Magnitude (M) — earthquake size; +1 on the scale ≈ 32× more energy. Negative values (e.g. M-0.5) are tiny, barely detectable quakes.

Depth — how deep the focus lies (km); automatic and often uncertain in preliminary data.

Swarm (hrina) — many earthquakes in a short time in the same area.

Cumulative count — count summed over time; a steep curve = a swarm.

Moment (Mo) — an energy measure; one M4 releases about a thousand times an M2. Cumulative moment shows real stress release, not just counts.

b-value — slope of the magnitude-frequency (Gutenberg-Richter) relation; a lower b can accompany higher stress. ± is the statistical uncertainty.

β (activity deviation) — how many standard deviations the last-24 h count is above the system 2-year mean (β≥2 = elevated, β≥4 = unusually high); a statistical swarm signal. Not to be confused with the b-value despite the similar symbol — β measures count, b measures the magnitude distribution.

Hypocenter migration — activity moving steadily in one direction can mark a dike intrusion (magma forcing its way); km/day measures the rate.

Magma (kvika) — molten rock below the surface; what becomes lava in an eruption.

Deformation (aflögun) — when the ground rises, sinks or shifts due to magma or stress; measured with GPS/GNSS.

GNSS (uplift) — precise GPS that measures whether the ground is rising or sinking, in mm per year.

Tremor (RSAM) — the Met Office's automatic tremor measure: continuous ground shaking, usually from weather and surf but sometimes from magma.

Volcanic system — a volcano together with its fissure and magma system.

Whether a quake is felt also depends on depth and distance from towns.