Geoscience Reference
In-Depth Information
there was “roaming” of large earthquakes between widespread fault systems. Their results
demonstrate the complex spatiotemporal patterns of intraplate earthquakes, the problems
of long recurrence time and short and incomplete records, and the need for a reassessment
of a seismic hazard in intraplate regions.
The largest and best studied intraplate event in the past two decades is arguably the
January 26, 2001 M 7.7 Bhuj earthquake in Western India. Soon after the earthquake, sci-
entists from several institutes carried out detailed aftershock studies. In the past 10 years,
additional deployment of large arrays of seismograph and GPS receivers has been comple-
mented with intensive geophysical and geological investigations providing an incredible
modern-day dataset. In Chapter 6, Rastogi, Mandal, and Biswas summarize the results of
these multidisciplinary investigations, providing for the first time an opportunity to study
the seismogenesis of a major intraplate earthquake. Using seismicity and seismic tomog-
raphy data they show that the earthquake nucleated at midcrustal depths in the vicinity
of a large, high-density, mafic body, in parts of which were pockets of fluids. The large
stress drop associated with it spawned a stress pulse that travelled at 25 km/yr. This stress
pulse, together with coseismic stress field changes, led to a sequential activation of mul-
tiple faults, both to the north and to the south of the epicenter, producing 20 M 4 to 5.1
earthquakes up to a distance of
250 km. The most distant M 5 earthquake occurred in
2011. GPS and InSAR studies revealed that, after the dissipation of the coseismic strain in
the first few years after the mainshock, the horizontal strain returned to a background value
of
2-5 mm/yr. However, InSAR and GPS surveys, 7 to 10 years after the mainshock,
provided evidence for localized pockets of high vertical strain rates located in the vicinity of
faults that had been activated by the seismicity that followed the 2001 mainshock. Insights
gathered from this contemporary study of a major earthquake can be used to analyze the
sparse data surrounding older intraplate earthquakes.
Arguably, the St. Lawrence Rift System in Eastern Canada and the New Madrid seismic
zone (NMSZ) in the Central United States are the most active seismic zones in eastern
North America and also the best studied. After a series of M
7 earthquakes in 1811-
1812, the continuing seismicity in the NMSZ has been at a lower magnitude level, and the
contemporary seismicity rarely exceeds M 4. The NMSZ also has been a subject of intense,
dedicated studies in the past few decades. Although the NMSZ is one of the best studied
cases of intraplate earthquakes, there is an absence of consensus about its cause and the
contemporary seismic hazard that it poses.
In Chapter 7, Van Arsdale presents a summary of these investigations, including a
detailed geological history of the region, and the results of historical and contemporary
seismological and paleoseismological investigations. Additional studies that are reviewed
includeGPS, geophysical, and geological investigations. These abundant data have spawned
a plethora of explanations, models, and speculations about the nature of the seismic haz-
ard in the NMSZ. Van Arsdale carefully presents these often conflicting explanations
and how they impact our current assessment of the seismic hazard. One of the sugges-
tions based on geological data is that the contemporary seismicity began in the Holocene,
and that the current GPS data indicate very low strain rates and a general absence of a
