Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

356 Robert A. UhrhammerTable 2. Rate of Seismicity1.5 31. 13.2.0 11. 4.82.5 4.2 2.03.0 1.5 .823.5 .57 .354.0 .21 .158. Analysis of Three-Component SeismogramsThe task is to determine the general characteristics of the local seismicity usingonly seismograms recorded at one station. A dense microearthquake network,consisting of many vertical-component short-period seismographs is commonly usedto determine the hypocentral locations, magnitude and focal mechanisms of localearthquakes (Lee and Stewart, 1981). This is readily accomplished using easilymeasurable parameters on the records, such as polarity and onset time of P-wavefirst-motion and coda duration. When only one three-component station is available,it is still possible to determine the magnitude and hypocentral location of anevent.Magnitude is determined, in the usual way, from the maximum trace amplitudeand the differential S-P travel-time interval (ts-p), Hypocentral locations are determinedby vectoring the first peak in the P-wave arrival into the apparent angle ofincidence (2,) and azimuth (Az) to define the raypath (for a given velocity model)and then using the differential travel time (ts-p) to determine the location of thefocus along the raypath. The precision of the hypocentral locations is approximately3 km and, even if a few of the locations were grossly in error, trends in the relativelocations of the hypocenters will be a robust indicator of the general characteristicsof the local seismicity.The focal depths of the events are estimated, from the records of a single threecomponentstation (SHS), by plotting the apparent angle of incidence (i,), measuredfrom vertical, versus the differential S-P (ts-p) travel-time difference for each eventas shown in Figure 6. i, is determined from the ratio of the horizontal (AH) andvertical (Av) component amplitudes of the first peak of the P-wave onset (Ewinget al., 1957; p. 27)i, = tan-' (AHIAV). (4)The uncertainty in estimating i, is about 5 degrees. The angle of incidence (i) ofthe P-wave is related to z, by (Walker, 1919)a22 sin 22 = -(I - cos ia),P2where a and P are the P- and S-wave velocities, respectively.Contours of constant focal depth, assuming a half-space velocity model (a = 5.6and ,b = 3.3, a/P = 1.7), are also plotted in Figure 6. The position of these contours(5)