Global Seismographic Network Research Articles

Centroid-moment-tensor analysis of the 2011 off the Pacific coast of Tohoku Earthquake and its larger foreshocks and aftershocks

Centroid moment tensors (CMTs) for the March 11, 2011, off the Pacific coast of Tohoku Earthquake and its larger foreshocks and aftershocks are presented. The CMTs are calculated using long-period data from the Global Seismographic Network retrieved in near-real time, and the inversion algorithm employed in the Global CMT Project. Owing to its large size and long duration, the mainshock CMT analysis is performed at periods longer than 300 s. The resulting moment tensor has a scalar moment of 5.3 · 1029 dyne-cm, and a geometry that indicates thrust motion of the Pacific Plate beneath the island of Honshu on a plane dipping 10°. The large scalar moment, translating to moment magnitude M W = 9.1, makes this the fourth largest earthquake in the last 100 years. The mainshock was preceded by several large foreshocks with similar underthrusting mechanisms, the largest of which having M W = 7.4. The aftershock sequence extends over a portion of the plate boundary more than 500 km long, and exhibits an unusual diversity of faulting geometries. Reverse, normal, and strike-slip faulting events with widely varying directions of maximum tension and compression occur throughout the upper plate. A M W = 7.9 aftershock occurred near the southern end of the aftershock zone approximately 30 minutes after the mainshock, and is, at the time of writing, the largest aftershock in the sequence. The second-largest aftershock is a M W = 7.6 normal-faulting outer-rise earthquake that occurred 40 minutes after the mainshock.

Great Earthquakes and Global Seismic Networks

Seismologists often jest that “the best way to stop earthquakes is to deploy seismic stations.” The laborious effort to install seismometers to record signals from earthquakes or to pursue targeted investigations of Earth's structure is sometimes confounded by the vagaries of earthquake occurrence, which is decidedly nonuniform in space and time. Of course, this often proves to be more of an anxiety than a reality, and most efforts, especially those employing multiyear installations, succeed in gathering valuable seismic data. One of the most gratifying examples of success is provided by the deployment of the Incorporated Research Institutions for Seismology–National Science Foundation–U.S. Geological Survey (IRIS-NSF-USGS) Global Seismographic Network (GSN) in parallel with numerous international deployments of broadband seismometers in the Federation of Digital Seismic Networks (FDSN). Between about 1982 and 2004, global installations of high-quality broadband digital seismometers proliferated, replacing the obsolete analog systems of the World-Wide Standardized Seismograph Network and upgrading sparse institutional observatories and first-generation digital networks. While deployments of important additional stations continue, by 2004 the GSN had achieved its basic design goals: openly available continuous broadband data from roughly 130 stations providing real-time global coverage and large dynamic range (Butler et al. 2004). Then the 26 December 2004 great Sumatra-Andaman earthquake ( Mw 9.2) occurred—the first event to exceed magnitude 9 since the 1964 Alaska earthquake. The GSN and FDSN provided unprecedented global seismic recordings for this event and enabled more detailed seismological investigation than was possible for any prior event of such size ( e.g. , Lay et al. 2005; Bilek et al. 2007). Those same globally distributed seismic stations have gathered data from a substantial number of great earthquakes; from 2001 to 2010, there have been 18 events with MS ≥ 8 and 13 events with Mw ≥ 8, whereas over the previous …

Self-Noise Models of Seismic Instruments

Based on coherence analysis methods we develop a method for computing low self-noise models of seismic sensors. We calculate self-noise models for 11 different production seismometers. This collection contains the majority of sensors currently in use at Global Seismographic Network Stations. By developing these noise models, with a standard estimation method, we are able to make absolute comparisons between different models of seismic sensors. This also provides a method of identifying quality variations between two or more of the same model sensor. Studying Earth's free oscillations requires a large amount of seismic data with a high signal-to-noise ratio at long periods (Laske 2004). Recent tomographic studies using ambient seismic noise (Shapiro et al. 2005) also require the self-noise of seismic instruments to be below that of the Earth's ambient background noise, because as they use Earth noise as the seismic signal. It is also important when making temporary sensor deployments that the instrument's noise levels are below that of the signals being used in the study (Wilson et al. 2002). In order to verify that seismic instruments meet the above demands and other user requirements it is important from a testing standpoint, that one be able to measure the self-noise of seismic sensors and develop baselines for different models of seismic instruments. The different methods used to estimate self-noise of seismic sensors have made it difficult to do side-by-side comparisons of their performance (Hutt et al. 2009). This lack of a self-noise estimate standard makes it difficult to assess when a sensor's self-noise is above the manufacturers' specifications, indicating a possible problem with the sensor or noisy site conditions. In sensor development it is important to be able to compare a prototype sensor's self-noise to that of known self-noise levels of a reference sensor. On top of these complications some …

