Moment Tensor Determination Research Articles

Improvements and Heterogeneities of the Global Centroid Moment Tensor Catalog

Abstract Earthquake catalogs are heterogeneous, especially those developed over long time spans. Changes in seismological monitoring, which provides the records on which these catalogs are based, are common. Typically, instruments and networks become more sensitive over time, allowing for the detection and characterization of smaller earthquakes. In pursuit of improvement, new methods for routine data analysis are occasionally introduced, modifying the procedures for catalog compilation. The resulting heterogeneities may not be evident to users, but they should be unveiled and considered in any application of the catalog, especially in statistical seismology, which analyzes large earthquake data sets. The Global Centroid Moment Tensor catalog is considered the most homogeneous database of global seismicity. However, a detailed analysis of its heterogeneities has been lacking. This work reviews changes in the catalog’s development from 1976 to 2023 and reveals how these have caused improvements and heterogeneities in the resulting data. Several periods are distinguished, separated by milestones in the methods employed for moment tensor inversion and catalog compilation, as well as by the advent of global broadband monitoring in 2004. These changes are shown to have caused variations in the catalog’s completeness and in the determinations of centroid depths, scalar seismic moments, and moment tensors. The magnitude of completeness is measured here in detail, both temporally and spatially. It has decreased over the years and shows spatial variations within each period, correlated to regional differences in network monitoring and compilation biases. Moment tensor determinations have been significantly different since 2004, resulting in a different frequency distribution of rake angles and a different dependence of the double-couple component as a function of rake. This work is expected to benefit all future uses of the catalog, enabling better characterization of seismicity properties and improved building and testing of models for earthquake occurrence.

An automated earthquake detection and characterization tool for rapid earthquake and tsunami response in Western Mediterranean

SUMMARYA method of moment tensor inversion with a grid search on the source location and time is presently considered by the French Tsunami Service Provider (TSP or CENALT in French) for rapid earthquake and tsunami warning in the western Mediterranean Sea and the North-East Atlantic Ocean. The approach follows the GRiD MT (Grid-based Real-time Determination of Moment Tensors) methodology already implemented in other regions. Here, we show developments made towards its implementation for moderate to large earthquakes in the Ibero–Maghreb region, which is prone to generate tsunamis in the western Mediterranean Sea. Results obtained for a dozen of events, contrasting in magnitudes, locations and focal mechanisms, validate the choices made for the inversion parameters (grid resolution, frequency band, velocity models, etc.). Their source solutions are compared to published solutions from seismological institutes including GFZ and USGS. Besides, we describe the special care needed for a correct real-time event detection analysis, and in particular for obtaining the most appropriate source solution out of the thousand ones explored by the method. Rapid GRiD MT solutions can in turn be used for rapid tsunami simulations run by the TSP operator.

Gisola: A High-Performance Computing Application for Real-Time Moment Tensor Inversion

Abstract Automatic moment tensor (MT) determination is essential for real-time seismological applications. In this article, Gisola, a highly evolved software for MT determination, oriented toward high-performance computing, is presented. The program employs enhanced algorithms for waveform data selection via quality metrics, such as signal-to-noise ratio, waveform clipping, data and metadata inconsistency, long-period disturbances, and station evaluation based on power spectral density measurements in parallel execution. The inversion code, derived from ISOLated Asperities—an extensively used manual MT retrieval utility—has been improved by exploiting the performance efficiency of multiprocessing on the CPU and GPU. Gisola offers the ability for a 4D spatiotemporal adjustable MT grid search and multiple data resources interconnection to the International Federation of Digital Seismograph Networks Web Services (FDSNWS), the SeedLink protocol, and the SeisComP Data Structure standard. The new software publishes its results in various formats such as QuakeML and SC3ML, includes a website suite for MT solutions review, an e-mail notification system, and an integrated FDSNWS-event for MT solutions distribution. Moreover, it supports the ability to apply user-defined scripts, such as dispatching the MT solution to SeisComP. The operator has full control of all calculation aspects with an extensive and adjustable configuration. MT’s quality performance, for 531 manual MT solutions in Greece between 2012 and 2021, was measured and proved to be highly efficient.

