Regional Seismic Arrays Research Articles

On the retrieval of body waves from ambient noise based on regional seismic arrays

SUMMARY Ambient noise technology can efficiently extract surface wave signals from seismic background noise and has been extensively utilized in imaging lithospheric structures. However, retrieving crustal body wave signals, such as PmP or SmS phases, still poses a challenge. Only a limited number of reports have successfully extracted these regional-scale body wave signals from ambient noise in only a few limited study areas. It remains unclear why these signals are difficult to retrieve from ambient noise data. To investigate the mechanism of recovering body wave signals in noise cross-correlations, we calculate cross-correlation functions at four regions and observe the similarity of the recovered body waves. Through a series of synthetic simulations, we demonstrate that the appearance of body wave signals in noise cross-correlations is closely related to the distribution of noise sources. Among these signals, the post-critical SmS wave proves to be the most readily recoverable from ambient noise data, primarily stemming from distant sources. In contrast, the recovery of P-wave requires the array to be in proximity to the sources. Our experiments also reveal that the main origin of PL waves is the multiple reflections of S-waves propagating in the crust.

Evaluating the effects of velocity models and array configuration on induced seismic event locations in the Permian Basin

In the final months of 2022, two substantial earthquakes with magnitudes exceeding ML 5.0 hit West Texas, USA, causing widespread public concerns about the effect of industrial activities on human and environmental safety. The monitoring of earthquakes in this region is largely based on the data of the public regional seismic array, which consists of stations spaced typically tens of kilometers apart. Accurate hypocenter determination of these induced events is crucial as it provides insight into the triggering mechanism, enabling operators and regulators to develop effective mitigation strategies. However, there are debates regarding the accuracy of the publicly reported induced event data due to discrepancies between public reports of event hypocenters and those determined using local dense arrays operated by private companies. The primary objective of this study is to identify the underlying causes of the discrepancies between results obtained from the public regional array in West Texas and local dense arrays. Through modeling and analysis of field data collected by a local dense array in the Permian Basin, we determine that three critical factors influence the reliability of induced event results. First, the accuracy of the velocity model used for the event location is the most crucial factor. Second, the distance between a station and an event plays a crucial role in determining the sensitivity of the data to the hypocenter depth. Finally, consistency between the observed and modeled wave propagation behavior is crucial for ensuring the validity of the objective function in the inversion. Our findings indicate that it is challenging to obtain reliable hypocenters using a regional sparse array with station spacing on the order of tens of kilometers. The best practice for obtaining an accurate event hypocenter and magnitude is to monitor induced seismicity using a local dense array and process the data with a velocity model that fully characterizes the local basin’s geology.

Seismic Structure of the Antarctic Upper Mantle Imaged with Adjoint Tomography

AbstractThe upper mantle and transition zone beneath Antarctica and the surrounding oceans are among the poorest‐imaged regions of the Earth's interior. Over the last 15 years, several large broadband regional seismic arrays have been deployed, as have new permanent seismic stations. Using data from 297 Antarctic and 26 additional seismic stations south of ~40°S, we image the seismic structure of the upper mantle and transition zone using adjoint tomography. Over the course of 20 iterations, we utilize phase observations from three‐component seismograms containing P, S, Rayleigh, and Love waves, including reflections and overtones, generated by 270 earthquakes that occurred from 2001–2003 and 2007–2016. The new continental‐scale seismic model (ANT‐20) possesses regional‐scale resolution south of 60°S. In East Antarctica, thinner continental lithosphere is found beneath areas of Dronning Maud Land and Enderby‐Kemp Land. A continuous slow wave speed anomaly extends from the Balleny Islands through the western Ross Embayment and delineates areas of Cenozoic extension and volcanism that span both oceanic and continental regions. Slow wave speed anomalies are also imaged beneath Marie Byrd Land and along the Amundsen Sea Coast, extending to the Antarctic Peninsula. These anomalies are confined to the upper 200–250 km of the mantle, except in the vicinity of Marie Byrd Land where they extend into the transition zone and possibly deeper. Finally, slow wave speeds along the Amundsen Sea Coast link to deeper anomalies offshore, suggesting a possible connection with deeper mantle processes.

