Geology topics

Susan E. Hough

Publications and source records attributed to Susan E. Hough.

At least 37 records · Page 2Linked to original sources

USGS2023

The 1886 Charleston, South Carolina, Earthquake: Relic railroad offset reveals rupture

In the absence of documented surface rupture during the 1 September 1886 Charleston earthquake, there has been considerable speculation about the location and mechanism of the causative fault. We use an inferred coseismic offset of the South Carolina Railroad and additional numerical constraints to develop an elastic deformation model—a west‐dipping fault following strands of two previously identified faults. The constraints are consistent with a blind rupture with 6.5 ± 0.3 m of dextral slip and 2 ± 0.5 m of reverse slip below 450 m depth. We propose that repeated slip on this fault has raised the Penholoway Marine Terrace >6 m since ∼770 ka. The inferred coseismic slip on the fault in an M w 7.3 earthquake is consistent with the distribution of damage in 1886.

South Carolina↗

USGS2023

Uncertainties in intensity-based earthquake magnitude estimates

Estimating the magnitude of historical earthquakes is crucial for assessing seismic hazard. Magnitudes of early‐instrumental earthquakes can be inferred using a combination of instrumental records, field observations, and the observed distribution of shaking intensity determined from macroseismic observations. For earthquakes before 1900, shaking intensity distributions often provide the only information to constrain earthquake magnitude. Considerable effort has been made to develop methods to estimate the magnitude of moderate‐to‐large historical earthquakes using shaking intensities derived from macroseismic data. In this study, we consider earthquakes in California with known instrumental magnitudes to explore uncertainties in estimating the magnitude of historical earthquakes from intensity information alone. We use three California‐specific intensity prediction equations (IPEs) and an IPE based on a global ground‐motion model (GMM) to determine optimum intensity‐based magnitudes for 33 moderate‐to‐large California earthquakes between 1979 and 2021. Intensity‐based magnitudes are close to instrumental magnitudes on average. However, intensity‐based magnitudes for individual events differ by as much as 2.2 magnitude units from instrumental magnitudes. This result reflects the weak dependence of ground motions and shaking intensities on moment magnitude and their strong dependence on stress drop. Considering the intensity distributions of the 1906 San Francisco and 1989 Loma Prieta earthquakes, we show that information that could constrain rupture length is discarded when considering only the 2D decay of intensity with distance. We also show that ground‐motion intensity conversion equations used in a GMM‐based approach may cause a systematic overestimation of large historical earthquake magnitudes. This study underscores both the reducible and potentially irreducible uncertainties associated with using intensity data to estimate magnitudes of historical earthquakes using IPEs and highlights the value of using additional information to constrain rupture dimensions. Using intensity observations alone, moment magnitude uncertainties are typically on the order of a full unit.

California↗

USGS2023

Modern products for a vintage event: An update on the 1933 Long Beach, California, earthquake

When a notable earthquake occurs in the United States, a range of familiar real‐ and near‐real‐time products are produced by the U.S. Geological Survey (USGS) Advanced National Seismic System (ANSS), and made available via the ANSS Comprehensive Earthquake Catalog. For historical and early instrumental earthquakes, similar results and products are developed depending on data availability and event significance, drawing from published later studies. The year 2023 marked the ninetieth anniversary of the 11 March 1933 Long Beach, California, earthquake. This anniversary provided the impetus to update ANSS products, drawing on archived and published data. Here, we describe the updated ShakeMap, shaking recordings and intensities, and retrospective aftershock forecast for the Long Beach, California, earthquake. In effect we have developed standard, modern ANSS products for an earthquake that occurred 90 yr ago. Our results show that the distributions of both the ground motions, anchored by three strong‐motion recordings, and aftershock magnitudes are consistent with expectations for an M 6.4 mainshock in Southern California. We show that, notwithstanding possible limitations, instrumentally recorded accelerations from the closest station are consistent with predicted shaking and directly estimated macroseismic intensities. Updated data products have been added to the USGS event page, where they are available for download. Public‐facing products were also created for the anniversary and are freely available on the USGS website.

