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Accurate regional seismic travel-time (RSTT) predictions rely on regional phases (e.g., Pg, Lg, Pn, Sn) to account for 3D effects in the crust and upper mantle that are not captured by 1D models traditionally used for real-time location. The RSTT prediction model accounts for regional-scale crust and upper mantle structure globally by incorporating regional seismic phases into its travel-time calculations. Previous versions of the RSTT model have used a constant grid cell size of 1°. To improve the tomographic accuracy of recovering velocity structure at regional scales, we perform data-driven grid refinement on the RSTT model down to a 0.125° grid (∼14 km) in pursuit of two main goals: (1) to test the limits of RSTT capability and accuracy of determined velocity structure through variable grid refinement and (2) to image smaller structures in Israel and the Middle East and illuminate upper mantle dynamics operating in this complex tectonic area. We investigate the effects of model parameterization as grid cell size decreases and the trade-offs between recovered velocity structures. Our final dataset includes 4751 events and 499 stations that recorded 79,344 Pn and 7489 Pg. The variable grid refinement method allows recovery of finer-scale velocity structures and reduces travel-time residuals in areas with the highest data coverage. At smaller grid cell sizes, longer paths need to be upweighted to stabilize the inversion. Results illuminate tectonic features undefined in coarser grid-size models; in particular, we observe mantle perturbations related to the subduction zone around the Cyprian arc and crustal anomalies near the Dead Sea fault and throughout the Anatolian plate.

An eruption of the Soufrière Hills Volcano (SHV) on the eastern Caribbean island of Montserrat began on 18 July 1995 and continued until February 2010. Within nine days of the eruption onset, an existing four-station analog seismic network (ASN) was expanded to 10 sites. Telemetered data from this network were recorded, processed, and archived locally using a system developed by scientists from the U.S. Geological Survey (USGS) Volcano Disaster Assistance Program (VDAP). In October 1996, a digital seismic network (DSN) was deployed with the ability to capture larger amplitude signals across a broader frequency range. These two networks operated in parallel until December 2004, with separate telemetry and acquisition systems (analysis systems were merged in March 2001). Although the DSN provided better quality data for research, the ASN featured superior real-time monitoring tools and captured valuable data including the only seismic data from the first 15 months of the eruption. These successes of the ASN have been rather overlooked. This article documents the evolution of the ASN, the VDAP system, the original data captured, and the recovery and conversion of more than 230,000 seismic events from legacy SUDS, Hypo71, and Seislog formats into Seisan database with waveform data in miniSEED format. No digital catalog existed for these events, but students at the University of South Florida have classified two-thirds of the 40,000 events that were captured between July 1995 and October 1996. Locations and magnitudes were recovered for ∼10,000 of these events. Real-time seismic amplitude measurement, seismic spectral amplitude measurement, and tiltmeter data were also captured. The result is that the ASN seismic dataset is now more discoverable, accessible, and reusable, in accordance with FAIR data principles. These efforts could catalyze new research on the 1995–2010 SHV eruption. Furthermore, many observatories have data in these same legacy data formats and might benefit from procedures and codes documented here.

The Joint Task Force, Science Monitoring And Reliable Telecommunications (SMART) Subsea Cables is working to integrate environmental sensors (temperature, pressure, seismic acceleration) into submarine telecommunications cables. This will support climate and ocean observation, sea-level monitoring, observations of Earth structure, tsunami and earthquake early warning, and disaster risk reduction. Recent advances include regional SMART pilot systems that are the initial steps to trans-ocean and global implementation. Building on the OceanObs'19conference and community white paper (https://doi.org/10.3389/fmars.2019.00424), this paper presents an overview of the initiative and a description of ongoing projects including: InSea wet demonstration project off Sicily; Vanuatu and New Caledonia; Indonesia; CAM-2 ring system connecting the Portuguese mainland, Azores, and Madeira; New Zealand; and Antarctica. In addition to the diverse scientific and societal benefits, the telecommunications industry's mission of societal connectivity will also benefit because environmental awareness improves both individual cable system integrity and the resilience of the overall global communications network.

Carrier screening, a well-established clinical initiative, has been slow to take advantage of the new possibilities offered by high-throughput next generation sequencing technologies. There is evidence of significant benefit in expanding carrier screening to include multiple autosomal recessive conditions and offering a ‘universal’ carrier screen that could be used for a pan-ethnic population. However, the challenges of implementing such a programme and the difficulties of demonstrating efficacy worthy of public health investment are significant barriers. In order for such a programme to be successful, it would need to be applicable and acceptable to the population, which may be ethnically and culturally diverse. There are significant practical and ethical implications associated with determining which variants, genes and conditions to include whilst maintaining adequate sensitivity and accuracy. Although preconception screening would maximise the potential benefits from universal carrier screening, the resource implications of different modes of delivery need to be carefully evaluated and balanced against maximising reproductive autonomy and ensuring equity of access. Currently, although a number of existing initiatives are increasing access to carrier screening, there is insufficient evidence to inform the development of a publicly funded, expanded, universal carrier screening programme that would justify investment over other healthcare interventions.

