ALBERT

Description: The core mantle boundary (CMB) features the most dramatic contrast in the physical properties within the Earth and plays a fundamental role in the understanding of the dynamic evolution of the Earth’s interior. Seismic core phases such as PKKP sample large area of the lowermost mantle and the uppermost core, thus providing valuable information of the velocity structures on both sides of the CMB. Diffraction Waves Well Beyond Cutoff Distance (PKKPab) is one branch of the triplicated PKKP that can be observed beyond its ray theoretical cutoff distance as a result of diffraction along the CMB. The travel time and slowness of the diffracted PKKPab (denoted as PKKPabdiff) can be used to constrain the P-wave velocities at the lowermost mantle, thus have been investigated in numerous studies. Previous results (Rost and Garnero, 2006) suggest that most of the observations of the PKKPabdiff waves are in the epicentral distance range of 95°–105° (minor arc convention) (PKKPabdiff diffraction length less than 10°). However, high-frequency (∼1 Hz) synthetic seismograms show that the PKKPabdiff waveforms could be observable at distance down to 65°, which indicates that the PKKPabdiff signals could be detected at distances less than 95° in observations. To explore the distance ranges in which PKKPabdiff is observable, we collected global three-component broadband waveforms from 246 events with source depth deeper than 100 km and magnitude above M 6 from 2007 to 2017 available at the Incorporated Research Institutions for Seismology Data Management Center. We analyzed the slowness, polarization, and amplitude of the candidate PKKPabdiff signals, and found 95 events with clear PKKPabdiffsignals, with nearly 60% of the events show PKKPabdiff diffraction lengths greater than 10°, and the longest diffraction distance is beyond 20°. These newly identified PKKPabdiff waves would substantially augment the dataset of core phases for improvements of the CMB velocity models.

Description: In this study, we investigate 70 days of distributed acoustic sensing (DAS) recordings in the Canary Islands using an undersea fiber-optic telecommunication cable that links the islands of Tenerife and Gran Canaria. Two DAS interrogators connected to both ends of the cable turned the fiber into an array of 11,968 strain sensors covering a total length of ∼120 km. We present the details of the experiment, noise analysis, and examples of recorded signals. Seismic ambient noise levels assessment indicates poor local coupling of the cable due to the irregular bathymetry that results in high-amplitude acoustic oscillations in some channels. The DAS array recorded several types of nonseismic (vehicles, surface gravity waves, ships) and seismic signals. Local and regional earthquakes were detected with magnitudes mbLg≥2. Surface waves from teleseismic events at a distance of ∼3000 km were also identified in the strain recordings. Here, we report the first observations with DAS of hydroacoustic T waves generated by oceanic earthquakes located at the Central Mid-Atlantic Ridge and the Cape St. Vincent region. Events had magnitudes from Mw 4.2 to 6.9, and the hydroacoustic waves were recorded at epicentral distances from 780 to 3400 km. Our findings show that submarine fiber-optic cables can effectively be used to assess the seismic activity in remote oceanic areas.

Description: Pulse-like near-source ground motions were observed by the local network during the 2015 Mw 8.3 Illapel, Chile earthquake. Such ground motions can be quite damaging to a wide range of infrastructures. The primary objective of this study is to provide a source model that can explain such ground motions. The isolated nature of the pulses indicated that the rupture of some small isolated region on the fault contributed to the generation of the pulse. Therefore, we considered such regions and termed them as Strong Motion Pulse Generation Areas (SPGAs). We used the corrected empirical Green's function (EGF) method because this method has been successfully applied to near-source pulse-like ground motions in previous studies. We simulated synthetic waveforms using the frequency dependent quality factor Q=239f0.71 and empirical site amplification factors, which we obtained by applying a generalized inversion technique to local weak-motion data. The result indicated that the observed ground motions from the Mw 8.3 Illapel earthquake can readily be explained with a source model that involves two SPGAs with dimensions of several kilometers in spite of the huge rupture zone of the earthquake. The source model can reproduce velocity waveforms, acceleration Fourier amplitude spectra (FAS) and pseudoacceleration response spectra. It also reproduces the duration of strong ground motions quite accurately. No significant bias was found with respect to distance and frequency. In conclusion, the corrected EGF method is a very efficient tool to simulate near-source ground motions of a subduction earthquake when it is combined with higher stress-drop subevents whose sizes are adjusted to the observed pulse widths.