Temporal Variations in Global Seismic Station Ambient Noise Power Levels

Research Article| July 01, 2010 Temporal Variations in Global Seismic Station Ambient Noise Power Levels A. T. Ringler; A. T. Ringler U.S. Geological Survey Albuquerque Seismological Laboratory P.O. Box 82010 Albuquerque, New Mexico 87198-2010 U.S.A. aringler@usgs.gov Search for other works by this author on: GSW Google Scholar L. S. Gee; L. S. Gee U.S. Geological Survey Albuquerque Seismological Laboratory P.O. Box 82010 Albuquerque, New Mexico 87198-2010 U.S.A. aringler@usgs.gov Search for other works by this author on: GSW Google Scholar C. R. Hutt; C. R. Hutt U.S. Geological Survey Albuquerque Seismological Laboratory P.O. Box 82010 Albuquerque, New Mexico 87198-2010 U.S.A. aringler@usgs.gov Search for other works by this author on: GSW Google Scholar D. E. McNamara D. E. McNamara Search for other works by this author on: GSW Google Scholar Author and Article Information A. T. Ringler U.S. Geological Survey Albuquerque Seismological Laboratory P.O. Box 82010 Albuquerque, New Mexico 87198-2010 U.S.A. aringler@usgs.gov L. S. Gee U.S. Geological Survey Albuquerque Seismological Laboratory P.O. Box 82010 Albuquerque, New Mexico 87198-2010 U.S.A. aringler@usgs.gov C. R. Hutt U.S. Geological Survey Albuquerque Seismological Laboratory P.O. Box 82010 Albuquerque, New Mexico 87198-2010 U.S.A. aringler@usgs.gov D. E. McNamara Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2010 by the Seismological Society of America Seismological Research Letters (2010) 81 (4): 605–613. https://doi.org/10.1785/gssrl.81.4.605 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. T. Ringler, L. S. Gee, C. R. Hutt, D. E. McNamara; Temporal Variations in Global Seismic Station Ambient Noise Power Levels. Seismological Research Letters 2010;; 81 (4): 605–613. doi: https://doi.org/10.1785/gssrl.81.4.605 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 SocietySeismological Research Letters Search Advanced Search Recent concerns about time-dependent response changes in broadband seismometers have motivated the need for methods to monitor sensor health at Global Seismographic Network (GSN) stations. We present two new methods for monitoring temporal changes in data quality and instrument response transfer functions that are independent of Earth seismic velocity and attenuation models by comparing power levels against different baseline values. Our methods can resolve changes in both horizontal and vertical components in a broad range of periods (∼0.05 to 1,000 seconds) in near real time. In this report, we compare our methods with existing techniques and demonstrate how to resolve... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Global trends in extremal microseism intensity

Globally ubiquitous seismic background noise peaks near 7 and 14 s period are generated via distinct mechanisms that transfer storm‐generated gravity wave energy to the seismic wave field. We utilize continuous digital ground motion data recorded by the Global Seismographic Network and precursor instrumentation to chronicle microseism power extreme events for 1972–2009. Because most land‐observed microseism surface‐wave energy is generated at or near coasts, microseism metrics are particularly relevant to assessing changes in coastal ocean wave energy. Extreme microseism winter storm season event counts reveal the widespread influence of the El Niño Southern Oscillation (ENSO). Individual station and ensemble slopes trend positive for this study period for Northern Hemisphere stations. The double‐frequency microseism is particularly volatile, suggesting that the weaker single‐frequency microseism directly generated by ocean swell at coasts is likely a more representative seismic proxy for broad‐scale ocean wave energy estimation.

