Christopher Madden

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.

Palaeoseismology of the Vilariça Segment of the Manteigas-Bragança Fault in northeastern Portugal

Abstract The Manteigas-Bragança fault is a major, 250-km-long, NNE-striking, sinistral strike-slip structure in northern Portugal. This fault has no historical seismicity for large earthquakes, although it may have generated moderate (M5+) earthquakes in 1751 and 1858. Evidence of continued left horizontal displacement is shown by the presence of Cenozoic pull-apart basins as well as late Quaternary stream deflections. To investigate its recent slip history, a number of trenches were excavated at three sites along the Vilariça segment, north and south of the Douro River. At one site at Vale Meão winery, the occurrence of at least two and probably three events in the past 14.5 ka was determined, suggesting an average return period of about 5–7 ka. All three events appear to have occurred as a cluster in the interval between 14.5 and 11 ka, or shortly thereafter, suggesting a return period of less than 2 ka between events within the cluster. In the same area, a small offset rill suggests 2–2.5 m of slip in the most recent event and about 6.1 m after incision below a c. 16 ka alluvial fill event along the Douro River. At another site along the Vilariça River alluvial plain, NE of the Vale Meão site, several trenches were excavated in late Pleistocene and Holocene alluvium, and exposed the fault displacing channel deposits dated to between 18 and 23 ka. In a succession of closely spaced parallel cuts and trenches, the channel riser was traced into and across the fault to resolve c. 6.5 m of displacement after 18 ka and c. 9 m of slip after c. 23 ka. These observations yield a slip rate of 0.3–0.5 mm/a, which is consistent with earlier estimates. Combining the information on timing at Vale Meão winery and displacement at Vilariça argues for earthquakes in the M7+ range, with coseismic displacements of 2–3 m. This demonstrates that there are potential seismic sources in Portugal that are not associated with the 1755 Lisbon earthquake or the Tagus Valley, and, although rare, large events on the Vilariça fault could be quite destructive for the region. This work provides an analogue for the study of active faulting in intracontinental settings and supports the view that earthquakes within intracontinental settings tend to cluster in time. In addition, this study highlights the usefulness and application of multiple field, remote sensing and geochronological techniques for seismic hazard mitigation.

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.

Holocene and Latest Pleistocene Activity on the Mesquite Lake Fault near Twentynine Palms, Eastern California Shear Zone: Implications for Fault Interaction

Paleoseismic data from the Mesquite Lake fault reveal evidence for up to four prehistoric earthquakes since the latest Pleistocene, with three large surface- rupturing events in about the past 10.2 ka. Three events were recorded in trenches excavated for this investigation on Mesquite Lake playa. A buried fault scarp brack- eted by accelerator mass spectrometer radiocarbon ages of detrital charcoal indicate that a mid to late Holocene event occurred between about 2.7 and 7.4 ka, most likely between about 3.9 and 4.6 ka. An early Holocene rupture defined by a subtle buried scarp and upward-terminating fissures is bracketed between about 7.7 and 10.2 ka. Evidence for an inferred late Pleistocene event prior to about 10.2 ka includes a subtle scarp, upward termination of faults, folding, and in-filled craters and fissures. A previous paleoseismic investigation just south (2 km) of the playa identified at least one event in the past 1.3 ka, although this event was not identified in this study at the playa site. Apparent vertical offsets of about 1.0 m and 1.2 m for the last two events at the playa site suggest a sequence of similar serial ruptures. These separa- tions are comparable to the height of surficial scarps just south of the playa. Because the vertical separations produced by each event are similar, the Mesquite Lake fault, at least locally, experiences similar slip-per-event. Since the initiation of Holocene playa development just over 10.2 ka ago, we estimate a horizontal slip rate of 0.7- 0.9 mm yr 1 that is consistent with slip rates for other faults in the eastern California shear zone. Despite large uncertainties, event stratigraphy at the playa site shows that periods of quiescence up to thousands of years separated large ruptures, similar to other faults in the eastern California shear zone. Comparison of paleoseismic studies in the eastern California shear zone suggest that the Mesquite Lake, and possibly the Bullion and Lavic Lake faults in the eastern part of the shear zone may have failed during periods of seismic quiescence in the western part of the zone, at about 1- 5 ka. This suggests that seismic strain release may vary spatially across the shear zone and that temporal clusters of earthquakes are not necessarily spatially limited to specific regions.