Seismic moment tensors from synthetic rotational and translational ground motion: Green’s functions in 1-D versus 3-D

SUMMARY Seismic moment tensors are an important tool and input variable for many studies in the geosciences. The theory behind the determination of moment tensors is well established. They are routinely and (semi-) automatically calculated on a global scale. However, on regional and local scales, there are still several difficulties hampering the reliable retrieval of the full seismic moment tensor. In an earlier study, we showed that the waveform inversion for seismic moment tensors can benefit significantly when incorporating rotational ground motion in addition to the commonly used translational ground motion. In this study, we test, what is the best processing strategy with respect to the resolvability of the seismic moment tensor components: inverting three-component data with Green’s functions (GFs) based on a 3-D structural model, six-component data with GFs based on a 1-D model, or unleashing the full force of six-component data and GFs based on a 3-D model? As a reference case, we use the inversion based on three-component data and 1-D structure, which has been the most common practice in waveform inversion for moment tensors so far. Building on the same Bayesian approach as in our previous study, we invert synthetic waveforms for two test cases from the Korean Peninsula: one is the 2013 nuclear test of the Democratic People’s Republic of Korea and the other is an Mw 5.4 tectonic event of 2016 in the Republic of Korea using waveform data recorded on stations in Korea, China and Japan. For the Korean Peninsula, a very detailed 3-D velocity model is available. We show that for the tectonic event both, the 3-D structural model and the rotational ground motion, contribute strongly to the improved resolution of the seismic moment tensor. The higher the frequencies used for inversion, the higher is the influence of rotational ground motions. This is an important effect to consider when inverting waveforms from smaller magnitude events. The explosive source benefits more from the 3-D structural model than from the rotational ground motion. Nevertheless, the rotational ground motion can help to better constraint the isotropic part of the source in the higher frequency range.

Enhancing Tsunami Warning UsingPWave Coda

AbstractMost large tsunamis are generated by earthquakes on offshore plate boundary megathrusts. The primary factors influencing tsunami excitation are the seismic moment, faulting geometry, and depth of the faulting. Efforts to provide rapid tsunami warning have emphasized seismic and geodetic methods for quickly determining the event size and faulting geometry. It remains difficult to evaluate the updip extent of rupture, which has significant impact on tsunami excitation. TeleseismicPwaves can constrain this issue; slip under deep water generates strongpwPwater reverberations that persist as ringingPcodaafter the directPphases from the faulting have arrived. Event‐averagedPcoda/Pamplitude measures at large epicentral distances (>80°), tuned to the dominant periods of deep waterpwP(~12–15 s), correlate well with independent models of whether slip extends to near the trench or not. Data at closer ranges (30° to 80°) reduce the time lag needed for inferring the updip extent of rupture to <15 min. Arrival ofPPandPPPphases contaminates closer distancePcodameasures, but this can be suppressed by azimuthal or distance binning of the measures. Narrowband spectral ratio measures and differential magnitude measures ofPcodaand directP(mB) perform comparably to broader band root‐mean‐square (RMS) measures.Pcoda/Plevels for large nonmegathrust events are also documented. Rapid measurement ofPcoda/Pmetrics after a large earthquake can supplement quick moment tensor determinations to enhance tsunami warnings; observation of largePcodalevels indicates that shallow submarine rupture occurred and larger than typical tsunami (for givenMW) can be expected.

Determination of moment tensor and location of microseismic events under conditions of highly correlated noise based on the maximum likelihood method

ABSTRACTWe examine the problem of localization of a single microseismic event and determination of its seismic moment tensor in the presence of strongly correlated noise. This is a typical problem occurring in monitoring of microseismic events from a daylight surface during producing or surface monitoring of hydraulic fracturing. We propose a solution to this problem based on the method of maximum likelihood. We discuss mathematical aspects of the problem, some features and weak points of the proposed approach, estimate the required computing resources, and present the results of numerical experiments. We show that the proposed approach is much more resistant to correlated noises than diffraction stacking methods and time reverse modeling.

