Are the number of recorded earthquakes rising?

[Sally Furniss]

Number Of Recorded Earthquakes Rises Sharply

By Mark Dunphy - Mon Jun 20, 10:09 pm

2011 is on target to record the largest number of earthquakes in a single year for at least 12 years.

Research by Irish Weather Online, using data from the US Geological Survey (USGS), has found that earthquake activity (5.0-9.9 magnitude) from 01 January to 19 June 2011 is already exceeding the total annual seismic activity for the years 2001, 2002 and 2003. 2011?s total number of recorded earthquakes is also expected to exceed the most seismically active year of the past 12 years, 2007.

A total of 1,445 earthquakes, ranging in magnitude from 5.0 to 9.9, have been recorded in the year up to 19 June. The total number of earthquakes recorded globally for the entire of 2007 was 2,270.

much more including graphs http://www.irishweatheronline.com/n...recorded-earthquakes-rises-sharply/20688.html

 

[Sally Furniss]

Re: Are the number of recorded earthquakes rising?

From the Ask An Expert column in The Press, Christchurch, June 22 2011.

Question: Haiti has had 42 aftershocks,... Japan 726, Canterbury has had over 7000

Answer: (John Ristau, GNS science seismologist) discrepancy in numbers is related to

1) density of seismic networks (number of seismic instruments)

2) location of the earthquakes relative to the seismic network.

Canterbury region has a very dense seismic network.

Haiti has poor seismic network, generally only magnitude 4.0+ are located.

Japan has a very dense seismic network but Japan's quake occurred off the coast, away from seismic instruments.

article available here http://www.stuff.co.nz/the-press/ne...5175031/Ask-an-Expert-why-so-many-aftershocks

 

[Sally Furniss]

Re: Are the number of recorded earthquakes rising?

Hattip Emily

Jun 22, 2011
A magnitude 6.7 earthquake rocked Japan today, the 75th aftershock of at least magnitude 6.0 from the devastating magnitude 9.0 quake on March 11 ...

The number of aftershocks seems staggering, but geologists are not surprised, especially for such large mainshock.

The aftershocks have been so plentiful that the world's premier earthquake-measurement service has since modified its alert system to filter out the smaller aftershocks.

The rule of thumb for aftershock strength is that the biggest aftershock will be about one magnitude smaller than the mainshock....

http://www.ouramazingplanet.com/75-aftershocks-from-japan-earthquake-110622-1667/

 

[Emily]

Re: Are the number of recorded earthquakes rising?

This is just a subset of all quake data, but it might show trends without the influence of very modern reporting technology and population growth.

If you just look at the USGS records kept since 1900 of mega-quakes, (most in the Ring of Fire), in chronological order, it looks like there are quiet periods and clusters. (That 39 year quiet period is when most of the seismic risk assessment was done for nuclear plant siting. )


http://earthquake.usgs.gov/earthquakes/world/10_largest_world.php

1906 01 31 8.8

.............................. 16 year gap

1922 11 11 8.5
1923 02 03 8.5

.............................. 15 year gap

1938 02 01 8.5

.............................. 14 year gap

1950 08 15 8.6
1952 11 04 9.0
1957 03 09 8.6
1960 05 22 9.5
1963 10 13 8.5
1964 03 28 9.2
1965 02 04 8.7

.............................. 39 year gap

2004 12 26 9.1
2005 03 28 8.6
2007 09 12 8.5
2010 02 27 8.8
2011 03 11 9.0

A Chinese expert noticed this trend. (He thinks it is normal in the context of geological time.)

http://english.cri.cn/6909/2010/03/05/53s554382.htm

Expert Says Chile Quake Part of New Activity Phase
2010-03-05 15:17:45 Xinhua Web Editor: Zhang

"Sun, a chief forecaster with CENC before retiring in October 2008, has been closely observing earthquake trends.

The Haiti and Chile earthquakes were related, Sun believed. "In terms of earthquakes, the world seems to have entered a new phase. It may have started in late 2004 when a 6.4-magnitude earthquake rocked the eastern part of Indonesia," he said."

 

[Sally Furniss]

Re: Are the number of recorded earthquakes rising?

Nature article

Seismologists debate whether the recent spate of megaquakes is a statistical fluke or something more.

 

[Sally Furniss]

Re: Are the number of recorded earthquakes rising?

Random Variability Explains Apparent Global Clustering of Large Earthquakes
Author: Andrew J. Michael

The occurrence of 5 Mw{greater than or equal to}8.5 earthquakes since 2004 has created a debate over whether or not we are in a global cluster of large earthquakes, temporarily raising risks above long-term levels.