SUMMARYEstimating subsurface density is important for imaging various geologic structures such as volcanic edifices, reservoirs and aquifers. Muon tomography has recently been used to complement traditional gravity measurements as a powerful method for probing shallow subsurface density structure beneath volcanoes. Gravity and muon data have markedly different spatial sensitivities and, as a result, the combination is useful for imaging structures on spatial scales that are larger than the area encompassed by crossing muon trajectories. Here we explore and test a joint inversion of gravity and muon data in a study area where there is an independently characterized target anomaly: a regionally extensive, high-density layer beneath Los Alamos, New Mexico, USA. We resolve the nearly flat-lying structure using a unique experimental set-up wherein surface and subsurface gravity and muon measurements are obtained above and below the target volume. Our results show that with minimal geologic (prior) constraints, the joint inversion correctly recovers salient features of the expected density structure. The results of our study illustrate the potential of combining surface and subsurface (e.g. borehole) gravity and muon measurements to invert for shallow geologic structures.

Imaging shallow subsurface density structure is an important goal in a variety of applications, from hydrogeology to seismic and volcanic hazard assessment. We assess the effectiveness of surface and subsurface gravity measurements in estimating the density structure of a well-characterized rock volume: the mesa (a small, flat-topped plateau) upon which the town of Los Alamos, New Mexico, USA is located. Our gravity measurements were made on the mesa surface above a horizontal tunnel and underground, within the tunnel. We demonstrate that, in the absence of other geophysical data such as seismic data or muon attenuation, subsurface (tunnel) gravity measurements are critical to accurately recovering geologic structure. Without the tunnel data, our resolution is limited to roughly the surface gravity station spacing, but by including the tunnel data we can resolve structure to a depth of ~ 10 times the surface gravity station spacing. Densities were obtained using both forward modeling and a Bayesian inverse modeling approach, incorporating relevant constraints from geologic observations. We find that Bayesian inversion, with geologically relevant prior, is a superior approach to the forward models in terms of both robustness and efficiency and correctly predicts the orientation and elevation of important geologic features.

Muons can be used to image the density of materials through which they pass, including geological structures. Subsurface applications of the technology include tracking fluid migration during injection or production, with increasing concern regarding such timely issues as induced seismicity or chemical leakage into aquifers. Current density monitoring options include gravimetric data collection and active or passive seismic surveys. One alternative, or complement, to these methods is the development of a muon detector that is sufficiently compact and robust for deployment in a borehole. Such a muon detector can enable imaging of density structure to monitor small changes in density – a proxy for fluid migration – at depths up to 1500 m. Such a detector has been developed, and Monte Carlo modeling methods applied to simulate the anticipated detector response. Testing and measurements using a prototype detector in the laboratory and shallow underground laboratory demonstrated robust response. A satisfactory comparison with a large drift tube-based muon detector is also presented.

We present an underground cosmic ray muon tomographic experiment imaging 3D density of overburden, part of a joint study with differential gravity. Muon data were acquired at four locations within a tunnel beneath Los Alamos, New Mexico, and used in a 3D tomographic inversion to recover the spatial variation in the overlying rock-air interface, and compared with a priori knowledge of the topography. Densities obtained exhibit good agreement with preliminary results of the gravity modeling, which will be presented elsewhere, and are compatible with values reported in the literature. The modeled rock-air interface matches that obtained from LIDAR within 4 m, our resolution, over much of the model volume. This experiment demonstrates the power of cosmic ray muons to image shallow geological targets using underground detectors, whose development as borehole devices will be an important new direction of passive geophysical imaging., 17 pages, 8 figures

Between May and June 2006, an earthquake swarm occurred near Volcan Tancitaro in Mexico, which was recorded by a temporary seismic deployment known as the MARS network. We located ∼1000 events from this seismic swarm. Previous earthquake swarms in the area were reported in the years 1997, 1999 and 2000. We relocate and analyze the evolution and properties of the 2006 earthquake swarm, employing a waveform cross-correlation-based phase repicking technique. Hypocenters from 911 events were located and divided into eighteen families having a correlation coefficient at or above 0.75. 90% of the earthquakes provide at least sixteen phase picks. We used the single-event location code Hypo71 and the P-wave velocity model used by the Jalisco Seismic and Accelerometer Network to improve hypocenters based on the correlation-adjusted phase arrival times. We relocated 121 earthquakes, which show clearly two clusters, between 9–10 km and 3–4 km depth respectively. The average location error estimates are

The internal structure of the Moon has been investigated over many years using a variety of seismic methods, such as travel time analysis, receiver functions, and tomography. Here we propose to apply body-wave seismic interferometry to deep moonquakes in order to retrieve zero-offset reflection responses (and thus images) beneath the Apollo stations on the nearside of the Moon from virtual sources colocated with the stations. This method is called deep-moonquake seismic interferometry (DMSI). Our results show a laterally coherent acoustic boundary around 50 km depth beneath all four Apollo stations. We interpret this boundary as the lunar seismic Moho. This depth agrees with Japan Aerospace Exploration Agency's (JAXA) SELenological and Engineering Explorer (SELENE) result and previous travel time analysis at the Apollo 12/14 sites. The deeper part of the image we obtain from DMSI shows laterally incoherent structures. Such lateral inhomogeneity we interpret as representing a zone characterized by strong scattering and constant apparent seismic velocity at our resolution scale (0.2–2.0 Hz).

