Global Centroid Moment Tensor Catalog Research Articles

The 3 August 2009 Mw 6.9 Canal de Ballenas Region, Gulf of California, Earthquake and Its Aftershocks

On 3 August 2009 an earthquake of magnitude Mw 6.9 occurred near Canal de Ballenas, in the north-central region of the Gulf of California, Mexico. The focal mechanism of the main event, reported in the Global Centroid Moment Tensor (CMT) catalog, is right lateral strike-slip with a strike of 216° and a dip of 78°. The initial location reported by the National Seismological Service of Mexico (Servicio Sismologico Nacional (SSN)) and the Array Network Facility (ANF) sug- gested that the epicenter was on the North American plate near the Tiburon fault, which is considered inactive. This earthquake was preceded by a magnitude mb 5.5 event that occurred about 5 min before. In the next 40 min after the main event two aftershocks with magnitudes mb 4.9 and Mw 6.2 occurred, and on 5 August a third aftershock of Mw 5.7 was located in the Canal de Ballenas region. The events of August 2009 were recorded by the regional stations of the broadband network Red Sismologica de Banda Ancha (RESBAN) that Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) operates and by stations of the SSN also located in the region of the Gulf of California. We used body-wave arrivals to determine precise epicentral locations and to estimate the rupture area of this impor- tant sequence of earthquakes. The resulting hypocentral coordinates indicate that the main event of this sequence occurred along the Canal de Ballenas transform fault, with a rupture length of 50 km. Based on the aftershock distribution, we estimate that the main event had a rupture area of approximately 600 km 2 , an average slip of 1.3 m, and

Global earthquake forecasts

SUMMARY We have constructed daily worldwide long- and short-term earthquake forecasts. These forecasts specify the earthquake rate per unit area, time and magnitude on a 0.5 degree grid for a global zone region between 75N and 75S latitude (301 by 720 grid cells). We use both the Global Centroid Moment Tensor (GCMT) and Preliminary Determinations of Epicenters (PDE) catalogues. Like our previous forecasts, the new forecasts are based largely on smoothed maps of past seismicity and assume spatial and temporal clustering. The forecast based on the GCMT catalogue, with the magnitude completeness threshold 5.8, includes an estimate of focal mechanisms of future earthquakes and of the mechanism uncertainty. The forecasted tensor focal mechanism makes it possible in principle to calculate an ensemble of seismograms for each point of interest on the Earth's surface. We also introduce a new approach that circumvents the need for focal mechanisms. This permits the use of the PDE catalogue that reliably documents many smaller quakes with a higher location accuracy. The result is a forecast at a higher spatial resolution and down to a magnitude threshold below 5.0. Such new forecasts can be prospectively tested within a relatively short time, such as a few years, because smaller events occur with greater frequency. The forecast's efficiency can be measured by its average probability gains per earthquake compared to the spatially or temporally uniform Poisson distribution. For the short-term forecast the gain is about 2.0 for the GCMT catalogue and 3.7 for the PDE catalogue relative to a temporally random but spatially localized null hypothesis. Preliminary tests indicate that for the long-term global spatial forecast the gain is of the order 20–25 compared to the uniform event distribution over the Earth's surface. We can also prospectively test the long-term forecast to check whether it can be improved.

The Puzzle of the 1996 Bardarbunga, Iceland, Earthquake: No Volumetric Component in the Source Mechanism

A volcanic earthquake with M w 5.6 occurred beneath the Bardarbunga caldera in Iceland on 29 September 1996. This earthquake is one of a decade-long sequence of ![Graphic][1] events at Bardarbunga with non-double-couple mechanisms in the Global Centroid Moment Tensor catalog. Fortunately, it was recorded well by the regional-scale Iceland Hotspot Project seismic experiment. We investigated the event with a complete moment tensor inversion method using regional long-period seismic waveforms and a composite structural model. The moment tensor inversion using data from stations of the Iceland Hotspot Project yields a non-double-couple solution with a 67% vertically oriented compensated linear vector dipole component, a 32% double-couple component, and a statistically insignificant (2%) volumetric (isotropic) contraction. This indicates the absence of a net volumetric component, which is puzzling in the case of a large volcanic earthquake that apparently is not explained by shear slip on a planar fault. A possible volcanic mechanism that can produce an earthquake without a volumetric component involves two offset sources with similar but opposite volume changes. We show that although such a model cannot be ruled out, the circumstances under which it could happen are rare. [1]: /embed/inline-graphic-1.gif

Comparison of Magnitude Estimates for New Zealand Earthquakes: Moment Magnitude, Local Magnitude, and Teleseismic Body-Wave Magnitude

New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M w>∼3.5–4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude ( M L), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M w with M L for New Zealand earthquakes. In addition to M w and M L, there are more than 330 teleseismic body-wave magnitude ( m b) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M w. These are used to examine the relationship between M w, M L, and m b for New Zealand earthquakes. There is a clear distinction in the relationship of M L to M w for shallow focus (≤33 km depth) and deep focus (>33 km depth) earthquakes. Shallow focus earthquakes show M L to be fairly consistent with M w, particularly for events with M w>∼4.5 and with ![Graphic][1] . Deep focus earthquakes have M L consistently larger than M w (more than a full magnitude unit for some events) with ![Graphic][2] . M w and m b are in fairly good agreement regardless of the depth, whereas m b estimates are consistently smaller than M L for deep events. This suggests that M L is overestimated for deep focus events and that the large M Ls are not the result of physical properties of the source. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Seeking Anderson's faulting in seismicity: A centennial celebration

