Plate-tectonic analysis of shallow seismicity:Apparent boundary width, beta, corner

magnitude, coupled lithosphere thickness, and coupling in 7 tectonic settings

Peter BirdYan Y. Kagan

UCLA Department of Earth & Space SciencesDec. 10, 2003

presented to AGU, San Francisco, CA

Seismic coupling and frequency/magnitude relations are best determined from global

statistics.• Different kinds of plate boundary clearly have

different seismicity characteristics.• But, no single subduction zone (etc.) has a

long enough seismic record to determine corner magnitude and coupling reliably [McCaffrey, 1997].

• We use the “ergodic assumption”: Global data over one century may substitute for local data covering thousands of years.

Plate boundary model PB2002 has 52 plates and 13 orogens: Bird [2003] An updated digital model of plate boundaries, Geochemistry Geophysics Geosystems, 4(3), 1027, doi:10.1029/2001GC000252.

The kinematic basis for the global calibration:

Every digitization step(from 1-109 km long)along every plate boundaryis classified as being oneof 7 types:

Using the Harvard CMT catalog of 15,015 shallow events:

We study histograms of earthquake* frequencyas a function of distance to thenearest plate boundary:

[*shallow earthquakes of appropriate focal mechanism, excluding those in orogens]

Note that the distribution forSUBduction zones is asymmetrical:

Formal assignment of an earthquake to a plate boundary stepis by a probabilistic algorithm that considers all available information: step type, spatial relations, EQ depth, and focalmechanism:

The A factor takes into account the length, velocity, and inherentseismicity of each candidate plate boundary step.The inherent seismicity levels of the 7 types of plate boundary(obtained by iteration of this classification algorithm)are valuable basic information for seismicity forecasts:

We allow for several different “model earthquake” focal mechanismson each plate boundary step. When the step is oblique to relativeplate motion (the general case), the Earth may produce either oblique-slip EQs, or sets of partitioned-slip EQs:

The final classification factor (E) “grades” a possible match on theangular discrepancy between actual and model focal mechanism:

The result…

Advantages ofthis distribution:

Simple (only one more parameter than G-R);Has a finite integrated moment (unlike G-R) for < 1;Fits global subcatalogs slightly better than the gamma distribution.

The maximum-likelihood method is used to determine theparameters of these tapered G-R distributions (and their uncertainties):

An ideal case(both parameters determined)

A typical case(corner magnitude unbounded

from above)

thresholdmagnitude

95%-confidencelower limit

95%-confidencelower limit

not to betaken literally!(“a largenumber”)

95%-confidenceupper limit

Review of results on spectral slope,

Although there are variations, none is significant with 95%-confidence.Kagan’s [1999] hypothesis of uniform still stands.

In many cases, subcatalogs obtained from the Harvard CMT catalog for non-orogen regions are not large enough to define

95%-confidence upper limits on the corner magnitudes.We next enlarged some of our subcatalogs in three ways:

included events of 1976 AD from catalog of Ekström & Nettles [1997] (mt 6.28);

included events of 1900-1975 AD from catalog of Pacheco & Sykes [1992] (mt 7.10);

included plate-boundary-associated events from within the 13 orogens of PB2002:

But, it is necessary to be careful:

• Catalog data from 1900-1975 is less accurate in every way (moment/magnitude, location, depth, focal mechanism-?), and therefore these events are more likely to be misclassified.

• The high catalog threshold (mt = 7.1) makes very hard to determine, and risks biasing mc values which are smaller.

We chose not to work with merged subcatalogs for OSR and OTF/medium-fast, where we already know that mc < 7.1.

We fix at the value determined from the 1977-2002 Harvard CMT catalog, and only optimize the corner magnitude mc.

IMPLICATIONS:1. Now that we know the coupled thickness of

seismogenic lithosphere in each tectonic setting, we can convert surface velocity gradients to seismic moment rates.

2. Now that we know the frequency/magnitude distribution in each tectonic setting, we can convert seismic moment rates to earthquake rate densities at any desired magnitude.

KinematicModel

MomentRates

Long-term-average(Poissonian)

seismicity maps

For SUB steps,the depth PDF function Dassociated with each“model earthquake” helpsto separate mechanismsexpected to be shallow(green curve)from those expected to be within the slab(blue curve, for case ofslab top at 25 km depth)and those thrust eventsexpected to lie along theslab-top plate interface(a Gaussian PDFcentered on this depth;not shown here).