Estimated likelihood of observing a large earthquake on a continental low‐angle normal fault and implications for low‐angle normal fault activity

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106807/1/grl51557.pd

The weight of the mountains: Constraints on tectonic stress, friction, and fluid pressure in the 2008 Wenchuan earthquake from estimates of topographic loading

Though it is widely recognized that large mountain ranges produce significant stresses in the Earth's crust, these stresses are not commonly quantified. Nonetheless, near large mountains topography may affect fault activity by changing the stress balance on the faults. In this work, we calculate the stress field from topography in the Longmen Shan (Sichuan, China) and resolve those stresses on several models of the faults that ruptured in the 2008 Mw 7.9 Wenchuan earthquake. We find that the topography results in shear stresses up to 20 MPa and normal stresses up to 80 MPa on the faults, with significant variability across the faults. Topographic stresses generally load the fault in a normal and left‐lateral shear sense, opposite to the inferred coseismic slip sense, and thus inhibit the coseismic slip. We estimate the tectonic stress needed to overcome topographic and lithostatic stresses by assuming that the direction of maximum shear accumulated on the faults is roughly collinear with the inferred coseismic slip. We further estimate the static friction and pore fluid pressure assuming that the fault was, on average, at Mohr‐Coulomb failure at the time of the Wenchuan earthquake. We use a Bayesian inversion strategy, yielding posterior probability distributions for the estimated parameters. We find most likely estimates of maximum tectonic compressive stress near 0.6 ρgz and oriented ∼E‐W, and minimum tectonic stress near 0.2 ρgz. Static friction on the fault is near 0.2, and pore fluid pressure is between 0 and 0.4 of the total pressure.Key PointsTopographic stress is significant on Wenchuan faultsTopographic stresses resist tectonic slipTectonic stress, friction, and fluid pressure estimatedPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111754/1/jgrb51070.pd

Inference of Multiple Earthquake-Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation

Characterizing surface deformation throughout a full earthquake cycle is a challenge due to the lack of high‐resolution geodetic observations of duration comparable to that of characteristic earthquake recurrence intervals (250–10,000 years). Here we approach this problem by comparing long‐term geologic slip rates with geodetically derived fault slip rates by sampling only a short fraction (0.001%–0.1%) of a complete earthquake cycle along 15 continental strike‐slip faults. Geodetic observations provide snapshots of surface deformation from different times through the earthquake cycle. The timing of the last earthquake on many of these faults is poorly known, and may vary greatly from fault to fault. Assuming that the underlying mechanics of the seismic cycle are similar for all faults, geodetic observations from different faults may be interpreted as samples over a significantly larger fraction of the earthquake cycle than could be obtained from the geodetic record along any one fault alone. As an ensemble, we find that geologically and geodetically inferred slip rates agree well with a linear relation of 0.94±0.09. To simultaneously explain both the ensemble agreement between geologic and geodetic slip‐rate estimates with observations of rapid postseismic deformation, we consider the predictions from simple two‐layer earthquake‐cycle models with both Maxwell and Burgers viscoelastic rheologies. We find that a two‐layer Burgers model, with two relaxation timescales, is consistent with observations of deformation throughout the earthquake cycle, whereas the widely used two‐layer Maxwell model with a single relaxation timescale, is not, suggesting that the earthquake cycle is effectively characterized by a largely stress‐recoverable rapid postseismic stage and a much more slowly varying interseismic stage.Earth and Planetary Science

Coseismic fault slip of the 2008 M w 7.9 Wenchuan earthquake estimated from InSAR and GPS measurements

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94993/1/grl26608.pd

A multiscale approach to estimating topographically correlated propagation delays in radar interferograms

When targeting small amplitude surface deformation, using repeat orbit Interferometric Synthetic Aperture Radar (InSAR) observations can be plagued by propagation delays, some of which correlate with topographic variations. These topographically-correlated delays result from temporal variations in vertical stratification of the troposphere. An approximate model assuming a linear relationship between topography and interferometric phase has been used to correct observations with success in a few studies. Here, we present a robust approach to estimating the transfer function, K, between topography and phase that is relatively insensitive to confounding processes (earthquake deformation, phase ramps from orbital errors, tidal loading, etc.). Our approach takes advantage of a multiscale perspective by using a band-pass decomposition of both topography and observed phase. This decomposition into several spatial scales allows us to determine the bands wherein correlation between topography and phase is significant and stable. When possible, our approach also takes advantage of any inherent redundancy provided by multiple interferograms constructed with common scenes. We define a unique set of component time intervals for a given suite of interferometric pairs. We estimate an internally consistent transfer function for each component time interval, which can then be recombined to correct any arbitrary interferometric pair. We demonstrate our approach on a synthetic example and on data from two locations: Long Valley Caldera, California, which experienced prolonged periods of surface deformation from pressurization of a deep magma chamber, and one coseismic interferogram from the 2007 Mw 7.8 Tocapilla earthquake in northern Chile. In both examples, the corrected interferograms show improvements in regions of high relief, independent of whether or not we pre-correct the data for a source model. We believe that most of the remaining signals are predominately due to heterogeneous water vapor distribution that requires more sophisticated correction methods than those described here

Southern San Andreas-San Jacinto fault system slip rates estimated from earthquake cycle models constrained by GPS and interferometric synthetic aperture radar observations

