Local Earthquake Locations Research Articles

Connection of Kryvbas tectonics with natural and technogenic seismicity

Purpose. Studying the tectonic features of the structure of the earth’s crust of Kryvyi Rih-Kremenchuk suture zone to clarify the nature of the origin of seismic events in the Kryvbas. Methodology. To analyze and generalize the data on the Kryvbas seismicity with reference to large-scale geological and tectonic maps of exploration work. To study its tectonic structure based on geological, geophysical studies and drilling of Kryvyi Rih superdeep well. Findings. In the period 2011–2021, about 1,200 seismic events were recorded on the territory of Kryvbas, the majority of which had a minor magnitude  2.0. Among them there were identified 13 powerful industrial explosions from mb = 2.7–3.5 and 20 earthquakes of tectonic origin with mb = 2.1–4.5. Powerful explosions in mines predominantly induce earthquakes. In recent years, local earthquakes began to occur in tectonic fault zones outside the ore mining area, which indicates a change in the elastic-deformation state of the geological environment. The analysis of the attributes of local earthquakes and their locations suggests that Kryvyi Rih tectonic system and the entire eastern flank of the earth’s crust of the West Inhulets-Kryvyi Rih-Kremenchuk suture zone are geodynamically active structures, where the processes of thrust and shear tectonics are also observed on a recent geological time scale. Originality. A detailed analysis of the earthquakes shows that some of them occur at significant depths in the zones of tectonic faults outside of Kryvbas. At the same time, faults in separate directions are activated, where brittle deformations and viscous-plastic formations are manifested in the past geological time. The location of local earthquakes made it possible to single out two sections and five linear zones of seismic activity in the region. Practical value. Based on the results from this research it is possible to create an applied model of the tectonic section of the earth’s crust to solve the problems of evolution and geodynamics of the lithosphere of the Ukrainian shield, mountain geology and to optimize mining. The identified faults of active inherited development are important in determining the development paths for quarries and mines in Kryvbas.

Central Baykal Crustal Earthquakes Location Method

Some methodological issues of the improved location of local earthquakes, recorded by the Baykal seismic network, are considered. Hypocenter location improvement is performed by combination of widely used techniques into the integrated technological line, moreover some of these techniques will be used in their new capacity. For example, for the first time for the Baykal area in the Hypoellipse software the following parameters, obtained from Wadati diagrams, will be used: origin time (To), defined with the peak determination factor, is to apply as an objective measure of the location accuracy; Vp/Vs ratio - as a fixed for each of the stations input parameter. In addition, to calculate the travel times in Hypoellipse software a multi-component velocity model (as equivalent of 3D model) have been used also for the first time for the area of investigations.

Epistemic Uncertainties in Local Earthquake Locations and Implications for Managing Induced Seismicity

ABSTRACTEarthquake hypocentral location is perhaps the most classical problem in seismology, the solution of which is often affected by significant uncertainty. In monitoring the effects of underground anthropogenic activities, the earthquake hypocentral location, magnitude, and ground motions are important parameters for managing induced seismicity (as e.g., for operating traffic-light systems). Such decisional systems define the operative reactions to be enacted once an earthquake, exceeding some magnitude or ground-motion threshold, occurs within a monitoring volume defined in the neighborhood of a certain anthropogenic underground activity. In this case, a reliable evaluation of the hypocentral location, along with its uncertainty, becomes crucial for rational decision making. In this article, we analyze different sources of uncertainty that can be relevant for the determination of earthquake source locations, and introduce a logic-tree-based ensemble modeling approach for framing the problem in a decision-making context. To demonstrate the performance of the proposed approach, we analyze uncertainties in the location of a seismic event that occurred on 22 July 2019 within the perimeter of the monitoring domain defined in the Val d’Agri oil field (southern Italy). We cast the result as a model ensemble that allows us to obtain samples from a parent distribution that better represents both aleatory and epistemic uncertainties of the earthquake location problem. We find that often-neglected epistemic uncertainties (i.e., those that arise when considering alternative plausible modeling approaches or data) can be considerably larger and more representative of the state of knowledge about the source location, than the standard errors usually reported by the most common algorithms. Given the consequential repercussions of decision making under uncertainty, we stress that an objective evaluation of epistemic uncertainties associated with any parameter used to support decisional processes must be a priority for the scientific community.

