Contribution of Slow Earthquake Study for Assessing the Occurrence Potential of Megathrust Earthquakes Review: Contribution of Slow Earthquake Study for Assessing the Occurrence Potential of Megathrust Earthquakes Kazushige Obara Earthquake Research Institute, The University of Tokyo 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan E-mail: [email protected] [Received February 17, 2014; accepted May 12, 2014] Studies of slow earthquakes during the last decade nomena, may be related to the stress regimes that cause have suggested a relationship between various types megathrust earthquakes, as the source regions of the two of earthquakes occurring at the interface between types of earthquakes are adjacent to one another. subducting oceanic plates and overlying continental Slow earthquakes can be broadly classified into seismic plates. Such a relationship has been postulated for and geodetic phenomena. For example, in 1992, an after- slow earthquakes, which are distributed between the slip event (in which the slip deficit accumulating adjacent stable sliding zone and the locked zone, and megath- to a large earthquake source fault is released) followed rust earthquakes, which are located in the locked by a magnitude (M) 6.9 earthquake off northern Honshu, zone. The adjacency of the respective sources of slow Japan, was first reported by Kawasaki et al. (1995) [1] and megathrust earthquakes suggests expected inter- as a geodetic slow earthquake (Fig. 2). The development actions between these two types of earthquakes. Ob- of GPS observation networks since the 1990s has enabled served interactions between different types of slow not only the detection of some afterslip events, but also earthquakes located at neighbor area suggest a com- the discovery of slow slip events (SSEs), in which small mon triggering mechanism in the seismogenic zone. amounts of crustal deformation occur spontaneously. Af- Also, it is expected that stress accumulations in the ter the construction of the GPS network GEONET, op- locked zone should influence stress regimes in sur- erated by the Geospatial Information Authority of Japan rounding regions; thus, slow earthquake activity in the (GSI) (Sagiya, 2004 [2]), an SSE lasting for about one stable sliding zone may change in response to stress year was detected in 1997 in the Bungo Channel between build-up in the locked zone. Numerical simulations Kyushu and Shikoku islands, southwestern Japan (Fig. 2). reproducing both megathrust and slow earthquakes Hirose et al. (1999) [3] estimated that the SSE event was show a shortening of the recurrence interval between generated by a thrusttype focal mechanism located on the slow earthquake episodes leading up to the occurrence downdip side of the Nankai earthquake seismogenic zone. of a megathrust earthquake. Similarities between the In the Tokai region, central Japan, similar SSEs were de- activities of slow and megathrust earthquakes, such as tected between late 2000 and 2005 (e.g., Ozawa et al., those related to periodicity and patterns of multiseg- 2002 [4]; Miyazaki et al., 2006 [5]). The Tokai SSE was ment ruptures, are useful for understanding megath- of interest because during the initial stage, the SSE oc- rust earthquakes, particularly given the higher fre- curred along the downdip part of the slab interface neigh- quency of occurrence of slow earthquakes. From this boring the source area of an anticipated Tokai earthquake. perspective, the continuous and accurate monitoring Similarly, SSEs with large magnitudes (M7) and long du- of slow earthquake activity is important for evaluating rations (months to years) have been detected in other sub- the occurrence potential of megathrust earthquakes. duction zones, for example in Alaska (e.g., Ohta et al., 2006 [6]) and in Mexico (e.g., Vergnolle et al., 2010 [7]) Keywords: slow earthquake, nonvolcanic tremor, slow (Fig. 3). slip, subduction zone, megathrust earthquake In the Cascadia subduction zone, along the western coast of North America, a significant SSE was discovered by a network of densely distributed GPS sites (Dragert 1. Introduction et al., 2001 [8]) (Fig. 3). This SSE (Mw6.7) was a reverse faulting event occurring on the interface of the deeper part Slow earthquakes, which are characterized by a wide from the megathrust seismogenic zone along the subduct- spectral range, and megathrust earthquakes both occur in ing plate interface. The duration of the SSE of weeks was subduction zones, along the interface between oceanic much shorter than those of the SSEs in Japan, Alaska, and and overlying plates (Fig. 1). The source of slow earth- Mexico described above. To discriminate SSEs based on quakes is in the transition zone between the locked and their durations, events such as the SSE in Cascadia