Phenomenology of deep slow earthquake family in southwest Japan: Spatiotemporal characteristics and segmentation

Abstract

The phenomenology of deep slow earthquakes, including low‐frequency tremors, very low frequency earthquakes, and short‐term slow slip events, on the subducting plate interface in southwest Japan is investigated on the basis of their spatiotemporal characteristics. The belt‐like distribution of tremors is divided into segments bounded by gaps. The repetition of tremor episode depends on the magnitude of the episode, which is defined by the number of detected tremors within each episode. Major tremor episodes with a large number of detected tremors recur at an interval of approximately 6 months, accompanying the short‐term slow slip events in large segments. In small segments, tremor episodes with small numbers of tremors have recurrence intervals of a few months. Some of them occasionally accompany the slight ground tilting caused by a small‐size slow slip event. Even in the same segment, there exist different recurrence intervals according to the magnitude of the tremor episode. The migration pattern varies greatly in initiation, termination, direction, and speed; however, the migration direction has a general tendency for each segment. On both sides of the gap, the tremor starts next to the gap and propagates to the other side. Assuming all tremor episodes are caused by slow slip events, the equivalent moment and slip are estimated from the number of detected tremor counts and the area of each tremor episode. The equivalent slip history estimated from all tremor episodes is almost constant in each segment. This indicates that tremor activity is a good proxy for slow slips.

Scaling Relationships of Source Parameters for Slow Slip Events

“…The rupture characteristics for SSEs and nonvolcanic tremor often exhibit complex propagation behavior, including propagation reversals to the major trend along strike, updip migration faster than the along-strike propagation, and fast-then-slow rupture pattern along strike McCausland et al, 2010;Obara, 2010;Houston et al, 2011). In this study, we only consider the average rupture velocity, which is calculated by dividing the rupture length by the event duration.…”

“…Determining the rupture velocity from tremor is more problematic. Most studies report the propagation of tremor streaks Obara, 2010), whereas few studies have resolved the rupture velocity of an individual event (Ito and Obara, 2006). Because of the difficulty in estimating the fault dimension and individual event duration from tremor, we focus on short-and long-term slow earthquakes (with durations longer than days, Fig.…”

“…Meade and Loveless (2009) considered the scaling law for the average particle velocity versus moment. Obara (2010) explored the relationship of moment versus fault area for tremors and inferred a static stress drop less than 0.1 MPa. Finally, Brodsky and Mori (2007) examined the relationship between fault slip, rupture length, and duration for creep events versus earthquakes.…”

Slow slip events in Costa Rica detected by continuous GPS observations, 2002–2011

“…In Japan, episodic tremors and slip events vary in duration and distribution [Obara, 2010;Liu et al, 2010]. The magnitudes and recurrence intervals of these events are important aspects of seismic hazard estimates [Peng and Gomberg, 2010;Obara, 2010]. To the extent that such events release a certain portion of accumulated strain due to plate motion on a locked plate interface, they may reduce the potential rupture area and magnitude of future large earthquakes.…”

“…In Cascadia, such events typically last 1-4 weeks, have centimeters of slip on the plate interface and have recurrence intervals of 11 to 18 months [Szeliga et al, 2008;Schmidt and Gao, 2010;Holtkamp and Brudzinski, 2010]. In Japan, episodic tremors and slip events vary in duration and distribution [Obara, 2010;Liu et al, 2010]. The magnitudes and recurrence intervals of these events are important aspects of seismic hazard estimates [Peng and Gomberg, 2010;Obara, 2010].…”