Fast inversion of rupture process of the 14 April 2010 Yushu, Qinghai, earthquake

Four results of the rupture process of 14 April 2010 Yushu, Qinghai, earthquake, obtained by inverting the broadband seismic data of Global Seismographic Network (GSN) based on the inversion method of earthquake rupture process, were compared and discussed. It is found that the Yushu earthquake has several basic characteristics as follows: ① There exist two principal sub-events which correspond to two slip-concentrated patches being located near the hypocenter and to the southeast of the epicenter. The rupture of the slip-concentrated patch to the southeast of the epicenter broke though the ground surface; ② The peak slip and peak slip-rate are about 2.1 m and 1.1 m/s, respectively, indicating that the Yushu earthquake is an event with large slip-rate on the fault plane; ③ Overall the Yushu earthquake is a unilateral rupture event with the rupture mainly propagating southeastward. The strong focusing of the seismic energy in the southeast of the epicenter due to the “seismic Doppler effect” reasonably accounts for the tremendous damage in the Yushu city.

Glacial Earthquakes in Greenland and Antarctica

Glacial earthquakes are a new class of seismic events, first discovered as signals in long-period seismograms recorded on the Global Seismographic Network. Most of these events occur along the coasts of Greenland, where they are spatially related to large outlet glaciers. Glacial earthquakes show a strong seasonality, with most events occurring during the late summer. The rate of glacial-earthquake occurrence increased between 2000 and 2005, with a stabilization of earthquake frequency at 2003–2004 levels in 2006–2008. Recent observations establish a strong temporal correlation between the distinct seismic signals of glacial earthquakes and large ice-loss events in which icebergs of cubic-kilometer scale collapse against the calving face, linking the seismogenic process to the force exerted by these icebergs on the glacier and the underlying solid earth. A sudden change in glacier speed results from these glacial-earthquake calving events. Seasonal and interannual variations in glacier-terminus position account for general characteristics of the temporal variation in earthquake occurrence. Glacial earthquakes in Antarctica are less well studied, but they exhibit several characteristics similar to glacial earthquakes in Greenland.

Estimation of crustal Qβ in the NW Himalaya using teleseismic broadband SH waveforms of the 8 October 2005 South Asian earthquake

The average value of the intrinsic shear wave quality factor, Qβ, for the 15 km of the upper crust in the NW Himalaya is estimated. Thirty-two teleseismic broadband SH and sSH waveforms of 8 October 2005 South Asian earthquake (Mw = 7.6), from sixteen Global Seismographic Network stations of the National Earthquake Information Center network have been used. The selected windows of the direct and depth phases have been Fourier transformed and smoothed using the Hamming and Tuckey technique. Ratio of the smoothed spectra of depth and direct phases are obtained in the frequency range 0.2–1.5 Hz. A straight line is fitted in the least-square sense to the spectral ratio versus frequency. The value of Qβ is estimated from the slope of the line. The frequency independent average value of Qβ is estimated to be 218±56.