I use three classes of statistical tests to determine if the record of M{greater than or equal to}7 earthquakes since 1900 can reject a null hypothesis of independent random events with a constant rate plus localized aftershock sequences.

The data cannot reject this null hypothesis. Thus, the temporal distribution of large global earthquakes is well-described by a random process, plus localized aftershocks, and apparent clustering is due to random variability.

Therefore the risk of future events has not increased, except within ongoing aftershock sequences, and should be estimated from the longest possible record of events.

Received 26 August 2011; accepted 5 October 2011.

GEOPHYSICAL RESEARCH LETTERS, doi:10.1029/2011GL049443

http://www.agu.org/pubs/crossref/pip/2011GL049443.shtml

 

[Emily]

Re: Are the number of recorded earthquakes rising?

There's an open access full text article available here.

Global risk of big earthquakes has not recently increased

Peter M. Shearera,1 and
Philip B. Starkb

+ Author Affiliations

aScripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0225; and
bDepartment of Statistics, University of California, Berkeley, CA 94720-3860

Contributed by Peter M. Shearer, November 12, 2011 (sent for review September 27, 2011)


The recent elevated rate of large earthquakes has fueled concern that the underlying global rate of earthquake activity has increased, which would have important implications for assessments of seismic hazard and our understanding of how faults interact. We examine the timing of large (magnitude M≥7) earthquakes from 1900 to the present, after removing local clustering related to aftershocks. The global rate of M≥8 earthquakes has been at a record high roughly since 2004, but rates have been almost as high before, and the rate of smaller earthquakes is close to its historical average. Some features of the global catalog are improbable in retrospect, but so are some features of most random sequences?if the features are selected after looking at the data. For a variety of magnitude cutoffs and three statistical tests, the global catalog, with local clusters removed, is not distinguishable from a homogeneous Poisson process. Moreover, no plausible physical mechanism predicts real changes in the underlying global rate of large events. Together these facts suggest that the global risk of large earthquakes is no higher today than it has been in the past.

In terms of human perception, it just does not seem that rates of megaquakes are almost as high as before since there was that almost 40 yr gap with none prior to 2004.

Here's an older paper that seems to take the position that there is clustering of megaquakes:

Copyright ? 2005 Bulletin of the Seismological Society of America

Evidence for a Global Seismic-Moment Release Sequence

Charles G. Bufe1 and
David M. Perkins1

- Author Affiliations

1U.S. Geological Survey
Central Region Geologic Hazards Team
P.O. Box 25046, MS966
Denver, Colorado 80225

Abstract

Temporal clustering of the larger earthquakes (foreshock-mainshock-aftershock) followed by relative quiescence (stress shadow) are characteristic of seismic cycles along plate boundaries. A global seismic-moment release history, based on a little more than 100 years of instrumental earthquake data in an extended version of the catalog of Pacheco and Sykes (1992), illustrates similar behavior for Earth as a whole. Although the largest earthquakes have occurred in the circum-Pacific region, an analysis of moment release in the hemisphere antipodal to the Pacific plate shows a very similar pattern. Monte Carlo simulations confirm that the global temporal clustering of great shallow earthquakes during 1952?1964 at M ≥ 9.0 is highly significant (4% random probability) as is the clustering of the events of M ≥ 8.6 (0.2% random probability) during 1950?1965. We have extended the Pacheco and Sykes (1992) catalog from 1989 through 2001 using Harvard moment centroid data. Immediately after the 1950?1965 cluster, significant quiescence at and above M 8.4 begins and continues until 2001 (0.5% random probability). In alternative catalogs derived by correcting for possible random errors in magnitude estimates in the extended Pacheco?Sykes catalog, the clustering of M ≥ 9 persists at a significant level. These observations indicate that, for great earthquakes, Earth behaves as a coherent seismotectonic system. A very-large-scale mechanism for global earthquake triggering and/or stress transfer is implied. There are several candidates, but so far only viscoelastic relaxation has been modeled on a global scale.

 

[Shannon Bennett]

Re: Are the number of recorded earthquakes rising?

I agreee with Bufe and Perkins. I don't find it at all surprising that one initial mega quake would trigger subsequent large, even mega, quakes. When a subduction fault ruptures as it did in Indonesia, the entire earth vibrates much like a bell does once it has rung. The vibration continues for several months even years. Pressure is released in that particuilar zone however it increases pressure elsewhere along the fault. Shifts in pressure also occur along other plate boundaries. Increased pressure as well as decreased pressure in conjuction with vibration would result in rock fracture. Rocks already under tremendous stress would be under even greater stress resulting in catastrophic fracture.