruption, and modeled the deformations using Mogi sources. We imaged the ground surface deformation associated with the 1996 seismic crisis over Akutan volcano. Although seismic swarm did not result in an eruption, we found that the western part of the volcano uplifted ???60 cm while the eastern part of the island subsided. The majority of the complex deformation field at the Akutan volcano was modeled by dike intrusion and Mogi inflation sources. Our InSAR results also indicate that the pyroclastic flows from last the last eruption have been undergoing contraction/subsidence at a rate of about 3 cm per year since 1992. InSAR measured no surface deformation before and during the 1999 eruption of Shishaldin and suggested the eruption may be a type of open system. Finally, we applied satellite radar interferometry to Westdahl volcano which erupted 1991 and has been quiet since. We discovered this volcano had inflated about 15 cm from 1993 to 1998. In summary, satellite radar interferometry can not only be used to study a volcanic eruption, but also to detect aseismic deformation at quiescent volcanoes preceding a seismic swarm; it is a useful technique to study volcanic eruptions as well as to guide scientists to better focus their monitoring efforts.

Alaska↗

USGS1997

Resolution of fault slip along the 470-km-long rupture of the great 1906 San Francisco earthquake and its implications

Data from all available triangulation networks affected by the 1906 earthquake have been combined to assess the trade-off between slip resolution and its uncertainty and to construct a conservative image of coseismic slip along the rupture. Because of varying network aperture and station density, slip resolution is very uneven. Although slip is determined within uncertainties of ??1.0 m along 60% of the fault, constraints are poor on the remaining, mostly offshore portions of the rupture. Slip decreases from maxima of 8.6 and 7.5 m at Shelter Cove and Tomales Bay to 4.5 m near Mount Tamalpais and 2.7 m at Loma Prieta. The geodetically derived slip distribution is in poor agreement with estimates based on analysis of S wave seismograms, probably because these waves register only 20-30% of the total seismic moment obtained from longer-period surface waves. Consideration of a range of fault geometries for 1906 slip near Loma Prieta indicates right-lateral motions lie between 2.3 and 3.1 m. These values are considerably greater than the 1.5 m of measured surface slip on which several assessments of high earthquake hazard for this fault segment were based. This factor, along with the absence of 1989 slippage where 1906 surface slip was used to make the forecasts, casts doubt on some claims of success in predicting the 1989 M = 6.9 Loma Prieta earthquake.

Journal of Geophysical Research B: Solid Earth↗

USGS1996

Detection of postseismic fault-zone collapse following the Landers earthquake

Stress changes caused by fault movement in an earthquake induce transient aseismic crustal movements in the earthquake source region that continue for months to decades following large events. These motions reflect aseismic adjustments of the fault zone and/or bulk deformation of the surroundings in response to applied stresses, and supply information regarding the inelastic behaviour of the Earth's crust. These processes are imperfectly understood because it is difficult to infer what occurs at depth using only surface measurements, which are in general poorly sampled. Here we push satellite radar interferometry to near its typical artefact level, to obtain a map of the postseismic deformation field in the three years following the 28 June 1992 Landers, California earthquake. From the map, we deduce two distinct types of deformation: afterslip at depth on the fault that ruptured in the earthquake, and shortening normal to the fault zone. The latter movement may reflect the closure of dilatant cracks and fluid expulsion from a transiently over-pressured fault zone.

Nature↗

USGS1994

Geodetic slip rate for the eastern California shear zone and the recurrence time of Mojave desert earthquakes

Where the San Andreas fault passes along the southwestern margin of the Mojave desert, it exhibits a large change in trend, and the deformation associated with the Pacific/North American plate boundary is distributed broadly over a complex shear zone. The importance of understanding the partitioning of strain across this region, especially to the east of the Mojave segment of the San Andreas in a region known as the eastern California shear zone (ECSZ), was highlighted by the occurrence (on 28 June 1992) of the magnitude 7.3 Landers earthquake in this zone. Here we use geodetic observations in the central Mojave desert to obtain new estimates for the rate and distribution of strain across a segment of the ECSZ, and to determine a coseismic strain drop of ~770 ??rad for the Landers earthquake. From these results we infer a strain energy recharge time of 3,500-5,000 yr for a Landers-type earthquake and a slip rate of ~12 mm yr-1 across the faults of the central Mojave. The latter estimate implies that a greater fraction of plate motion than heretofore inferred from geodetic data is accommodated across the ECSZ.

Nature↗

USGS1991

Fault orientations in extensional and conjugate strike-slip environments and their implications

Seismically active conjugate strike-slip faults in California and Japan typically have mutually orthogonal right- and left-lateral fault planes. Normal- fault dips at earthquake nucleation depths are concentrated between 40° and 50°. The observed orientations and their strong clustering are surprising, because conventional faulting theory suggests fault initiation with conjugate 60° and 120° intersecting planes and 60° normal-fault dip or fault reactivation with a broad range of permitted orientations. The observations place new constraints on the mechanics of fault initiation, rotation, and evolutionary development. We speculate that the data could be explained by fault rotation into the observed orientations and deactivation for greater rotation or by formation of localized shear zones beneath the brittle-ductile transition in Earth's crust. Initiation as weak frictional faults seems unlikely.

Geology↗