Glenn P. Biasi

Major Earthquakes Occur Regularly on an Isolated Plate Boundary Fault

The Sedimentary Life of Earthquakes Estimating the hazards associated with possible large earthquakes depends largely on evidence of prior seismic activity. The relatively new global seismic networks installed to monitor earthquakes, however, have only captured the very recent history of fault zones that can remain active for thousands of years. To understand the recurrence of large earthquakes along the Alpine Fault in New Zealand, Berryman et al. (p. 1690) looked to the sediments near an old creek for evidence of surface ruptures and vertical offset. Along this fault segment, 24 large earthquakes seem to have occurred over the last 6000 years, resulting in a recurrence interval of ∼329 years. The activity is more regular than other similar strike-slip faults, such as the San Andreas Fault in California. Evidence of past earthquakes from sediments along New Zealand’s Alpine Fault improves seismic hazard estimates. The scarcity of long geological records of major earthquakes, on different types of faults, makes testing hypotheses of regular versus random or clustered earthquake recurrence behavior difficult. We provide a fault-proximal major earthquake record spanning 8000 years on the strike-slip Alpine Fault in New Zealand. Cyclic stratigraphy at Hokuri Creek suggests that the fault ruptured to the surface 24 times, and event ages yield a 0.33 coefficient of variation in recurrence interval. We associate this near-regular earthquake recurrence with a geometrically simple strike-slip fault, with high slip rate, accommodating a high proportion of plate boundary motion that works in isolation from other faults. We propose that it is valid to apply time-dependent earthquake recurrence models for seismic hazard estimation to similar faults worldwide.

Evidence for Large Earthquakes on the San Andreas Fault at the Wrightwood, California, Paleoseismic Site: a.d. 500 to Present

We present structural and stratigraphic evidence from a paleoseismic site near Wrightwood, California, for 14 large earthquakes that occurred on the southern San Andreas fault during the past 1500 years. In a network of 38 trenches and creek-bank exposures, we have exposed a composite section of interbedded debris flow deposits and thin peat layers more than 24 m thick; fluvial deposits occur along the northern margin of the site. The site is a 150-m-wide zone of deformation bounded on the surface by a main fault zone along the northwest margin and a secondary fault zone to the southwest. Evidence for most of the 14 earthquakes occurs along structures within both zones. We identify paleoearthquake horizons using in-filled fissures, scarps, multiple rupture terminations, and widespread folding and tilting of beds. Ages of stratigraphic units and earthquakes are constrained by historic data and 72 14C ages, mostly from samples of peat and some from plant fibers, wood, pine cones, and charcoal. Comparison of the long, well-resolved paleoseimic record at Wrightwood with records at other sites along the fault indicates that rupture lengths of past earthquakes were at least 100 km long. Paleoseismic records at sites in the Coachella Valley suggest that each of the past five large earthquakes recorded there ruptured the fault at least as far northwest as Wrightwood. Comparisons with event chronologies at Pallett Creek and sites to the northwest suggests that approximately the same part of the fault that ruptured in 1857 may also have failed in the early to mid-sixteenth century and several other times during the past 1200 years. Records at Pallett Creek and Pitman Canyon suggest that, in addition to the 14 earthquakes we document, one and possibly two other large earthquakes ruptured the part of the fault including Wrightwood since about a.d. 500. These observations and elapsed times that are significantly longer than mean recurrence intervals at Wrightwood and sites to the southeast suggest that at least the southermost 200 km of the San Andreas fault is near failure.

Paleoseismic Event Dating and the Conditional Probability of Large Earthquakes on the Southern San Andreas Fault, California

