Geology topics

Frederick Pollitz

Publications and source records attributed to Frederick Pollitz.

At least 55 records · Page 3Linked to original sources

USGS2017

Connecting crustal seismicity and earthquake-driven stress evolution in Southern California

Tectonic stress in the crust evolves during a seismic cycle, with slow stress accumulation over interseismic periods, episodic stress steps at the time of earthquakes, and transient stress readjustment during a postseismic period that may last months to years. Static stress transfer to surrounding faults has been well documented to alter regional seismicity rates over both short and long time scales. While static stress transfer is instantaneous and long lived, postseismic stress transfer driven by viscoelastic relaxation of the ductile lower crust and mantle leads to additional, slowly varying stress perturbations. Both processes may be tested by comparing a decade-long record of regional seismicity to predicted time-dependent seismicity rates based on a stress evolution model that includes viscoelastic stress transfer. Here we explore crustal stress evolution arising from the seismic cycle in Southern California from 1981 to 2014 using five M ≥6.5 source quakes: the M 7.3 1992 Landers, M6.5 1992 Big Bear, M 6.7 1994 Big Bear, M 7.1 1999 Hector Mine, and M 7.2 2010 El Mayor-Cucapah earthquakes. We relate the stress readjustment in the surrounding crust generated by each quake to regional seismicity using rate-and-state friction theory. Using a log likelihood approach, we quantify the potential to trigger seismicity of both static and viscoelastic stress transfer, finding that both processes have systematically shaped the spatial pattern of Southern California seismicity since 1992.

California↗

USGS2017

Geodetic slip model of the 3 September 2016 Mw 5.8 Pawnee, Oklahoma, earthquake: Evidence for fault‐zone collapse

The 3 September 2016 M w 5.8 Pawnee earthquake in northern Oklahoma is the largest earthquake ever recorded in Oklahoma. The coseismic deformation was measured with both Interferometric Synthetic Aperture Radar and Global Positioning System (GPS), with measureable signals of order 1 cm and 1 mm, respectively. We derive a coseismic slip model from Sentinel‐1A and Radarsat 2 interferograms and GPS static offsets, dominated by distributed left‐lateral strike slip on a primary west‐northwest–east‐southeast‐trending subvertical plane, whereas strike slip is concentrated near the hypocenter (5.6 km depth), with maximum slip of ∼1 m located slightly east and down‐dip of the hypocenter. Based on systematic misfits of observed interferogram line‐of‐sight (LoS) displacements, with LoS based on shear‐dislocation models, a few decimeters of fault‐zone collapse are inferred in the hypocentral region where coseismic slip was the largest. This may represent the postseismic migration of large volumes of fluid away from the high‐slip areas, made possible by the creation of a temporary high‐permeability damage zone around the fault.

Oklahoma↗

USGS2017

Implications of the earthquake cycle for inferring fault locking on the Cascadia megathrust

GPS velocity fields in the Western US have been interpreted with various physical models of the lithosphere-asthenosphere system: (1) time-independent block models; (2) time-dependent viscoelastic-cycle models, where deformation is driven by viscoelastic relaxation of the lower crust and upper mantle from past faulting events; (3) viscoelastic block models, a time-dependent variation of the block model. All three models are generally driven by a combination of loading on locked faults and (aseismic) fault creep. Here we construct viscoelastic block models and viscoelastic-cycle models for the Western US, focusing on the Pacific Northwest and the earthquake cycle on the Cascadia megathrust. In the viscoelastic block model, the western US is divided into blocks selected from an initial set of 137 microplates using the method of Total Variation Regularization, allowing potential trade-offs between faulting and megathrust coupling to be determined algorithmically from GPS observations. Fault geometry, slip rate, and locking rates (i.e. the locking fraction times the long term slip rate) are estimated simultaneously within the TVR block model. For a range of mantle asthenosphere viscosity (4.4 × 10 18 to 3.6 × 10 20 Pa s) we find that fault locking on the megathrust is concentrated in the uppermost 20 km in depth, and a locking rate contour line of 30 mm yr −1 extends deepest beneath the Olympic Peninsula, characteristics similar to previous time-independent block model results. These results are corroborated by viscoelastic-cycle modelling. The average locking rate required to fit the GPS velocity field depends on mantle viscosity, being higher the lower the viscosity. Moreover, for viscosity ≲ 10 20 Pa s, the amount of inferred locking is higher than that obtained using a time-independent block model. This suggests that time-dependent models for a range of admissible viscosity structures could refine our knowledge of the locking distribution and its epistemic uncertainty.