Finite frequency effects on globalSdiffracted traveltimes

SUMMARY Many seismic observations have shown that strong heterogeneities exist in the bottom few hundreds kilometres of the mantle. Among different seismic phases, this region, that is, the D″ layer, can be most globally sampled by diffracted waves along the core mantle boundary. Here, we assess the amplitude and distribution of S-wave velocity variations in the D″ layer of an existing tomographic model. We compare observed SHdiff traveltime anomalies to synthetic ones obtained using (1) the coupled spectral element method (CSEM), which is our reference exact method, (2) non-linear asymptotic coupling theory (NACT) and (3) 1-D ray theory. Synthetic waveforms are calculated down to 0.057 Hz with a corner frequency at 0.026 Hz. In the first part of this paper, we compare the traveltime anomaly predictions from the three different methods. The anomalies from CSEM and NACT are obtained by taking cross-correlations of the 3-D and 1-D synthetic waveforms. Both NACT and standard ray theory, which are used in other recent tomographic models, suffer from biases in traveltime predictions for vertically varying structure near the core–mantle boundary: NACT suffers from saturation of traveltimes, due to the portion in the kernel calculation that is based on the reference 1-D model, while ray theory suffers from wave front healing effects in the vertical plane, exacerbated in the presence of thin low velocity layers. In the second part, we compare observed traveltime anomalies and predictions from CSEM. The data consists of 506 Sdiff traveltime anomalies from 15 events, obtained form global seismograph network records. The tomographic model does a good job at predicting traveltimes of Sdiff phases especially when the path mostly samples fast S velocity regions at the base of the mantle, such as beneath India, China, North America and Northern Pacific. The underprediction of the positive observed traveltime anomalies seems to occur in regions where the paths sample close to the border of the large low shear velocity provinces, such as beneath Southern Pacific, South of Australia, Philippine Sea plate and Northwestern Pacific Ocean. Inspection of the observed seismograms at higher frequencies indicate that (1) the gradient of structure is underestimated and (2) there are post-cursors to the Sdiff which bias the lower frequency traveltime measurements, and which are not taken into account by the tomographic inversion, because of the approximate theoretical framework.

Quick moment tensor solution for 6 April 2009, L’Aquila, Italy, earthquake

A MW6.4 earthquake occurred in L’Aquila, central Italy at 1:32:42 (UTC), April 6, 2009. We quickly obtained the moment tensor solution of the earthquake by inverting the P waveforms of broadband recordings from the global seismographic network (GSN) stations using the quick technique of moment tensor inversion, and further inferred that the nodal plane of strike 132°, dip 53° and rake −103° is the seismogenic fault.

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

A Method to Establish Seismic Noise Baselines for Automated Station Assessment

Currently, the United States Geological Survey (USGS) National Earthquake Information Center (NEIC) receives more than 4,000 channels of seismic data from roughly 900 seismic stations distributed around the world in near real time. As the NEIC develops increasingly sophisticated methods for automated data processing, it becomes equally important to develop tools for ensuring that only the highest-quality data are used in rapid earthquake products. In this paper, we present methods and applications for computing long- and short-term, station-specific noise baselines for broadband seismic data channels. Long-term baselines (one year or greater) are useful for determining ambient noise conditions, while short-term baselines (one hour to one month) are useful for monitoring changing station performance and noise characteristics. This method is currently in testing and development at the NEIC for evaluation of instrument responses, assessment of data quality for real-time earthquake processing systems, new station site evaluation, and assessment of gross network characteristics. Real-time earthquake processing systems at the NEIC rely on high-quality seismic data to compute accurate earthquake locations and magnitudes, moment-tensor solutions, finite-fault models, and shaking intensity. The NEIC receives real-time seismic data from a variety of contributors including: global networks (Global Seismographic Network [GSN], GEOSCOPE, GEOFON), national networks (ANSS, Australia, Spain, Canada, Switzerland), regional networks (ANZA, PRSN, CISN, PNWSN, AEIC), government agencies (National Oceanic and Atmospheric Administration [NOAA] tsunami warning centers, the International Monitoring System [IMS]), and temporary deployments such as the EarthScope transportable array (TA). On most days, approximately 10% of contributed instrument responses are either not known or are erroneous and cannot be used for accurate real-time amplitude information. Maintaining accurate and current metadata for roughly 900 contributed stations is difficult when instrumentation upgrades or other changes occur regularly. Automated processing of real-time seismic data necessitates the development of automated tools and procedures to quickly identify changes …

Differential Energy Radiation from Two Earthquakes in Japan with Identical Mw: The Kyushu 1996 and Tottori 2000 Earthquakes

