TY - JOUR A1 - Nooshiri, Nima A1 - Saul, Joachim A1 - Heimann, Sebastian A1 - Tilmann, Frederik A1 - Dahm, Torsten T1 - Revision of earthquake hypocentre locations in global bulletin data sets using source-specific station terms JF - Geophysical journal international N2 - Global earthquake locations are often associated with very large systematic travel-time residuals even for clear arrivals, especially for regional and near-regional stations in subduction zones because of their strongly heterogeneous velocity structure. Travel-time corrections can drastically reduce travel-time residuals at regional stations and, in consequence, improve the relative location accuracy. We have extended the shrinking-box source-specific station terms technique to regional and teleseismic distances and adopted the algorithm for probabilistic, nonlinear, global-search location. We evaluated the potential of the method to compute precise relative hypocentre locations on a global scale. The method has been applied to two specific test regions using existing P- and pP-phase picks. The first data set consists of 3103 events along the Chilean margin and the second one comprises 1680 earthquakes in the Tonga-Fiji subduction zone. Pick data were obtained from the GEOFON earthquake bulletin, produced using data from all available, global station networks. A set of timing corrections varying as a function of source position was calculated for each seismic station. In this way, we could correct the systematic errors introduced into the locations by the inaccuracies in the assumed velocity structure without explicitly solving for a velocity model. Residual statistics show that the median absolute deviation of the travel-time residuals is reduced by 40-60 per cent at regional distances, where the velocity anomalies are strong. Moreover, the spread of the travel-time residuals decreased by similar to 20 per cent at teleseismic distances (>28 degrees). Furthermore, strong variations in initial residuals as a function of recording distance are smoothed out in the final residuals. The relocated catalogues exhibit less scattered locations in depth and sharper images of the seismicity associated with the subducting slabs. Comparison with a high-resolution local catalogue reveals that our relocation process significantly improves the hypocentre locations compared to standard locations. KW - Seismicity and tectonics KW - Computational seismology KW - Subduction zone processes KW - Pacific Ocean KW - South America Y1 - 2016 U6 - https://doi.org/10.1093/gji/ggw405 SN - 0956-540X SN - 1365-246X VL - 208 IS - 2 SP - 589 EP - 602 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Letort, Jean A1 - Retailleau, Lise A1 - Boue, Pierre A1 - Radiguet, Mathilde A1 - Gardonio, Blandine A1 - Cotton, Fabrice Pierre A1 - Campillo, Michel T1 - Lateral variations of the Guerrero-Oaxaca subduction zone (Mexico) derived from weak seismicity (M(b)3.5+) detected on a single array at teleseismic distance JF - Geophysical journal international N2 - Detections of pP and sP phase arrivals (the so-called depth phases) at teleseismic distance provide one of the best ways to estimate earthquake focal depth, as the P-pP and the P-sP delays are strongly dependent on the depth. Based on a new processing workflow and using a single seismic array at teleseismic distance, we can estimate the depth of clusters of small events down to magnitude M-b 3.5. Our method provides a direct view of the relative variations of the seismicity depth from an active area. This study focuses on the application of this new methodology to study the lateral variations of the Guerrero subduction zone (Mexico) using the Eielson seismic array in Alaska (USA). After denoising the signals, 1232 M-b 3.5 + events were detected, with clear P, pP, sP and PcP arrivals. A high-resolution view of the lateral variations of the depth of the seismicity of the Guerero-Oaxaca area is thus obtained. The seismicity is shown to be mainly clustered along the interface, coherently following the geometry of the plate as constrained by the receiver-function analysis along the Meso America Subduction Experiment profile. From this study, the hypothesis of tears on the western part of Guerrero and the eastern part of Oaxaca are strongly confirmed by dramatic lateral changes in the depth of the earthquake clusters. The presence of these two tears might explain the observed lateral variations in seismicity, which is correlated with the boundaries of the slow slip events. KW - North America KW - Time-series analysis KW - Body waves KW - Earthquake source observations KW - Seismicity and tectonics Y1 - 2018 U6 - https://doi.org/10.1093/gji/ggy035 SN - 0956-540X SN - 1365-246X VL - 213 IS - 2 SP - 1002 EP - 1012 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Hendriyana, Andri A1 - Bauer, Klaus A1 - Muksin, Umar A1 - Weber, Michael T1 - AIC-based diffraction stacking for local earthquake locations at the Sumatran Fault (Indonesia) JF - Geophysical journal international N2 - We present a new workflow for the localization of seismic events which is based on a diffraction stacking approach. In order to address the effects from complex source radiation patterns, we