Predicting the Endpoints of Earthquake Ruptures

Steven G. WesnouskyNature, 444, 358-360, 2006

doi:10.1038/nature05275

Introduction

Active fault traces are:• Generally not continuous• Segmented by discontinuitiesStress concentrations resulting from slip at

discontinuities may slow or stop rupture propagation and hence play a controlling role in limiting the length of earthquake rupture.

Data

Examined mapped surface ruptures of:• 22 historical strike-slip earthquakes• 10-420 km rupture lengths

Date Location Type Length Mw1859 Jan 09 San Andreas, California SSR 360 7.9

1891 Oct 28 Neo-Dani, Japan SSL 80 7.3

1930 Nov 02 Kita-Izu, Japan SSL 35 6.7

1939 Dec 25 Erzincan, Turkey SSR 300 7.7

1940 May 19 Imperial, California SSR 60 6.9

1942 Dec 20 Erbaa Niskar, Turkey SSR 28 6.8

1943 Sep 10 Tottori, Japan SSL 10.5 6.2

1943 Nov 26 Tosya, Turkey SSR 275 7.5

1944 Feb 01 Gerede Bolu, Turkey SSR 135 7.3

1967 Jul 22 Mudurnu, Turkey SSR 60 6.9

1968 Apr 08 Borrego Mountain, California SSR 31 6.1

1979 Oct 15 Imperial, California SSR 36 6.2-6.4

1981 Jul 29 Sirch, Iran SS 64 6.2

1987 Nov 23 Superstition Hills, California SSR 25 6.2-6.4

1990 Jul 16 Luzon, Philippines SSL 112 6.9

1992 Jun 28 Landers, California SSR 77 7.2

1998 Mar 14 Fandoqa, Iran SSN 25 6.6

1999 Aug 17 Izmit, Turkey SSR 145 7.1

1999 Oct 16 Hector Mine, California SSR 44 6.9

1999 Nov 12 Duzce, Turkey SSR 40 7.0

2001 Nov 14 Kunlun, China SSL 421 7.8

2002 Nov 03 Denali, Alaska SSR 302 7.6

Mapping Fault Geometry

• Began circa 1968 (Borrego Mtn)• Clear that active fault traces are

discontinuous• Discontinuities in fault traces often

associated with endpoints of earthquake ruptures

• Focus on continental SS ruptures of length greater ~15 km

• Depth dimension of brittle failure during SS earthquakes limited to ~15 km owing to physical and frictional constraints

• Direction of rupture propagation may be viewed as principally horizontal for each event

Figure 1. Map of 1968 Borrego Mountain earthquake surface trace.

Adjacent and continuing traces of active faults that did not rupture during the earthquake are shown as dotted lines. Also annotated are the dimensions of fault steps measured approximately perpendicular to the fault strike and the distance to the nearest-neighboring fault from the 1968 rupture endpoints. The star is the earthquake epicenter.

Figure 2. Synopsis of the observations bearing on relationship of geometrical discontinuities along fault strike to the endpoints of historical earthquake ruptures.

• Approximately 2/3 of terminations of SS ruptures are also associated with geometrical steps in fault trace or the termination of the active fault on which they occurred

Figure 3. Geometrical discontinuities as a function of size.

Discussion

• Histogram gives a statistical idea as to whether or not an earthquake rupture will be associated with particular fault step or fault terminus

• A transition exists between 3 and 4 km, above which rupture has not been observed to propagate through

• Transition seems largely independent of rupture length

• Suggests that magnitude of stress changes and the volume effected by those stress changes at the leading edge of propagating ruptures are largely invariable during the rupture process

• Recent theoretical work implies that releasing steps should be easier to rupture through than restraining steps although it is releasing steps that are observed more frequently at the endpoints of earthquake ruptures

• Releasing steps outnumber restraining steps by six to one in the data set

• Observations indicate that deformation processes attendant on cumulative SS displacements are relatively more efficient in the creation and maintenance of extensional steps and analogously in the linkage and removal of restraining steps