A consistent and uniform research earthquake catalog for the AlpArray region (original) (raw)
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Geophysical Journal International, 2019
We present an iterative classification scheme using interevent cross-correlation to update an existing earthquake catalogue with similar events from a list of automatic seismic event detections. The algorithm automatically produces catalogue quality events, with improved hypocentres and reliable P-and S-arrival time information. Detected events are classified into four event categories with the purpose of using the top category, with the highest assessed event quality and highest true-to-false ratio, directly for local earthquake tomography without additional manual analysis. The remaining categories have varying proportions of lower quality events, quality being defined primarily by the number of observed phase onsets, and can be viewed as different priority groups for manual inspection to reduce the time spent by a seismic analyst. A list of 3348 event detections from the geothermally active volcanic region around Hengill, southwest Iceland, produced by our migration and stack detector (Wagner et al. 2017), was processed using a reference catalogue of 1108 manually picked events from the same area. P-and S-phase onset times were automatically determined for the detected events using correlation time lags with respect to manually picked phase arrivals from different multiple reference events at the same station. A significant improvement of the initial hypocentre estimates was achieved after relocating the detected events using the computed phase onset times. The differential time data set resulting from the correlation was successfully used for a double-difference relocation of the final updated catalogue. The routine can potentially be implemented in real-time seismic monitoring environments in combination with a variety of seismic event/phase detectors.
Arxiv preprint arXiv:1007.0201, 2010
In this paper we present two new procedures for automatic detection and picking of P-wave arrivals. The first involves the application of kurtosis and skewness on the vector magnitude of three component seismograms. Customarily, P-wave arrival detection techniques use vertical component seismogram which is appropriate only for teleseismic events. The inherent weakness of those methods stems from the fact that the energy from P-wave is distributed among horizontal and vertical recording channels. Our procedure, however, uses the vector magnitude which accommodates all components. The results show that this procedure would be useful for detecting/picking of P-arrivals from local and regional earthquakes and man-made explosions. The second procedure introduces a new method called "Ratios in Higher Order Statistics (RHOS)." Unlike commonly used techniques that involve derivatives, this technique employs ratios of adjacent kurtosis and skewness values to improve the accuracy of the detection of the P onset. RHOS can be applied independently on vertical component seismogram as well as the vector magnitude for improved detection of P-wave arrivals. PACS numbers: 91.30.-f, 93.85.Rt
Optimizing Event Detection and Location in Low‐Seismicity Zones: Case Study from Western Switzerland
Bulletin of the Seismological Society of America, 2016
Obtaining robust event catalogs in regions of low seismicity can be timeconsuming, because quality events are less frequent and sensor coverage is generally sparse. Optimizing event detection and location in such regions is all the more crucial because these areas tend to host a higher density of sensitive infrastructures. The methodology proposed consists of reprocessing existing data recorded by a permanent network and boosting the final catalog resolution by temporarily deploying portable sparse mini-arrays in the target area. Sonogram analysis is applied on both existing and new datasets to detect waveforms barely emerging from the background noise. A visual interactive event analysis module is used to test for phase picking, event association, waveform cross correlation, and location ambiguities. It also estimates back azimuth and slowness when sparse array data are available. The method is applied to a low-seismicity region in the western Swiss Molasse basin where two sparse miniarrays were temporarily deployed. The detection of earthquakes is improved by a factor of 9 when reprocessing four yrs (2009-2013) of available data recorded by two accelerometers and one broadband station in a 2500 km 2 target area. Magnitude estimations are empirically calibrated over four magnitude units, down to −1:7 M L , lowering the existing catalog completeness by close to one magnitude unit. After validating picking and location accuracies with a standard residual-based scheme, 174 newly detected events are relocated, illuminating zones of previously undetected microseismic activity.