Automatic moment tensor determination for the Hellenic Unified Seismic Network.

Modern seismic networks with broadband sensors and real time digital telemetry made Moment Tensor (MT) determination a routine procedure. Automatic MT’s are now provided by global networks and a few very dense regional networks, within minutes after a significant event. An automatic MT determination wasn’t possible for the broader Hellenic area since seismic station density wasn’t sufficient. The creation of the Hellenic Unified Seismic Network (HUSN) provided the opportunity to apply an automated MT procedure using the available broad band data from almost one hundred stations. Thus the ISOLA code was extended towards the automatic operation based on Linux OS shell scripts, stand alone Fortran codes and SAC2000. Software supports both manual and automatic mode; at the first case, the user manually runs the program with the desired input parameters while at the latter, the system monitors a mailbox or RSS feed and if it receives an appropriate notification triggers the MT inversion procedure based on certain conditions. As it is setup now it calculates automatically the moment tensor of earthquakes larger than 3.5M w using data from HUSN. Application of an automated MT inversion procedure for HUSN will provide important real time information for studies like ground motion evaluation, tsunami warning etc.

The first month of the 2016 central Italy seismic sequence: fast determination of time domain moment tensors and finite fault model analysis of the ML 5.4 aftershock

<p>We present the revised Time Domain Moment Tensor (TDMT) catalogue for earthquakes with M_L larger than 3.6 of the first month of the ongoing Amatrice seismic sequence (August 24th - September 25th). Most of the retrieved focal mechanisms show NNW–SSE striking normal faults in agreement with the main NE-SW extensional deformation of Central Apennines. We also report a preliminary finite fault model analysis performed on the larger aftershock of this period of the sequence (M_w 5.4) and discuss the obtained results in the framework of aftershocks distribution.</p>

Path-specific, dispersion-based velocity models and moment tensors of moderate events recorded at few distant stations: Examples from Brazil and Greece

Centroid moment tensor (CMT) determination in intraplate regions like Brazil can be very difficult, because earthquakes are often recorded just at few and distant stations. This paper introduces a methodology for datasets like that. The methodology is based on waveform inversion in which each source-station path has its own velocity model. The 1-D path-specific velocity models are derived from the Rayleigh- and Love-wave dispersion curves. The waveform inversion is accompanied by posterior check of numerous P-wave first-motion polarities. An important innovation is the use of so-called frequency range test. The test basically consists in calculating CMT's for many different frequency ranges to assess the stability and uncertainty of the solution. The method is validated on two Brazilian earthquakes and a well-known Greek event. An offshore event (mb 5.2) in SE Brazil is inverted with four stations, at epicentral distances 300–400 km. The other Brazilian earthquake (mb 4.8 in Central Brazil) is even more challenging – only two broadband stations at 800–1300 km are at disposal for waveform inversion. The paper unambiguously demonstrates that the path-specific velocity models significantly increase the reliability of the CMT's. While standard models (e.g. IASP91) typically allow waveform modeling up to epicentral distances of the order of a few (∼10) minimum shear wavelengths (MSW), using the path-specific velocity models we successfully inverted waveforms up to > 20 MSW. Single-station waveform inversions are thoroughly tested, but multi-station joint inversions are shown to be preferable. The new methodology of this paper, providing a reasonable estimate of focal mechanisms and their uncertainties in case of highly limited waveform data, may find broad applicability in Brazil and elsewhere.