are re- stable sliding zones. Therefore, the occurrence of slow ferred to as short-term SSEs, while SSEs with durations earthquakes, which are considered as transitional phe- of months to years (such as those in the Bungo Channel, Journal of Disaster Research Vol.9 No.3, 2014 317 Obara, K. Near Downdip side Nankai Trough Characteristic time (tc) Sensor Network Long-term slow slip event GPS GSI 100day (tc:0.5~5years) GEONET Short-term slow slip event 1day Tiltmeter (tc:2~6days) 1000sec Deep very low frequency Tiltmeter NIED Earthquake䠄VLF䠅 (tc:20sec䠅 Hi-net Shallow very low 10sec frequency earthquake Deep low frequency tremor Short-period (S-VLF) (tc:10sec䠅 0.1sec (tc:1.5~5Hz) seismometer Accretionary prism Nankai Trough Long-term SSE Episodic Tremor and Slip Short-term SSE S-VLF VLF Tremor Fig. 1. Cross sectional schematic illustration of slow earthquakes in southwest Japan. On the top panel, the difference in characteristic time of each slow earthquake is shown. On the right side of the top panel, adequate sensor and observation network for each slow earthquake are indicated. Tokai, Alaska, and Mexico) are referred to as long-term SSEs. Interestingly, the short-term SSE in Cascadia was Afterslip characterized by an along-strike migration of the source Long-term SSE location at a speed of several km per day. After the dis- covery of the Cascadia SSE, Miller et al. (2002) [9] found Boso-type SSE 1 that it recurred at an interval of about 14 months. There- Shallow VLF Tokachi-oki fore, this SSE is interpreted as a stickslip phenomenon ETS(Deep VLF, tremor, 11 Short-term SSE) occurring on the downdip side of the megathrust seis- 2 Sanriku-oki mogenic zone, which is estimated to have ruptured in Tohoku SSE AD 1700 based on historical descriptions of tsunami in 15 Japan (Satake et al., 2003 [10]) (Fig. 4). 3 Miyagi-oki A nonvolcanic tremor was the first seismic slow earth- 4 quake to be recorded in southwestern Japan (Obara, Fukushima-oki 2002 [11]; Obara and Shiomi, 2009 [12]) (Figs. 1 and 2) Tokai 10 14 based on the analysis of Hi-net operated by National Re- Bungo Cha. 6 search Institute for Earth Science and Disaster Prevention 14 Boso 14 (NIED) (Okada et al., 2004 [13]; Obara et al., 2005 [14]). 7 12 12 8 12 12 The tremor was distributed along a belt-shaped zone on 5 13 9 the downdip side of the seismogenic zone of the Nankai 12 Trough megathrust earthquake, parallel to the strike of the Hyuga-nada subducting Philippine Sea Plate. The tremor source lo- cation usually migrated along strike at a velocity of ap- Fig. 2. Distribution of slow earthquakes in Japan. proximately 10 km/day. Based on the similarity of the Afterslip: 1, Miyazaki et al. (2004) [16]; 2, Kawasaki et short-term SSEs in Cascadia and the tremor in south- al. (1995) [1]; 3, Miura et al. (2006) [17]; 4, Suito et al. western Japan, with respect to their locations relative to (2011) [18]; 5, Yagi and Kikuchi (2003) [19], Long-term the seismogenic zone and to their migration properties, SSE: 6, Miyazaki et al. (2006) [5]; 7, Kobayashi (2012) [20]; Rogers and Dragert (2003) [15] searched for a seismic 8, Hirose et al. (1999) [3]; 9, Yarai and Ozawa (2013) [21], tremor during the occurrence period of the SSEs in Cas- Boso-type SSE: 10, Hirose et al. (2012) [22], Shallow VLF cadia and discovered a coupled phenomenon composed earthquake: 11, Asano et al. (2008) [23]; 12, Obara and Ito of a geodetic SSE and a seismic tremor, referred to as (2005) [24]; 13, Hirose et al. (2010) [25], ETS: 14, Obara et episodic tremor and slip (ETS) (Fig. 4). A coupling phe- al. (2004) [26], Ito et al. (2007) [27], Tohoku SSE: 15, Kato nomenon that includes both short-term SSE and tremor et al. (2012) [28], Ito et al. (2013) [29]. 318 Journal of Disaster Research Vol.9 No.3, 2014 Contribution of Slow Earthquake Study for Assessing the Occurrence Potential of Megathrust Earthquakes ETS(Episodic tremor and slip) Alaska Deep VLF earthquake 21 14 9 60N Aleutian Queen Charlotte Fault Ambient tremor without SSE 9 (including triggered tremor) 2 Cascadia 8 San Andreas Fault Triggered tremor only Japan 30N Mexico Taiwan 4 19 20 Long-term SSE 13 9 10 1 31517 Haiti Boso-type shallow SSE 22 Costa Rica 0 Shallow VLF earthquake 12 7 18 Tremor excited by SSE 30S Shallow VLF excited by SSE New Zealand 6 Southern Chile 11 5 16 60S 0 60E 120E 180W 120W 60W Fig. 3. World-wide distribution of various types of slow earthquakes. ETS: 1, Obara et al. (2004) [26], Maeda and Obara (2009) [30]; 2, Rogers and Dragert (2003) [15], Deep VLF earthquake: 3, Ito et al. (2007) [27], Ambient tremor: 4, Peng and Chao (2008) [31], Chao et al. (2013) [32]; 5, Ide (2012) [33], Kim et al. (2011) [34]; 6, Gallego et al. (2013) [35]; 7, Brown et al. (2009) [36]; 8, Nadeau and Dolenc (2005) [37], Triggered tremor: 9, Chao et al.