Alaska Earthquake Research Articles

Strike-slip 23 January 2018 MW 7.9 Gulf of Alaska rare intraplate earthquake: Complex rupture of a fracture zone system

Large intraplate earthquakes in oceanic lithosphere are rare and usually related to regions of diffuse deformation within the oceanic plate. The 23 January 2018 MW 7.9 strike-slip Gulf of Alaska earthquake ruptured an oceanic fracture zone system offshore Kodiak Island. Bathymetric compilations show a muted topographic expression of the fracture zone due to the thick sediment that covers oceanic basement but the fracture zone system can be identified by offset N-S magnetic anomalies and E-W linear zones in the vertical gravity gradient. Back-projection from global seismic stations reveals that the initial rupture at first propagated from the epicenter to the north, likely rupturing along a weak zone parallel to the ocean crustal fabric. The rupture then changed direction to eastward directed with most energy emitted on Aka fracture zone resulting in an unusual multi-fault earthquake. Similarly, the aftershocks show complex behavior and are related to two different tectonic structures: (1) events along N-S trending oceanic fabric, which ruptured mainly strike-slip and additionally, in normal and oblique slip mechanisms and (2) strike-slip events along E-W oriented fracture zones. To explain the complex faulting behavior we adopt the classical stress and strain partitioning concept and propose a generalized model for large intra-oceanic strike-slip earthquakes of trench-oblique oriented fracture zones/ocean plate fabric near subduction zones. Taking the Kodiak asperity position of 1964 maximum afterslip and outer-rise Coulomb stress distribution into account, we propose that the unusual 2018 Gulf of Alaska moment release was stress transferred to the incoming oceanic plate from co- and post-processes of the nearby great 1964 MW 9.2 megathrust earthquake.

Late Holocene activity of Sherman and Sheridan glaciers, Prince William Sound, Alaska

Two adjacent glaciers in the Chugach Mountains of south-central Alaska have markedly different histories on decadal to perhaps centennial timescales. Sheridan Glacier has advanced and retreated hundreds of metres during the latest Holocene. Its recent fluctuations have markedly altered local base level of Sherman River, which drains Sherman Glacier and flows into Sheridan Lake. Sheridan Glacier advanced to its greatest extent during the Little Ice Age, raising base level of Sherman River and inducing aggradation there of up to 17 m of sediment. Retreat of Sheridan Glacier formed a series of lakes that have coalesced. As lower lake outlets have become available, base level of Sherman River has dropped, resulting in the evacuation of substantial volumes of sediment from Sherman River valley. In about 2000, the terminus of Sheridan Glacier began to disintegrate; retreat accelerated dramatically in 2010. By 2016, the glacier had retreated an average of 600 m from its 2010 terminus, although some areas retreated up to 1.9 km and others did not retreat at all. Meanwhile, Sherman Glacier continued a slow advance initiated by a rock avalanche that blanketed much of its ablation area in the 1964 Alaska earthquake.

Repeated Selection of Alternatively Adapted Haplotypes Creates Sweeping Genomic Remodeling in Stickleback.

Heterogeneous genetic divergence can accumulate across the genome when populations adapt to different habitats while still exchanging alleles. How long does diversification take and how much of the genome is affected? When divergence occurs in parallel from standing genetic variation, how often are the same haplotypes involved? We explore these questions using restriction site-associated DNA sequencing genotyping data and show that broad-scale genomic repatterning, fueled by copious standing variation, can emerge in just dozens of generations in replicate natural populations of threespine stickleback fish (Gasterosteus aculeatus). After the catastrophic 1964 Alaskan earthquake, marine stickleback colonized newly created ponds on seismically uplifted islands. We find that freshwater fish in these young ponds differ from their marine ancestors across the same genomic segments previously shown to have diverged in much older lake populations. Outside of these core divergent regions the genome shows no population structure across the ocean-freshwater divide, consistent with strong local selection acting in alternative environments on stickleback populations still connected by significant gene flow. Reinforcing this inference, a majority of divergent haplotypes that are at high frequency in ponds are detectable in the sea, even across great geographic distances. Building upon previous population genomics work in this model species, our data suggest that a long history of divergent selection and gene flow among stickleback populations in oceanic and freshwater habitats has maintained polymorphisms of alternatively adapted DNA sequences that facilitate parallel evolution.

