Left heart radical, the side-form of 心. Marks 惊 as an emotional character: shock, alarm, startle are heart-felt reactions. Same radical anchors 怕 (fear), 慌 (panic), 怒 (anger), 悲 (grief).
Right-side 京 supplies the sound, matching the parent's jīng exactly with no tone shift. The 'capital' meaning plays no role here - pure sound borrowing. Simplified from traditional 驚 where the phonetic was 敬 (jìng). Same jīng phonetic series: 凉 (cool), 景 (scenery), 鲸 (whale).
