Graciela Rojo Limón


Last Name

Rojo Limón

First Name

Graciela

ORCID

0000-0002-7730-3734

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2020 - 20234
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Publications 1 - 4 of 4
  • Local magnitude determination for Turkey from two independent seismic networks and a consistent M4 + catalogue
    Item type: Journal Article
    Rojo Limón, Graciela; Ouillon, Guy; Kissling, Eduard; et al. (2023)
    Geophysical Journal International
    Turkish seismicity is routinely recorded by two broad-band seismic networks: the Turkish National Seismic Network (AFAD 'TU' network) and the Bogazici University Kandilli Observatory And Earthquake Research Institute (KOERI 'KO' network). Each of them has over 200 seismic stations distributed across the country. We obtain a new uniform and consistent local magnitude (Ml) scale for Turkey using data from both networks, as well as the M4 + catalogue with Ml values that results from applying said scale to the events that occurred between 2007 and 2016 within the country. We measure the half peak-to-peak amplitude of the vertical component seismograms and estimate the attenuation parameters through an iterative procedure that fits the logarithm of the amplitude as a function of the hypocentral distance, the local magnitude, the station corrections and a constant accounting for possible systematic differences between amplitudes measured on horizontal seismographs and those measured on vertical seismographs. We additionally scale Ml to the moment magnitude measured by an independent source, obtaining a magnitude relationship accounting for attenuation adjustments, as well as differences between maximum horizontal and vertical amplitudes. The estimated Ml formula shows that station corrections from both networks regionally agree, and that non-zero corrections are centred on zero, within-0.48 and 0.46 magnitude units (corresponding to the 2.5 and the 97.5 quantiles). The new M4 + catalogue with consistent Ml values for AFAD and KOERI shows magnitude residuals that cannot be explained by a Gaussian distribution. We also show a very good agreement between our new Ml value and the original Ml values of AFAD and KOERI, with median differences below 0.1 magnitude units.
  • An updated parametric catalog of historical earthquakes around the Dead Sea Transform Fault Zone
    Item type: Journal Article
    Grigoratos, Iason; Poggi, Valerio; Danciu, Laurentiu; et al. (2020)
    Journal of Seismology
    In this study, we present the work done to review the existing historical earthquake information of the Dead Sea Transform Fault Zone (DSTFZ). Several studies from various sources have been collected and reassessed, with the ultimate goal of creating of new homogenized parametric earthquake catalog for the region. We analyze 244 earthquakes between 31 BC and 1900, which are associated with the geographical buffer extending from 27 N to 36 N and from 31 E to 39 E. Of these, 93 were considered real seismic events with moment magnitude (Mw) greater than 5 that indeed occurred within this zone. While we relied on past parametric data and did not assign new macroseismic intensities, magnitude values, or epicenters for several controversial events, we did however resort to the primary sources to obtain a more critical perspective for the various assigned macroseismic intensities. In order to validate the derived parametric information, we tried to associate the events present in the historical records, with any evidence coming from past field investigations, i.e., geological or archaeological studies. Acknowledging the uneven quality and quantity of data reporting each event, we provided each entry with an uncertainty range estimate. Our catalog lists 33 events of Mw ≥ 6 absent from the latest published compilation with compatible time span and areal coverage. The whole catalog is considered complete down to Mw 7 and in certain areas down to Mw 6 after the year 1000, with majority of the larger earthquakes located in the part of DSTFZ, which extends from the southeast part of Dead Sea lake till Antioch.
  • A consistent and high-quality M4+ earthquake catalogue for Turkey 2007–2016 from two independent catalogues
    Item type: Journal Article
    Rojo Limón, Graciela; Kissling, Eduard; Ouillon, Guy; et al. (2021)
    Geophysical Journal International
    We describe the process of pairing M4+ earthquakes reported in the routine catalogues of two seismic networks in Turkey and present the resulting products: (i) hypocentre locations and magnitudes and (ii) minimum 1-D velocity model with station delays for the combined station network. The two networks are the Kandilli Observatory (KOERI) and the National Earthquake Department (AFAD) and the data cover the decade 2007–2016. The seismic stations of these two networks spatially overlap within Turkey, and with their average station spacing of about 60 km, they potentially detect all events of magnitude ≥4 independently. We first obtain separate AFAD and KOERI catalogues using a series of joint hypocentre inversions that allow us to detect outliers and problematic events. We establish separate minimum 1-D velocity models and station delays for each catalogue and reduce the average inversion residuals by 75 per cent compared to our first joint hypocentre inversion using the original catalogue event locations. We then pair M4+ events from AFAD (1219) and KOERI (1271) catalogues using a spatiotemporal window and test if the combined observations indeed belong to the same event. In these tests, we compare the hypocentre location residuals of the combined observations to those of the two individual events, apply epicentral distance limits between the individual and the paired events and assess qualitatively a simple probabilistic solution of the events. As a result, we get 1145 paired events. We finally obtain a minimum 1-D velocity model with delays for all seismic stations of AFAD and KOERI combined and relocate the entire set of events. These include all paired events plus M4+ earthquakes reported by AFAD or KOERI that could not be paired (500). The velocity model has a well-defined Moho at a depth of 32 km. The station delays for both networks regionally correspond very well and exhibit some correlation with the regional subsurface temperature field. The well-locatable hypocentres of the new M4+ catalogue have an epicentre uncertainty of 3 km and a depth uncertainty of 4 km. We ascribe to each paired earthquake a magnitude equal to the average of the reported magnitudes by AFAD and KOERI. The new magnitude frequency distribution is similar to those of AFAD and KOERI for M4+ events. Except for events of magnitude larger than 5.5, there is no systematic bias in the magnitudes reported by AFAD and by KOERI, and the standard deviation is 0.3.