A Closer Look at an Undersea Source of Alaskan Earthquakes

A systematic survey offers a striking portrait of movement along a 500-kilometer-long undersea section of the Queen Charlotte–Fairweather fault off the coast of southeastern Alaska.

The 1904Ms 7.3 Earthquake in Central Alaska

Abstract On 27 August 1904, seismic stations from around the globe recorded an M >7 earthquake originating from central Alaska. Very little was known about this earthquake. One felt report from Rampart, Alaska, had been attributed to the notes of Harry Fielding Reid, yet its original source was unknown. Here, we present five felt reports for the 1904 earthquake that show evidence of felt shaking across most of central Alaska. Using the 1904 arrival‐time data, we estimate an epicentral location near Lake Minchumina at the northeastern extent of the Iditarod–Nixon fault. Our preferred fault for the 1904 earthquake is the right‐lateral Iditarod–Nixon fault, which, though relatively seismically quiet, generated an M 6.2 earthquake in 1935. Paleoseismic investigations are needed to search for evidence of fault activity, including the 1904 earthquake rupture, in the tectonically complex region of the 1904 earthquake. Electronic Supplement: Tables of arrival time, figures of station registers, visualization of NonLinLoc (NLL) solution for the 1904 Alaska earthquake, distribution of depths of the posterior probability for the 1904 and 1935 events, epicenter and samples of the posterior probability distribution for the 1904 and 1935 earthquakes, map of southward shift of epicenters, and estimated epicenters for the 3 February 2000 Kaltag earthquake.

Detection of co-seismic earthquake gravity field signals using GRACE-like mission simulations

After launching the GRACE satellite mission in 2002, the earth’s gravity field and its temporal variations are measured with a closer inspection. Although these variations are mainly because of the mass transfer of land water storage, they can also happen due to mass movements related to some natural phenomena including earthquakes, volcanic eruptions, melting of polar ice caps and glacial isostatic adjustment. Therefore this paper shows which parameters of an earthquake are more sensitive to GRACE-Like satellite missions. For this purpose, the parameters of the Maule earthquake that occurred in recent years and Alaska earthquake that occurred in 1964 have been chosen. Then we changed their several parameters to serve our purpose. The GRACE-Like sensitivity is observed by using the simulation of the earthquakes along with gravity changes they caused, as well as using dislocation theory under a half space earth. This observation affects the various faulting parameters which include fault length, width, depth and average slip. These changes were therefore evaluated and the result shows that the GRACE satellite missions tend to be more sensitive to Width among the Length and Width, the other parameter is Dip variations than other parameters. This article can be useful to the upcoming scenario designers and seismologists in their quest to study fault parameters.

A phase coherence approach to identifying co-located earthquakes and tremor

We present and use a phase coherence approach to identify seismic signals that have similar path effects but different source time functions: co-located earthquakes and tremor. The method used is a phase coherence-based implementation of empirical matched field processing, modified to suit tremor analysis. It works by comparing the frequency-domain phases of waveforms generated by two sources recorded at multiple stations. We first cross-correlate the records of the two sources at a single station. If the sources are co-located, this cross-correlation eliminates the phases of the Green’s function. It leaves the relative phases of the source time functions, which should be the same across all stations so long as the spatial extent of the sources are small compared with the seismic wavelength. We therefore search for cross-correlation phases that are consistent across stations as an indication of co-located sources. We also introduce a method to obtain relative locations between the two sources, based on back-projection of interstation phase coherence. We apply this technique to analyse two tremor-like signals that are thought to be composed of a number of earthquakes. First, we analyse a 20 s long seismic precursor to a M 3.9 earthquake in central Alaska. The analysis locates the precursor to within 2 km of the mainshock, and it identifies several bursts of energy—potentially foreshocks or groups of foreshocks—within the precursor. Second, we examine several minutes of volcanic tremor prior to an eruption at Redoubt Volcano. We confirm that the tremor source is located close to repeating earthquakes identified earlier in the tremor sequence. The amplitude of the tremor diminishes about 30 s before the eruption, but the phase coherence results suggest that the tremor may persist at some level through this final interval.

