Global Network Stations Research Articles

Revised Distance and Depth Corrections for Use in the Estimation of Short-Period P-Wave Magnitudes

A new short-period seismic magnitude measure based on uniform, automatic processing of digital P -wave data recorded on the standardized International Monitoring System (IMS) global network of stations has been formulated and tested. The test data consisted of over 220,000 single-station P -wave amplitude and period observations from approximately 25,000 events that were reported in the Reviewed Event Bulletin of the prototype International Data Center, which was established to evaluate various concepts proposed for use in the monitoring of nuclear explosions. Large-scale statistical analyses of these data have been used to define improved corrections for the effects of epicentral distance and focal depth, distance weighting factors for use in the estimation of a new, generalized m b measure ( m b ) that incorporates data observed over the expanded epicentral distance range extending from 2° to 180°, as well as preliminary global station corrections for a number of existing and proposed IMS station sites. The station corrections are considered to be preliminary at this time since possible maximum-likelihood effects are not explicitly considered in the present analysis. However, evidence is presented that indicates that such effects are not significantly biasing the derived distance/depth corrections and that the magnitude of their influence on the estimated station corrections is likely to be rather small with respect to the range encompassed by these corrections. Tests of this new, generalized m b magnitude measure have indicated that it can provide consistent and easily reproducible measures of seismic event magnitude, even for events that are not well recorded at traditional teleseismic distances.

On F2 response to geomagnetic storm

The interaction of solar wind with earth’s magnetic field leads to a number of modifications of the F region global structure. Furthermore, the absorption of solar wind energy has the same phase all over the globe throughout the year. All these imply that there could be simultaneity in UT in global response of some F2 parameters to geomagnetic storms. Presently, F2 region global structure response to geomagnetic storm was studied using foF2 data obtained during the very intense geomagnetic storm (A p > 196) of 13–15 March 1989 from a global network of stations. The ionosonde stations located are at Ouagadougou (12.4), Manila (14.7°N), Chung-Li (25.0°N), Kokubunji (35.7°N), Akita (39.7°N), Boulder (40.0°N), Rome (41.8°N), Ottawa (45.4°N), Wakkanai (45.4°N), and Slough (54.5°N) and Churchill (58.8°N). The present result obtained using derived ratios R (foF2) show that all the stations showed some high degree of simultaneity in the depletion of foF2 between 0000 and 1200 UT on 14 March.

An Assessment of Seismic Body-wave Magnitudes Published by the Prototype International Data Centre

A significant initiative in the reporting of global seismicity began in January 1995 with the publication of daily “Reviewed Event Bulletins” (REB's), a few days in arrears. This work was and still is done in the context of international efforts aimed at verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). When it was finalized in 1996 the CTBT specified that the seismic monitoring network of the International Monitoring System (IMS) would consist of 50 primary stations contributing data continuously to an International Data Centre (IDC) in Vienna, plus 120 auxiliary stations that record continuously but contribute data to IDC only on request (Richards, 2002). Until IDC began daily operations on 21 February 2000, the daily REB was published by the Prototype International Data Centre (PIDC) and a predecessor organization. We shall refer to all the REB's from 1 January 1995 to 20 February 2000 as having been published by PIDC. This paper focuses on the PIDC teleseismic body-wave magnitudes ( mb ). They are important as a modern effort to measure short-period magnitudes based on what is expected to become a fixed network of global stations, all operating in a standardized way. Although seismic magnitudes are intended to be a characteristic of the source, it is well known that in practice they are influenced by numerous additional factors such as the particular set of stations reporting individual magnitudes, the instrument responses, and the details of how each station magnitude is assigned. Global bulletins of seismicity, including mb values, have long been reported, months in arrears, by the Preliminary Determination of Epicenters (PDE) and Earthquake Data Report (EDR) of the U.S. Geological Survey's National Earthquake Information Center (USGS NEIC). They have also been reported, about two years in arrears, by the International Seismological Centre (ISC), based in England. To obtain its …

A coarse grid three‐dimensional global inverse model of the atmospheric transport: 2. Inversion of the transport of CO2 in the 1980s

