急激
きゅうげき · kyuugeki · JLPT N3 · na-adj
sudden
🎢 急 sudden/urgent + 激 fierce/violent = a sudden, violent change
急 is the kyuu of 急ぐ (to hurry) and 救急車's urgency; 激 is the geki of 激しい (intense). Together they describe change that is both fast and forceful, like 急激な変化 (a radical change) or 急激に増える (to rise sharply).
急 — heart radical 心 + 及 (reach)
A semantic-phonetic compound: the heart radical 心 supplies the meaning (an emotional state) while 及 ('to reach, catch up') gives the sound. Originally it depicted a racing, anxious heart — hence the modern senses 'urgent,' 'hurried,' and 'impatient.'