Seismic strain rates and the state of tectonic stress in the southern California region

Seismic hazard, active faults, fault slip rates, earthquake recurrence

Huang, Weishi ; Kanamori, Hiroo (advisor)

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Seismic hazard, active faults, fault slip rates, earthquake recurrence;   

Others  :  https://thesis.library.caltech.edu/4372/5/Huang_w_1995.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

I determine 505 fault plane solutions from the first motions of P-waves for thebackground seismicity (3.0≤ M≤6.0, 1981-1991) and collect mechanisms of majorearthquakes (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 intotal) by analyzing these mechanism data. The seismic strain tensors are obtained bytensorial summation of individual seismic moment tensors. The tectonic stress tensorsare determined by performing numerical inversions of the slip vector data, usingAngelier'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-slipfaulting (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, seismicconsistency (Sc) introduced by Apperson (1991). It is defined as the ratio of thescalar moment of the total moment tensor to the sum of the scalar moments ofindividual 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 betweenthe above two values;

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

(4) The axes of the maximum principal stress, δ₁. are oriented N6°E ±11°, whereasthose 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), SanJacinto (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 smallvalues 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 0.5such as the SJF, ETR, WWF, ESF, BYF, and MVE domains are associated withmore than 47% of SF events. The SSNF domain has a large Φ (>0.65) andshows 49% of NF events, Variation of the state of stress appears to be in theTransverse Ranges where hypocenters are generally deep. Other areas show arelatively stable state of stress throughout the seismogenic depth;

(6) Seismic fraction of deformation, η, is a measure of the deformation mode. It isdefined as the ratio of seismic strain rate to the total deformation rate. Because ofthe limited seismic data, we can usually estimate the apparent instead of the realseismic fraction of deformation. Therefore, caution must be exercised in applyingthe values of η to evaluations of seismic potential, In southern California, thereare some indications that areas in which seismic deformation nearly accounts forthe total deformation are typically associated with cold and rigid batholithic rocksor high seismic velocity anomalies such as in the SJF, south central MVE, WWF,and possibly the ETR domains. However, areas with low seismic velocityanomalies 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 themissing deformation is being released aseismically or has accumulated elasticallyremains to be resolved.