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USGS2018

The 2013–2016 induced earthquakes in Harper and Sumner Counties, southern Kansas

We examine the first four years (2013–2016) of the ongoing seismicity in southern Kansas using high‐precision locations derived from a local seismometer network. The earthquakes occur almost exclusively in the shallow crystalline basement, below the wastewater injection horizon of the Arbuckle Group at the base of the sedimentary section. Multiple lines of evidence lead us to conclude that disposal of wastewater from the production of oil and gas by deep injection is the probable cause for the surge of seismicity that began in 2013. First, the seismicity correlates in space and time with the injection. We observe increases in seismicity subsequent to increases in injection and decreases in seismicity in response to decreases in injection. Second, the earthquake‐rate change is statistically improbable to be of natural origin. From 1974 through the time of the injection increase in 2012, no M L "> M L 4 or larger earthquakes occurred in the study area, while six occurred between 2012 and 2016. The probability of this rate change occurring randomly is ∼ 0.16 % "> ∼ 0.16 % . Third, the other potential industrial drivers of seismicity (hydraulic fracturing and oil production) do not correlate in space or time with seismicity. Local geological conditions are important in determining whether injection operations will induce seismicity, as shown by absence of seismicity near the largest injection operations in the southwest portion of our study area. In addition to local operations, the presence of seismicity 10+ km from large injection wells indicates that regional injection operations also need to be considered to understand the effects of injection on seismicity.

Kansas↗

USGS2017

Improvements in absolute seismometer sensitivity calibration using local earth gravity measurements

The ability to determine both absolute and relative seismic amplitudes is fundamentally limited by the accuracy and precision with which scientists are able to calibrate seismometer sensitivities and characterize their response. Currently, across the Global Seismic Network (GSN), errors in midband sensitivity exceed 3% at the 95% confidence interval and are the least‐constrained response parameter in seismic recording systems. We explore a new methodology utilizing precise absolute Earth gravity measurements to determine the midband sensitivity of seismic instruments. We first determine the absolute sensitivity of Kinemetrics EpiSensor accelerometers to 0.06% at the 99% confidence interval by inverting them in a known gravity field at the Albuquerque Seismological Laboratory (ASL). After the accelerometer is calibrated, we install it in its normal configuration next to broadband seismometers and subject the sensors to identical ground motions to perform relative calibrations of the broadband sensors. Using this technique, we are able to determine the absolute midband sensitivity of the vertical components of Nanometrics Trillium Compact seismometers to within 0.11% and Streckeisen STS‐2 seismometers to within 0.14% at the 99% confidence interval. The technique enables absolute calibrations from first principles that are traceable to National Institute of Standards and Technology (NIST) measurements while providing nearly an order of magnitude more precision than step‐table calibrations.

Bulletin of the Seismological Society of America↗

USGS2018

Turing-style tests for UCERF3 synthetic catalogs

Epidemic-Type Aftershock Sequence (ETAS) catalogs generated from the 3rd Uniform California Earthquake Rupture Forecast (UCERF3) model are unique in that they are the first to combine a complex, fault-based long-term forecast with short-term earthquake clustering statistics. We present Turing-style tests to examine whether these synthetic catalogs can successfully imitate observed earthquake behavior in California. We find that UCERF3-ETAS is more spatially diffuse than the observed historic catalog in California and that it is lacking quiet periods that are present in the real catalog. While mean aftershock productivity of the observed catalog is matched closely by UCERF3-ETAS, the real catalog has more inter-sequence productivity variability and small mainshocks have more foreshocks. In sum, we find that UCERF3-ETAS differs from the observed catalog in ways that are foreseeable from its modeling simplifications. The tests we present here can be used on any model which produces suites of synthetic catalogs; as such, in addition to providing avenues for future improvements to the model, they could also be incorporated into testing platforms such as Collaboratory for the Study of Earthquake Predictability (CSEP).

