Olaf Zielke

Slip in the 1857 and Earlier Large Earthquakes Along the Carrizo Plain, San Andreas Fault

Slip, Tripped, and Faulted Earthquake risk assessment can be improved if we were able to quantify the recurrence and magnitude of slip events. Until recently though, a lack of sophisticated seismometers has forced us to rely on anecdotal evidence from those who survived major earthquakes or to look for clues in the landscape. Zielke et al. (p. 1119, published online 21 January; see the Perspective by Scharer) analyzed high-resolution images of the San Andreas Fault in southern California. The data showed that major surface ruptures, such as the 1857 Fort Tejon earthquake, resulted from slips of only about 5 meters; much less than previously thought. In a study that lends support to this discovery, Grant Ludwig et al. (p. 1117, published online 21 January; see the Perspective by Scharer) suggest from analysis of the geomorphic features of this region that several smaller earthquakes have occurred during recent centuries rather than infrequent but larger movements. The Perspective by Scharer (p. 1089) discusses how paleoseismological studies like these may be valuable for feeding data into earthquake prediction. The historical behavior of the San Andreas Fault may have been dominated by smaller, more frequent slip events. The moment magnitude (Mw) 7.9 Fort Tejon earthquake of 1857, with a ~350-kilometer-long surface rupture, was the most recent major earthquake along the south-central San Andreas Fault, California. Based on previous measurements of its surface slip distribution, rupture along the ~60-kilometer-long Carrizo segment was thought to control the recurrence of 1857-like earthquakes. New high-resolution topographic data show that the average slip along the Carrizo segment during the 1857 event was 5.3 ± 1.4 meters, eliminating the core assumption for a linkage between Carrizo segment rupture and recurrence of major earthquakes along the south-central San Andreas Fault. Earthquake slip along the Carrizo segment may recur in earthquake clusters with cumulative slip of ~5 meters.

Century-long average time intervals between earthquake ruptures of the San Andreas fault in the Carrizo Plain, California

Paleoseismological data constrain the age, location, and associated magnitude of past surface-rupturing earthquakes; these are critical parameters for developing and testing fault behavior models and characterizing seismic hazard. We present new earthquake evidence and radiocarbon analyses that refi ne the chronology of the six most recent earthquakes that ruptured the south-central San Andreas fault in the Carrizo Plain (California, United States) at the Bidart Fan site. Modeled 95 percentile ranges of the earthquakes prior to the A.D. 1857 earthquake are A.D. 1631‐1823, 1580‐1640, 1510‐1612, 1450‐1475, and 1360‐1452. The average time interval between the last six earthquakes that ruptured the San Andreas fault in the Carrizo Plain is 88 ± 41 yr. This is less than the time since the most recent A.D. 1857 earthquake, less than all reported average intervals of prehistoric earthquakes along the entire San Andreas fault, and signifi cantly shorter than the 235 yr average used in recent seismic hazard evaluations. The new chronological data combined with recent slip studies imply that the magnitudes of the earthquakes that ruptured the southern San Andreas fault in the Carrizo Plain since ca. A.D. 1360 were variable, and suggest that the widely held view of rare but great surface rupturing earthquakes along this portion of the southern San Andreas fault should be reevaluated. 118°

Climate-modulated channel incision and rupture history of the San Andreas Fault in the Carrizo Plain