Improved earthquake location in the gold mining regions of South Africa using new velocity models

AbstractReliable local earthquake locations depend on many factors of which a major one is the velocity model. Currently the Council for Geoscience (CGS) has been using the global IASP91 velocity model for earthquake locations in the cluster networks. To continue improving the earthquake locations it is necessary that new velocity models are determined for each cluster region (Central and East Rand - CERAND, the Klerksdorp – Orkney – Stilfontein – Hartebeesfontein – KOSH and the Far West and West Rand - WRAND). The availability of good quality data recorded by the cluster networks since their inception in 2010 provides an opportunity to conduct this work. Thus data from the cluster networks database were selected according to set quality criteria to obtain parametric data for 130 earthquakes in the CERAND region, 404 in the KOSH region and 1024 in the WRAND region. These data were used to determine a minimum 1-D velocity model with associated station corrections for each of the regions using the VELEST software package. Comparison of epicentres obtained using the new velocity models to epicentres previously published by the CGS, showed improvement in the quality of the new locations. Thus, the new models will be implemented in the day-to-day analysis of data recorded in the three study regions by the cluster network of stations.

AIC-based diffraction stacking for local earthquake locations at the Sumatran Fault (Indonesia)

AIC-based diffraction stacking for local earthquake locations at the Sumatran Fault (Indonesia)

Apparently-deep events in the Middle Atlas resolved to be shallow: Implications for lithospheric deformation

The occurrence of intermediate depth seismicity in intracontinental settings is rare, but it has been postulated with various degrees of certainty in several regions. One such region is the Middle Atlas of Morocco. Since 1960, 75 intermediate depth earthquakes have been reported in this region by Spain's National Geographic Institute (IGN). The apparent deep nature of these events is hard to reconcile with well-established geophysical evidence of a thin lithosphere under the Middle Atlas, but has been associated with mantle delamination processes. We relocate 4 events with IGN-reported depths >80km that were recorded by a relatively dense temporary deployment; using a recent regional 3D velocity model obtained through teleseismic body and surface wave tomography. The relocation procedure uses a grid-search approach to minimize the mean normalized misfit, where each travel-time misfit is normalized by the estimated pick uncertainty. We find that our observed arrivals are much better fit by shallow (<5km) depths than the reported depths of >80km. We propose that these shallow foci earthquakes are the result of regional crustal deformation of this region caused by the present convergence between Africa and Eurasian Plate. We infer that if there are any ongoing delamination processes in the area, they are aseismic. This study is an example of how local earthquake locations in tectonically complex areas can be significantly improved by using a dense local seismic array and a well-constrained 3-D velocity model.

1-D Velocity Model for Use by the SANSN in Earthquake Location

Reliable local earthquake locations depend on many factors, a major one of which is the velocity model. Locating earthquakes using unreliable models contributes in part to the uncertainties of active fault mapping and unexplained scatter of s eismic locations. Given that we strive to continually improve our locating abilities, it is necessary to always improve on the model, when possible. The South African National Seismograph Network (SANSN) has been using a model (Saunders et al. 2006) from work published by Wright et al. (2002) and refined by Brandt (2005). Even though there have been many crust and mantle structure studies in South Africa ( e.g. , Nguuri et al. 2001; Simon et al. 2002; Wright et al. 2002; James et al. 2004), the models obtained are too complicated for use in seismic event locations, especially when using the earthquake analysis software SEISAN (Havskov and Ottemoller 2008). The HYPOCENTER program (Lienert 1994), which is the location program in SEISAN, cannot use models with low-velocity layers, because these usually introduce instabilities. Modification/refinement of the results of one of these studies (Wright et al. 2002) by Brandt (2005) produced a model that the SANSN is currently using in earthquake locations. This model can therefore be used as a starting point in conducting local earthquake tomography to obtain a more realiable model. Our study aims to compute a 1-D velocity model for South Africa by inverting P -wave travel times recorded by SANSN. Ideally, SANSN should use different models for the different major tectonic units in South Africa, i.e. , Kaapvaal craton, Namaqualand, and Cape Fold belt. However, available data dictates what can be done, and for now we can only produce a 1-D model whose velocities are approximately equal to the average velocity of the 3-D structure within the same …

