Yukutake and Iio Earth, Planets and Space (2017) 69:68 DOI 10.1186/s40623-017-0650-2 FULL PAPER Open Access Why do aftershocks occur? Relationship between mainshock rupture and aftershock sequence based on highly resolved hypocenter and focal mechanism distributions Yohei Yukutake1* and Yoshihisa Iio2 Abstract In order to clarify the origin of aftershocks, we precisely analyze the hypocenters and focal mechanisms of the after- shocks following the 2000 Western Tottori Earthquake, which occurred in the western part of Japan, using data from dense seismic observations. We investigate whether aftershocks occur on the mainshock fault plane on which coseis- mic slip occurred or they represent the rupture of fractures surrounding the mainshock fault plane. Based on the hypocenter distribution of the aftershocks, the subsurface fault structure of the mainshock is estimated using princi- pal component analysis. As a result, we can obtain the detail fault structure composed of 8 best-ft planes. We demon- strate that the aftershocks around the mainshock fault are distributed within zones of 1.0–1.5 km in thicknesses, and their focal mechanisms are signifcantly diverse. This result suggests that most of the aftershocks represent the rupture of fractures surrounding the mainshock fault rather than the rerupture of the mainshock fault. The aftershocks have a much wider zone compared with the exhumed fault zone in feld observations, suggesting that many aftershocks occur outside the fault damage zone. We fnd that most aftershocks except in and around the large-slip region are well explained by coseismic stress changes. These results suggest that the thickness of the aftershock distribution may be controlled by the stress changes caused by the heterogeneous slip distribution during the mainshock. The aftershock is also distributed within a much wider zone than the hypocenter distribution observed in swarm activity in the geothermal region, which is thought to be caused by the migration of hydrothermal fuid. This result implies a diference in generation processes: Stress changes due to the mainshock contribute primarily to the occurrence of aftershocks, whereas earthquake swarms in the geothermal region are caused by fuid migration within the localized zone. Keywords: Aftershock, Fault plane of mainshock, Fault damage zone, Hypocenter distribution, Focal mechanism Background of the mainshock slip (e.g., Das and Henry 2003) and to Understanding why aftershocks occur is one of the major be triggered by strength reductions due to high pore fuid unsolved problems in seismology, and several genera- pressure caused by the mainshock slip (e.g., Miller et al. tion mechanisms of aftershocks have been proposed. For 2004). Although all of these mechanisms have reasonable example, aftershocks have been thought to be reruptures physical bases, which mechanism is actually at work has of remnants on the fault surface that were not broken by rarely been confrmed based on precise aftershock data. the mainshock, to be generated by stress concentrations Te hypocenter distribution of aftershocks has been recognized to refect the structure of the fault plane that was ruptured during the mainshock. Tus, a number of *Correspondence: [email protected] previous studies estimated the subsurface structure of the 1 Hot Springs Research Institute of Kanagawa Prefectural Government, Odawara, Japan mainshock fault plane based on the aftershock distribu- Full list of author information is available at the end of the article tion (e.g., Shibutani et al. 2005a; Kato 2005). On the other © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Yukutake and Iio Earth, Planets and Space (2017) 69:68 Page 2 of 15 hand, feld observation of an exhumed outcrop has dem- was released on the fault plane in the southern part of onstrated that a fault damage zone is developed around the aftershock region, whereas afterslip occurred on the a fault core that is the localized slip zone and contains fault planes in the northern part of the aftershock region. high-strain products, such as gouges, cataclasites, and Fukuyama et al. (2003) estimated the fault model of the ultracataclasites (Mitchell and Faulkner 2009). Te fault Western Tottori Earthquake using the detailed distribu- damage zone has also been identifed using geophysical tion of the aftershock hypocenters and their centroid data, such as seismic travel times or trapped waves (e.g., moment tensor (CMT) solutions. However, they used Cochran et al. 2009; Ben-Zion et al. 2003). We have not only seismic data obtained by permanent stations in clarifed what an aftershock distribution represents, spe- and around the aftershock region with a station inter- cifcally, whether aftershocks refect the rerupture of the val of approximately 20 km. Terefore, the locations of mainshock