We introduce a quantitative approach to paleoearthquake dating and apply it to paleoseismic data from the Wrightwood and Pallett Creek sites on the southern San Andreas fault. We illustrate how stratigraphic ordering, sedimentolog- ical, and historical data can be used quantitatively in the process of estimating earth- quake ages. Calibrated radiocarbon age distributions are used directly from layer dating through recurrence intervals and recurrence probability estimation. The method does not eliminate subjective judgements in event dating, but it does provide a means of systematically and objectively approaching the dating process. Date dis- tributions for the most recent 14 events at Wrightwood are based on sample and contextual evidence in Fumal et al. (2002) and site context and slip history in Weldon et al. (2002). Pallett Creek event and dating descriptions are from published sources. For the five most recent events at Wrightwood, our results are consistent with pre- viously published estimates, with generally comparable or narrower uncertainties. For Pallett Creek, our earthquake date estimates generally overlap with previous results but typically have broader uncertainties. Some event date estimates are very sensitive to details of data interpretation. The historical earthquake in 1857 ruptured the ground at both sites but is not constrained by radiocarbon data. Radiocarbon ages, peat accumulation rates, and historical constraints at Pallett Creek for event X yield a date estimate in the earliest 1800s and preclude a date in the late 1600s. This event is almost certainly the historical 1812 earthquake, as previously concluded by Sieh et al. (1989). This earthquake also produced ground deformation at Wrightwood. All events at Pallett Creek, except for event T, about A.D. 1360, and possibly event I, about A.D. 960, have corresponding events at Wrightwood with some overlap in age ranges. Event T falls during a period of low sedimentation at Wrightwood when conditions were not favorable for recording earthquake evidence. Previously pro- posed correlations of Pallett Creek X with Wrightwood W3 in the 1690s and Pallett Creek event V with W5 around 1480 (Fumal et al., 1993) appear unlikely after our dating reevaluation. Apparent internal inconsistencies among event, layer, and dating relationships around events R and V identify them as candidates for further inves- tigation at the site. Conditional probabilities of earthquake recurrence were estimated using Poisson, lognormal, and empirical models. The presence of 12 or 13 events at Wrightwood during the same interval that 10 events are reported at Pallett Creek is reflected in mean recurrence intervals of 105 and 135 years, respectively. Average Poisson model 30-year conditional probabilities are about 20% at Pallett Creek and 25% at Wrightwood. The lognormal model conditional probabilities are somewhat higher, about 25% for Pallett Creek and 34% for Wrightwood. Lognormal variance rln estimates of 0.76 and 0.70, respectively, imply only weak time predictability. Conditional probabilities of 29% and 46%, respectively, were estimated for an em- pirical distribution derived from the data alone. Conditional probability uncertainties are dominated by the brevity of the event series; dating uncertainty contributes only secondarily. Wrightwood and Pallett Creek event chronologies both suggest varia- tions in recurrence interval with time, hinting that some form of recurrence rate modulation may be at work, but formal testing shows that neither series is more ordered than might be produced by a Poisson process.

Estimating Surface Rupture Length and Magnitude of Paleoearthquakes from Point Measurements of Rupture Displacement

We present a method to estimate paleomagnitude and rupture extent from measurements of displacement at a single point on a fault. The variability of historic ruptures is summarized in a histogram of normalized slip, then scaled to give the probability of finding a given displacement within a rupture for any magnitude considered. The histogram can be inverted assuming any magnitude earthquake is as likely as another, yielding probability density functions of magnitude and rupture length for any given displacement measurement. To improve these distributions we include a term to account for the probability that the earthquake would cause ground rupture and two alternative distributions of earthquake magnitude. The Gutenberg- Richter magnitude distribution predicts shorter rupture lengths and smaller magnitudes than does a uniform distribution where any magnitude earthquake is considered equally likely. Longer ruptures and larger magnitudes than the uniform model are predicted by an alternative magnitude distribution designed to return site average displacement. This model is a generalization of the characteristic earthquake model, and reasonably describes paleoseismic findings on the southern San Andreas fault, where slip is accommodated by average displacements of a few meters and earthquake recurrence times of 100–250 years. Our results should increase the value of paleoseismic displacement measurements for hazard assessment. In particular, they quantify probability estimates of earthquake magnitude and rupture length where point observations of rupture displacement are available, and so can contribute to probabilistic seismic-hazard analyses. Online material: Rupture profiles used to sample rupture variability.

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.

Quasi-periodic recurrence of large earthquakes on the southern San Andreas fault

It has been 153 yr since the last large earthquake on the southern San Andreas fault (California, United States), but the average interseismic interval is only ∼100 yr. If the recurrence of large earthquakes is periodic, rather than random or clustered, the length of this period is notable and would generally increase the risk estimated in probabilistic seismic hazard analyses. Unfortunately, robust characterization of a distribution describing earthquake recurrence on a single fault is limited by the brevity of most earthquake records. Here we use statistical tests on a 3000 yr combined record of 29 ground-rupturing earthquakes from Wrightwood, California. We show that earthquake recurrence there is more regular than expected from a Poisson distribution and is not clustered, leading us to conclude that recurrence is quasi-periodic. The observation of unimodal time dependence is persistent across an observationally based sensitivity analysis that critically examines alternative interpretations of the geologic record. The results support formal forecast efforts that use renewal models to estimate probabilities of future earthquakes on the southern San Andreas fault. Only four intervals (15%) from the record are longer than the present open interval, highlighting the current hazard posed by this fault.