Description: Ancient authors report the destruction and drowning in 373 B.C. in the Gulf of Corinth (Greece) of Helike (Helice and Eliki), an important, nearly coastal town, and of Boura (Bura, Buris, Bouris, and Voura), another town in the hinterland, as a punishment by the ancient God Poseidon because of a serious crime committed in his shrine. This narrative has been regarded as a description of a true event, though with some exaggerations, and the 373 B.C. event is included in earthquake and tsunami catalogs. In the first part of this article, it is shown that (1) local natural hazards exclude the possibility (risk) of total loss of the ancient “polis” (town state) of ancient Helike because of its vulnerability due to its geography. (2) Systematic geoarchaeological studies confirm this prediction because they reveal essentially undisturbed archaeological layers predating and postdating 373 B.C., with no signs of a tsunami. (3) Archaeological excavations have recently brought to light, among other findings, remains of the harbor of Boura and of the shrine of Poseidon at Helike, as well as coins issued by Helike several decades after its alleged loss. This evidence permits a reconsideration of ancient texts related to the loss of Helike in a supervised learning-type approach. It was found that genuine ancient Greek texts do not mention any catastrophe of Helike, but rather that the legend of its total loss appeared several centuries later in Roman times, in local legends, rumors, and forged or manipulated ancient texts (e.g., by pseudo-Aristotle). The ancient legend became important because it explained the collapse of the town state of Helike and it fit ancient religious ideas in a tectonically active region because of the rapid burial (“disappearance”) of ancient Greek remains under sediments in a young delta and because of the prominent location of Helike in the seafaring route between Rome and the eastern Mediterranean. For earthquakes before our era, historical and archaeological data have attracted interpretations… attributing to earthquakes… the demise of flourishing city-states. …The reason for the revival of catastrophe hypotheses is perhaps that they are easy to explain. They are too simple, too obvious and too coincidental and chiefly because they have become fashionable in recent years. (Ambraseys, 2006)

Description: We describe the construction and performance of a new high-precision ground- or platform-rotation sensor called the Quartz Rotation Sensor (QRS). The QRS is a mechanical angular accelerometer that senses rotational torque with an inherently digital, load-sensitive resonant quartz crystal. The noise floor is measured to be ∼45 pico-radians/root (Hz) near 1 Hz, and the resonant period of the sensor is about 10 s, making it a broadband sensor. Among similarly sized broadband rotation sensors, this represents more than two orders of magnitude improvement in noise floor near 0.1 Hz. We present measurements of rotational components of teleseismic waves recorded with the sensor at a vault. The QRS is useful for rotational seismology and for improving low-frequency seismic isolation in demanding applications such as the Laser Interferometer Gravitational-Wave Observatories.

Description: Clustering is a fundamental feature of earthquakes that impacts basic and applied analyses of seismicity. Events included in the existing short-duration instrumental catalogs are concentrated strongly within a very small fraction of the space–time volume, which is highly amplified by activity associated with the largest recorded events. The earthquakes that are included in instrumental catalogs are unlikely to be fully representative of the long-term behavior of regional seismicity. We illustrate this and other aspects of space–time earthquake clustering, and propose a quantitative clustering measure based on the receiver operating characteristic diagram. The proposed approach allows eliminating effects of marginal space and time inhomogeneities related to the geometry of the fault network and regionwide changes in earthquake rates, and quantifying coupled space–time variations that include aftershocks, swarms, and other forms of clusters. The proposed measure is used to quantify and compare earthquake clustering in southern California, western United States, central and eastern United States, Alaska, Japan, and epidemic-type aftershock sequence model results. All examined cases show a high degree of coupled space–time clustering, with the marginal space clustering dominating the marginal time clustering. Declustering earthquake catalogs can help clarify long-term aspects of regional seismicity and increase the signal-to-noise ratio of effects that are subtler than the strong clustering signatures. We illustrate how the high coupled space–time clustering can be decreased or removed using a data-adaptive parsimonious nearest-neighbor declustering approach, and emphasize basic unresolved issues on the proper outcome and quality metrics of declustering. At present, declustering remains an exploratory tool, rather than a rigorous optimization problem, and selecting an appropriate declustering method should depend on the data and problem at hand.

Description: 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.