scholarly journals Complementing regional moment magnitudes to GCMT: a perspective from the rebuilt International Seismological Centre Bulletin

2021 ◽  
Vol 13 (5) ◽  
pp. 1957-1985
Author(s):  
Domenico Di Giacomo ◽  
James Harris ◽  
Dmitry A. Storchak
Keyword(s):  
Body Wave ◽  
Moment Tensor ◽  
Specific Area ◽  
Centroid Moment Tensor ◽  
Earthquake Catalogues ◽  
Origin Time ◽  
Earthquake Parameters ◽  
Short Period ◽  
The Moment ◽  
International Seismological Centre

Abstract. Seismologists and geoscientists often need earthquake catalogues for various types of research. This input usually contains basic earthquake parameters such as location (longitude, latitude, depth, and origin time), as well as magnitude information. For the latter, the moment magnitude Mw has become the most sought after magnitude scale in the seismological community to characterize the size of an earthquake. In this contribution we provide an informative account of the Mw content for the newly rebuilt Bulletin of the International Seismological Centre (ISC, http://www.isc.ac.uk, last access: May 2021), which is regarded as the most comprehensive record of the Earth's seismicity. From this data, we extracted a list of hypocentres with Mw from a multitude of agencies reporting data to the ISC. We first summarize the main temporal and spatial features of the Mw provided by global (i.e. providing results for moderate to great earthquakes worldwide) and regional agencies (i.e. also providing results for small earthquakes in a specific area). Following this, we discuss their comparisons, by considering not only Mw but also the surface wave magnitude MS and short-period body wave magnitude mb. By using the Global Centroid Moment Tensor solutions as an authoritative global agency, we identify regional agencies that best complement it and show examples of frequency–magnitude distributions in different areas obtained both from the Global Centroid Moment Tensor alone and complemented by Mw from regional agencies. The work done by the regional agencies in terms of Mw is fundamental to improve our understanding of the seismicity of an area, and we call for the implementation of procedures to compute Mw in a systematic way in areas currently not well covered in this respect, such as vast parts of continental Asia and Africa. In addition, more studies are needed to clarify the causes of the apparent overestimation of global Mw estimations compared to regional Mw. Such difference is also observed in the comparisons of Mw with MS and mb. The results presented here are obtained from the dataset (Di Giacomo and Harris, 2020, https://doi.org/10.31905/J2W2M64S) stored at the ISC Dataset Repository (http://www.isc.ac.uk/dataset_repository/, last access: May 2021).

scholarly journals Complementing regional moment magnitudes to GCMT: a perspective from the rebuilt ISC Bulletin

2021 ◽  
Author(s):  
Domenico Di Giacomo ◽  
James Harris ◽  
Dmitry A. Storchak
Keyword(s):  
Body Wave ◽  
Moment Tensor ◽  
Specific Area ◽  
Centroid Moment Tensor ◽  
Earthquake Catalogues ◽  
Origin Time ◽  
Earthquake Parameters ◽  
Short Period ◽  
The Moment ◽  
Work Done

Abstract. Seismologists and geoscientists in general often need earthquake catalogues for various types of research. This input usually contains basic earthquake parameters such as location (longitude, latitude, depth and origin time) as well as magnitude information. For the latter, the moment magnitude Mw has became the most sought after magnitude scale in the seismological community to characterize the size of an earthquake. In this contribution we provide an informative account of the Mw content for the newly rebuilt Bulletin of the International Seismological Centre (ISC, http://www.isc.ac.uk/), which is regarded as the most comprehensive record of the Earth's seismicity. From it, we extracted a list of hypocentres with Mw from a multitude of agencies reporting data to the ISC. We first summarize the main temporal-spatial features of the Mw provided by global agencies (i.e., providing results for moderate to great earthquakes worldwide) and regional ones (i.e., also providing results or small earthquakes in a specific area). Then we discuss their comparisons, not only by considering Mw but also the surface wave magnitude MS and short-period body wave magnitude mb. By using the Global Centroid Moment Tensor solutions as authoritative global agency, we identify regional agencies that best complement it and show examples of frequency-magnitude distributions in different areas obtained both from the Global Centroid Moment Tensor alone and complemented by Mw from regional agencies. The work done by the regional agencies in terms of Mw is fundamental to improve our understanding of the seismicity of an area and we call for the implementation of procedures to compute Mw in a systematic way in areas currently not well covered in this respect, such as vast parts of continental Asia and Africa. In addition, more studies are needed to clarify the causes of the apparent overestimation of global Mw estimations compared to regional Mw. Such difference is also observed in the comparisons of Mw with MS and mb. The results presented here are obtained from the dataset (Di Giacomo and Harris, 2020, https://doi.org/10.31905/J2W2M64S) stored at the ISC Dataset Repository (http://www.isc.ac.uk/dataset_repository/).