Adaptive Techniques for Estimating and Locating Small Events Using Regional Waveform Data
2000
Attempts at locating and identifying small events could benefit greatly from a better appreciation of regional seismograms. Essentially, we need more information from the few regional seismograms available than just arrival times and amplitude ratios. Here we discuss a method of estimating source parameters using as few as one broadband station in conjunction with travel-time and polarities from at least one more station. The method employs an adaptive grid-search of matching three-component waveform records against synthetics to establish source location and depth. The basic matching procedure contains a trade-off of source mechanism with location. The better the constraint on mechanism and depth, the better the location. It appears that refined depths and origin times can be obtained for GT5 events if the first-motion polarities are available to constrain the focal plane. The locations of GT10 and GT25 and EHB (Engdahl, Kennett, and Buland) events can also be improved if CMT's and other independent depth estimates are available. Preliminary results applied to the PASSCAl data recorded in Pakistan and Tibet (China) and TriNet (CAL) proved highly effective when locating events located approximately between two stations. If both stations have usable three-component data, we can determine depth and source mechanism and refine the location. If we assume the mechanism and depth are known (Master Reference Event), we can relocate the event using just one broadband station and the P-wave arrival time pick from the other, or we can refine the crustal model. We find good agreement when testing these results against those from the entire array. Crustal complexities can change the separation between key phases (Pnl, Snl and surface waves) which may require some calibration for maximum sensitivity. In many situations, distortions in wave-shapes may be severe enough to require 2D and 3D Green's functions. To accomplish this objective, we are developing an adaptive method for recovering 2D structure using Waveform Tomography. Our approach uses individual generalized rays computed from a layered model. The model is divided into blocks with variable velocity perturbations such that ray responses are allowed to shift relative to each other to maximize synthetic waveform fits to data. An efficient simulated annealing algorithm is employed in this search. The technique is applied to a collection of 25 aftershocks (Landers EQ) as recorded at two stations, GSC and PFO, separated by about 200 km which bracket the event population along the Landers fault system. The events are assumed to have known mechanisms and epicenters, but both their depths and origin times are allowed to vary. The results indicate considerable variation, especially in the top layer (up to ± 13%) which mirrors surface geology. A similar experiment involving a profile of observations across Tibet indicates substantial deep crustal variation. But given such large changes in velocities requires regeneration of the ray responses and the process repeated. Presently, we are at this stage of development.
Robust and Reliable Epicenter Determinations: Envelope Processing of Local Network Data
Local event recordings are complex in the sense that relevant P and S phases vary in an unpredictable manner even between closely spaced stations; thus, manual analysis of such records is still commonplace. Our approach to solving this long-standing problem of observational seismology is to bandpass filter (3 to 6 Hz) to ensure good signal-to-noise ratio SNR and then form envelopes to ensure simple signals across a seismograph network. The physical basis is that Pg and Lg are crustal wave-guide phases reflecting P-and S-energy propagation. Extensive tests on envelope analysis of local records from different areas found that arrival times of the maxima of Pg and Lg envelopes increase very consistently with distance even in different tectonic regimes, typical velocities being 6.1 and 3.5 km/sec, respectively. These arrival-time parameters are easy to extract in a semi-automatic manner and are highly suitable for local epicenter determinations. Extensive tests on locating mining explosions were conducted, and on average, the "envelope" location errors relative to "true" locations were similar to those in bulletins that are based on conventional phase pickings. Occasionally, the Pg/Pn envelope may be very weak but can be replaced by the easily pickable (non-envelope) Pn phase. Additional advantages with envelope locations are transportability (not overly sensitive to details of crustal structure), and that envelope amplitudes can be directly converted to ground motion and magnitudes. For modern stations, envelopes can be formed in situ with low sampling rates of 1 to 2 Hz, thus greatly reducing transmission costs.
