Outline of the 2011 off the Pacific coast of Tohoku Earthquake (M_w 9.0) —Earthquake Early Warning and observed seismic intensity—

In Japan, the JMA intensity scale is widely used to measure seismic intensity. Since 1996 this scale has been based on instrumental measurements in which not only the amplitude but also the frequency and duration of the shaking are considered (JMA, 1996; Hoshiba et al., 2010). The 10-degree JMA intensity scale rounds off the instrumental intensity value to the integer. Intensities of 5 and 6 are divided into two degrees, namely 5-lower, 5-upper, 6-lower and 6-upper, respectively. Intensity 1 corresponds to ground motion that people can barely detect, and 7 is the upper limit. At present, seismic intensity is measured at more than 4,000 places throughout Japan by JMA, municipalities, local governments and the National Research Institute for Earth Science and Disaster Prevention (NIED). When an earthquake occurs, the intensity data are transmitted to JMA and summarized. The summary is broadcast through multiple media starting within two minutes after the earthquake, and it is updated as the data increase.

EEW aims at mitigating an earthquake disaster by giving people enough time to take appropriate safety measures in advance of strong shaking. It has been operational nationwide in Japan by JMA since October, 2007. For JMA EEW, the hypocenter is determined by a combination of several techniques (Hoshiba et al., 2008), using approximately 1,100 stations from the JMA network and the Hi-net network of NIED; magnitude is mainly from maximum displacement amplitudes (Kamigaichi et al., 2009). The JMA EEWs are divided into two grades: “forecast” and “warning” (Hoshiba et al., 2008; Kamigaichi et al., 2009; Doi, 2010). The EEW “forecast” is issued to advanced users when events are estimated to be M 3.5 or larger, or when the expected seismic intensity is 3 or greater. In the “forecast”, the regions are particularly specified where seismic intensity 4 or greater is expected. When intensity is expected to be 5-lower or greater at any observation station of the seismic intensity networks, “warning” is issued to the general public in regions where intensity 4 or greater is expected. The “warning” is broadcast in various ways, such as by TV, radio and cellular phone mails. JMA EEWs are updated as available data increases with elapsed time. Accordingly EEWs are issued repeatedly with improving accuracy. The “warning” is updated when the seismic intensity is expected to be 5-lower or greater in regions where the intensity was estimated to be less than 4 in the first “warning”: that is, where new regions are subject to shaking above the threshold. Even when the update causes the expected intensity to fall below 5-lower at any stations, the “warning” is not canceled so as to avoid confusion if the expected intensity rises again. In the updated “warning”, the newly added regions are described. At present operation, an update of the “warning” is given only when the elapsed time is less than 60 s from the first trigger, to avoid too late a warning due to the fluctuation of gradually increasing amplitude of later phases (e.g., M 5.8 events of May 8, 2008; JMA, 2008).

Figure 3 shows the distribution of the observed seismic intensities of the March 11 mainshock, and the 2003 Tokachi Oki earthquake (M_w 8.0). The intensity contours are based on the observed intensities from the dense seismic intensity networks after taking into account the site amplification factors, which is similar to the idea of ShakeMap (Wald et al., 2006). The area of strong ground motion (6-upper and 6-lower) extends from the Tohoku to the Kanto district, over an area of approximately 400 km × 100 km, which is much larger than the corresponding area of the 2003 Tokachi Oki earthquake. K-NET station MYG004 in Kurihara city, at which intensity 7 was recorded, is 175 km from the epicenter, and some stations at which 6-lower intensity were observed in the Kanto district are more than 350 km from the epicenter.

In addition to the wide area of strong ground motion, a long duration was characteristic of the event. Figure 4 shows the acceleration at stations OURI (relatively near the epicenter, 138 km) and IYASAT(relatively far, 315 km), along with the seismic intensity measured in real-time manner using Kunugi et al.’s (2008) method. For comparison, acceleration and intensity at KiK-net station IWTH25 are also shown at the time of the 2008 Iwate-Miyagi Nairiku earthquake (M_w 7.0, 3 km). At OURI, two peaks are apparent in the acceleration envelope, which is probably due to the complicated source process. The second peak was 50 s after the first peak. At IYASAT, strong ground motion was apparent from later phases, well after the direct S phase, and it took 80 s to increase from intensity 1 (barely felt) to 5-lower, the level at which most people were frightened. The duration is very long as compared to 1 s which was observed at IWTH25 at the M_w 7.0 event. Ground motions corresponding to intensity 4, or greater, continued for 120 to 190 s at many observation stations in the Tohoku and Kanto districts (JMA, 2011c).

The EEW system expected intensity of 4 in the Tokyo region in the twelfth to fifteenth (final) issues (Fig. 2). This was an underestimation. Actual observations reached 5-upper, which is greater than the criterion of the EEW “warning”. The underestimation can probably be attributed to the large extent of the later fault rupture. For the northern part of Ibaraki prefecture (around IYASAT), where intensity expected in the first warning (fourth “forecast”) was less than 4, the expected intensity rose to 5-lower by the fourteenth “forecast”, but it was too late to update the “warning”, because it was issued 105 s after the trigger, which is later than the 60 s criteria at which upgrades are stopped.

The 2011 Tohoku Earthquake (M_w 9.0) generated widespread strong ground motion, and seismic intensities of 6-lower and 6-upper were recorded in the Tohoku and Kanto districts over an area of approximately 400 km × 100 km. The durations of strong ground motion were very long.

The JMA EEW system issued one “warning” to the general public in the Tohoku district before the start of strong ground motion. It was earlier than the S wave arrival and 15 s earlier than strong ground motion (intensity 5-lower or greater) at the closest station to the epicenter.

After the mainshock, the EEW system did not work well for several hours because of high background noise from the coda waves of the mainshock and active aftershocks, and because of power failure and wiring disconnections. For several days, when earthquakes sometimes occurred simultaneously over the wide source region, the system became confused, and did not always determine the location and magnitude correctly. In 19 days from the mainshock to March 29, 2011, JMA appropriately issued EEW “warning” for 15 of the 22 events for which seismic intensity 5-lower or greater was actually observed. On the other hand, during the same time, 45 EEW “warnings” were issued, but actual observed intensities did not exceed 2 at any observation stations in 11 of the 45 events (JMA, 2011d).