A teleseismic body-wave analysis of the May 1980 Mammoth Lakes, California, earthquakes

1983 ◽  
Vol 73 (2) ◽  
pp. 419-434
Author(s):  
Jeffery S. Barker ◽  
Charles A. Langston
Keyword(s):  
Body Wave ◽  
Moment Tensor ◽  
Normal Fault ◽  
Strike Slip ◽  
Body Waves ◽  
P Wave ◽  
Moment Tensors ◽  
Double Couple ◽  
The Moment ◽  
Mammoth Lakes

abstract Teleseismic P-wave first motions for the M ≧ 6 earthquakes near Mammoth Lakes, California, are inconsistent with the vertical strike-slip mechanisms determined from local and regional P-wave first motions. Combining these data sets allows three possible mechanisms: a north-striking, east-dipping strike-slip fault; a NE-striking oblique fault; and a NNW-striking normal fault. Inversion of long-period teleseismic P and SH waves for the events of 25 May 1980 (1633 UTC) and 27 May 1980 (1450 UTC) yields moment tensors with large non-double-couple components. The moment tensor for the first event may be decomposed into a major double couple with strike = 18°, dip = 61°, and rake = −15°, and a minor double couple with strike = 303°, dip = 43°, and rake = 224°. A similar decomposition for the last event yields strike = 25°, dip = 65°, rake = −6°, and strike = 312°, dip = 37°, and rake = 232°. Although the inversions were performed on only a few teleseismic body waves, the radiation patterns of the moment tensors are consistent with most of the P-wave first motion polarities at local, regional, and teleseismic distances. The stress axes inferred from the moment tensors are consistent with N65°E extension determined by geodetic measurements by Savage et al. (1981). Seismic moments computed from the moment tensors are 1.87 × 1025 dyne-cm for the 25 May 1980 (1633 UTC) event and 1.03 × 1025 dyne-cm for the 27 May 1980 (1450 UTC) event. The non-double-couple aspect of the moment tensors and the inability to obtain a convergent solution for the 25 May 1980 (1944 UTC) event may indicate that the assumptions of a point source and plane-layered structure implicit in the moment tensor inversion are not entirely valid for the Mammoth Lakes earthquakes.

The Global Centroid Moment Tensor Catalog: Heterogeneities and improvements

2021 ◽  
Author(s):  
Álvaro González
Keyword(s):  
Subduction Zones ◽  
Moment Tensor ◽  
Statistical Seismology ◽  
Centroid Moment Tensor ◽  
Moment Tensors ◽  
Global Database ◽  
Intermediate Period ◽  
Data Compilation ◽  
Forecast Models ◽  
The Moment

<p>Statistical seismology relies on earthquake catalogs as homogeneous and complete as possible. However, heterogeneities in earthquake data compilation and reporting are common and frequently are not adverted.</p><p>The Global Centroid Moment Tensor Catalog (www.globalcmt.org) is considered as the most homogeneous global database for large and moderate earthquakes occurred since 1976, and it has been used for developing and testing global and regional forecast models.</p><p>Changes in the method used for calculating the moment tensors (along with improvements in global seismological monitoring) define four eras in the catalog (1976, 1977-1985, 1986-2003 and 2004-present). Improvements are particularly stark since 2004, when intermediate-period surface waves started to be used for calculating the centroid solutions.</p><p>Fixed centroid depths, used when the solution for a free depth did not converge, have followed diverse criteria, depending on the era. Depth had to be fixed mainly for shallow earthquakes, so this issue is more common, e.g. in the shallow parts of subduction zones than in the deep ones. Until 2003, 53% of the centroids had depths calculated as a free parameter, compared to 78% since 2004.</p><p>Rake values have not been calculated homogenously either. Until 2003, the vertical-dip-slip components of the moment tensor were assumed as null when they could not be constrained by the inversion (for 3.3% of the earthquakes). This caused an excess of pure focal mechanisms: rakes of -90° (normal), 0° or ±180° (strike-slip) or +90° (thrust). Even disregarding such events, rake histograms until 2003 and since 2004 are not equivalent to each other.</p><p>The magnitude of completeness (<em>M</em><sub>c</sub>) of the catalog is analyzed here separately for each era. It clearly improved along time (average <em>M</em><sub>c</sub> values being ~6.4 in 1976, ~5.7 in 1977-1985, ~5.4 in 1986-2003, and ~5.0 since 2004). Maps of <em>M</em><sub>c</sub> for different eras show significant spatial variations.</p>