We use ground geodetic and interferometric synthetic aperture radar satellite observations across the southern San Andreas (SAF)-San Jacinto (SJF) fault systems to constrain their slip rates and the viscosity structure of the lower crust and upper mantle on the basis of periodic earthquake cycle, Maxwell viscoelastic, finite element models. Key questions for this system are the SAF and SJF slip rates, the slip partitioning between the two main branches of the SJF, and the dip of the SAF. The best-fitting models generally have a high-viscosity lower crust (η = 10^(21) Pa s) overlying a lower-viscosity upper mantle (η = 10^(19) Pa s). We find considerable trade-offs between the relative time into the current earthquake cycle of the San Jacinto fault and the upper mantle viscosity. With reasonable assumptions for the relative time in the earthquake cycle, the partition of slip is fairly robust at around 24–26 mm/a for the San Jacinto fault system and 16–18 mm/a for the San Andreas fault. Models for two subprofiles across the SAF-SJF systems suggest that slip may transfer from the western (Coyote Creek) branch to the eastern (Clark-Superstition hills) branch of the SJF from NW to SE. Across the entire system our best-fitting model gives slip rates of 2 ± 3, 12 ± 9, 12 ± 9, and 17 ± 3 mm/a for the Elsinore, Coyote Creek, Clark, and San Andreas faults, respectively, where the large uncertainties in the slip rates for the SJF branches reflect the large uncertainty in the slip rate partitioning within the SJF system

An open source Bayesian Monte Carlo isotope mixing model with applications in Earth surface processes

The implementation of isotopic tracers as constraints on source contributions has become increasingly relevant to understanding Earth surface processes. Interpretation of these isotopic tracers has become more accessible with the development of Bayesian Monte Carlo (BMC) mixing models, which allow uncertainty in mixing end‐members and provide methodology for systems with multicomponent mixing. This study presents an open source multiple isotope BMC mixing model that is applicable to Earth surface environments with sources exhibiting distinct end‐member isotopic signatures. Our model is first applied to new δ18O and δD measurements from the Athabasca Glacier, which showed expected seasonal melt evolution trends and vigorously assessed the statistical relevance of the resulting fraction estimations. To highlight the broad applicability of our model to a variety of Earth surface environments and relevant isotopic systems, we expand our model to two additional case studies: deriving melt sources from δ18O, δD, and 222Rn measurements of Greenland Ice Sheet bulk water samples and assessing nutrient sources from ɛNd and 87Sr/86Sr measurements of Hawaiian soil cores. The model produces results for the Greenland Ice Sheet and Hawaiian soil data sets that are consistent with the originally published fractional contribution estimates. The advantage of this method is that it quantifies the error induced by variability in the end‐member compositions, unrealized by the models previously applied to the above case studies. Results from all three case studies demonstrate the broad applicability of this statistical BMC isotopic mixing model for estimating source contribution fractions in a variety of Earth surface systems.Key Points:Open source BMC model determines source contributions in Earth surface systemsEffectively applied to stable and radiogenic isotope systems in various settingsModel able to encompass end‐member uncertainties and multiple isotopic systemsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111937/1/ggge20708.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/111937/2/ggge20708-sup-0001-2014GC005683-ts01.pd

EULAR recommendations for the management of rheumatoid arthritis with synthetic and biological disease-modifying antirheumatic drugs: 2019 update

Objectives: To provide an update of the European League Against Rheumatism (EULAR) rheumatoid arthritis (RA) management recommendations to account for the most recent developments in the field. Methods: An international task force considered new evidence supporting or contradicting previous recommendations and novel therapies and strategic insights based on two systematic literature searches on efficacy and safety of disease-modifying antirheumatic drugs (DMARDs) since the last update (2016) until 2019. A predefined voting process was applied, current levels of evidence and strengths of recommendation were assigned and participants ultimately voted independently on their level of agreement with each of the items. Results: The task force agreed on 5 overarching principles and 12 recommendations concerning use of conventional synthetic (cs) DMARDs (methotrexate (MTX), leflunomide, sulfasalazine); glucocorticoids (GCs); biological (b) DMARDs (tumour necrosis factor inhibitors (adalimumab, certolizumab pegol, etanercept, golimumab, infliximab), abatacept, rituximab, tocilizumab, sarilumab and biosimilar (bs) DMARDs) and targeted synthetic (ts) DMARDs (the Janus kinase (JAK) inhibitors tofacitinib, baricitinib, filgotinib, upadacitinib). Guidance on monotherapy, combination therapy, treatment strategies (treat-to-target) and tapering on sustained clinical remission is provided. Cost and sequencing of b/tsDMARDs are addressed. Initially, MTX plus GCs and upon insufficient response to this therapy within 3 to 6 months, stratification according to risk factors is recommended. With poor prognostic factors (presence of autoantibodies, high disease activity, early erosions or failure of two csDMARDs), any bDMARD or JAK inhibitor should be added to the csDMARD. If this fails, any other bDMARD (from another or the same class) or tsDMARD is recommended. On sustained remission, DMARDs may be tapered, but not be stopped. Levels of evidence and levels of agreement were mostly high. Conclusions: These updated EULAR recommendations provide consensus on the management of RA with respect to benefit, safety, preferences and cost