Omori Utsu Research Articles

Model Ensembles for Prediction of Wenchuan Aftershock Activity

This paper considers forecasting the number of large aftershocks in a given interval of time following the 12 May 2008 Wenchuan earthquake ( M w 7.9). First, we investigate the Gutenberg–Richter frequency distribution as a potential forecasting technique. The parameters of the Gutenberg–Richter distribution are calculated using short periods of time. We then combine well-known models describing aftershock sequences, Omori–Utsu and the epidemic-type aftershock sequence (ETAS), with the Gutenberg–Richter forecast to create a consensus forecast of the expected number of aftershocks above a designated magnitude. The Omori–Utsu or ETAS parameters are calculated using all available data, and therefore combining the Gutenberg–Richter predictions with the Omori–Utsu or ETAS forecasts can be considered as creating a forecast ensemble that contains both short-term and long-term information about the aftershock sequence. The results are illustrated over a variety of interval lengths of 10 to 20 days. The results show that the forecasts created by the windowed Gutenberg–Richter model perform as well as the more common aftershock forecasting methods. The ensemble forecasts are also shown to produce reliable predictions and are more accurate when forecasting the number of aftershocks greater than or equal to M 5.

Space-time heterogeneity in aftershock activity

For a number of aftershock sequences in and around Japan, we examine detrended space—time coordinates after fitting the Omori–Utsu occurrence rate to each aftershock sequence. The case studies in conjunction with the fault locations and orientations of the main shocks and focal large aftershocks indicate that the region of deficit and surplus in the aftershock activity well corresponds to the decreased and increased region of Coulomb failure stress, respectively, caused by various aseismic slips. We illustrate the relation between the transient stress changes and the anomalous aftershock activity in a local subregion by six scenarios derived from the rate and state dependent friction law of Dietrich.

Mixture Models for Improved Short-Term Earthquake Forecasting

Abstract The short-term earthquake probability (STEP) forecasting model applies the Omori–Utsu aftershock-decay relation and the Gutenberg–Richter frequency-magnitude relation to clusters of earthquakes. It is mainly intended to forecast aftershock activity and depends on a time-invariant background model to forecast most of the major earthquakes. On the other hand, the long-range earthquake forecasting model EEPAS (every earthquake a precursor according to scale) exploits the precursory scale increase phenomenon and associated predictive scaling relations to forecast the major earthquakes months, years, or decades in advance, depending on magnitude. Both models are shown to be more informative than time-invariant models of seismicity. By forming a mixture of the two, we aim to create an even more informative short-term forecasting model. Using the Advanced National Seismic System catalog of California over the period 1984–2004, the optimal mixture model for forecasting earthquakes with M ≥5.0 is a convex linear combination consisting of 0.42 of the EEPAS forecast and 0.58 of the STEP forecast. This mixture gives an average probability gain of more than 2 compared to each of the individual models. Several different mixture models will be submitted to the CSEP Testing Center at the Southern California Earthquake Center to ascertain whether or not this result is borne out by real-time tests of the models against future earthquakes.

Omori-Utsu Law c-Values Associated with Recent Moderate Earthquakes in Japan

We investigate the early aftershock activity associated with four moderate earthquakes ( M w 6.6–6.7) that occurred recently in Japan. For each aftershock sequence, we examine continuous high-pass filtered seismograms recorded at seismic stations nearby the main fault to identify as many early events as possible. The magnitude of these events is calibrated using aftershocks that are listed in the earthquake catalog of Japan Meteorological Agency (JMA). The analysis of the aftershock decay rates reveals a power-law time dependence with a scaling exponent close to 1.0 that starts from about one minute from the mainshock. Our results demonstrate that the c -value of the Omori–Utsu law is very small, although a lower bound is not established due to completeness problems in the first minute after the mainshock and statistical fluctuations.