Seismic and hydroacoustic observations of the 2016–17 Bogoslof eruption

In mid-December 2016, Bogoslof volcano, Alaska, began an 8.5-month-long eruption that produced at least 70 explosions and three lava domes. Bogoslof is an emergent submarine volcano with only the very top forming a small island, so the eruptive activity mostly occurred from a vent submerged beneath a bay or lagoon. The Bogoslof eruption was recorded on regional seismic and infrasound arrays as well as by a hydrophone that was deployed locally during the second half of the eruption. Since few emergent volcanic eruptions have seismo-acoustic recordings, these observations, taken in context with satellite and other observations, provide an opportunity to greatly improve our understanding of the seismo-acoustic signals produced by these eruptions. The instruments detected a range of activity including earthquakes, tremor, and mass flow events. Earthquakes occurred before, during, after, and unassociated with explosions and also often occurred in swarms. The 47 seismically-detected swarms and 27 additional hydroacoustically detected swarms can be further broken into four types: precursory, post-eruptive, general, and tremor-dominated. Most seismic swarms were less than 10 h long, had fewer than 50 earthquakes, and had average earthquake rates below 20/h. For the explosions, we calculate a tremor magnitude, frequency index, and average frequency. The tremor magnitudes determined from the hydrophone data show a roughly linear relation to explosion plume height, though this relation was not clear in the more limited seismic data. Lastly, we categorize the activity into five eruption phases based on seismo-acoustic character—precursory, opening, explosive I, pause, and explosive II.

Relative Relocations of the North Korean Nuclear Tests From 2006 to 2017 Using the Hi‐Net Array in Japan

AbstractPrecisely locating sites of nuclear tests is important for the Comprehensive Nuclear‐Test‐Ban Treaty and structure studies. Here we apply a relative relocation method to a dense regional seismic array, Hi‐net, to determine the relative locations of the sites of the North Korean nuclear tests from 2006 to 2017. Our results show that the sites of the 2013 to 2017 events are closely located together and within 600 m of the site of the 2009 event. The site of the 2006 event is located ~2 km southeast to the site of the 2009 event. A bootstrap resampling of azimuthal subsets of the array adds confidence to the relative locations. Seismic data recorded at regional distance by a dense network with azimuthal coverage as limited as 80° can resolve the relative locations of the nuclear tests to within a few hundred meters for all but the 2006 event. This approach can be extended to locating natural earthquakes as well.

Influence of wind turbines on seismic stations in the upper rhine graben, SW Germany

By analysing long- and short-term seismological measurements at wind farms close to the town of Landau, SW Germany, we present new insights into ground motion signals from wind turbines (WTs) at local seismic stations. Because of their need to be located in similar regions with sparsely anthropogenic activities, wind turbines impact seismic stations and their recordings in a way that is not yet fully understood by researchers. To ensure the undisturbed recording tasks of a regional seismic array or a single station by a protected area around those endangered stations, it is very important to investigate the behavior of WTs as a seismic source. For that reason, we calculate averaged one-hour long spectra of the power spectral density (PSD) before and after the installation of a new wind farm within the investigated area. These PSD are ordered according to the rotation speed. We observe a clear increase of the PSD level after the WT installation in a frequency range of 0.5 to 10 Hz up to a distance of 5.5 km away from the WT. By analysing seismic borehole data, we also observe a decrease of the PSD of wind dependent signals with depth. The impact of wind-dependent signals is found to be much more pronounced for the shallower station (150 m depth) than for the deeper one (305 m depth). Using short-term profile measurements, we fit a power-law decay proportional to 1/r b to the main WT-induced PSD peaks and differentiate between near-field and far-field effects of ground motions. For low frequencies in the range from 1 to 4 Hz, we determine a b value of 0.78 to 0.85 for the far field, which is consistent with surface waves. The b value increases (up to 1.59) with increasing frequencies (up to 5.5 Hz), which is obviously due to attenuating effects like scattering or anelasticity. These results give a better understanding of the seismic wavefield interactions between wind turbines (or wind farms) with nearby seismic stations, including borehole installations, in a sedimentary setting.

