This is a seismogram from the Borgarfjordur seismic station in Iceland. The clusters of noise are from an earthquake swarm in Iceland in early July 2023. As of 12 noon, local time, 5 July 2023, about 2,200 earthquakes occurred in Iceland's Reykjanes Peninsula (= Southern Peninsula), southwest of the town of Reykjavik since the 4th of July. The three most powerful events on the chart are magnitude 4.5. Hypocenters of early quakes were about 8 kilometers deep. Later quakes were shallow, ranging from about 4 to 2 kilometers deep. The Icelandic Meteorological Office interprets the activity as an igneous dike emplacement event, similar to three dike intrusion events in the same region in 2021 and 2022. Two previous seismic swarms / dike emplacement events resulted in basaltic surface eruptions.
Update: a basaltic fissure eruption commenced in the late afternoon, local time, on 10 July 2023 next to a hill called Litli Hrútur.
Iceland is a volcanic island in the North Atlantic Ocean between Britain and Greenland. It is situated on the Mid-Atlantic Ridge, along which the North American Plate and the Eurasian Plate separate and new oceanic crust forms. Seismicity in the area is due to subsurface magma movement from hotspot (mantle plume) activity, as well as seafloor spreading (= tectonic divergence).
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Info. at:
en.wikipedia.org/wiki/Iceland_hotspot
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An earthquake is a natural shaking or vibrating of the Earth caused by sudden fault movement and a rapid release of energy. Earthquake activity is called "seismicity". The study of earthquakes is called "seismology". The actual underground location of an earthquake is the hypocenter, or focus. The site at the Earth's surface, directly above the hypocenter, is the epicenter. Minor earthquakes may occur before a major event - such small quakes are called foreshocks. Minor to major quakes after a major event are aftershocks.
Most earthquakes occur at or near tectonic plate boundaries, such as subduction zones, mid-ocean ridges, collision zones, and transform plate boundaries. They also occur at hotspots - large subsurface mantle plumes (Examples: Hawaii, Yellowstone, Iceland, Afar).
Earthquakes generate four types of shock waves: P-waves, S-waves, Love waves, and Rayleigh waves. P-waves and S-waves are body waves - they travel through solid rocks. Love waves and Rayleigh waves travel only at the surface - they are surface waves. P-waves are push-pull waves that travel quickly and cause little damage. S-waves are up-and-down waves (like flicking a rope) that travel slowly and cause significant damage. Love waves are side-to-side surface waves, like a slithering snake. Rayleigh waves are rotational surface waves, somewhat like ripples from tossing a pebble into a pond.
Earthquakes are associated with many specific hazards, such as ground shaking, ground rupturing, subsidence (sinking), uplift (rising), tsunamis, landslides, fires, and liquefaction.
Some famous major earthquakes in history include: Shensi, China in 1556; Lisbon, Portugal in 1755; New Madrid, Missouri in 1811-1812; San Francisco, California in 1906; Anchorage, Alaska in 1964; and Loma Prieta, California in 1989.