Impact of Ground Freezing on Seismic Performance of Bridge Pile Foundations Subject to Liquefaction-Induced Lateral Spread

Lessons were learned from two major earthquakes in Alaska: the freezing of ground crust had a great influence on bridge pile foundation behavior during lateral spread. Based on a typical bridge from the Alaska Department of Transportation and Public Facilities bridge inventory, supported on commonly used pile foundations that penetrate a liquefiable sand layer, finite element (FE) modeling was used to study seismic performance of bridge foundations subject to liquefaction-induced lateral spread in two scenarios: the frozen case, in which a frozen ground crust represented typical winter conditions, and the unfrozen case, in which an unfrozen ground crust represented summer conditions. Soil reactions and pile responses were analyzed with solid–fluid coupled FE modeling. Results for both cases illustrate the key characteristics of pile performance. In addition, the effectiveness of the p-y approach for predicting the pile response under such a complex loading combination was confirmed by a comparison of the results from LPILE with those from coupled FE modeling, and this approach is recommended for practice. Guidelines are proposed for design practitioners to assess the freezing ground impact on pile foundations with the p-y approach.

Hypothetical Tsunami Scenarios and Evacuation Intention: A Case Study of Orange County, California

Historically, California has had 14 tsunamis since 1812 where the heights of the waves were higher than three feet. One of California’s major tsunamis followed the 1964 Alaskan earthquake and caused 12 deaths, with damages totaling to at least 17 million dollars. According to Jose Borrero and Costas Synolakis in their report, “Tsunami Inundation Mapping, Field Survey Report and Final Recommendations for Orange County”, the worst case Tsunami scenario in Orange County, California, is a wave of approximately 10 meters or 32.81 feet (Tsunami Annex, 2009). Having evidence of historical tsunami events, even if not major ones, in addition to the probabilities of tsunami occurrences in the future requires the attention of both responsible administrative officials and the foundation of sound research directed toward planning for such events. The goal of this study is to measure how the surveyed population would respond in different, hypothetical tsunami scenarios should Orange County Officials advise the population to evacuate. The methodology consists of a survey of 235 people who live along the coastal area of Orange County, California. Method of Splitting Tsunami (MOST) model maps and GIS were used to delineate the evacuation and shadow evacuation zones in order to locate the spatial distribution of the population within these two zones. The results of this research indicated that the evacuation participation rate increases as the heights of the tsunami waves increase. The average socioeconomic characteristics of the sampled population vary from the sampled population when compared with the 2010 US Census data. This research may be useful for emergency managers, policy and decision makers.

Detection limits of tidal-wetland sequences to identify variable rupture modes of megathrust earthquakes

Recent paleoseismological studies question whether segment boundaries identified for 20th and 21st century great, >M8, earthquakes persist through multiple earthquake cycles or whether smaller segments with different boundaries rupture and cause significant hazards. The smaller segments may include some currently slipping rather than locked. In this review, we outline general principles regarding indicators of relative sea-level change in tidal wetlands and the conditions in which paleoseismic indicators must be distinct from those resulting from non-seismic processes. We present new evidence from sites across southcentral Alaska to illustrate different detection limits of paleoseismic indicators and consider alternative interpretations for marsh submergence and emergence. We compare predictions of coseismic uplift and subsidence derived from geophysical models of earthquakes with different rupture modes. The spatial patterns of agreement and misfits between model predictions and quantitative reconstructions of coseismic submergence and emergence suggest that no earthquake within the last 4000 years had a pattern of rupture the same as the Mw 9.2 Alaska earthquake in 1964. From the Alaska examples and research from other subduction zones we suggest that If we want to understand whether a megathrust ruptures in segments of variable length in different earthquakes, we need to be site-specific as to what sort of geological-based criteria eliminate the possibility of a particular rupture mode in different earthquakes. We conclude that coastal paleoseismological studies benefit from a methodological framework that employs rigorous evaluation of five essential criteria and a sixth which may be very robust but only occur at some sites: 1 – lateral extent of peat-mud or mud-peat couplets with sharp contacts; 2 – suddenness of submergence or emergence, and replicated within each site; 3 – amount of vertical motion, quantified with 95% error terms and replicated within each site; 4 – syncroneity of submergence and emergence based on statistical age modelling; 5 – spatial pattern of submergence and emergence; 6 – possible additional evidence, such as evidence of a tsunami or liquefaction concurrent with submergence or emergence. We suggest that it is possible to consider detection limits as low as 0.1–0.2 m coseismic vertical change. • Revised methodological framework of 6 criteria for coastal paleoseismic studies. • New evidence from southcentral Alaska to illustrate different detection limits. • No earthquake within last 4000 years had same rupture pattern as 1964 earthquake. • Possible to consider detection limits of 0.1–0.2 m of coseismic land-level change.

