GEOPHYSICAL RESEARCH LETTERS, VOL. 19, NO. 3, PAGES 301-304, FEBRUARY ?, 1992 TSUNAMI FROM THE MARIANA EARTHQUAKE OF APRIL 5,1990: ITS ABNORMAL PROPAGATION AND IMPLICATIONS FOR TSUN• POTENTIAL FROM OI.J'•R-•SE EARTHQUAKES Kenji Satake Department of Geological Sciences, University of Michigan Yasuhiro Yoshida and Katsuyuki Abe Earthquake Research Institute, University ofTokyo Abstract. Thetsunami generated by theMariana earthquake ofApril5, 1990wasobserved on theJapanese and Pacific islands as far as Hawaii.The observed tsunami amplitudes are not a simple function of distance from the source but vary with large-scale bathymetry. Numerical computations of tsunami propagation are made foractual bathymetry and the computed amplitudes are compared with the observations. Fromthe comparisons, the seismic moment is estimated to be 1.4 x !020 Nm, very similar to that from seismic waves, and indicates that the seismic waves and tsunami are equally excited. The numerical'computations also show that the nunand propagated towardthe Japanese coast through two different paths: one is through the trench system with high velocity and smallamplitude, theother along the ridge system with low velocity but large amplitude. The two kinds of tsunami are identified on recently-installed ocean bottom pressure gauges. Since theMariana earthquake was anouter- rise earthquake in a weakly coupled subduction zone, where the size of outer-rise event is larger than underthrust•g events, it ispossible thateven larger earthquakes occur in this region. Thetsunami potential from such events mustbe considered, including the unusual tsunami propagation. Introduction A moderate-size tsunami wascaused by anearthquake that occurred in the Mariana region on April 5, 1990.Although the size of the tsunami was not large enough to cause damage, it was clearly recorded ontide gauges onthe Japanese coast and in the Pacific islands as far asHawaii. The amplitude of the tsunami shows abnormal geographical distribution. The earthquake occurred atthe deep Mariana trench (Figure I) where theoldest partof thePacific plate is subducting beneath the Philippine Seaplate.A series of trenches (Mariana andIzu-Bonin trenches) extends from the source area to thenorth up theJapanese coast. A ridge(Izu-Bonin ridge) system runs parallel to the west. These topographic features might have affected the tsunami propagation. An unusual tsunami was also observed from the 1984 Torishima earthquake which occurreA on the Izu-Bonin ridge. Satake and Kanamori [1991] found that the abnormally large tsunami from the Torishima event was due to both the abnormal propagation along the ridge system and an unusual source process with a non-fault origin, most likely an injection of material into a shallow sediment layer. Thesource process of the Mariana earthquake was Copyright 1992 bythe American Geophysical Union. Paper number 91GLO2493 0094-8534/92/91GL_02493503.00 301 1300 140 ø 150øE ''( .... •"',"' , , 'F"'''' •OON• • .,.:,._,.... mdllSau?J ' : pa•z •?. . # Saipan's• • •_ • _ • 5 •h,, ,,,, ,;[,, ,•• E:',,• •o o Fig. 1. Topography aroundthe Izu-Bonin-Marianatrench system. The computation of tsunami is made in thisarea with a grid sizeof 1 min. The starsymbols show the sources of the 1984 Torishima and 1990 Mariana tsunamis. The solid triangle shows approximate location of ocean bottom tsunami gauges and tide gauge (Mera) that record trenchand ridge waves. The contour interval for bathymetry is 2000 m. investigated in detail by Yoshida et al.[!992].Theyfound that the earthquake wascaused by a normal fault motion within the subducting Pacific plate. The seismic moment was1.4 x 1020 Nm or Mw=7.3. Thisearthquake is an example of an outer-rise event in a subduerion zone where the seismic coupling between subducting and overlying plates is weak. In this paper,we make numerical computations of the tsunami on actual bathymetry. The tsunami source model can beestimated fromthe comparison between thecomputed and observed tsunami amplitudes. The numerical computations also show the bathymetric effecton thetsunami propagation. We further discussfuture tsunami potential from an earthquake in this region. Observed Tsunami The tsunami fromthe Marianaearthquake was recorded on many tide gauges in Japan aswell asin Trak (the maximum peak-to-peak amplitude was3 era),WakeIsland (4 cm), Midway (6 cm)and Kailua-Kona onthe Island of Hawaii (24 cm).Figure 2 shows themaximum amplitudes of tsunami observed ontidegauges in Japan. Thefigure also shows the tsunami heights computed by a method wewill discuss later. Thetsunami amplitude is large on the islands along the Izu- Bonin ridge system, ranging from 18 cm atIzu-Osh'maa to42 cm at Kozu-jima.On the southern coast of Japan, the