Donald V. Helmberger
Smits Family Professor of Geophysics, Emeritus
Theory and Application of Synthetic Seismograms
Our understanding of the detailed nature of seismic sources and earth structure has increased dramatically with our increased ability to interpret seismograms. One of the best ways to demonstrate understanding of a seismogram is to generate a numerical one where the various processes effecting the source excitation and wave-propagation are accounted for and modeled. Professor Helmberger and his colleagues continue to develop more powerful codes to treat wave-fields interacting with complex structures.
The application of waveform modeling in exploring the nature of the various boundaries in the earth continues to draw considerable attention; this is especially true of the region just above the core-mantle boundary (CMB). The CMB is of fundamental importance to the earth sciences because of its impact on many geophysical disciplines, involving the thermal and magnetic fields. This zone is particularly interesting because recent studies suggest that it is laterally varying, [Garnero and Helmberger (1993), Garnero et al., (1993a, 1993b), Song and Helmberger (1993a)].
The basic evidence is revealed in differential times between branches of SKS and PKP. An example of the latter is displayed in the accompanying figure which shows the travel paths and a seismic record section of observations. These seismograms of a Fiji-Tonga deep earthquake were recorded in Europe; COP(Netherlands), ATU(Greece), PTO(Portugal), TOL(Toledo, northern Spain), MAL(Malaga, southern Spain). Near 144 deg all three branches of PKP (PKP-AB a path in outer core, PKP-BC a path in lower outer core, and PKP-DF a path through the inner core) have the same travel time and arrive at the same time (COP) to produce a strong simple pulse. At larger ranges these three branches separate in time with PKP-BC following PKP-DF closely and decaying rapidly near 155 deg where it reaches the outer-inner core boundary (ICB) and no longer exists as a geometric arrival. Record sections such as this are being modeled to determine the detailed nature of the ICB, [Song and Helmberger (1992)]. The dotted plots are synthetics generated for our favored 1D model of the core. The observations are plotted in time after subtracting the origin time. The synthetics are shifted slightly to align them with DF individually. The two stations PTO and TOL were shifted about 2 secs forward in this particular case which then makes the AB branch appear late. The ray paths for these paths cross the mantle-core boundary between Fiji and Hawaii and exit this boundary near Greenland. In this case the anomalous behavior is associated with a slow velocity zone at the base of the mantle between Fiji and Hawaii, [see Song and Helmberger (1993a)].
Record sections similar to this but sampling paths that follow along the spin-axis, polar paths of events and stations near the poles, show the DF branch to be consistently fast by about 3 secs. This is produced by a thin layer roughly 300 km thick that surrounds the inner core that acts like an anisotropic zone [see Song and Helmberger, (1993b)].