Foreshock sequence of the 1992 Landers, California, earthquake and its implications for earthquake nucleation

Abstract

Abstract. The June 28, 1992, Landers, California, earthquake (Mw=7.3) was preceded for about 7 hours by a foreshock sequence consisting of at least 28 events. In this study we examine the geometry and temporal development of the foreshocks using high-precision locations based on cross correlation of waveforms recorded at nearby stations. By aligning waveforms, rather than trying to obtain travel time picks for each event independently, we are able to improve the timing accuracy greatly and to make very accurate travel time picks even for emergent arrivals. We perform a joint relocation using the improved travel times and reduce the relative location errors to less than 100 rn horizontally and less than 200 rn vertically. With the improved locations the geometry of the foreshock sequence becomes clear. The Landers foreshocks occurred at a fight step of about 500 rn in the mainshock fault plane. The nucleation zone as defined by the foreshock sequence is southeast trending to the south and nearly north trending to the north of the right step. This geometry is confirmed by the focal mechanisms of the foreshock sequence, which are fightlateral and follow the trend as determined by the foreshock locations on the two straight segments of the fault, and are rotated clockwise for foreshocks that occur within the step. The extent of the foreshock sequence is approximately 1 km both vertically and horizontally. Modeling of the Coulomb stress changes due to all previous foreshocks indicates that the foreshocks probably did not trigger each other. This result is particularly clear for the Mw=4.4 immediate foreshock. Since stress transfer in the sequence appears not to have played a significant role in its development, we infer an underlying aseismic nucleation process, probably aseismic creep. Other studies have shown that earthquake nucleation may be controlled by fault zone irregularities. This appears to be true in the case of the Landers earthquake, although the size of the irregularity is so small that it is not detectable by standard location techniques.

Evolving towards a critical point: A possible electromagnetic way in which the critical regime is reached as the rupture approaches

“…The K-G Earthquake was preceded within 30 min of the main shock by five foreshocks with M > 3.5 clustered within 2 km of one another about 5 to 10 km from the main shock epicentre (Bernard et al, 1997). This size of foreshocks clustering fits well the correlation law with the main-shock magnitude obtained by Dodge et al (1995) for Californian Earthquakes (Bernard et al, 1997). This unusual foreshock activity for the aseismic K-G region, which was completely quiet for a long period before the EQ, may be possibly regarded as a fingerprint of local dynamic rupture of asperities in the weak zone associated with the nucleation of the main rupture.…”

“…Perhaps the most convincing evidence that there is a nucleation stage before major Earthquakes comes from observations of foreshoks (Dodge et al, 1995). The K-G Earthquake was preceded within 30 min of the main shock by five foreshocks with M > 3.5 clustered within 2 km of one another about 5 to 10 km from the main shock epicentre (Bernard et al, 1997).…”

Seismic Constraints on a Double‐Layered Asymmetric Whole‐Mantle Plume Beneath Hawai‘i

“…Observational seismology has, in some cases, observed foreshock sequences preceding larger mainshock events (e�g�, Dodge et al, 1995) not dissimilar to our preliminary laboratory results� While the scale of the two results are distinctly different (in both space and time) we have currently begun an investigation that employs similar techniques and models to characterize our experimental findings� These techniques may help develop scaling relations from the laboratory to the field that have been difficult to characterize in the past� These foreshock bursts may be useful contributors to short-term earthquake probability estimates (Chen and Shearer, 2013 Figure 2�22�2 determined using P-wave travel times from multiple sensors� The locations were superimposed on the pressure measurements obtained from the pressure sensitive film� Locations exhibiting higher normal stress appear red while the blue background assumes no normal stress was transferred (i�e�, no contact occurred)� The inlay shows an enlarged view of the movement of the foreshock sequence where the distribution appears to be random in space�…”

Southern California Hypocenter Relocation with Waveform Cross-Correlation, Part 1: Results Using the Double-Difference Method

“…Waveform cross-correlation is an increasingly important tool for characterizing event similarity, improving earthquake locations, and studying source properties (Nakamura, 1978;Poupinet et al, 1984;Got et al, 1994;Dodge et al, 1995;Nadeau et al, 1995;Gillard et al, 1996;Rubin et al, 1999;Waldhauser et al, 1999). However, it is not yet used routinely for network data because of its greater computational requirements compared with standard processing based on phase picks.…”