A New Strategy for Weak Events in Sparse Networks: The First-Motion Polarity Solutions Constrained by Single-Station Waveform Inversion

Moment tensor determinations of small earthquakes are quite challenging. It is because their signal‐to‐noise ratio is satisfactory only at frequencies above the microseismic noise peak (∼0.2 Hz), and waveforms can be modeled only up to ∼1–2 Hz at relatively near stations (epicentral distance of a few kilometers). Therefore, the availability of high‐quality waveforms at near stations is the most critical issue. Dense local networks enable solving specific issues like non‐double‐couple (non‐DC) components (e.g., Vavrycuk, 2011), relation with tectonics (Serpetsidaki et al. , 2013), and/or even space–time clustering of moment tensors (Cesca et al. , 2014). On the contrary, records of weak events at very few local stations hardly allow us to get even DC components of moment tensors. For example, focal mechanism of two microearthquakes, M w 0.2 and 0.4, were obtained by the full‐waveform inversion in the frequency range below 4 Hz and distance below 3.7 km by Benetatos et al. (2013). Fojtikova et al. (2010) studied shallow earthquakes of M w 1.2–3.4 at 0.8–1.6 Hz. A probabilistic nonlinear inversion of local waveforms was developed by Weber (2009) and applied to weak events in Hungary. Various constraints for determining focal mechanisms of small local earthquakes monitored by sparse networks were adopted. For example, Nakamura (2002) suggested a joint use of the P ‐ and S ‐wave polarities. Li et al. (2011) combined the phase and amplitude waveform matching with the first‐motion P polarities and average S / P amplitude ratios. Delouis and Legrand (1999) emphasized inclusion of the near‐field Green’s function terms. Pinar et al. (2003) made tests according to which some single‐station inversions are feasible. Busfar and Toksoz (2013) gave examples of focal mechanisms calculated in good velocity models from less than four stations. Godano et al. (2010, 2011) studied …

Combined Inversion of Broadband and Short-Period Waveform Data for Regional Moment Tensors: A Case Study in the Alborz Mountains, Iran

Abstract In this study, we suggest a novel approach for the retrieval of regional moment tensors for earthquakes with small to moderate magnitudes. The first modification is the combined inversion of broadband and short‐period waveform data. The broadband waveforms are inverted in a frequency range suitable for surface waves, whereas for the short‐period data a frequency range suitable for body waves is applied. The second modification is the use of first‐motion body‐wave polarities to select the most probable solution out of all solutions from inversion. To combine three different criteria for selecting the most probable solution (i.e., residual from inversion, double‐couple content of solution, number of nonmatching first‐motion body‐wave polarities), the L2 norm is applied to the normalized parameters. We chose five earthquakes within the Alborz mountains, Iran, as a case study (3.1≤ M w ≤4.1). In this area, several factors exacerbate the difficulty of performing inversion for moment tensors, for example, a heterogeneous station network and large azimuthal gaps. We have demonstrated that our approach supplies reliable moment tensors when inversion from broadband data alone fails. In one case, we successfully retrieved a stable solution from short‐period waveform data alone. Thus, our approach enables successful determination of seismic moment tensors wherever a sparse network of broadband stations has thus far prevented it.

On the Systematic Long-Period Noise Reduction on Ocean Floor Broadband Seismic Sensors Collocated with Differential Pressure Gauges

Vertical-component broadband seismic data collected on the ocean floor at regional distances from the shore and water depths on the order of 1-2 km are polluted by noise due to ocean infragravity (IG) waves in a period range (20-200 s) that is critical for regional structure studies and the determination of moment tensors for regional earthquakes. Using broadband seismic and differential pressure gauge data collected at the Monterey Ocean Bottom Broadband (MOBB) observatory, which has been operational since 2002, we identify the optimal length of time window to obtain the transfer function between the vertical seismic and pressure records. The maximum reduction in IG-wave-induced noise is obtained by using a one-day stack of transfer function inferred from data segment collected in the last 24 hours prior to data in which the noise removal method is applied. We show that a one-year stack of transfer function effectively reduces the noise level in the period band 50-100 s by 20 dB and by up to 15 dB at shorter periods. This resultant 20 dB noise reduction is comparable to that obtained from the one-day stack of transfer function. Our result suggests that the removal of noise induced by IG waves on MOBB vertical-component data can be done routinely without continuously calibrating the transfer function, also in near-real time, and is therefore also useful for routine regional moment tensor de- termination. This noise removal method has been implemented in the moment tensor determination system of the Northern California Seismic System.