Imaging subsurface rock formations or geological objects like oil and gas reservoirs, mineral deposits, cavities or even magmatic plumbing systems under active volcanoes has been for many years a major quest of geoscientists. Since these subsurface objects cannot be observed directly, different indirect methods have been developed. These methods are all based on variations of certain physical properties of the subsurface materials that can be detected from the ground surface or from boreholes. To determine the density distribution, a new imaging technique using cosmic-ray muon detectors deployed in a borehole has been developed and a first prototype of a borehole muon detector successfully tested. In addition to providing a static image of the subsurface density in three dimensions (or three-dimensional tomography), borehole muography can also inform on the variations of density with time, which recently became of major importance with the injection of large volumes of fluids, mainly water and CO 2 , in porous subsurface reservoirs (e.g. aquifer storage and recovery, wastewater disposal, enhanced oil recovery and carbon sequestration). This raises several concerns about the risk of leakage and the mechanical integrity of the reservoirs. Determining the field scale induced displacement of fluids by geophysical methods like muography is thus a priority. This article is part of the Theo Murphy meeting issue ‘Cosmic-ray muography'.

We present analyses of the 2D seismic structure beneath Source Physics Experiments (SPE) geophone lines that extended radially at 100 m spacing from 100 to 2000 m from the source borehole. With seismic sources at only one end of the geophone lines, standard refraction profiling methods cannot resolve seismic velocity structures unambiguously. In previous work, we demonstrated overall agreement between body‐wave refraction modeling and Rg dispersion curves for the least complex of the five lines. A more detailed inspection supports a 2D reinterpretation of the structure. We obtained Rg phase velocity measurements in both the time and frequency domains, then used iterative adjustment of the initial 1D body‐wave model to predict Rg dispersion curves to fit the observed values. Our method applied to the most topographically severe of the geophone lines is supplemented with a 2D ray‐tracing approach, whose application to P ‐wave arrivals supports the Rg analysis. In addition, midline sources will allow us to refine our characterization in future work.

We investigate the merits of the more recently developed finite-frequency approach to tomography against the more traditional and approximate ray theoretical approach for state of the art seismic models developed for western North America. To this end, we employ the spectral element method to assess the agreement between observations on real data and measurements made on synthetic seismograms predicted by the models under consideration. We check for phase delay agreement as well as waveform cross-correlation values. Based on statistical analyses on S wave phase delay measurements, finite frequency shows an improvement over ray theory. Random sampling using cross-correlation values identifies regions where synthetic seismograms computed with ray theory and finite-frequency models differ the most. Our study suggests that finite-frequency approaches to seismic imaging exhibit measurable improvement for pronounced low-velocity anomalies such as mantle plumes.

Online Material: Table of station parameters; figures of mean acceleration power differences, interpolated noise maps. Seismographic coverage of Antarctica prior to 2007 consisted overwhelmingly of a handful of long running and sporadically deployed transient stations, many of which were principally collocated with scientific research stations. Despite very cold temperatures, sunless winters, challenging logistics, and extreme storms, recent developments in polar instrumentation driven by new scientific objectives have opened up the entirety of Antarctica to year‐round and continuous seismological observation (e.g., Nyblade et al. , 2012). Motivations for these recent studies include improved understanding of seismogenic, volcanic, tectonic and glaciological processes, heat flow, dynamic glaciological/ocean interactions, and mantle viscosity. Such studies contribute generally to improvements in understanding the geophysical, geological, and glaciological history of the continent and how these processes interact with the past and present state of the glaciological and climate system (e.g., Winberry et al. , 2009; Hansen et al. , 2010; West et al. , 2010; Winberry et al. , 2011; Heeszel et al. , 2013; Lough et al. , 2013; Chaput et al. , 2014; Accardo et al. , 2014), including processes relevant to glacial isostatic adjustment and sea level rise (Intergovernmental Panel on Climate Change [IPCC] Report, 2007). In addition, microseisms arising from ocean wave activity contain useful climate proxy information on the state and variability of the relatively poorly sensed southern oceans (Aster et al. , 2008; Stutzmann et al. , 2009; Aster et al. , 2010), and such observations are sensitive to sea ice concentration and areal coverage in the polar regions (Grob et al. , 2011; Tsai and McNamara, 2011; Koch et al. , 2013). This characterization of the seismic noise environment of Antarctica, documentation of instrument performance, and comparisons of installation conditions (e.g., ice vaults vs. rock sites) is intended to facilitate optimization …