The COMPLOC Earthquake Location Package

This article describes the programs included in the COMPLOC computer program package that are designed to apply the source-specific station term (SSST) method to solve for local earthquake locations using P- and S- wave phase data. These programs can greatly improve the relative location accuracy of nearby events by applying empirical corrections for the biasing effects of three-dimensional velocity structure. They have been tested on data from both the Southern California Seismic Network (SCSN) and Northern California Seismic Network (NCSN) The SSST method (Richards-Dinger and Shearer 2000; Lin and Shearer 2005) works by assigning each station a travel-time correction that varies as a function of source position. This approach provides relative location accuracy comparable to master event or hypocentroidal decomposition (Jordan and Sverdrup 1981) methods within compact event clusters, but is applicable to distributed seismicity. It has some similarities to the double-difference algorithm (Waldhauser and Ellsworth 2000, 2002) and can be shown to give comparable results in tests on synthetic data (Lin and Shearer 2005). However, the SSST approach has computational advantages for big data sets because the location and station term parts of the computation are separate, so that large matrix inversions are not necessary. In addition, our implementation of SSST provides the option to use L1-norm misfit measures, which are more robust than least squares in the case of occasional timing errors or bad phase picks. We implement the SSST approach by selecting nearby events located within a sphere of specified radius r max around the target event. The station term for the target event is then computed as the median (or mean) residual of these events. Different results will be obtained depending upon the size of the cutoff distance r max. If r max is set to a large enough distance, then the SSST …

Earthquake Relocation and Focal Mechanism Determination Using Waveform Cross Correlation, Nicoya Peninsula, Costa Rica

The Nicoya Peninsula in Costa Rica directly overlies the seismogenic zone of the Middle America Trench, making it an ideal location for geophysical investigations of shallow subduction zone earthquake processes. As part of the collaborative Costa Rica Seismogenic Zone Experiment (crseize), a seismic network consisting of 20 land and 14 ocean-bottom seismometers recorded small magnitude local earthquakes along the Nicoya Peninsula from December 1999 to June 2001. Previous studies have used these data to compute local earthquake locations and 3D velocity structure to identify plate boundary seismicity and to investigate seismogenic behavior. Here we utilize waveform cross-correlation and clustering techniques in an attempt to improve earthquake relocations and determine first-motion focal mechanisms to validate, refine, and expand on existing models. Due to the high quality of the original locations and the small cross-correlation P -wave arrival time adjustments, large differences between the previously determined and the cross-correlated earthquake locations are not observed. However, focal mechanism determinations using cross-correlated P waveforms are significantly enhanced. Approximately 90% of the focal mechanisms computed for events previously identified as interplate earthquakes are consistent with underthrusting. Focal mechanisms for continental intraplate events indicate dextral strike-slip motion in the central region and normal faulting at the southern tip of the peninsula. These motions may be associated with oblique convergence and seamount subduction, respectively. Within the subducting plate, steep P and T axes of earthquakes below 50 km depth are consistent with unbending of the slab.

ASDP: a PC-based program using a multi-algorithm approach for automatic detection and location of local earthquakes

A few automated data acquisition and processing systems operate on mainframes, some are run on UNIX-based workstations and others on personal computer, equipped with either DOS or UNIX-derived operating systems. Several large and complex software packages for automatic and interactive analysis of seismic data have been developed in recent years mainly for UNIX-based systems, and some of these programs use a variety of artificial intelligence techniques. Here, the first operational version of a new software package, named PC-Seism, for analyzing seismic data from a local network is presented. This package, composed of three separate modules, provides an example of a new generation of visual object-oriented programs for interactive and automatic seismic data processing run on a personal computer. In particular, we discuss the automatic procedures implemented in the ASDP (Automatic Seismic Data Processing) module. A multi-algorithm approach to the on-line detection and location of local earthquakes is adopted in ASDP, and its operative mode is similar to that used in more complex systems, where the algorithms run on different processors and parallel computations are generally performed. Since highly complex computation routines may still be prohibitive for current PC when the number of analyzing traces becomes large, we have opted for simplicity and have planned three main routines (working in cascade mode) and a multi-station analysis (MSA) procedure in ASDP, to recognize phase picking, declare and locate earthquakes. Basically, signal detection on a single-component trace is obtained by a short-term average to long-term average ratio (STA/LTA) taken along a characteristic function (CF) envelope generated from the seismogram. To confirm and identify earthquake phase arrivals and to discard noise disturbances, two other sections of analysis are applied on short signal windows after the declared triggers. A spectral analysis is applied as detector of earthquake phase arrivals. Furthermore, for three-component data, the application of a polarization filter is used to identify P- and S-phases. Therefore in order to optimize the efficiency of a seismic monitoring system, the software is provided with full detection capability and makes use of a high degree of automation, aimed at minimizing the analyst's workload. PC-Seism and, in particular, the ASDP module are under constant review. In fact, the modular structure of the program will allow us to develop it with new and improved capabilities. Future developments improvements in ASDP will concern the application of other detection algorithms and routines to recognize secondary phases in regional and teleseismic earthquakes.

