**8. Dealing with false alarms**

The first tsunami alert is indicated when the *q*‐factor exceeds the preset limit. Due to the varying background current and noise/interference effects described in the last section, the detection might be a false alarm. The trade‐off in the selection of the *q*‐factor threshold is as follows: if the *q*‐factor limit is set too low, the peak will certainly produce a *q*‐factor alert, but many non‐ tsunami false‐alarm detections may be generated, degrading performance and operational acceptability. If it is set too high, false alarms will be eliminated, but then the first actual tsunami peak may be missed. This is the classic "probability‐of‐detection vs. false‐alarm rate" trade‐off encountered by warning sensor systems. An acceptably low false‐alarm rate for a given tsunami intensity and detection distance from shore are the parameters to be optimized at each site. Judging by the strength of the *q*‐factor signals seen in the Japan tsunami, it would appear that a tsunami having a run‐up height of 1 m should easily be detectable with very low false‐ alarm rates in uncontaminated radar spectra, using the detection methods described earlier. The value of the *q*‐factor limit defining a tsunami detection is site specific and needs to be studied for the site under consideration. The most effective way to handle this for a given site is to study the background currents/extraneous effects and how the *q*‐factor algorithm responds to them over a several month period.

We discuss two approaches for the reduction of false alarms:
