Geology topics
R.E. Abercrombie
Publications and source records attributed to R.E. Abercrombie.
The enigma of the Arthur's Pass, New Zealand, earthquake: 1. Reconciling a variety of data for an unusual earthquake sequence
The 1994 Arthur's Pass earthquake ( M W 6.7) is the largest in a recent sequence of earthquakes in the central South Island, New Zealand. No surface rupture was observed, the aftershock distribution was complex, and routine methods of obtaining the faulting orientation of this earthquake proved contradictory. We use a range of data and techniques to obtain our preferred solution, which has a centroid depth of 5 km, M 0 =1.3 × 10 19 N m, and a strike, dip, and rake of 221°, 47°, 112°, respectively. Discrepancies between this solution and the Harvard centroid moment tensor, together with the Global Positioning System (GPS) observations and unusual aftershock distribution, suggest that the rupture may not have occurred on a planar fault. A second, strike slip, subevent on a more northerly striking plane is suggested by these data but neither the body wave modeling nor regional broadband recordings show any complexity or late subevents. We relocate the aftershocks using both one-dimensional and three-dimensional velocity inversions. The depth range of the aftershocks (1–10 km) agrees well with the preferred mainshock centroid depth. The aftershocks near the hypocenter suggest a structure dipping toward the NW, which we interpret to be the mainshock fault plane. This structure and the Harper fault, ∼15 km to the south, appear to have acted as boundaries to the extensive aftershock zone trending NNW-SSE. Most of the M L ≥ 5 aftershocks, including the two largest ( M L 6.1 and M L 5.7), clustered near the Harper fault and have strike slip mechanisms consistent with motion on this fault and its conjugates. Forward modeling of the GPS data suggests that a reverse slip mainshock, combined with strike slip aftershock faulting in the south, is able to match the observed displacements. The occurrence of this earthquake sequence implies that the level of seismic hazard in the central South Island is greater than previous estimates.
Depth dependence of earthquake frequency-magnitude distributions in California: Implications for rupture initiation
Statistics of earthquakes in California show linear frequency-magnitude relationships in the range of M 2.0 to M 5.5 for various data sets. Assuming Gutenberg-Richter distributions, there is a systematic decrease in b value with increasing depth of earthquakes. We find consistent results for various data sets from northern and southern California that both include and exclude the larger aftershock sequences. We suggest that at shallow depth (∼0 to 6 km) conditions with more heterogeneous material properties and lower lithospheric stress prevail. Rupture initiations are more likely to stop before growing into large earthquakes, producing relatively more smaller earthquakes and consequently higher b values. These ideas help to explain the depth-dependent observations of foreshocks in the western United States. The higher occurrence rate of foreshocks preceding shallow earthquakes can be interpreted in terms of rupture initiations that are stopped before growing into the mainshock. At greater depth (9–15 km), any rupture initiation is more likely to continue growing into a larger event, so there are fewer foreshocks. If one assumes that frequency-magnitude statistics can be used to estimate probabilities of a small rupture initiation growing into a larger earthquake, then a small ( M 2) rupture initiation at 9 to 12 km depth is 18 times more likely to grow into a M 5.5 or larger event, compared to the same small rupture initiation at 0 to 3 km.
Occurrence patterns of foreshocks to large earthquakes in the western United States
Observations of foreshocks preceding large earthquakes provide one of the few well documented cases of premonitory events that are clearly related to a subsequent earthquake. Unfortunately, the apparent randomness of foreshock occurrence—they precede some events and not others—has severely hampered their use in reliable earthquake prediction. Understanding the factors that control foreshock occurrence is critical for determining how large earthquakes initiate and whether reliable short-term prediction will ever be possible 1 . Here we report the results of a comprehensive study of the occurrence patterns of foreshocks to large earthquakes in the western United States. The incidence of foreshocks decreases with increasing depth of the mainshock, and also depends on the mainshock slip orientation. This pattern of occurrence may be explained by a decrease in small-scale crustal heterogeneity with increasing depth, and suggests that increasing normal stress (both regional tectonic stress and lithostatic load) inhibits the occurrence of foreshocks. No relationship is observed between any aspect of foreshock occurrence and the magnitude of the subsequent mainshock, suggesting that the eventual size of the mainshock may be independent of the earthquake nucleation process, or that foreshocks are not part of this process.