The significance of the crustal velocity model in local earthquake locations from a case example of a PANDA experiment in the central United States

Abstract Reliable local earthquake locations depend on many factors including the quality of earthquake data, the spatial distribution of network stations, the location algorithms, and the local and regional seismic velocity models. The uncertainties of P- and S-wave arrival times at various seismic stations are, in large part, responsible for the uncertainty in the determination of crustal velocity models and earthquake locations. We took the advantage of the high-quality and high-dynamic range PANDA data from a 33-month seismic array experiment in the central United States to investigate the importance of crustal velocity models, especially the S-wave model, on earthquake locations. Several crustal models are used to relocate a group of selected earthquakes from the PANDA database. It is clear that earthquake depths are determined less reliably than the epicenters when the shear-wave velocity model is not properly determined. The mislocations in some examples are significant that the importance of determining reliable P and S velocity models independently cannot be overlooked. If the crustal P- and S-wave velocity structures in the region can be well determined, reliable earthquake locations are possible even with only a few accurate P and S readings.

Improving local earthquake locations using the L1 norm and waveform cross correlation: Application to the Whittier Narrows, California, aftershock sequence

Experiments with different earthquake location methods applied to aftershocks of the October 1, 1987, Whittier Narrows earthquake in California (ML=5.9) suggest that local event locations can be greatly improved through the use of the L1 norm, station corrections and waveform cross correlation. The Whittier Narrows sequence is a compact cluster of over 500 events at 12 to 18 km depth located within the dense station coverage of the Southern California Seismic Network (SCSN), a telemetered network of several hundred short‐period seismographs. SCSN travel time picks and waveforms obtained through the Southern California Earthquake Center are examined for 589 earthquakes between 1981 and 1994 in the vicinity of the mainshock. Using a smoothed version of the standard southern California velocity model and the existing travel time picks, improved location accuracy is obtained through use of the L1 norm rather than the conventional least squares (L2 norm) approach, presumably due to the more robust response of the former to outliers in the data. A large additional improvement results from the use of station terms to account for three‐dimensional velocity structure outside of the event cluster. To achieve greater location accuracy, waveforms for these events are resampled and low‐pass filtered, and thePandSwave cross‐correlation functions are computed at each station for every event pair. For those events with similar waveforms, differential times may be obtained from the cross‐correlation functions. These times are then combined with the travel time picks to invert for an adjusted set of picks that are more consistent than the original picks and include seismograms that were originally unpicked. Locations obtained from the adjusted picks show a further improvement in accuracy. Location uncertainties are estimated using a bootstrap technique in which events are relocated many times for sets of picks in which the travel time residuals at the best fitting location are used to randomly perturb each pick. Improvements in location accuracy are indicated by the reduced scatter in the residuals, smaller estimated location errors, and the increased tendency of the locations to cluster along well‐defined fault planes. Median standard errors for the final inversion are 150 m in horizontal location and 230 m in vertical location, although the relative locations within localized clusters of similar events are better constrained. Seismicity cross sections resolve the shallow dipping fault plane associated with the mainshock and a steeply dipping fault plane associated with aML=5.3 aftershock. These fault planes appear to cross, and activity began on the secondary fault plane prior to the large aftershock.

Interactive local earthquake location on a PC

Two small and fast programs for interactive location of local earthquakes are presented; one considers the medium as a homogeneous halfspace, the other works with velocity models representing laterally (and for some special situations, radially) homogeneous media. Both programs feature automatic (least-squares damped iterative residual minimization in the distance domain) location; however, their main feature is the graphic interactive PC version of the old map and compass location method, which lets the user apply his/her expertise and easily incorporate supplementary data or information to the location process, and gives a clear picture of the goodness of the locations and their possible errors. Because both station and arrival times (phase) files can be created or modified directly from the programs, data checking and error correction can be carried out interactively.