Paleoseismic potential of sublacustrine landslide records in a high-seismicity setting (south-central Alaska)

Sublacustrine landslide stratigraphy is considered useful for quantitative paleoseismology in low-seismicity settings. However, as the recharging of underwater slopes with sediments is one of the factors that governs the recurrence of slope failures, it is not clear if landslide deposits can provide continuous paleoseismic records in settings of frequent strong shaking. To test this, we selected three lakes in south-central Alaska that experienced a strong historical megathrust earthquake (the 1964 Mw9.2 Great Alaska Earthquake) and exhibit high sedimentation rates in their main basins (0.2cmyr−1–1.0cmyr−1). We present high-resolution reflection seismic data (3.5kHz) and radionuclide data from sediment cores in order to investigate factors that control the establishment of a reliable landslide record. Seismic stratigraphy analysis reveals the presence of several landslide deposits in the lacustrine sedimentary infill. Most of these landslide deposits can be attributed to specific landslide events, as multiple landslide deposits sourced from different lacustrine slopes occur on a single stratigraphic horizon. We identify numerous events in the lakes: Eklutna Lake proximal basin (14 events), Eklutna Lake distal basin (8 events), Skilak Lake (7 events) and Kenai Lake (7 events). The most recent event in each basin corresponds to the historic 1964 megathrust earthquake. All events are characterized by multiple landslide deposits, which hints at a regional trigger mechanism, such as an earthquake (the synchronicity criterion). This means that the landslide record in each basin represents a record of past seismic events. Based on extrapolation of sedimentation rates derived from radionuclide dating, we roughly estimate a mean recurrence interval in the Eklutna Lake proximal basin, Eklutna Lake distal basin, Skilak Lake and Kenai Lake, at ~250yrs., ~450yrs., ~900yrs. and ~450yrs., respectively. This distinct difference in recording can be explained by variations in preconditioning factors like slope angle, slope recharging (sedimentation rate) and the sediment source area: faster slope recharging and a predominance of delta and alluvial fan failures, increase the sensitivity and lower the intensity threshold for slope instability. Also, the seismotectonic setting of the lakes has to be taken into account. This study demonstrates that sublacustrine landslides in several Alaskan lakes can be used as reliable recorders of strong earthquake shaking, when a multi-lake approach is used, and can enhance the temporal and spatial resolution of the paleoseismic record of south-central Alaska.

Seismic moment tensors and estimated uncertainties in southern Alaska

AbstractWe present a moment tensor catalog of 106 earthquakes in southern Alaska, and we perform a conceptually based uncertainty analysis for 21 of them. For each earthquake, we use both body waves and surface waves to do a grid search over double couple moment tensors and source depths in order to find the minimum of the misfit function. Our uncertainty parameter or, rather, our confidence parameter is the average value of the curve , where is the posterior probability as a function of the fractional volume V of moment tensor space surrounding the minimum misfit moment tensor. As a supplemental means for characterizing and visualizing uncertainties, we generate moment tensor samples of the posterior probability. We perform a series of inversion tests to quantify the impact of certain decisions made within moment tensor inversions and to make comparisons with existing catalogs. For example, using an L1 norm in the misfit function provides more reliable solutions than an L2 norm, especially in cases when all available waveforms are used. Using body waves in addition to surface waves, as well as using more stations, leads to the most accurate moment tensor solutions.