Automated determination of magnitude and source length of large earthquakes using backprojection andPwave amplitudes

AbstractFast estimates of magnitude and source extent of large earthquakes are fundamental for disaster mitigation. However, resolving these estimates within 10–20 min after origin time remains challenging. Here we propose a robust algorithm to resolve magnitude and source length of large earthquakes using seismic data recorded by regional arrays and global stations. We estimate source length and source duration by backprojecting seismic array data. Then the source duration and the maximum amplitude of the teleseismicPwave displacement waveforms are used jointly to estimate magnitude. We apply this method to 74 shallow earthquakes that occurred within epicentral distances of 30–85° to Hi‐net (2004–2014). The estimated magnitudes are similar to moment magnitudes estimated from W‐phase inversions (U.S. Geological Survey), with standard deviations of 0.14–0.19 depending on the global station distributions. Application of this method to multiple regional seismic arrays could benefit tsunami warning systems and emergency response to large global earthquakes.

3D imaging of the Earth's lithosphere using noise from ocean waves

The lithosphere is the rigid outer shell of the Earth, composed of the crust and the rigid uppermost mantle. The lateral variation of lithosphere is believed to be closely related to the foci of intraplate earthquakes and volcanism, the location of large ore deposits and diamond-bearing kimberlites, and the formation of oil/gas-bearing sedimentary basins. Understanding the fine-scale structure of the lithosphere is therefore one of the most fundamental tasks in Earth sciences, and has important implications for society in terms of economic prosperity and hazard mitigation. Seismic tomography is the main technique available to image the subsurface structure of the Earth across a range of scales. Conventional seismic tomography exploits the seismic waves emitted by earthquakes. However, because most large earthquakes occur at plate boundaries and most continents, including Australia, are not seismically active, earthquake-based tomography suffers from uneven distributions of earthquakes and has difficulties in deciphering fine-scale structures of the Earth in seismically quiet continents. In the past decade, the advent of ambient noise tomography (ANT) has revolutionized seismic tomography because it can overcome the limitations of conventional earthquake surface wave tomography. This technique uses diffuse background seismic energy, mostly comes from the interaction of ocean waves with the crust. Empirical Green’s functions of surface waves passing between a pair of stations are extracted by cross-correlating continuous time series of ambient noise. Within a regional seismic array, all inter-station measurements of surface-wave dispersion can be measured and tomography can be performed to image the underling lithospheric structure. In this study, we demonstrate the applications of broadband ANT in mapping fine-scale lithosphere structures around the world using continuous seismic data.

Multiarray rupture imaging of the devastating 2015 Gorkha, Nepal, earthquake sequence

AbstractA rapid, robust multiarray backprojection method was applied to image the rupture pattern of the 2015 Gorkha, Nepal Mw7.8 main shock and its Mw7.3 aftershock. Backprojected teleseismic P wave trains from three regional seismic arrays in Europe, Australia, and Alaska show that both earthquakes ruptured unilaterally and primarily eastward, with rupture speeds potentially decreasing with depth. The rupture of the main shock first extended ESEward at ∼3.5 km/s over ∼120 km, with later rupture propagation further downdip on the eastern segment at ∼2.1 km/s. The aftershock ruptured the fault SE of the main shock's ruptured plane. It began to rupture updipward for ∼20 km at a speed around 1.2 km/s, then it may have accelerated to 3.5 km/s for the next 50 km. The apparent depth‐dependent rupture speeds of the two earthquakes may be caused by along‐dip heterogeneities in fault strength, with a higher stress concentration on the updip part of the Nepalese Main Himalayan Thrust.

