Earthquake location in tectonic structures of the Alpine Chain: the case of the Constance Lake (Central Europe) seismic sequence

A set of four magnitude Ml ≥ 3.0 earthquakes including the magnitude Ml = 3.7 mainshock of the seismic sequence hitting the Lake Constance, Southern Germany, area in July–August 2019 was studied by means of bulletin and waveform data collected from 86 seismic stations of the Central Europe-Alpine region. The first single-event locations obtained using a uniform 1-D velocity model, and both fixed and free depths, showed residuals of the order of up ± 2.0 s, systematically affecting stations located in different areas of the study region. Namely, German stations to the northeast of the epicenters and French stations to the west exhibit negative residuals, while Italian stations located to the southeast are characterized by similarly large positive residuals. As a consequence, the epicentral coordinates were affected by a significant bias of the order of 4–5 km to the NNE. The locations were repeated applying a method that uses different velocity models for three groups of stations situated in different geological environments, obtaining more accurate locations. Moreover, the application of two methods of relative locations and joint hypocentral determination, without improving the absolute location of the master event, has shown that the sources of the four considered events are separated by distances of the order of one km both in horizontal coordinates and in depths. A particular attention has been paid to the geographical positions of the seismic stations used in the locations and their relationship with the known crustal features, such as the Moho depth and velocity anomalies in the studied region. Significant correlations between the observed travel time residuals and the crustal structure were obtained.

Spatio-temporal clustering of successive earthquakes: analysis of a global CMT catalog.

Spatio-temporal clustering of seismicity features is an interesting phenomenon that is relevant for earthquake generation process and operational earthquake forecasting. We analyze successive earthquakes that closely occur in space and time in order to clarify how large earthquakes successively occur. We use the Global Centroid Moment Tensor catalog for the period from 1976 to 2016. Shallow earthquakes with a moment magnitude, Mw , of larger than or equal to 5.0 are analyzed. We first sort all of the earthquakes in time to select a master event from the beginning. Then, we group the earthquakes that occur within a horizontal distance ( D ) and a lapse time ( Ta ) from the master event into a cluster. Next master event is selected from the catalog in order, and the same procedure is repeated. We count the number of the clusters, which represent the successive earthquakes, for different D and Ta To examine whether or not successive earthquakes randomly occur, we compare the results with simulations in which earthquakes are set to occur randomly in time but at the locations same with the estimated centroid. The results show that the cumulative numbers of clusters for the simulation more rapidly increase with the horizontal distance than those for real data at short distance ranges, and the formers approach to the latter at long distance range. The triggering distance, at which the cumulative numbers of real and simulation data merge, increases with increasing the magnitude of master event. The triggering distance becomes smaller as the lapse time increases, which implies that the seismic activity turns to become the normal condition in which the occurrence time intervals of large earthquakes obey a Poisson distribution. The triggering distance increases with being almost proportional to the 1/3 of the seismic moment of master earthquake, and the number of earthquakes occurring in the region with positive Coulomb stress change (ΔCFF) are more than 60-80% of the total number of the successive earthquakes. These results suggest that static stress change introduced by a master event is one of the triggering mechanism of successive earthquakes.

Black Protest Events In The Us 1994-2010: Issues, Campaigns And Trends

This paper fills a major empirical gap in providing the first systematic data about US Black movement protests between the 1994 and 2010. There is a dearth of systematic information about the Black movement and Black protests between 1995 and its reemergence in Black Lives Matter in 2012. Using a new automated system, we identified 1017 events in 1049 news wire stories relevant to Black protest between 1994 and 2010 from the Annotated English Gigaword file. These data provide insight into Black activism in the between 1994 and 2010 and identify themes that are salient for both the resurgence of the Black Lives Movement after 2012 and the rise of White supremacist movements. Over a third of all protest events are about policing, generally about police violence. Other major themes include direct struggles over White vs. Black symbolic representations and access to resources, including protests over Confederate symbols, abolition of affirmative action programs, and White supremacists. The 1995 Million Man March shows up as having a significant connection to subsequent mobilization, a relationship that is confirmed by historical accounts. A third of the protests are around a wide variety of other specific issues that will require more detailed classification. Time trends show active and rising Black mobilization in the 1990s through 2001 that was interrupted by the 9/11 terror attacks and not significantly resumed until 2006-7. Overall, most protests in this era appear to have been reacting against attacks on existing benefits rather than proactively seeking new benefits. Methodologically, we show the importance of maintaining information about both events and coverage of events and of identifying the relations between events using the concepts of campaign, master event, and subevent. NOTE: This is a draft paper based on a preliminary analysis of data. We are continuing to refine the analysis and revise the paper. Please check with us for the latest version if you wish to cite the paper. This is a wholesale revision of a 2017 paper based on what turned out to be only a subset of the events in the newswire stories, due to an error in the selection of stories fed into the MPEDS classifier.