The effects of crack anisotropy on the hypocentral locations of local earthquakes

Summary. Exact modelling of the earthquake location process in regions of dilatancy-anisotropy shows that failure to take account of the velocity anisotropy in the determination of local hypocentres can result in erroneous and misleading locations. In particular, the locations can indicate spurious migrations of foci from the true epicentral positions and the true depths of foci. These spurious locations may indicate planes of hypocentres deviating from the true fault plane. Observations of such phenomena have been noted several times in the literature. However, once the anisotropic model is known, a simple location program, incorporating the anisotropic velocity-variations, permits accurate location of local earthquakes using P and first S-wave arrival times.

Differences in time terms between New Zealand seismograph stations and implications for crustal structure

Abstract Differences between time terms roughly equal to the crustal contributions to Pn and Sn travel times have been calculated for pairs of stations of the New Zealand seismograph network. The maximum differences found between 2 stations are 2.6 s for P-waves and 6.8 s for S-waves. These differences reflect major differences in crustal structure and, although they cannot be interpreted unambiguously in terms of crustal thickness and/or crustal seismic wave velocities, they provide important constraints on structure not evident from surface geology. The results will ultimately be important in improving the accuracy of local earthquake locations.

An algorithm for the optimum distribution of a regional seismic network — II. An analysis of the accuracy of location of local earthquakes depending on the number of seismic stations

Seven optimal networks consisting of 4 to 10 stations are compared for a given region, where velocity-depth profiles and the distribution of seismic intensity are known. Assuming that the standard error of arrival time is σ t =0.05 s and the standard errors of the parameters of velocity-depth profiles are equal to 5% of their values, the average standard errors of the origin time $$\langle \sigma _{t_0 } \rangle $$ and focus coordinates $$\langle \sigma _{x_0 } \rangle , \langle \sigma _{y_0 } \rangle , \langle \sigma _{z_0 } \rangle $$ are estimated. The application of optimum methods to the planning of seismic networks in the Lublin Coal Basin is presented, and maps of standard errors of origin time $$(\sigma _{t_0 } )$$ , depth $$(\sigma _{z_0 } )$$ and epicenter (σ xy ) for the case of an optimum network of 6 seismic stations are given.

Time-term analysis and geological interpretation of seismic travel-time data from the Coast Ranges of central California

Time terms resulting from an analysis of Pg seismic travel-time data from the Coast Ranges of central California yield a pattern that is remarkably consistent with other geological and geophysical estimates of depth to basement. A set of high-quality data for more than 330 paths was selected from all available travel-time observations from quarries and explosions. To assure a statistical estimate of the uncertainty of each time term, time terms were calculated only for sites recording arrivals from more than one source. Time terms ranged from 0.0 to 1.5 sec, indicating depths to the Pg refractor of near-surface to more than 6 km, depending on assumed near-surface velocities. The Pg refractor is presumed to be granitic and metamorphic rocks southwest of the San Andreas Fault and volcanic or ultrabasic rocks within or beneath the Franciscan Formation northeast of the San Andreas Fault. Small time terms (< 0.4 sec) are associated with exposed granitic basement in the Santa Cruz and Gabilan ranges and with exposures of the Franciscan Formation in the central Diablo Range and in the Santa Teresa Hills (south of San Jose). Large time terms are associated with large thicknesses of upper Cretaceous and younger sedimentary rocks in the northern Santa Cruz Mountains, in the northern part of the Diablo Range, in structural troughs between the Zayante and San Andreas faults, and between the Sargent and San Andreas faults and in the Hollister Trough. Time terms for sites athwart structural discontinuities, particularly the San Andreas Fault, vary with the direction of the approaching ray. Time terms for sites on the relatively homogeneous granitic mass of the Gabilan Range increase with elevation. The large differences in magnitude of these time terms and the nature of the distribution clearly demonstrate that the laterally inhomogeneous Earth's crust in this area must be taken into account in the location of local earthquakes where high accuracy is required. These time terms provide a firm basis for the necessary station corrections.