Intraplate earthquakes in North China - Intraplate Earthquakes - page 117

ruptured the southern segment of the Longmenshan fault, also assigned with moderate to
low risk on the hazard map ( Figure 5.10 ) . The Wenchuan and Lushan earthquakes show
clearly that having no previous large earthquakes does not mean having no future large
earthquakes.
Within North China, the hazard map assigns the highest risk to the Tangshan, Xingtai,
and Tancheng regions, apparently because of the 1966 Xingtai and the 1976 Tangshan
earthquakes, and because of the big Tancheng earthquake (M 8.5) in 1668. The Shanxi rift
zone was assigned with lower risk, presumably because it has been seismically quiescent
for the past 300 years. The Weihe rift zone was assigned a low risk because the 1556
Huaxian earthquake (M 8.0) was the last large earthquake within the rift.
The nearly 3,000 years of earthquake records in North China, however, indicate an
earthquake behavior more complicated than that assumed in the current practice of hazard
assessment. Because the large earthquakes tend to roam between widespread fault systems,
previous large earthquakes may not be a good indicator of where a future large earthquake
will occur; because strain can accumulate in the fault zones for thousands of years before
being released by a large earthquake, low slip rates do not mean being safe; and not having
previous large earthquakes does not mean no large earthquakes in the future. And finally,
because it usually take thousands of years for intracontinental fault zones to accumulate
enough strain energy for a large earthquake, places where large earthquakes occurred in the
recent past are not necessarily more dangerous than other places.
Another challenge for earthquake hazard assessment in North China, and in other mid-
continents, is the long sequences of aftershocks (Stein and Liu, 2009 ) . Small earthquakes
are often regarded as signs of stress building up towards the next big earthquake; their
occurrence in source regions of previous large earthquakes therefore often causes alarm.
One example is the recent sequence of moderate sized earthquakes in the Tangshan region,
which includes an M 4.8 event on May 28 and an M 4.0 event on June 18, 2012. These
earthquakes caused widespread concerns and heated debate in China: are they aftershocks
of the great 1976 Tangshan earthquake, or are they harbingers of a new period of active
seismicity in Tangshan and the rest of North China, where seismic activity seems fluctuate
between highs and lows over periods of a few decades (Ma, 1989 ) ?
Liu and Wang ( 2012 ) showed that this recent seismicity in Tangshan is likely the
aftershocks of the 1976 Tangshan earthquake for the following reasons: (1) The seismicity
rate in the Tangshan region has been decaying since 1976, following Omori's law, but is still
clearly above the background level ( Figure 5.11 ) . (2) The seismicity rates of the Tangshan,
Xingtai, and Haicheng regions for 1986-2010 (i.e., 10 years after the Tangshan earthquake
and 20 years after the Xingtai earthquake) are clearly higher than the average value for
the North China Plain. This indicates either that these regions are tectonically more active
than the rest of the North China Plain, or the continuing influence of aftershocks of the
large earthquakes decades ago. Because these regions showed no sign of abnormal tectonic
activity before the large earthquakes, aftershocks are more likely the cause. (3) Strain rates
calculated from GPS data are higher in the Tangshan and Xingtai regions than that of the