The Data Management Problem Research in seismology commonly involves three main tasks: data acquisition, data management, and data analysis. Acquisition has been greatly advanced by institutions such as the Incorporated Research Institutions for Seismology (IRIS) and the Network of Research Infrastructures for European Seismology (NERIES), accelerated by facilities such as the USArray Transportable Array (TA), and facilitated by tools such as Standing Order for Data (Owens et al. , 2004) and ObsPyLoad (Scheingraber et al. , 2013). The availability of abundant broadband data has fueled novel analysis tools (Lou et al. , 2013), techniques (Levander and Nolet, 2005), and data products (Trabant et al. , 2012) previously not possible with earlier limited datasets. It has also highlighted, however, the long‐standing data management problem for many researchers in seismology. In the face of the data flood, traditional workflows, such as a combination of shell scripts, text files, miniSEED or Seismic Analysis Code (SAC) files, and C, Fortran, or MATLAB analysis codes, can quickly become untenable. This is partly because, for each ad hoc storage scheme, the researcher must hand code what is essentially a database query/command in order to select (meta)data or to store measurements. This makes obtaining basic information such as event times, station availability, and phase arrival times burdensome and error prone. In this paper, we present Pisces, a data management library that utilizes databases, leverages existing free and open‐software tools and standards, and frees the user to both manage and analyze data with a single easy‐to‐use language, Python. ### Relational Databases Relational databases have long existed to help with the data management problem. Versions of the Center for Seismic Studies (CSS 3.0) seismic database schema (Anderson et al. , 1990), a set of standard tables holding seismological data, have been adopted by United States national laboratories, national and international data centers, and in some commonly used software, such as the …

No conclusive evidence has been presented to date for tectonic tremor (TT) in the vicinity of central Chile, where the Nazca Plate is subducting beneath the South American Plate. Subduction in our experimental location (roughly 35.5° S, 70.5° W) is steep and fairly unobstructed compared to the flattened and more seismogenic behavior to the north. We seek to identify TT in our experimental area, whose geodynamics are comparable to tremor-rich subduction zones such as Cascadia and the Nankai Trough. Our method combines time-series visual inspection, frequency-spectrum analysis, waveform cross-correlation, and 3-component (3C) signal covariance to explore the presence of TT in this region. We have identified TT using stations in central Chile and the Malargue region, Argentina. The TT exhibits similar features to other TT observations worldwide. Waveform characteristics for the TT in our study, particularly dimension of the 3C signal covariance, vary as a function of apparent source location. The duration of one episode of identified TT was about 10 h, which may indicate that the plate interface where tremor generates is strongly coupled. We conclude that our observations reflect features of the local propagation, rather than the tremor source itself.

We analyze event archives and continuous waveform data recorded by the Cooperative New Madrid Seismic Network from 1995 to 2008 in conjunction with waveform cross‐correlation techniques to investigate the spatiotemporal distribution of small‐magnitude ( M D

Seismicity in the Himalaya indicates that relatively deep earthquakes (focal depth 40∼100 km) occur in specific regions beneath the High Himalaya and Nepal. This study focuses on these specific regions to estimate the detailed velocity structure of the lower crust and upper mantle. We selected 202 earthquakes from the Himalaya Nepal Tibet Seismic Experiment (HIMNT) and relocated these earthquakes accurately by applying a genetic algorithm locator, GA-MHYPO (Kim et al., 2006). The detailed onedimensional P-velocity structure is estimated based on travel-time inversion using hypocentral parameters determined by GA-MHYPO. Computational results show two velocity anomalies exhibiting upper-mantle velocities at depth of about 45–50 and 60 km, respectively, beneath the High Himalayan region, whereas a single Moho exists at about 55 km depth beneath Nepal. To validate the stability of these results, a bootstrapping method was used for the inversion.

SUMMARY In this paper, we present inversion results for regional Pg and Sg phases for the Eurasian continent to explore its use for understanding upper crustal velocity structure. Tomographic inversion of traveltimes from first arriving compressional and shear waves for velocity structure has been applied with great success at all length scales, ranging from the laboratory benchtop to the entire Earth, while inversion of later arriving phases has been much more limited. Our inversion is performed using a damped, smoothed LSQR implementation that solves for site and event terms as well as for velocity along great circle paths between the source and receiver. Results are broadly consistent with published upper crustal velocities for Eurasia, with predominantly high velocities in cratonic regions. Generally, but not always, lower velocities are observed in orogenic and extensional areas and in deep sedimentary basins. A spotcomparison of VP/VS from local and regional studies compares well with the ratio of observed Pg to Sg velocities from our study where resolution is high. Resolution is determined through the use of checkerboard tests, and these suggest that in regions where data density is high we can resolve features down to at least 2 ◦ , with 4 ◦ possible over broader areas. rms residual reductions are on the order of 25 per cent for Sg and 30 per cent for Pg.