The Meckering earthquake of 14 October 1968: A possible downward propagating rupture

1987 ◽  
Vol 77 (5) ◽  
pp. 1558-1578
Author(s):  
Kristín S. Vogfjörd ◽  
Charles A. Langston
Keyword(s):  
Moment Tensor ◽  
Source Parameters ◽  
Time History ◽  
Vertical Displacement ◽  
Body Waves ◽  
Finite Fault ◽  
Short Period ◽  
Double Couple ◽  
The Moment ◽  
Fault Length

Abstract Average source parameters of the 1968 Meckering, Australia earthquake are obtained by the inversion of body waves. The objectives of the inversion are the elements of the moment tensor and the source-time history. An optimum source depth of 3 km is determined, but because of source complexity the point source assumption fails and the moment tensor obtained at that depth has a large nondouble-couple term, compensated linear vector dipole = 34 per cent. The source parameters of the major double-couple are: strike = 341°; dip = 37°; rake = 61°; and seismic moment = 8.2 ×1025 dyne-cm. The source-time function is of approximately 4 sec duration, with a long rise time and a sharp fall-off. The fault length is constrained on the surface by the observed surface break, and results from vertical displacement modeling suggest a width of approximately 10 km in the middle, assuming a dip of 37°. That restricts the entire faulted area to lie above 6 km depth. Two finite fault models for the earthquake are presented, with rupture initiating at a point (1) near the top of the fault and (2) at the bottom of the fault. Both models produce similar long-period synthetics, but based on the short-period waveforms, model 1 is favored. It is argued that such a rupture process is the most reasonable in this cold shield region.

Inversion of teleseismic body waves for the moment tensor of the 1978 Thessaloniki, Greece, earthquake

1981 ◽  
Vol 71 (5) ◽  
pp. 1423-1444
Author(s):  
Jeffrey S. Barker ◽  
Charles A. Langston
Keyword(s):  
Point Source ◽  
Moment Tensor ◽  
Source Parameters ◽  
Source Model ◽  
Body Waves ◽  
Pass Filter ◽  
Low Pass Filter ◽  
Initial Rupture ◽  
Short Period ◽  
The Moment

abstract Seismograms from WWSSN and Canadian network stations were modeled to determine the source parameters of the 20 June 1978 Thessaloniki, Greece, earthquake (Ms = 6.4). The depth of the initial rupture was constrained to 11 ± 1 km by comparison of the arrival times of surface reflections with synthetic short-period seismograms. A focal sphere plot of first motion polarities provided little constraint on other focal parameters, except to indicate that predominantly normal faulting was involved. A generalized inverse technique utilizing the moment tensor formalism was applied to teleseismic P and SH waves for six increments of depth. The moment tensor obtained indicated a nearly horizontal, N-trending tension axis and a nearly vertical compression axis, and yielded the following double-couple source parameters: strike 280° ± 7°; dip 55° ± 3°; rake −65° ± 5°; seismic moment 5.7 × 1025 dyne-cm; and a skewed triangular source time function with a rise time of about 1 sec and duration of 6 to 8 sec. Due to indications of multiple or finite source effects for this event, and the assumption in the moment tensor formalism of a point source, a low-pass filter was applied to the data and the inversions were repeated. The results were nearly identical with those of the original inversion, suggesting that any individual sources had similar mechanisms, or that the point source model is sufficient for this earthquake.

scholarly journals An updated and extended earthquake catalogue for Greece and adjacent areas since 1900

