Random Catalogs Research Articles

Suppressed radio emission in supercluster galaxies: enhanced ram pressure in merging clusters?

Context. Continuum radio emission of galaxies is related to AGN activity and to starbursts, both of which require a supply of gas, respectively to the central black hole and to molecular clouds. Aims. The environmental influence on the 21 cm (1.4 GHz) continuum radio emission of galaxies is analyzed in a 600 deg 2 region of the local Universe containing the Shapley Supercluster (SSC), whose core is thought to be the site of cluster-cluster merging. Methods. Galaxies in the FLASH and 6dFGS optical/NIR redshift surveys are cross-identified with NVSS radio sources, selected in a subsample doubly complete in volume and luminosity, and classified as starbursts or AGN according to their radio luminosity. We study radio luminosities as well as radio loudness (luminosities normalized by stellar mass) RK . Environmental effects are studied through a smoothed density field (normalized to that obtained from random catalogs with the same survey edges and redshift selection function) and through the projected distance to the nearest cluster (in units of its virial radius, whose relation to the aperture velocity dispersion is quantified). Results. The fraction of high RK galaxies in the dense 10 Mpc Abell 3558 cluster complex at the core of the SSC (SSC-CR) is half as large than elsewhere. Moreover, radio loudness in the SSC-CR is anti-correlated with the density of the large-scale environment and correlated with clustercentric radius: central brightest cluster galaxies (BCGs) in the SSC-CR are an order of magnitude less radioloud than BCGs elsewhere, with signs of suppressed radio loudness also present beyond the BCGs, out to at least 0.3r200. The gradual suppression of radio loudness from inner cluster regions to the cluster centers highlights a significant correlation of radio loudness with clustercentric radius, not seen outside the SSC-CR. This correlation is nearly as strong as the tight correlation of K-band luminosity, LK, with clustercentric radius (K-luminosity segregation), inside the SSC-CR, with a mild K-luminosity segregation outside the SSC-CR. Conclusions. The suppression of radio loudness in SSC-CR BCGs can be attributed to major cluster-cluster mergers that destroy the cool core and thus the supply of gas to the central AGN. We analytically demonstrate that the low radio loudness of non-BCG galaxies within SSC-CR clusters cannot be explained by direct major galaxy mergers or rapid galaxy flyby collisions, but by the loss of gas supply through the enhanced ram pressure felt when these galaxies cross the shock front between the two merging clusters and are afterwards subjected to the stronger wind from the second cluster.

Increased correlation range of seismicity before large events manifested by earthquake chains

“Earthquake chains” are clusters of moderate-size earthquakes which extend over large distances and are formed by statistically rare pairs of events that are close in space and time (“neighbors”). Earthquake chains are supposed to be precursors of large earthquakes with lead times of a few months. Here we substantiate this hypothesis by mass testing it using a random earthquake catalog. Also, we study stability under variation of parameters and some properties of the chains. We found two invariant parameters: they characterize the spatial and energy scales of earthquake correlation. Both parameters of the chains show good correlation with the magnitudes of the earthquakes they precede. Earthquake chains are known as the first stage of the earthquake prediction algorithm reverse tracing of precursors (RTP) now tested in forward prediction. A discussion of the complete RTP algorithm is outside the scope of this paper, but the results presented here are important to substantiate the RTP approach.

Tests of the precursory accelerating moment release model using a synthetic seismicity model for Wellington, New Zealand

We have constructed a synthetic seismicity model of the Wellington region, New Zealand, including seven superfaults and 500 subfaults which are randomly positioned. From this model, a synthetic catalogue of 2000 years duration, containing events of magnitude 5.0 or more, has been generated. The properties of the catalogue, such as the long‐term slip rates, b value, average activity rate, and hypocenter distribution, are in accord with paleoseismic studies and the real seismicity over the last 40 years. Such a synthetic catalogue can replace the short, incomplete, and inhomogeneous historic and instrumental records in research which needs a long time duration and many strong shocks. We have used our catalogue to examine tests for the existence of accelerating moment release (AMR) before large events and compared the results with those from random (Poisson) catalogues. We find that (1) the apparent success rate is very dependent on the rules used to define the test window; (2) when appropriately defined, the AMR pattern occurs before about 20% of the strong (M ≥ 7.0) shocks with a typical precursor time of about 22 years; (3) the AMR pattern is found almost equally frequently before large events in random catalogues; and (4) there are some false alarms (AMR pattern without a large event). This extended study reinforces the conclusion in our preliminary report: that in synthetic catalogues of the kind we have constructed, the AMR pattern is essentially an artefact of the method of sampling.