Seismic structure in southern Peru: evidence for a smooth contortion between flat and normal subduction of the Nazca Plate

Rapid changes in slab geometry are typically associated with fragmentation of the subducted plate; however, continuous curvature of the slab is also possible. The transition from flat to normal subduction in southern Peru is one such geometrical change. The morphology of the subducted Nazca Plate along this transition is explored using intraslab earthquakes recorded by temporary regional seismic arrays. Observations of a gradual increase in slab dip coupled with a lack of any gaps or vertical offsets in the intraslab seismicity suggest warping of the slab. Concentrations of focal mechanisms at orientations which are indicative of slab bending are also observed along the change in slab geometry. The presence of a thin ultra-slow velocity layer (USL) atop the horizontal Nazca slab is identified and located. The lateral extent of this USL is coincident with the margin of the projected linear continuation of the subducting Nazca Ridge, implying a causal relationship wherein increased hydration of the ridge results in the formation of the USL downdip. Waveform modelling of the 2-D structure in southern Peru using a finite-difference algorithm provides constraints on the velocity and geometry of the slab's seismic structure and confirms the absence of any tears in the slab. The seismicity and structural evidence suggests smooth contortion of the Nazca Plate along the transition from flat to normal subduction. The slab is estimated to have experienced 10 per cent strain in the along-strike direction across this transition.

Seismicity and structure in central Mexico: Evidence for a possible slab tear in the South Cocos plate

AbstractThe morphology of the transition from flat to normal subduction in eastern central Mexico is explored using intraslab earthquakes recorded by temporary and permanent regional seismic arrays. Observations of a sharp transition in slab dip near the abrupt end of the Trans‐Mexican Volcanic Belt (TMVB) suggest a possible slab tear located within the subducted South Cocos plate. The eastern lateral extent of a thin ultra‐slow velocity layer (USL) imaged atop the Cocos slab in recent studies along the Meso America Subduction Experiment array is examined here using additional data. We find an end to this USL which is coincident with the western boundary of a zone of decreased seismicity and the end of the TMVB near the sharp transition in slab dip. Waveform modeling of the 2‐D structure in this region using a finite difference algorithm provides constraints on the velocity and geometry of the slab's seismic structure and confirms the location of the USL. Analysis of intraslab seismicity patterns reveals clustering, sudden increase in depth, variable focal mechanism orientations and faulting types, and alignment of source mechanisms along the sharp transition in slab dip. The seismicity and structural evidence suggests a possible tear in the South Cocos slab. This potential tear, together with the tear along the Orozco Fracture Zone to the northwest, indicates a slab rollback mechanism in which separate slab segments move independently, allowing for mantle flow between the segments.