2012 ◽  
Vol 12 (5) ◽  
pp. 1425-1430 ◽  
Author(s):  
K. Makropoulos ◽  
G. Kaviris ◽  
V. Kouskouna
Keyword(s):  
Seismic Hazard ◽  
Body Wave ◽  
Moment Tensor ◽  
Earthquake Catalogue ◽  
Time Span ◽  
Regression Equation ◽  
Moment Tensor Inversion ◽  
Inversion Method ◽  
The Moment ◽  
Body Wave Magnitude

Abstract. A homogeneous earthquake catalogue for Greece and adjacent areas covering the period 1900–2009 is presented, to be used for reliable seismic hazard studies. The catalogues of Makropoulos and Burton (1981) and Makropoulos et al. (1989), covering the time span 1900–1985, were updated for the period 1986–2009 using instrumentally determined focal coordinates, except for the magnitude from the bulletin of the ISC. For Ms, which is the magnitude scale included in the previous versions, the same procedure applied since 1964 was adopted, using the ISC body wave magnitude (mb) and a regression equation. In the present update, Mw is also calculated for the entire period 1900–2009 using a formula derived from all available moment magnitudes and directly determined by the moment tensor inversion method. Thus, a magnitude homogeneous catalogue concerning both Ms and Mw scales is presented. The extended catalogue contains 7352 events, 70% more than the 4310 events of the previous published (1989) version. The completeness test revealed that the catalogue is complete for magnitudes above 4 for the last 34 yr and that no earthquake with magnitude 6 or greater has been omitted in the whole instrumental era (1900–2009).

Uncertainties in Seismic Moment Tensors Inferred from Differences between Global Catalogs

2021 ◽  
Author(s):  
Boris Rösler ◽  
Seth Stein ◽  
Bruce D. Spencer
Keyword(s):  
Moment Tensor ◽  
Centroid Moment Tensor ◽  
Moment Tensors ◽  
Double Couple ◽  
Source Mechanisms ◽  
Order Of Magnitude ◽  
The Difference ◽  
The Moment ◽  
Source Geometry ◽  
Scalar Moment

Abstract Catalogs of moment tensors form the foundation for a wide variety of seismological studies. However, assessing uncertainties in the moment tensors and the quantities derived from them is difficult. To gain insight, we compare 5000 moment tensors in the U.S. Geological Survey (USGS) and the Global Centroid Moment Tensor (Global CMT) Project catalogs for November 2015–December 2020 and use the differences to illustrate the uncertainties. The differences are typically an order of magnitude larger than the reported errors, suggesting that the errors substantially underestimate the uncertainty. The catalogs are generally consistent, with intriguing differences. Global CMT generally reports larger scalar moments than USGS, with the difference decreasing with magnitude. This difference is larger than and of the opposite sign from what is expected due to the different definitions of the scalar moment. Instead, the differences are intrinsic to the tensors, presumably in part due to different phases used in the inversions. The differences in double-couple components of source mechanisms and the fault angles derived from them decrease with magnitude. Non-double-couple (NDC) components decrease somewhat with magnitude. These components are moderately correlated between catalogs, with correlations stronger for larger earthquakes. Hence, small earthquakes often show large NDC components, but many have large uncertainties and are likely to be artifacts of the inversion. Conversely, larger earthquakes are less likely to have large NDC components, but these components are typically robust between catalogs. If so, these can indicate either true deviation from a double couple or source complexity. The differences between catalogs in scalar moment, source geometry, or NDC fraction of individual earthquakes are essentially uncorrelated, suggesting that the differences reflect the inversion rather than the source process. Despite the differences in moment tensors, the location and depth of the centroids are consistent between catalogs. Our results apply to earthquakes after 2012, before which many moment tensors were common to both catalogs.

