Quake watch

How the estimate is made

Each station's vertical channel is converted to ground velocity, filtered to 1 to 15 Hz (the band of local earthquakes and of vibration people notice), and reduced to its peak in every second.

The stations are made comparable before they are combined. Each sensor's poles and zeros are used to equalize its response, so a 1 Hz geophone, a 2 Hz geophone, and a broadband read the same across the band. All channels are put on one 50 samples per second time base. Each station's site term, fitted from a year of nearby earthquakes, is divided out, and stations whose readings scatter less around that fit weigh more. When a network event matches a cataloged earthquake, each station's level is carried to home's distance from the source.

The wiggles at stations a few kilometers apart are unrelated at these frequencies, so they cannot be summed into a true waveform. Their peak levels can be combined. Each station's level is held for the few seconds a wave needs to cross between it and home. In quiet times the estimate is the weighted middle value, so a car passing one station does not move it. During a network event it is the weighted log average, and every station clear of its own background counts by distance.

The vertical trace is a stand-in for a seismometer at home, not a recording. Its wiggles are the sum of the station waveforms with the same weights, so the nearest stations set its timing, within about a second of home. Because the stations do not move in step, the sum comes out too small, and a slow gain (0.5 to 4) brings its size to the combined station level. The level chart holds each peak for a few seconds, so it can read up to about twice the trace.

Two lines. The ShakeMap felt line, 215 µm/s, is the top of "not felt" on the USGS ShakeMap intensity scale: where a community starts to report the faintest shaking, fitted on larger quakes and on horizontal motion, here applied to a vertical estimate. The at-rest line, 50 µm/s, is about where one person lying still at night in a wood-frame house notices vibration (the ISO 2631-2 floor curve over a 2 to 3 times floor amplification). Each network event's peak carries a range: the stations' disagreement, the unknown ground under the house (about a factor 1.6 either way), and, for an automatic solution without a constrained depth, source depths from 2 to 8 km. An event is called possibly felt at rest when that range reaches 50 µm/s, or when the source is closer to home than to any station, so the estimate is an extrapolation inward.

Carrying a station's reading to home needs the source distance. Automatic solutions with a depth under 2 km are treated as depth unknown and carried at 5 km, the usual depth of Hayward fault quakes, and no source is treated as closer than 1 km. Inside an event a reading carried a long way in distance from where it was made counts less.

A row in the list says "Reached home" when at least three stations rose above their own background together, or when the two stations nearest home did and a cataloged earthquake within 15 km explains the timing. One noisy site (a truck, a train) does not count. "Sharp and close" means the nearest station's 15 to 40 Hz motion was at least a quarter of its 1 to 15 Hz motion: the signature of a small quake a few kilometers away, the kind that rattles a house. Anything that starts within a few hundred meters of home (construction, traffic, an appliance) does not reach a station kilometers away and will not show here.