California↗

USGS2022

Rupture scenarios for the 3 June 1770 Haiti earthquake

The 2010 M 7.0 Haiti earthquake provided the impetus to reconsider historical earthquakes in Hispaniola ( Bakun et al. , 2012 ). That earthquake also shed new light on complex fault systems along Haiti’s southern peninsula ( Douilly et al. , 2013 ; Saint Fleur et al. , 2015 ). Recently, the 2021 M 7.2 Nippes earthquake ( Calais et al. , 2022 ; Douilly et al. , 2022 ), and a recent study reconsidering the 1860 sequence ( Martin et al. , 2022 ) further underscored the complexity of fault systems and large earthquake ruptures along the peninsula. Motivated by these studies and recent geological investigations ( Prentice et al. , 2010 ; Saint Fleur et al. , 2020 ), we reconsider the 3 June 1770 Haiti earthquake to explore the conventional assumption that it was the last major ( M ≥7.5) earthquake along the Enriquillo–Plantain Garden fault (EPGF). Accounts provide compelling evidence for substantial liquefaction in the Cul‐de‐Sac plain, one or more likely landslide‐driven tsunami in Gonaïves Bay, and extensive landsliding that created at least three documented landslide dams. We consider three end‐member rupture scenarios that are consistent with available constraints: two scenarios with M 7.7 and rupture lengths of 150–170 km, and one scenario with a ∼90 km rupture and M 7.5. Absent future work to identify and date paleoevents along the southern peninsula, none of these scenarios can be ruled out. Our preferred rupture model extends from the Miragoâne pull‐apart to near la Selle mountain, with a rupture length of 127 km, M 7.6, and a high stress drop. Rupture could have been on the EPGF or on an oblique thrust fault associated with overthrusting of the Massif de la Selle. The results do support the conclusion that the 1770 earthquake was the last major earthquake in southern Haiti, with a magnitude upward of M 7.5 and significantly more severe shaking in southern Haiti than during the 2010 earthquake.

Bulletin of the Seismological Society of America↗

USGS2022

On the documentation, independence, and stability of widely used seismological data products

Earthquake scientists have traditionally relied on relatively small data sets recorded on small numbers of instruments. With advances in both instrumentation and computational resources, the big-data era, including an established norm of open data-sharing, allows seismologists to explore important issues using data volumes that would have been unimaginable in earlier decades. Alongside with these developments, the community has moved towards routine production of interpreted data products such as seismic moment tensor catalogs that have provided an additional boon to earthquake science. As these products have become increasingly familiar and useful, it is important to bear in mind that they are not data, but rather interpreted data products. As such, they differ from data in ways that can be important, but not always appreciated. Important - and sometimes surprising - issues can arise if methodology is not fully described, data from multiple sources are included, or data products are not versioned (time-stamped). The line between data and data products is sometimes blurred, leading to an underappreciation of issues that affect data products. This note illustrates examples from two widely used data products: moment tensor catalogs and Did You Feel It? (DYFI) macroseismic intensity values. These examples show that increasing a data product’s documentation, independence, and stability can make it even more useful. To ensure the reproducibility of studies using data products, time-stamped products should be preserved, for example as electronic supplements to published papers, or, ideally, a more permanent repository.

Frontiers in Earth Science↗

USGS2022

A study on the effect of site response on California seismic hazard map assessment

Prior studies have repeatedly shown that probabilistic seismic hazard maps from several different countries predict higher shaking than that observed. Previous map assessments have not, however, considered the influence of site response on hazard. Seismologists have long acknowledged the influence of near-surface geology, in particular low-impedance sediment layers, on earthquake ground-motion at frequencies of engineering concern. Although the overall effects of site response are complex, modern ground-motion models (GMMs) account for site effects using terms based on V S30 , the time-averaged shear-wave velocity in the upper 30 m of the Earth’s surface. In this study, we consider general implications of incorporating site terms from modern GMMs using site-specific V S30 as a proxy in probabilistic seismic hazard maps for California. At the long periods (1–5 s) that affect tall buildings, site terms amplify the mapped hazard by factors of 1–3 at many sites relative to maps calculated for the standard reference soft-rock site condition, V S30 = 760 m/s. However, at the short periods of ground-motion that are the main contributors to peak ground acceleration (PGA) and thus affect smaller structures, only negligible effects occur due to nonlinear deamplification of strong ground-motion at high frequencies. Nonlinear deamplification increases as the shaking level increases. For very strong shaking, deamplification can overcome the linear amplification, yielding net deamplification. We explore the implications of these results for the evaluation of hazard maps. Because site effects do not change the maps appreciably at short periods, we can exclude site response as an explanation for why the maps overpredict historically observed shaking as captured by the California Historical Intensity Mapping Project (CHIMP) dataset. The results are expected to be generalizable to regions that are comparable to California in terms of structure and seismicity rates. In low-to-moderate-seismicity regions where the hazard reflects weaker shaking, nonlinear site response is expected to be less important for the hazard.

