Allan Rubin, Professor of Geosciences, The Department of Geosciences
Earthquake Relocation and Fault Mechanics
Earthquake catalogs are always dominated by the smallest recorded events, e.g. magnitude 1–2 earthquakes for well-instrumented regions such as can be found in California. Routine catalog location errors for such events are of order 1 km, much larger than some interesting fault-zone structures and the earthquakes themselves (several tens of meters). This severely limits their usefulness in studies of earthquake mechanics. However, earthquakes that have similar locations and focal mechanisms generate seismograms at a given station that appear similar (Figure 1). By cross-correlating these seismograms one can measure extremely accurate relative arrival times (in the best cases with errors as small as 1/40th of the sampling rate), and from these one can obtain relative locations that are factor of 10 to more than 100 times better than in the catalog (Figure 2). These improved locations have allowed us to image fault-zone structures that are invisible in the original catalogs (Figure 3).
The origin of the seismically-defined lineations in Figures 2 and 3 is currently a subject of great interest. Because we see similar slip-parallel lineations on normal and thrust faults in Kilauea Volcano (basically a pile of basaltic rubble), we conclude that they result from a structural organization of the fault surface, and not (for example) an inherited sedimentary layering. We first observed such streaks in the East Rift Zone of Kilauea Volcano, Hawaii (Figure 4), where from geodetic data we inferred that they represented the edge of a locked, shallow region that was being slowly eroded by steady creep at greater depth (Figure 5). This scenario is consistent with the vertical extent of the lineation (~100 m) and the increase in earthquake rate and moment rate since the previous dike intrusion in 1982. A similar explanation has been proposed for the streaks in central California, but the evidence there is more equivocal.
Earthquake Mechanics From Precise Relative Locations
Because the relative location errors between the nearest events in our relocated catalogs (meters) are now much smaller than the rupture dimensions (tens of meters), these catalogs contain a wealth of information concerning earthquake interaction. One of our most surprising observations is that within about 1 day of a microearthquake on the central San Andreas fault, aftershocks are more likely to occur immediately to the NW than to the SE by a factor of nearly 3:1 (Figure 6). On a vertical strike-slip fault in a homogeneous body there is no mechanism for producing such asymmetry. However, the San Andreas fault in this region juxtaposes two very different rock types, with higher seismic wavespeeds on the Pacific plate, and this breaks the symmetry. We have used analytical and numerical models of elastodynamic ruptures to explain the aftershock asymmetry in terms of how the material contrast across the fault affects the dynamics of the mainshock (Figure 7). Understanding earthquake behavior on a fault separating different rock types has implications for seismic hazards because such material contrasts can also give rise to rupture directivity, which has a significant impact on ground shaking.