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Results for “Bulletin Seismological Society America”

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USGS2017

Combining multiple earthquake models in real time for earthquake early warning

The ultimate goal of earthquake early warning (EEW) is to provide local shaking information to users before the strong shaking from an earthquake reaches their location. This is accomplished by operating one or more real‐time analyses that attempt to predict shaking intensity, often by estimating the earthquake’s location and magnitude and then predicting the ground motion from that point source. Other EEW algorithms use finite rupture models or may directly estimate ground motion without first solving for an earthquake source. EEW performance could be improved if the information from these diverse and independent prediction models could be combined into one unified, ground‐motion prediction. In this article, we set the forecast shaking at each location as the common ground to combine all these predictions and introduce a Bayesian approach to creating better ground‐motion predictions. We also describe how this methodology could be used to build a new generation of EEW systems that provide optimal decisions customized for each user based on the user’s individual false‐alarm tolerance and the time necessary for that user to react.

Bulletin of the Seismological Society of America↗

USGS2018

Paleoseismic results from the Alpine site, Wasatch fault zone: Timing and displacement data for six holocene earthquakes at the Salt Lake City–Provo segment boundary

To improve the characterization of Holocene earthquakes on the Wasatch fault zone (WFZ), we conducted light detection and ranging (lidar)‐based neotectonic mapping and excavated a paleoseismic trench across an 8‐m‐high fault scarp near Alpine, Utah, located < 1    km "> < 1 km <1 km south of the boundary between the Salt Lake City and Provo segments (SLCS and PS). We document evidence for six paleoearthquakes (AL6–AL1) from scarp‐derived colluvial wedges and crosscutting relations. A ground‐penetrating radar survey across the scarp resolved fault‐zone width, but not paleoearthquake stratigraphy. Bayesian (OxCal) modeling of 13 radiocarbon and 13 optically stimulated luminescence ages indicates that six earthquakes occurred ∼ 6.2 – 0.4    ka "> ∼ 6.2 – 0.4 ka ∼6.2–0.4 ka . Interseismic recurrence ranges from 0.2 to 1.8 ky (mean 1.2 ky). We estimate 6.5 ± 0.7    m "> 6.5 ± 0.7 m 6.5±0.7 m of cumulative vertical tectonic displacement across the ≥ 14 ‐ m ‐ wide "> ≥ 14 ‐ m ‐ wide ≥14‐m‐wide fault zone used near‐field observations of scarp‐derived colluvial‐wedge thicknesses, antithetic faulting, and graben backtilting. This is similar to our independent estimate of 6.5 ± 0.5    m "> 6.5 ± 0.5 m 6.5±0.5 m using far‐field observations of the offset ground surface and correlation of alluvial‐fan stratigraphy across the WFZ. These results suggest that colluvial‐wedge thickness at the Alpine site approximates one‐half the original west‐facing fault scarp height. Per‐event vertical displacements range from 0.8 to 1.2 m (mean 1.1 m), which we use to estimate surface rupture lengths that may exceed 50 km from earthquakes as large as moment magnitude ( M w "> M w Mw ) ∼ 7.0 "> ∼ 7.0 ∼7.0 . The late Holocene average vertical slip rate is 0.9    mm / yr "> 0.9 mm / yr 0.9 mm/yr ( 0.7 – 1.2    mm / yr "> 0.7 – 1.2 mm / yr 0.7–1.2 mm/yr range). Earthquake frequency has increased in the past ∼ 1    ky "> ∼ 1 ky ∼1 ky , whereas displacement per event has been similar for the past ∼ 6    ky "> ∼ 6 ky ∼6 ky , suggesting that strain accumulation is not the sole factor that controls the frequency and size of earthquakes at the SLCS–PS segment boundary. These findings can be used for a more nuanced characterization of earthquakes at the SLCS–PS boundary and improve earthquake hazard assessments along the Wasatch Front.

Utah↗

USGS2017

Repeatability of testing a small broadband sensor in the Albuquerque Seismological Laboratory Underground Vault

Variability in seismic instrumentation performance plays a fundamental role in our ability to carry out experiments in observational seismology. Many such experiments rely on the assumed performance of various seismic sensors as well as on methods to isolate the sensors from nonseismic noise sources. We look at the repeatability of estimating the self‐noise, midband sensitivity, and the relative orientation by comparing three collocated Nanometrics Trillium Compact sensors. To estimate the repeatability, we conduct a total of 15 trials in which one sensor is repeatedly reinstalled, alongside two undisturbed sensors. We find that we are able to estimate the midband sensitivity with an error of no greater than 0.04% with a 99th percentile confidence, assuming a standard normal distribution. We also find that we are able to estimate mean sensor self‐noise to within ±5.6 dB with a 99th percentile confidence in the 30–100‐s‐period band. Finally, we find our relative orientation errors have a mean difference in orientation of 0.0171° from the reference, but our trials have a standard deviation of 0.78°.