Seismic attenuation ( Q -value) can be estimated by extracting the amplitude-frequency information contained in seismic waveforms. We apply the attenuation tomography method of Pei et al. (2006) using M L amplitude data to estimate attenuation within the upper crust in Japan. More than 60,000 Sg -wave maximum amplitude readings from 5559 events, recorded by 971 stations, were selected from the dense High-Sensitivity Seismography Network (Hi-net) under the condition that epicentral distance is less than 2o and event depth is less than 10 km. The lateral S -wave Q variations of the upper crust at 1 Hz in Japan were obtained. The results indicate that low Q -values exist in the central Japanese islands, with almost the same distribution as volcanoes, while high Q -values exist mainly between the front of volcanoes and the Japanese east coast. In addition, a low Q was found between the eastern coast and the subducted trench. Most large crustal earthquakes occur in or around zones of low Q or the boundaries between areas of low and high Q , which will be very helpful in estimating the risk of large earthquakes.

Between 1986 and 1990 the eruptive activity of Erebus volcano was monitored by a video camera with on-screen time code and recorded on video tape. Corresponding seismic and acoustic signals were recorded from a network of 6 geophones and 2 infrasonic microphones. Two hundred Strombolian explosions and three lava flows which were erupted from 7 vents were captured on video. In December 1986 the Strombolian eruptions ejected bombs and ash. In November 1987 large bubble-bursting Strombolian eruptions were observed. The bubbles burst when the bubble walls thinned to ∼ 20 cm. Explosions with bomb flight-times up to 14.5 s were accompanied by seismic signals with our local size estimate, “unified magnitudes” ( m u ), up to 2.3. Explosions in pools of lava formed by flows in the Inner Crater were comparatively weak. Changes in eruptive activity occurred in 1987 when the lava lake was buried by a landslide from the crater wall. Two new vents formed and seismic activity peaked as the landslide was ingested. Lava flows from a vent on the eastern side of the crater formed small lakes and a vent on the north began to flow in 1990. By December 1990 the entire floor of the Inner Crater was buried by up to 20 000 m 3 of new lava. Different families of nearly identical eruption earthquakes occurred each year, whose foci were contained within small, shallow volumes. Immediately after several bubble-bursting eruptions, clear views of the empty vent were recorded. The vent was seen to taper downwards to about half its diameter at the bottom. Our observations confirm models of Strombolian eruptions suggesting they arise from gas slugs rising through a conduit into a flared vent.

The estimation of earthquake occurrence rates and seismic hazard analyses, along with treaty verification requirements to identify and characterize seismic sources in particular regions, requires removing mine blasts from seismic catalogs (Mackey et al. 2003). While efforts progress to formulate generalized discriminants based solely on seismic data, these efforts are often hampered by poor data quality and a significant need for custom processing due to the widely ranging propagation characteristics of each region (Hartse et al. 1997; Taylor and Hartse 1998; Stump et al. 2002). Many regions of interest to researchers and to the treaty verification community are sparsely instrumented, geologically complex, and contain both natural and anthropogenic seismicity. A number of temporal methods have been used to detect and remove anthropogenic seismicity from seismic catalogs; many of these rely on earthquake metadata such as epicenter location and depth, the quality of which may be poor. Mackey et al. (2003) performed simple time-of-day analysis on events from a number of Russian regional catalogs, including the annual Earthquakes of the USSR catalog. By investigating regions with local daytime seismicity greater than 65%, several areas of industrial seismic contamination were identified. Wiemer and Baer (2000) mapped normalized daytime to nighttime ratio of events, Rq , and calculated the probability of occurrence within a tight spatial grid to remove likely mining seismicity from catalogs in Switzerland and North America. This method was successful at highlighting regions of anomalously high daytime seismicity and removing those events, but it also removed a small percentage of tectonic events, which occur with fairly even time distribution throughout the day. Additionally, events with bulletin depths greater than 30 km were not considered. Significant inaccuracies in depth or epicenter location may inappropriately exclude industrial events from identification. Relatively straightforward waveform correlation and clustering analyses, however, have proven …

Seismicity in the Rio Grande rift in central New Mexico, southwestern United States, has been dominated by microearthquakes occurring above the midcrustal Socorro magma body (SMB) for at least the past century. The SMB is a sill-like feature ≥3400 km2 in area, with a top surface at 19-km depth and centered below the inner rift half-graben system. A recent swarm of microearthquakes above the magma body began on 20 October 2005. The activity intensified toward 30 October 2005, with the largest event being a shallow felt local magnitude ( M L) 2.4 earthquake at 02:57:35 (UTC). Seismicity increased in the region following this event, with over 1600 earthquakes detectable at a minimum of one local seismic station during the subsequent month. Seismic waveforms for the earthquakes in this sequence are remarkably similar, implying consistently similar location and faulting geometry. The time–space distribution of earthquakes indicates that this sequence is highly swarmlike, in contrast to tectonic mainshock–aftershock sequences. Given the local tectonic setting and shallow (