A submarine landslide source for the devastating 1964 Chenega tsunami, southern Alaska

During the 1964 Great Alaska earthquake (Mw 9.2), several fjords, straits, and bays throughout southern Alaska experienced significant tsunami runup of localized, but unexplained origin. Dangerous Passage is a glacimarine fjord in western Prince William Sound, which experienced a tsunami that devastated the village of Chenega where 23 of 75 inhabitants were lost – the highest relative loss of any community during the earthquake. Previous studies suggested the source of the devastating tsunami was either from a local submarine landslide of unknown origin or from coseismic tectonic displacement. Here we present new observations from high-resolution multibeam bathymetry and seismic reflection surveys conducted in the waters adjacent to the village of Chenega. The seabed morphology and substrate architecture reveal a large submarine landslide complex in water depths of 120–360 m. Analysis of bathymetric change between 1957 and 2014 indicates the upper 20–50 m (∼0.7 km3) of glacimarine sediment was destabilized and evacuated from the steep face of a submerged moraine and an adjacent ∼21 km2 perched sedimentary basin. Once mobilized, landslide debris poured over the steep, 130 m-high face of a deeper moraine and then blanketed the terminal basin (∼465 m water depth) in 11±5 m of sediment. These results, combined with inverse tsunami travel-time modeling, suggest that earthquake-triggered submarine landslides generated the tsunami that struck the village of Chenega roughly 4 min after shaking began. Unlike other tsunamigenic landslides observed in and around Prince William Sound in 1964, the failures in Dangerous Passage are not linked to an active submarine delta. The requisite environmental conditions needed to generate large submarine landslides in glacimarine fjords around the world may be more common than previously thought.

Evolution of stickleback in 50 years on earthquake-uplifted islands

How rapidly can animal populations in the wild evolve when faced with sudden environmental shifts? Uplift during the 1964 Great Alaska Earthquake abruptly created freshwater ponds on multiple islands in Prince William Sound and the Gulf of Alaska. In the short time since the earthquake, the phenotypes of resident freshwater threespine stickleback fish on at least three of these islands have changed dramatically from their oceanic ancestors. To test the hypothesis that these freshwater populations were derived from oceanic ancestors only 50 y ago, we generated over 130,000 single-nucleotide polymorphism genotypes from more than 1,000 individuals using restriction site-associated DNA sequencing (RAD-seq). Population genomic analyses of these data support the hypothesis of recent and repeated, independent colonization of freshwater habitats by oceanic ancestors. We find evidence of recurrent gene flow between oceanic and freshwater ecotypes where they co-occur. Our data implicate natural selection in phenotypic diversification and support the hypothesis that the metapopulation organization of this species helps maintain a large pool of genetic variation that can be redeployed rapidly when oceanic stickleback colonize freshwater environments. We find that the freshwater populations, despite population genetic analyses clearly supporting their young age, have diverged phenotypically from oceanic ancestors to nearly the same extent as populations that were likely founded thousands of years ago. Our results support the intriguing hypothesis that most stickleback evolution in fresh water occurs within the first few decades after invasion of a novel environment.

Diurnal periodicity of the flow of Alaska earthquakes

The Alaska earthquake catalog has been analyzed in detail to find and study the diurnal periodicity of earthquake events. For this purpose, a set of spatially and temporally homogeneous samples of earthquakes with the well-known magnitude of completeness (Mc) has been prepared. For each sample, the spectra have been considered, the average diurnal variations in the number of earthquakes have been calculated, and their amplitudes were determined. The average diurnal variations were compared. For representative earthquakes in Alaska, no significant diurnal variation has been found. In subrepresentative samples, either the diurnal variation is insignificant or the signal-to-noise ratio only slightly exceeds 1.3–1.9. The diurnal variation is significant (a signal-to-noise ratio of 2.0–4.5) only for the samples of weak earthquakes with magnitudes of no more than 1.4, which is 0.5 units less than the strong (i.e., guaranteed for the entire sample area) completeness threshold. The results are consistent with the hypothesis explaining the diurnal periodicity of earthquakes by noise-discrimination effects. However, a comparative analysis of the diurnal variation parameters estimated over a large number of spatially and temporally homogeneous samples of earthquakes in the Alaska, southern California, and Greece catalogs shows that all of these results cannot be explained by this model.

Are there new findings in the search for ULF magnetic precursors to earthquakes?