Bulletin of the Seismological Society of America↗

USGS2017

Seismic response of soft deposits due to landslide: The Mission Peak, California, landslide

The seismic response of active and intermittently active landslides is an important issue to resolve to determine if such landslides present an elevated hazard in future earthquakes. To study the response of landslide deposits, seismographs were placed on the Mission Peak landslide in the eastern San Francisco Bay region for a period of one year. Numerous local and near‐regional earthquakes were recorded that reveal a complexity of seismic response phenomena using the horizontal‐to‐vertical spectral ratio method. At lower frequencies, a clear spectral peak is observed at 0.5 Hz common to all four stations in the array and is attributed to a surface topographic effect. At higher frequencies, other spectral peaks occur that are interpreted in terms of local deposits and structures. Site amplification from the standard reference site method shows the minimum amplification with a factor of 2, comparing a site on and off the landslide. A site located on relatively homogeneous deposits of loose soils shows a clear spectral peak associated with the thickness of the deposit. Another site on a talus‐filled graben near the headscarp shows possible 2D or 3D effects from subsurface topography or scattering within and between buried sandstone blocks. A third site on a massive partially detached block below the crown of the headscarp shows indications of resonance caused by the reverberation of shear waves within the block. The varied seismic response of different parts of this complex landslide is consistent with other studies which found that, although landslide response is commonly enhanced in the downslope direction of landslide movement, such a response does not occur uniformly or consistently. When it does occur, enhanced site response parallel to the direction of landslide movement would contribute to landslide reactivation during significant earthquakes.

California↗

USGS2017

Determination of earthquake magnitude for early warning from the time-dependence of P-wave amplitudes

We propose a method that utilizes the time dependence of P ‐wave displacement amplitudes to estimate the final magnitude ( ⁠M⁠ ) for earthquake early warning (EEW) before the arrival of the peak amplitude. A relation between M and P ‐wave displacement amplitude is employed for the method. Its value is set as a function of time from the P arrival, and is determined using a K‐NET dataset recorded in Japan from a scaling relation between M and the time dependence of P ‐wave displacement. A test to check the performance of the proposed equation demonstrates in a statistical sense that this technique enables us to estimate M more rapidly than conventional methods without loss of accuracy. We conclude that the approach proposed in this article effectively gains a longer lead time as well as reduces the blind zone for EEW.

Bulletin of the Seismological Society of America↗

USGS2017

Evaluating a kinematic method for generating broadband ground motions for great subduction zone earthquakes: Application to the 2003 Mw 8.3 Tokachi‐Oki earthquake

We compare broadband synthetic seismograms with recordings of the 2003 M w "> M w Mw 8.3 Tokachi‐Oki earthquake to evaluate a compound rupture model, in which slip on the fault consists of multiple high‐stress‐drop asperities superimposed on a background slip distribution with longer rise times. Low‐frequency synthetics ( < 1    Hz "> < 1 Hz <1 Hz ) are calculated using deterministic, 3D finite‐difference simulations and are combined with high‐frequency ( > 1    Hz "> > 1 Hz >1 Hz ) stochastic synthetics using a matched filter at 1 Hz. We show that this compound rupture model and overall approach accurately reproduces waveform envelopes and observed response spectral accelerations (SAs) from the Tokachi‐Oki event. We find that sufficiently short subfault rise times (i.e., < ∼ 1 – 2    s "> < ∼ 1 – 2 s <∼1–2 s ) are necessary to reproduce energy ∼ 1    Hz "> ∼ 1 Hz ∼1 Hz . This is achieved by either (1) including distinct subevents with short rise times, as may be suggested by the Tokachi‐Oki data, or (2) imposing a fast‐slip velocity over the entire rupture area. We also include a systematic study on the effects of varying several kinematic rupture parameters. We find that simulated strong ground motions are sensitive to the average rupture velocity and coherence of the rupture front, with more coherent ruptures yielding higher response SAs. We also assess the effects of varying the average slip velocity and the character (i.e., area, magnitude, and location) of high‐stress‐drop subevents. Even in the absence of precise constraints on these kinematic rupture parameters, our simulations still reproduce major features in the Tokachi‐Oki earthquake data, supporting its accuracy in modeling future large earthquakes.

