Earthquake clusters in southern California II: Classification and relation to physical properties of the crust

Abstract

[1] This is a second paper in a study of statistical identification and classification of earthquake clusters using a relocated catalog of 1981-2011 seismicity in southern California and synthetic catalogs produced by the Epidemic Type Aftershock Sequence model. Here we focus on classification of event families-statistically significant clusters composed of foreshocks, mainshocks, and aftershocks-that are detected with the methodology discussed in part I of the study. The families are analyzed using their representation as time oriented tree graphs. The results (1) demonstrate that the clustering associated with the largest earthquakes, m > 7, is statistically different from that of small-to-medium earthquakes; (2) establish the existence of two dominant types of small-to-medium magnitude earthquake families-burst-like and swarm-like sequences-and a variety of intermediate cluster forms obtained as a mixture of the two dominant types; (3) suggest a simple new quantitative measure for identifying the cluster type based on its topological structure; (4) demonstrate systematic spatial variability of the cluster characteristics on a scale of tens of kilometers in relation to heat flow and other properties governing the effective viscosity of a region; and (5) establish correlation between the family topological structure and a dozen of metric properties traditionally considered in the literature (number of aftershocks, duration, spatial properties, b-value, parameters of Omori-Utsu and Båth law, etc.). The burst-like clusters likely reflect highly brittle failures in relatively cold regions, while the swarm-like clusters are likely associated with mixed brittle-ductile failures in regions with relatively high temperature and/or fluid content. The results of this and paper I may be used to develop improved region-specific hazard estimates and earthquake forecasts.Citation: Zaliapin, I., and Y. Ben-Zion (2013), Earthquake clusters in southern California II: Classification and relation to physical properties of the crust,

Triggering Processes in Rock Fracture

“…We find that pronounced event-event triggering is only observable in the presence of large-scale imperfections such as a notch or inclusion in the sample. Triggering cascades tend to propagate away from the imperfections, and large cascades have a more swarmlike topology as observed for earthquakes in areas of high heat flow [29]. The triggered events typically have a focal mechanism more similar to their trigger than expected by chance, and they follow a GR distribution with a significantly lower b value than observed for background events (events without a trigger)-both features have also been reported for earthquakes [10,30].…”

“…week ending 11 AUGUST 2017 068501-3 values indicate that the cascades tend to follow a swarmlike structure and that the triggering process does not obey a simple branching process as often assumed [29]. Another feature of the underlying triggering process is that the distribution of 3D rotation angles δ between the trigger and triggered event is typically smaller than for random events, see Fig.…”

“…These randomized versions also allow one to establish a suitable threshold value for n à . To quantify the topological structure of the triggering cascades, we use the concept of vertex depth-the minimal number d of links that connects a given triggered event to the tree root (the first event in a given triggering cascade) [29]. A useful scalar measure suitable for the following analysis is the average leaf depth hdi f -the vertex depth d averaged over the leaves of a given tree or triggering cascade (events that do not trigger themselves).…”

“…A useful scalar measure suitable for the following analysis is the average leaf depth hdi f -the vertex depth d averaged over the leaves of a given tree or triggering cascade (events that do not trigger themselves). A high hdi f indicates a swarmlike activity, while a low hdi f indicates a burstlike activity [29].…”

A tentative model for the explanation of Båth law using the order parameter of seismicity in natural time

“…We note that Zaliapin and Ben-Zion (Zaliapin and Ben-Zion 2013a) have observed that for earthquake sequences in southern California DM follows an almost uniform distribution in the range [0,2] with an exponential tail for larger values of DM, thus supporting the value hDMi = 1.1 which has been found (Zaliapin and Ben-Zion 2013a) in the observed seismicity of southern California. They have also identified (Zaliapin and Ben-Zion 2013b) that there exist two basic types of earthquake clustering: the burst-like sequences which are consistent with highly brittle behaviour and the swarm-like sequences consistent with mixed brittle-ductile behaviour. The latter sequences result in smaller DM values than those of the former.…”