San Andreas Fault Rupture Scenarios from Multiple Paleoseismic Records: Stringing Pearls

We present a new method to objectively combine paleoseismic event data from multiple sites into rupture scenarios and apply it to the southern San Andreas fault (SSAF) of California. First, a pool is constructed of all ruptures between sites allowed by fault geometry and available event age probability distribution functions (PDFs). Scenarios constructed by drawing from this pool are evaluated by the average quality of agreement event dating evidence, by the degree of misfit of cumulative displacement over all ruptures compared to a prediction from the fault slip rate and elapsed time, and by the number of events in the scenario. Three slip-rate models from the 2008 Working Group on California Earthquake Probabilities (WGCEP) were considered. Scenarios with full fault length ruptures tend to be inconsistent with low slip rates through the San Bernardino and San Gorgonio sections of the SSAF. Favorable scenarios tend to include 1857-like ruptures for three of the most recent five prehistoric ruptures in the northern half of the SSAF from Carrizo to approximately Wrightwood. They also include one to two ruptures that involve the southern half of the SSAF, but most earthquakes there appear shorter and exhibit less consistency from event to event. By combining paleoseismic data into ensembles of scenarios and selecting viable scenarios using external constraints, our method provides rupture histories useful for seismic hazard assessment without having to first settle which event at a site correlates with those at adjoining sites. This opens a way for paleoseismic data to be used with greater power to understand the seismic hazard posed by faults like the southern San Andreas.

Paleoearthquakes on the Southern San Andreas Fault, Wrightwood, California, 3000 to 1500 b.c.: A New Method for Evaluating Paleoseismic Evidence and Earthquake Horizons

We present evidence of 11–14 earthquakes that occurred between 3000 and 1500 b.c. on the San Andreas fault at the Wrightwood paleoseismic site. Earthquake evidence is presented in a novel form in which we rank (high, moderate, poor, or low) the quality of all evidence of ground deformation, which are called “event indicators.” Event indicator quality reflects our confidence that the morphologic and sedimentologic evidence can be attributable to a ground-deforming earthquake and that the earthquake horizon is accurately identified by the morphology of the feature. In four vertical meters of section exposed in ten trenches, we document 316 event indicators attributable to 32 separate stratigraphic horizons. Each stratigraphic horizon is evaluated based on the sum of rank (Rs), maximum rank (Rm), average rank (Ra), number of observations (Obs), and sum of higher-quality event indicators (Rs >1 ). Of the 32 stratigraphic horizons, 14 contain 83% of the event indicators and are qualified based on the number and quality of event indicators; the remaining 18 do not have satisfactory evidence for further consideration. Eleven of the 14 stratigraphic horizons have sufficient number and quality of event indicators to be qualified as “probable” to “very likely” earthquakes; the remaining three stratigraphic horizons are associated with somewhat ambiguous features and are qualified as “possible” earthquakes. Although no single measurement defines an obvious threshold for designation as an earthquake horizon, Rs, Rm, and Rs >1 correlate best with the interpreted earthquake quality. Earthquake age distributions are determined from radiocarbon ages of peat samples using a Bayesian approach to layer dating. The average recurrence interval for the 10 consecutive and highest-quality earthquakes is 111 (93–131) years and individual intervals are ±50% of the average. With comparison with the previously published 14–15 earthquake record between a.d. 500 and present, we find no evidence to suggest significant variations in the average recurrence rate at Wrightwood during the past 5000 years.

A Comparison of Spectral Parameter Kappa from Small and Moderate Earthquakes Using Southern California ANZA Seismic Network Data

Kappa is a one-parameter estimator of the spectral amplitude decay with frequency of a seismogram. Low values (∼5 ms) indicate limited attenuation of high- frequency energy whereas higher values (∼40 ms) indicate high-frequency energy has been removed. Kappa is often assumed to be a site term and used in seismic designs. We address two key questions about kappa: (1) how to identify source, path, and site contributions to kappa; and (2) can kappa estimates from smaller earthquakes, and more readily accessible weak-motion recordings, be reasonably extrapolated to esti- mate kappa of larger earthquakes? The use of small earthquakes (ML 3:5 earthquakes inside the network. We find kappa from small earthquakes predicts the relative values of kappa for larger earthquakes (e.g., measurements at stations PFO and KNW are small compared with those at stations TRO and SND). For the SND and TRO data, however, kappa values from small earth- quakes overpredict those from moderate and large earthquakes. Site effects are the most important contributor to kappa estimates, but the scatter within kappa measure- ments at a given station is likely caused by a significant contribution from near the source, perhaps related to near-source scattering. Because of this source-side varia- bility, care is recommended in using individual small events as Greens functions to study source-time effects of moderate and large events.

The Length to Which an Earthquake Will Go to Rupture

Abstract Using a database of 22 historical strike-slip surface rupture earthquakes, an apparent upper limit is observed in the number of steps through which an earthquake is likely to rupture. The number of ruptures is also observed to decrease as a function of the number of steps through which the respective earthquakes propagated. The observations may be important for assessing the expected length of earthquake ruptures where fault sections interact on mapped fault systems.