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2011

Plenkers, K., D. Schorlemmer, G. Kwiatek, and JAGUARS Research Group: On the Probability of Detecting Picoseismicity, Bull. Seismol. Soc. Am., 101, 2579-2591, 10.1785/0120110017, 2011.
Abstract: Estimation of the recording completeness of seismic catalogs recorded with small networks in a heterogeneous observation volume, for example, in mines, is difficult. Local heterogeneities have a strong influence on the wave path and attenuation and must be taken into account. In order to analyze the spatially varying completeness of such catalogs in three dimensions, we present a new approach based on the probability-based magnitude of completeness (PMC) method of Schorlemmer and Woessner (2008). We demonstrate that the traditional approach of Schorlemmer and Woessner (2008) is insufficient in very complex and heterogeneous settings. To account for this problem, we extend the PMC method, taking into account the direction of incoming seismic waves. This allows us to analyze the influence of small heterogeneities on the recording completeness in high resolution. We compare the results with results obtained by a traditional Gutenberg–Richter frequency-magnitude analysis for the JAGUARS catalog: The Japanese German Underground Acoustic Emission Research in South Africa (JAGUARS) project recorded approximately 500,000 seismic events with magnitudes -5 < Mw < -1 in the Mponeng gold mine (Carletonville, South Africa) at -3.5 km depth. The network is surrounded by several cavities and is located partly in the Pink-Green dike, a local geological feature. We estimate that the completeness of the JAGUARS catalog varies significantly in space. In the center of the network, we estimate a magnitude of completeness of MP = -4.8, whereas at stope level (approximately 100 m from the network), the magnitude of completeness is only MP = -3.1. Variations due to the influence of local heterogeneities, for example, tunnels, are clearly resolvable.
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Gentili, S., M. Sugan, L. Peruzza, and D. Schorlemmer: Probabilistic completeness assessment of the past 30 years of seismic monitoring in northeastern Italy, Phys. Earth Planet. Inter., 186, 81-96, :10.1016/j.pepi.2011.03.005, 2011.
Abstract: We investigate detection probabilities and recording completeness of the seismic network in north-eastern Italy, operated by the OGS (Istituto Nazionale di Oceanografia e Geofisica Sperimentale) during the years 1977–2007, using the Probability-based Magnitude of Completeness (PMC) method by Schorlemmer and Woessner (2008). Completeness of the dataset is varying in space and time due to the evolution of network geometry, instrumental characteristics, and monitoring and processing strategies over time: this is a common feature for all the regional and national instrumental catalogues that should be properly accounted for in seismicity rate evaluations. For the first time, we quantify with the new PMC method detection probabilities of a regional network operating since the late 70s, also including paper records. PMC is based on empirical data and requires the earthquake catalog containing phase picks, the station locations and on-/off-times, and the attenuation relation used to compute local magnitudes. In the 30-year period, we identified four time windows, roughly corresponding to the main changes in the acquisition system. We reconstructed on-/off-times by merging the available information on instruments with the analysis of inter-pick times at each station. We revised the relationship between duration and local magnitudes as the calibration of duration magnitude demonstrated to be not homogeneous among the different acquisition systems. Moreover, we introduced some constraints on the application of the PMC method to improve its performance for networks with sparse data and show the role of missing events and of temporary networks on regional completeness maps. The results demonstrate that the OGS network is detecting earthquakes completely down to magnitude 1.5 for a large part of the Friuli-Venezia-Giulia region since the earliest stages of its functionality; the OGS instrumental catalog is therefore the most precise and complete dataset available for this area. Our analysis shows that the change from analog to digital systems does not necessarily correspond to an improvement in detection capabilities. The computed time series of completeness maps are available and should be considered for any seismicity study involving data from the OGS network.
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2010

