Lisa Grant Ludwig

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.

Potential for a large earthquake near Los Angeles inferred from the 2014 La Habra earthquake.

Abstract Tectonic motion across the Los Angeles region is distributed across an intricate network of strike‐slip and thrust faults that will be released in destructive earthquakes similar to or larger than the 1933 M6.4 Long Beach and 1994 M6.7 Northridge events. Here we show that Los Angeles regional thrust, strike‐slip, and oblique faults are connected and move concurrently with measurable surface deformation, even in moderate magnitude earthquakes, as part of a fault system that accommodates north‐south shortening and westerly tectonic escape of northern Los Angeles. The 28 March 2014 M5.1 La Habra earthquake occurred on a northeast striking, northwest dipping left‐lateral oblique thrust fault northeast of Los Angeles. We present crustal deformation observation spanning the earthquake showing that concurrent deformation occurred on several structures in the shallow crust. The seismic moment of the earthquake is 82% of the total geodetic moment released. Slip within the unconsolidated upper sedimentary layer may reflect shallow release of accumulated strain on still‐locked deeper structures. A future M6.1–6.3 earthquake would account for the accumulated strain. Such an event could occur on any one or several of these faults, which may not have been identified by geologic surface mapping.

Disaster preparedness as social control

ABSTRACT This paper discusses research on disaster management institutions, as well as members of the vulnerable public in an area of significant seismic risk. Our main theoretical conclusion is that disaster preparedness is a problem because people do not do what they are supposed to in the ways they are told by authority figures. They have breached a social norm. The need for public preparedness is constructed as a fundamental part of management efforts, where the vulnerable public is thought of as both complacent and potentially threatening in post-disaster contexts. The authors find institutional perspectives idealize public preparedness as a major goal, but in doing so also emphasize social control. This is in contrast to public reflections that demonstrate the potential for collective self-organization and altruistic behavior when they imagine how they would respond in a disaster. The institutional need for compliance in acts of preparing is built on fear of the public and resonates with the growth of disaster management practices inspired by military structures (i.e. the Incident Command System). Here, we suggest the paradigm of preparedness needs to be re-framed to more accurately reflect the reality of public behavior during such events.

QuakeSim: Integrated modeling and analysis of geologic and remotely sensed data

The QuakeSim Project improves understanding of earthquake processes by integrating model applications and various heterogeneous data sources within a web services environment. The project focuses on the earthquake cycle and related crustal deformation. Spaceborne GPS and Interferometric Synthetic Aperture data provide information on near-term crustal deformation, while paleoseismic geologic data provide longer-term information on earthquake fault processes. These data sources are integrated into QuakeSims QuakeTables database and are accessible by users or various model applications. An increasing amount of UAVSAR data is being added to the QuakeTables database through a map browsable interface. Model applications can retrieve data from QuakeTables or remotely served GPS velocity data services or users can manually input parameters into the models. Pattern analysis of GPS and seismicity data has proved useful for mid-term forecasting of earthquakes and for detecting subtle changes in crustal deformation. The GPS time series analysis has also proved useful for detecting changes in processing of the data. Development of the QuakeSim computational infrastructure has benefitted greatly from having the user in the development loop. Improved visualization tools enable more efficient data exploration and understanding. Tools must provide flexibility to science users for exploring data in new ways, but also must facilitate standard, intuitive, and routine uses for end users such as emergency responders.

A Distributed Approach to Computational Earthquake Science: Opportunities and Challenges

Advances in understanding earthquakes require the integration of models and multiple distributed data products. Increasingly, data are acquired through large investments, and utilizing their full potential requires a coordinated effort by many users, independent researchers, and groups who are often distributed both geographically and by expertise.

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.

Integrating remotely sensed and ground observations for modeling, analysis, and decision support

Earthquake science and emergency response require integration of many data types and models that cover a broad range of scales in time and space. Timely and efficient earthquake analysis and response require automated processes and a system in which the interfaces between models and applications are established and well defined. Geodetic imaging data provide observations of crustal deformation from which strain accumulation and release associated with earthquakes can be inferred. Data products are growing and tend to be either relatively large in size, on the order of 1 GB per image with hundreds or thousands of images, or high data rate, such as from 1 second GPS solutions. The products can be computationally intensive to manipulate, analyze, or model, and are unwieldy to transfer across wide area networks. Required computing resources can be large, even for a few users, and can spike when new data are made available or when an earthquake occurs. A cloud computing environment is the natural extension for some components of QuakeSim as an increasing number of data products and model applications become available to users. Storing the data near the model applications improves performance for the user.

An Applied Method for General Regional Seismic Loss Assessment—With A Case Study in Los Angeles County

ABSTRACT We describe the formulation and application of an integrated general regional seismic loss assessment (RSLA) method for buildings in seismic regions. An efficient method for RSLA is valuable for engineers involved in city planning, risk management, and insurance dealings. In contrast to previously reported methods, the framework presented herein is hazard-based and utilizes a regional rapid seismic hazard deaggregation tool that allows regional assessment to be conducted more efficiently. The proposed technique is implemented as an example to assess general regional seismic loss in Los Angeles County for a ground motion hazard with 10% probability of exceedance in 50 years.