[1] In this study, a large collection of 41,141 S-P times from the untapped records of the Taiwan Strong Motion Instrumentation Program (TSMIP) network is combined with the P and S wave arrival times from the Taiwan Central Weather Bureau Seismic Network (CWBSN) to image the V p and V p /V s structures beneath Taiwan. The records from the 680 TSMIP stations throughout Taiwan in the past 15 years enhance the path coverage and the resolution in the tomography inversions tremendously. Our result for the V p structure largely confirms previous studies but brings better constraint on the V p /V s structure. The colliding Luzon volcanic arc is characterized by a belt of high V p and high V p /V s with high seismicity that includes the offshore islands of Lutao and Lanyu and the Coastal Ranges in eastern Taiwan, at the depth between about 13 and 25 km. This high V p /V s belt can be traced to the subduction zone in the region between Hualian and Ilan in the deeper portion. The shallow portions of the southwestern coastal plain and the Pingtung region are also characterized by a belt of high V p /V s with lower seismicity. Most of the events occurred at the base of the high V p /V s zones. We suggest that material strength in those regions may be too low to accumulate stress, which may indicate water-saturated young sediments. Finally, the Central Range region is characterized by a low V p /V s belt.

[1] Pd is the peak amplitude of displacement in the first three seconds after the arrival of the P wave. We investigated the attenuation of Pd with the hypocentral distance R in southern California as a function of magnitude M, and obtained the following relationship: log (P d ) = À3.463 + 0.729 Â M À 1.374 Â log (R) ± 0.305. Given an earthquake location determined by the P-wave arrival times at stations close to the epicenter, this relationship can be used to define a so-called ''Pd magnitude'' of earthquakes. Our result shows that for earthquakes in southern California the Pd magnitudes agree with the catalog magnitudes with a standard deviation of 0.18 for events less than magnitude 6.5. Therefore, Pd is a robust measurement for estimating the magnitudes of earthquakes and has practical application in earthquake early warning systems. Citation: Wu, Y.-M., and L. Zhao (2006), Magnitude estimation using the first three seconds P-wave amplitude in earthquake early warning, Geophys.

Summary Seismic traveltimes are the most widely exploited data in seismology. Their Fréchet or sensitivity kernels are important tools in tomographic inversions based on the Born or single‐scattering approximation. The current study is motivated by a paradox posed by two seemingly irreconcilable observations in the numerical calculations for the sensitivity kernels of the traveltime perturbations. Calculations of kernels for 2‐D media by the normal‐mode approach indicate that traveltimes are most sensitive to the structure on and around the geometrical ray paths corresponding to the seismic arrivals, whereas calculations for 3‐D media by geometrical ray theory predict exactly zero sensitivities on the ray paths. In the current work, we employ these two completely different wave‐propagation approaches, the more efficient geometrical ray theory and the more accurate normal‐mode theory, to investigate the 3‐D sensitivities of the delay times to shear‐wave speed variations. Expressions for the delay‐time Fréchet kernels are presented for both methods, and extensive numerical experiments are conducted for various types of seismic phases as well as for different reference earth models. The results show that the contradictory observations are but two examples of a wide range of behaviours in the delay‐time sensitivity. For most of the seismic phases in realistic reference models with multiple discontinuities, wave‐speed gradients and low‐velocity zones, the wavefields are highly complicated and ray theory, which describes the response by the contributions of a few geometrical rays between the source and receiver, produces qualitatively different delay‐time kernels from those obtained by the normal‐mode theory, which includes essentially all contributions.

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