fault plane or the rupture of the damage zone the aftershock hypocenters in their study contain large surrounding the mainshock fault plane or whether after- uncertainty. shocks occur outside the damage zone. Tis is important to clarifying the generation mechanism of aftershocks. Data and methods Liu et al. (2003) investigated the hypocenter catalog Determination of hypocenters and focal mechanisms of the aftershock sequence following the 1993 Mw 7.3 A total of 48 temporary stations were installed in and Landers earthquake and concluded that only 5% of after- around the aftershock region during the period from 13 shocks occurred on the mainshock fault plane. However, October to early December 2000 (Shibutani et al. 2005b). since the location errors of the aftershock hypocenters in Together with data from seven permanent stations, the their study were up to 1 km, discussion based on more dense seismic observation network was developed with precisely determined hypocenters is essential. Moreo- an average station spacing of 4–5 km near the center of ver, a focal mechanism solution of aftershocks provides the aftershock region (Fig. 1). We manually picked the additional information associated with the orientation of arrival time of the P- and S-waves and the polarities of the fault plane. We analyzed the aftershock sequence of the P-waves for 4770 local events that occurred during the 2000 Western Tottori Earthquake that occurred in an the period from October 15, to November 31, 2000. Te intraplate region. A dense seismic observation network local magnitudes of these events ranged from 0.4 to 3.9. composed of 48 temporary stations was installed imme- In order to obtain the initial hypocenter locations, we diately after the mainshock. Te high-quality observation selected 4190 events for which the P- or S- wave arrival data provide an excellent opportunity to clarify the above times were obtained at eight or more stations. Te initial issue. hypocenter locations were obtained by applying the one- In the present study, we estimate the hypocenter loca- dimensional velocity model and the station corrections tions and focal mechanisms of the aftershocks of the estimated by Shibutani et al. (2005b) to the method of 2000 Western Tottori Earthquake using dense seismic hypocenter determination developed by Kawanishi et al. observations. We examine the fault model of the main- (2009), which is a modifed version of the Hypomh pro- shock based on precisely determined hypocenter loca- gram code (Hirata and Matsu’ura 1987). tions. Moreover, we discuss the relationship between the Te double-diference relocation (DD) method (Wald- subsurface fault structure of the mainshock and the after- hauser and Ellsworth 2000) was applied to the ini- shock distribution. tial hypocenter data. Te diferential arrival time data for the manually picked P- and S-waves consisted of Overview of the 2000 Western Tottori Earthquake 597,744 and 270,359 pairs, respectively. We also used Te 2000 Western Tottori Earthquake occurred on Octo- the diferential arrival times obtained by cross-corre- ber 6, 2000, in the western part of Tottori Prefecture in lation analysis. Te correlation analysis was conducted southwestern Honshu, Japan (Fig. 1). Te moment mag- using a velocity waveform with a time window of 0.5 s, nitude was determined to be 6.6 (Fukuyama et al. 2003). including the manually picked arrival times, applying a Iwata and Sekiguchi (2002) estimated the slip distribution band-pass flter between 3 and 20 Hz. We used only the of the mainshock using near-feld strong motion wave- diferential arrival time data with normalized cross-cor- forms and geodetic data and found that coseismic slip relation coefcients greater than or equal to 0.8. Using occurred on the southern part of the aftershock region, this threshold, we obtained cross-correlation data con- while there was little moment release on the north- sisting of 307,447 pairs for the P-wave and 177,677 pairs ern part of the aftershock region (Fig. 2b). Sagiya et al. for the S-wave. After application of the DD method, (2002) also estimated the slip distribution using global 4089 events were successfully relocated (Fig. 2). Te navigation satellite system (GNSS) data and reported root-mean-square (RMS) of the double-diference time that most of the seismic moment during the mainshock residual decreased from 95 to 30 ms for the manually Yukutake and Iio Earth, Planets and Space (2017) 69:68 Page 3 of 15 0 km 30 35.6˚N 35.4˚ 35.2˚ Mainshock Temporary station a First motion b CMT Permanent station Active fault 133.0˚E 133.2˚ 133.4˚ 133.6˚ Fig. 1 Map showing the spatial distribution of seismic stations (blue: temporary stations, white: permanent stations). The red star indicates the start- ing point of rupture during the 2000 Western Tottori Earthquake.