Bulletin of the Seismological Society of America↗

USGS2017

Aftershocks, earthquake effects, and the location of the large 14 December 1872 earthquake near Entiat, central Washington

Reported aftershock durations, earthquake effects, and other observations from the large 14 December 1872 earthquake in central Washington are consistent with an epicenter near Entiat, Washington. Aftershocks were reported for more than 3 months only near Entiat. Modal intensity data described in this article are consistent with an Entiat area epicenter, where the largest modified Mercalli intensities, VIII, were assigned between Lake Chelan and Wenatchee. Although ground failures and water effects were widespread, there is a concentration of these features along the Columbia River and its tributaries in the Entiat area. Assuming linear ray paths, misfits from 23 reports of the directions of horizontal shaking have a local minima at Entiat, assuming the reports are describing surface waves, but the region having comparable misfit is large. Broadband seismograms recorded for comparable ray paths provide insight into the reasons why possible S – P times estimated from felt reports at two locations are several seconds too small to be consistent with an Entiat area epicenter.

Washington↗

USGS2018

Holocene fault reactivation in the eastern Cascades, Washington

Significant uncertainty remains concerning how and where crustal shortening occurs throughout the eastern Cascade Range in Washington State. Using light detection and ranging (lidar) imagery, we identified an ∼ 5 ‐ km ‐ long "> ∼5‐km‐long lineament in Swakane canyon near Wenatchee, roughly coincident with a strand of the Entiat fault. Topographic profiles across the lineament reveal a southwest‐side‐up break in slope, with an average of 2–3 m of vertical separation of the hillslope surface. We consider a range of possible origins for this feature, including differential erosion across a fault‐line scarp, slope failure (sackung or landslide), and surface deformation across an active fault strand. Based on trenching, radiocarbon and luminescence dating, and ground‐penetrating radar (GPR) across the lineament, we conclude that warped saprolite observed in the shallow subsurface is most consistent with southwest‐side‐up folding caused by blind reverse faulting at depth. Following this reasoning, dating of overlying colluvial deposits suggests that at least one Holocene earthquake occurred on this strand of the southern Entiat fault, with an approximate vertical separation of ≥ 1    m "> m ≥1 m . GPR reveals up to 4 m of cumulative vertical separation of the saprolite, suggesting a history of multiple earthquakes on the structure. Taken in context with other potential fault‐related lineaments along the Entiat fault, our interpretation of Holocene earthquakes in Swakane canyon could suggest reactivation of longer sections of the Entiat fault, as well as of other bedrock faults in the eastern Cascades. Although active erosion and slow strain rates lead to a subdued geomorphic expression of recent deformation, we conclude that the reactivated Entiat fault represents a seismogenic structure that should be considered in regional seismic hazard analyses. The difficulty of recognizing low‐slip‐rate structures in forested and mountainous terrain underscores the importance of additional lidar surveys and geological and geophysical studies for fully understanding seismic hazard in regions with infrequent but potentially large earthquakes.

Washington↗

USGS2017

Effects of thermal variability on broadband seismometers: Controlled experiments, observations, and implications

Isolating seismic instruments from temperature fluctuations is routine practice within the seismological community. However, the necessary degree of thermal stability required in broadband installations to avoid generating noise or compromising the fidelity in the seismic records is largely unknown and likely application dependent. To quantify the temperature sensitivity of seismometers over a broad range of frequencies, we artificially induced local temperature changes on three different models of seismometers to measure the effect of thermal variations on seismometer output. We found that diurnal temperature changes above 0.002°C root mean square (rms) showed significant changes in velocity and acceleration output in comparison to thermally stable reference measurements. We also found that sensor incoherent self‐noise increased with temperature variation; these increases in noise can be modeled as 1 / f "> 1 / f noise (pink noise), and are unlikely to be easily corrected for. These experimental results are compared with the data from Incorporated Research Institutions for Seismology (IRIS) U.S. Geological Survey (USGS) Global Seismographic Network (GSN) station TUC (Tucson, Arizona). This station is well instrumented with temperature sensors and has three different broadband seismometers, each of which uses a different method of thermal isolation. We show that the water bricks and borehole installations give ample temperature attenuation to thermally isolate seismometers from diurnal thermal variability that would compromise seismic data. We find that seismometer installations that provide thermal stability below 0.002°C rms could help to improve long‐period vertical seismic data across the GSN by decreasing temperature‐driven 1 / f "> 1 / f noise.