Relocation of clustered earthquakes in the Groningen gas field

Previous locations of earthquakes induced by depletion of the Groningen gas field were not accurate enough to infer which faults in the reservoir are reactivated. A multiplet analysis is performed to identify clusters of earthquakes that have similar waveforms, representing repeating rupture on the same or nearby faults. The multiplet analysis is based on the cross-correlation of seismograms to assess the degree of similarity. Using data of a single station, six earthquake clusters within the limits of the Groningen field were identified for the period 2010 to mid-2014. Four of these clusters were suitable for a relocation method that is based on the difference in travel time between the P- and the S-wave. Events within a cluster can be relocated relative to a master event with improved accuracy by cross-correlating first arrivals. By choosing master events located with a new dense seismic network, the relocated events likely not only have better relative, but also improved absolute locations. For a few clusters with sufficient signal-to-noise detections, we show that the relocation method is successful in assigning clusters to specific faults at the reservoir level. Overall, about 90% of the events did not show clustering, despite choosing low correlation thresholds of 0.5 and 0.6. This suggests that different faults and/or fault segments with likely varying source mechanisms are active in reservoir sub-regions of a few square kilometres.

Peningkatan Akurasi Lokasi Gempa Mikro Dengan Menggunakan Metoda Double-Difference Dan Korelasi Silang Master Waveform

Perekaman gempa mikro menjadi suatu metode yang umum dilakukan untuk memperkirakan struktur geologi pada suatu lapangan panas bumi. Data hasil perekaman gempa mikro di Lapangan Panas Bumi ARD, sebelum proses pemboran eksplorasi, diolah dan dianalisis dalam penelitian ini dengan tujuan untuk memetakan struktur bawah permukaan. Jaringan seismik pada Lapangan Panas Bumi ARD terdiri dari 26 stasiun yang dipasang selama 8 bulan sejak Agustus 2011 dalam radius 20 km dari target reservoir. Terdapat 637 gempa mikro lokal yang berhasil terdeteksi dan terpetakan selama pengukuran berlangsung. Jumlah ini relatif besar untuk sebuah daerah yang masih dalam tahap awal eksplorasi panas bumi. Akan tetapi, akurasi penentuan lokasi hiposenter gempa mikro perlu ditingkatkan mengingat keberadaan kesalahan penentuan waktu tiba dan model kecepatan. Studi ini bertujuan untuk meminimalkan kesalahan akibat dua parameter tersebut dengan menggunakan metode korelasi silang dan double dierence. Korelasi silang waveform dengan master event diterapkan untuk meningkatkan kualitas penentuan waktu tiba. Terdapat 559 dan 527 pasangan waveform yang memiliki korelasi diatas 0.76. Gempa-gempa mikro yang muncul diinterpretasikan berasal dari bidang patahan yang sama. Koreksi waktu tiba rata-rata yang dihasilkan adalah sebesar 0.2 detik. Setelah dilakukan relokasi dengan metode double dierence, lokasi gempa bergeser sekitar 200 hingga 1500 m dan dihasilkan cluster gempa mikro yang lebih tajam. Kluster gempa mikro tersebut bersesuaian dengan struktur berarah Timur laut - Barat daya di lapangan ARD. Residual travel time gempa mikro juga menunjukkan penurunan yang signikan. Dari hasil penelitian dapat disimpulkan bahwa relokasi hiposenter dengan metode korelasi silang master event dan double difference berhasil mereduksi ketidakpastian lokasi hiposenter di lapangan ARD.

Seismological investigation of the 2016 January 6 North Korean underground nuclear test

Seismology plays an important role in characterizing potential underground nuclear tests. Using broad-band digital seismic data from Northeast China, South Korea and Japan, we investigated the properties of the recent seismic event occurred in North Korea on 2016 January 6. Using a relative location method and choosing the previous 2006 explosion as the master event, the 2016 event was located within the North Korean nuclear test site, with its epicentre at latitude 41.3003°N and longitude 129.0678°E, approximately 900 m north and 500 m west of the previous event on 2013 February 12. Based on the error ellipse, the relocation uncertainty was approximately 70 m. Using the P/S spectral ratios, including Pg/Lg, Pn/Lg and Pn/Sn, as the discriminants, we identify the 2016 event as an explosion rather than an earthquake. The body-wave magnitude calculated from regional wave Lg is mb(Lg) equal to 4.7 ± 0.2. Adopting an empirical magnitude–yield relation, and assuming that the explosion is fully coupled and detonated at a normally scaled depth, we find that the seismic yield is about 4 kt, with the uncertainties allowing a range from 2 to 8 kt.

An improved method for hydrofracture-induced microseismic event detection and phase picking

The ability to detect small microearthquakes and identify their P- and S-phase arrivals is a key issue in hydrofracture downhole monitoring because of the low signal-to-noise ratios (S/N). An array-based waveform correlation approach (matched filter) is applied to improve the detectability of small magnitude events with mechanisms and locations similar to a nearby master event. After detecting the weak events, a transformed spectrogram method is used to identify the phase arrivals. The technique has been tested on a downhole monitoring data set of the microseismic events induced by hydraulic fracturing. It is shown that, for this case, one event with a S/N around [Formula: see text], which is barely detectable using an array-stacked short-time average/long-time average (STA/LTA) detector under a reasonable false alarm rate, is readily detected on the array-stacked correlation traces. The transformed spectrogram analysis of the detected events improves P- and S-phase picking.