Kaj M. Johnson

Long-Term Time-Dependent Probabilities for the Third Uniform California Earthquake Rupture Forecast (UCERF3)

The 2014 Working Group on California Earthquake Probabilities (WGCEP 2014) presents time-dependent earthquake probabilities for the third Uniform California Earthquake Rupture Forecast (UCERF3). Building on the UCERF3 time-in- dependent model published previously, renewal models are utilized to represent elastic- rebound-implied probabilities. A new methodology has been developed that solves applicability issues in the previous approach for unsegmented models. The new meth- odology also supports magnitude-dependent aperiodicity and accounts for the historic open interval on faults that lack a date-of-last-event constraint. Epistemic uncertainties are represented with a logic tree, producing 5760 different forecasts. Results for a variety of evaluation metrics are presented, including logic-tree sensitivity analyses and comparisons to the previous model (UCERF2). For 30 yr M ! 6:7 probabilities, the most significant changes from UCERF2 are a threefold increase on the Calaveras fault and a threefold decrease on the San Jacinto fault. Such changes are due mostly to differences in the time-independent models (e.g., fault-slip rates), with relaxation of segmentation and inclusion of multifault ruptures being particularly influential. In fact, some UCERF2 faults were simply too long to produce M 6.7 size events given the segmentation assumptions in that study. Probability model differences are also influential, with the implied gains (relative to a Poisson model) being generally higher in UCERF3. Accounting for the historic open interval is one reason. Another is an effective 27% increase in the total elastic-rebound-model weight. The exact factors influencing differences between UCERF2 and UCERF3, as well as the relative im- portance of logic-tree branches, vary throughout the region and depend on the evalu- ation metric of interest. For example, M ! 6:7 probabilities may not be a good proxy for other hazard or loss measures. This sensitivity, coupled with the approximate nature of the model and known limitations, means the applicability of UCERF3 should be evaluated on a case-by-case basis.

Reconciling geologic and geodetic model fault slip-rate discrepancies in Southern California: Consideration of nonsteady mantle flow and lower crustal fault creep

In Southern California, slip rates derived from geodesy-constrained elastic models are lower than geologic rates along the Mojave and San Bernardino segments of the San Andreas fault and the Garlock fault. In contrast, the summed geodetic rate across the Mojave eastern California shear zone (ECSZ) is significantly higher than the summed geologic rate. We show that geodetic and geologic slip rates in Southern California can be reconciled using a viscoelastic earthquake cycle model that explicitly incorporates time-dependent deformation due to nonsteady interseismic fault creep in the lower crust and viscous flow in the upper mantle. To reconcile geologic and geodetic model rates, our model requires that the southern San Andreas fault and the Garlock fault are in the late stages of the earthquake cycle, resulting in lower current deformation rates than the cycle-averaged rate. Our model implies that the ECSZ and the San Jacinto faults are in the early stages of the earthquake cycle, resulting in high current deformation rates.

Influence of lithosphere viscosity structure on estimates of fault slip rate in the Mojave region of the San Andreas fault system

[1] It is well known that slip rate estimates from geodetic data are nonunique because they depend on model assumptions and parameters that are often not known a priori. Estimates of fault slip rate on the Mojave segment of the San Andreas fault system derived from elastic block models and GPS data are significantly lower than estimates from geologic data. To determine the extent to which the slip rate discrepancy might be due to the oversimplified models of the rheology of the lithosphere, we develop a two-dimensional linear Maxwell viscoelastic earthquake cycle model and simultaneously estimate fault slip rates and lithosphere viscosity structure in the Mojave region. The model consists of episodic earthquakes in an elastic crust overlying layers with different viscosities that represent the lower crust, uppermost mantle, and upper mantle. We use GPS measurements of postseismic relaxation following the 1992 Landers earthquake, triangulation measurements spanning 1932-1977, GPS measurements of the contemporary velocity field, and paleoseismic data along the San Andreas fault. We infer lower crustal (15-30 km depth) viscosity of ∼1019-10 20 Pa s, uppermost mantle (30-60 km) viscosity of ∼10 20-22 Pa s, and underlying upper mantle viscosity of ∼10 18 -10 19 Pa S, consistent with inferences from laboratory experiments of relatively high-viscosity lithospheric mantle and lower-viscosity lower crust and underlying asthenospheric mantle. We infer a 20-30 mm/yr slip rate on the San Andreas fault, in agreement with the lower end of geologic estimates. Inversions of geodetic data with models that do not incorporate layered viscosity structure may significantly misestimate slip rates.