Geophysical Journal International↗

USGS2017

Shallow microearthquakes near Chongqing, China triggered by the Rayleigh waves of the 2015 M7.8 Gorkha, Nepal earthquake

We present a case of remotely triggered seismicity in Southwest China by the 2015/04/25 M7.8 Gorkha, Nepal earthquake. A local magnitude ML3.8 event occurred near the Qijiang district south of Chongqing city approximately 12 min after the Gorkha mainshock. Within 30km of this ML3.8 event there are 62 earthquakes since 2009 and only 7 ML>3events, which corresponds to a likelihood of 0.3% for a ML>3on any given day by a random chance. This observation motivates us to investigate the relationship between the ML3.8 event and the Gorkha mainshock. The ML3.8 event is listed in the China Earthquake National Center (CENC) catalog and occurred at shallow depth (∼3km). By examining high-frequency waveforms, we identify a smaller local event (∼ML2.5) ∼15s before the ML3.8 event. Both events occurred during the first two cycles of the Rayleigh waves from the Gorkha mainshock. We perform seismic event detection based on envelope function and waveform matching by using the two events as templates. Both analyses found a statistically significant rate change during the mainshock, suggesting that they were indeed dynamically triggered by the Rayleigh waves. Both events occurred during the peak normal and dilatational stress changes (∼10–30 kPa), consistent with observations of dynamic triggering in other geothermal/volcanic regions. Although other recent events (i.e., the 2011 M9.1 Tohoku-Oki earthquake) produced similar peak ground velocities, the 2015 Gorkha mainshock was the only event that produced clear dynamic triggering in this region. The triggering site is close to hydraulic fracturing wells that began production in 2013–2014. Hence we suspect that fluid injections may increase the region’s susceptibility to remote dynamic triggering.

Chongqing↗

USGS2016

A note on adding viscoelasticity to earthquake simulators

Here, I describe how time‐dependent quasi‐static stress transfer can be implemented in an earthquake simulator code that is used to generate long synthetic seismicity catalogs. Most existing seismicity simulators use precomputed static stress interaction coefficients to rapidly implement static stress transfer in fault networks with typically tens of thousands of fault patches. The extension to quasi‐static deformation, which accounts for viscoelasticity of Earth’s ductile lower crust and mantle, involves the precomputation of additional interaction coefficients that represent time‐dependent stress transfer among the model fault patches, combined with defining and evolving additional state variables that track this stress transfer. The new approach is illustrated with application to a California‐wide synthetic fault network.