SUMMARY We processed data from a small, five-station temporary seismic network deployed from January 2002 until March 2003 within the Kingdom of Bhutan. We detected, associated, and located approximately 2,100 teleseismic, regional, and local events; approximately 900 were not in the United States Geological Survey (USGS) Earthquake Data Report catalog. We supple mented our data for these 900 events with data from the Global Seismographic Network (GSN) stations in the region. After relocation of these events, we focused on approximately 175 events that occurred near or within the borders of Bhutan. We reviewed each solution, manually timing the P- and S-waves for each event, and inverted for event locations and an average 1-D velocity model for the region. Our model was tested with other models appropriate for the region. We found a high amount of microseismicity throughout southern Bhutan and almost no seismicity under northern Bhutan and southern Tibet. Our results showed that analysis of data from small in-country seismic networks resulted in new scientific findings. In this case, we found the crust under southern Bhutan brittlely deforming, and there was evidence for strike-slip faulting, supporting previous results for the region.

We have selected 10,899 M L amplitude readings from 1732 events re- corded by 91 stations, as reported in the Annual Bulletin of Chinese Earthquakes (ABCE), and have used tomographic imaging to estimate the lateral variations of the quality factor Q0 (Q at 1 Hz) within the crust of Northern China. Estimated Q0 values vary from 115 to 715 with an average of 415. Q0 values are consistent with tectonic and topographic structure in Eastern China. Q0 is low in the active tectonic regions having many faults, such as the Shanxi and Yinchuan Grabens, Bohai Bay, and Tanlu Fault Zone, and is high in the stable Ordos Craton. Q0 values are low in several topographically low-lying areas, such as the North China, Taikang-Hefei, Jianghan, Subei-Yellow Sea, and Songliao basins, whereas it is high in mountainous and uplift regions exhibiting surface expressions of crystalline basement rocks: the Yinshan, Yanshan, Taihang, Qinlin, Dabie and Wuyi Mountains, and Luxi and Jiaoliao Uplifts. Quality-factor estimates are also consistent with Pn- and Sn-velocity patterns. High- velocity values, in general, correspond with high Q0 and low-velocity values with low Q0. This is consistent with a common temperature influence in the crust and uppermost mantle.

On 3 November 2002, a M w 7.9 earthquake, the largest continental strike-slip earthquake in North America since the 1857 Fort Tejon, California, event, occurred in central Alaska. The earthquake began with reverse faulting on a ∼40-km extent of the previously unknown Susitna Glacier fault, but rupture transferred eastward to the right-lateral Denali fault and continued for over 200 km, finally transferring to rupture ∼70 km of the Totschunda fault. This large, complex event we term the Denali fault earthquake (dfe), after the major crustal fault that carried most of the displacement. The initiation of the rupture, the Susitna Glacier fault, is in a remote region of central Alaska that under normal circumstances is sparsely instrumented. On 23 October of that year, however, a large earthquake of M w 6.7, referred to as the Nenana Mountain earthquake (nme), occurred only 22 km to the west of the dfe epicenter. The nme, in hindsight recognized as a foreshock to the dfe, prompted deployment of a temporary network by the Alaska Earthquake Information Center (aeic). Hence, the area was under significantly enhanced seismic surveillance at the time of the dfe, 10 days later, which was further augmented by the addition of 19 more stations following the dfe mainshock. As a result, high-quality data were available in the near field, providing enhanced coverage for aftershock activity from the Susitna Glacier fault initiation point, along the Denali fault as far as the western portion of the Totschunda fault, to augment regional and teleseismic data for this sequence. As the rupture proceeded eastward, the Richardson Highway, one of the two north–south roads connecting the central and southern parts of the state, was disrupted where it crosses the Denali fault trace. Also significantly displaced was the Trans-Alaska Pipeline, operated by the Alyeska Pipeline …

We have relocated a subset of events from the digital waveform catalogue of ∼17,000 volcanic microearthquakes recorded between July 1995 and February 1996 at Soufriere Hills Volcano (SHV), Montserrat, using a cross-correlation-based phase repicking technique with a joint location method. Hypocenters were estimated for 3914 earthquakes having five or more corrected P-wave picks. The seismic source region collapsed to a volume of ∼1 km 3 from an initial ∼100 km 3 . Relocated events represent 36 swarms, each containing nearly identical waveforms, having source dimensions of 10 to 100 m in diameter and spatial separations on the order of 500 m or less. Each swarm occurred over a span of several hours to a few days. Triggered data appear to miss between 65% and 98% of the events that occur within these swarms, based on review of helicorder records. Visual estimates of summit dome growth show a rough correspondence between episodes of intense swarming and increases in extruded magma, although dome observations are too sparse to make a direct comparison for this time period. The limited depth range over which dome-growth-related events occur is consistent with a dynamic model of cyclic plug extrusion behavior in the shallow conduit, governed by magma supply rate, overpressure buildup and physical properties of the magma and conduit geometry. Seismic sources may occur in locally overpressured regions that result from microlite formation in a zone of rapid decompression; we propose that this zone exists in the vicinity of a detachment plane associated with the cyclic plug extrusion.