Evaluating Centroid-Moment-Tensor Uncertainty in the New Version of ISOLA Software

Online Material: Additional figures. Focal mechanism and moment tensor determinations based on regional and local waveforms have become routine tasks in seismology. In recent years, considerable effort has been dedicated to extend software capabilities, in particular towards the inclusion of finite‐extent sources and easy, user‐friendly use. Two examples of these efforts are the FMNEAREG and KIWI software packages. Focal Mechanism from NEAr source to REGional distance records (FMNEAREG; Delouis et al. , 2008; Maercklin et al. , 2011) performs a grid search in order to find pure double‐couple sources, based on a 1D (line) finite‐source representation and complete elastodynamic wave field. The KIWI software (KInematic Waveform Inversion; Cesca et al. , 2010) allows the automatic retrieval of point‐source parameters, and for magnitudes above a given threshold kinematic finite‐fault rupture models can also be obtained. KIWI relies on a pre‐calculated database of Green’s functions. KIWI performs the moment tensor inversion in two steps, the first of which is based on the fit of amplitude spectra. The second step is based on time‐domain waveform fits. The ISOLA, ISOLated Asperities, software package (Sokos and Zahradnik, 2008) also performs waveform inversions to find source parameters. ISOLA is based on FORTRAN codes and offers a user‐friendly MATLAB graphical interface (GUI; www.mathworks.com/products/MATLAB). ISOLA allows for both single‐ and multiple‐point‐source iterative deconvolution (Kikuchi and Kanamori, 1991) inversion of complete regional and local waveforms. The moment tensor is retrieved through a least squares inversion, whereas the position and origin time of the point sources are grid searched. The computation options include inversion to retrieve the full moment tensor (MT), deviatoric MT, and pure double‐couple MT. Finite‐extent source inversions may also be performed by prescribing a priori the double‐couple mechanism (which will remain homogeneous over the fault plane). Green’s functions, including near‐field terms, are calculated using the discrete wavenumber method (Bouchon, …

OGS improvements in the year 2011 in running the Northeastern Italy Seismic Network

Abstract. The Centro di Ricerche Sismologiche (CRS, Seismological Research Center) of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS (Italian National Institute for Oceanography and Experimental Geophysics) in Udine (Italy) after the strong earthquake of magnitude Mw = 6.4 occurred in 1976 in the Italian Friuli-Venezia Giulia region, started to operate the Northeastern Italy Seismic Network: it currently consists of 12 very sensitive broad band and 21 simpler short period seismic stations, all telemetered to and acquired in real time at the OGS-CRS data centre in Udine. Real time data exchange agreements in place with other Italian, Slovenian, Austrian and Swiss seismological institutes lead to a total number of 93 seismic stations acquired in real time, which makes the OGS the reference institute for seismic monitoring of Northeastern Italy, as shown in Fig. 1 (Bragato et al., 2011; Saraò et al., 2010). Since 2002 OGS-CRS is using the Antelope software suite as the main tool for collecting, analyzing, archiving and exchanging seismic data, initially in the framework of the EU Interreg IIIA project "Trans-national seismological networks in the South-Eastern Alps" (Bragato et al., 2010; Pesaresi et al., 2008). SeisComP is also used as a real time data exchange server tool. In order to improve the seismological monitoring of the Northeastern Italy area, at OGS-CRS we tuned existing programs and created ad hoc ones like: a customized web server named PickServer to manually relocate earthquakes, a script for automatic moment tensor determination, scripts for web publishing of earthquake parametric data, waveforms, state of health parameters and shaking maps, noise characterization by means of automatic spectra analysis, and last but not least scripts for email/SMS/fax alerting. A new OGS-CRS real time seismological website (http://rts.crs.inogs.it/) has also been operative since several years.

Determination of full moment tensors of microseismic events in a very heterogeneous mining environment

The determination of source parameters and full moment tensors in mines is a difficult task, especially, when the velocity model of the mining environment is complex and strongly heterogeneous. The heterogeneities in the velocity model are usually caused by the presence of ore bodies, host rocks, tunnel systems and large excavations due to mining activity. The mined-out cavities introduce strong velocity contrasts in the model, cause multiple scattering of waves and result in a complex wave field with long coda waves. We have analysed five blasts and five induced microseismic events recorded at the Pyhäsalmi ore mine, Finland, and suggest a strategy of successfully inverting for the seismic moment tensors. We compute accurate locations using an eikonal solver and perform the time-domain moment tensor inversion from full waveforms using a generalized linear inversion. Green's functions are computed using a 3-D finite difference visco-elastic code capable of reproducing complex interactions of waves and structures. To suppress the sensitivity of the inversion to inaccuracies of locations and the velocity model, we analyse the data in the frequency range from 30 to 80Hz. The analysis of blasts and microseismic events proves that the moment tensor inversion is successful. The moment tensors of blasts display a high percentage of positive isotropic components. However, the presence of minor shear faulting triggered during blasting cannot be excluded. On the other hand, the moment tensors of microseismic events display significant negative isotropic and compensated linear vector dipole components. This indicates that the predominant mechanism of the events is probably related to the collapse of rock due to mining activity.

Determination and uncertainty of moment tensors for microearthquakes at Okmok Volcano, Alaska

Efforts to determine general moment tensors (MTs) for microearthquakes in volcanic areas are often hampered by small seismic networks, which can lead to poorly constrained hypocentres and inadequate modelling of seismic velocity heterogeneity. In addition, noisy seismic signals can make it difficult to identify phase arrivals correctly for small magnitude events. However, small volcanic earthquakes can have source mechanisms that deviate from brittle double-couple shear failure due to magmatic and/or hydrothermal processes. Thus, determining reliable MTs in such conditions is a challenging but potentially rewarding pursuit. We pursued such a goal at Okmok Volcano, Alaska, which erupted recently in 1997 and in 2008. The Alaska Volcano Observatory operates a seismic network of 12 stations at Okmok and routinely catalogues recorded seismicity. Using these data, we have determined general MTs for seven microearthquakes recorded between 2004 and 2007 by inverting peak amplitude measurements of P and S phases. We computed Green's functions using precisely relocated hypocentres and a 3-D velocity model. We thoroughly assessed the quality of the solutions by computing formal uncertainty estimates, conducting a variety of synthetic and sensitivity tests, and by comparing the MTs to solutions obtained using alternative methods. The results show that MTs are sensitive to station distribution and errors in the data, velocity model and hypocentral parameters. Although each of the seven MTs contains a significant non-shear component, we judge several of the solutions to be unreliable. However, several reliable MTs are obtained for a group of previously identified repeating events, and are interpreted as compensated linear-vector dipole events.

The global CMT project 2004–2010: Centroid-moment tensors for 13,017 earthquakes

Earthquake moment tensors reflecting seven years of global seismic activity (2004–2010) are presented. The results are the product of the global centroid-moment-tensor (GCMT) project, which maintains and extends a catalog of global seismic moment tensors beginning with earthquakes in 1976. Starting with earthquakes in 2004, the GCMT analysis takes advantage of advances in the mapping of propagation characteristics of intermediate-period surface waves, and includes these waves in the moment-tensor inversions. This modification of the CMT algorithm makes possible the globally uniform determination of moment tensors for earthquakes as small as MW=5.0. For the period 2004–2010, 13,017 new centroid-moment tensors are reported.

W phase source inversion for moderate to large earthquakes (1990-2010)