California↗

USGS2022

The 8 April 1860 Jour de Pâques earthquake sequence in southern Haiti

The grave threat posed by the Enriquillo‐Plantain Garden fault zone (EPGFZ) and other fault systems on the Tiburon Peninsula in southern Haiti was highlighted by the catastrophic M 7.0 Léogâne earthquake on 12 January 2010 and again by the deadly M 7.2 Nippes earthquakes on 14 August 2021. Early Interferometric Synthetic Aperture Radar observations suggest the 2021 earthquake broke structures associated with this fault system farther west of the 2010 event, but the rupture zones of both events are separated by a ∼50 km gap. This sequence provided the impetus to reconsider a nineteenth century earthquake that may have occurred within this gap. Though previous studies identified a single moderately large event on 8 April 1860, original sources describe a complex and distributed seismic sequence to the west of Port‐au‐Prince. These provide evidence for an initial event to the west of Les Cayes, on the southern coast of the Tiburon Peninsula. This was followed on the morning of 8 April 1860 by a damaging earthquake near l’Anse‐à‐Veau along the northern coast of the peninsula, which was succeeded 14 hr later by a larger mainshock to the east. Although locations cannot be determined precisely from extant macroseismic data, our preferred scenario includes an intensity magnitude ( M I ) ∼ 6.2 "> ( M I ) ∼ 6.2 (MI)∼6.2 foreshock, followed by an M I ∼ 6.8 "> M I ∼ 6.8 MI∼6.8 mainshock wholly or partially on the EPGFZ to the east of the foreshock. Our results suggest that, although earthquakes tend to trigger subsequent earthquakes on immediately neighboring fault segments, sequences can include gaps due to strain release by poorly characterized earthquakes in the historic past. We also document 83 aftershocks to the 1860 earthquake sequence, including a previously unknown damaging shock, estimated M I ∼ 6.6 "> M I ∼ 6.6 MI∼6.6 ⁠ , on 10 April 1860 near Jérémie, farther west of l’Anse‐à‐Veau.

Tiburon Peninsula↗

USGS2021

Contributed reports of widely felt earthquakes in California, United States: If they felt it, did they report it?

In a recent study, Hough and Martin (2021) considered the extent to which socioeconomic factors influence the numbers and distribution of contributed reports available to characterize the effects of both historical and recent large earthquakes. In this study I explore the question further, focusing on analysis of widely felt earthquakes near major population centers in northern and southern California since 2002. For most of these earthquakes there is a correlation between average household income in a postal ZIP code and the population-normalized rate of responses to the DYFI system. As past studies have demonstrated, there is also a strong correlation between DYFI participation and the severity of shaking. This first-order correlation can obscure correlations with other factors that influence participation. Focusing on five earthquakes between 2011 and 2021 that generated especially uniform shaking across the greater Los Angeles, California, region, response rate varies by two orders of magnitude across the region, with a clear correlation with demographics, and consistent spatial patterns in response rate for earthquakes 10 years apart. While there is no evidence that uneven DYFI participation in California impacts significantly the reliability of intensity data collected, the results reveal that DYFI participation is significantly higher in affluent parts of southern California compared to economically disadvantaged areas.