Bulletin of the Seismological Society of America↗

USGS2018

Improving near‐real‐time coseismic landslide models: Lessons learned from the 2016 Kaikōura, New Zealand, earthquake

The U.S. Geological Survey (USGS) is developing near‐real‐time global earthquake‐triggered‐landslide products to augment the USGS Prompt Assessment of Global Earthquakes for Response (PAGER) system. The 14 November 2016 M w "> M w Mw 7.8 Kaikōura, New Zealand, earthquake provided a test case for evaluating the performance and near‐real‐time response applicability of three published global seismically induced landslide models. All three models obtain shaking estimates from the USGS ShakeMap, which is updated and sometimes changes significantly in the hours to days after an earthquake. The Kaikōura earthquake is a particularly valuable event that helps us better understand how changes to the ShakeMap affect the landslide models because the ShakeMap evolved significantly over several weeks as multifault rupture and seismic data were incorporated. We used the detailed landslide inventory available for this event for qualitative landslide model evaluation. We found that once a point source was replaced with an approximate rupture extent in ShakeMap, the landslide models were all successful at roughly identifying the area of highest hazard. This is notable, given that the models are relatively simple, coarse in resolution, and are based solely on input proxies that are globally available. However, all of the models dramatically overpredicted the hazard level, which indicates that improvements can be made. Subsequent updates to the ShakeMap resulted in improvements to model performance by some metrics and declining performance by others. In all cases, details of the ShakeMap strongly controlled the spatial pattern, even when those details were erroneous, such as the inclusion of a fault segment that did not rupture. If maps of landslide hazard are to be used effectively for rapid response, then we need to understand and clearly communicate the control that ShakeMap has over the models and how that typically evolves with time and is (or is not) reflected in reported uncertainties.

Bulletin of the Seismological Society of America↗

USGS2018

Combining conflicting Bayesian models to develop paleoseismic records—An example from the Wasatch Fault Zone, Utah

Bayesian statistical analyses of paleoseismic data result in the probabilistic determination of earthquake times using geochronological data evaluated in the context of a stratigraphic model. However, a fundamental problem in paleoseismology is how to use the Bayesian approach to model sparse and/or conflicting geochronological datasets, such as those derived from sites exhibiting episodic sedimentary and pedogenic processes in moderate‐ to high‐energy environments (e.g., a normal‐faulted alluvial fan). Using paleoseismic data for the Corner Canyon site on the Salt Lake City segment of the Wasatch fault zone (Utah), we develop an approach by which multiple Bayesian models are combined to generate an earthquake history at a site. This approach accommodates mutually exclusive interpretations of the geochronological data and thereby limits the influence of sparse data, stratigraphically inconsistent ages, or a single, subjective model interpretation. For the Corner Canyon site, we integrate four OxCal Bayesian models to generate a chronology of six events between ∼ 4.8 "> ∼ 4.8 and ∼ 0.5    ka "> ∼ 0.5 ka ⁠ . Late Holocene (post‐5 ka) mean recurrence and vertical slip‐rate estimates are ∼ 0.9    ky "> ∼ 0.9 ky (0.7–1.0 ky; 95% confidence) and 1.1    mm / yr "> 1.1 mm / yr ( ⁠ 0.8 – 1.7    mm / yr "> 0.8 – 1.7 mm / yr range), respectively. Although our method increases the uncertainty in the timing of individual earthquakes, it more objectively accounts for potential geochronological errors and different interpretations of stratigraphic age control. By relaxing the need to select a single age model, our approach yields more accurate earthquake‐timing results that will better facilitate evaluations of along‐fault event correlation and earthquake rupture length.