Bulletin of the Seismological Society of America↗

USGS2017

Detection and characterization of pulses in broadband seismometers

Pulsing - caused either by mechanical or electrical glitches, or by microtilt local to a seismometer - can significantly compromise the long‐period noise performance of broadband seismometers. High‐fidelity long‐period recordings are needed for accurate calculation of quantities such as moment tensors, fault‐slip models, and normal‐mode measurements. Such pulses have long been recognized in accelerometers, and methods have been developed to correct these acceleration steps, but considerable work remains to be done in order to detect and correct similar pulses in broadband seismic data. We present a method for detecting and characterizing the pulses using data from a range of broadband sensor types installed in the Global Seismographic Network. The technique relies on accurate instrument response removal and employs a moving‐window approach looking for acceleration baseline shifts. We find that pulses are present at varying levels in all sensor types studied. Pulse‐detection results compared with average daily station noise values are consistent with predicted noise levels of acceleration steps. This indicates that we can calculate maximum pulse amplitude allowed per time window that would be acceptable without compromising long‐period data analysis.

Bulletin of the Seismological Society of America↗

USGS2017

Exploring the historical earthquakes preceding the giant 1960 Chile earthquake in a time‐dependent seismogenic zone

New documentary findings and available paleoseismological evidence provide both new insights into the historical seismic sequence that ended with the giant 1960 south-central Chile earthquake and relevant information about the region’s seismogenic zone. According to the few available written records, this region was previously struck by earthquakes of varying size in 1575, 1737, and 1837. We expanded the existing compilations of the effects of the two latter using unpublished first-hand accounts found in archives in Chile, Peru, Spain, and New England. We further investigated their sources by comparing the newly unearthed historical data and available paleoseismological evidence with the effects predicted by hypothetical dislocations. The results reveal significant differences in the along-strike and depth distribution of the ruptures in 1737, 1837, and 1960. While the 1737 rupture likely occurred in the northern half of the 1960 region, on a narrow and deep portion of the megathrust, the 1837 rupture occurred mainly in the southern half and slipped over a wide range of depth. Such a wide rupture in 1837 disagrees with the narrow and shallow seismogenic zone currently inferred along this region. If in fact there is now a narrow zone where 200 years ago there was a wider one, it means that the seismogenic zone changes with time, perhaps between seismic cycles. Such change probably explains the evident variability in both size and location of the great earthquakes that have struck this region over the last centuries, as evidenced by written history, and through millennia, as inferred from paleoseismology.

Bulletin of the Seismological Society of America↗

USGS2017

Evaluating spatial and temporal relationships between an earthquake cluster near Entiat, central Washington, and the large December 1872 Entiat earthquake

We investigate spatial and temporal relations between an ongoing and prolific seismicity cluster in central Washington, near Entiat, and the 14 December 1872 Entiat earthquake, the largest historic crustal earthquake in Washington. A fault scarp produced by the 1872 earthquake lies within the Entiat cluster; the locations and areas of both the cluster and the estimated 1872 rupture surface are comparable. Seismic intensities and the 1–2 m of coseismic displacement suggest a magnitude range between 6.5 and 7.0 for the 1872 earthquake. Aftershock forecast models for (1) the first several hours following the 1872 earthquake, (2) the largest felt earthquakes from 1900 to 1974, and (3) the seismicity within the Entiat cluster from 1976 through 2016 are also consistent with this magnitude range. Based on this aftershock modeling, most of the current seismicity in the Entiat cluster could represent aftershocks of the 1872 earthquake. Other earthquakes, especially those with long recurrence intervals, have long‐lived aftershock sequences, including the M w "> M w Mw 7.5 1891 Nobi earthquake in Japan, with aftershocks continuing 100 yrs after the mainshock. Although we do not rule out ongoing tectonic deformation in this region, a long‐lived aftershock sequence can account for these observations.