We examine two closely located earthquakes in Japan that had identical moment magnitudes M w but significantly different energy magnitudes M e . We use teleseismic data from the Global Seismograph Network and strong-motion data from the National Research Institute for Earth Science and Disaster Prevention’s K-Net to analyze the 19 October 1996 Kyushu earthquake ( M w 6.7, M e 6.6) and the 6 October 2000 Tottori earthquake ( M w 6.7, M e 7.4). To obtain regional estimates of radiated energy E S we apply a spectral technique to regional (<200 km) waveforms that are dominated by S and Lg waves. For the thrust-fault Kyushu earthquake, we estimate an average regional attenuation Q ( f )=230 f 0.65. For the strike-slip Tottori earthquake, the average regional attenuation is Q ( f )=180 f 0.6. These attenuation functions are similar to those derived from studies of both California and Japan earthquakes. The regional estimate of E S for the Kyushu earthquake, 3.8×1014 J, is significantly smaller than that for the Tottori earthquake, E S 1.3×1015 J. These estimates correspond well with the teleseismic estimates of 3.9×1014 J and 1.8×1015 J, respectively. The apparent stress ( τ a = μE S / M , with μ equal to rigidity) for the Kyushu earthquake is 4 times smaller than the apparent stress for the Tottori earthquake. In terms of the fault maturity model, the significantly greater release of energy by the strike-slip Tottori earthquake can be related to strong deformation in an immature intraplate setting. The relatively lower energy release of the thrust-fault Kyushu earthquake can be related to rupture on mature faults at a subduction environment. The consistence between teleseismic and regional estimates of E S is particularly significant as teleseismic data for computing E S are routinely available for all large earthquakes whereas often there are no near-field data.

Anomalous earthquakes associated with Nyiragongo Volcano: Observations and potential mechanisms

A series of five unusual earthquakes (4.6 ≤ M ≤ 5.3) has been recently located near Nyiragongo volcano (D. R. Congo) in the Western Rift Valley of the East African Rift. Despite their moderate size, these earthquakes do not appear in global seismicity catalogs, but were located using long-period surface waves primarily recorded on the Global Seismographic Network. Three of the events occurred in the week following the January 2002 eruption of Nyiragongo, while the final two occurred in 2003 and 2005 respectively, and are not linked to a major eruption at Nyiragongo or its neighboring volcano, Nyamuragira. Several common techniques were used to investigate the characteristics of these seismic sources in the context of the volcanic activity of the region. The frequency content of the five anomalous earthquakes was compared to that from local events found in global catalogs, and the newly detected earthquakes were shown to be slow events, being greatly depleted in frequencies above 0.1 Hz. Each of the newly detected earthquakes was modeled by a series of forces and by a centroid-moment tensor. A deviatoric moment tensor was shown to provide a better fit to the data. The newly detected earthquakes are highly non-double-couple in nature, each having a large compensated-linear-vector-dipole component of the moment tensor. Drawing on models based on similar observations from other active volcanoes, we propose that the earthquakes are caused by slip on non-planar faults beneath the volcano. We suggest a mechanism in which these newly detected earthquakes are generated by the collapse of the roof of a shallow magma chamber along an inward-dipping cone-shaped ring fault. Diking events, which result in magma evacuation from shallow magma chambers, could trigger such earthquakes. Our results provide new constraints on the dynamics of the poorly understood magma system beneath Nyiragongo, an active volcano that is a significant threat to life and property.

Glacial rumblings from Jakobshavn ice stream, Greenland

AbstractThe steep increase in Greenland’s glacial earthquake activity detected by the Global Seismographic Network since the late 1990s suggests that a close inspection of these events might provide clues to the nature and origin of such seismic activity. Here we discuss the detection of large, unexpected seismic events of extraordinarily long duration (10–40 min) occurring about once every 2 days, and localized in the ice stream that feeds the Earth’s fastest-moving glacier (Jakobshavn Isbræ) from the east. These ‘glacial rumblings’ represent an ice-mass wasting process that is greater and more frequent than glacial earthquakes have suggested. Probably triggered by calving, the rumblings are all very similar regardless of duration, and all end with a sharp, earthquake-like event in which the largest seismic amplitude is in the rumbling and that might signal the collapse of large ice masses upstream. By calculating the total amount of seismic energy released as rumblings, we estimate that the maximum seasonal amount of ice moved seismogenically down the ice stream is up to 12 km3, or ∼30% of the average annual iceberg discharge in Jakobshavn.