Landslides in Eastern Honshu Induced by the 2011 Tohoku Earthquake

Research Article| May 01, 2013 Landslides in Eastern Honshu Induced by the 2011 Tohoku Earthquake Joseph Wartman; Joseph Wartman Department of Civil and Environmental Engineering, University of Washington, 201 More Hall, Box 352700, Seattle, Washington 98195‐2700wartman@uw.eduladunham49@gmail.com Search for other works by this author on: GSW Google Scholar Lisa Dunham; Lisa Dunham Department of Civil and Environmental Engineering, University of Washington, 201 More Hall, Box 352700, Seattle, Washington 98195‐2700wartman@uw.eduladunham49@gmail.com Search for other works by this author on: GSW Google Scholar Binod Tiwari; Binod Tiwari Department of Civil and Environmental Engineering, California State University, Fullerton, 800 N State College Blvd., E‐419, Fullerton, California 92834btiwari@fullerton.edu Search for other works by this author on: GSW Google Scholar Daniel Pradel Daniel Pradel University of California, Los Angeles, Civil and Environmental Engineering Department, 5732 Boelter Hall, Box 951593, Los Angeles, California 90095‐1593dpradel@ucla.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Joseph Wartman Department of Civil and Environmental Engineering, University of Washington, 201 More Hall, Box 352700, Seattle, Washington 98195‐2700wartman@uw.eduladunham49@gmail.com Lisa Dunham Department of Civil and Environmental Engineering, University of Washington, 201 More Hall, Box 352700, Seattle, Washington 98195‐2700wartman@uw.eduladunham49@gmail.com Binod Tiwari Department of Civil and Environmental Engineering, California State University, Fullerton, 800 N State College Blvd., E‐419, Fullerton, California 92834btiwari@fullerton.edu Daniel Pradel University of California, Los Angeles, Civil and Environmental Engineering Department, 5732 Boelter Hall, Box 951593, Los Angeles, California 90095‐1593dpradel@ucla.edu Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2013) 103 (2B): 1503–1521. https://doi.org/10.1785/0120120128 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joseph Wartman, Lisa Dunham, Binod Tiwari, Daniel Pradel; Landslides in Eastern Honshu Induced by the 2011 Tohoku Earthquake. Bulletin of the Seismological Society of America 2013;; 103 (2B): 1503–1521. doi: https://doi.org/10.1785/0120120128 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract In this article we discuss the character and spatial pattern of coseismic landslides from the eastern Honshu region of Japan, which was strongly shaken in the 2011 Tohoku earthquake. We developed a detailed geospatial database of 3477 landslides based on postearthquake field surveys and examination of high‐resolution satellite imagery across a 28,380 km2 landslide study area. Analysis of the database shows that a substantial majority (80%) of landslides occurred in Quaternary soil and Neogene rock units. Despite their abundance in the study area, relatively few landslides occurred in pre‐Neogene rocks (i.e., older than 23 Ma). Further examination of the data showed that the most common types of landslides were (1) disrupted landslides in Neogene sedimentary rocks and (2) lateral spreading in Quaternary sediments. However, we found that coseismic landslide erosion (i.e., debris mobilization) was almost fully dominated by lateral spreading within Quaternary sediments. When comparing the landslide inventory with ground motions recorded by dense regional seismic arrays, we found no statistically significant correlation between landslide intensity and ground motion within the study area.Online Material: Derivation of empirical parameters used in the area to volume transformations for lateral spreads. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

High‐resolution backprojection at regional distance: Application to the HaitiM7.0 earthquake and comparisons with finite source studies

A catastrophicMw7 earthquake ruptured on 12 January 2010 on a complex fault system near Port‐au‐Prince, Haiti. Offshore rupture is suggested by aftershock locations and marine geophysics studies, but its extent remains difficult to define using geodetic and teleseismic observations. Here we perform the multitaper multiple signal classification (MUSIC) analysis, a high‐resolution array technique, at regional distance with recordings from the Venezuela National Seismic Network to resolve high‐frequency (about 0.4 Hz) aspects of the earthquake process. Our results indicate westward rupture with two subevents, roughly 35 km apart. In comparison, a lower‐frequency finite source inversion with fault geometry based on new geologic and aftershock data shows two slip patches with centroids 21 km apart. Apparent source time functions from USArray further constrain the intersubevent time delay, implying a rupture speed of 3.3 km/s. The tips of the slip zones coincide with subevents imaged by backprojections. The different subevent locations found by backprojection and source inversion suggest spatial complementarity between high‐ and low‐frequency source radiation consistent with high‐frequency radiation originating from rupture arrest phases at the edges of main slip areas. The centroid moment tensor (CMT) solution and a geodetic‐only inversion have similar moment, indicating most of the moment released is captured by geodetic observations and no additional rupture is required beyond where it is imaged in our preferred model. Our results demonstrate the contribution of backprojections of regional seismic array data for earthquakes down toM≈ 7, especially when incomplete coverage of seismic and geodetic data implies large uncertainties in source inversions.