Washington↗

USGS2017

A comprehensive analysis of geodetic slip rate estimates and uncertainties in California

Developing a comprehensive model of tectonic continental deformation requires assessing (1) fault‐slip rates, (2) off‐fault deformation rates, and (3) realistic uncertainties. Fault‐slip rates can be estimated by modeling fault systems, based on space geodetic measurements of active surface ground displacement such as Global Navigation Satellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR). Geodetic slip‐rate estimates may vary widely due to measurement and epistemic (model) uncertainties, presenting a challenge for both estimating slip rates and accurately characterizing uncertainties: models may vary in the number of faults represented and the precise location of those faults. Since 2003, 33 published geodetic deformation models have produced slip‐rate estimates within California. Variability among these models represents variability among valid model choices and may be considered a proxy for model uncertainties in geodetic slip‐rate estimates. To enable rigorous comparison between geodetic slip‐rate estimates, I combine models on a georeferenced grid and find an average standard deviation on slip rate of ∼1.5 mm/yr over 542 grid cells (average area of 1304 km 2 /cell⁠ ). Furthermore, the average strike‐slip and tensile‐slip rates over all 33 studies, in each grid cell, may then be projected onto Unified California Earthquake Rupture Forecast (UCERF) v.3.1 faults for a single summary model of geodetic slip rates. Slip rates that do not project perfectly onto UCERF3.1 faults form a summary model of off‐modeled‐fault (OMF) deformation. Most of this OMF deformation occurs in grid cells that intersect UCERF3.1 faults, suggesting that off‐fault deformation may be, in part, a product of epistemic uncertainty in geodetic slip‐rate estimates and may be physically accommodated on, or very near, UCERF faults.

California↗

USGS2017

Estimating rupture distances without a rupture

Most ground motion prediction equations (GMPEs) require distances that are defined relative to a rupture model, such as the distance to the surface projection of the rupture (RJB) or the closest distance to the rupture plane (RRUP). There are a number of situations in which GMPEs are used where it is either necessary or advantageous to derive rupture distances from point-source distance metrics, such as hypocentral (RHYP) or epicentral (REPI) distance. For ShakeMap, it is necessary to provide an estimate of the shaking levels for events without rupture models, and before rupture models are available for events that eventually do have rupture models. In probabilistic seismic hazard analysis, it is often convenient to use point-source distances for gridded seismicity sources, particularly if a preferred orientation is unknown. This avoids the computationally cumbersome task of computing rupture-based distances for virtual rupture planes across all strikes and dips for each source. We derive average rupture distances conditioned on REPI, magnitude, and (optionally) back azimuth, for a variety of assumed seismological constraints. Additionally, we derive adjustment factors for GMPE standard deviations that reflect the added uncertainty in the ground motion estimation when point-source distances are used to estimate rupture distances.

Bulletin of the Seismological Society of America↗

USGS2018

Holocene surface ruptures on the Salinas Fault and southeastern Great Southern Puerto Rico Fault Zone, South Coastal Plain of Puerto Rico

We analyzed light detection and ranging (lidar) data and aerial photography to locate active faults near the south coast of Puerto Rico and excavated paleoseismic trenches across the Salinas fault and the Great Southern Puerto Rico fault zone (GSPRFZ). We document evidence for two Holocene surface‐rupturing earthquakes along both faults. Two earthquakes on the Salinas fault occurred after the deposition of sediments that are 7400–10,400 yrs old. No quantitative ages constrain the timing of the two earthquakes on the southeast GSPRFZ, but we interpret both events to have occurred during the Holocene or latest Pleistocene, based on the similarities in the characteristics of the faulted sediment and the development of soils exposed in both trenches. Stratigraphic and geomorphic evidence suggests components of both vertical and lateral slip on both faults. These results show that onshore active faults in Puerto Rico are more common than previously recognized and highlight the need for additional study to search for Holocene active faults and for an updated seismic hazard analysis for the island.

Puerto Rico↗

USGS2018

Spatial and spectral interpolation of ground-motion intensity measure observations

Following a significant earthquake, ground‐motion observations are available for a limited set of locations and intensity measures (IMs). Typically, however, it is desirable to know the ground motions for additional IMs and at locations where observations are unavailable. Various interpolation methods are available, but because IMs or their logarithms are normally distributed, spatially correlated, and correlated with each other at a given location, it is possible to apply the conditional multivariate normal (MVN) distribution to the problem of estimating unobserved IMs. In this article, we review the MVN and its application to general estimation problems, and then apply the MVN to the specific problem of ground‐motion IM interpolation. In particular, we present (1) a formulation of the MVN for the simultaneous interpolation of IMs across space and IM type (most commonly, spectral response at different oscillator periods) and (2) the inclusion of uncertain observation data in the MVN formulation. These techniques, in combination with modern empirical ground‐motion models and correlation functions, provide a flexible framework for estimating a variety of IMs at arbitrary locations.

Bulletin of the Seismological Society of America↗

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↗