Bulletin of the Seismological Society of America↗

USGS2016

Persistent slip rate discrepancies in the eastern California (USA) shear zone

Understanding fault slip rates in the eastern California shear zone (ECSZ) using GPS geodesy is complicated by potentially overlapping strain signals due to many sub-parallel strike-slip faults and by inconsistencies with geologic slip rates. The role of fault system geometry in describing ECSZ deformation may be investigated with total variation regularization, which algorithmically determines a best-fitting geometry from an initial model with numerous faults, constrained by a western United States GPS velocity field. The initial dense model (1) enables construction of the first geodetically constrained block model to include all ECSZ faults with geologic slip rates, allowing direct geologic-geodetic slip rate comparisons, and (2) permits fault system geometries with many active faults that are analogous to distributed interseismic deformation. Beginning with 58 ECSZ blocks, a model containing 10 ECSZ blocks is most consistent with geologic slip rates, reproducing five of 11 within their reported uncertainties. The model fits GPS observations with a mean residual velocity of 1.5 mm/yr. Persistent geologic-geodetic slip rate discrepancies occur on the Calico and Garlock faults, on which we estimate slip rates of 7.6 mm/yr and <2 mm/yr, respectively, indicating that inconsistencies between geology and geodesy may be concentrated on or near these faults and are not due to pervasive distributed deformation in the region. Discrepancies may in part be due to postseismic relaxation following the A.D. 1992 M w 7.3 Landers and 1999 M w 7.1 Hector Mine earthquakes. Otherwise, resolving geologic-geodetic discrepancies would require as much as 11.4 mm/yr of off-fault deformation within <10 km of the main ECSZ faults, with ∼5 mm/yr concentrated near the Calico fault.

California↗

USGS2016

The Earthquake‐Source Inversion Validation (SIV) Project

Finite‐fault earthquake source inversions infer the (time‐dependent) displacement on the rupture surface from geophysical data. The resulting earthquake source models document the complexity of the rupture process. However, multiple source models for the same earthquake, obtained by different research teams, often exhibit remarkable dissimilarities. To address the uncertainties in earthquake‐source inversion methods and to understand strengths and weaknesses of the various approaches used, the Source Inversion Validation (SIV) project conducts a set of forward‐modeling exercises and inversion benchmarks. In this article, we describe the SIV strategy, the initial benchmarks, and current SIV results. Furthermore, we apply statistical tools for quantitative waveform comparison and for investigating source‐model (dis)similarities that enable us to rank the solutions, and to identify particularly promising source inversion approaches. All SIV exercises (with related data and descriptions) and statistical comparison tools are available via an online collaboration platform, and we encourage source modelers to use the SIV benchmarks for developing and testing new methods. We envision that the SIV efforts will lead to new developments for tackling the earthquake‐source imaging problem.

Seismological Research Letters↗

USGS2016

Postseismic gravity change after the 2006–2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 M w 8.3 thrust and 2007 M w 8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in Gravity Recovery and Climate Experiment (GRACE) but without a discernible coseismic gravity change. The gravity increase of ~4 μGal, observed consistently from various GRACE solutions around the epicentral area during 2007–2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25–35 km for the elastic thickness and ~10 18 Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.

Geophysical Research Letters↗

USGS2016

Seismic velocity structure of the crust and shallow mantle of the Central and Eastern United States by seismic surface wave imaging

Seismic surface waves from the Transportable Array of EarthScope's USArray are used to estimate phase velocity structure of 18 to 125 s Rayleigh waves, then inverted to obtain three-dimensional crust and upper mantle structure of the Central and Eastern United States (CEUS) down to &sim;200 km. The obtained lithosphere structure confirms previously imaged CEUS features, e.g., the low seismic-velocity signature of the Cambrian Reelfoot Rift and the very low velocity at >150 km depth below an Eocene volcanic center in northwestern Virginia. New features include high-velocity mantle stretching from the Archean Superior Craton well into the Proterozoic terranes and deep low-velocity zones in central Texas (associated with the late Cretaceous Travis and Uvalde volcanic fields) and beneath the South Georgia Rift (which contains Jurassic basalts). Hot spot tracks may be associated with several imaged low-velocity zones, particularly those close to the former rifted Laurentia margin.

