{"id":66224,"date":"2026-05-31T05:00:43","date_gmt":"2026-05-31T09:00:43","guid":{"rendered":"https:\/\/swling.com\/blog\/?p=66224"},"modified":"2026-05-31T07:16:54","modified_gmt":"2026-05-31T11:16:54","slug":"did-the-miyako-earthquake-affect-medium-wave-reception-at-a-japanese-dxpedition","status":"publish","type":"post","link":"https:\/\/swling.com\/blog\/2026\/05\/did-the-miyako-earthquake-affect-medium-wave-reception-at-a-japanese-dxpedition\/","title":{"rendered":"Did the Miyako earthquake affect Medium Wave reception at a Japanese DXpedition?"},"content":{"rendered":"

by Satoshi Miyauchi, JP1SCQ, with Nick Hall-Patch, VE7DXR<\/strong><\/p>\n

Introduction<\/b><\/p>\n

In early November 2025, several members of our Totsuka DXers Circle in Japan (TDXC\u00a0 https:\/\/www.tdxc.net\/abouttdxc\/<\/b><\/a> ) traveled from the Tokyo area to Tanohata village in Iwate prefecture on northern Honshu island in order to take part in a medium wave (MW) DXpedition that took place on the 8th and 9th of the month.\u00a0 The site was about 500m (1\/3 mile) from the Pacific Ocean, overlooking Kitayamazaki cliffs, a very scenic area (Figure 1<\/b>), but also one from which a great deal of long-haul DX had been heard in the past, including trans-polar WBZ-1030kHz, as well as the farthest possible Antipodes DX such as R. Nacional in Argentina on 870kHz and Radio Monte Carlo in Uruguay on 930kHz.<\/p>\n

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Figure 1<\/p><\/div>\n

Our listening post was a meeting room in the Tanohata Nature Training Center, where we set up our receivers, such as Perseus and Airspy HF+discovery, plus our recording gear and accessories (Figure 2<\/b>).<\/p>\n

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Figure 2<\/p><\/div>\n

My recording software was SDR Console, but playback and analysis also used WavViewDX.\u00a0 We set up a TDDF (Twisted Double Delta Flag) antenna with a northeast directional pattern in order to receive medium-wave broadcasts from North America. (Figure 3)<\/b><\/p>\n

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Figure 3 \u2013 TDDF antenna; note that low-noise pre-amplifier with bias-T is a must.<\/p><\/div>\n

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Directional patterns from Kazu GOSUI<\/p><\/div>\n

On the second evening, November 9th, while enjoying the reception, an emergency earthquake alert was issued, and shaking struck. Inside our building, nearly 200 meters above sea level on the solid bedrock of Kitayamazaki, the shaking felt less intense than the reported magnitude of 6.9, even with an epicenter only 140km away. (Figure 4<\/b>)<\/p>\n

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Figure 4<\/p><\/div>\n

However, since earthquakes had been occurring even before that day and numerous aftershocks were felt afterward, it left us with a vague sense of unease. Later, a tsunami advisory was announced on the radio, plus the Tohoku Shinkansen train back to Tokyo had also stopped, and I myself couldn’t help worrying about whether it might affect my return home the following day. At that moment, I had a conversation with the members there, thinking, “If there’s something related to the earthquake recorded, that would be amazing.” However, during the real-time reception, we were targeting signals from North America in 10kHz steps, and there was no effect noticed upon those receptions.<\/p>\n

Unusual Signal Dropouts Observed<\/b><\/p>\n

I played back the SDR files using WavViewDX (https:\/\/rweiss.de\/dxer\/tools.html<\/b><\/a>), a software with many capabilities, including a choice of displaying all signals across the MW band at 9 or 10kHz channel spacing, but, because I was looking for North American DX, I only realized a week after returning home that the reception conditions for the 9kHz spaced domestic Japanese stations had significantly changed around 0715 to 0745UT (16:15 to 16:45 Japan time) on 9 November, based on our recordings. The dropouts on various channels over 0715 to 0745UT are quite obvious in Figure 5<\/b>; I had never seen such sudden attenuation before.\u00a0 For those not familiar with WavViewDX, the green vertical lines on the display represent stronger signals being received on broadcast channels, while gray or black areas represent weak or no signal. (For a more detailed description of WavViewDX and its capabilities, see https:\/\/swling.com\/blog\/2025\/10\/an-introduction-to-wavviewdx-sdr-playback-software-a-totsuka-dxers-circle-article-by-kazu-gosui<\/b><\/a><\/p>\n

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Figure 5 – WavViewDX display of signal dropouts. X-axis is frequency of received signal, Y-axis is time UTC<\/p><\/div>\n

A first look at the data led to a couple of other observations:<\/p>\n

    \n
  1. Signals originating north of the receiving site, primarily from the island of Hokkaido, were largely unaffected by the attenuation. (It is true that our antenna’s directionality was northeast, but it also received the stronger domestic stations from southwest of the antenna.)<\/li>\n
  2. Regarding signals from North America, even during the same time period, the intense attenuation observed in domestic stations was generally not seen. It is unclear, however, whether some dips in North American signals around that time were due to normal fading or to the same cause that brought about the attenuation in domestic stations.<\/li>\n<\/ol>\n

    What Could Have Caused These Dropouts?<\/b><\/p>\n

    Local sunset?<\/p>\n

    These sudden drops in signal strength corresponded quite closely with local sunset at 0722UT, normally a time of disturbed propagation (see Figure 6<\/b>), so the most straightforward possibility is simply the well-known change in ionospheric propagation conditions that occurs at sunset. \u00a0 Was that all that there was to it?\u00a0 However, we had been listening and recording the previous day as well, and analyzing those recordings with WavViewDX yielded no sign of dropouts in domestic signal strength at sunset on that day. \u00a0 Examining recordings that had been made at the same site, using similar equipment, on 24 October 2024, also showed no dropouts taking place at local sunset.<\/p>\n

