Quake watch

Each station's vertical channel is converted to ground velocity, equalized for its sensor, filtered to 1 to 15 Hz, the band of local earthquakes and of vibration people notice, and reduced to its peak in every second. A site term fitted from a year of nearby earthquakes is divided out, and stations whose readings scatter less around that fit weigh more. Between events the home estimate is the weighted middle value of the stations, so a car passing one station does not move it. During a network event, at least three stations rising above their own background together, or the two nearest home when a cataloged earthquake within 15 km explains the timing, it is the weighted log average, with each station's level carried to home's distance from the source; an automatic solution without a constrained depth is carried at 5 km, the usual depth of Hayward fault quakes. The at-rest line, 50 µm/s, is about where one person lying still at night in a wood-frame house notices vibration. The ShakeMap felt line, 215 µm/s, is where a community starts to report the faintest shaking, a horizontal figure applied here to vertical motion. Anything that starts within a few hundred meters of home, such as construction, traffic, or an appliance, does not reach a station kilometers away and will not show here.