Utah↗

USGS2018

Landslides triggered by the 14 November 2016 Mw 7.8 Kaikōura Earthquake, New Zealand

The 14 November 2016 M w "> M w Mw 7.8 Kaikōura earthquake generated more than 10,000 landslides over a total area of about 10 , 000    km 2 "> 10 , 000 km 2 10,000 km2 , with the majority concentrated in a smaller area of about 3600    km 2 "> 3600 km 2 3600 km2 . The largest landslide triggered by the earthquake had an approximate volume of 20 ( ± 2 )    M   m 3 "> 20 ( ± 2 ) M m 3 20(±2) M m3 , with a runout distance of about 2.7 km, forming a dam on the Hapuku River. In this article, we present version 1.0 of the landslide inventory we have created for this event. We use the inventory presented in this article to identify and discuss some of the controls on the spatial distribution of landslides triggered by the Kaikōura earthquake. Our main findings are (1) the number of medium to large landslides (source area ≥ 10 , 000    m 2 "> ≥ 10 , 000 m 2 ≥10,000 m2 ) triggered by the Kaikōura earthquake is smaller than for similar‐sized landslides triggered by similar magnitude earthquakes in New Zealand; (2) seven of the largest eight landslides (from 5 to 20    M   m 3 "> 20 M m 3 20 M m3 ) occurred on faults that ruptured to the surface during the earthquake; (3) the average landslide density within 200 m of a mapped surface fault rupture is three times that at a distance of 2500 m or more from a mapped surface fault rupture; (4) the “distance to fault” predictor variable, when used as a proxy for ground‐motion intensity, and when combined with slope angle, geology, and elevation variables, has more power in predicting landslide probability than the modeled peak ground acceleration or peak ground velocity; and (5) for the same slope angles, the coastal slopes have landslide point densities that are an order of magnitude greater than those in similar materials on the inland slopes, but their source areas are significantly smaller.

Bulletin of the Seismological Society of America↗

USGS2018

The case for mean rupture distance in ground‐motion estimation

This article advocates for the use of mean rupture distances that we contend are more physically representative of the distance to an earthquake and are simpler than minimum distances. Many current ground‐motion models (GMMs) rely on numerous modifications of minimum rupture distances to accurately model near‐source ground motions. These modifications, that include additional distance definitions and saturation terms, result in complicated functional forms and are often not easily understood on a seismological basis, such as the magnitude‐dependent near‐fault saturation term. The use of mean distance represents the location of a station in relation to the entire rupture plane and results in a simpler, more physically meaningful GMM that models near‐source ground motion as accurately as other GMMs that have more inputs and more complex functional forms. We demonstrate the use of mean distance by developing a GMM for shallow‐crustal earthquakes with the Next Generation Attenuation‐West2 (NGA‐West2) project database. Specifically, we use the generalized mean distance, also known as the power mean, in which the power varies with frequency. We show that this new GMM fits the NGA‐West2 database as well as the NGA‐West2 GMMs and exhibits similar near‐source amplitude scaling. An additional benefit of mean distance is that it can provide a mechanism to account for spatially variable slip. We prospectively validate this GMM against the 2016 M "> M 7.8 Kaikōura, New Zealand, earthquake, which was not used in model development. To better understand the magnitude dependence of geometrical spreading, we employ a simple conceptual model based on fundamental principles to show that the GMM is consistent with common seismological understanding.

Bulletin of the Seismological Society of America↗

USGS2018

The intensity signature of induced seismicity

We analyze a comprehensive database of ∼ 63 , 000 "> ∼ 63 , 000 geocoded community intensity observations from > 400 "> > 400 earthquakes of moment magnitude M ≥ 3.5 "> M ≥ 3.5 in Oklahoma from 2010 to 2016 to define the intensity signature of induced events. We show that natural and induced events have similar average intensities within 10 km of the epicenter. At greater distances, induced events have low‐average intensities compared with deeper natural events. These trends are predictable based on ground‐motion prediction equations. They are a consequence of two focal‐depth effects that have offsetting impacts on the strength of ground motion: (1) the epicenter is near the source for shallow events, and (2) the stress parameter scales with focal depth.

