Weak motion attenuation of the North Island, New Zealand

Abstract

Attenuation relations using weak-motion peak ground acceleration recordings have been determined using data from the New Zealand National Seismograph Network and several temporary seismograph deployments. Models have been developed for earthquake sources in four regions: the Eastern North Island Shallow; the Eastern North Island Deep; the Central Volcanic Region Shallow and the Central Volcanic Region Deep. Deep events were those with hypocenters below 33 km. Both volcanic and non-volcanic travel paths are considered. Regression coefficients have been determined for each of the regions using the attenuation models of Joyner and Boore (1981) and Molas and Yamazaki (1995). For earthquakes in the Eastern North Island Shallow region, and excluding data from stations in the volcanic region, the resulting Joyner and Boore (1981) model equation is: log10 A = -5.56 + 0.983M - 0.0028r - log10 r where A is the peak ground acceleration in g, M is the local magnitude, ML, and r is the hypocentral distance. Similarly, those for the Eastern North Island Deep region and the Central Volcanic Region Deep are respectively: log10 A = -5.27 + 0.944M - 0.0027r -log10 r and log10 A = -5.69 + 1.015M - 0.0019r - log10 r. Anelastic attenuation was found to be greatest in the Central Volcanic Region at a rate of -0.067 g/km. This is of the order of twenty times greater than that for the Eastern North Island. Rates for both deep and shallow earthquakes in the Eastern North Island are similar. The lowest rate of attenuation (-0.002 g/km) was noted for seismic waves originating deep within the subducting slab beneath the Central Volcanic Region which do not travel through the mantle or crust of the Central Volcanic Region. This is in agreement with the hypothesis that seismic waves travelling through the subducting slab of the Pacific Plate experience less attenuation than those travelling through the overlying crust. Although the attenuation rate in the above Eastern North Island expressions are comparable to that of Joyner and Boore (1981), the absolute level of the strong-motion attenuation curves differs greatly from those of the weak-motion, particularly at low magnitudes. This may be due to a number of factors. Unlike models within the literature, weak-motion far-field data have been used for this study, rather than strong-motion, near-field data. A second possible explanation for the difference in absolute level is the difference in frequency characteristics of large and small earthquakes. The corner frequency, fc, marks the change in spectra from an increasing level of acceleration to a constant level. Between the corner frequency, fc, and fmax the acceleration spectra is flat, while above fmax, the high frequency content of the waveform decays exponentially. With increasing magnitude, the corner frequency increases, while fmax remains constant. Thus it is possible that fc might exceed fmax for low magnitude events. In this case the observed peak ground motion would be less than that expected for the magnitude of the event. Most importantly, differences may result from the use of ML rather than the surface-wave magnitude, MS, or moment magnitude, MW, as are used in the strong motion models. The magnitude scale difference appears to be the primary reason for the difference in absolute level between the weak-motion and strong-motion relations. This means that while the constant and magnitude term in the above expression are different from strong motion models, the anelastic attenuation term should be directly comparable. Azimuthal dependence of peak ground acceleration is evident within each of the regions. Within the Eastern North Island, the attenuation rate is lowest in the direction of 30 - 60° from North, which is roughly along the strike of the subducting Pacific plate. A similar azimuthal dependence was also noted within the deep Central Volcanic Region, while a slightly different minimum direction (5°) was determined for the shallow Central Volcanic Region.