Waveform cross-correlation based repicking of P arrival times and subsequent relocation of 187 earthquakes that occurred near the summit of Mauna Loa, Hawaii, prior to the March 1984 eruption has illuminated a previously obscured structure beneath the northwestern flank. Simultaneous inversion for hypocenters and velocity model parameters using the refined arrival times resulted in well-constrained relative earthquake locations with very low arrival time misfits (average RMS 0.041 s). Pre-eruption seismicity from this time period occurred in two groups: a shallow group located near Mauna Loa’s summit, at depths of 1–3 km, and a deeper group (5–10 km) located 4–6 km northwest of the summit. After relocation, we found that most of the northwest flank earthquakes occurred along a 3-km-long planar feature striking about 60° West of North in a thin band about 1 km thick. There is a temporal migration of epicenters from the northwestern edge of the zone migrating towards the summit. Focal mechanisms in this zone reveal a change in faulting from strike-slip in the southeast to a mix of strike-slip and normal faulting in the northwest. This feature we interpret to be related to a rift zone that was stunted by the buttressing of the adjacent volcanoes Hualalai and Mauna Kea. Previous gravity and magnetic studies provide supporting evidence for the existence of a rift zone. There was only a modest increase in clustering of the summit events. After relocation, hypocenters that were previously located within a diffuse zone (4.5 km) were reduced to a 2.5-km-wide zone. They extend from a depth of 6 km to about sea level, trending toward the summit. The focal mechanisms from the summit events are mix of faulting types without a consistent pattern.

We have developed an automatic P -wave arrival detection and picking algorithm based on the wavelet transform and Akaike information criteria (AIC) picker. Wavelet coefficients at high resolutions show the fine structure of the time series, and those at low resolutions characterize its coarse features. Primary features such as the P -wave arrival are retained over several resolution scales, whereas secondary features such as scattered arrivals decay quickly at lower resolutions. We apply the discrete wavelet transform to single-component seismograms through a series of sliding time windows. In each window the AIC autopicker is applied to the thresholded absolute wavelet coefficients at different scales, and we compare the consistency of those picks to determine whether a P -wave arrival has been detected in the given time window. The arrival time is then determined using the AIC picker on the time window chosen by the wavelet transform. We test our method on regional earthquake data from the Dead Sea Rift region and local earthquake data from the Parkfield, California region. We find that 81% of picks are within 0.2-sec of the corresponding analyst pick for the Dead Sea dataset, and 93% of picks are within 0.1 sec of the analyst pick for the Parkfield dataset. We attribute the lower percentage of agreement for the Dead Sea dataset to the substantially lower signal-to-noise ratio of those data, and the likelihood that some percentage of the analyst picks are in error.

We have developed an adaptive, automatic, correlation- and clustering- based method for greatly reducing the degree of picking inconsistency in large, digital seismic catalogs and for quantifying similarity within, and discriminating among, clusters of disparate waveform families. Innovations in the technique include (1) the use of eigenspectral methods for cross-spectral phase estimation and for providing subsample pick lag error estimates in units of time, as opposed to dimensionless relative scaling of uncertainties; (2) adaptive, cross-coherency-based filtering; and (3) a hierarchical waveform stack correlation method for adjusting mean intercluster pick times without compromising tight intracluster relative pick estimates. To solve the systems of cross-correlation lags we apply an iterative, optimized conjugate gra- dient technique that minimizes an L1-norm misfit. Our repicking technique not only provides robust similarity classification-event discrimination without making a priori assumptions regarding waveform similarity as a function of preliminary hypocenter estimates, but also facilitates high-resolution relocation of seismic sources. Although knowledgeable user input is needed initially to establish run-time parameters, sig- nificant improvement in pick consistency and waveform-based event classification may be obtained by then allowing the programs to operate automatically on the data. The process shows promise for enhancing catalog reliability while at the same time reducing analyst workload, although careful assessment of the automatic results is still important.

— An automatic, adaptive, correlation-based algorithm for adjusting phase picks in large digital seismic data sets provides significant improvement in resolution of microseismic structures using only a small fraction of the time and manpower which would be required to re-analyze waveforms manually or semi-automatically. We apply this technique to induced seismicity at the Soultz-sous-Forets geothermal site, France. The method is first applied to a small, previously manually repicked subset of the catalogue so that we may compare our results to those obtained from painstaking, visual, cross-correlation-based techniques. Relative centroid-adjusted hypocenters show a decrease in median mislocation from 31 to 7 m for preliminary and automatically adjusted picks, respectively, compared to the manual results. Narrow, intersecting joint features not observed in the preliminary hypocenter cloud, but revealed through manual repicking, are also recovered using the automatic method. We then address a larger catalogue of ∼7000 microearthquakes. After relocating the events using automatic repicks, the percentage of events clustering within 5 m of their nearest neighbor increases form 5 to 26% of the catalogue. Hypocenter relocations delineate narrow, linear features previously obscured within the seismic cloud, interpreted as faults or fractures which may correspond to fluid propagation paths, or to changes in stress as a result of elevated pore pressures. RMS travel-time residuals for the larger data set are reduced by only 0.2%; however, phase-pick biases in the preliminary catalogue have influenced both the velocity model and station correction calculations, which will affect location residuals. These pick biases are apparent on the adjusted, stacked waveforms and correcting them will be important prior to future velocity model refinements.