Slip, Tripped, and Faulted Earthquake risk assessment can be improved if we were able to quantify the recurrence and magnitude of slip events. Until recently though, a lack of sophisticated seismometers has forced us to rely on anecdotal evidence from those who survived major earthquakes or to look for clues in the landscape. Zielke et al. (p. 1119, published online 21 January; see the Perspective by Scharer) analyzed high-resolution images of the San Andreas Fault in southern California. The data showed that major surface ruptures, such as the 1857 Fort Tejon earthquake, resulted from slips of only about 5 meters; much less than previously thought. In a study that lends support to this discovery, Grant Ludwig et al. (p. 1117, published online 21 January; see the Perspective by Scharer) suggest from analysis of the geomorphic features of this region that several smaller earthquakes have occurred during recent centuries rather than infrequent but larger movements. The Perspective by Scharer (p. 1089) discusses how paleoseismological studies like these may be valuable for feeding data into earthquake prediction. The historical behavior of the San Andreas Fault may have been dominated by smaller, more frequent slip events. The spatial and temporal distribution of fault slip is a critical parameter in earthquake source models. Previous geomorphic and geologic studies of channel offset along the Carrizo section of the south central San Andreas Fault assumed that channels form more frequently than earthquakes occur and suggested that repeated large-slip earthquakes similar to the 1857 Fort Tejon earthquake illustrate typical fault behavior. We found that offset channels in the Carrizo Plain incised less frequently than they were offset by earthquakes. Channels have been offset by successive earthquakes with variable slip since ~1400. This nonuniform slip history reveals a more complex rupture history than previously assumed for the structurally simplest section of the San Andreas Fault.

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.

LaDiCaoz and LiDARimager—MATLAB GUIs for LiDAR data handling and lateral displacement measurement

Light detection and ranging (LiDAR), high-resolution topographic data sets enable remote identification of submeter-scale geomorphic features and have proven very valuable in geologic, paleoseismic, and geomorphologic investigations. They are also useful for studies of hydrology, timber evaluation, vegetation dynamics, coastal monitoring, hill-slope processes, or civil engineering. One application for LiDAR data is the measurement of tectonically displaced geomorphic markers to reconstruct paleoearthquake slip distributions—currently a cornerstone in the formulation of earthquake recurrence models and the understanding of seismic fault behavior. With this publication we provide two MATLAB-based graphical user interfaces (GUIs) and corresponding tutorials: LiDARimager—a tool for LiDAR data handling and visualization (e.g., data cropping, generation of map- and oblique-view plots of various digital elevation model [DEM] derivatives, storable as *.jpg or *.kmz files); and LaDiCaoz—a tool to determine lateral displacements of offset sublinear geomorphic features such as stream channels or alluvial fan edges. While application of LaDiCaoz is closely linked to tectonogeomorphic studies, LiDARimager may find application in a wide range of studies that utilize LiDAR data visualizations. A key feature of LaDiCaoz, not available in standard geographic information system (GIS) packages, is DEM slicing and (laterally) back slipping for visual offset reconstruction assessment, improving measurement accuracy and precision. Comparison of offset measurements, made by different individuals, showed good measurement repeatability with LaDiCaoz for morphologically simple channels. Offset estimates began to vary distinctly for morphologically more complex features, attributed to different assumptions of pre-earthquake morphology and underlining the importance of a sound understanding of pre-earthquake site morphology for meaningful offset measurements.

Applications of airborne and terrestrial laser scanning to paleoseismology

Paleoseismic investigations aim to document past earthquake characteristics such as rupture location, frequency, distribution of slip, and ground shaking intensity—critical parameters for improved understanding of earthquake processes and refined earthquake forecasts. These investigations increasingly rely on high-resolution ( 2 /m. This situation refines interpretations of PBR exhumation rates and thus their effectiveness as paleoseismometers. Given that earthquakes disrupt Earth9s surface at centimeter to meter scales and that depositional and erosional responses typically operate on similar scales, ALS and TLS provide the absolute measurement capability sufficient to characterize these changes in challenging geometric arrangements, and thus demonstrate their value as effective analytical tools in paleoseismology.

Recurrence of large earthquakes in magmatic continental rifts : insights from a paleoseismic study along the Laikipia-Marmanet Fault, Subukia Valley, Kenya Rift