Seismicity changes associated with the 2000 earthquake swarm in the Izu Island region

A statistical method taking into account the information of magnitude, occurrence time and location of earthquakes was applied to the earthquake data (1977–2000) of the Japan Meteorological Agency (JMA) to investigate seismicity changes in the Izu Island region. The analysis indicated that a quiescence anomaly started about 1.5 years before the occurrence of the largest swarm in the summer of 2000 in the Izu Island region. Close investigations of the possible artifacts due to the selection of model parameters and the improvement of the seismological network lead to the conclusion that the above quiescence anomaly is unlikely a man-made change. The further stochastic test using 1000 random earthquake catalogs supports the idea that the above anomaly is significant. The spatial distribution of the seismic quiescence, which is quantified by a newly developed Q-parameter, revealed a clear anomalous region around the epicentral area of the above earthquake swarm. The seismicity revealed by the normalized parameter tends to increase just before the swarm. As the first test of the above statistical method for investigating seismicity changes of earthquake swarms, this study indicates that the increased RTL parameter would be a new potentially useful index for the risk alarm of earthquake swarms.

An observational test of the stress accumulation model based on seismicity preceding the 1992 Landers, CA earthquake

We test the Bowman and King [Bowman, D.D., King, G.C.P., 2001a, Accelerating seismicity and stress accumulation before large earthquakes. Geophys. Res. Lett., 28 (21), 4039–4042, Bowman, D.D., King, G.C.P., 2001b. Stress transfer and seismicity changes before large earthquakes. C. R. Acad. Sci. Paris, 333, 591–599] Stress Accumulation model by examining the evolution of seismicity rates prior to the 1992 Landers, California earthquake. The Stress Accumulation (SA) model was developed to explain observations of accelerating seismicity preceding large earthquakes. The model proposes that accelerating seismicity sequences result from the tectonic loading of large fault structures through aseismic slip in the elasto-plastic lower crust. This loading progressively increases the stress on smaller faults within a critical region around the main structure, thereby causing the observed acceleration of precursory activity. A secondary prediction of the SA model is that the precursory seismicity rates should increase first at the edges of the critical region, with the rates gradually rising over time at closer distances to the main fault. We test this prediction by examining year-long seismicity rates between 1960 and 2004, as a function of distance from the Landers rupture. To quantify the significance of trends in the seismicity rates, we auto-correlate the data, using a range of spatial and temporal lags. We find weak evidence for increased seismicity rates propagating towards the Landers rupture, but cannot conclusively distinguish these results from those obtained for a random earthquake catalog. However, we find a strong indication of periodicity in the rate fluctuations, as well as high correlation between activity 130–170 km from Landers and seismicity rates within 50 km of the Landers rupture temporally offset 1.5–2 years. The implications of this spatio–temporal correlation will be addressed in future studies.

Tidal triggering of earthquakes in Japan related to the regional tectonic stress

We observe a correlation between the Earth tide and earthquake occurrence that is closely related to the regional tectonic stress. We investigate the direction of the tidal compressional stress using shallow earthquakes occurring in 100 subregions of Japan for nearly five years. The azimuthal distribution of the compressional stress obtained for the observed earthquake data is compared with that synthesized for random earthquake occurrence. Statistical analysis confirms a significant difference between the observed and random catalogs for 13 subregions, which include the areas where unusual seismic activities took place recently, and where the possibility of future large earthquakes has been argued. For these subregions, earthquakes preferentially occur when the tidal compressional stress is near the dominant direction of P-axes of focal mechanisms obtained in the corresponding subregions. This suggests that the tidal stress may encourage earthquake occurrence when it acts in the direction to increase the regional tectonic stress.