Amplitude and Q of 0S0 from the Sumatra Earthquake as Recorded on Superconducting Gravimeters and Seismometers

It has been known for some time that superconducting gravimeters (SGs) can provide excellent recordings of the seismic normal modes. Studies by Banka and Crossley (1999) and van Camp (1999) established the low noise level of SGs in the seismic normal mode band, especially at periods longer than 100 s. In an extensive study of all the currently operating SGs (about 18), Rosat et al. (2003) showed that the best SGs have lower noise than the New Low Noise Model (Peterson 1993) at periods longer than 1,000 s. Also, several studies have compared the performance of SGs and broadband seismometers at long periods and shown that the correction of meteorological influence on the SGs can be more effective than on seismometers (Freybourger et al. 1997; Hinderer et al. 2002). Zurn and Widmer-Schnidrig (2006) demonstrated the need to correct seismometers for pressure effects at long periods. In the frequency domain the sensitivity of SGs reaches 1 nanogal (10–11ms–2) or less, especially at periods longer than 100 s. Widmer-Schnidrig (2003) summarized the advantages of using SGs for long-period seismology, noting that in addition to sensitivity the SGs are calibrated to DC with an amplitude accuracy of better than 0.1%. Here we combine and compare, for the first time, seismic data from the Incorporated Research Institutions for Seismology (IRIS) Data Management System and data from SGs contributing to the Global Geodynamics Project (GGP) (Crossley et al. 1999). The 26 December 2004 Sumatra earthquake is the largest event to be recorded both by the Global Seismographic Network (GSN) and SGs, with a magnitude larger than 9.0. It therefore provides a very good opportunity to test the calibration levels of our instruments (Davis et al. 2005; Park, Song, et al. 2005). Rosat et al. (2007).recently published a study that …

Challenges Ahead for the Global Seismographic Network

Twenty-five years ago, a new global digital seismic network was identified by the National Academies' Committee on Science, Engineering, and Public Policy as one of five research areas in which significant dividends could be expected as a result of incremental federal investment. That recommendation, through coordinated planning and support by scientists, mission agencies, and international partners, spawned the development of the Global Seismographic Network (GSN). The GSN is a joint program of the National Science Foundation (NSF) and the U.S. Geological Survey (USGS). The NSF program is implemented by the Incorporated Research Institutions for Seismology (IRIS) and the University of California at San Diego (UCSD); the USGS program is implemented by the Albuquerque Seismological Laboratory. Network completion is a beginning, not an end, and there is a need for continual rejuvenation of such a network, both to improve operational capabilities and to open new avenues for scientific exploration. The GSN has now far exceeded initial plans and expectations, both in terms of its operational scope and performance and the scientific applications of the data it produces. Originally envisioned as a 100-station network to gather data mainly for research applications, the network has evolved into a real-time Earth-observing system also supporting earthquake alerts, tsunami warning, nuclear treaty verification, and other uses. Several recent publications have documented the GSN's remarkable successes. Butler et al. (2004) emphasized that the GSN had exceeded its original technical design goal of 128 globally distributed stations—last year the core network plus affiliates reached 150 stations, strategically scattered across continents and islands from the Arctic to the South Pole. One in four of these stations has evolved into a cooperative “geophysical observatory,” which may include a range of sensor types (gravimeters, GPS receivers, magnetometers, microbarographs, etc.) in addition to the broadband and very-broadband seismometers that are capable of …

A Study of the Regional Variation of Low-Frequency around the Korean Peninsula

We studied regional ![Graphic][1] at 1 Hz (![Graphic][2] ) around the Korean Peninsula based on broadband vertical component seismic records of six Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network stations and 19 Korean stations of the Korea Institute of Geoscience and Mineral Resources (KIGAM). Using 177 seismic events with magnitudes between 5.3 and 5.7 and depths less than 50 km, the reversed two station method was applied, and 94 high quality interstation paths were selected from 869 possible pairs. These results show high ![Graphic][3] paths around the Sea of Japan (East Sea) reflecting the typical oceanic structure and low ![Graphic][4] paths around northeastern China related to inactive seismicity. Assigning these path values into 193 cells around South Korea with a size of 1° by 1°, we observed that the regional ![Graphic][5] decreased gradually from east to west between 2 and 1×10-3. [1]: /embed/inline-graphic-2.gif [2]: /embed/inline-graphic-3.gif [3]: /embed/inline-graphic-4.gif [4]: /embed/inline-graphic-5.gif [5]: /embed/inline-graphic-6.gif