The seismicity of the Kenya rift is characterized by high-frequency low-magnitude events concentrated along the rift axis. Its seismic character is typical for magmatically active continental rifts, where igneous material at a shallow depth causes extensive grid faulting and geothermal activity. Thermal overprinting and dike intrusion prohibit the buildup of large elastic strains, therefore prohibiting the gen- eration of large-magnitude earthquakes. On 6 January 1928, the MS 6:9 Subukia earthquake occurred on the Laikipia-Marmanet fault, the eastern rift-bounding struc- ture of the central Kenya rift. It is the largest instrumentally recorded seismic event in the Kenya rift, standing in contrast to the current model of the rifts seismic character in which large earthquakes are not anticipated. Furthermore, the proximity of the rup- tured fault and the rift axis is intriguing: The rift-bounding structure that ruptured in 1928 remains seismically active, capable of generating large-magnitude earthquakes, even though thermally weakened crust and better oriented structures are present along the rift axis nearby, prohibiting any significant buildup of elastic strain. We excavated the surface rupture of the 1928 Subukia earthquake to find evidence for preceding ground-rupturing earthquakes. We also made a total station survey of the site topography and mapped the site geology. We show that the Laikipia-Marmanet fault was repeatedly activated during the late Quaternary. We found evidence for six ground-rupturing earthquakes, includ- ing the 1928 earthquake. The topographic survey around the trench site revealed a degraded fault scarp of ≈7:5 m in height, offsetting a small debris slide. Using scarp-diffusion modeling, we estimated an uplift rate of U 0:09-0:15 mm=yr, con- straining the scarp age to 50-85 ka. Assuming an average fault dip of 55°-75°, the preferred uplift rate (0:15 mm=yr) accommodates approximately 10%-20% of the recent rate of extension (0:5 mm=yr) across the Kenya rift.

Validation of meter-scale surface faulting offset measurements from high-resolution topographic data

Studies of active fault zones have flourished with the availability of high-resolution topographic data, particularly where airborne light detection and ranging (lidar) and structure from motion (SfM) data sets provide a means to remotely analyze submeter-scale fault geomorphology. To determine surface offset at a point along a strike-slip earthquake rupture, geomorphic features (e.g., stream channels) are measured days to centuries after the event. Analysis of these and cumulatively offset features produces offset distributions for successive earthquakes that are used to understand earthquake rupture behavior. As researchers expand studies to more varied terrain types, climates, and vegetation regimes, there is an increasing need to standardize and uniformly validate measurements of tectonically displaced geomorphic features. A recently compiled catalog of nearly 5000 earthquake offsets across a range of measurement and reporting styles provides insight into quality rating and uncertainty trends from which we formulate best-practice and reporting recommendations for remote studies. In addition, a series of public and beginner-level studies validate the remote methodology for a number of tools and emphasize considerations to enhance measurement accuracy and precision for beginners and professionals. Our investigation revealed that (1) standardizing remote measurement methods and reporting quality rating schemes is essential for the utility and repeatability of fault-offset measurements; (2) measurement discrepancies often involve misinterpretation of the offset geomorphic feature and are a function of the investigator’s experience; (3) comparison of measurements made by a single investigator in different climatic regions reveals systematic differences in measurement uncertainties attributable to variation in feature preservation; (4) measuring more components of a displaced geomorphic landform produces more consistently repeatable estimates of offset; and (5) inadequate understanding of pre-event morphology and post-event modifications represents a greater epistemic limitation than the aleatoric limitations of the measurement process.

Fault roughness and strength heterogeneity control earthquake size and stress drop

