b'Data trends Table 2.List of some elevation products, their geodetic datum and geoid. after removing natural canopy and horizontal positions tweaked by ATRF.Product Datum Elevations LiDAR (Light Detection and Ranging) GPS WGS84 EGM96 produces a DSM referenced to a datum.Shuttle Radar WGS84 WGS84 ellipsoid fit to EGM96This summary raises questions about Geoid how precise we can be with historical GLONASS PZ 90 (WGS84 with a north poledata and how well would knowing the tweak) age, origin and settings of a GPS help AUSPOS WGS84 with ITRF2014 AHD AUSGeoid2020 inform heights. A more practical question already raised is can we see examples Australian Vertical WorkingGDA2020 AGQGSurface of the various calculated heights that a single position has had over the last 40 years? That would be a great exercise, SRTM from NASA is a DSM in WGS84usgs-eros-archive-digital-elevation- and if you know of any such data, please and heights relative to EGM96 https:// shuttle-radar-topography-mission- contact the Committee at technical-www.usgs.gov/centers/eros/science/ srtm-1. GAs version is mostly a DEMstandards@aseg.org.au.Henderson Byte: Earthquake prediction and controversyFor decades, early warning signs of impending earthquakes better than a shaking sensation under your feet have not been reliable. Some such signs have involved unusual electromagnetic effects in the upper atmosphere, changes in the geochemistry of groundwater, and even changes in animal behaviour (horses agitated, snakes exiting the ground, etc.). These signs are difficult to explain scientifically, and events with a physical basis are preferred.A reliable early warning system based on the use of seismic signals is ShakeAlert. It is operated by the USGS and was developed initially on the USA West Coast, a particularly high-risk area associated with the San Andreas Fault. It identifies early seismic P-wave events and issues alerts before more serious events occur. Using microseismometers alone, 1675 units are needed to be effective just for the West Coast. The addition of GPS data in conjunction with the seismic data may be about to begin.A universal precursor of quakes independent of region is needed, ideally defining the location and time of the onset, and preferably with enough warning to enable evacuation of humans to safety. It should be applicable to individual quakes.A recent paper in Science (Bletery and Nocquet 2023) claims to have found a reliable precursor, better than seismographs, using precise GPS to detect movement on faults. By analysing the changes in the horizontal position of 3026 geodetic stations measured using GPS near 90 different earthquakes, they found that, on average, movement of plate slip accelerated exponentially in the last two hours before the ground ruptures. The odds that this increase is merely a coincidence was found to be negligible. However, detection of this precursor by this global analysis of 90 quakes is not yet able to be used for individual quakes. That would require GPS sensors that are 100 times more precise than those in use today and many more GPS stations. This is a demanding requirement as many of the worlds earthquake zones are just not instrumented in this way. Nevertheless, the analysis has at least shown that the quakes are predictable in nature.Bletery and Nocquets analysis has sparked controversy in the literature among other geophysicists in the field. For example, there is some concern that contamination of the GPS data from small foreshocks has not been successfully removed. There is also the concern about how the arbitrary time scale of two hours was derived, given the diversity of earthquake behaviour.Thus, until GPS positioning becomes much more precise, the search for a usable precursor for individual earthquakes is still on-going.ReferenceBletery, Q. and J-M. Nocquet, 2023. The precursory phase of large earthquakes, Science, 381 (6655), 20 July 2023, p.297-301. doi.org/gshpgs.Roger Henderson rogah@tpg.com.au35 PREVIEW OCTOBER 2023'