suggest to compute diffraction stacking from a characteristic function (CF) instead of stacking the original waveform data. A new CF, which is called in the following mAIC (modified from Akaike Information Criterion) is proposed. We demonstrate that both P- and S-wave onsets can be detected accurately. To avoid cross-talk between P and S waves due to inaccurate velocity models, we separate the P and S waves from the mAIC function by making use of polarization attributes. Then, the final image function is represented by the largest eigenvalue as a result of the covariance analysis between P-and S-image functions. Results from synthetic experiments show that the proposed diffraction stacking provides reliable results. The workflow of the diffraction stacking method was finally applied to local earthquake data from Sumatra, Indonesia. Recordings from a temporary network of 42 stations deployed for nine months around the Tarutung pull-apart basin were analysed. The seismic event locations resulting from the diffraction stacking method align along a segment of the Sumatran Fault. A more complex distribution of seismicity is imaged within and around the Tarutung basin. Two lineaments striking N-S were found in the centre of the Tarutung basin which support independent results from structural geology. KW - Time-series analysis KW - Body waves KW - Computational seismology KW - Earthquake source observations KW - Seismicity and tectonics Y1 - 2018 U6 - https://doi.org/10.1093/gji/ggy045 SN - 0956-540X SN - 1365-246X VL - 213 IS - 2 SP - 952 EP - 962 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Krüger, Frank A1 - Kulikova, Galina A1 - Landgraf, Angela T1 - Magnitudes for the historical 1885 (Belovodskoe), the 1887 (Verny) and the 1889 (Chilik) earthquakes in Central Asia determined from magnetogram recordings JF - Geophysical journal international N2 - Six large magnitude earthquakes in Central Asia which occurred at the end of the 19th century were recorded on early magnetographs in Great Britain. Scalar seismic moment estimates of the 1911 Chon-Kemin, the 1902 Atushi and the 1907 Karatag earthquakes in Central Asia were recently determined by historical seismogram modelling. For those events, we find agreement between moment magnitudes estimated from seismograms and from magnetograms. This supports the assumption of linear scaling of magnetogram amplitudes as function of M-0, which we then use to estimate the moment magnitudes for earlier large-magnitude events, that is, the 1885 Belovodskoe, 1887 Verny and 1889 Chilik earthquakes. The magnetometer data imply that the Chilik earthquake had M(W)7.9, slightly smaller than the Chon-Kemin event with M(W)8.0. The Verny earthquake, however, for which we estimate M(W)7.7, is likely larger than listed in catalogues (M7.3). Similarly, we find a larger magnitude M(W)7.6 (instead of the previous M6.9) for the Belovodskoe earthquake, but this remains uncertain due to measurement imprecision. KW - Earthquake source observations KW - Seismicity and tectonics KW - Intraplate processes Y1 - 2018 U6 - https://doi.org/10.1093/gji/ggy377 SN - 0956-540X SN - 1365-246X VL - 215 IS - 3 SP - 1824 EP - 1840 PB - Oxford Univ. Press CY - Oxford ER - TY - GEN A1 - Passarelli, Luigi A1 - Hainzl, Sebastian A1 - Cesca, Simone A1 - Maccaferri, Francesco A1 - Mucciarelli, Marco A1 - Roessler, Dirk A1 - Corbi, Fabio A1 - Dahm, Torsten A1 - Rivalta, Eleonora T1 - Aseismic transient driving the swarm-like seismic sequence in the Pollino range, Southern Italy (vol 201, pg 1553, 2015) T2 - Geophysical journal international KW - Seismicity and tectonics KW - Statistical seismology KW - Dynamics: seismotectonics Y1 - 2016 U6 - https://doi.org/10.1093/gji/ggv425 SN - 0956-540X SN - 1365-246X VL - 204 SP - 365 EP - 365 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Kulikova, Galina A1 - Schurr, Bernd A1 - Krüger, Frank A1 - Brzoska, Elisabeth A1 - Heimann, Sebastian T1 - Source parameters of the Sarez-Pamir earthquake of 1911 February 18 JF - Geophysical journal international N2 - The Ms ∼ 7.7 Sarez-Pamir earthquake of 1911 February 18 is the largest instrumentally recorded earthquake in the Pamir region. It triggered one of the largest landslides of the past century, building a giant natural dam and forming Lake Sarez. As for many strong earthquakes from that time, information about source parameters of the Sarez-Pamir earthquake is limited due to the sparse observations. Here, we present the analysis of analogue seismic records of the Sarez-Pamir earthquake. We have collected, scanned and digitized 26 seismic records from 13 stations worldwide to relocate the epicentre and determine the event's depth (∼26 km) and magnitude (mB7.3 and Ms7.7). The unusually good quality of the digitized waveforms allowed their modelling, revealing an NE-striking sinistral strike-slip focal mechanism in accordance with regional tectonics. The shallow depth and magnitude (Mw7.3) of the earthquake were confirmed. Additionally, we investigated the possible contribution of the landslide to the waveforms and present an alternative source model assuming the landslide and earthquake occurred in close sequence. KW - Earthquake source observations KW - Seismicity and tectonics KW - Body waves KW - Theoretical seismology Y1 - 2016 U6 - https://doi.org/10.1093/gji/ggw069 SN - 0956-540X SN - 1365-246X VL - 205 SP - 1086 EP - 1098 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Kulikova, Galina A1 - Krüger, Frank T1 - Source process of the 1911 M8.0 Chon-Kemin earthquake: investigation results by analogue seismic records JF - Geophysical journal international N2 - Several destructive earthquakes have occurred in Tien-Shan region at the beginning of 20th century. However, the detailed seismological characteristics, especially source parameters of those earthquakes are still poorly investigated. The Chon-Kemin earthquake is the strongest instrumentally recorded earthquake in the Tien-Shan region. This earthquake has produced an approximately 200 km long system of surface ruptures along Kemin-Chilik fault zone and killed about similar to 400 people. Several studies presented the different information on the earthquake epicentre location and magnitude, and two different focal mechanisms were also published. The reason for the limited knowledge of the source parameters for the Chon-Kemin earthquake is the complexity of old analogue records processing, digitization and analysis. In this study the data from 23 seismic stations worldwide were collected and digitized. The earthquake epicentre was relocated to 42.996NA degrees and 77.367EA degrees, the hypocentre depth is estimated between 10 and 20 km. The magnitude was recalculated to m(B) 8.05, M-s 7.94 and M-w 8.02. The focal mechanism, determined from amplitude ratios comparison of the observed and synthetic seismograms, was: str = 264A degrees, dip = 52A degrees, rake = 98A degrees. The apparent source time duration was between similar to 45 and similar to 70 s, the maximum slip occurred 25 s after the beginning of the rupture. Two subevents were clearly detected from the waveforms with the scalar moment ratio between them of about 1/3, the third subevent was also detected with less certainty. Taking into account surface rupture information, the fault geometry model with three patches was proposed. Based on scaling relations we conclude that the total rupture length was between similar to 260 and 300 km and a maximum rupture width could reach similar to 70 km. KW - Earthquake source observations KW - Seismicity and tectonics KW - Body waves KW - Theoretical seismology Y1 - 2015 U6 - https://doi.org/10.1093/gji/ggv091 SN - 0956-540X SN - 1365-246X VL - 201 IS - 3 SP - 1891 EP - 1911 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Passarelli, Luigi A1 - Hainzl, Sebastian A1 - Cesca, Simone A1 - Maccaferri, Francesco A1 - Mucciarelli, Marco A1 - Rößler, Dirk A1 - Corbi, Fabio A1 - Dahm, Torsten A1 - Rivalta, Eleonora T1 - Aseismic transient driving the swarm-like seismic sequence in the Pollino range, Southern Italy JF - Geophysical journal international N2 - Tectonic earthquake swarms challenge our understanding of earthquake processes since it is difficult to link observations to the underlying physical mechanisms and to assess the hazard they pose. Transient forcing is thought to initiate and drive the spatio-temporal release of energy during swarms. The nature of the transient forcing may vary across sequences and range from aseismic creeping or transient slip to diffusion of pore pressure pulses to fluid redistribution and migration within the seismogenic crust. Distinguishing between such forcing mechanisms may be critical to reduce epistemic uncertainties in the assessment of hazard due to seismic swarms, because it can provide information on the frequency-magnitude distribution of the earthquakes (often deviating from the assumed Gutenberg-Richter relation) and on the expected source parameters influencing the ground motion (for example the stress drop). Here we study the ongoing Pollino range (Southern Italy) seismic swarm, a long-lasting seismic sequence with more than five thousand events recorded and located since October 2010. The two largest shocks (magnitude M-w = 4.2 and M-w = 5.1) are among the largest earthquakes ever recorded in an area which represents a seismic gap in the Italian historical earthquake catalogue. We investigate the geometrical, mechanical and statistical characteristics of the largest earthquakes and of the entire swarm. We calculate the focal mechanisms of the M-l > 3 events in the sequence and the transfer of Coulomb stress on nearby known faults and analyse the statistics of the earthquake catalogue. We find that only 25 per cent of the earthquakes in the sequence can be explained as aftershocks, and the remaining 75 per cent may be attributed to a transient forcing. The b-values change in time throughout the sequence, with low b-values correlated with the period of highest rate of activity and with the occurrence of the largest shock. In the light of recent studies on the palaeoseismic and historical activity in the Pollino area, we identify two scenarios consistent with the observations and our analysis: This and past seismic swarms may have been 'passive' features, with small fault patches failing on largely locked faults, or may have been accompanied by an 'active', largely aseismic, release of a large