Bulletin of the Seismological Society of America↗

USGS2018

Observations of rotational motions from local earthquakes using two temporary portable sensors in Waynoka, Oklahoma

Characterizing rotational motions from earthquakes at local distances has the potential to improve earthquake engineering and seismic gradiometry by better characterizing the complete seismic wavefield. Applied Technology Associates (ATA) has developed a proto‐seismic magnetohydrodynamic (SMHD) three‐component rotational rate sensor. We deploy two ATA rotational rate sensors at a temporary aftershock station in Waynoka, Oklahoma. From 27 April to 6 June 2017, we recorded the translational and rotational motions of 155 earthquakes of ML≥2.0 within 220 km of the station. Using the recorded events, we compare peak ground rotation rate (⁠ PGω˙ ⁠) with peak ground velocity (PGV) and with peak ground acceleration (PGA). Our results support previously identified potential relationships between the two quantities. We also compare peak ground rotations (⁠ PGω ⁠) as a function of seismic moment and distance. We found that PGω˙ decays with an exponent of approximately −4.0km−1 for both horizontal and vertical components. On the other hand, PGA decays with an exponent of approximately −1.8km−1 for all components. We compute apparent phase velocity directly from the rotational data for both horizontally polarized shear waves (SH; 379m/s with a standard deviation of 114m/s ⁠) and vertically polarized compression and shear waves (P‐SV; 387m/s with a standard deviation of 121m/s ⁠). Finally, by comparing various rotational and translational components, we look at potential implications for estimating local event source parameters. We found that the absolute correlation of nearby earthquakes decays at a rate of approximately 0.39/km for rotational sensors. This decay rate of absolute correlation is faster on translational sensors with a decay rate of 0.44/km. The latter may help in identifying phenomena such as repeating earthquakes by using differences in correlations as a function of distance and how these differences compare with translational correlations.

Oklahoma↗

USGS2018

Decomposing leftovers: Event, path, and site residuals for a small magnitude ANZA region GMPE

Ground‐motion prediction equations (GMPEs) are critical elements of probabilistic seismic hazard analysis (PSHA), as well as for other applications of ground motions. To isolate the path component for the purpose of building nonergodic GMPEs, we compute a regional GMPE using a large dataset of peak ground accelerations (PGAs) from small‐magnitude earthquakes ( ⁠0.5 ≤ M ≤ 4.5 with > 10,000 events, yielding ∼120,000 recordings) that occurred in 2013 centered around the ANZA seismic network (hypocentral distances ≤180 km⁠ ) in southern California. We examine two separate methods of obtaining residuals from the observed and predicted ground motions: a pooled ordinary least‐squares model and a mixed‐effects maximum‐likelihood model. Whereas the former is often used by the broader seismological community, the latter is widely used by the ground‐motion and engineering seismology community. We confirm that mixed‐effects models are the preferred and most statistically robust method to obtain event, path, and site residuals and discuss the reasoning behind this. Our results show that these methods yield different consequences for the uncertainty of the residuals, particularly for the event residuals. Finally, our results show no correlation (correlation coefficient [CC] <0.03⁠ ) between site residuals and the classic site‐characterization term V S30 ⁠ , the time‐averaged shear‐wave velocity in the top 30 m at a site. We propose that this is due to the relative homogeneity of the site response in the region and perhaps due to shortcomings in the formulation of V S30 ⁠ and suggest applying the provided PGA site correction terms to future ground‐motion studies for increased accuracy.

