Kazuhiko Goto

Earthquake generating stresses in a descending slab

Abstract A numerical study of stress within descending slabs beneath island arcs provides a general interpretation for the occurrence of deep- and intermediate-focus earthquakes. The computation of stress due to several factors, which have been proposed for the interpretation of the double-planed seismicity of intermediate-depth events, shows that thermal expansion and olivine-spinel phase change play an important role in earthquake generation. The stress field due to the combination of these two causes gives the following results; in the depth range 50–300 km, the compressional and tensional stress predominates in the upper and central part of the slab respectively. Maximum shear stress amounting to 1.6 GPa in compressional and 0.4 GPa in tensional fields occurs at a depth of 200 km. The stress is generally very small at around 300–350 km. Below 400 km, compressional stress exists at the central part of the slab and tensional stress appears near the upper and lower surfaces of the slab. Maximum shear stress is 2.8 GPa in compression and 1.6 GPa in tension at a depth of 500 km. The principal axes in these high stress fields align almost parallel to the dip direction of the slab. These stress features explain well such observed seismological facts beneath island arcs as the seismicity-depth relationship, planer configuration of seismic zone, existence or non-existence of double-planed seismicity and focal mechanisms of intermediate and deep focus earthquakes.

Factors responsible for the limited inland extent of sand deposits on Leyte Island during 2013 Typhoon Haiyan: LIMITED INLAND EXTENT OF STORM DEPOSITS

Previous geological studies suggest that the maximum inland extent of storm-induced sand deposits is shorter, but their thickness is larger, than those of tsunami-induced sand deposits. However, factors that determine the maximum extent and thickness of storm deposits are still uncertain. We conducted numerical simulations of storm surge, waves, and sediment transport during Typhoon Haiyan in order to understand the distribution and sedimentary processes responsible for storm deposits. Numerical results showed that wave-induced currents slightly offshore were strong, but attenuated rapidly in the inland direction after wave breaking. Therefore, sediments were not transported far inland by waves and storm surge. Consequently, the maximum inland extent of storm deposits was remarkably shorter than the inland extent of inundation. We also revealed that vegetation (roughness coefficient) and typhoon intensity greatly affect the calculation of maximum extent and thickness distribution of storm deposits. As the duration of wave impact on a coast is relatively long during a storm (hours, compared to minutes for a tsunami), sediments are repeatedly supplied by multiple waves. Therefore, storm deposits tend to be thicker than tsunami deposits, and multiple layers can form in the internal sedimentary structure of the deposits. We infer that limitation of the sand deposit to within only a short distance inland from the shoreline and multiple layers found in a deposit can be used as appropriate identification proxies for storm deposits.

Determination of Earthquake Magnitude from Amplitude in Coda-Portion of S-Wave

A new empirical formula to determine the earthquake magnitude from the amplitude in the coda-portion of S-wave is obtained. The formula has a form, MC=alogA+blogT+c, where A is the rms amplitude in the coda-portion of S-wave at the lapse time, T, from the origin time of an earthquake. The coefficient, a, is around 1. 0 and the coefficients, 5 and c, depend on the observational site condition, in particular, characteristics of the seismometer. The formula is applicable to A more than twice as large as the noise level and to T more than 1. 6 times as long as the S-wave travel time. The magnitude derived from the above formula, the coda magnitude, gives a close approximation to the magnitude determined by the Japan Meteorological Agency in the magnitude range from 1. 3 to 4. 6, when the seismometer of its natural frequency, either 1. 0 Hz or 4. 5 Hz, is used. The coda magnitude has the advantage of covering a wide range of magnitude, because the amplitude in the coda-portion of S-wave can be used even when the maximum amplitude is out of scale. It also has a high reliability, because the coda magnitude is not largely affected by the characteristics of radiation at the source. Present method is very easy to carry out, especially when the digital data are available, as the coda magnitude can be determined by an easy calculation of rms amplitude.