Models of atmospheric transport can be used to interpret spatiotemporal differences in the observed concentrations of CO2 in terms of its surface exchange fluxes. Inversion of the atmospheric transport is the systematic search for both a flux field that yields an optimal match between modeled and observed concentrations and, equally importantly, the uncertainties in this inferred flux field. The present inversion study combines observations of the CO2 concentration at the global station network of the NOAA/CMDL in the 1980s with a representation of the atmospheric transport model TM2 by its Jacobian matrix, which has been previously computed by the adjoint model of TM2. This Jacobian matrix maps monthly fluxes on the approximately 8° latitude by 10° longitude horizontal model grid onto the resulting changes in the monthly CO2 concentration at every station. Since the number of observational sites is much smaller than the number of grid cells, the inverse problem is highly underdetermined. A unique solution is determined by including a priori information on the surface exchange fluxes derived from output of high‐resolution models of both the terrestrial biosphere and the ocean, combined with statistics of fossil fuel burning and land use change. Performing a Bayesian synthesis inversion, for a target period in the 1980s, the average seasonal cycle and the mean annual magnitude of CO2 surface fluxes on the TM2 grid are inferred. The resulting simulated concentration compares well with independent observations. On a global scale, an oceanic sink of 1.5±0.4 gigatons of carbon (GtC) is estimated. This sink is stronger in the northern than in the southern hemisphere. On a regional scale, however, the inferred exchange fluxes exhibit high uncertainty, indicating a low capacity of the global observational network to monitor regional trace gas emissions. These findings are relatively insensitive to the year of meteorological driving data, suggesting interannual changes in concentration should primarily result from source not transport changes.

Searching for slow and silent earthquakes using free oscillations

Our knowledge of Earth structure can be used to distinguish far‐field seismic signals from near‐field noise. In particular, earthquakes excite ground motion at the eigenfrequencies of the Earth's normal modes. By observing the spectral levels in small bands centered on the known frequencies of the normal mode resonance peaks and comparing them to the spectral levels of ambient noise, it is possible to detect earthquakes at low frequencies without knowing anything about them at high frequencies. We have used this fact to develop an earthquake detection algorithm that compares the signal level in the mode bands, as measured by the modulus of zeroth‐order moment of the Fourier spectrum, with that in the noise bands using an F test. The excitation of each mode over a global network of stations and of all the modes over the network is evaluated from a summed‐score statistic using a binomial test and a Markov test, respectively, which are robust with respect to deviations from the Gaussian noise model. We apply the technique to the Earth's fundamental spheroidal modes 0S8‐0S43 as recorded by the IDA network for the 2‐year interval 1978–1979. We calculate the level of excitation at 3‐hour intervals using a time domain integration technique designed to optimize the signal‐to‐noise ratio for each mode. The algorithm detects 50% of all teleseismic earthquakes having seismic moments M0 ≥ 1×1018 N m (Mw ≥ 6.0), and it detects all events having M0 ≥ 3×1018 N m (Mw ≥ 6.3). Slow earthquakes, i.e., those having anomalously large characteristic durations, are observed to be enriched in low‐frequency mode excitation relative to other events of comparable magnitude. Using this criterion, we have found a number of slow earthquakes that had not been previously identified as anomalous. Most of these newly identified slow earthquakes occurred on oceanic transform faults. In addition to the cataloged normal and slow earthquakes, we observe 27 episodes of substantial mode excitation that are reasonably well isolated from significant earthquakes. Such anomalies may represent “silent earthquakes,” events with propagation velocities sufficiently low that they do not generate globally detectable wave trains on high‐frequency seismometers.

Experience in operating a limited global network of stations measuring full-disc oscillations of the Sun

Details are given about the operation of a two station network and of a new semi-automatic station which has recently been added. Comparison is made with predicted duty cycles. A possible way of quantifying the sky quality is also given.

Enhanced isotropic cosmic ray intensity waves during cosmic ray storms

A study of the abnormal cosmic ray storm that followed the solar particle ground level event (GLE) on September 23, 1978, has revealed that the protracted recovery phase is impregnated with wavelike modulations that are associated with the rotational period of the sun. The analysis of data from the global network of stations has demonstrated that these large‐amplitude waves are isotropic. From an analysis of axial and longitudinal anisotropies it is concluded that the observed fluctuations did not originate from periodic changes in the cosmic ray density gradient (▽n) either in or perpendicular to the plane of the ecliptic. Although these long‐duration waves cannot be ascribed to singular features on the sun, it is speculated that they may reflect changes in the topology of active regions on the visible disk. In any case, it is clear that the scope of the classical picture, in which Forbush decreases are associated with short‐term anisotropies, must be expanded to include long‐duration wave phenomena. These new observations provide an important clue in understanding the physical mechanism that gives rise to long‐duration cosmic ray storms.

The behaviour of the F-layer at a low midlatitude station on winter and summer nights and comparison with the results of a thermospheric model

The action of neutral air winds and of influx of plasma from outside the F-layer are shown to be important in understanding its observed nighttime behaviour at a lower midlatitude station. On winter nights the problem is to understand why F-layer electron concentrations are as high as they are. On the other hand on summer nights the problem is to understand how the F-layer decays as quickly as it does after large increases in F-layer electron concentrations which are observed to occur near midnight. The use of radio observations from a global network of stations could possibly lead to a model capable of predicting a wider range of ionospheric phenomena than is currently possible.