California↗

USGS2021

Reply to “comment on ‘which earthquake accounts matter?’ by Susan E. Hough and Stacey S. Martin” by David J. Wald

We thank David Wald ( Wald, 2021 ; henceforth, W21) for his interest in our recent article ( Hough and Martin, 2021 ; henceforth, HM21). Although different perspectives are vital in science, we are concerned that W21 misrepresents HM21 as an oblique criticism of the U.S. Geological Survey “Did You Feel It?” (DYFI) system, calling for HM21 to be retracted. Readers who are interested in the issues raised by HM21 and the statements made by us therein are referred to that article. In this brief reply, we respond to specific accusations made by W21 and return to the focus of HM21, calling attention to the extent to which macroseismic data sets and inferences drawn from them can be shaped by a lack of representation among individuals whose observations are available to science. HM21 never questioned the benefits of the community science DYFI project to science. HM21 noted, however, and we reiterate here, that community science also potentially benefits the community. Whether or not it matters for science, if participation in community science projects is unrepresentative across socioeconomic groups, it underscores the need for the scientific community to be proactive in its efforts to reach out to groups that have been underserved by current outreach and education programs. We appreciate this opportunity to continue the important conversation about representation.

Seismological Research Letters↗

USGS2021

The 6 May 1947 Milwaukee, Wisconsin, earthquake

The State of Wisconsin is not known for earthquake activity. The authoritative public‐facing U.S. Geological Survey Comprehensive Catalog of earthquakes includes only three small (magnitude < 2) earthquakes in the state, all instrumentally recorded. Although other catalogs include more events in Wisconsin, experience has shown that many types of events, such as explosions and cryoseisms, have made their way into earthquake catalogs in this region. In this short report, I summarize available information about an earthquake that was felt in eastern Wisconsin at 15:27 local time on 6 May 1947. As what appears to be the largest historical earthquake in the State of Wisconsin, it is of public interest, its modest size notwithstanding. It appears that no useful instrumental records exist, due in part to a teleseismic event that occurred approximately 3 min later, generating surface waves that were recorded on early long‐period instruments in the region. Instrumental data may exist for this event but have not been found. Comparing the felt area with information from recent earthquakes in the region, I estimate an intensity magnitude of 3.8 for the event, with a subjectively estimated uncertainty range 3.5–4.1. Relatively strong effects, including reports of broken dishes in Milwaukee, and shaking described as short but especially sharp, suggest that the event may have been among the sprinkling of shallow earthquakes now known to occur in the upper Great Lakes region.

Wisconsin↗

USGS2021

When Punjab cried wolf: How a rumor triggered an “earthquake” in India

In recent years, earthquake felt reports contributed via online systems have provided increasingly valuable sources of data to characterize earthquakes and their effects. Contributed felt reports are accompanied by increases in website traffic, which are themselves potentially useful for the early detection of seismic events. In February 2017 the European‐Mediterranean Seismic Centre detected an unusual surge in traffic from the Punjab region in northwestern India, although no nearby seismic event was detected instrumentally. Had crowdsourcing detected a felt earthquake that instruments had missed? Or did Punjab cry wolf? In this Earthquake Lites report, we describe the sleuthing endeavor undertaken to find an answer.

Punjab↗

USGS2021

Which earthquake accounts matter?

Earthquake observations contributed by human observers provide an invaluable source of information to investigate both historical and modern earthquakes. Commonly, the observers whose eyewitness accounts are available to scientists are a self‐selected minority of those who experience a given earthquake. As such these may not be representative of the overall population that experienced shaking from the event. Eyewitness accounts can contribute to modern science only if they are recorded in the first place and archived in an accessible repository. In this study, we explore the extent to which geopolitics and socioeconomic disparities can limit the number of earthquake observers whose observations can contribute to science. We first revisit a late nineteenth‐century earthquake in the central United States in 1882 that provides an illustrative example of an event that has been poorly characterized due to a reliance on English‐language archival materials. For modern earthquakes, we analyze data collected for recent earthquakes in California and India via the online “Did You Feel It?” (DYFI) system. In California, online data‐collection systems appear to be effective in gathering eyewitness accounts from a broad range of socioeconomic groups. In India, however, responses to the DYFI system reveal a strong bias toward responses from urban areas as opposed to rural settlements, as well a bias with literacy rate. The dissimilarity of our results from modern earthquakes in the United States and India provides a caution that, in some parts of the world, contributed felt reports can still potentially provide an unrepresentative view of earthquake effects, especially if online data collection systems are not designed to be broadly accessible. This limitation can in turn potentially shape our understanding of an earthquake’s impact and the characterization of seismic hazard.

