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After 120 years of unsuccessful endeavor, a paradigm shift is required before earthquakes can be predicted. The most sensitive diagnostic of low-level changes of stress in in situ rock, variations in microcrack geometry, can be monitored by analyzing shear-wave splitting. The suggested paradigm shift is that, instead of investigating the source zone, we monitor stress accumulation before earthquakes at, possibly, substantial distances from the source. Characteristic temporal variations of shear-wave time delays have been observed in retrospect before 14 earthquakes worldwide. On one occasion, when changes were recognized early enough, the time, magnitude, and fault break of an M = 5 earthquake in southwest Iceland were successfully stress-forecast in a narrow time-magnitude window. Such stress accumulation can be theoretically modeled and is believed to be at least partially understood. When sufficient shear-wave source earthquakes are available, increasing time delays also show an abrupt decrease shortly before the impending earthquake occurs. This is not fully understood but is thought to be caused by stress relaxation as microcracks coalesce onto the eventual fault break. The new result confirming these ideas, and justifying the paradigm shift, is that logarithms of the durations of both increases and decreases in time delays are found to be proportional (self-similar) to the magnitudes of impending earthquakes. Keywords: stress accumulation, paradigm shift, shear-wave splitting, stress-forecasting earthquakes.

Borehole logs from the northern Barbados accretionary prism show that the plate-boundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.

The 1999 February 25, 1858 hr, Southern Lake Baikal earthquake is evidently a two-stage rupture when compared with its prominent foreshock at 0537 hr on the same day and a prominent nearby earthquake on 2000 May 31. The foreshock and 2000 May 31 earthquake have similar waveforms recorded at short-period array stations at . The main shock P wave train (including the surface reflections pP and sP) appears most similar to that from the 2000 May 31 earthquake after a shift of ∼8.5 s from the P wave first arrival. A model of two-stage rupture with a smaller stage followed 8.5 s later by a larger rupture fits many aspects of the seismograms of the main shock, whereas the 2000 May 31 seismograms can be fitted with a single source. The main shock first-stage rupture has moment ∼17 per cent of that of the second-stage rupture. Their hypocentres are at depth km, from modelling pP and sP phases using a crustal model of a low-wave speed sedimentary basin at the epicentral region. The hypocentres of the two stages are the same to within the resolution of the modelling. The 2000 May 31 hypocentre is at depth ∼13 km in the same model. Model seismograms at stations to the northwest and southeast have different time intervals between initial P and the surface reflections: the longer P– pP interval for the northwest stations can be explained by a thicker sedimentary basin at pP bounce points northwest of the epicentre. The two-stage rupture of the main shock could be responsible for the wide range of depths (7–21 km) and (4.9–6.1) published in the International Seismological Centre catalogue. [ABSTRACT FROM AUTHOR]

A high-frequency Rayleigh wavetrain with frequency ~ 0.2 Hz is the largest-amplitude signal observed in broadband instrument-corrected displacement seismograms at three regional stations, MDJ, USRK and INCN, at ~400-600 km from the six announced underground nuclear explosions by the Democratic People's Republic of Korea ("DPRK1-6"). The Rayleigh wave was identified in particle motions and is concentrated in the frequency band 0.1-0.5 Hz (period 2-10 s). MDJ and USRK, northeast of the test site, show similar spectrograms for DPRK2-6 with small positive dispersion. At these two stations there is a second, lower-amplitude Rayleigh wavetrain with frequency 0.4 Hz, following after ~5-7 s. INCN, southwest of the site, shows a "C"-shaped spectrogram of the first wavetrain, with minimum travel time at ~0.3 Hz, but the second wavetrain is not visible above noise except in the record from the largest explosion, DPRK6. Station KSRS, ~100 km east of INCN but with raypath with an oceanic segment, shows no clear Rayleigh phases at these frequencies. Waveforms and spectrograms at each station are very similar for tests DPRK2-5, with only a slight decrease overall in frequency for DPRK6. Log amplitude/period (log A/T) values in the band in which the waves have maximum amplitude, 0.125-0.25 Hz (4-8 s), show a roughly linear relationship with station surface-wave magnitude (Ms) measured on Rayleigh Airy phases with period 11-13 s.©British Crown Owned Copyright 2019/AWE [ABSTRACT FROM AUTHOR]

Alan Reid reports on the BGA seminar on aeromagnetic data interpretation.MEETING REPORT Alan Reid reports on a successful British Geophysical Association seminar at Burlington House that attracted more than 300 participants worldwide. [ABSTRACT FROM AUTHOR]