Geophysical Research Letters↗

USGS2016

Lithospheric rheology constrained from twenty-five years of postseismic deformation following the 1989 Mw 6.9 Loma Prieta earthquake

The October 17, 1989 M w 6.9 Loma Prieta earthquake provides the first opportunity of probing the crustal and upper mantle rheology in the San Francisco Bay Area since the 1906 M w 7.9 San Francisco earthquake. Here we use geodetic observations including GPS and InSAR to characterize the Loma Prieta earthquake postseismic displacements from 1989 to 2013. Pre-earthquake deformation rates are constrained by nearly 20 yr of USGS trilateration measurements and removed from the postseismic measurements prior to the analysis. We observe GPS horizontal displacements at mean rates of 1–4 mm/yr toward Loma Prieta Mountain until 2000, and ∼2 mm/yr surface subsidence of the northern Santa Cruz Mountains between 1992 and 2002 shown by InSAR, which is not associated with the seasonal and longer-term hydrological deformation in the adjoining Santa Clara Valley. Previous work indicates afterslip dominated in the early (1989–1994) postseismic period, so we focus on modeling the postseismic viscoelastic relaxation constrained by the geodetic observations after 1994. The best fitting model shows an elastic 19-km-thick upper crust above an 11-km-thick viscoelastic lower crust with viscosity of ∼6 × 10 18 Pas, underlain by a viscous upper mantle with viscosity between 3 × 1018 and 2 × 10 19 Pas. The millimeter-scale postseismic deformation does not resolve the viscosity in the different layers very well, and the lower-crustal relaxation may be localized in a narrow shear zone. However, the inferred lithospheric rheology is consistent with previous estimates based on post-1906 San Francisco earthquake measurements along the San Andreas fault system. The viscoelastic relaxation may also contribute to the enduring increase of aseismic slip and repeating earthquake activity on the San Andreas fault near San Juan Bautista, which continued for at least a decade after the Loma Prieta event.

California↗

USGS2015

Postearthquake relaxation evidence for laterally variable viscoelastic structure and water content in the Southern California mantle

I reexamine the lower crust and mantle relaxation following two large events in the Mojave Desert: the 1992 M 7.3 Landers and 1999 M 7.1 Hector Mine, California, earthquakes. Time series from continuous GPS sites out to 300 km from the ruptures are used to constrain models of postseismic relaxation. Crustal motions in the Mojave Desert region are elevated above background for several years following each event. To account for broadscale relaxation of the lower crust and mantle, the Burgers body model is employed, involving transient and steady state viscosities. Joint afterslip/postseismic relaxation modeling of the GPS time series up to one decade following the Hector Mine earthquake reveals a significant rheological contrast between a northwest trending “southwest domain” (that envelopes the San Andreas fault system and western Mojave Desert) and an adjacent “northeast domain” (that envelopes the Landers and Hector Mine rupture areas in the central Mojave Desert). The steady state viscosity of the northeast domain mantle asthenosphere is inferred to be ∼4 times greater than that of the southwest domain. This pattern is counter to that expected for regional heat flow, which is higher in the northeast domain, but it is explicable by means of a nonlinear rheology that includes dependence on both strain rate and water concentration. I infer that the southwest domain mantle has a relatively low steady state viscosity because of its high strain rate and water content. The relatively low mantle water content of the northeast domain is interpreted to result from the continual extraction of water through igneous and volcanic activity over the past ∼20 Myr.

California↗

USGS2015

Regional seismic-wave propagation from the M5.8 23 August 2011, Mineral, Virginia, earthquake

The M5.8 23 August 2011 Mineral, Virginia, earthquake was felt over nearly the entire eastern United States and was recorded by a wide array of seismic broadband instruments. The earthquake occurred ~200 km southeast of the boundary between two distinct geologic belts, the Piedmont and Blue Ridge terranes to the southeast and the Valley and Ridge Province to the northwest. At a dominant period of 3 s, coherent postcritical P-wave (i.e., direct longitudinal waves trapped in the crustal waveguide) arrivals persist to a much greater distance for propagation paths toward the northwest quadrant than toward other directions; this is probably related to the relatively high crustal thickness beneath and west of the Appalachian Mountains. The seismic surface-wave arrivals comprise two distinct classes: those with weakly dispersed Rayleigh waves and those with strongly dispersed Rayleigh waves. We attribute the character of Rayleigh wave arrivals in the first class to wave propagation through a predominantly crystalline crust (Blue Ridge Mountains and Piedmont terranes) with a relatively thin veneer of sedimentary rock, whereas the temporal extent of the Rayleigh wave arrivals in the second class are well explained as the effect of the thick sedimentary cover of the Valley and Ridge Province and adjacent Appalachian Plateau province to its northwest. Broadband surface-wave ground velocity is amplified along both north-northwest and northeast azimuths from the Mineral, Virginia, source. The former may arise from lateral focusing effects arising from locally thick sedimentary cover in the Appalachian Basin, and the latter may result from directivity effects due to a northeast rupture propagation along the finite fault plane.

