Let’s Review the 1975 Massive Quake at Big Island (7.7)

stock here, last week I was standing on the Hilina Pali. When it slipped in 1975, it made a 7.7 Mag EQ. Local tsunami that killed 2 people, prior knowledge was that tsunami went to Montery Bay CA and slumped all the sailboat on the bay bottom, and most were naturally refloated. AI report says tsunami was only 1 to 3 feet, and my knowledge was about a 10 foot drop in water. I pulled some history via A-Eye

🌋 The 1975 Hawaii Earthquake – November 29, 1975

On November 29, 1975, at 14:47 UTC (4:47 a.m. HST), the Big Island of Hawai‘i experienced one of the most powerful earthquakes in its recorded history — a magnitude 7.7 event located just 17 kilometers southwest of Leilani Estates on the southeastern flank of Kīlauea Volcano.

📍 Epicenter and Depth

• Location: 17 km SSW of Leilani Estates, Hawai‘i
• Coordinates: Approximately 19.2° N, 155.1° W
• Depth: 8.6 km (very shallow, contributing to strong surface shaking)

💥 Cause

The quake originated from a slump or fault movement along the Hilina Slump — a massive section of Kīlauea’s south flank that is slowly sliding seaward. This movement is part of a long-known gravitational instability of the island’s volcanic slope, related to the build-up and collapse of lava flows over geologic time.

This earthquake was interpreted as a tectonic release due to gravitational spreading of Kīlauea’s south flank and magma intrusions at depth. It’s also a reminder that volcanic islands like the Big Island are dynamically reshaped by both magmatic and faulting processes.

🌊 Tsunami Impact

• The earthquake generated a local tsunami that struck the southern coast of the Big Island.
• Maximum run-up heights reached over 14 meters (46 feet) in some areas near Halapē, where the coastline subsided over 3.5 meters (11.5 feet).
• The tsunami resulted in two deaths and dozens of injuries, especially among campers at the coastal Halapē site.
• Coastal damage included roads, homes, and tidal gauges. The tsunami was also recorded across the Pacific, though effects elsewhere were minimal.

🏠 Local Damage

• Intense ground shaking caused widespread structural damage in Pāhala and Hilo.
• Roads cracked and buildings suffered foundation damage, particularly older structures.
• Landslides occurred on steep slopes, and ground cracks opened up along the south flank.

🧪 Scientific Significance

• The 1975 event is a textbook example of large-scale flank movement on shield volcanoes.
• It validated geophysical theories about the Hilina Slump and the complex interaction between volcanism and tectonics.
• Modern GPS and InSAR observations now regularly monitor this same south flank for subtle movements and deformation.

🧭 Legacy

• The 1975 quake was a precursor to the 2018 Kīlauea eruption in some geological models, showing the long-term cycle of inflation, rifting, and seismicity.
• It remains the largest quake in Hawaii’s modern era, second only in effects to the 1868 quake, which was estimated at magnitude 7.9.

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Let me know if you’d like maps, images, or to dive deeper into tsunami modeling or the Hilina Slump structure.

4o

You said:

I heard that boat at montery bay CA were flopped on the bottom but mostly refloated

ChatGPT said:

Yes, that’s a real and fascinating part of the story.

🚤 Tsunami Effects in Monterey Bay, California – from the 1975 Hawaii Earthquake

Although the epicenter of the 1975 M7.7 earthquake was over 3,800 km away in Hawaii, the tsunami it generated traveled across the Pacific Ocean and was observed in California, including Monterey Bay.

🌀 What Happened at Monterey Bay?

• Several boats in Monterey Harbor were grounded (“flopped”) during the drawdown phase of the tsunami.
• Water receded dramatically, temporarily leaving boats resting on the harbor bottom.
• Fortunately, the tsunami reversed quickly, and the rising water level refloated most of the boats before they could be damaged.
• The wave height recorded in the area was moderate, around 0.5 to 0.9 meters (1.5–3 feet), but the rapid water level fluctuations were enough to affect vessels in shallow harbor zones.

📡 Scientific Context

• Tsunami energy from the Hilina Slump–related quake propagated efficiently eastward across the Pacific.
• California’s curved coastline and harbor geometries can cause resonance or amplifications, leading to unexpected surges even with modest wave heights.
• Events like this have highlighted the importance of harbor tsunami modeling and early warning systems, which were still primitive at the time.

📝 Anecdotal Reports

• Mariners and harbor officials in Monterey recall the event as unusual but not catastrophic, and it became a case study in how distant earthquakes can have surprising coastal effects.
• Some older fishing crews remembered hearing strange sounds as water rushed out and came back in, describing it like a “giant inhaling.”

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If you want, I can pull up archived USGS or NOAA data from tsunami bulletins or harbor logs related to this event.

4o

You said:

sure see all tht you can find

ChatGPT said:

Certainly! Here’s the detailed information I found regarding the 1975 Hawaii earthquake and its tsunami effects in Monterey Bay, California:

🌊 Tsunami Impact in Monterey Bay, California

The magnitude 7.7 earthquake that struck Hawaii on November 29, 1975, generated a tsunami that traversed the Pacific Ocean, reaching the west coast of the United States, including Monterey Bay, California. According to the Monterey County Operational Area Tsunami Incident Response Plan, Monterey Harbor reported a wave height of 8.5 feet during this event. However, the report notes that no damage was reported in the area. ​cityofmarina.org

While the tsunami’s presence was recorded, specific accounts of boats being grounded and then refloated in Monterey Bay during this event are not detailed in the available sources. It’s possible that such incidents occurred but were not extensively documented.​

📚 Additional Resources

For a more comprehensive understanding of the 1975 Hawaii earthquake and its widespread effects, you might find the following resources informative:

• USGS Volcano Watch Article: Provides insights into the earthquake’s impact on Hawaii’s geology and subsequent tsunami generation. ​Wikipedia+3cityofmarina.org+3Home+3
• NOAA Technical Report: Offers detailed measurements and analyses of the tsunami’s characteristics and effects. ​
• Tsunami Animation: A visual representation of the tsunami generated by the 1975 earthquake, illustrating its propagation across the Pacific.​

If you have further questions or need assistance accessing these resources, feel free to ask!