  • High quality parametric seismic catalogues from several sources around the Dead Sea Fault Zone and in Turkey
    Item type: Doctoral Thesis
    Rojo Limón, Graciela (2022)
    Large earthquakes pose a serious threat to humanity, especially since potentially active faults are hidden under the surface and have left no obvious traces of rupture, and because fault interaction and multiple rupture mechanisms are poorly understood. To study said big events, and to improve the forecasting capabilities of current statistical models, we need to study past seismicity. In recent years there has been a large effort towards making seismic data available to the geosciences research communities. Open-access initiatives like the European Integrated Data Archive and the datasets built by the Global Earthquake Model combine and provide earthquake information for regions beyond geopolitical borders, for extended periods of time and magnitude ranges. Users can compare and combine information to create custom earthquake catalogues. The result is a compendium of several sources, or a preliminary meta-analysis on singular events. As a consequence, single events may be reported more than once: in historical times with different sources reporting effects of ground shaking; in instrumental catalogues multiple reporting may also happen when earthquakes are detected by more than one seismic network. Because each catalogue source has different procedures to calculate locations and magnitudes, the resulting parameters may be different and it is not straightforward to extract useful information or to quantify their error. There has also been much progress in models that use seismic catalogues, and their statistical byproducts, for seismic hazard and forecasting. These include the 2020 European Seismic Hazard Model and all recent developments on epidemic type aftershock sequence (ETAS) simulations. The problem is that hazard and forecasting experts use seismic catalogues without knowing the details about their limitations. Possible artifacts in depth, epicentral location and magnitude undetected by the user can negatively impact forecasting and hazard. Earthquake catalogues represent an opportunity for observational seismologists, especially if they cover large areas or long time periods. There is great potential in historical earthquake records in seismically active regions that have been populated for thousands of years, and also in regions that have today a seismic station coverage that records all moderate to large earthquakes. Both historical and instrumental catalogues when combined have the potential to reveal pivotal information for hazard analysis if they are carefully built and analysed. The initial goal of this thesis was to tackle the problem of fault segments interaction allowing an extreme release of seismic moment either within a single event, or within a more erratic sequence. For this ambitious goal, we accomplished the first step: generating high-quality seismic catalogues of earthquake location and magnitude. I focused on two strike-slip fault systems where earthquakes were documented by different sources, using pre-instrumental records in the Dead Sea Transform Fault Zone (DSTFZ), and a decade of instrumental records in Turkey. We did it following a consistent and repeatable procedure in the parameter assessment (historical) and calculation (instrumental), providing uncertainties for the location and magnitude, a known completeness level, and ensuring the catalogue is free of repeated or fake events. Quality of historical earthquake catalogues needs to be revised, in particular in regions with a vast written registry of earthquakes. While instrumental seismicity has only recorded one earthquake of moment magnitude equal or larger than 7 in the DSTFZ, tectonic forces in the region have caused centuries of documented incidence of large earthquakes, and remote sensing supports the idea that the area is still in motion and could host large events in the near future. In chapter 2, we build a high-quality historical catalogue for the region between 27-36N and 31-39 E. Historical records in the DSTFZ come from a multitude of sources that don’t share the spatiotemporal location, the magnitude, and sometimes neither the certitude of occurrence of all events. We obtain parametric earthquake data from previous studies, validate the historical records with field investigations (geological and archaeological studies) and analyse several events in depth with regards to their macroseismic intensities. We further identify how does the magnitude and location correlate with the nearby identified faults, and assess their relation to soft-sedimentary deformation structures in the Dead Sea. We convert all magnitudes to Mw and class the magnitude and location uncertainty among three categories. The result is a new historical parametric catalogue from between 31 BCE and 1900 that contains 93 earthquakes of magnitude equal or larger than 5 in moment magnitude scale, complete down to Mw=7. To the North of the DSTFZ, Turkey is routinely surveyed by two agencies: The Kandilli Observatory (BOUN KOERI Regional Earthquake-Tsunami Monitoring Center) and the Turkish National Network (Disaster and Emergency Management Authority Earthquake Department; AFAD). Both have installed new broadband stations in recent years and nowadays cover the country with an average station spacing of about 60 km. We create a consistent M4+ high-quality catalogue for Turkey within 34° –43°N and 25°–46°E for the time period 2007–2016, with relocated hypocentres that combines observations from KOERI and AFAD using a repeatable procedure, providing quantitative location uncertainties and a magnitude estimate (the average of the magnitudes reported by each network). We simultaneously invert for a new 1D velocity model using a set free from outlier observations. The resulting seismic catalogue has 1645 earthquakes of magnitude equal or larger than 4, with an epicentral precision (for the well- locatable events) of 3 km and a depth precision of 4 km. Both AFAD and KOERI report routine magnitudes in several types, requiring to propose a new local magnitude (Ml) formula for Turkey using vertical amplitudes. This led to a set of consistent magnitudes for all earthquakes in the 10-year instrumental catalogue we relocated before. To calculate the magnitudes, we iteratively inverted for two parameters related to the distance between the source and the station, for individual station effects and for magnitudes themselves, after converting each seismogram to its Wood-Anderson equivalent. The final value of Ml is then scaled to fit with the moment magnitude. Our resulting catalogue with recalculated magnitudes is now complete down to magnitude 4.2 with a b-value of 0.97.
Publications 1 - 4 of 4