Historical accounts of pre-eruption seismicity of Katla, Hekla, Öræfajökull and other volcanoes in Iceland

Detecting unusual activity leading to the outbreak of a volcanic eruption is of vital importance for the short-term warning to the local population of an impending eruption. The varied volcano types of Iceland and range of ambient conditions at which eruptions occur offer an unusually wide spectrum of volcanic phenomena and volcanic hazards during the initial phase of eruptions. A recent study of eruptions in Iceland during the last four decades of instrumental observations has revealed that all the eruptions had a detectable precursory seismic activity, that under favourable conditions can be used to issue short-term warnings to the surrounding communities. Considerable documentary data also exist for pre-instrumental times extending several centuries back in time, that can be compared to the instrumental experience. This is true in particular for two of the most active volcanoes, Katla and Hekla, that are sufficiently close to the populated areas of the country. All seven confirmed eruptions of Katla since 1625 were preceded by felt earthquakes, beginning one to nine hours before the eruption was detected and two to over twelve hours before a jökulhlaup from this partly sub-glacial volcano reached the inhabited areas. The behaviour of Hekla is quite different. Large eruptions from the main edifice of Hekla since 1510 were generally accompanied by rather weak seismic activity. Earthquakes are usually felt only minutes before the first explosion occurs, in the 1947 case even several minutes after the first explosion of the volcano. Eruptions of the Hekla volcanic system outside the main edifice are, on the other hand, accompanied by considerable seismic activity, and the precursor times may be more than three hours, even much longer. The two historical eruptions of Öræfajökull, in 1362 and 1727, were apparently preceded by felt seismicity, sufficient to alarm the local population.

Long-term seismic forecast (lTSF) for the Kuril-Kamchatka arc VI 2019–V 2024; properties of precursory seismic activity in I 2017–V 2019. Development and practical application of the LTSF method

The paper presents the results of the ongoing work on the method of the long-term seismic forecast (LTSF) for the Kuril-Kamchatka Arc. The method is based on seismic gaps and seismic cycle patterns. The work also reveals the most important trends of the method development over the prior decade. Based on the main methodology the long-term forecast is given for the next 5 years from VI 2019 to V 2024 period, for the most active part of the regions seismogenic zone. The seismic cycle stages are predicted for the next five years, the normalized characteristic of the weak earthquakes (A10) amount, the earthquakes with the medium magnitudes expecting with the 0.8, 0.5 and 0.15 probabilities, the maximum expected magnitudes and the strongest with the M 7.7 earthquakes probabilities for 20 of its zones. The famous works of S.A. Fedotov resulted in further research of the regional seismic processs spatial and temporal features within the 2017 period, including the strongest (M = 7.7) July 17, 2017 Near-Aleutian Earthquake. The results confirm close seismic process relation in the most seismically dangerous, according to the LTSF data zones and major events in the region itself and the adjacent seismic regions, as well as the current very high seismic hazard in some zones of the Kuril-Kamchatka Arc and the need to continue and increase the works being done for earthquake resistance and seismic safety in the most endangered zones and of course in administrative center of Kamchatka the Petropavlovsk-Kamchatskii city.

Patterns of local seismicity preceding the petatlan earthquake of 14 March 1979

abstract Local seismic activity (M > 2.3) during the 412-day period preceding the Petatlan earthquake of 14 March 1979 (Ms = 7.6) was monitored by a seismic network deployed by the Hawaii Institute of Geophysics. These data allow us to revise the location of that event, and to study its pattern of foreshocks. The foreshock hypocenters are concentrated above the ocean/continent plate boundary and landward of the hypocenter of the main shock. The spatial distribution of foreshocks suggests that the foreshocks occurred along existing zones of weakness within the continental plate. During the 112-day period preceding the Petatlan event, foreshocks were concentrated within 100 km of the epicenter of that earthquake. The pattern of precursory seismic activity we observed during that period is similar to that observed during the same time period prior to the Oaxaca earthquake of 29 November 1978. However, the 17-hr period of seismic quiescence (for events with M > 2.8) preceding the Oaxaca earthquake is not found in our data where a magnitude 4 foreshock occurs within 28 min of and 2 km from the Petatlan event. This suggests that the spatially larger set of foreshocks may have greater diagnostic value than does the sequence of precursory events within 30 km of the main shock.