As discussed in Estimates of the regional and global frequency of tsunamis (especially giant tsunamis), and in tsunami earthquakes many large earthquakes do not in fact produce tsunamis whereas a smaller proportion produce unexpectedly large waves. For this latter reason the threshold of earthquake magnitude for instituting a tsunami watch is set conservatively low, although the problems of detecting tsunami earthquakes with conventional seismometers has led some seismologists to question the reliance of the PTWC upon these instruments and to propose the establishment of built - for - the - purpose broadband seismometers. The high proportion of false alarms (as many as 80% of tsunami watches are false alarms, although the proportion of warnings that are false alarms is lower) causes problems with the public perception of the system and public response to warnings: although it should be noted that the false warning rate could be as high as 90% and the costs of evacuations would still be small compared to the cost of tsunamis impacting unevacuated areas.