Global Ground Truth Data Set with Waveform and Improved Arrival Data
2006
The main objective of the three-year research project is to produce a quality controlled global GT0-5 event set, accompanied with waveform and groomed arrival time data sets. Our efforts are directed toward developing and refining methodologies for generating new GT events through multiple event location analysis. Multiple event location techniques, such as Hypocentroidal Decomposition (HDC) (Jordan and Sverdup, 1981; Engdahl et al., 2004), provide precise relative locations within an event cluster. However, the absolute locations could still be biased. In order to get accurate absolute locations, independent GT information is needed. We have developed a novel multiple-event location technique, Reciprocal Cluster Analysis (RCA), which combines local data with regional/teleseismic HDC results and uses local stations as GT0 constraints to obtain accurate absolute locations. We have validated the HDC-RCA methodology using an event cluster of GT0 nuclear explosions and GT5 earthquakes which occurred within the Nevada Test Site (NTS). We demonstrated that the HDC-RCA method requires neither dense local networks, nor prior GT information. It relies on a few local stations, provided that the station centroid is inside the event cluster. We showed that absolute locations obtained from the HDC-RCA analysis are consistent with the true GT locations as RCA reduces the regional/teleseismic bias to less than 5 km. Monte Carlo simulations demonstrated that the RCA error ellipses are conservative estimates of the absolute location uncertainties. This allows us to identify GT5 events based on the semi-major axis of their error ellipses scaled to the 95% confidence level. Using this criterion, we identified 21 out of 24 GT events in the NTS cluster. The size of the 95% confidence error ellipses is mainly driven by the reading errors. We utilize waveform crosscorrelation to reduce reading errors, and possibly identify phases not reported in bulletins. Waveform correlation also offers a way to flag and correct phase identification errors. We follow a rigorous statistical approach by using the significance of the cross-correlation to assess the similarity of waveforms. Arrival times are automatically adjusted according to the optimal alignment derived from the waveform cross-correlation, thus resulting in accurate phase picks with reduced measurement errors. We further demonstrate the potential of the HDC-RCA approach on selected event clusters (Chi-Chi, Taiwan; Afar triangle, Africa) and we are prepared to process candidate event clusters selected from an updated EHB (Engdahl et al., 1998) bulletin. 27th Seismic Research Review: Ground-Based Nuclear Explosion Monitoring Technologies Extreme Value PDF Extreme Value Cumulative Distribution Z-transformed max(correlation) Maximum correlation Normal Distribution Measured Correlations Z-transformed max(correlation) Significance=0.995
Remote Triggering Not Evident Near Epicenters of Impending Great Earthquakes
Bulletin of the Seismological Society of America, 2013
Recently, there have been numerous great (M w ≥ 8), devastating earthquakes, with a rate in the last seven years that is 260% of the average rate during the 111-year seismological history. Each great earthquake presents an opportunity to study a major fault at the very beginning and end of the inferred seismic cycle. In this work, we use these events as both targets and sources to probe susceptibility to dynamic triggering in the epicentral region before and after a large earthquake. This study also carefully addresses the possibility that large earthquakes interact in a cascade of remotely triggered sequences that culminates in further large earthquakes. We seek evidence of triggering associated with the 16 great M w ≥ 8 events that occurred between 1998 and 2011, using regional and global earthquake catalogs to measure changes in interevent time statistics. Statistical significance is calculated with respect to a nonstationary reference model that includes mainshock-aftershock clustering. We find limited evidence that a few great earthquakes triggered an increase in seismicity at the site of the next great earthquake in the sequence. However, this evidence is not corroborated by all statistical tests nor all earthquake catalogs. Systematic triggered rate changes in the years to decades before each great earthquake are less than 19% at the 95% confidence level, too small to explain the observed rate increase. The catalogs are insufficient for the purpose of resolving more moderate triggering expected from previous studies. We calculate that an improvement in completeness magnitude from 3.7 to 3.5 could resolve the expected triggering signal in the International Seismic Center (ISC) catalog taken as a whole, but an improvement to M 2.0 would be needed to consistently resolve triggering on a regional basis.
Regional Magnitude Research Supporting Broad-Area Monitoring of Small Seismic Events
The Los Alamos National Laboratory's Ground-Based Nuclear Explosion Monitoring Research and Engineering (GNEMRE) Program has a long-standing magnitude research project in support of regional yield estimation and source discrimination. This project has developed magnitude-yield scaling relationships based on regional P, S phases and coda waves to improve our capabilities to monitor nuclear explosions over broad areas and at low yields. Due to the great variability of regional seismograms, the methods developed for broad-area monitoring must be adaptable to different phases and frequency bands. These requirements, along with an understanding of the transportability of scaling relationships, pose a significant challenge from both practical and theoretical standpoints since any broad-area method needs a sound physical basis to be ultimately successful.
Energy-stack and Kurtosis: the dynamic duo for microseismic event identification
2018
Event detection, validation and arrival-time picking are critical components for processing microseismic monitoring data. The energy-stack has been developed as a fast and efficient scanning method for potential microseismic events. Among a large number of tested metrics used in arrival-time picking, the use of kurtosis stands out due its accuracy and the onset sharpness of either Pand S-waves in real-world noisy datasets. This paper combines energy stacking and kurtosis as the core of a new workflow based on: 1) building a potential-event catalog based in energy-stack and peak properties; 2) analysis of the signal of each peak to refines the initial catalog; and 3) quality-control for removal of false and poor quality events. Benchmarking of the potential-event detector shows excellent results compared with several other existing methods. In particular, the energy-stack approach collects all quasi-simultaneous perturbations in the geophone array, while the kurtosis approach collect...