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    Figure 6<\/p><\/div>\n

    In fact, over many years in Japan, not only at this location but across various areas, records have been accumulated during the same time window, because good trans-Pacific DX occurs around local sunset.\u00a0 Nowhere in these records has a situation such as observed this time\u2014a significant attenuation of domestic stations at local sunset\u2014been found. Therefore, it seemed unlikely that sunset was the cause of the dropouts, but what else could it have been?<\/p>\n

    The earthquake?<\/p>\n

    What about the occurrence of the large earthquake at 0803UT, also noted in Figure 6,<\/b> which was within the hour after the observed attenuation events?\u00a0 Although there have been reports of changes in very low frequency (VLF) signals and noise before earthquakes, we had not heard of signal strength precursors having been observed at medium frequencies.\u00a0 That caused us to look more closely at which signals were actually getting attenuated.<\/p>\n

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    Table 1<\/p><\/div>\n

    First, we’ve listed domestic Japanese stations that experienced significant attenuation during the 0700-0800UT period, as well as those with almost no change, representing them as RED (attenuation present) in Table 2<\/b> and BLUE (attenuation absent) in Table 1<\/b>.<\/p>\n

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    Table 2<\/p><\/div>\n

    Significant attenuation was observed for nearly all signals from stations south of a line drawn between Akita in the northwest of Honshu and our site at Tanohata in the northeast. In contrast, stations where no severe attenuation was observed were those north of the receiving location, specifically from the northern tip of Honshu to the Sea of Japan.\u00a0 These were mainly stations from various locations across Hokkaido.\u00a0 The two areas are represented by the red and blue circles in Figure 7<\/b>.\u00a0 In addition, a number of signals from the northeastern portion of the People\u2019s Republic of China also suffered attenuation at about the same time as signals were attenuated in southern Japan. (Table 3<\/b>).<\/p>\n

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    Table 3<\/p><\/div>\n

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    Figure 7<\/p><\/div>\n

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    Figure 8<\/p><\/div>\n

    We have noted that the attenuation in signal strengths occurred before the earthquake.\u00a0 But the only easily available data about the earthquake\u2019s effects on Japan\u2019s land mass were from various seismographic records at the time of the earthquake itself.\u00a0 So, we started with those records, portrayed on a map in Figure 8<\/b>), alongside a similar Google map of Japan with pins indicating effects on signal strength (red pins for attenuated signals and blue pins for unaffected signals).\u00a0 Was there at least some correlation between the intensity of the earthquake at various sites and the sites of observed attenuation?\u00a0 The earthquake was felt less strongly in areas further to the north of the receiving site, and signals were not attenuated from there. \u00a0 However, the earthquake was not felt particularly strongly further south in Japan, and signals from that part of the country had definitely been attenuated. \u00a0 Perhaps before we started looking for seismographic datasets from before the earthquake occurred, we should look for some more obvious cause of the signal drop-outs that we had missed?<\/p>\n

    Further signal analysis<\/b><\/p>\n

    So, we looked more closely at the strength of the attenuations relative to the locations of the transmitter sites.\u00a0 In fact, it was found that more distant signals from the south of Japan and from northeast China were attenuated even more<\/i> than signals nearer the earthquake epicenter.<\/p>\n

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    Figure 9<\/p><\/div>\n

    Figure 9 <\/b>based on Google Maps shows this clearly, where the yellow pins indicate greater than 25dB declines in signal strength from various transmitter sites during the attenuation event, while the red pins indicate signal attenuations of less than 25dB.<\/p>\n

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    Figure 10<\/p><\/div>\n

    To illustrate more clearly the difference in the characteristics of the attenuation observed in signals received in these two categories, we chose receptions from JOER-1350kHz in Hiroshima, at 1040 km from Tanohata, and JODR-1116kHz in Niigata at 340 km away (Figure 10<\/b>). \u00a0 We then created graphical representations of the signal strength variations in those receptions, as seen in Figure 11<\/b>, using Carrier Sleuth\u2019s signal strength extraction capabilities (https:\/\/www.blackcatsystems.com\/software\/medium_wave_carrier_display_app.html<\/b><\/a>).\u00a0 On JODR\u2019s chart, we have also superimposed the signal strength of a nearby relay of Japan\u2019s NHK 1 network (now NHK AM) on 1224kHz.<\/p>\n

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    Figure 11 \u2013 Signal strengths JOER-1350kHz (red trace), JODR-1116kHz (blue trace), and NHK1-1224kHz (purple trace)<\/p><\/div>\n

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    There was much deeper attenuation of the more distant JOER\u2019s signal compared with that from JODR\u2019s closer transmitter.\u00a0 These attenuations took place during the normal increase in signal strength from distant MW transmitters that occurs as sunset approaches, indicating that these two signals were almost certainly skywave, not ground wave.\u00a0 In comparison, the signal on 1224kHz from 5km away was almost certainly ground wave only, and showed little change in signal strength as sunset approached, nor did it show any sign of the attenuation observed on JOER and JODR\u2019s signals.\u00a0 It seems likely then that the attenuations observed were a skywave phenomenon, yet the skywave signals from north of Tanohata were unaffected.<\/p>\n

    Perhaps the position of the sunset terminator was more important than the time of the earthquake after all; we\u2019ve indicated the sunset terminator in Figure 9 <\/b>as it passed through Tanohata at 0722UT on 9 November, a few minutes after the attenuation event began.\u00a0 These points are clear:<\/p>\n