AbstractMoore (1964) in a letter published in Nature reported disturbances in geomagnetic field data prior to the 27 March 1964 Alaska earthquake. After the publication of this report, many papers have shown magnetic changes preceding earthquakes. However, a causal relationship between preearthquake magnetic changes and impending earthquakes has never been demonstrated. As a consequence, after 50 years, magnetic disturbances in the geomagnetic field are still candidate precursory phenomena. Some researchers consider the investigation of ultra low frequency (ULF: 0.001–10 Hz) magnetic data the correct approach for identifying precursory signatures of earthquakes. Other researchers, instead, have recently reviewed many published ULF magnetic changes that preceded earthquakes and have shown that these are not actual precursors. The recent studies by Currie and Waters (2014) and Han et al. (2014) aim to provide relevant new findings in the search for ULF magnetic precursory signals. However, in order to contribute to science, alleged precursors must be shown to be valid and reproducible by objective testing. Here we will briefly discuss the state of the art in the search for ULF magnetic precursors, paying special attention to the recent findings of Currie and Waters (2014) and Han et al. (2014). We do not see in these two reports significant evidence that may support the observation of precursory signatures of earthquakes in ULF magnetic records.

Landslides and Megathrust Splay Faults Captured by the Late Holocene Sediment Record of Eastern Prince William Sound, Alaska

Abstract We present new marine seismic‐reflection profiles and bathymetric maps to characterize Holocene depositional patterns, submarine landslides, and active faults beneath eastern and central Prince William Sound (PWS), Alaska, which is the eastern rupture patch of the 1964 M w 9.2 earthquake. We show evidence that submarine landslides, many of which are likely earthquake triggered, repeatedly released along the southern margin of Orca Bay in eastern PWS. We document motion on reverse faults during the 1964 Great Alaska earthquake and estimate late Holocene slip rates for these growth faults, which splay from the subduction zone megathrust. Regional bathymetric lineations help define the faults that extend 40–70 km in length, some of which show slip rates as great as 3.75 mm/yr. We infer that faults mapped below eastern PWS connect to faults mapped beneath central PWS and possibly onto the Alaska mainland via an en echelon style of faulting. Moderate ( M w >4) upper‐plate earthquakes since 1964 give rise to the possibility that these faults may rupture independently to potentially generate M w 7–8 earthquakes, and that these earthquakes could damage local infrastructure from ground shaking. Submarine landslides, regardless of the source of initiation, could generate local tsunamis to produce large run‐ups along nearby shorelines. In a more general sense, the PWS area shows that faults that splay from the underlying plate boundary present proximal, perhaps independent seismic sources within the accretionary prism, creating a broad zone of potential surface rupture that can extend inland 150 km or more from subduction zone trenches.