Seismicity Pattern Changes Prior to Large Earthquakes An Approach of the RTL Algorithm

A statistical method, which is called the Region-Time-Length (RTL) algorithm and takes into account information such as magnitude, occurrence time and place of earthquakes, was applied to earthquake data to investigate seismicity pattern changes prior to large earthquakes. Based on the RTL algorithm and some newly developed parameters such as the Qparameter (average of the RTL values over some time window) and S-parameter (an index of seismic activation), I quantified both the temporal and spatial characteristics of seismicity pattern changes in various tectonic regions. The results indicated that seismic quiescence anomalies generally started a few years before the occurrence of the earthquakes and lasted from 1 to 2.5 years. The duration of the subsequent stage of seismic activation generally lasted several months. The linear dimension of the quiescence zone reached a few hundred kilometers (several times the rupture dimension of the mainshock), while the activation zone was generally in order of several tens of kilometers (comparable to the rupture dimension). An earthquake is most likely to occur once the relevant source region has passed through the quiescence and activation stages. Close investigation of possible artifacts due to the selection of model parameters and the changes of seismological networks are important in identifying real seismicity changes from man-made ones. Further stochastic testing using random earthquake catalogs was also done and it supports that the anomalies revealed in my works are significant. Besides studying on seismicity changes before large earthquakes, I also performed the first test of the above statistical method for investigating seismicity changes of earthquake swarms. It indicated that an increased RTL parameter would be a new potentially useful index for the risk alarm of earthquake swarms.

Accelerating seismic crustal deformation before strong mainshocks in Adriatic and its importance for earthquake prediction

Time accelerating Benioff strain releasebefore the mainshock has been observed inall five cases of strong (M > 6.0) shallowmainshocks, which have occurred during thelast four decades in the area surroundingthe Adriatic Sea. This observation supportsthe idea that strong mainshocks arepreceded by accelerating seismic crustaldeformation due to the generation ofintermediate magnitude shocks (preshocks).It is further shown that the values ofparameters calculated from these datafollow appropriately modified relations,which have previously been proposed asadditional constraints to the criticalearthquake model and to the correspondingmethod of intermediate term earthquakeprediction. Thus, these results show thatthe identification of regions wheretime-accelerating Benioff strain followssuch constraints may lead to usefulinformation concerning the epicenter,magnitude and origin time of oncomingstrong mainshocks in this area. Theprocedure for identification of thetime-acceleration is validated byappropriate application on synthetic butrealistic random catalogues. Largerdimension of critical regions in Adriaticcompared to such regions in the Aegean isattributed to an order of magnitude smallerseismic deformation of the crust in theformer in comparison to the latter.

A systematic spatiotemporal test of the critical point hypothesis for large earthquakes

The critical point hypothesis for large earthquakes predicts two different precursory phenomena in space and time, an accelerating moment release and the growth of the spatial correlation length. The objective of this work is to investigate both methods with respect to their predictive power. A systematic statistical test based on appropriate random earthquake catalogs allows to quantify the correlations of a precursory pattern with the subsequent mainshock activity. The analysis of target earthquakes in California since 1960 with magnitudes M ≥ Mcut reveals that these correlations increase systematically with growing Mcut, and correlations at greater than 95% confidence are observed for Mcut ≥ 6.5 in the case of the spatial correlation length. In particular, the seismicity patterns are found to be significantly correlated with each of the largest earthquakes (M ≥ 7.0), individually. The acceleration of the moment release has a similar trend, but is less significant.

Accelerating Seismic Crustal Deformation in the Southern Aegean Area

A region of intense accelerating seismic crustal deformation has been identified in the southwestern part of the Hellenic arc (broader area of Cythera island). The identification is performed using a detailed parametric grid search of the broader southern Aegean area for accelerating energy release behavior. The identified region has similar properties with past preshock (critical) regions, which have been identified for strong mainshocks in the Aegean area. Based on such observations, which suggest that this region is at a critical state that can lead to a critical point, that is, to the generation of a mainshock, an estimation is made of the possible epicenter coordinates, magnitude, and origin time of this oncoming large (M ∼7.0) earthquake. The estimation procedure is validated on the basis of retrospective analysis of strong events in the Aegean area, as well as by appropriate application on synthetic random catalogs. These results, the existence of similar observations of accelerating seismic deformation in eastern part of southern Aegean and independent information on the time distribution of large earthquakes (M ≥6.8) for the whole southern Aegean indicate that the generation of strong earthquakes in this area in the next few years must be considered as very probable.

Accelerating Seismic Energy Release and Evolution of Event Time and Size Statistics: Results from Two Heterogeneous Cellular Automaton Models

The evolution of event time and size statistics in two heterogeneous cellular automaton models of earthquake behavior are studied and compared to the evolution of these quantities during observed periods of accelerating seismic energy release Drier to large earthquakes. The two automata have different nearest neighbor laws, one of which produces self-organized critical (SOC) behavior (PSD model) and the other which produces quasi-periodic large events (crack model). In the PSD model periods of accelerating energy release before large events are rare. In the crack model, many large events are preceded by periods of accelerating energy release. When compared to randomized event catalogs, accelerating energy release before large events occurs more often than random in the crack model but less often than random in the PSD model; it is easier to tell the crack and PSD model results apart from each other than to tell either model apart from a random catalog. The evolution of event sizes during the accelerating energy release sequences in all models is compared to that of observed sequences. The accelerating energy release sequences in the crack model consist of an increase in the rate of events of all sizes, consistent with observations from a small number of natural cases, however inconsistent with a larger number of cases in which there is an increase in the rate of only moderate-sized events. On average, no increase in the rate of events of any size is seen before large events in the PSD model.