Rapid Earthquake Source Description Using Variometric-Derived GPS Displacements toward Application to the 2019 Mw 7.1 Ridgecrest Earthquake

2021 ◽  
Author(s):  
Jianfei Zang ◽  
Caijun Xu ◽  
Yangmao Wen ◽  
Xiaohang Wang ◽  
Kefeng He
Keyword(s):  
Real Time ◽  
Early Warning ◽  
Moment Tensor ◽  
Continuous Phase ◽  
Earthquake Source ◽  
High Rate ◽  
Convergence Time ◽  
Centroid Moment Tensor ◽  
Origin Time ◽  
Broadcast Ephemeris

Abstract Using near-field high-rate Global Positioning System (GPS) displacements to invert for earthquake fault slips in real time has the potential to improve the accuracy of earthquake early warning or tsunami early warning. For such applications, real-time retrieval of high-accuracy GPS displacements is essential. Here, we report on rapid modeling of the 2019 Mw 7.1 Ridgecrest earthquake with real-time GPS displacements derived from a variometric approach with readily available broadcast ephemeris. This method calculates station variations in real time by differencing continuous phase observations and does not rely on precise orbit and clock information. The phase ambiguity is also removed, and thus the method does not suffer from a relatively long convergence time. To improve the accuracy of variometric displacements, we use a local spatial filter to decrease the influence of residual errors that cannot be removed completely by the time difference. We invert for the centroid moment tensor, static fault slips, and fault rupture process from the derived displacements. Our results show that all inverted models are available within about 65 s after the origin time of the earthquake and are comparable with models inverted by real-time precise point positioning displacements. This study highlights the great value of variometric displacements for the rapid earthquake source description with only broadcast ephemeris.

On an improved method to obtain the moment tensor and depth of earthquakes from the amplitude spectrum of Rayleigh waves

1983 ◽  
Vol 73 (6A) ◽  
pp. 1513-1526
Author(s):  
Barbara Romanowicz ◽  
Gerardo Suárez
Keyword(s):  
Rayleigh Waves ◽  
Variance Reduction ◽  
Body Wave ◽  
Moment Tensor ◽  
Amplitude Spectrum ◽  
Focal Depth ◽  
Vertical Component ◽  
Tien Shan ◽  
Period Range ◽  
The Moment

Abstract A new method is presented to invert for the moment tensor and depth using the amplitude spectra of vertical-component Rayleigh waves in the period range 20 to 100 sec. The technique follows a similar approach to that suggested by Romanowicz (1982a) to invert for the moment tensor from the complex spectra of Rayleigh waves and presents some distinct advantages to the method proposed originally by Mendiguren (1977). It eliminates some biases and errors in the data arising, for example, from inaccurate propagation corrections. Furthermore, it is substantially faster computationally and permits us to study independently the variance reduction as a function of depth of each of the momenttensor elements, resulting in better focal depth resolution. The method is applied to three earthquakes in the Tien Shan Mountains of Central Asia, the Eastern Cordillera of Peru, and the Gibbs Transform fault in the North Atlantic. In all three cases, the results of the moment-tensor inversion agree with those determined using long-period body-wave modeling.

Waveform Energy Focusing Tomography for Passive Sources

2020 ◽  
Author(s):  
Yueqiao Hu ◽  
Junlun Li ◽  
Haijiang Zhang
Keyword(s):  
Moment Tensor ◽  
Waveform Inversion ◽  
Velocity Model ◽  
Time Function ◽  
Source Information ◽  
Source Time Function ◽  
Origin Time ◽  
Small Magnitude ◽  
Short Period ◽  
Energy Focusing

<p>Full waveform inversion (FWI) is one of the most attractive geophysical inversion methods that reconstruct models with higher quality by exploiting the information of full wave-field. Despite its high resolution and successful practical applications, there still exist several obstacles to the successful application of FWI for passive earthquake sources, such as the high non-linearity for model convergence and demand for accurate source information, such as the moment tensor, the source time function, etc. To alleviate the requirement for a priori source information in waveform inversion, we propose a new method called Waveform Energy Focusing Tomography (WEFT), which backpropagates the observed wavefield from the receivers, not the data residuals like in conventional FWI, and tries to maximize the back-propagated wavefield energy around the source location over a short period around the origin time. Therefore, there is no need to provide the focal mechanism and source time function in advance. To better reconstruct the passive sources, the least-squares moment tensor migration approach is used, and the Hessian matrix is approximated using either analytic expression or raytracing. Since waveform fitting is superseded by simpler energy maximization, the nonlinearity of WEFT is weaker than that of FWI, and even less-accurate initial velocity model can be used. These advantages of WEFT make it more practical  for challenging earthquake data, especially for local small magnitude earthquakes where both velocity model and earthquake source information are unknown.</p>

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