A Regional Seismic Array of Three-Component Stations in Central Saudi Arabia

Seismic arrays are used to enhance signals and suppress noise as well as to estimate the slowness and azimuth of signals. Seismic monitoring traditionally has relied upon seismic arrays to reduce detection (magnitude) thresholds by increasing signal-to-noise ratios (SNR) and to contribute to event formation by improving phase identification and association. A seismic array differs from a local network of seismic stations mainly by the techniques used for data analysis. In principle, a network of seismic stations can be used as an array, and data from an array can be analyzed the same as from a network. However, most array processing techniques require high signal coherency across the array, and this puts important constraints on the array geometry, spatial extent, and data quality. Furthermore, proper analysis of array data is dependent on a stable, highly precise relative timing of all array elements. This is required because the measurement of (usually very small) time differences in the arrival of seismic signals between the different sensors plays an important role in all array processing techniques. The Department of Geology of King Saud University (KSU) has established a small-aperture (∼3.5 km) three-component seismic array in central Saudi Arabia at a hard rock site on the Arabian shield (Figure 1A). The facility consists of well-constructed vaults in hard rock outcrops in a region that has very low background noise. The deployed equipment consists of a broadband three-component sensor (STS-2) at the center element and eight short-period three-component sensors at the outlying elements (Figure 1B). The objectives of this deployment are to provide data for research into three-component seismic array processing and structural studies and are not related to any real-time monitoring efforts ( e.g. , the International Monitoring System). Specifically, the array project is intended to improve constraints on the structure and physical state of …

Estimation of rupture processes of the 2008 Wenchuan Earthquake from joint analyses of two regional seismic arrays

We estimated the source rupture of the 2008 Wenchuan Earthquake (Ms 8), China, based on a back-projection of seismic waves to their source plane, using data from regional broadband arrays in Taiwan and northern Vietnam. Observations from these arrays, located at different azimuths, were processed to evaluate the spatio-temporal rupture behavior of the fault. The seismic energy spot converted from windowed array waveforms was back-projected to the rupture plane of the earthquake, to image the instantaneous slip on the fault plane. Rupture processes were reconstructed based on the imaged time-dependent seismic energy radiating from the earthquake fault plane. The high station density of both arrays enabled a detailed examination of the rupture processes. The results indicate that the 2008 Wenchuan Earthquake had a rupture duration of approximately 100s, with major asperities radiating energy at 50s after the initiation of rupture. The location of asperities is in good agreement with sections of the earthquake fault along which major damage occurred, such as along the Yingxiu–Beichuan Fault. The radiated seismic energy shows a complex spatial distribution on the fault plane. Our analysis indicates that the rupture initiated at the epicenter and extended to the northeast for approximately 280km. The average rupture velocity of this earthquake was approximately 2.8km/s.

Wave separation for the great Sumatra-Andaman earthquake with regional seismic array

We analyzed the seismic waveforms from the December 26, 2004 Sumatra-Andaman earthquake recorded at broadband seismic stations in western Europe. Previous studies involving of the beam-forming technique and high frequency analysis suggest that the earthquake ruptured with a duration of around 500 s. This very long duration makes P wave overlap with later arrivals such as PP wave, which follows P in about 200 s. Since P waves are crucial for modeling earthquake processes, we propose an iterative method to separate P and PP waveforms. The separated P waveform confirms a second large energy release around 300 s after the initial rupture. The iterative signal separation technique is particularly useful for mixed signals that are not independent and the number of recording stations far exceeds number of mixed signal sources.

Preface to the Special Issue on “Arrays and Array Methods in Global Seismology”

