Seismic Strain Rates and the State of Tectonic Stress in the Southern California Region

Citation

Huang, Weishi (1995) Seismic Strain Rates and the State of Tectonic Stress in the Southern California Region. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/P2W6-3155. https://resolver.caltech.edu/CaltechETD:etd-11022007-093001

Abstract

I determine 505 fault plane solutions from the first motions of P-waves for the background seismicity (3.0≤ M≤6.0, 1981-1991) and collect mechanisms of major earthquakes (M≥6.0, 1927-1994) from the literature in the southern California region. Then I study the seismic strain and tectonic stress fields in individual domains (ten in total) by analyzing these mechanism data. The seismic strain tensors are obtained by tensorial summation of individual seismic moment tensors. The tectonic stress tensors are determined by performing numerical inversions of the slip vector data, using Angelier's (1990) method. The findings are summarized as follows:

(1) Of the 505 fault plane solutions for the 1981-1991 background seismicity, 54% are strike-slip (SF), 21 % reverse (RF), 17% normal (NF), and 8% oblique-slip faulting (OS) events. The catalog of the major earthquakes for the period 1927- 1994 also displays similar proportions of the faulting mechanisms;

(2) The similarity of the focal mechanisms can be measured by a parameter, seismic consistency (Sc) introduced by Apperson (1991). It is defined as the ratio of the scalar moment of the total moment tensor to the sum of the scalar moments of individual moment tensors. In southern California, the Brawley fault (BYF) domain shows the highest Sc (0.70), whereas the White Wolf fault (WWF) domain displays the lowest Sc (0.44). Sc values in other domains vary between the above two values;

(3) The depths of possible low-angle faults inferred from the fault plane solutions vary from 20 km in the Transverse ranges where N-S convergence dominates, to only 1 km in the southern Sierra Nevada fault (SSNF) domain where E-W divergence dominates. Our current data do not show the existence of a sigle unified seismically-active master detachment in the seismogenic zone;

(4) The axes of the maximum principal stress, δ₁. are oriented N6°E ±11°, whereas those of the maximum principal strain, ε₁ are oriented N5°E ±21°;

(5) The strain and stress tensors are similar to each other in the Mojave (MVE), San Jacinto (SJF), Elsinore (ESF), BYF, western and eastern Transverse Ranges (WTR, ETR) domains, but dissimilar in the central Transverse Ranges (CTR), Newport-Inglewood fault (NIF), WWF, and SSNF domains. Areas with small values of Φ = (δ₂ - δ₃)/(δ₁ - δ₃) (<0.35) such as the WTR, CTR, and NIF domains are associated with more than 40% of RF events. Areas with Φ values around 0.5 such as the SJF, ETR, WWF, ESF, BYF, and MVE domains are associated with more than 47% of SF events. The SSNF domain has a large Φ (>0.65) and shows 49% of NF events, Variation of the state of stress appears to be in the Transverse Ranges where hypocenters are generally deep. Other areas show a relatively stable state of stress throughout the seismogenic depth;

(6) Seismic fraction of deformation, η, is a measure of the deformation mode. It is defined as the ratio of seismic strain rate to the total deformation rate. Because of the limited seismic data, we can usually estimate the apparent instead of the real seismic fraction of deformation. Therefore, caution must be exercised in applying the values of η to evaluations of seismic potential, In southern California, there are some indications that areas in which seismic deformation nearly accounts for the total deformation are typically associated with cold and rigid batholithic rocks or high seismic velocity anomalies such as in the SJF, south central MVE, WWF, and possibly the ETR domains. However, areas with low seismic velocity anomalies are not free of earthquakes as seen, for example, in the BYF domain, which shows η = 0.6-1.0. Other domains show η < 0.4. The problem of whether the missing deformation is being released aseismically or has accumulated elastically remains to be resolved.

Thesis Files