Seismic hazard in Hawaii: High rate of large earthquakes and probabilisticground-motion maps

The seismic hazard and earthquake occurrence rates in Hawaii are locally as high as that near the most hazardous faults elsewhere in the United States . We have generated maps of peak ground acceleration (PGA) and spectral acc eleration (SA) (at 0.2, 0.3 and 1.0 sec, 5% critical damping) at 2% and 10% exceedance probabilities in 50 years. The highest hazard is on the south s ide of Hawaii Island, as indicated by the M-I 7.0, M-S 7.2, and M-I 7.9 ear thquakes, which occurred there since 1868. Probabilistic values of horizont al PGA (2% in 50 years) on Hawaii's south coast exceed 1.75g. Because some large earthquake aftershock zones and the geometry of flank bl ocks slipping on subhorizontal decollement faults are known, we use a combi nation of spatially uniform sources in active flank blocks and smoothed sei smicity in other areas to model seismicity. Rates of earthquakes are derive d from magnitude distributions of the modern (1959-1997) catalog of the Haw aiian Volcano Observatory's seismic network supplemented by the historic (1 868-1959) catalog. Modern magnitudes are M-L measured on a Wood-Anderson se ismograph or M-S. Historic magnitudes may add M-L measured on a Milne-Shaw or Bosch-Omori seismograph or M-I derived from calibrated areas of MM inten sities. Active flank areas, which by far account for the highest hazard, are charac terized by distributions with b slopes of about 1.0 below M 5.0 and about 0 .5 above M 5.0. The kinked distribution means that large earthquake rates w ould be grossly underestimated by extrapolating small earthquake rates, and that longer catalogs are essential for estimating or verifying the rates o f large earthquakes. Flank earthquakes thus follow a semicharacteristic mod el, which is a combination of background seismicity and an excess number of large earthquakes. Flank earthquakes are geometrically confined to rupture zones on the volcano flanks by barriers such as rift zones and the seaward edge of the volcano, which may be expressed by a magnitude distribution si milar to that including characteristic earthquakes. The island chain northwest of Hawaii Island is seismically and volcanically much less active. We model its seismic hazard with a combination of a line arly decaying ramp fit to the cataloged seismicity and spatially smoothed s eismicity with a smoothing half-width of 10 km. We use a combination of up to four attenuation relations for each map because for either PGA or SA, th ere is no single relation that represents ground motion for all distance an d magnitude ranges. Great slumps and landslides visible on the ocean floor correspond to catastrophes with effective energy magnitudes M-E above 8.0. A crude estimate of their frequency suggests that the probabilistic earthqu ake hazard is at least an order of magnitude higher for flank earthquakes t han that from submarine slumps.