Earthquake-cycle deformation and fault slip rates in northern Tibet

Fault slip rate estimates along the Altyn Tagh and Kunlun strike-slip faults in northern Tibet vary considerably between short-term geodetic and long-term geologic studies. Here we reanalyze and model all global positioning system (GPS) data from northern Tibet to determine if these differences might be explained by previously unmodeled transient processes associated with the earthquake cycle, which can bias slip-rate estimates from geodetic data. We find that these effects cannot reconcile the geodetic data with the lowest bounds on the geologic slip rates along these faults, even in the presence of low (<1018 Pa s) viscosities within the mid-crust or crust and mantle lithosphere. Surface velocities derived from GPS measurements are best reproduced with models with a high-viscosity (≥1018 Pa s) middle to lower crust and mantle lithosphere.

A decadal‐scale deformation transient prior to the 2011 Mw 9.0 Tohoku‐oki earthquake

GPS time series in northeast Japan exhibit nonlinear trends from 1996 to 2011 before the Mw 9.0, 2011 Tohoku-oki earthquake. After removing reference frame noise, we model time series as linear trends plus constant acceleration, correcting for coseismic and postseismic effects from the numerous Mw ∼ 6.5+ earthquakes during this period. We find spatially coherent and statistically significant accelerations throughout northern Honshu. Large areas of Japan outside the Tohoku region show insignificant accelerations, demonstrating that the observation is not due to network-wide artifacts. While the accelerations in northern Tohoku (Sanriku area) can be explained by decaying postseismic deformation from pre-1996 earthquakes, the accelerations in south-central Tohoku appear unrelated to postseismic effects. The latter accelerations are associated with a decrease in average trench-normal strain rate and can be explained by increasing slip rate on the Japan trench plate interface and/or updip migration of deep aseismic slip in the decades before the 2011 Tohoku-oki earthquake.

Fault coupling and potential for earthquakes on the creeping section of the central San Andreas Fault

The 150 km long central section of the San Andreas Fault (CSAF) in central California creeps at the surface and has not produced a large earthquake historically. However, sections of the San Andreas Fault to the north and south are known to have ruptured repeatedly in M~7–8 earthquakes. It is currently unclear whether the creeping CSAF could rupture in large earthquakes, either individually or along with earthquakes on the locked sections to the north and south. We invert Global Positioning System and interferometric synthetic aperture radar data with elastic block models to estimate the degree of locking on the CSAF and place bounds on the moment accumulation rate on the fault. We find that the inferred moment accumulation rate is highly dependent on the long-term fault slip rate, which is poorly constrained along the CSAF. The inferred moment accumulation rate, normalized by shear modulus, ranges from 3.28 × 104 to 5.85 × 107 m3/yr, which is equivalent to a Mw = 5.5–7.2 earthquake every 150 years for a long-term slip rate of 26 mm/yr and Mw = 7.3–7.65 for a long-term slip rate of 34 mm/yr. The comparisons of slip distributions with microseismicity and repeating earthquakes indicate a possible locked patch between 10 and 20 km depth on the CSAF that could potentially rupture with Mw = 6.5.