The increased availability and fidelity of broadband seismic instruments have played a key role in bridging the gap between exploration and global seismic applications. Long gone are the days when global structure mapping and source characterization/monitoring relied on the manual digitization of WWSSN (World-Wide Standard Seismographic Network) analogue records or hours in front of microfiche machines. Even the GSN (Global Seismographic Network), a milestone of the digital era, now pales in comparison with the scope and density of regional seismic arrays that dominate the global data stream. Today, the nominal resolution estimated from the ratio between wavelength (approximately tens to hundreds of kilometers) and target dimension (often of continental scale) in properly designed global experiments could rival the typical resolution of a controlled source seismic survey. Vastly improved data coverage, highlighted by multi-disciplinary initiatives such as the USArray project, has overcome many conceptual (as well as practical) barriers between exploration and global seismic applications. A direct consequence is that the ‘‘global community’’ can now take full advantage of important array methods that are predicated upon superior data density and distribution. In fact, it could legitimately be argued that exploration seismology is fast becoming a realistic, scaled-down model for global seismic surveys. This Special Issue reviews the assumptions, algorithms and prospects of several important methodologies in today’s global and regional seismic surveys. A key objective is to highlight developments and novel applications that improve our ability to determine seismic structures across all spatial scales. We mainly emphasize those methodologies and applications that are relevant to structure imaging (note: source characterization may be detailed in a future Special Issue). Apart from the obvious scale differences between exploration (\20 km) and global (typically [ several 100 km) problems, we demonstrate that key objectives such as signal isolation, correlation, reconstruction and noise reduction

Moment release rate of Cascadia tremor constrained by GPS

A comparison of GPS and seismic analyses of 23 distinct episodic tremor and slip events, located throughout the Cascadia subduction zone over an 11‐year period, yields a highly linear relationship between moment release, as estimated from GPS, and total duration of nonvolcanic tremor, as summed from regional seismic arrays. The events last 1–5 weeks, typically produce ∼5 mm of static forearc deformation, and show cumulative totals of tremor that range from 40 to 280 h. Moment released by each event is estimated by inverting GPS‐measured deformation, which is sensitive to all rates of tremor‐synchronous faulting, including aseismic creep, for total slip along the North American‐Juan de Fuca plate interface. Tremor, which is shown to be largely invariant in amplitude and frequency content both between events and with respect to its duration, is quantified using several different parameterizations that agree to within 10%. All known Cascadia events detected since 1997, which collectively span the Cascadia arc from northern California to Vancouver Island, Canada, release moment during tremor at a rate of 5.2 ± 0.4 × 1016 N m per hour of recorded tremor. This relationship enables estimation of moment dissipation, via seismic monitoring of tremor, along the deeper Cascadia subduction zone that poses the greatest threat to its major metropolitan centers.

Imaging the solid Earth with seismic noise

The correlation of ambient vibrations of the Earth yields the impulse response between two passive sensors as if one was a source. This provides the opportunity to image the Earth beneath regional seismic arrays. In this paper, we present imaging of the lithosphere in Europe based on surface Rayleigh waves extracted from continuous record of the seismic noise. We also recall a brief history of passive imaging in seismology. The link between correlations and the green function is investigated from a mathematical point of view, and through laboratory experiments.

The structure of the base of the outer core inferred from seismic waves diffracted around the inner core

We systematically searched for seismograms of waves diffracted around the inner core (PKPCdiff) from all the temporary seismic arrays with data currently available at the IRIS DMC, as well as some permanent regional seismic arrays including F‐NET in Japan and GRF in Germany, to assemble the largest high‐quality PKPCdiff database ever created. PKPCdiff waves preferentially sample the base of the outer core and so contain important clues about Earth structure in this region. We measured PKPDF–PKPCdiff differential traveltimes and PKPCdiff/PKPDF amplitude ratios in the distance range of 154°–160° and modeled the observations using grid searches and full wave theory synthetic seismograms. The optimum model found by fitting the differential traveltimes has relatively low velocity at the base of the outer core as in AK135, which is consistent with many previous traveltime studies. However, the optimum model found by fitting the amplitude ratios (PKPCdiff/PKPDF) does not exhibit this feature, and instead is closer to PREM. The discrepancy may be explained by two likely causes. One is that small‐scale topography or roughness on the ICB tends to scatter energy away from PKPCdiff waves by generating trailing coda waves. The other is that there exists a thin layer with relatively low Q at the base of the outer core. This might be expected if there are suspended solid particles at the base of the outer core, as proposed decades ago. Both mechanisms could generate smaller PKPCdiff amplitudes without significantly affecting PKPCdiff traveltimes.