Tales of Misery and Woe

Tales of Misery and Woe Matthew Mulcahy (bio) Kathleen Donegan. Seasons of Misery: Catastrophe and Colonial Settlement in Early America. Philadelphia: University of Pennsylvania Press, 2014. 288pages. Notes and index. $49.95. Amy Mitchell-Cook. A Sea of Misadventures: Shipwreck and Survival in Early America. Columbia: University of South Carolina Press, 2013. ix + 229 pages. Illustrations, notes, bibliography, and index. $34.95. Almost thirty years ago, John C. Burnham wrote a short essay in Perspectives entitled “A Neglected Field: The History of Natural Disasters.”1 Burnham had become interested in the topic after being invited to serve on a team of academics investigating the impact of the 1964 Alaskan earthquake. He rushed to the library in search of materials on the history of earthquakes, but he found only antiquarian or journalistic (and often sensationalist) accounts with no real analysis. Feeling he had little to contribute as a historian, Burnham stayed home from the trip, but the experience alerted him to the important work on disasters being undertaken by sociologists, political scientists, and others. A few “pioneering” historical studies had appeared by the time Burnham taught a graduate seminar in the mid-1980s, but the field remained relatively underdeveloped. He put out his call for more research in 1988. Burnham’s essay is not cited as frequently as it should be, but his call for greater attention to historical disasters has been more than fulfilled. Far from being neglected, the history of disasters has become a vibrant and innovative subfield. A flood (avalanche?—pick your disaster metaphor) of works has appeared on library shelves in recent years. In some cases, historians have focused on individual events, such as the Great Chicago Fire or the 1927 Mississippi River flood, narrating both the event itself as well as tracing its impact on social relations, political policies, or urban design, among other topics. Others use various disasters as a lens through which to explore larger preexisting social, political, economic, and cultural conditions at a particular time or place. Still others have investigated the place of disasters in American popular consciousness across time. Much of this work is interdisciplinary in the best sense, drawing on diverse sources and literatures in an effort to make [End Page 434] sense of disasters and their effects on past societies. Much of it has also focused on the late nineteenth or twentieth centuries (some variant on “the making of modern America” often appears in subtitles), but scholars increasingly are turning to earlier periods as well, including the colonial era. The books under review here are part of this expanding literature and share several features. Both focus on a select group of texts: in Mitchell-Cook’s case, 100 shipwreck narratives; in Kathleen Donegan’s, a few classic accounts written by colonists detailing the early history of colonial settlements. Both are concerned with issues of identity in one form or other. And both focus on the failure or near-failure of voyages and settlements. Nevertheless, these are quite different books, covering different time periods, asking different questions, and employing different methodologies. They also differ in their approach to the idea of disaster. Mitchell-Cook focuses on a single kind of event—the shipwreck—and traces how survivors (and readers) made sense of such events across several centuries. Donegan offers a more expansive view of disaster in a shorter time frame. For her, catastrophe refers to a variety of events—“mass starvation, sieges, mutinies, desecrated corpses, slaughtered children, shipwrecks, fire, delusional fantasies, and abandonment” (p. 5)—but also to the general state of affairs in the early years of Roanoke, Jamestown, Plymouth, and Barbados, where such events seemed to come one after another. She focuses less on the history of particular events than on how such events created conditions that overwhelmed colonists, overturned their understanding of “settlement,” undermined their links to England, and shaped a distinct colonial identity. It is not a history of disasters so much as an engaging and thought-provoking account of the place of disaster and failure in the colonial origins of the American imagination. Donegan argues that while historians have long highlighted the failure of Roanoke, the litany of troubles and temporary abandonment of Jamestown, and...

A two-asperity fault model with wave radiation

We study the dynamics of a fault containing two asperities having different strengths and subject to a constant strain rate. The fault is modeled by a discrete dynamical system with two degrees of freedom that are the slip deficits of the asperities. The radiation of elastic waves during the slipping modes is taken into account by introducing a term proportional to slip rate in the equations of motion. We give a complete analytical solution of the four dynamic modes of the system. Any seismic event can be expressed as a sequence of modes, for which the moment rate, the spectrum and the total seismic moment can be calculated. We consider the energy budget of the event and calculate the seismic efficiency. The presence of radiation may remarkably change the evolution of the system from a given state, since it moves the boundaries between the different subsets of the sticking region. In addition, the slip amplitude in a seismic event is smaller, while the slip duration is longer in the presence of radiation. The shape of the moment rate function depends on the seismic efficiency and the seismic moment decreases with increasing efficiency, at constant radiated energy. As an application of the model, we consider the 1964 Great Alaska earthquake and calculate the mode durations, the slip distribution, the moment rate function and the seismic moment of the earthquake, obtaining values that are consistent with observations.

Stress states and moment rates of a two-asperity fault in the presence of viscoelastic relaxation

Abstract. A fault containing two asperities with different strengths is considered. The fault is embedded in a shear zone subject to a constant strain rate by the motions of adjacent tectonic plates. The fault is modelled as a discrete dynamical system where the average values of stress, friction and slip on each asperity are considered. The state of the fault is described by three variables: the slip deficits of the asperities and the viscoelastic deformation. The system has four dynamic modes, for which analytical solutions are calculated. The relationship between the state of the fault before a seismic event and the sequence of slipping modes in the event is enlightened. Since the moment rate depends on the number and sequence of slipping modes, the knowledge of the source function of an earthquake constrains the orbit of the system in the phase space. If the source functions of a larger number of consecutive earthquakes were known, the orbit could be constrained more and more and its evolution could be predicted with a smaller uncertainty. The model is applied to the 1964 Alaska earthquake, which was the effect of the failure of two asperities and for which a remarkable post-seismic relaxation has been observed in the subsequent decades. The evolution of the system after the 1964 event depends on the state from which the event was originated, that is constrained by the observed moment rate. The possible durations of the interseismic interval and the possible moment rates of the next earthquake are calculated as functions of the initial state.