Small interseismic asperities and widespread aseismic creep on the northern Japan subduction interface

The canonical model of fault coupling assumes that slip is partitioned into fixed asperities that display stick-slip behavior and regions that creep stably. We show that this simple asperity model is inconsistent with GPS-derived deformation in northern Japan associated with interseismic coupling on the subduction interface and the transient response to Mw 6.3–7.2 earthquakes during 2003–2011. Comparisons of GPS data with simulations of earthquakes on asperities and associated velocity-strengthening afterslip require that afterslip overlaps areas of the fault that ruptured in previous earthquakes, including the 2011 Mw 9 Tohoku-oki earthquake. Whereas about 55% of the plate interface ruptured in earthquakes during 2003–2011, we infer that only 9% of the plate interface was fully locked between earthquakes. Inferred locked asperities are roughly 25% the size of rupture areas determined by seismic source inversions. These smaller asperities are consistent with interseismic strain accumulation in 2009, although more extensive locking is required a decade earlier in 1998.

Active back thrust in the eastern Taiwan suture revealed by the 2013 Rueisuei earthquake: Evidence for a doubly vergent orogenic wedge?

Rapid exhumation of 3–10 mm/yr of the Taiwan metamorphic range is often explained as the unroofing of the retrowedge of a doubly vergent mountain belt. Yet, to date, the Central Range fault forming the boundary of the retrowedge has displayed no definitive evidence for recent seismic activity and no unambiguous geomorphic expression over much of the fault. The 2013 M6.4 Rueisuei reverse-faulting earthquake nucleated at the eastern boundary of the retrowedge and appears to illuminate the west dipping Central Range fault. We estimate the fault geometry and coseismic slip distribution using a uniform stress drop slip inversion and surface displacements derived from GPS and strong-motion data. We identify a ~42° dipping blind reverse fault, consistent with the previously proposed buried Central Range fault beneath the highly active Longitudinal Valley fault. This earthquake may be the first indication that rapid exhumation and uplift occur along a distinct fault structure bounding the eastern margin of the Taiwan retrowedge.

A Synoptic View of the Third Uniform California Earthquake Rupture Forecast (UCERF3)

ABSTRACT Probabilistic forecasting of earthquake‐producing fault ruptures informs all major decisions aimed at reducing seismic risk and improving earthquake resilience. Earthquake forecasting models rely on two scales of hazard evolution: long‐term (decades to centuries) probabilities of fault rupture, constrained by stress renewal statistics, and short‐term (hours to years) probabilities of distributed seismicity, constrained by earthquake‐clustering statistics. Comprehensive datasets on both hazard scales have been integrated into the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3). UCERF3 is the first model to provide self‐consistent rupture probabilities over forecasting intervals from less than an hour to more than a century, and it is the first capable of evaluating the short‐term hazards that result from multievent sequences of complex faulting. This article gives an overview of UCERF3, illustrates the short‐term probabilities with aftershock scenarios, and draws some valuable scientific conclusions from the modeling results. In particular, seismic, geologic, and geodetic data, when combined in the UCERF3 framework, reject two types of fault‐based models: long‐term forecasts constrained to have local Gutenberg–Richter scaling, and short‐term forecasts that lack stress relaxation by elastic rebound.

Space Geodetic Data Improve Seismic Hazard Assessment in California: Workshop on Incorporating Geodetic Surface Deformation Data Into UCERF3; Pomona, California, 1–2 April 2010

A workshop was held to begin scientific consideration of how to incorporate space geodetic constraints on strain rates and fault slip rates into the next generation Uniform California Earthquake Rupture Forecast, version 3 (UCERF3), due to be completed in mid-2012. Principal outcomes of the meeting were (1) an assessment of secure science ready for UCERF3 applications within the next year, and (2) an agenda of new research objectives for the Southern California Earthquake Center (SCEC), the U.S. Geological Survey (USGS), and others in support of UCERF3 and related probabilistic seismic hazard assessments (PSHA). A number of goals potentially achievable within a year were identified, including (1) slip rate and fault locking depth estimates, with uncertainties or ranges, for all major and some minor faults of the extended San Andreas system; (2) strain rate estimates or bounds on rates for selected regions lying off the major faults of the San Andreas system; and (3) corrections or bounds on perturbing effects of postseismic deformation and elastic modulus heterogeneities on the observed Global Positioning System (GPS) velocity field (needed as input to models for estimating fault slip and strain rates in goals 1 and 2 above).