California↗

USGS2018

Broadband ground‐motion simulation of the 2011 Mw 6.2 Christchurch, New Zealand, earthquake

This study presents the details and results of hybrid broadband (0–10 Hz) ground‐motion simulations for the 2011 M w "> M w Mw 6.2 Christchurch, New Zealand, earthquake. The simulations utilize a 3D velocity model and a kinematic source model with stochastic realizations of the slip amplitude, rise time, and rake angle. The resulting ground motions capture the salient basin amplification effects that are seen in the observed ground motions in central Christchurch city. Quantitative comparisons of the simulations with both observed recordings and empirical ground‐motion models (GMMs), considering peak ground acceleration, 5% damped pseudospectral acceleration, and 5%–95% significant duration, indicate that the simulations exhibit lower bias than empirical GMMs over the T = 1 – 10    s "> T = 1 – 10 s T=1–10 s period range, and are comparable at short periods ( ⁠ T < 1    s "> T < 1 s T<1 s ⁠ ). Sensitivity analyses suggest that the effect of stochastic realizations of different slip distributions is relatively small because of the fault dimensions. It is also illustrated that the effect of slip distribution variability is only a small component of the total uncertainty in ground‐motion simulation. As well as the important implications toward ground‐motion simulation validation, the presented simulations provide ground‐motion time series that can be used for forensic structural and geotechnical case histories that are located sufficiently far from strong‐motion station recordings.

Christchurch↗

USGS2018

Broadband synthetic seismograms for magnitude 9 earthquakes on the Cascadia Megathrust based on 3D simulations and stochastic synthetics, Part 2: Rupture parameters and variability

We used a combination of 3D finite‐difference simulations ( < 1    Hz "> < 1 Hz ) and 1D stochastic synthetics ( > 1    Hz "> > 1 Hz ) to generate broadband (0–10 Hz) synthetic seismograms for numerous M w "> Mw 9 earthquake rupture scenarios on the Cascadia megathrust. Slip consists of multiple high‐stress‐drop subevents ( M w "> Mw 8) with short rise times on the deeper portions of the fault, superimposed on a background slip distribution with longer rise times. We identify key rupture parameters that control the intensity of ground shaking and resulting seismic hazard; these include the hypocenter location, the down‐dip limit of slip, the average rupture velocity, and the character (i.e., location, magnitude, and stress drop) of subevents. Extending the down‐dip limit of rupture to the top of the nonvolcanic tremor zone results in localized regions with a factor of 5–10 increase in spectral acceleration (SA) for periods < 5    s "> <5 s , compared to a rupture that is completely offshore. This is primarily due to the closer proximity of high‐stress‐drop subevents to inland locations when the rupture is allowed to extend deeper. Similarly, we find that the hypocenter location can result in a variation in the intensity of ground motions of a factor of > 10 "> >10 , due to the effects of rupture directivity (i.e., SA at periods > 1    s "> >1 s ). We also observe a coupling between rupture directivity and basin amplification. The intensity of ground motions is also strongly affected by the magnitude, stress drop, and location of high‐stress‐drop subevents, which are poorly constrained. Overall, our results quantify the effect of kinematic rupture parameters on ground motions for an M w "> Mw 9 earthquake in Cascadia and emphasize the need for further constraints on these parameters to improve seismic hazard estimates in the Pacific Northwest.

Bulletin of the Seismological Society of America↗

USGS2018

Broadband synthetic seismograms for magnitude 9 earthquakes on the Cascadia megathrust based on 3D simulations and stochastic synthetics, part 1: Methodology and overall results

