Clustering and Scattering of Intraplate Earthquakes in the

NMSZ and Other Regions: Preliminary Results From Geodynamic Modeling

Qingsong LiLunar and Planetary Institute, USRA

NMSZ Seismicity

Is the NMSZ the permanent site of concentrated seismicity in the central US?

or

Is seismicity in the NMSZ transient and may migrate to elsewhere?

Spatiotemporal Patterns of Intraplate Seismicity

CEUS seismicity is clustered around several active seismic zones.

Australia seismicity seems be randomly scattered, lacking large earthquakes.

In North China, earthquakes migrate eastward from the Shanxi Graben to the North China Plain in the past 200 years.

CEUS North China

Australia

Geodynamic Model (1) and Mechanisms (Horizontally homogeneous rheology)

Conceptual Stress Cycles

0

12

0 1 2 3 4

Time

stre

ss

Strength

Mechanisms incorporated include:

1. Far field boundary loading.

2. Each element fails when stress reaches strength (an earthquake).

3. Coseismic and postseismic stress evolution (butterfly patterns)

1 mm/yr

(1)

(3)

(2)

Stress Evolution Cartoon

Seismicity in successive time periods (5000 years)

Seismicity in a longer time period (30000 years)

Geodynamic Model (2)(Strength anomaly in the upper crust)

1 mm/yr

A weak zone, with lower strength in the upper crust, is added at the center of the model region.

Seismicity in successive time periods(5000 years)

Seismicity in a longer time period(30000 years)

Seismicity in much shorter time periods(500 years)

a b c

fed

Seismicity in the CEUS, including the NMSZ

or

Transient activity of earthquakes along a much longer seismic belt.

The NMSZ seismicity reflects either

a temporary feature of spatially scattered seismicity in the CEUS over long-terms

Seismicity in the CEUS, including the NMSZ

or

transient activity of earthquakes along a much longer seismic belt.

The NMSZ seismicity reflects either

a temporary feature of spatially scattered seismicity in the CEUS over long-terms

Summary• In a “homogeneous” intraplate region, seismicity may appear

spatially clustered in “seismic zones” during a short period, but over long time they will become randomly scattered.

• Conversely, a weak zone in the upper crust can cause persistent clustering of earthquakes that tend to migrate back and forth along a belt that extends from the weak zone.

• Recent clustering of seismicity in the NMSZ may reflect a short-term feature of scattered seismicity over long-terms, or transient activity of earthquakes along a much longer seismic belt.

• In either case, future earthquakes may not be limited to the presently active NMSZ.