Arkansas, Oklahoma, Texas↗

USGS2020

The normal faulting 2020 Mw5.8 Lone Pine, Eastern California earthquake sequence

The 2020 M w "> M w 5.8 Lone Pine earthquake, the largest earthquake on the Owens Valley fault zone, eastern California, since the nineteenth century, ruptured an extensional stepover in that fault. Owens Valley separates two normal‐faulting regimes, the western margin of the Great basin and the eastern margin of the Sierra Nevada, forming a complex seismotectonic zone, and a possible nascent plate boundary. Foreshocks began on 22 June 2020; the largest M w "> M w 4.7 foreshock occurred at ∼ 6    km "> ∼6 km depth, with primarily normal faulting, followed ∼ 40    hr "> ∼ 40 hr later on 24 June 2020 by an M w "> M w 5.8 mainshock at ∼ 7    km "> ∼ 7 km depth. The sequence caused overlapping ruptures across a ∼ 0.25    km 2 "> ∼0.25 km 2 area, extended to ∼ 4    km 2 "> ∼ 4 km 2 , and culminated in an ∼ 25    km 2 "> ∼25 km 2 aftershock area. The mainshock was predominantly normal faulting, with a strike of 330° (north‐northwest), dipping 60°–65° to the east‐northeast. Comparison of background seismicity and 2020 Ridgecrest aftershock rates showed that this earthquake was not an aftershock of the Ridgecrest mainshock. The M w "> M w M w – m B "> – m B relationship and distribution of ground motions suggest typical rupture speeds. The aftershocks form a north‐northwest‐trending, north‐northeast‐dipping, 5 km long distribution, consistent with the rupture length estimated from analysis of regional waveform data. No surface rupture was reported along the 1872 scarps from the 2020 M w "> M w 5.8 mainshock, although, the dipping rupture zone of the M w "> M w 5.8 mainshock projects to the surface in the general area. The mainshock seismic energy triggered rockfalls at high elevations ( ⁠ > 3.0    km "> > 3.0 km ⁠ ) in the Sierra Nevada, at distances of 8–20 km, and liquefaction along the western edge of Owens Lake. Because there were ∼ 30 % "> ∼ 30 % fewer aftershocks than for an average southern California sequence, the aftershock forecast probabilities were lower than expected. ShakeAlert, the earthquake early warning system, provided first warning within 9.9 s, as well as subsequent updates.

California↗

USGS2020

Revisiting California’s past great earthquakes and long-term earthquake rate

In this study, we revisit the three largest historical earthquakes in California—the 1857 Fort Tejon, 1872 Owens Valley, and 1906 San Francisco earthquakes—to review their published moment magnitudes, and compare their estimated shaking distributions with predictions using modern ground‐motion models (GMMs) and ground‐motion intensity conversion equations. Currently accepted moment magnitude estimates for the three earthquakes are 7.9, 7.6, and 7.8, respectively. We first consider the extent to which the intensity distributions of all three earthquakes are consistent with a moment magnitude toward the upper end of the estimated range. We then apply a GMM‐based method to estimate the magnitudes of large historical earthquakes. The intensity distribution of the 1857 earthquake is too sparse to provide a strong constraint on magnitude. For the 1872 earthquake, consideration of all available constraints suggests that it was a high stress‐drop event, with a magnitude on the higher end of the range implied by scaling relationships, that is, higher than moment magnitude 7.6. For the 1906 earthquake, based on our analysis of regional intensities and the detailed intensity distribution in San Francisco, along with other available constraints, we estimate a preferred moment magnitude of 7.9, consistent with the published estimate based on geodetic and instrumental seismic data. These results suggest that, although there can be a tendency for historical earthquake magnitudes to be overestimated, the accepted catalog magnitudes of California’s largest historical earthquakes could be too low. Given the uncertainties of the magnitude estimates, the seismic moment release rate between 1850 and 2019 could have been either higher or lower than the average over millennial time scales. It is further not possible to reject the hypothesis that California seismicity is described by an untruncated Gutenberg–Richter distribution with a b "> b ‐value of 1.0 for moment magnitudes up to 8.0.