Static stress transfer and earthquake triggering: No lower threshold in sight?

The main objective of this study is to see if a lower threshold for earthquake triggering exists. Resolving this issue is important for the understanding of earthquake mechanics and for the purpose of hazard analysis. We compute the cumulative static stress changes imposed on 63 M ≥ 4.5 earthquakes in central California between 1969 and 1998, by adding the stress changes imposed by all previous M ≥ 4.5 earthquakes as a function of time prior to the events. We find that 85% of the cumulative stress changes at the time of rupture are positive for stress change magnitudes of >10 kPa (>0.1 bar), and 70% are positive for stress changes of <10 kPa (< 0.1 bar) as well as <1 kPa (<0.01 bar). A comparison between these results and those obtained for synthetic catalogs, in which the timing or focal mechanisms of the earthquakes were randomized, shows that this degree of triggering is very unlikely to be found in a random catalog. Thus we conclude that no lower threshold for earthquake triggering in central California has been found. We show that the temporal distribution of stress changes that discourage failure is consistent with the theoretical prediction that the time delay increases with the magnitude of the stress change.

An observational test of the critical earthquake concept

We test the concept that seismicity prior to a large earthquake can be understood in terms of the statistical physics of a critical phase transition. In this model, the cumulative seismic strain release increases as a power law time to failure before the final event. Furthermore, the region of correlated seismicity predicted by this model is much greater than would be predicted from simple elastodynamic interactions. We present a systematic procedure to test for the accelerating seismicity predicted by the critical point model and to identify the region approaching criticality, based on a comparison between the observed cumulative energy (Benioff strain) release and the power law behavior predicted by theory. This method is used to find the critical region before all earthquakes along the San Andreas system since 1950 with M≥6.5. The statistical significance of our results is assessed by performing the same procedure on a large number of randomly generated synthetic catalogs. The null hypothesis, that the observed acceleration in all these earthquakes could result from spurious patterns generated by our procedure in purely random catalogs, is rejected with 99.5% confidence. An empirical relation between the logarithm of the critical region radius (R) and the magnitude of the final event (M) is found, such that log R∝0.5M, suggesting that the largest probable event in a given region scales with the size of the regional fault network.

A catalogue of the voids in the IRAS 1.2- Jy survey

Using the VOID FINDER algorithm we have compiled a catalogue of voids in the IRAS 1.2-Jy sample. The positions of the voids correspond well to underdense regions seen in the IRAS smoothed density map. However, since in our analysis no smoothing is used, all structures appear much sharper: walls are not smeared and the voids are not artificially reduced by them. Therefore the current method based on the galaxy point distribution is better suited to determine the diameter of voids in the galaxy distribution. We have identified 24 voids, covering more than 30% of the volume considered. By comparing the results with equivalent random catalogues we have determined that 12 voids are significant at a 0.95 confidence level, having an average diameter of 40+-6 h^{-1} Mpc. Our results serve not only for charting the cosmography of the nearby Universe, but also to give support to the results recently obtained with the SSRS2 sample, suggesting a void-filled Universe. Moreover, our results indicate that the voids detected have a similar scale, demonstrating that both optically and IRAS-selected galaxies delineate the same large-scale structures.

The supercluster–void network

We investigate the distribution of superclusters and voids using a new catalogue of superclusters of rich clusters of galaxies which extends up to a redshift of . The new catalogue contains 220 superclusters of rich clusters, of which 90 superclusters have been determined for the first time. Among them there are several very rich superclusters, containing at least eight member clusters. We demonstrate that two thirds of very rich superclusters are concentrated to a Dominant Supercluster Plane which is situated at a right angle with respect to the plane of the Local Supercluster and adjacent nearby superclusters. We apply several methods to estimate the characteristic distance between superclusters. The results indicate consistently the presence of a quite regular supercluster-void network with scale of h-1 Mpc. Comparison with random supercluster catalogues shows significant differences between spatial distributions of real and random superclusters. We determine the selection function of the sample of clusters and suggest that the mean true space density of Abell clusters is , twice the conventionally used value.