The reactivation of faults with near‐optimal orientations is commonly considered to control the state of stress in the crust. Near the surface, where a principal stress direction is vertical, the attitude of such faults is explained by Anderson's theory. This raises the questions of how prevalent this type of faulting actually is in current seismicity, down to what depth it frequently occurs, and what range of friction angles explains it best. The Global Centroid Moment Tensor catalog is analyzed to address these questions. Dip‐slip and strike‐slip mechanisms are dominant, and oblique slips are relatively rare for well‐constrained events with depths shallower than 30 km. Preferred Andersonian faulting is the simplest, but not the unique, explanation for this dominance. Isolating reverse, strike‐slip, and normal events reveals an asymmetry in the distribution of nodal plane dips and plunges of the P, B, and T axes between reverse and normal faulting. Assuming that the most frequent events correspond to reactivation near optimal orientations yields 40–60° and 0–20° friction angles for reverse and normal faults, respectively. This indicates that reverse and normal faulting mechanics are not symmetrical with respect to stress configuration as predicted by Anderson's theory.

Foreshock Rates from Aftershock Abundance

The question whether foreshocks behave like mainshocks that happen to have larger aftershocks is important for the understanding of earthquake nucleation and the modeling of earthquake clustering. Many models, including the short-term earthquake probability (STEP) model, for forecasting short-term earthquake hazard in California, and the epidemic-type aftershock sequence (ETAS) model, for the description of earthquake clustering, imply that all earthquakes of magnitude M , including foreshocks, can trigger earthquakes according to a magnitude dependent triggering ability 10 αM . The magnitudes of the triggered earthquakes are proportional to 10- bM , according to the Gutenberg–Richter relation for the magnitude-frequency distribution, and therefore can be larger than the magnitude of the initiating event. However, numerous studies observe a lower foreshock occurrence than predicted from aftershock models. In this article we show that, for homogeneous earthquake catalogs and a consistent definition of foreshocks and aftershocks, foreshock rates can be predicted from aftershock characteristics. For Southern California the probability that the initiating event of an earthquake cluster in the magnitude range 2.0–4.5 is followed by at least one larger event within one day is around 4%. This agrees well with corresponding data from the global centroid moment tensor (CMT) catalog where the probability that the initiating earthquake of a cluster is followed by at least one larger event within 30 days is around 3%. These foreshock rates are significantly lower than previously reported, and we discuss possible reasons for this.

Comparison of seismic moment release rates along different types of plate boundaries

This is a global survey of seismic moment release rates (scalar moment/length/time) along five categories of plate boundaries; for shallow earthquakes we evaluate divergent, transcurrent and convergent boundaries; for deep earthquakes we consider separately boundaries where the deepest seismicity is intermediate (<300 km) or deep-focus (500–600 km). The objective is to evaluate the typical range of rates observed and present the results in a form where it is straightforward to compare factors that affect rates in the different environments. For earthquakes occurring 1977–2005, the data are scalar moments from the global centroid moment tensor catalogue; for earthquakes 1900–1976, we use a standard formula to obtain moment from magnitudes in the Engdahl—Villaseñor Centennial catalogue. Moment release rates correlate with relative velocity between plates along divergent and transcurrent boundaries; focal depth influences moment release rates along intermediate and deep-focus boundaries. After accounting for these effects and adjusting rates for the influence of the short (29–106 yr) data record, within each category the typical observed variation is about an order of magnitude. We estimate seismic coupling coefficients, that is, the proportions of plate tectonic slip accommodated as earthquake rupture and reflected in the moment rates. Coupling may be 0.7 or greater along shallow convergent boundaries where at least one plate is continental. For all other boundary categories coupling is generally less than 0.4; it is 0.2 or less along most divergent boundaries and for intermediate and deep boundaries at depths exceeding 100 km. These results confirm that there are fundamental mechanical differences in the failure processes acting along different categories of plate boundaries. One implication is that only along a few highly coupled boundaries is it plausible that analysis of seismicity alone might lead to successful algorithms to predict large earthquakes.

A global index earthquake approach to probabilistic assessment of extremes

This paper applies recent innovations in flood frequency analysis to regional series of earthquake magnitudes in the global Centroid Moment Tensor (CMT) catalog from 1976 to 2005. Probability plot correlation coefficient hypothesis tests and L‐moment goodness‐of‐fit evaluations reveal that the Gumbel (GUM) distribution provides a good approximation to the probability distribution function (pdf) of series of annual maximum (AM) earthquake magnitudes. Homogeneity tests based on the theory of L‐moments further reveal that broad regions of the globe are homogeneous in the sense that the AM observations of earthquake magnitudes are well approximated by a GUM pdf with fixed upper moments. The homogeneity of global earthquake data across broad tectonic environments enables us to pool data into a regional pdf of earthquake magnitudes, termed an index earthquake distribution. Research in hydrology has shown that frequency analysis based on pooling of data using an analogous index‐flood method are much more accurate than frequency analysis based on site or region specific data. The index earthquake distribution is a dimensionless GUM distribution with fixed scale parameter so only the mean earthquake magnitude must be estimated for a region to define the frequency distribution of large earthquakes. We show how the degree of spatial homogeneity of earthquake magnitudes across broad tectonic environments can be exploited to yield improved estimates of the risk posed by extreme earthquake magnitudes.