An earthquake’s stress drop is related to the frictional breakdown during sliding and constitutes a fundamental quantity of the rupture process. High-speed laboratory friction experiments that emulate the rupture process imply stress drop values that greatly exceed those commonly reported for natural earthquakes. We hypothesize that this stress drop discrepancy is due to fault-surface roughness and strength heterogeneity: an earthquake’s moment release and its recurrence probability depend not only on stress drop and rupture dimension but also on the geometric roughness of the ruptured fault and the location of failing strength asperities along it. Using large-scale numerical simulations for earthquake ruptures under varying roughness and strength conditions, we verify our hypothesis, showing that smoother faults may generate larger earthquakes than rougher faults under identical tectonic loading conditions. We further discuss the potential impact of fault roughness on earthquake recurrence probability. This finding provides important information, also for seismic hazard analysis. 1. Background and Motivation Earthquakes can be regarded as frictional phenomena that release tectonically or otherwise accumulated stresses in the form of slip along generally preexisting fault surfaces [e.g., Scholz, 2002; Aki and Richards, 2009]. The coseismically released static stress drop Δτ—defined as the average change in shear stress on a rupture surface before and after a slip event—is a fundamental quantity of the rupture process, bearing information on an earthquake’s frictional breakdown during sliding, its seismic energy release, the frequency content of radiated seismic waves, and earthquake recurrence probability [e.g., Reid, 1910; Brune, 1970; Scholz, 2002; Aki and Richards, 2009]. Static stress drop is relevant for hazard assessment and the general understanding of earthquake physics. Estimates of Δτ based on seismological observations employ a simplified representation of the earthquake source that correlates fault slip, moment release, or frequency content of radiated seismic waves to stress drop [e.g., Brune, 1970; Kanamori and Anderson, 1975; Hanks, 1977; Aki and Richards, 2009; Allmann and Shearer, 2009]. The corresponding values of Δτ are centered at ~3–4MPa and do not change systematically with earthquake size, which is taken as evidence for self-similar earthquake scaling [e.g., Kanamori and Anderson, 1975; Hanks, 1977; Allmann and Shearer, 2009]. On the other hand, laboratory friction experiments indicate an almost complete breakdown in frictional resistance during sliding when coseismic slip velocities are reached [e.g., Han et al., 2007; Di Toro et al., 2011]. The observed large change in friction (typically Δμ ≥ 0.5) in such experiments, combined with effective normal stresses at seismogenic depths (σeff), yields coseismic stress drops Δτ =Δμσeff that exceed those derived from seismological observations by multiples of 10. Consequently, laboratoryand field-based estimates of coseismic stress drop Δτ are incompatible, questioning the validity of current Δτ estimates and the conclusions that are based on them. We conjecture that the strong discrepancy in Δτ estimates is due to the nonplanarity of natural rupture surfaces [e.g., Power et al., 1988; Sagy et al., 2007; Candela et al., 2012; Brodsky et al., 2016], the spatial heterogeneity of rock strength on the fault (here strength refers to a fault’s potential to sustain some amount of shear stress before slippage occurs [e.g., Ripperger and Mai, 2004; Konca et al., 2008; Mai and Thingbaijam, 2014]), and their combined effect on an earthquake’s slip distribution and moment release. We employ large-scale numerical simulations to investigate how the surface roughness of a fault and its strength heterogeneity affect average slip D and seismic moment M0 that are associated to stress drop Δτ. After describing the numerical model that was used in this study, we present our results and conclusion. The online supporting information contains additional data on model formulation and adopted physical parameters. ZIELKE ET AL. FAULT ROUGHNESS AND EARTHQUAKE STRESS DROP 777 PUBLICATIONS Geophysical Research Letters

Three‐Dimensional Investigation of a 5 m Deflected Swale along the San Andreas Fault in the Carrizo Plain

Topographic maps produced from Light Detection and Ranging (LiDAR) data are useful for paleoseismic and neotectonic research because they pro- vide submeter representation of faulting-related surface features. Offset measurements of geomorphic features, made in the field or on a remotely sensed imagery, commonly assume a straight or smooth (i.e., undeflected) pre-earthquake geometry. Here, we present results from investigation of an ∼20 cm deep and >5 m wide swale with a sharp bend along the San Andreas fault (SAF) at the Bidart fan site in the Carrizo Plain, California. From analysis of LiDAR topography images and field measure- ments, the swale was initially interpreted as a channel tectonically offset ∼4:7 m. Our observations from exposures in four backhoe excavations and 25 hand-dug trenchettes show that even though a sharp bend in the swale coincides with the trace of the A.D. 1857 fault rupture, the swale formed after the 1857 earthquake and was not tectonically offset. Subtle fractures observed within a surficial gravel unit overlying the 1857 rupture trace are similar to fractures previously documented at the Phelan fan and LY4 paleoseismic sites 3 and 35 km northwest of Bidart fan, respectively. Collectively, the fractures suggest that a post-1857 moderate-magnitude earthquake caused ground cracking in the Carrizo and Cholame stretches of the SAF. Our obser- vations emphasize the importance of excavation at key locations to validate remote and ground-based measurements, and we advocate more geomorphic characterization for each site if excavation is not possible. Online Material: Figures of trench logs.