California↗

USGS2020

Introduction to the Special Issue on the 2019 Ridgecrest, California, Earthquake Sequence

No abstract available.

California↗

USGS2020

California Historical Intensity Mapping Project (CHIMP): A consistently reinterpreted dataset of seismic intensities for the past 162 years and implications for seismic hazard maps

Historical seismic intensity data are useful for myriad reasons, including assessment of the performance of Probabilistic Seismic Hazard Assessment (PSHA) models and corresponding hazard maps by comparing their predictions to a dataset of historically observed intensities in the region. To assess PSHA models for California, a long and consistently interpreted intensity record is necessary. For this purpose, the California Historical Intensity Mapping Project (CHIMP) has compiled a dataset that combines and reinterprets intensity information that has been stored in disparate and sometimes hard-to-access locations. The CHIMP dataset also includes new observations of intensity from archival research and oral history collection. Version 1 of the dataset includes 46,502 intensity observations for 62 earthquakes with estimated magnitudes ranging from 4.7 to 7.9. The 162 years of shaking data show observed shaking lower than expected from seismic hazard models. This discrepancy is reduced, but persists, if historical intensity data for the largest earthquakes are smoothed to reduce the effects of spatial under-sampling. Possible reasons for this discrepancy include other limitations of the CHIMP dataset, the hazard models, and the possibility that California seismicity throughout the historical period has been lower than the long-term average. Some of these issues may also explain similar discrepancies observed for Italy and Japan.

California↗

USGS2020

The 1933 Long Beach Earthquake (California, USA): Ground motions and rupture scenario

We present a synoptic analysis of the ground motions from the 11 March 1933 Mw 6.4 Long Beach, California, earthquake, the largest known earthquake within the central Los Angeles Basin region. Our inferred shaking intensity pattern supports the association of the earthquake with the Newport-Inglewood fault; it further illuminates the concentration of severe damage in the town of Compton, where accounts suggest vertical ground motions exceeding 1 g . We use a broadband simulation approach to develop a rupture scenario for this earthquake, informed by the damage distribution. The predicted shaking for a 25-km-long fault matches the intensity distribution, with an indication that non-linear site response on soft sediments in some near-field regions was stronger than predicted using a simple model to account for non-linearity. Our results suggest that the concentration of damage near Compton can be explained by a combination of local site amplification, source-controlled directivity, and three-dimensional basin effects whereby energy was channeled towards the deepest part of the Los Angeles Basin.

California↗

USGS2020

Mild displacements of boulders during the 2019 Ridgecrest Earthquakes

Strong seismic waves from the July 2019 Ridgecrest, California, earthquakes displaced rocks in proximity to the M 7.1 mainshock fault trace at several locations. In this report, we document large boulders that were displaced at the Wagon Wheel Staging Area (WWSA), approximately 4.5 km southeast of the southern terminus of the large M 6.4 foreshock rupture (hereafter “the large foreshock”) and 9 km southwest of the nearest approach of the M 7.1 mainshock surface rupture. Some boulders appear to have slid along essentially flat surfaces, which suggest that dynamic stresses overcame the coefficient of friction. Other boulders appear to have rocked within their sockets. In both cases, we use simple mechanical models to estimate total peak dynamic accelerations between 0.5 and 1g⁠, commensurate with modified Mercalli intensity 9. It is unclear if the strongest shaking at this location occurred during the large foreshock or the M 7.1 mainshock. The inferred accelerations are higher than predicted mainshock ground motions at WWSA, although local high accelerations could have been generated by path, site, or source effects. Gaps between boulders and their sockets are easily visible in the immediate aftermath of earthquakes and provide a quick indication of strong shaking. More importantly, the gaps quickly fill with surficial organic debris, including seeds and leaves of the year, that quickly become entombed. Boulders may thus potentially be extracted to examine gap fillings associated with past earthquakes, providing a new datable paleoseismic method.

California↗