Voids in the distribution of galaxies: an assessment of their significance and derivation of a void spectrum

An algorithm to search for nearly spherical regions which are devoid of bright galaxies is presented. The presence of empty regions, or voids, is an intrinsic property of any spatial point data. Some means must thus be found of distinguishing between voids which have been produced by some physical mechanism. A method is developed to assess the significance of the candidate voids by means of Monte Carlo simulations, using random catalogues which mimic the selection effects of the actual data set. The resultant list of more than 100 voids throughout the entire sky is used to derive a spectrum of voids size.

Superclustering of Galaxy Clusters

The use of Abell's (1958) complete redshift sample of rich clusters of galaxies with distance class less than or equal to 4 to calculate the spatial correlation function of the rich clusters of galaxies is described. The distance and separations for the clusters of galaxies are estimated. It is observed that the mean density remains constant for small redshifts and then the density falls off due to the limited accuracy of the estimated distance. The calculated density data are compared with random catalog data; the observed density selection function is applied to the development of a supercluster catalog. 18 references.

A supercluster catalog

The largest available complete redshift sample of rich clusters of galaxies is used to construct a three-dimensional catalog of superclusters complete to redhsift of about 0.08 or less. The criterion of membership in the catalog is the density enhancement of clusters over the mean density; the catalog is presented for various values of this parameter. The dependence of the identified sample of superclusters on the overdensity is studied, and the sensitivity of the identified superclusters to various quantities and selection procedures is investigated. The validity of the identification procedure is established by comparisons with random catalogs.

Filamentary structure in the Shane-Wirtanen galaxy distribution

view Abstract Citations (50) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Filamentary structure in the Shane-Wirtanen galaxy distribution. Kuhn, J. R. ; Uson, J. M. Abstract The paper considers a statistic that is sensitive to differences in the galaxy distribution of the Shane-Wirtanen catalog and similar random catalogs with the same low-order galaxy autocorrelation functions. It has been indicated by numerical experiments that this statistic is a measure of the filamentary structure in these catalogs. The present results show that there is significantly more filamentary structure in the Shane-Wirtanen galaxy distribution than in model catalogs without added chainlike structure. Publication: The Astrophysical Journal Pub Date: December 1982 DOI: 10.1086/183921 Bibcode: 1982ApJ...263L..47K Keywords: Autocorrelation; Galactic Clusters; Spatial Distribution; Statistical Analysis; Astronomical Maps; Monte Carlo Method; Random Walk; Astrophysics; Clusters of Galaxies:Large-Scale Structure full text sources ADS |

A method of studying variations in the clustering property of earthquakes: Application to the analysis of global seismicity

We propose a formal algorithm for identification of aftershocks following a main event. Our approach is based on the analysis of the local statistics: the number of events following the ‘initial’ earthquake displayed as a function of distance and time. By comparing this distribution with the random catalog, we establish, for several magnitude ranges, spatiotemporal thresholds for detection of aftershocks. High‐confidence thresholds correspond to the limit at which the number of events derived from the actual catalog (corrected for the background seismicity) exceeds by a factor of 10 the expected level of background seismicity; marginal confidence thresholds correspond to the spatiotemporal boundary along which these two values are approximately equal. After several iterations, the final thresholds are derived. The swarming parameter is defined as the number of aftershocks, with a cut‐off lower magnitude range, following an initial earthquake of certain magnitude. If a correction for the recurrence law is applied, we find that the swarming parameter is magnitude independent. This allows us to apply this technique to the analysis of global seismicity. Using a catalog derived from the ISC Bulletin for the years from 1964 to 1975, we determine for shallow earthquakes the average values of the swarming parameter in 9°×9° cells. A global map produced in this manner shows a very wide range of variations in the swarming property, undoubtedly reflecting variations in the local geological conditions. Triple junctions are among the areas with the highest swarming values. At the present time there is an insufficient amount of data to study regional variations in the swarming property of deep earthquakes. Considering the clustering as a function of depth alone, we find detectable swarming for two depth ranges: from 100 to 250 km and from 450 to 650 km. Not a single aftershock has been detected among nearly 200 earthquakes withmb≥5 in a depth range from 250 to 450 km. The average swarming parameter for earthquakes deeper than 100 km is 10 times smaller than that for the shallow earthquakes.