portion of the accumulated tectonic strain. Those scenarios have very different implications for the seismic hazard of the area. KW - Seismicity and tectonics KW - Statistical seismology KW - Dynamics: seismotectonics Y1 - 2015 U6 - https://doi.org/10.1093/gji/ggv111 SN - 0956-540X SN - 1365-246X VL - 201 IS - 3 SP - 1553 EP - 1567 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Ghods, Abdolreza A1 - Shabanian, Esmaeil A1 - Bergman, Eric A1 - Faridi, Mohammad A1 - Donner, Stefanie A1 - Mortezanejad, Gholamreza A1 - Aziz-Zanjani, Asiyeh T1 - The Varzaghan-Ahar, Iran, Earthquake Doublet (M-w 6.4, 6.2): implications for the geodynamics of northwest Iran JF - Geophysical journal international N2 - On 2012 August 11, a pair of large, damaging earthquakes struck the Varzaghan-Ahar region in northwest Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. To investigate the active tectonics of the source region we applied a combination of seismological methods (local aftershock network, calibrated multiple event relocation and focal mechanism studies), field observations (structural geology and geomorphological) and inversions for the regional stress field. The epicentral region is north of the North Tabriz Fault. The first main shock is characterized by right-lateral strike-slip motion on an almost E-W fault plane of about 23 km length extending from the surface to a depth of about 14 km. The second main shock occurred on an ENE-striking fault that dips at 60-70A degrees to the NW. Independent inversions of focal mechanisms and geologically determined fault kinematic data for the active stress state yield a transpressional tectonic regime with sigma(1) oriented N132E. For the region northeast of the North Tabriz Fault, the presence of rigid lithosphere of the South Caspian Basin implies the kinematic adjustment by northward transferring of the contracted masses through both distributed deformation and structural deflections. Our results suggest that the kinematic adjustment inside a contracting wedge may occur along interacting crosswise or conjugate faults to accommodate low rates of internal deformation. At a global scale, our results indicate that despite the basic assumption of 'rigid blocks' in geodetic plate modelling, internal deformation of block-like regions could control the kinematics of deformation and the level of seismic hazard within and around such regions of low deformation rate. KW - Earthquake source observations KW - Seismicity and tectonics KW - Continental neotectonics KW - Continental tectonics: compressional KW - Dynamics: seismotectonics KW - Asia Y1 - 2015 U6 - https://doi.org/10.1093/gji/ggv306 SN - 0956-540X SN - 1365-246X VL - 203 IS - 1 SP - 522 EP - 540 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Grigoli, Francesco A1 - Cesca, Simone A1 - Amoroso, Ortensia A1 - Emolo, Antonio A1 - Zollo, Aldo A1 - Dahm, Torsten T1 - Automated seismic event location by waveform coherence analysis JF - Geophysical journal international N2 - Automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring. Since microseismic records are generally characterized by low signal-to-noise ratio, automated location methods are requested to be noise robust and sufficiently accurate. Most of the standard automated location routines are based on the automated picking, identification and association of the first arrivals of P and S waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases. Although current methods can accurately pick P onsets, the automatic picking of the S onset is still problematic, especially when the P coda overlaps the S wave onset. In this paper, we propose a picking free earthquake location method based on the use of the short-term-average/long-term-average (STA/LTA) traces at different stations as observed data. For the P phases, we use the STA/LTA traces of the vertical energy function, whereas for the S phases, we use the STA/LTA traces of a second characteristic function, which is obtained using the principal component analysis technique. In order to locate the seismic event, we scan the space of possible hypocentral locations and origin times, and stack the STA/LTA traces along the theoretical arrival time surface for both P and S phases. Iterating this procedure on a 3-D grid, we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event. A pilot application was performed in the Campania-Lucania region (southern Italy) using a seismic network (Irpinia Seismic Network) with an aperture of about 150 km. We located 196 crustal earthquakes (depth < 20 km) with magnitude range 1.1 < M-L < 2.7. A subset of these locations were compared with accurate manual locations refined by using a double-difference technique. Our results indicate a good agreement with manual locations. Moreover, our method is noise robust and performs better than classical location methods based on the automatic picking of the P and S waves first arrivals. KW - Time-series analysis KW - Inverse theory KW - Earthquake source observations KW - Seismicity and tectonics KW - Early warning Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggt477 SN - 0956-540X SN - 1365-246X VL - 196 IS - 3 SP - 1742 EP - 1753 PB - Oxford Univ. Press CY - Oxford ER -