Schorlemmer, D., F. Mele, and W. Marzocchi: A completeness analysis of the National Seismic Network of Italy, J. Geophys. Res., 115, B04308, 10.1029/2008JB006097, 2010.
Abstract: We present the first detailed study of earthquake detection capabilities of the Italian National Seismic Network and of the completeness threshold of its earthquake catalog. The network in its present form started operating on 16 April 2005 and is a significant improvement over the previous networks. For our analysis, we employed the PMC method as introduced by Schorlemmer and Woessner (2008). This method does not estimate completeness from earthquake samples as traditional methods, mostly based on the linearity of earthquake size distributions. It derives detection capabilities for each station of the network and synthesizes them into maps of detection probabilities for earthquakes of a given magnitude. Thus, this method avoids the many assumptions about earthquake distributions that traditional methods make. The results show that the Italian National Seismic Network is complete at M = 2.9 for the entire territory excluding the islands of Sardinia, Pantelleria, and Lampedusa. At the M = 2.5 level, which is the reporting threshold level of the Italian Civil Protection, the network may miss events in southern parts of Apulia and the western part of Sicily. The stations are connected through many different telemetry links to the operational data center in Rome. Scenario computations show that no significant drop in completeness occurs if one of the three major links fails, indicating a well-balanced network setup.
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Nanjo, K.Z., D. Schorlemmer, J. Woessner, S. Wiemer, and D. Giardini: Earthquake detection capability of the Swiss Seismic Network, Geophys. J. Int., 181, 1713-1724, 10.1111/j.1365-246X.2010.04593.x, 2010.
Abstract: A reliable estimate of completeness magnitudes is vital for many seismicity- and hazard-related studies. Here we adopted and further developed the Probability-based Magnitude of Completeness (PMC) method (Schorlemmer & Woessner 2008). This method determines network detection completeness (MP) using only empirical data: earthquake catalog, phase picks, and station information. To evaluate the applicability to low- or moderate-seismicity regions, we performed a case study in Switzerland. The Swiss Seismic Network (SSN) at present is recording seismicity with one of the densest networks of broad-band sensors in Europe. Based on data from 1 January 1983 to 31 March 2008, we found strong spatio-temporal variability of network completeness: the highest value of MP in Switzerland at present is 2.5 in the far southwest, close to the national boundary, while MP is lower than 1.6 in high-seismicity areas. Thus, events of magnitude 2.5 can be detected in all of Switzerland. We evaluated the temporal evolution of MP for the last 20 years, showing the successful improvement of the SSN. We next introduced the calculation of uncertainties to the probabilistic method using a bootstrap approach. The results show that the uncertainties in completeness magnitudes are generally less than 0.1 magnitude units, implying that the method generates stable estimates of completeness magnitudes. We explored the possible use of PMC (1) as a tool to estimate the number of missing earthquakes in moderate-seismicity regions and (2) as a network planning tool with simulation computations of installations of one or more virtual stations to assess the completeness and identify appropriate locations for new station installations. We compared our results with an existing study of the completeness based on detecting the point of deviation from a power-law in the earthquake-size distribution. In general, the new approach provides higher estimates of the completeness magnitude than the traditional one. We associate this observation with the difference in the sensitivity of the two approaches in periods where the event detectability of the seismic networks is low. Our results allow us to move towards a full description of completeness as a function of space and time, which can be used for hazard-model development and forecast-model testing, showing an illustrative example of the applicability of the PMC method to regions with low to moderate seismicity.
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2008

Schorlemmer, D. and J. Woessner: Probability of Detecting an Earthquake, Bull. Seismol. Soc. Am., 98(5), 2103-2117, 10.1785/0120070105, 2008.
Abstract: We present a new method for estimating earthquake detection probabilities that avoids assumptions about earthquake occurrence, e.g., the frequency-size distribution, and uses only empirical data: phase data, station information, and network specific attenuation relations. First, we determine the detection probability for each station as a function of magnitude and hypocentral distance, using data from past earthquakes. Second, we combine the detection probabilities of stations using a basic combinatoric procedure to determine the probability that a hypothetical earthquake with a given size and location could escape detection. Finally, we synthesize detection probability maps for earthquakes of particular magnitudes and probability-based completeness maps. Because the method relies only on detection probabilities of stations, it can also be used to evaluate hypothetical additions or deletions of stations as well as scenario computations of a network crisis. The new approach has several advantages: completeness is analyzed as a function of network properties instead of earthquake samples, and thus no event-size distribution is assumed. Estimating completeness is becoming possible in regions of sparse data where methods based on parametric earthquake catalogs fail. We find that the catalog of the SCSN has, for most of the region, a lower magnitude of completeness than that computed using traditional techniques, although in some places traditional techniques provide lower estimates. The network reliably records earthquakes smaller than magnitude 1.0 in some places, and 1.0 in the seismically active regions. However, it does not achieve the desired completeness of magnitude ML=1.8 everywhere in its authoritative region. A complete detection is achieved at ML=3.4 in the entire authoritative region; thus, at the boundaries, earthquakes as large as ML=3.3 might escape detection.
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