P-Wave Detection — How Earthquake Early Warning Systems Buy You Time
When an earthquake strikes, you don't get much warning. But there is a window — measured in milliseconds to seconds — between the first detectable seismic signal and the destructive shaking that follows. P-wave earthquake detection is what makes it possible to act within that window.
Two types of seismic wave
Every earthquake generates two main types of body wave: P-waves (primary waves) and S-waves (secondary waves).
- P-waves travel faster — typically 5–8 km/s through rock. They are compressional waves, pushing and pulling the ground in the direction of travel. The shaking they cause is relatively minor.
- S-waves travel slower — roughly 60% of P-wave speed. They move the ground perpendicular to their direction of travel. S-waves carry the energy that damages structures, ruptures pipes, and collapses buildings.
The time gap between P-wave arrival and S-wave arrival depends on distance from the epicentre. At 50 km out, you might have 8–10 seconds. At 10 km, it's closer to 2 seconds. That gap is the opportunity.
Why frequency response matters
P-waves fall primarily in the 2–10 Hz frequency range. An electronic seismic detector needs a flat response across this band to reliably identify P-wave energy and distinguish it from background vibration — trucks, machinery, foot traffic.
The Solid State Seismic Shutoff — MK6 — is tuned with a flat frequency response from 2 to 10 Hz. When seismic energy in that band exceeds the configured threshold, the output relay fires within 10 milliseconds. That response is fast enough to trigger gas isolation before the S-wave shaking arrives in most real-world scenarios.
Why mechanical shutoff valves don't use P-wave detection
A mechanical seismic shutoff valve responds to physical movement — it contains a ball or mass that displaces when shaken hard enough. There is no signal processing, no frequency discrimination, no sensitivity tuning. (See the full electronic vs mechanical comparison.)
This means a mechanical valve cannot detect a P-wave as a distinct seismic signal. It waits for the shaking itself to trip the mechanism. By definition, it responds to destructive motion — not to the early warning that precedes it.
An electronic trigger like the MK6 can be configured to fire on P-wave detection, giving automated systems a head start before peak ground acceleration arrives.
Sensitivity settings and threshold selection
The MK6 offers five sensitivity settings via DIL switch: 0.012g, 0.025g, 0.050g, 0.100g, and 0.200g. The appropriate threshold depends on the application and site seismicity.
In high-risk or critical-infrastructure installations — hospitals, gas network control points, industrial facilities — a lower threshold captures more events, including moderate earthquakes that may still pose a gas fire risk. In locations with significant ambient vibration, a slightly higher threshold avoids nuisance trips.
The relay holds for 8 seconds after the last threshold exceedance, so continued shaking within the same event doesn't prematurely reset gas isolation. The MK6 is supplied in AC (mains, with battery backup) and DC (no-break) variants — see the MK6 variants and specifications to match one to your installation.
Event recording
Every relay activation is logged by the MK6's built-in event counter. This provides a verifiable record of activations — useful for post-event review, maintenance scheduling, and demonstrating to insurers or building compliance assessors that the system responded correctly.
The case for electronic seismic detection
P-wave detection is not available in passive mechanical systems. It requires a sensor with appropriate frequency response, threshold discrimination, and fast relay output. For a broader overview, see how an earthquake trigger system works. For engineers and specifiers who need a system that reacts to the earthquake's first signal — not its peak destructive energy — an electronic trigger is the correct specification.
The MK6 has been doing this from Lower Hutt since 1971. For specifications, installation guidance, or technical enquiries, contact us or visit the MK6 product page. For application-specific guidance, see the applications overview.