The reflection seismic method is the most frequently used exploration method for imaging and monitoring subsurface structures with high resolution. It has proven its qualities from the scale of regional seismology to the scale of near-surface applications that look just a few meters below the surface. The reflection method uses controlled active sources at known positions to give rise to reflections recorded at known receiver positions. The reflections’ two-wave travel time is used to extract desired information about and image the subsurface structures. When active sources are unavailable or undesired, one can retrieve body-wave reflections from application of seismic interferometry (SI) to sources of opportunity—quakes, tremors, ambient noise, or even man-made sources not connected to the exploration campaign. We show examples of imaging of subsurface structures using reflections retrieved from quakes and ambient noise. We apply SI by autocorrelation to global earthquake to image seismic and aseismic pa...

Volcanic activity at Mount Erebus is dominated by eruptive activity within a phonolitic summit lava lake. Common eruption styles range from passive degassing to Strombolian explosions, which typically occur several times daily, and occasionally in swarms of up to 900 per day. Shallow explosions, although generally the result of steady exsolution of volatiles from depth, can be triggered by surficial input of H 2O through mass wasting of rock, snow and ice from the crater walls. Broadband observations of Strombolian explosions document very-long-period (VLP) signals with strong spectral peaks near 20, 12 and 7 s, which are polarized in the vertical/radial plane. These signals precede lava lake surface explosions by ,1.5 s, are highly repeatable, and persist for up to 200 s. First motions indicate a deflationary source, with any precursory inflation being below the ,30 s passband of our instruments. Particle motions suggest a VLP source residing up to 800 m below the lava lake surface; however, this depth could be exaggerated by near-field radial tilt. Seismic and acoustic signals associated with lava lake explosions commonly show evidence for multiple bubble bursts in corresponding complexity features resulting from varying time delays and relative sizes of superimposed bursts. A systematic decrease in seismic/acoustic ratio for smaller surface explosions suggests that either the seismic energy from the smallest, shallowest bubble bursts experiences much greater seismic attenuation than energy arising from larger events which may involve a deeper, less attenuative portion of the magma column, and/or that the shallowest layer is seismically isolated from deeper parts of a stratified magma column, which are not excited by the smallest explosions due to sharp impedance contrasts across distinct layers. Tremor at Erebus is uncommon, with only a few isolated instances identified in five years of monitoring. Some tremor events are nearly monochromatic, and some exhibit numerous gliding harmonic spectral lines. q 2000 Elsevier Science B.V. All rights reserved.

A new method for improving relative locations of clustered earthquakes is presented and applied to a suite of microearthquakes induced by hydraulic fracturing. The method is based on the assumption that clustering of earthquake hypocenters is obscured by the uncorrelated scatter of individual hypocenters. The method is implemented as an additional constraint in a Joint Hypocenter Determination (JHD) scheme. The method shifts event hypocenters toward the center of mass of the events within some volume surrounding the event location if the RMS misfit between predicted and measured arrival times does not increase significantly. The method uses the same basic assumption of Jones and Stewart (1997), which is that there is greater clustering in actual earthquake locations than there is in locations determined using conventional techniques. Our method differs in that it is implemented as part of the JHD process so it operates on raw travel-time data rather than on derived hypocenters. The method produces hypocenters from a demonstration field dataset that are similar to those obtained by Phillips et al. (1997), from time-consuming precise manual repicking of relative arrival times of events. The clustering constraint can easily be incorporated as an additional constraint in earthquake location/velocity tomography codes and may lead to improved velocity structure determination and earthquake location pattern identification and interpretation.

In December 1996 and January 1997, broadband seismometers were deployed on the summit plateau of Mount Erebus at radial distances of 0.7, 1.4 and 1.9 km from the central crater and lava lake. Strombolian explosions at Erebus previously have been observed to produce seismic and acoustic energy between 1 and 6 Hz. New observations document significant energy with spectral peaks as grave as 20 s. Nearly identical very-long-period (VLP) signals begin ∼1.5 s prior to explosions, have dilatational onsets and persist for up to 150 s. Similar VLP waveforms were recorded at all three stations, indicating that the seismograms are essentially source-dominated. Particle motions suggest an initial depth for the VLP source of up to several hundred meters, migrating deeper in the course of ∼15 s. Such explosion-associated VLP signals may indicate a nondestructive lossy resonance or nonlinear fluid-flow excitation within the shallow magmatic system.