GSA Special Papers↗

USGS2014

Post-earthquake relaxation using a spectral element method: 2.5-D case

The computation of quasi-static deformation for axisymmetric viscoelastic structures on a gravitating spherical earth is addressed using the spectral element method (SEM). A 2-D spectral element domain is defined with respect to spherical coordinates of radius and angular distance from a pole of symmetry, and 3-D viscoelastic structure is assumed to be azimuthally symmetric with respect to this pole. A point dislocation source that is periodic in azimuth is implemented with a truncated sequence of azimuthal order numbers. Viscoelasticity is limited to linear rheologies and is implemented with the correspondence principle in the Laplace transform domain. This leads to a series of decoupled 2-D problems which are solved with the SEM. Inverse Laplace transform of the independent 2-D solutions leads to the time-domain solution of the 3-D equations of quasi-static equilibrium imposed on a 2-D structure. The numerical procedure is verified through comparison with analytic solutions for finite faults embedded in a laterally homogeneous viscoelastic structure. This methodology is applicable to situations where the predominant structure varies in one horizontal direction, such as a structural contrast across (or parallel to) a long strike-slip fault.

Geophysical Journal International↗

USGS2014

The profound reach of the 11 April 2012 M 8.6 Indian Ocean earthquake: Short‐term global triggering followed by a longer‐term global shadow

The 11 April 2012 M 8.6 Indian Ocean earthquake was an unusually large intraoceanic strike‐slip event. For several days, the global M ≥4.5 and M ≥6.5 seismicity rate at remote distances (i.e., thousands of kilometers from the mainshock) was elevated. The strike‐slip mainshock appears through its Love waves to have triggered a global burst of strike‐slip aftershocks over several days. But the M ≥6.5 rate subsequently dropped to zero for the succeeding 95 days, although the M ≤6.0 global rate was close to background during this period. Such an extended period without an M ≥6.5 event has happened rarely over the past century, and never after a large mainshock. Quiescent periods following previous large ( M ≥8) mainshocks over the past century are either much shorter or begin so long after a given mainshock that no physical interpretation is warranted. The 2012 mainshock is unique in terms of both the short‐lived global increase and subsequent long quiescent period. We believe that the two components are linked and interpret this pattern as the product of dynamic stressing of a global system of faults. Transient dynamic stresses can encourage short‐term triggering, but, paradoxically, it can also inhibit rupture temporarily until background tectonic loading restores the system to its premainshock stress levels.

Bulletin of the Seismological Society of America↗

USGS2014

Seismic structure of the central US crust and upper mantle: Uniqueness of the Reelfoot Rift

Using seismic surface waves recorded with Earthscope's Transportable Array, we apply surface wave imaging to determine 3D seismic velocity in the crust and uppermost mantle. Our images span several Proterozoic and early Cambrian rift zones (Mid-Continent Rift, Rough Creek Graben—Rome trough, Birmingham trough, Southern Oklahoma Aulacogen, and Reelfoot Rift). While ancient rifts are generally associated with low crustal velocity because of the presence of thick sedimentary sequences, the Reelfoot Rift is unique in its association with low mantle seismic velocity. Its mantle low-velocity zone (LVZ) is exceptionally pronounced and extends down to at least 200 km depth. This LVZ is of variable width, being relatively narrow ( ∼ 50 km "> ∼50km wide) within the northern Reelfoot Rift, which hosts the New Madrid Seismic Zone (NMSZ). We hypothesize that this mantle volume is weaker than its surroundings and that the Reelfoot Rift consequently has relatively low elastic plate thickness, which would tend to concentrate tectonic stress within this zone. No other intraplate ancient rift zone is known to be associated with such a deep mantle low-velocity anomaly, which suggests that the NMSZ is more susceptible to external stress perturbations than other ancient rift zones.