We have produced a large set of broadband (0–10 Hz) synthetic seismograms for Mw 9.0 earthquakes on the Cascadia megathrust by combining synthetic seismograms derived from 3D finite‐difference simulations ( ⁠≤1 Hz⁠ ) with finite‐source, stochastic synthetics ( ⁠≥1 Hz⁠ ). We used a compound rupture model consisting of high stress drop M w 8 subevents superimposed on large, shallower slip with long‐slip duration, informed by observations of the M w 9.0 Tohoku, Japan, and M w 8.8 Maule, Chile, earthquakes. Thirty 3D simulations were run, considering a variety of rupture parameters, to determine the range of expected ground motions. For sites not in sedimentary basins, the spectral accelerations of the synthetics are similar to the BC Hydro ground‐motion prediction equations (GMPEs) for periods of 0.1–6 s, but exceed them at periods greater than 6 s. Response spectra from the synthetics at sites in the Seattle and Tacoma sedimentary basins show large amplifications of factors of 2–5 at periods of 1–10 s. This basin amplification is substantially larger than that found for crustal earthquakes in the Next Generation Attenuation‐West2 (NGA‐West2) GMPEs. Basin amplification is caused by basin‐edge generated surface waves and by amplification and focusing of S waves and surface waves by the 3D basin structure. The synthetic seismograms show effective average durations of strong motions of about 70 s for coastal sites, increasing to about 120 s at 200 km distance. We find that the interevent and intraevent standard deviations of the spectral amplitudes of the synthetics are larger for sites closer to the rupture, because they are more sensitive to the location of subevents and rupture directivity.

California, Oregon, Washington↗

USGS2019

Combining dynamic rupture simulations with ground motion data to characterize seismic hazard from Mw 3-5.8 earthquakes in Oklahoma and Kansas

Many seismically active areas suffer from a lack of near‐source ground‐motion recordings, making ground‐motion prediction difficult at distances within ∼ 40    km "> ∼ 40 km ∼40 km from an earthquake. We aim to aid the development of near‐source ground‐motion prediction equations (GMPEs) by generating synthetic ground‐motion data via simulation. Building on previous work using point‐source moment tensor sources to simulate small ( ⁠ M w "> M w Mw 3–4) earthquakes for a target region encompassing north central Oklahoma and south central Kansas, we perform dynamic rupture simulations of earthquakes up to M w "> M w Mw 5.8. We introduce complexity into the rupture process by adding stochastically generated heterogeneity to initial stress conditions. Our simulated ground‐motion data are added to a catalog of recorded ground‐motion data to construct a composite recorded‐simulated ground‐motion catalog that we use to develop a GMPE for the target region. This procedure can be generalized and used for GMPE development in other regions with near‐source ground‐motion data scarcity, which could directly benefit critical applications such as the National Seismic Hazard Maps produced by the U.S. Geological Survey (USGS).

Kansas, Oklahoma↗

USGS2018

Characterizing and imaging sedimentary strata using depth-converted spectral ratios: An example from the Atlantic Coastal Plain of the Eastern U.S.

Unconsolidated, near-surface sediments can have a profound influence on the amplitudes and frequencies of ground shaking during earthquakes, and these effects should be accounted for when using amplitude observations for seismic hazard assessments. This study explores methods to use teleseismic arrivals recorded on linear receiver arrays to characterize widespread, shallow sedimentary deposits, including estimation of the velocities, determination of the fundamental resonance peaks, and imaging of the major reflectors. The examples used are the extensive Atlantic Coastal Plain (ACP) and associated Mississippi Embayment (ME) strata of the Central and Eastern United States. The large contrast in material properties at the bedrock surface beneath these sediments produces a strong fundamental resonance peak in the 0.2 to 4 Hz frequency range, which is estimated here by computing spectral ratios at each receiver site relative to bedrock sites at the ends of the receiver arrays. Sediment thicknesses derived from published contour maps made from drill hole data allow for the computation of average velocities to match the observed frequencies of resonance peaks with theoretical values at each receiver site, with the sloping bedrock surface allowing for computation of an average velocity versus depth function if horizontal layers are assumed. The velocity function is then used to convert the spectral ratios from frequency to depth, resulting in an image of the subsurface similar to that of a seismic reflection profile. The results demonstrate the use of teleseismic signals for characterizing and imaging shallow sedimentary strata.

Atlantic Coastal Plain↗