SIGNIFICANT EARTHQUAKES NEAR THE CITY OF THESSALONIKI (NORTHERN GREECE) AND PROBABILITY DISTRIBUTION ON FAULTS

Στόχος της παρούσας εργασίας είναι η εκτίμηση της χρονικά εξαρτώμενης πιθανότητας γένεσης ισχυρών σεισμών (M≥6.0) στα ενεργά ρήγματα γύρω από την πόλη της Θεσσαλονίκης, περιοχή που στο παρελθόν έχει πληγεί από αρκετούς κατατροφικούς σεισμούς με τελευταίο αυτόν του 1978. Με τη χρήση της μεθόδου υπολογισμού μακροσεισμικού επικέντρου και μεγέθους χρησιμοποιώντας μακροσεισμικές εντάσεις και από τις μακροσεισμικές περιγραφές παλαιότερων σεισμών με M≥6.0 που έγιναν στην περιοχή τα τελευταία 500 χρόνια καταβλήθηκε προσπάθεια επαναπροσδιορισμού των εστιακών παραμέτρων των ιστορικών σεισμών με σκοπό τον υπολογισμό μέσης περιόδου επανάληψης και του χρόνου από τον τελευταίο σεισμό σ’ ένα ρήγμα. Για να γίνει η εκτίμηση της πιθανότητας λήφθηκε υπόψη η μεταβολή της τάσης που προκύπτει μετά από κάθε ισχυρό σεισμό και η οποία έχει ως αποτέλεσμα να επιταχύνει ή να επιβραδύνει τη γένεση ενός επόμενου σεισμού. Ενσωματώθηκε έτσι στους υπολογισμούς της χρονικά εξαρτώμενης πιθανότητας, η μεταβολή της τάσης που προκλήθηκε από το σεισμό της 20 Ιουνίου 1978 με Μ=6.5. Η συνδυαστική πιθανότητα Poisson για τα επόμενα 30 χρόνια (2015-2045) βρέθηκε ίση με ~35%, η δεσμευμένη πιθανότητα εκτιμήθηκε από 0% έως 15% και από0% έως 20% με το συνδυασμό των τάσεων.Stress triggering must be incorporated into quantitative earthquake probability estimate, given that faults are interacted though their stress field. Using time dependent probability estimates this work aims at the evaluation of the occurrence probability of anticipated earthquakes near the city of Thessaloniki, an urban center of 1 million people located in northern Greece, conditional to the time elapsed since the last stronger event on each fault segment of the study area. A method that calculates the macroseismic epicenter and magnitude according to macroseismic intensities is used to improve the existing earthquake catalog (from AD 1600 - 2013 with M≥6.0) in order to compute new interevent and elapsed time values which form the basis for time-dependent probability estimates. To investigate the effects of stress transfer to seismic hazard, the probabilistic calculations presented here employ detailed models of coseismic stress associated with the 20 June 1978 M=6.5 Thessaloniki which is the latest destructive earthquake in the area in the instrumental era. The combined 2015-2045 regional Poisson probability of M≥6.0 earthquakes is ~35% the regional time-dependent probability varies from 0% to 15% and incorporation of stress transfer from 0% to 20% for each fault segment

Forming short period sub-stellar companions in 47 Tucanae -- II. Analytic expressions for the orbital evolution of planets in dense environments

Short period, massive planets, known as hot Jupiters (HJs), have been discovered around $\sim 1$ percent of local field stars. The inward migration necessary to produce HJs may be `low eccentricity', due to torques in the primordial disc, or `high eccentricity' (HEM). The latter involves exciting high orbital eccentricity, allowing sufficiently close passages with the host star to raise circularising tides in the planet. We present an analytic framework for quantifying the role of dynamical encounters in high density environments during HEM. We show that encounters can enhance or suppress HEM, depending on the local stellar density and the initial semi-major axis $a_0$. For moderate densities, external perturbations can excite large eccentricities that allow a planet to circularise over the stellar lifetime. At extremely high densities, these perturbations can instead result in tidal disruption of the planet, thus yielding no HJ. This may explain the apparent excess of HJs in M67 compared with their local field star abundance versus their apparent deficit in 47 Tuc. Applying our analytic framework, we demonstrate that for an initial massive planet population similar to the field, the expected HJ occurrence rate in 47 Tuc is $f_\mathrm{HJ}=2.2\times 10^{-3}$, which remains consistent with present constraints. Future large (sample sizes $\gtrsim 10^5$) or sensitive transit surveys of stars in globular clusters are required to refute the hypothesis that the initial planet population is similar to the solar neighbourhood average. Non-detection in such surveys would have broad consequences for planet formation theory, implying planet formation rates in globular clusters must be suppressed across a wide range of $a_0$.Comment: Accepted for publication in MNRAS - 28 pages, 17 figure

Limits on the Mass and Initial Entropy of 51 Eri b from Gaia EDR3 Astrometry

51 Eri b is one of the only young planets consistent with a wide range of possible initial entropy states, including the cold-start scenario associated with some models of planet formation by core accretion. The most direct way to constrain the initial entropy of a planet is by measuring its luminosity and mass at a sufficiently young age that the initial conditions still matter. We present the tightest upper limit on 51 Eri b's mass yet (M < 11 Mjup at 2$\sigma$) using a cross-calibration of Hipparcos and Gaia EDR3 astrometry and the orbit-fitting code orvara. We also reassess its luminosity using a direct, photometric approach, finding log(Lbol/Lsun) = -5.5$\pm$0.2 dex. Combining this luminosity with the 24$\pm$3 Myr age of the $\beta$ Pic moving group, of which 51 Eri is a member, we derive mass distributions from a grid of evolutionary models that spans a wide range of initial entropies. We find that 51 Eri b is inconsistent with the coldest-start scenarios, requiring an initial entropy of >8 $k_B$/baryon at 97% confidence. This result represents the first observational constraint on the initial entropy of a potentially cold-start planet, and it continues the trend of dynamical masses for directly imaged planets pointing to warm- or hot-start formation scenarios.Comment: Accepted for publication in MNRAS (9 pages, 6 figures