The Flash animation above shows the travel-time residuals observed (some more detailed notes below). The circle at lower left represents the horizontal distance a ray travels from 200 km depth to the station (station at the center) with the azimuth reflecting the azimuth back from the station to the earthquakes. The outer circle has closer events (rays are more horizontal), inner circle are more distant events (more nearly vertical rays). Inner circle is about 50 km in radius, second circle 100 km and outer circle 120 km radius. Clicking or dragging into a gray region will show the travel time residuals, blue arrivals are observed before the predicted IASPEI time, red are late relative to IASPEI. Clicking down and dragging across the gray areas will animate the residuals. So moving along one of the circles (through events at common distances) shows how an anomaly moves with changes in backazimuth: shallow anomalies should not move at all, while deeper anomalies will move substantially. Moving on a radius changes the incidence angle of rays and provides information on both depth and shape. The green wedge is the bin that produced the anomalies displayed.

So, for instance, the characteristics of the Isabella anomaly near 36 N, 240E are nicely illustrated. Clicking down just outside the outer circle near 120 degrees and then dragging through the gray areas towards the center will show the blue dots (early arrivals) moving steadily from SE to NW, indicating that this anomaly has to be at depths below 100 km--continued dragging of the mouse through the gray areas towards 310 continues to display movement of the early arrivals. Dragging across the gray zones at a constant distance (say from SW toward NW) shows motion of the anomaly also consistent with its presence near 36, 240.5 and at depth. Some changes in amplitude are probably reflecting directions where rays travel along the length of an anomaly vs. rays travelling across it (e.g., compare the 2 bins in the 80-90° BAZ direction).

Other anomalies worth examining include a low parallel to the Sierran crest that also exhibits considerable variation (and the earlier arrivals on the east side of this anomaly are quite robust, revealing a significant contradiction with surface wave anomalies in the area), the Foothills anomaly on the west side of the Sierra (most evident from NW backazimuths), and the profound changes in the north associated with the subducting slab (e.g., arrivals from the east and northeast reveal a relatively narrow band of extremely early arrivals that travelled up the slab, while arrivals from the west have a broader and lower amplitude set of early-ish arrivals well to the east from rays crossing the slab and low-wavespeed upper mantle).

Each image is actually a combination of several events, generally as few as 3 for some of the eastern backazimuths and over 20 events for several of the western backazimuths. Events were combined by minimizing the median difference in residual between common stations; this includes common stations between SNEP, the 1997 SPE and 1988 deployments, thus leveraging these older datasets. Potential blunders in arrival times were removed by eye in the process of combining events using an interactive code (these blunders remained in the dataset, however, that was inverted); these were never more than ~10% of all arrivals and were generally only a problem for the more monochromatic and emergent arrivals from events to the east (BAZs from 0 to about 120). Many of these presumably reflect cycle-skips caused by the indistinct nature of these arrivals. 1988 picks were from Jones et al., 1994; 1997 picks done by C. Jones with dbxcor; 2005-2007 SNEP picks by H. Reeg with dbxcor (Reeg MS thesis, 2008). Travel times to stations above known thicknesses of Great Valley sediment were corrected using delays calculated from the depth of basement and standard "rule of thumb" velocity-depth relations used in interpreting seismic profiles in the Great Valley.

You can download the Flash file directly to open with the Flash viewer or within many webbrowsers, where you can resize the window to your preferred size.