Complex seismic anisotropy at the border of a very low velocity province at the base of the Earth's mantle

We constrain the anisotropy associated with a very low velocity province (VLVP) at the base of the Earth's mantle using the SKS and SKKS waves sampling the region. Our selected high‐quality data sets consist of 415 SKS and 111 SKKS waveforms for 127 deep earthquakes recorded at distances between 90° and 150° by the seismic stations in three temporary broadband PASSCAL seismic arrays: the Kaapvaal seismic array (1997–1999), the Tanzania seismic array (1994–1995), and the Ethiopia/Kenya seismic array (1999–2001), as well as the permanent stations in the Global Seismographic Network. These seismic data provide good sampling coverage for some portion of the VLVP and its surrounding areas. Our results show, when the SKS or SKKS phases sample the regions away from the border of the VLVP (inside or outside the VLVP), the apparent splitting parameters inferred from the SKS phases are consistent with those inferred from the SKKS phases, and their variations strongly correlate with seismic stations but not with the exit points at the core‐mantle boundary of these seismic phases. However, when the SKS or SKKS phases sample near the border of the VLVP, the apparent splitting parameters inferred from the SKS phases and SKKS phases are different, and their variations no longer correlate with seismic stations. These features indicate that part of the shear wave splitting for the seismic data sampling the border of the VLVP has to originate from deep mantle, most likely near the border of the VLVP. We assume that the anisotropy in the shallow mantle beneath seismic stations has a horizontal hexagonal symmetry axis and infer the splitting parameters associated with the shallow anisotropy beneath the seismic stations using the SKS and SKKS waveforms for the seismic data sampling the regions away from the border of the VLVP. We then obtain the splitting parameters associated with the lowermost mantle anisotropy using the SKS and SKKS waveforms corrected for the inferred shallow anisotropy beneath seismic stations, assuming that the medium in the lowermost mantle also has a horizontal hexagonal symmetry axis. Our results reveal a complex anisotropy pattern near the border of the VLVP. Such a complex anisotropy pattern may be explained by lattice‐preferred orientation of anisotropic mantle aggregates aligned by a complex mantle flow near the VLVP margins. The complex flow pattern near the VLVP margins may indicate strong interactions between the VLVP, a compositional anomaly, and the surrounding normal mantle, and may provide an explanation to the concentration of some hot spots geographically near the borders of the VLVPs in the lowermost mantle.

Calibration of the Global Seismographic Network Using Tides

The Incorporated Research Institutions for Seismology/United States Geological Survey (IRIS/USGS) Global Seismographic Network (GSN) is now a fully operational scientific facility routinely used to investigate earth structure and seismicity throughout the world (Butler et al. 2004). Data recorded at GSN stations are rapidly transmitted to international agencies charged with hazard assessment and response as well as to individual scientists eager to study in detail the properties of the most recent earthquake. All who use these recordings depend upon the accuracy of the GSN's metadata provided to them through the IRIS Data Management System (http://www.iris.edu). The IRIS DMS in turn distributes response information supplied by the GSN network operators, the University of California San Diego's Project IDA (International Deployment of Accelerometers) and the USGS's Albuquerque Seismological Laboratory (ASL). To maximize the scientific return from these data and to ensure that accurate measurements may be made at the various monitoring centers that use GSN data in their analyses, both network operators subject the equipment at each GSN station to a series of stringent tests before field deployment. These tests include careful calibration of the data acquisition system. A goal of the GSN is to publish instrument responses accurate to 1%. In principle, the Earth's' tides provide an independent check of the laboratory-determined instrument response. The bulk properties of the Earth are well-enough known to compute the solid earth's response to tides to a high degree of accuracy. However, until the advent of improved ocean tidal models based upon analysis of TOPEX/Poseidon orbit cycles, the effects of ocean loading were too poorly known to be used for this purpose. Using the current generation of tidal models that effectively incorporate ocean loading as well as the solid earth tide, one may now compute a tidal series accurate to 1% in many regions. In this …