Rapid characterization of the earthquake source and of its effects is a growing field of interest. Until recently, it still took several hours to determine the first-order attributes of a great earthquake (e.g. M_w ≥ 7.5), even in a well-instrumented region. The main limiting factors were data saturation, the interference of different phases and the time duration and spatial extent of the source rupture. To accelerate centroid moment tensor (CMT) determinations, we have developed a source inversion algorithm based on modelling of the W phase, a very long period phase (100–1000 s) arriving at the same time as the P wave. The purpose of this work is to finely tune and validate the algorithm for large-to-moderate-sized earthquakes using three components of W phase ground motion at teleseismic distances. To that end, the point source parameters of all M_w ≥ 6.5 earthquakes that occurred between 1990 and 2010 (815 events) are determined using Federation of Digital Seismograph Networks, Global Seismographic Network broad-band stations and STS1 global virtual networks of the Incorporated Research Institutions for Seismology Data Management Center. For each event, a preliminary magnitude obtained from W phase amplitudes is used to estimate the initial moment rate function half duration and to define the corner frequencies of the passband filter that will be applied to the waveforms. Starting from these initial parameters, the seismic moment tensor is calculated using a preliminary location as a first approximation of the centroid. A full CMT inversion is then conducted for centroid timing and location determination. Comparisons with Harvard and Global CMT solutions highlight the robustness of W phase CMT solutions at teleseismic distances. The differences in M_w rarely exceed 0.2 and the source mechanisms are very similar to one another. Difficulties arise when a target earthquake is shortly (e.g. within 10 hr) preceded by another large earthquake, which disturbs the waveforms of the target event. To deal with such difficult situations, we remove the perturbation caused by earlier disturbing events by subtracting the corresponding synthetics from the data. The CMT parameters for the disturbed event can then be retrieved using the residual seismograms. We also explore the feasibility of obtaining source parameters of smaller earthquakes in the range 6.0 ≤_Mw < 6.5. Results suggest that the W phase inversion can be implemented reliably for the majority of earthquakes of Mw= 6 or larger.

Real-time centroid moment tensor determination for large earthquakes from local and regional displacement records

SUMMARY We present an algorithm to rapidly determine the moment tensor and centroid location for large earthquakes employing local and regional real-time high-rate displacement records from GPS. The algorithm extracts the coseismic offset from the displacement waveforms and uses the information to invert for the moment tensor. The Green's functions for a layered earth are obtained numerically from open source code EDGRN. To determine the centroid, multiple inversions are simultaneously performed within a grid of inversion nodes, and the node with the smallest misfit is then assigned the centroid location. We show results for two large earthquakes replayed in simulated real-time mode using recorded 1 Hz GPS displacements: the 2003 Mw 8.3 Tokachi-oki and the 2010 Mw 7.2 El Mayor-Cucapah earthquakes. We demonstrate that it is feasible to obtain accurate CMT solutions within the first 2–3 min after rupture initiation without any prior assumptions on fault characteristics, demonstrating an order of magnitude improvement in latency compared to existing seismic methods for the two earthquakes studied. This methodology is useful for rapid earthquake response, tsunami prediction and as a starting point for rapid finite fault modelling.

Acquiring, archiving, analyzing and exchanging seismic data in real time at the Seismological Research Center of the OGS in Italy

After the 1976 Friuli earthquake (Ms = 6.5) in north-eastern Italy that caused about 1,000 casualties and widespread destruction in the Friuli area, the Italian government established the Centro di Ricerche Sismologiche (CRS). This is now a department of the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), and it is specifically devoted to the monitoring of the seismicity of north-eastern Italy. Since its inception, the North-East Italy Seismic Network has grown enormously. Currently, it consists of 14 broad-band and 20 short-period seismic stations, all of which are telemetered to and acquired in real time at the OGS-CRS data center in Udine. Data exchange agreements in place with other Italian, Slovenian, Austrian and Swiss seismological institutes lead to a total number of 94 seismic stations acquired in real time, which confirms that the OGS is the reference institute for seismic monitoring of north-eastern Italy. Since 2002, CRS has been using the Antelope software suite as the main tool for collecting, analyzing, archiving and exchanging seismic data. SeisComP is also used as a real-time data exchange server tool. A customized webaccessible server is used to manually relocate earthquakes, and automatic procedures have been set-up for moment-tensor determination, shaking-map computation, web publishing of earthquake parametric data, waveform drumplots, state-of-health parameters, and quality checks of the station by spectra analysis. Scripts for email/SMS/fax alerting to public institutions have also been customized. Recently, a real-time seismology website was designed and set-up (http://rts.crs.inogs.it/).