Reelfoot Rift↗

USGS2013

Annual modulation of non-volcanic tremor in northern Cascadia

Two catalogs of episodic tremor events in northern Cascadia, one from 2006 to 2012 and the other from 1997 to 2011, reveal two systematic patterns of tremor occurrence in southern Vancouver Island: (1) most individual events tend to occur in the third quarter of the year; (2) the number of events in prolonged episodes (i.e., episodic tremor and slip events), which generally propagate to Vancouver Island from elsewhere along the Cascadia subduction zone, is inversely correlated with the amount of precipitation that occurred in the preceding 2 months. We rationalize these patterns as the product of hydrologic loading of the crust of southern Vancouver Island and the surrounding continental region, superimposed with annual variations from oceanic tidal loading. Loading of the Vancouver Island crust in the winter (when the land surface receives ample precipitation) and unloading in the summer tends to inhibit and enhance downdip shear stress, respectively. Quantitatively, for an annually variable surface load, the predicted stress perturbation depends on mantle viscoelastic rheology. A mechanical model of downdip shear stress on the transition zone beneath Vancouver Island—driven predominantly by the annual hydrologic cycle—is consistent with the 1997–2012 tremor observations, with peak-to-peak downdip shear stress of about 0.4 kPa. This seasonal dependence of tremor occurrence appears to be restricted to southern Vancouver Island because of its unique situation as an elongated narrow-width land mass surrounded by ocean, which permits seasonal perturbations in shear stress at depth.

Journal of Geophysical Research B: Solid Earth↗

USGS2013

Toward a time-dependent probabilistic seismic hazard analysis for Alaska

We report on a time-dependent seismic hazard analysis for Alaska and the Aleutians to complement our recently completed time-independent map. Whereas the time-independent map treats all sources as statistically independent, the time-dependent analysis is based on calculations of the conditional probability of occurrence for the next 50 years by using a Brownian Passage Time model for the seismic sources judged to be characteristic. We then consider how those probabilities are modified by coseismic and postseismic stress changes resulting from large regional earthquakes occurring from 1938 to 2002. Recombining the time-dependent probabilities with time-independent truncated Gutenberg–Richter and smoothed seismicity sources leads to our time-dependent probabilistic seismic hazard results. We find that when accounting for time dependence without stress changes, earthquake probabilities can be significantly altered, reducing probabilities to near zero or increasing them to several times the time-independent values. In addition, accounting for coseismic stress changes tends to have a local influence on earthquake probabilities, whereas postseismic effects can be far-reaching in both time and space. In sum, however, since we combine time-dependent and time-independent sources, the modification to seismic hazard is relatively minor, increasing or decreasing hazard adjacent to characteristic faults by about 10%. Most cities, located far from characteristic faults, are little affected.

Alaska↗

USGS2012

ViscoSim Earthquake Simulator

Synthetic seismicity simulations have been explored by the Southern California Earthquake Center (SCEC) Earthquake Simulators Group in order to guide long‐term forecasting efforts related to the Unified California Earthquake Rupture Forecast ( Tullis et al. , 2012a ). In this study I describe the viscoelastic earthquake simulator (ViscoSim) of Pollitz, 2009 . Recapitulating to a large extent material previously presented by Pollitz (2009 , 2011 ) I describe its implementation of synthetic ruptures and how it differs from other simulators being used by the group.

Seismological Research Letters↗